Rock sponges (lithistid Demospongiae) of the Northeast Atlantic seamounts, with description of ten new species

Background Lithistid demosponges, also known as rock sponges, are a polyphyletic group of sponges which are widely distributed. In the Northeast Atlantic (NEA), 17 species are known and the current knowledge on their distribution is mainly restricted to the Macaronesian islands. In the Mediterranean Sea, 14 species are recorded and generally found in marine caves. Methods Lithistids were sampled in nine NEA seamounts during the scientific expeditions Seamount 1 (1987) and Seamount 2 (1993) organized by the MNHN of Paris. Collected specimens were identified through the analyses of external and internal morphological characters using light and scanning electron microscopy, and compared with material from various museum collections as well as literature records. Results A total of 68 specimens were analysed and attributed to 17 species across two orders, seven families, and seven genera, representing new records of distribution. Ten of these species are new to science, viz. Neoschrammeniella inaequalis sp. nov., N. piserai sp. nov., N. pomponiae sp. nov., Discodermia arbor sp. nov., D. kellyae sp. nov., Macandrewia schusterae sp. nov., M. minima sp. nov., Exsuperantia levii sp. nov., Leiodermatium tuba sp. nov. and Siphonidium elongatus sp. nov., and are here described and illustrated. New bathymetric records were also found for D. ramifera, D. verrucosa and M. robusta. The Meteor seamount group has a higher species richness (15 species) compared to the Lusitanian seamount group (six species). The majority of the species had their distribution restricted to one seamount, and ten are only known from a single locality, but this can be a result of sample bias. Discussion The number of species shared between the seamounts and the Macaronesian islands is very reduced. The same pattern repeats between the NEA and Mediterranean Sea. This study demonstrates that NEA seamounts are ecosystems with a higher diversity of lithistids than previously thought, increasing the number of lithistids known to occur in the NEA and Mediterranean Sea from 26 to 36 species.

In this study, we describe the lithistid demosponges collected during the French expeditions Seamount 1 and Seamount 2. New records of geographic distribution are reported, ten new species for science are described and illustrated, and the diversity and biogeographic patterns discussed. An identification key of all lithistid species reported for the NEA and Mediterranean is also provided.

MATERIALS AND METHODS
The material examined in this study was collected during Seamount 1 and Seamount 2 scientific expeditions undertaken by the MNHN of Paris to several NEA seamounts ( Fig. 1; Supplemental Material S1). The main aims of these campaigns were to study the patterns of faunal diversity and endemism found on isolated seamounts in comparison to continental areas and the relation with the dispersal capacity of the various taxonomic groups. The Seamount 1 campaign, coordinated by Dr. Philippe Bouchet, took place in 1987 onboard of the research vessel L. Noroît, and explored the Galicia Banks and the Lusitanian Seamounts (Gorringe, Josephine, Ampère, Lion and Seine) (Bouchet & Métivier, 1988). The second campaign, Seamount 2, this time lead by Dr. Serge Gofas, explored the Meteor Seamounts group (Great Meteor, Hyères, Irving, Cruiser, Plato, Atlantis and Tyro) and the Antialtair Seamount on board of the RV L. Suroît, sampling 165 stations also at depths above 1,000 m (Gofas, 1993). Lithistids were collected in 10 stations on Seamount 1 (11%) and in 42 stations on Seamount 2 (32%) between 280 and 1,035 m depth using various sampling gears (beam trawl (CP), epibenthic dredge (DE) and Warén dredge (DW)), and preserved in formalin onboard. The specimens examined are deposited in the 'zoothèque' of the MNHN in Paris, and stored at room temperature in ethanol 70%. Detailed information regarding the collection of the specimens studied here, is deposited in PANGAEA Ò Data Publisher (www.pangaea.de) under the digital object identifier (DOI): https://doi.pangaea.de/10.1594/PANGAEA.896492.
The specimens were analysed through the use of Light Microscopy (LM) and Scanning Electron Microscopy (SEM). For light microscopy, cross sections and slides of loose spicules were mounted in Canada Balsam Ò Sigma-Aldrich or Eukit Ò Sigma-Aldrich following standards procedures (Boury-Esnault & Rutzler, 1997). In addition, a few specimens, representative of each species, were selected and prepared for SEM. For this purpose, pieces of both the ectosome and choanosome of the sponge were excised and then either directly mounted or digested in nitric acid, washed several times with distilled water and then fixed in ethanol. The spicules were then placed on a stub and covered with gold-paladium. Thirty spicules of each spicule type were measured using the Leica Application Suite (LAS v. 4.5), for individual specimens. Minimum, mean and maximum values are presented for the measurements obtained for each analysed specimen. For the higher taxa classification, we followed the revised Demospongiae classification (Morrow & Cárdenas, 2015).
Due to the formalin fixation, we were not able to extract DNA for molecular analysis, and any attempts to barcode the mitochondrial COI gene, including the mini-barcode protocol used in other tetractinellids (Cárdenas & Moore, 2017) were unsuccessful.
The electronic version of this article in PorTable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by Other material. MNHN IP-2018-86 (1988  Diagnosis. Cup-shaped Neoschrammeniella with rounded edges and smooth surfaces; dicranoclone desmas of vine-like appearance; irregular dichotriaenes. Description (holotype MNHN-IP-2018-84). Massive, flattened cup-shaped, with a concave centre, 73 mm length, 29 mm high and 64 mm wide ( Fig. 2A); top surface is smooth with some oxeas perforating the surface and several small openings evenly distributed; walls are rounded and thick, 14-17 mm wide; bottom surface is also smooth, full of little openings dispersed throughout the entire surface, 31-56 mm in diameter, and some oxeas (Fig. 2B); colour is light brown in ethanol; the smooth surfaces could indicate that these specimens were not attached to any substrate, and therefore had a free living mode (Fig. 2B).
Skeleton. Ectosomal skeleton composed of smooth dichotriaenes of variable shape and size, along with a dense layer of microscleres (Figs. 4A and 4B); long-shafted triaenes or under-developed dichotriaenes, can also be observed (Fig. 4E); choanosomal skeleton is made of an irregular and loose network of dicranoclone desmas (Figs. 4C and 4D), spirasters and metasters; oxeas can be observed crossing the skeleton and projecting the surface.
Distribution. N. inaequalis sp. nov. was found in the Gorringe Seamount between 460 and 675 m depth.
Etymology. From the latin inaequalis = unequal, due to the uneven and irregular cladomes of the dichotriaenes.
Remarks. N. inaequalis sp. nov. is a distinct species due to (1) the growth form, being flattened cup-shaped with a concave center; (2) the fact that both surfaces were completely smooth may indicate that the sponge is free-living, i.e., not attached to the substrate; (3) triaenes can be present, although rare, being the second time this kind of spicule is reported for the genus (see illustration of the redescription of N. moreti (Lévi & Lévi, 1988)) in Systema Porifera (Pisera & Lévi, 2002b); (4) the vine-like desmas also resemble the desmas found in the genus Isabella (Carvalho, Pomponi & Xavier, 2015;Ekins et al.,  2016; Schlacher-Hoenlinger, Pisera & Hooper, 2005); (5) the shape and ornamentation of desmas are distinct from the other Neoschrammeniella species (see descriptions below and Remarks under N. pomponiae sp. nov.). It is also important to note that this species presents dichotriaenes very variable in size and shape (cladomes are irregular and unequal, and rhabdomes can present small protuberances or branches), so far only found in Isabella spp. (Carvalho, Pomponi & Xavier, 2015;Schlacher-Hoenlinger, Pisera & Hooper, 2005). These irregularities can be attributed to a pathologic development.   Holotype. MNHN-IP-2008-233 (1993 Diagnosis. Neoschrammeniella with a cup-rounded shape and a rugose surface, fixed to the substratum by a small pedicel; dicranoclones are densely covered by numerous and ornamented tubercles with a rugose appearance. Description (holotype MNHN-IP-2008-233). Large sponge, 54 mm height and 81 mm in diameter, with a small pedicel 23 mm wide; its external morphology resembles a bowl; walls are about 11 mm thick; the surfaces of the sponge are rugose, and hispid due to oxeas protruding the surface; openings are small and evenly spread on both surfaces, 40-87 mm in diameter; colour is brown in ethanol (Fig. 2D).
Skeleton. Ectosome is composed of a layer of dichotriaenes perpendicular to the surface that is covered by various microscleres (Figs. 8A and 8B); choanosome composed of a dense mesh of dicranoclone desmas, oxeas crossing the choanosome protruding the surface (Fig. 8A), and several microscleres spread through the skeleton.
Spicules (holotype MNHN-IP-2008-233). Remarks. The genus Neoschrammeniella was erected by Pisera & Lévi (2002b) to accommodate Corallistidae with smooth dichotriaenes and two to three types of microscleres. This genus is widely distributed, with records spanning the Southern Ocean, SW Pacific, Mediterranean Sea and NEA. Until now, six species were described and only one, N. bowerbankii (Johnson, 1863), was known to occur in the Mediterranean Sea (Pisera & Vacelet, 2011) and the NEA in the Madeira archipelago (Carvalho & Pisera, 2019;Johnson, 1863). In the present work, we described and illustrate three new species of Neoschrammeniella, that can mainly be distinguished by their habitus, sculpture of the desmas, presence or absence of oxeas, and, shape and size of the dichotriaenes. The external morphology of N. pomponiae sp. nov. resembling a bowl, contrasts with the cup-shaped to contorted lamellate masses with thick walls in N. bowerbankii, the flattened cup-shaped with a concave centre in N. inaequalis sp. nov. and the large cup-rectangular shape in N. piserai sp. nov. The sculpture of the desmas is also very distinct among all these species, while N. bowerbankii has very tuberculated dicranoclones divided into smaller and irregular lobes/tubercles (redescription in Pisera & Vacelet, 2011), N. inaequalis sp. nov. presents a distinct shape of desmas with vine-like appearance and few to several tubercles, N. piserai sp. nov. has irregular and compact dicranoclones  Diagnosis. Theonellidae with discotriaenes exclusively as ectosomal megascleres and choanosomal tetraclone desmas; microscleres are acanthoxeas and acanthorhabds.
Definition. Polymorphic sponges, from massive irregular to cup-shaped, branched or cylindrical; ectosomal megascleres are smooth discotriaenes; choanosomal megascleres are tetraclone desmas (regular or irregular) that can be smooth or tuberculated, and oxeotes or stylotes; microscleres are acanthoxeas and acanthorhabds (Kelly, 2007;Pisera & Lévi, 2002c;Pisera & Vacelet, 2011 Diagnosis. Small Discodermia, elongated to branching in shape, with smooth tetraclone desmas. Description (MNHN-IP-2008-213). Elongated and branched, small sponge, 15-29 mm high and 3-10 mm thick (Fig. 2E); surface is smooth and transparent, where it is possible to see the subdermal water canals, that gives a striated appearance to the sponge when observed under a magnifier; openings form a small elevation on the sponges' surface; colour is beige to light yellow in ethanol. Skeleton. Ectosome is composed of a layer of overlapping discotriaenes and abundant microscleres such as acanthomicroxeas and acanthorhabds, spread through this part of the skeleton; choanosomal skeleton has tetraclone desmas ( Fig. 10), smooth oxeas and some microscleres spread through the entire sponge; desmas form an irregular and compact net on the choanosome but a loose mesh near the ectosome with big spaces between them; oxeas can be observed crossing the interior of the skeleton.
3. Oxeas, long, smooth with rounded extremities (Fig. 10C); the vast majority of oxeas were broken, thus measurements of these megascleres are not presented here.
Distribution. Specimens were collected at the Great Meteor Seamount between 300 and 335 m depth.
Remarks. D. ramifera was described by Topsent (1892) from material collected in the Azores (318 m depth), and later re-collected in the same archipelago at 98 m depth (Topsent, 1904). So far, these were the only records in the North Atlantic. Here we discover for the first time the presence of this species in the Great Meteor seamount (between 300 and 335 m depth). The specimens analysed in this work have a similar external morphology compared to the ones described by Topsent (i.e., small, elongated to branching sponge with finger-like extensions), and similar spicule composition. However, the spicules' sizes are in general smaller from those presented in the original description (Table 2). Discotriaenes have a smaller cladome, 124-213 µm in the analysed material versus the 300 µm in diameter in the original description; acanthomicroxeas (22.8-32.6 µm vs 40-45 µm long) and acanthorhabds are also smaller (3.9-13.9 µm vs 20-25 µm long), but see Discussion for more details on these differences. Description (MNHN-IP-2008-210). Small fragment, 20 × 10 mm in size, of elongated shape, with a smooth surface; subdermal water canals are visible, giving a striated appearance to the sponge; colour is beige in ethanol.
Skeleton. Ectosomal skeleton is formed by a layer of overlapped discotriaenes, and several microscleres spread through the surface; choanosome is formed by irregular tetraclone desmas, oxeas crossing the interior of the sponge and numerous microscleres spread through the interior of the sponge.
Remarks. Although the external morphology, type of spicules and desma ornamentation are in agreement with the description of D. ramifera, the spicules sizes of this specimen are significantly larger when compared to the ones found in the Great Meteor (Table 2). For this reason, we consider this species as D. cf. ramifera.
Description (MNHN-IP-2008-205). Spherical polymorphic sponge with several round protuberances, 15-20 mm high and 12-13 mm wide, with a rough surface (Fig. 2F); pores cannot be seen with naked eye; colour varies from whitish to light brown in ethanol.
Skeleton. Ectosome composed of a compact layer of discotriaenes, usually overlapping each other, numerous microscleres (acanthomicroxeas and acanthorhabds) spread through the surface, and oxeas perforating the sponges' surface; occasionally, bundles of oxeas can be observed; choanosome with strongly tuberculated and compact tetraclone desmas ( Fig. 12), forming an irregular net with dispersed microscleres in the interior of the sponge.
Distribution. Specimens of D. verrucosa were found in Atlantis and Plato Seamounts between 338 and 580 m depth.
Remarks. Discodermia verrucosa was first found in the Canary Islands and described by Topsent (1928). The species differs from the D. ramifera on the habitus and sculpture of desmas. D. verrucosa has a cup to spherical shape with several rounded protuberances/warts and strongly tuberculated tetraclones. On the other hand, D. ramifera has an elongated to branching shape and smooth tetraclone desmas only tuberculated in the extremities.
The specimens analysed in this study overall match the description of D. verrucosa, apart from two differences: (1) the discotriaenes are much smaller and (2) the microscleres present a wider size range when compared to the original description (see Table 2).  Diagnosis. Discodermia of tree-like appearance; discotriaenes vary from square to circular shape and can also be indented.
Description (holotype MHNH-IP-2008-211). Discodermia of tree-like appearance ( Fig. 2G), with a relatively long stem, 15 mm, where it extends on top into three branches; the stem is wider at the base, 12 mm, and thinner on top, 7.5 mm; branches are irregular and 13-28 mm long; surface is smooth but some rugosities/protuberances are visible; full sponge length is 58 mm; the sponge was attached to the substrate by the stem; colour is beige in ethanol.
Skeleton. Ectosome has a layer of overlapped discotriaenes of variables sizes (Figs. 14A and 14B) with numerous microscleres beneath them; choanosome is composed of an irregular net of tetraclone desmas (Figs. 14C and 14D) and spread microscleres; near the surface, tetraclones are more intricate, rugose, with very complex and strong zygoses near the water canals (Fig. 14C); in the interior part of the sponge, the tetraclones still form an intricate and irregular net, but there is more space between the desmas. Spicules (holotype MHNH-IP-2008-211).  Diagnosis. Massive, spherical, irregular, Discodermia of bulb appearance, with smooth tetraclone desmas.
Description (holotype MNHN-IP-2008-208). Massive sponge, irregular appearance, with large protuberances of round shape, 53 mm high and 31 mm wide; surface is irregular with a rugose appearance; the basal part of the sponge is not evident, since there is no obvious mark in the sponge that shows where it was attached to the substrate; colour is beige to light brown in alcohol (Fig. 2H).  Distribution. D. kellyae sp. nov. is only known from its type locality, the Plato Seamount at 580 m depth.
Remarks. The identification of species belonging to the genus Discodermia is particularly challenging due to the few and very variable morphological characters used for the distinction of species (Pisera & Vacelet, 2011). Moreover, for some species we are limited to the original descriptions where detailed information of skeletal composition and spicule sizes, or images are lacking.
In the North Atlantic and Mediterranean Sea, a total of nine species have been described, including the two described species in this study (Table 2). Despite the high plasticity of morphological characters, the main differences between species are (1) habitus, (2) the sculpture and size of the desmas, (3) size and shape of the discotriaenes, and (4) size and shape of the microscleres. We propose D. kellyae sp. nov. as a new species based on (1) the habitus of this sponge: the polymorphic sponge of bulb appearance contrasts with the massively encrusting shape of D. adhaerens, the spherical to irregular masses in D. polymorpha, the cup-shaped with numerous warts/protuberances in D. verrucosa, the elongated with several finger-like extensions in D. ramifera, the tree-like shape of D. arbor, the cluster of knobby fingers in D. dissoluta and the irregular mushroom shape of D. polydiscus; (2) tetraclones of D. kellyae sp. nov. have similar ornamentation to the ones found in D. ramifera (tetraclones with smooth clones that are tuberculated in the zygomes), however, they are more compact and thicker (24-32-48 µm vs 20-42-76 µm) resembling the ones present in D. verrucosa; the other species have slender and smooth desmas without strong/complex zygoses; (3) the intraspecific size range of discotriaenes is usually wide, and similar between the different species, but in D. kellyae sp. nov. the size range of the cladomes is very large, 121-425 µm, and this can only be observed in D. verrucosa (200-560 µm) and D. arbor sp. nov. (148-396 µm); besides that, the shape of the rhabdome is also variable in D. kellyae sp. nov., where the tips of the rhabdomes can be blunt or sharp; (4) the size of the acanthomicroxeas in D. kellyae sp. nov. is larger (16.7-43.2-66.5 µm) compared to the other species, except when compared to D. dissoluta (41.6-68.0 µm; however, these values were taken from Pisera & Pomponi, 2015 where the authors presented a detailed description of the species, since in the original description, the species was poorly described and no measurements were given); (5) D. kellyae sp. nov., along with D. arbor sp. nov., are the only species with a wide acanthorhabds size range (5.3-13.3-24.9 µm and 6.7-16.1-25.9 µm, respectively) while the other species have a considerably narrower range ( Table 2).
The species D. inscripta (Schmidt, 1879) was not included here for comparison because the type material was deciduous and the species is therefore considered incertae sedis (Pisera & Lévi, 2002d).
Remarks. Pisera & Lévi (2002d) re-described and illustrated the holotype of M. azorica, a specimen collected in the Azores archipelago. Since we also had access to the holotype of M. azorica we have made new measurements of the spicules, in order to fill the gaps of some spicule's measurements missing in the redescription. The comparison of the holotype of M. azorica with the specimens collected during the campaigns Seamount 1 and 2, lead us to consider these specimens as M. cf. azorica. Although very similar in the habitus they differ from the holotype in two features: (1) (Table 3). Nineteen large specimens were found in the same station in the Hyères seamount (station DW202), suggesting that the species may be forming a sponge ground in this area of the seamount.
Macandrewia robusta Topsent, 1904 Figures 2J, 20-21 and Table 3 Material Diagnosis. Small ficiform to globular shape Macandrewia with a flattened top and a short and thick pedicel.
Description (MNHN-IP-2008-216). Small sponges with a ficiform to globular shape, 18-20 × 14-22 mm in size, attached to the substrate by a short and thick pedicel (8 mm in height and 16 mm width) (Fig. 2J); top of the sponge is flattened, smooth, where openings can be observed in small clusters leading to water canals giving a striated appearance to the sponge; openings and the subdermal water canals visible to the naked eye; lateral walls of the sponge are smooth with small openings spread evenly through this surface; in some individuals, the top or upper surface has a slight depression; colour varies from beige to light brown in alcohol.
Skeleton. Ectosome is composed of a layer of overlapped phyllotriaenes and numerous microxeas; these microxeas surround the openings radially; choanosomal skeleton formed by desmas, oxeas and dispersed microxeas; desmas form an irregular and very dense mesh (Fig. 20).
Remarks. In the specimens here examined, phyllotriaenes (165-230 µm vs 154-309; Table 3) and oxeas (330-400 vs 203-309; Table 3) are smaller when compared to previous records for the species (Topsent, 1904). However, M. robusta has a very distinct habitus in relation to the other Macandrewia described for the North Atlantic Ocean (Table 3). Its ficiform to globular shape, with a short and thick pedicel, contrasts with the cyathiform to flabellate shape with undulating rounded margins in M. azorica, the encrusting with standing trunks of M. ramosa, the foliate with thick lamellas in M. schusterae sp. nov., or the globular shape with a small pedicel as in M. minima sp. nov. (descriptions of the latter two below). Differences in spicule sizes were observed in another species analysed in this work as well as in other studies (see 'Spicules dimensions' section in the Discussion for further information regarding this topic). Two specimens from the Seamount 2 collection could not be confidently identified down to species level (MNHN-IP-2008-228 andMNHN-IP-2018-94). They are very small fragments, seemingly encrusting, and most likely it is a Macandrewia at an early stage of   Diagnosis. Foliate to vase shaped Macandrewia with thick, irregular and undulated lamellas, with a small pedicel.
Distribution. This specimen was found on Gorringe, Tyro and Plato Seamounts between 520 and 805 m depth.
Remarks. M. schusterae sp. nov. is here proposed as a new species due to its particular habit, the sculpture of the desmas and size of the spicules. M. schusterae sp. nov. has a foliate shape with contorted lamellas, sometimes resembling a Leiodermatium sp., that contrasts with the ficiform to globular shape with a flattened top in M. robusta, the flabellate to undulate masses with thin lamellas in M. azorica, the ramose shape in M. ramosa Topsent, 1904 and the small ball shape in M. minima sp. nov. The desmas have a different sculpture compared to the other Macandrewia species, as the zygomes have extremely ramified long and thin branches, forming a very strong zygosis (Figs. 22C-22E). This new species also presents a relatively wide range of spicule sizes, mainly on phyllotrianes (cladome: 177-304-420 µm; rhabdome: 67-119-178 µm), oxeas (263-437-620 µm) and microxeas (43.8-67.9-95.2 µm), a feature that is not so common on the other species (Table 3).   Skeleton. Ectosome has a layer of phyllotriaenes covered by large amounts of microxeas; microxeas surround the openings radially (Figs. 24A and 24B); choanosome has desmas, with a triaenose crepsis, forming a compact and irregular network (Fig. 24C); oxeas and microxeas are spread through the choanosome but in small amounts compared to the ectosome.
Etymology. From the Latin minima = small.
Distribution. Only known from its type locality, the Great Meteor Seamount at 615 m depth.
Remarks. M. minima sp. nov. differs from the other Macandrewia in the considerably smaller size of its spicules (see Table 3), its globular shape and in the characteristic tubercles of the phyllotriaenes (only observed in this species).

*
Measurements of spicules from the holotype presented here, were measured for this study, they were not taken from the redescription of the holotype.
'-' no information/not mentioned.  Diagnosis. Columnar to ficiform Exsuperantia with trider-type desmas that have smooth tubercles (few presenting rugosities). -2008-196). Small phymarapiniid 22-23 × 8-18 mm in size, columnar to ficiform in habitus, with or without lateral protuberances (Fig. 3A); some specimens have a "V" shape morphology; surface is smooth with conspicuous subdermal water canals giving a striped appearance to the sponge; oscula or pores are not visible; colour beige in ethanol.

Description (MNHN-IP
Skeleton. Ectosome is formed by a layer of phyllotriaenes covered by large amounts of microscleres: openings are surrounded by these microscleres; choanosomal skeleton is mainly built of trider-type desmas, that form a regular network with large spaces in between (Fig. 26); some subtylostyles ( Fig. 26B) and microscleres are also present and spread through the skeleton.
Distribution. E. archipelagus was found in Tyro, Hyères, Atlantis, and Plato Seamounts between 280 and 1,000 m depth and also in Gran Canaria island at 660 m depth.
Remarks. The size of the spicules measured in these specimens are considerable smaller when compared to the type material (Carvalho & Pisera, 2019) ( Diagnosis. Clusters of globular to ficiform knob-like short fingers with apical osculum; phyllo-to discotriaenes as ectosomal megascleres. Description (holotype MNHN-IP-2008-201). Clusters of globular to ficiform knob-like short fingers, 30 mm in length and 29 mm wide; oscula, approximately 2 mm in diameter, are located on the top of the knobs (Fig. 3B); surface is rugose with a striated appearance due to the visible subdermal water canals; colour is brown in ethanol.
Skeleton. Ectosome is composed by phyllo-to discotriaenes that are very variable in shape, and several microscleres; choanosomal skeleton has regular and articulated triders, forming an irregular and relatively loose network (Fig. 28); subtylostyles are present crossing the skeleton (Figs. 28A and 28B); microscleres are present and very abundant, except for streptasters that are less numerous.  Remarks. Recently, a revision of the genus Exsuperantia allowed to clarify some taxonomic problems by establishing two species, E. clava (NWA) and E. archipelagus (NEA), that were previously considered a single species (Carvalho & Pisera, 2019). According to the authors, the main differences between these two species are the desmas morphology and ornamentation.
Here we propose E. levii sp. nov. as a new species, third of the genus, based not only on desmas morphology and ornamentation, but also on the habitus of this new species. The trider-type desmas on E. levii sp. nov. resemble the ones found in E. clava, i.e., the tubercles are ornamented and the tip of the trider has a tubercle, while in E. archipelagus it usually has a conical shape. In general, the size of the spicules of E. levii sp. nov. is smaller when compared to the holotype E. archipelagus (unfortunately the size of spicules of the E. clava is not known, with exception of the desmas, since the type material was deciduous and microscleres were not present (Pisera & Lévi, 2002f)), however, the most distinct feature is the habitus of E. levii sp. nov.: a cluster of globular knob-like fingers with large apical oscula on top, contrasting with the columnar to ficiform morphology of the other two species. Bergquist & Hogg, 1969Family Azoricidae Sollas, 1888Genus Leiodermatium Schmidt, 1870 Diagnosis. Azoricidae with spiny rhizoclones and diactines as megascleres; ectosomal spicules and microscleres are absent (Pisera & Lévi, 2002i).

Suborder Spirophorina
Definition. Lamellate, plate-like, foliose, vase-or ear-shaped Azoricidae; oscules are visible; choanosomal desmas are spiny rhizoclones; megascleres are diactines; microscleres are absent (Kelly, 2007; modified from Pisera & Lévi, 2002h   Diagnosis. Foliate to undulate polymorphic masses, with large openings in the outer surface of the sponge and small openings in the inner surface. Description (MNHN-IP-2018-93). Large foliate to undulate irregular masses, with thick lamellas, 5-12 mm, that in some cases can form cups/funnels (Fig. 3C); inner and outer surfaces are different from each other, and it is possible to distinguished at naked eye; outer surface has larger openings slightly elevated from the surface, 243-269 µm in diameter, (Figs (Fig. 31E).
Distribution. These specimens were found on the Gorringe and Hyères Seamounts, between 305 and 480 m depth.
Remarks. Within Tetractinellida, the genus Leiodermatium is particularly difficult from a taxonomic standpoint, given the few characters available to distinguish and describe the different species. In the North Atlantic, only two species have been described to date-L. lynceus Schmidt, 1870 andL. pfeifferae (Carter, 1873); the former from specimens collected off the coast of Portugal, and the later from Madeira island i.e. both from the NEA but unknown depths. Later, Carter (1876) formally explained the differences between these two species: (1) L. lynceus has large oscula located on outer surface while in L. pfeifferae they are on the inner surface; (2) L. pfeifferae has numerous fusiform oxeas on the edge of the laminae, while L. lynceus has "isolated acerates" (Schmidt, 1870) (however they were not found in the redescription of the holotype L. lynceus (Pisera & Lévi, 2002i)). Another important detail, is the difference between the thickness of the laminae on both species, L. lynceus has thinner (3-4 mm) laminae compared to L. pfeifferae (6-17 mm; see Table 5).
In addition to these two currently recognized species, Poritella deciduum (Schmidt, 1879), was also assigned to this genus (Lendenfeld, 1903) but this allocation is considered questionable (Pisera & Lévi, 2002i). Also, Sollas (1888) reported a number of varieties of L. pfeifferae from the material collected in the course of the Challenger expedition in the Atlantic, viz. A. pfeifferae tenuilaminaris (Bahia, Brazil, unknown depth) and A. pfeifferae tenuilaminaris osculis disjunctis (Bermuda, 795-1965 m depth). However, the material was deciduous and therefore the descriptions are incomplete (see also review in Kelly, 2007). Records of L. lynceus and L. pfeifferae for the western Atlantic (e.g. Van Soest & Stentoft, 1988) need to be carefully re-assessed, as they may represent different and likely undescribed species given that several putatively new Leiodermatium species have been reported for the tropical western Atlantic (Schuster et al., 2019) but still lack formal description. Topsent (1892) reports one specimen of Azorica pfeifferae for the Azores (st. 234, 454 m depth) with a strong blue coloration. However, from the illustration provided, it appears that the specimen has elevated openings on the external surface, thereby conforming to L. lynceus.
The specimens analysed in this study are very similar to the holotype of L. lynceus regarding the morphology, surfaces and the ornamentation of the desmas. The only difference lays on the size of the openings: the holotype has large oscula on the outer surface, 500-750 µm in diameter, while in our specimen oscula are 243-269 µm in diameter; the same happens in relation to the pores of the inner surface of the holotype, which are 156-188 µm in diameter, against 68-145 µm in our specimen (Table 5).
Leiodermatium tuba sp. nov. Figures 3D, 32-33 and Table 5  Diagnosis. Massive lamellate vase to contorted walls, sometimes forming a cone, with smooth and similar surfaces.
Description (holotype MNHN-IP-2018-72). Lamellate vase with contorted thin walls, 4-5 mm, occasionally forming a cone (Fig. 3D); this specimen consists of three fragments, the largest one is 138 mm long and 93 mm wide; surfaces are identical when observed with the naked-eye, given they are both smooth, but some differences can be noticed when observed under the stereomicroscope: outer surface has slightly larger depressed openings, 266-322 µm in diameter, (Figs. 32A and 32C) and a striated appearance due to the water canals underneath the surface; inner surface (Figs. 32B and 32D) has a whitish appearance caused by the presence of numerous oxeas covering the smaller depressed openings; openings are 186-261 µm in diameter; specimen coloration varies from light beige to brown in ethanol.
Skeleton. There is no clear distinction between the ectosome and choanosome since there is no special arrangement of spicules or different spicules in the ectosome; choanosomal skeleton is composed by very spiny rhizoclones desmas, forming a complex, branching and compact network (Figs. 33A and 33B); other megascleres are oxeas across the skeleton; microscleres are not present.  Etymology. From the Latin tubae= trumpet; since some lamellas in this species have a conical shape resembling a trumpet.
Distribution. The type locality is the Gorringe Seamount at 805-830 m depth. Other specimens were found in Plato, Hyères, Atlantis and Gorringe Seamounts between 545 and 1,035 m, and in Gran Canaria at 660 m. Remarks. L. tuba sp. nov. exhibits a distinct external morphology and surface ornamentation compared to the other two Leiodermatium species recorded for the North Atlantic, i.e. L. lynceus and L. pfeifferae. Firstly, in L. tuba sp. nov. both surfaces look similar at the naked eye (smooth and with slightly depressed openings; Fig. 32) whereas in L. lynceus and L. pfeifferae, the openings are elevated (depending on the surface) and this is a very distinctive feature (see above remarks under L. lynceus). Additionally, the inner surface of L. tuba sp. nov. is pierced by numerous oxeas providing a whitish colour to this surface. Pisera & Lévi (2002h) discussed the possibility of a new species of  (1873). 3 Pisera & Lévi (2002d): where the authors state that Poritella decidua Schmidt, 1879 seems to be a synonym of Leiodermatium, but the specimens are considered incertae sedis due the bad condition of the material. '-' indicates the information was not given in the description.
Leiodermatium being reported as L. lynceus due to the absence of larger oscules on the outer side. However, it is not clear from their account to which specimens they were referring to nor their characteristics. Perhaps they conform to L. tuba sp. nov. here described.
Another important observation is the bathymetric range where the Leiodermatium spp. were collected in this study. L. tuba sp. nov. was usually found deeper (330-830 m depth) than L. lynceus (305-320 m depth) (see "Diversity" section and Supplemental Material S1).
Description (holotype MNHN-IP-2008-236). Polymorphic sponge, cylindrical to arborescent Siphonidiidae, sometimes of bulb shape, attached by the base to the substrate; small, 33-49 mm high, thin, 2-9 mm wide (but can be 14 mm wide); surface is smooth and exhibits fistules spread through the sponge pointed in several directions, 1-8 mm long and 1-4 thick (Fig. 3E); fistules are usually close-ended, but when open, it is possible to see the subdermal water canals emerging from the interior of the sponge; extremely hard sponge (stony consistency); colour varies from beige to brown in ethanol.

Skeleton.
No clear distinction of the spicules between the ectosome and choanosome, with exception of the desmas of the surface that are different from the interior of the skeleton: a layer of flattened, fused and modified desmas, resembling a puzzle, constitutes the surface of the sponge (Figs. 34B and 35B); these modified desmas, resembling a shield, contribute to the hardness of this species; some wrinkles can also be observed on the surface of the sponge (Fig. 35A); choanosome is formed by an extremely dense, compact and irregular net of rhizoclone desmas, exotylostyles and rarely styles, crossing through the skeleton; several water canals can be observed in a cross section of the sponge, as large holes (Figs. 34A and 34C) surrounded by the desmas that here are slightly more elongated (Fig. 34C); desmas from the fistules are different from the ones in the 'body' of the sponge, i.e., usually the desmas of the fistules are longer and looser (Fig. 35C) while in the 'body' they are very dense and compact (Fig. 34C). 2. Exotylostyles, pin-shaped, with spiny heads and pointed tips, straight or slightly curved, not very abundant, 173-363-504 mm in length and 2.9-5.1-6.6 mm in width (Figs. 35F and 35G); some exotylostyles look underdeveloped and resemble styles. Distribution. Siphonidium elongatus sp. nov. was found in the Atlantis, Hyéres, Lion, and Gorringe seamounts, and in Gran Canaria, between 470 and 675 m depth Etymology. From the latin elongatus = elongated, due to an elongated shape of the desmas, especially those composing the fistules.
Remarks. Three species of Siphonidium have been described in the Atlantic Ocean, and only one, S. ramosum, has been reported for both sides of the North Atlantic (Schmidt, 1879; Topsent, 1928Topsent, , 1904Topsent, , 1892Van Soest, 2017;Van Soest & Stentoft, 1988) and Mediterranean Sea (Longo, Mastrototaro & Corriero, 2005;Vacelet, 1969;Zibrowius & Taviani, 2005). With the redescription of S. ramosum in (Pisera & Lévi, 2002g), a detailed account of the external morphology and spicules was given, allowing a better definition of the species. Despite the relatively similar habitus of S. ramosum and S. elongatus sp. nov., the main difference between these two species relies on the desmas morphology and ornamentation: S. elongatus sp. nov. has very spiny rhizoclones with slim arms ornamented with microspines on the edges, contrasting with the tuberculated rhizoclones of S. ramosum. Another distinct feature, is the presence of styles in S. elongatus sp. nov.
(even though they are rare) that were never mentioned in the redescription of S. ramosum. Furthermore, when S. elongatus sp. nov. is compared with the other North Atlantic species, its external morphology and spicules differ: S. dubium Lévi, 1959 is a massive sponge with a large base, subdivided into three lobes and the only one within the genus with strongyles; S. geminum (Schmidt, 1879) has a flat and irregular incrusting base with simple or bifurcated cone shape. Topsent (1904) presented a small description of S. ramosum from several specimens found in the Azores. In his account, the shape and the ornamentation of the desmas are not explicitly described or illustrated, but the spicules sizes are given and are much larger than the ones described by Schmidt (1879) from material collected in the Gulf of Mexico (Table 6). The spicules sizes in S. elongatus sp. nov. are more similar to the ones in S. ramosum described by Schmidt than to the one described by Topsent. It was previously stated by Van Soest (2017), that the S. ramosum reported from the Azores, is most likely a different species due to the difference in the spicules sizes when compared to the type material. A revision of Topsent's material would be required to clarify this question.
Definition. Massive, encrusting or globular in shape, with or without fistule-like papillae. Surface smooth, hispid, conules can be present. Compressible to rigid, or soft to fragile sponges. Acrepid or monocrepid smooth desmas, branched in several planes. Desmas can be isolated, non-articulated, fused, or dispersed in the ectosome and choanosome; zygomes vary from simple to complex; zygosis when present, is rarely fully articulated in the skeleton turning into a loose skeleton. Other megascleres are oxeas, where the tips can vary from sharp to blunt. Microscleres not present (List-Armitage & Hooper, 2002;Muricy et al., 2001;Pisera & Lévi, 2002h) Type species. Petromica (Petromica) grimaldii Topsent, 1898 (type by monotype).

Subgenus Petromica Topsent, 1898
Diagnosis. Firm and rigid sponge, with or without papillae, with acrepid or monocrepid desmas that can form a loose or well-formed skeleton. Oxeas present and variable in size (List-Armitage & Hooper, 2002).
Spicules (MNHN-IP-2018-92  Distribution. This specimen was found on the Gorringe seamount between 255 and 265 m depth. Remarks. Petromica is a widely distributed genus, and so far, eight species have been described. In the North Atlantic, three species have been reported, P. (Chaladesma) ciocalyptoides and P. (Chaladesma) citrina to the NWA and P. (Petromica) grimaldii from the NEA and MED (Table 7). P. (P.) grimaldii was first described from the Azores archipelago by Topsent (1898) where it was found to be a very common sponge, collected throughout the archipelago between 200 and 914 m depth (Topsent, 1928(Topsent, , 1904(Topsent, , 1898. This species has been also reported from the MED (Boury-Esnault, Pansini & Uriz, 1994;Pulitzer-Finali, 1972) and since microspine desmas' terminations were absent, P. (P.) massalis Dendy, 1905 (a species from the Indian Ocean) and P. (P.) grimaldii were synonymized (Pulitzer-Finali, 1972). According to Muricy et al. (2001), these microspines are not present in all desmas in the same specimen and they can be rare. Therefore, the absence of microspines in the desmas is not enough to distinguish one species from another. A more detailed examination of the specimens from the MED would be necessary to allow to clarify this uncertainty (Muricy et al., 2001) and make sure the Petromica found in MED are in fact P. (P.) grimaldii. In the specimen examined in this study spicules sizes are very similar to those of the holotype (from the Azores) and the microspines in the termination of the desmas are present and very evident (Fig. 37).

DIVERSITY
The specimens described in the present work constitute the first records of lithistid demosponges for these two groups of NEA seamounts, except for Exsuperantia archipelagus. The Meteor seamount group harbours a more diverse lithistid fauna, 15 species, compared to the Lusitanian seamount group, where six species are recorded (Table 8). At a smaller scale, the Hyères seamount is the most diverse where eight species, namely N. pomponiae sp. nov., M. cf. azorica, M. robusta., E. archipelagus, E. levii sp. nov., L. lynceus, L. tuba sp. nov. and S. elongatus sp. nov. were found, followed by the Gorringe and Atlantis (six species), Plato and Great Meteor (five species), Tyro (three species) and Lion seamount (one species). Two specimens were found on the Antialtair and Ampère seamount (one on each) but it was not possible to identify them down to species level because they were small and incrusting specimens, possibly young individuals of M. robusta. The majority of the species have a restricted distribution  (Fig. 38). Some of the examined material was of very small size and/or in poor condition, which hampered its identification to lower taxonomic levels. These specimens were therefore not identified and are not included in this manuscript (see Supplemental Material).  (1870); Topsent (1889Topsent ( , 1892Topsent ( , 1898Topsent ( , 1904Topsent ( , 1928; Vacelet (1969). * Var tenuilaminare (Topsent, 1928).
?The assignment of the specimens examined by Topsent in Azores need to be revised in order to clarify if it is in fact S. ramosum.

Diversity and biogeographic patterns
With the present work, we describe for the first time the lithistid fauna of two seamount groups of the NEA, the Great Meteor and the Lusitanian seamounts. All of the 17 species here reported constitute new records for these seamounts and ten are new to science. The only exception is E. archipelagus previously reported for the Great Meteor Seamount as Exsuperantia sp. (Cárdenas et al., 2011). These 10 newly described species add to the 17 species previously reported for the NEA, representing an increase of approximately 60% of the lithistid diversity of this area. These findings show how understudied the fauna of these ecosystems is and suggests that additional species are likely to be found as survey efforts increase. It also concurs with previous studies made for other invertebrate groups based on material collected from the same seamounts where several new species were described (Berning, Harmelin & Bader, 2017;Cárdenas et al., 2018;George & Schminke, 2002;Gofas, 2007;Souto, Berning & Ostrovsky, 2016). The Great Meteor group, appears to harbour a more diverse lithistid fauna, with a total of 15 species (nine new to science), whereas in the Lusitanian group, six species were recorded (four new to science). Interestingly, only a relatively small proportion of the lithistid species known from the NEA (7 out of 17) were found during the present study. Finally, the finding of 19 large specimens of M. cf. azorica in the same station in the Hyères Seamount (st. DW202), suggests that this species may occur in relatively larger densities, possibly forming a sponge ground in this area. However, this would require verification with other sampling and observation tools such as remotely operated or autonomous underwater vehicles (ROV/AUV). Such finding would add on to the aggregations dominated by Leiodermatium pfeifferae, recently reported on three seamounts in the Western Mediterranean Sea (Maldonado et al., 2015), which suggests that some extant lithistids may still form highly structured habitats comparable to the Mesozoic reefs (Maldonado et al., 2015;Reid, 1967), Several paradigms in seamount ecology, including the seamount endemism hypothesis, have been heavily debated in recent years, with some authors considering seamounts as places of high endemism (de Forges, Koslow & Poore, 2000), while others attributed the observed patterns to sample bias (Samadi et al., 2006;see also McClain, 2007;Rowden et al., 2010). In our study, the majority of the species (Neoschrammeniella inaequalis sp. nov., N. piserai sp. nov., N. pomponiae sp. nov., Discodermia. arbor sp. nov., D. kellyae sp. nov., D. ramifera, Macandrewia minima sp. nov., M. robusta, Leiodermatium lynceus and Exsuperantia levii sp. nov.) were only found on one of the seamounts. These findings concur with a study on lithistids of the Norfolk Ridge (New Caledonia) where the authors reported 16 species (seven new to science, including a new genus) with the half of the species (eight) restricted to one seamount (Schlacher-Hoenlinger, Pisera & Hooper, 2005). On the other hand, five species, M. cf. azorica, M. schusterae sp. nov., S. elongatus sp. nov., L. tuba sp. nov. and E. archipelagus, have a wider distribution (found in three to five seamounts), and the latter three are shared between the two seamount groups. The differences in diversity and distribution found in our study may be a result of uneven sampling effort between the different seamounts (between 2 and 35 stations) and the two seamount groups (92 stations in Seamount 1 vs 131 stations in Seamount 2).
When examined at a larger scale, seamounts share most species with the Azores and Canary archipelagos, with seven (D. ramifera, D. verrucosa, M. azorica, M. robusta, E. archipelagus, L. lynceus, P. (P.) grimaldii) and six species (D. verrucosa, M. azorica, E. archipelagus, L. lynceus, L. tuba sp. nov. and S. elongatus sp. nov.) shared, respectively. Given the relative proximity between localities and the oceanographic setting, it would be expected that the Azores would share more species with the Great Meteor group, instead of the Canaries, Madeira, Selvagens and the continental shelf of the Lusitanian group (Fig. 1). However, this is not observed in our study as only two species (D. ramifera and M. robusta) are exclusively shared between the Azores and the Meteor Seamount group. One species (L. lynceus) is common to Azores, Madeira, Canaries and the two groups of seamounts, and two species (M. azorica and D. verrucosa) are shared between the Meteor group and the oceanic islands. All the species found in the Lusitanian group are shared with the archipelagos and/or the Meteor Seamount, with only one exception, N. inaequalis sp. nov. that is exclusively known from the Gorringe Seamount. However, none of the species reported from the Portuguese (Corallistes elegantior Schmidt, 1870) and Moroccan continental shelves (Theonella annulata Lendenfeld, 1907) were found to occur in the Lusitanian seamounts group. It should be noted that the description of C. elegantior is vague and does not provide a detailed characterization of all spicules. Moreover, this species was never observed since its description by Schmidt (1870) in Portugal or in the surrounded areas, thus it should be considered a taxon inquirendum.
Neophrissospongia nolitangere Pisera & Vacelet, 2011 a species reported from all oceanic islands (Carvalho, Pomponi & Xavier, 2015;Cruz, 2002;Topsent, 1904) and the Mediterranean sea (Manconi, 2011;Pisera & Vacelet, 2011) and Corallistes masoni Bowerbank, 1869 reported from Madeira (Bowerbank, 1869;Carvalho, Pomponi & Xavier, 2015) and Canary Islands, were also not found in this study. If we compare the diversity between NEA and the Mediterranean Sea, only five species, viz. N. nolitangere, Neoschrammeniella bowerbankii (Johnson, 1863), L. lynceus, L. pfeifferae (Carter, 1876) and Siphonidium ramosum (Schmidt, 1870) out of 36, are shared between these two areas. Finally, whether some of the species here described for the first time are shared with the Northwest Atlantic and/or the Caribbean Sea also remains to be assessed, since the lithistid fauna of these areas is known to be far more diverse than currently reported but awaits formal description (A. Pisera, 2018, personal communication;Schuster et al., 2019). Therefore, and given the still limited and uneven sampling of the various areas, we refrain from considering the species herein described endemic to these seamounts or seamount groups.
Future studies employing a more comprehensive sampling design and modern technologies would be required to test the extent to which an interplay between intrinsic (dispersal potential) and extrinsic (seamount age, isolation and area) factors underpin and shape the observed diversity and endemism patterns of the fauna of these seamounts.

Spicules dimensions
Several morphological features are used in taxonomy and classification of Porifera and among them, the skeletal elements (spicules, fibres) and their arrangement are the most used. This is mainly due to historical reasons, since specimens would be sent for taxonomic assignment, sometime after collection and preservation, and usually having lost some of its live characteristics such as colour or consistency (Bergquist, 1970). Spicules sizes, which occur over a relatively large range are also important for species determination (Bergquist, 1970), altough some studies have shown that biophysical environmental conditions and life cycle can lead to some intraspecific varibaility (Bavestrello, Bonito & Sarà, 1993;Cárdenas & Rapp, 2013;Mercurio et al., 2000). In the case of lithistids sponges, the identification is mainly based on the shape and development of desmas and other accompanying spicules (Bergquist, 1970;Lévi, 1991).
Whether spicule size is as relevant for lithistids as in other taxonomic groups remains to be assessed. However, in the material examined in our study, we have found some differences in the size of the spicules for some species in comparison with the type material. Examples include D. ramifera, D. verrucosa, M. cf. azorica, M. robusta, E. archipelagus and P. (P). grimaldii. Specimens of D. ramifera and D. verrucosa despite being slightly larger than the holotypes and having been sampled at similar depths, present smaller cladomes of the discotriaenes, as well as their acanthomicroxeas and acanthorhabds (Table 2). In the case of M. cf. azorica and M. robusta the same pattern repeats, with exception of the microxeas on both specimens analysed here which are larger than those in the respective holotypes (Table 3). Finally, in E. archipelagus all the spicules are smaller than those in the holotype, even though the specimen itself has nearly the same size as the type material (Table 4). P. (P.) grimaldii is the only one that has slightly larger spicules compared with the type material (Table 7). These variations were also found in other deep water tetractinellids and were assumed to be related to the depth and/or silica concentration, where deeper specimens have larger spicules due to the availability of silica in the water (Cárdenas & Rapp, 2013). However, one cannot find a correlation with the depth since: (1) D. ramifera and D. verrucosa were sampled at similar depths as the holotypes, (2) the depth at which the type material of M. azorica was sampled is unknow preventing us to make any assumption, (3) M. robusta was found at shallower depths in the Hyères seamount and yet its spicules were in general smaller, (4) P. (P.) grimaldii was found within the same depth range as the holotype and has larger spicules, thus the depth seems to not be related with the size of the spicules. The amount of silica in the water does not seem to be related either since these two groups of seamounts have many lithistids, and they possibly require large amounts of silica to build their skeleton. Another explanation is that lithistids are very efficient at removing the silica from the water thus, not requiring large amounts of this element (Alvarez et al., 2017;Maldonado et al., 2015). Since there is no data regarding the biogeochemical parameters of the water column upon the time of collection of the material, it remains unclear if the cause of this variation are abiotic factors or intraspecific variation due to distinctive geographical area, as it was also observed in other astrophorins (Van Soest, Beglinger & de Voogd, 2010) including lithistids (Pisera & Vacelet, 2011).

CONCLUSIONS AND IDENTIFICATION KEY
The discovery of ten new lithistid species in the NE Atlantic seamounts and the additional record of another seven species, emphasises how diverse these ecosystems are and how our knowledge on the diversity of this group of sponges is still limited. Whether the patterns of distribution here reported are due to sampling bias, or true cases of endemism, requires further investigation.
The factors behind the variability on the spicules sizes, found in some species compared to those of the holotypes, remain unclear and more studies are needed in order to shed light on the factors behind this variability. This is particularly important on the field of sponge taxonomy since spicules are a key element for their identification. Future expeditions to these seamounts, with the use of ROVs, will allow us to have a better picture of this diversity and confirm if there are sponge grounds dominated by lithistids in the area.
An identification key of all lithistid species reported to date for the NE Atlantic and Mediterranean Sea is presented below (Table 9). Desmas have a root/vine-like appearance, microscleres are two types of microacanthoxeas, spirasters and streptasters 6. Isabella Two types of microsclers (metasters and spirasters), oxeas usually present 7. Neoschrammeniella

5.
No proper description has been given to this species in the original description and there are no more records of this species. The type material should be re-examined

C. elegantior
Sinuously fan-shaped with rounded and thin walls; microscleres are spirasters with long and thin arms C. masoni 6. Irregular rounded sponge of dark purple-brown colour; ectosomal spicules are irregular dichotriaenes, short-and long-shafted triaenes; two types of long oxeas (type I: long and thick with blunt tips; type II long, thin, curved with acerate tips) I. harborbranchi 7. Cup-shaped to contorted lamellate masses with thick walls; smooth surface; several thin oxeas in the inner surface; dicranoclones have irregular and high tubercles, that can be subdivided into several smaller tubercles

N. bowerbankii
Cup-to flattened cup-shaped with a concave center and rounded edges; smooth surfaces; dichotriaenes are very variable in shape and size; long-shafted triaenes can be present; oxeas are large and thin; dicranoclones of vine-like appearance, with some tubercles that are smooth or rugose

N. inaequalis
Large cup-rectangular in shape with smooth surfaces; dicranoclones are irregular, compact, usually smooth, with few tubercles that are usually smooth; no oxeas; some microscleres are irregular, resembling irregular rhabds with spiny tips

N. piserai
Cup-rounded in shape with a small pedicel; surfaces are crumble and hispid; oxeas are long with sharp tips; dicranoclones are compact, densely covered by numerous and ornamented tubercles N. pomponiae 8. Ectosomal spicules are discotriaenes, desmas are tetraclones, oxeas usually present, microscleres are acanthoxeas and acanthorhabds

Discodermia
Ectosomal spicules are phyllotriaenes to discotriaenes, microscleres are acanthorhabds 10. Theonella 9. Tree-like shaped, with a long stem smooth surface with some rugosities/protuberances; discotriaenes of "square" to "circular" shape or with "idented" cladomes; oxeas not present; tetraclones very tuberculated near the surface and smoother in the inner part of the sponge.

D. arbor
Massive, irregular in shape, with large protuberances of round shape; rugose surface; discotriaenes very variable in the shape of the cladomes varying from oval to indented, and size of rhabdomes; strongyles with one tip rounded and the other sharp D. kellyae Table 9 (continued ).
Small irregular mushroom shaped, with a concave upper side, a short stem and smooth surface; discotriaenes with a round to oval cladome; tetraclones with smooth rays and strongly branched and tuberculated zygomes; oxeas

D. polydiscus
Small, polymorphic, varying from spherical to irregular masses with protuberances, attached by a short pedicel; smooth surface; discotriaenes have very variable cladomes, from circular and concave to oval with irregular margins; tetraclones are smooth and irregular; oxeas not present

D. polymorpha
Small, elongated and branched with a smooth surface; discotriaenes have a round/oval to irregular and indented cladome; oxeas; tetraclones have smooth rays and tuberculated zygoses, that are usually smooth

D. ramifera
Cup-shaped to spherical polymorphic, with several round protuberances; discotriaenes are round/oval, smooth, often indented; oxeas; tetraclones are large, robust densely covered by tubercles D. verrucosa 10. Tetraclones are tuberculated but sometimes smooth in the center; phyllotriaenes have a simple or bifurcated cladome with rounded edges, and a short rhabdome * T. annulata 11. Dentate ectosomal phyllotriaenes/discotriaenes, smooth oxeas, microscleres are smooth microxeas. 12. Macandrewia 12. Cyathiform to flabellate, with undulating rounded margins and a short stem; outer surface is smooth with small pores and inner surface is smooth but the oscules have slightly raised margins; desmas are smooth, either resembling tetraclones or rhizoclones, very branched at the end

M. azorica
Small round-globular shaped, with a very short and slender pedicel and smooth surface; phyllotriaenes have incised and tuberculated cladomes; desmas with triaenose crepsis, usually smooth but some rugosities can be present

M. minima
Sponge with a vast base where it stands two or more truncks of cylindrical shape, with the top divided into short and obtuse branches

M. ramosa
Ficiform to globular in shape, with a thick and short pedicel; top of the sponge can be curved or slightly depressed; monocrepid desmas are smooth, with short and thick tubercles
MNHN Paris for hosting JRX and PC in the museum and making the material available for the present study. To Shirley Pomponi from the Harbour Branch Oceanographic Institute (Florida Atlantic University, USA) for sharing some material that was helpful to compare with Seamount 1 and 2 specimens. To Emma Sherlock and Ana Riesgo for hosting FCC at the NHM for the study of the lithistid collection of the museum. To Nicole de Voogd for hosting FCC at the Naturalis Biodiversity Center. To Irene Heggstad and Pedro Ribeiro for the support with the SEM and the map of the study area, respectively. To Christina Nagler and Daniel Kersken for the translations (from German) of several original species descriptions. We also thank the reviewers for their constructive comments to previous version of this manuscript. Lastly, we dedicate this study to all taxonomists and museum curators, who despite the many challenges, dedicate their efforts to discovering, identifying, describing, and documenting the diversity of our planet, and ensure proper archiving of biological specimens and associated scientific knowledge. This output reflects only the authors' views and the Executive Agency for Small and Medium-sized Enterprises (EASME) is not responsible for any use that may be made of the information it contains.