A collection of hexactinellids (Porifera) from the deep South Atlantic and North Pacific: new genus, new species and new records

This article describes or redescribes four hexactinellid sponges, namely Poliopogon amadou, Euplectella sanctipauli sp. nov., Bolosoma perezi sp. nov. and Advhena magnifica gen. et sp. nov. P. amadou, E. sanctipauli sp. nov. and B. perezi sp. nov. represent new findings for the South Atlantic deep-sea fauna, including the first record of Bolosoma for this ocean. Advhena magnifica gen. et sp. nov., on the other hand, was collected by NOAA oceanographic expeditions in the North Pacific (Pigafetta Guyot).


INTRODUCTION
In deep-sea environments, corals and sponges form ecologically important structures that provide habitats for other organisms (Fuller et al., 2008), and as a result are commonly associated with biodiversity hotspots (Freiwald et al., 2004). Hexactinellids or glass sponges (Porifera, Hexactinellida) are siliceous marine sponges that occur on both hard and soft sediments. These sponges are diverse and abundant in deep-sea environments, frequently being the dominant component in deep benthic communities (200-6000 m depth), where they secrete considerable amounts of silica (Reiswig, 2002). The class Hexactinellida has classically been divided into two subclasses, Amphidiscophora and Hexasterophora, based on microsclere form, amphidiscs or hexasters, respectively. Whereas Amphidiscophora has contained one order only (Amphidiscosida), Hexasterophora has traditionally been divided into four orders (Aulocalycoida, Hexactinosida, Lychniscosida, Lyssacinosida). Parts of this classification have recently been altered to reflect phylogenetic considerations (Dohrmann et al., 2017;Kersken et al., 2018;Dohrmann, 2019). While the subclass designation into two reciprocally monophyletic groups, Amphidiscophora and Hexasterophora, has been retained, present classification divides the latter into orders Lychniscosida, Lyssacinosida, Sceptrulophora and a diverse assemblage of Hexasterophora incertae sedis (Reiswig, 2002;Van Soest et al., 2019).
DISTRIBUTION AND ECOLOGY. Former records from the Azores (Thomson, 1873), and the Canary Islands (Thomson, 1877, Schulze, 1887, 2,789-4,022 m deep (Tabachnick & Menshenina, 2002). This is the first finding of the species in the South Atlantic, where it is thus far only known from the São Paulo Ridge (SW Atlantic) (present paper), at 3,060 m depth. The single specimen collected was growing on basaltic rock with other sponges likely of the same species, albeit in very low densities. No epibiontic macro-or megafauna could be seen from the images obtained ( Fig. 2).

Remarks
The single specimen collected by the Iata Piuna expedition matches the original description of Thomson (1877) and redescription by Tabachnick & Menshenina (2002). The latter authors mentioned that whereas macramphidiscs are clearly separable from the smaller amphidiscs, separation of second and third categories as mesamphidiscs and micramphidiscs is not always straightforward. It is not clear to us whether Tabachnick & Menshenina (2002) were referring to every specimen as a rule, the holotype included. Regardless, in our material the three categories were clearly separable, if not entirely on the basis of dimensions, but surely when the micromorphology of these microscleres was taken into account. The rare hexadiscs seen in our specimen had previously been reported from a single specimen from the Great Meteor Bank (29 58′51.5604″N/28 29′12.6456″W, 2,480-2,550 m depth; Tabachnick & Menshenina, 2002 Remarks: We emended the diagnosis to include the present new species which lacks a sieve-plate. The new species fits with all the remaining aspects of the previously accepted diagnosis of Euplectella (cf. Tabachnick, 2002).
DISTRIBUTION AND ECOLOGY. Known only from its type locality, São Paulo Ridge (Southwest Atlantic; 4,061 m depth). The single specimen was collected from soft sediment, apparently the sole megafauna in several meters of terrain, and seemingly devoid of any macro-epibionts.
ETYMOLOGY. The specific epithet is used as a noun in apposition, derived from the species having been found at the São Paulo Ridge (deep SW Atlantic).

Remarks
Euplectella comprises 18 species around the world (Reiswig & Kelly, 2018), classified by Tabachnick & Collins (2008) into four main groups recognizable on the basis of the architecture of their principal skeleton: (1) mainly stauractins, (2) mainly hexactins with a reduced proximal ray and some stauractins, (3)    ETYMOLOGY. The specific epithet honors Prof. Dr. Angel Perez (UNIVALI, Brazil) for granting us access to Rio Grande Rise materials, and for our long-standing collaboration in the study of South Atlantic deep-sea ecosystems.

Remarks
Bolosoma comprises eight species (see comparative Table 1), up to now recorded only from the Pacific Ocean. The new species presents a unique set of spicules, indicating its distinctiveness from the remaining species in the genus. None of its congeners present the combination of delicate codonhexasters and "ball-like" discohexasters, which confers B. perezi sp. nov. obvious status as a new species. This is the first record of Bolosoma for the Atlantic Ocean.

Etymology
Feminine gender. Modified from the latin, 'Advena', meaning stranger, foreigner, alien, newcomer, guest, in reference to the sponge shape, which calls to mind aliens from various movies. An 'h' was added in 'Advena' (Advhena) to distinguish the name from that of the helicarionid gastropod Advena Gude, 1913, in order to make sure there will be no overlapping with valid names.

Remarks
The new specimen cannot be accommodated in any of the currently accepted genera in the subfamily (Table 2). It is clearly a long stalked Bolosominae individuum, with diactins as the main spicules, plus hexactins and pentactins. It is distinguished from other Bolosominae by the set of microscleres present, namely discasters, discohexasters, codonhexasters and derivatives, calycocomes, and graphiocomes, which render it unique among bolosomines, and justifies the proposal of a new genus.  EXTERNAL MORPHOLOGY. Pedunculate sponge characterized by mushroom-like shape (136 mm in diameter and 75 mm thick) with lateral openings concentrated on one side of the body, each one with 50 and 18 mm in diameter, and peduncle longer than 154 mm in length (broken) and 16 mm in diameter (at least four times the body size).  (1 cm  diameter); discohexasters (50-75 mm diameter); codonstaurasters (103-160 mm diameter) usually with four to six central axis and five to seven secondary rays (sometimes bent over), discs with eleven teeth; calycocomes (138-255 mm diameter), and graphiocomes (150 mm

GENERAL DISCUSSION
With our observations, South Atlantic hexactinellid diversity still only amounts to 34 species. Most of these observations are from the continental slope in the SW (20 spp) and SE Atlantic (three spp), areas much more easily accessible due to the relatively small navigational distances involved. The present study constitutes a~27% increase in the knowledge of South Atlantic deep-sea hexactinellids not associated with continental margins (three new records, raising the total to 14 species). The area comprised by abyssal plains, the mid-Atlantic ridge, Rio Grande Rise, São Paulo Plateau, and many additional important geomorphologic features not associated to continental margins, is an enormous part of the South Atlantic (~26.46 km 2 × 10 6 ; Watling et al., 2013), but limited observations hinder understanding of hexactinellid and other biodiversity. This is obviously a major frontier for deep-sea exploration. The opportunity brought up by Brazil's research obligations, in exchange for the permission granted by the International Seabed Authority (International Seabed Authority, 2015) for the exploration of mineral crusts at the Rio Grande Rise, is a considerable opportunity to pursue an exponential increase in the knowledge of deep-sea benthic biodiversity in this large area of the SW Atlantic. The present study brought together materials obtained through dredging and manned-submersible ("Shinkai 6500") collections, between 3,060 and 4,061 m depth, important tools for building a comprehensive understanding of deep-sea communities in the Rio Grande Rise. At present, only the tip of the iceberg has been touched upon. As dredging has been conducted with geological priorities in mind, biological samples are few and mostly severely damaged. On the other hand, samples obtained by the "Shinkai 6500" are of good quality by nature. Although very few in number, those samples, together with records of environmental parameters, can greatly improve understanding of deep-sea biodiversity, habitats and ecosystems. New collections should be planned with the use of ROVs, essential to establish the true dimensions of the recently described Sarostegia Topsent (1904) sponge garden, for example (Hajdu et al., 2017), as well as to determine whether other potentially vulnerable marine habitats occur in the Rio Grande Rise.
Poliopogon amadou is reported here for the first time from the South Atlantic. Xavier, Tojeira & Van Soest (2015) reported upon large aggregations of this species in the NE Atlantic (Meteor Bank). It is not clear to us how abundant the species is in the São Paulo Plateau, something worth investigating further, as part of the expected commitment to the study of deep-sea communities in the neighboring Rio Grande Rise over the next decades.
A new species of Bolosoma is described here, raising to nine the number of known species in this genus, and expanding the genus' known distribution to the Atlantic Ocean. A better assessment of deep-sea sponge biodiversity in the Atlantic and the Pacific will only be possible with additional efforts, both in enabling new expeditions, but also in granting support to the necessary work on proper identifications of collections already available. A considerable number of deep-sea sponge specimens is deposited in scientific collections around the world, calling for taxonomists to work on them and furnish not only inventory data, but also reveal surprising animals, whose integrative study can help us build a sharper Porifera Tree of Life.
Finally, a new genus in the Pacific Ocean is proposed here within Bolosominae, Advhena gen. nov. The last decade has seen a significant increase in recognition of higher taxa within this subfamily, with four new genera proposed, as well as eleven new species. Including Advhena gen. nov., all of the recently recognized bolosomine diversity has been reported from the Pacific (Amphoreus Reiswig & Kelly, 2018, Rhizophyta Shen et al., 2019and Trychella Reiswig & Kelly, 2018, further highlighting the richness of this ocean basin, which is also apparent in the number of Bolosoma spp. described thus far (cf. above). Although Bolosominae was found to be non-monophyletic (Dohrmann et al., 2017), the new genus fits best within it taxonomically until a detailed integrative revision of Euplectellidae and its subfamilies is completed.