The genus Parapseudomma from the East Atlantic deep sea, with description of a new species (Mysida: Mysidae)

Abstract Revised diagnoses are given for the mysid genus Parapseudomma Nouvel and Lagardère, 1976, and its type species Parapseudomma calloplura (Holt and Tattersall, 1905) from the Northeast-Atlantic, Mediterranean and waters off Japan, depth range 94-1200 m. Parapseudomma stenurum n. sp. from the sea floor off Cape Verde Islands and the Angola Basin, depth range 3825-5460 m, is described as the second species of its genus. It differs from the type species mainly by an unsegmented antennal scale, the carpus of thoracic endopods 3-8 separated from the propodus by a transverse articulation, the digitus mobilis of the right mandible reduced to a slender spine, and by a more slender telson. Reduction of the right digitus is also found in Abyssomysis and Xenomysis, the latter with potential genetic affinity to Parapseudomma. The three genera also share reduced eyes as in many other deep-sea Erythropinae. Both species of Parapseudomma share recently detected, probably sensorial structures: a pit-like depression here named 'antennular bursa', dorsally on the basal segment of the antennula, and a median 'eye-cyst' inside the fused 'eye-bar', the cyst opening to the surface by a duct. The evidence in Parapseudomma raises expectations that these sensorial organs are potentially present in a great variety of Erythropinae taxa.


INTRODUCTION
Among 171 currently (29 Sep 2022) acknowledged Recent genera in the family Mysidae, the previously monotypic genus Parapseudomma Nouvel and Lagardère, 1976, shows the greatest similarity with the species-rich Pseudomma G.O. Sars, 1870.In retrospective view it was hidden for seven decades in the shadows of the latter genus as Pseudomma calloplura Holt and Tattersall, 1905.Already at its first description in 1905, Holt and Tattersall expressed some doubts on the affiliation of this species and finally decided to include it in the genus Pseudomma based on the highly similar structure of the eye rudiment.As late as 1976, Nouvel and Lagardère upgraded it to genus level as Parapseudomma by claiming that it cannot remain in the genus Pseudomma due to a two-segmented antennal scale, terminal margin of the telson with minute median spine flanked by large rugged spines, telson without setae, short rostral process of the labrum, modified seta arising from the penultimate segment of the endopod of pleopod 4 in adult males, and thoracic sternal processes in non-adults of both sexes.As many as 34 currently acknowledged species of Pseudomma were described up to 1976, thereafter fewer, yielding a total of 46 species.Both counts include the poorly known Pseudomma oculospinum W.M. Tattersall, 1951 (syn.Scolamblyops oculospinum emend.Murano, 1974) based on eye morphology; not acknowledged by Wittmann et al. (2014) as pertaining to the genus Scolamblyops Murano, 1974.Due to the comparatively great number of species, it is unsurprising that most of the individual characters claimed by Nouvel and Lagardère (1976) (unlike certain combinations of characters) are actually shared by one or more species of Pseudomma.Only the process of the labrum and the sternal processes in non-adults remain as autapomorphic characters of Parapseudomma (for additional characters see below), with the reservation that the respective characters are insufficiently known in many species of Pseudomma.Nonetheless, Kou et al. (2020) depicted Parapseudomma and Pseudomma in different clusters based on molecular phylogenetic analysis, though based on only a few species.
A single specimen of Parapseudomma with unusually slender telson is identified in the present contribution based on materials taken in 1982 by the CANCAP-VI expedition to the deep sea off Cape Verde Islands (equatorial NE-Atlantic).The structure of the telson is reminiscent of mysids taken in 2000 by the DIVA-1 expedition to the deep sea floor of the Angola Basin (equatorial SE-Atlantic) and determined by Wittmann (2020) as Parapseudomma calloplura (Holt and Tattersall, 1905).Hence the Angola material was recalled from the Zoological Museum Hamburg and by re-examination confirmed as fitting with the morphology of the specimen from the Cape Verdes.All other so far published records of P. calloplura came from more northern, less deep sublittoral to bathyal waters of the NE-Atlantic (and off Japan).The previously published data did not suffice to evaluate whether there are latitudinal clines rather than a clear-cut separation of morphological clusters.In order to shed light on this question, requests for loan of northern material were sent to museums and respective experts.The Hamburg Museum provided a single, somewhat damaged, specimen (not labeled as type) sampled by E.W.L. Holt, first author of the first description of P. calloplura, in the Celtic Sea near the range estimated by Wittmann (2020) for the originally undefined type locality.Inmaculada Frutos from the University of Lódz kindly provided 15 specimens sampled in the frame of the ECOMARG 2004 project in the Bay of Biscay (I. Frutos, pers. communication).Particularly, the examination of dissected materials revealed clearcut morphological differences between northern and southern specimens, the latter described as a new species of Parapseudomma herein.

MATERIALS AND METHODS
Materials were obtained on request from the Zoological Museum Hamburg, from the Naturalis Biodiversity Center in Leiden, and from Inmaculada Frutos, University of Lódz.Materials, including the here defined types, were returned in vials with aqueous solution of ethanol 80% with propylene glycol 10% unless explicitly stated as having been dissected and mounted on slides.Terminology, measurements, preparation, microscopy, and documentation as in Wittmann and Chevaldonné (2021).Nomenclature from genus to family level according to Wittmann et al. (2014).All here studied eye rudiments are proximally united in a transverse bar rather than a plate, therefore termed 'eye-bars' rather than using the traditional term 'eyeplates'.Unlike 'claws', the 'nails' are not hook-like; only nails were found in the here treated species.Wittmann and Chevaldonné (2021) described a pit-like depression with striated pad on the bottom, probably a sensorial structure, dorsally on the basal segment of the antennula.This organ is here termed 'antennular bursa' (Fig. 4E).Wittmann and Chevaldonné (2021) also described an 'eyeplate cyst', for conformity now termed 'eye-cyst' (Fig. 2C) found inside fused eye-bar; this cyst opens to the outside or into the eye cleft by a duct.Diagnosis of this subfamily as in Wittmann et al. (2014).Worldwide key to its currently acknowledged 57 genera plus three new ones will be provided in a forthcoming report about Mysidae of three ANDEEP expeditions to the Antarctic deep sea.

Revised diagnosis. The diagnosis by
Potential additional diagnostic characters.Adult male pleopods in type species with large sympod, endopods 2-5 and all exopods multisegmented; pleopod 4 the largest, penultimate segment of its endopod with large, modified seta.Adult males unknown in P. stenurum n. sp.
Distribution.Type locality not stated by Holt and Tattersall (1905).A rough estimate by Wittmann (2020) suggests that the type locality is off Ireland in the range of 52-54°N 12-13°W, depth 363-696 m.The present sample from the Celtic Sea was taken near this range by E.W.L. Holt, the first author of the first description.This species is known from the NE-Atlantic and Mediterranean, 35-58°N 013°W-014°E, depth 94-1200 m (references listed above); the here presented records are within these ranges.Murano (1970) reported this species as Pseudomma calloplura from the NW-Pacific off Japan, 35-37°N 137-140°E, depth 220-660 m.Records of Parapseudomma calloplura reported by Wittmann (2020) from the Angola Basin are here attributed to the below described Parapseudomma stenurum n. sp.Revised diagnosis.Based on adults of both sexes.All features within the limits of the genus diagnosis.Carapace without rostrum; disto-lateral edges angular (right-angled to wide-angled).Transverse extension of eye-bar 2.3-3.5 times length of distal segment of antennular trunk.Eyes fused at proximal third, while distally separated by narrow median cleft (Fig. 2A).Eyes dorsoventrally flattened by factor of 1.4-2.2measured at maximum width in dorsal view.Each eye microserrated (Fig. 2B) along central third to half of its anterior margin.Disto-mesial lobe dorsally at basal segment of antennular trunk reaching to half-length of median segment.Disto-lateral edge of antennal sympod with tooth-like projection.Antennal scale with small apical segment separated by tiny (in part inconspicuous) suture.Length of scale 3.5-5.0times maximum width; mesial margin setose; straight lateral margin bare, ending in tooth; distal segment not reaching to tip of this tooth.Scale extending 40-55% its length beyond antennal peduncle and 35-50% beyond antennular trunk.Labrum mid-rostrally slightly prolonged by small acute projection.Mandibles normal, right digitus mobilis smaller than the left one (Fig. 1C).Unsegmented carpus of thoracic endopods 3-8 separated from two-segmented propodus by oblique articulation.Male pleopod 4 the largest, penultimate segment of its endopod with large smooth (modified) seta in adults longer than endopod.Exopod of uropods extending beyond endopod; exopod and endopod beyond telson (Fig. 1A, B).Endopod with 1 small spine on mesial margin below statocyst.Telson (Fig. 2E) with slightly sigmoid, almost straight lateral margins and convex, broadly rounded terminal margin.Telson length 1.7-2.9times maximum width near basis and 4-6 times width at disto-lateral edge.Proximal 29-45% of lateral margins smooth, remaining distal portion of each margin with 10-15 spines distally somewhat discontinuously increasing in size.Terminal margin with minute (often inconspicuous) median spine flanked by 3-4 pairs of large spines; these spines bilaterally armed along their basal to subapical portions with stiff bristles; largest spine 23-34% telson length.
Male sexual characteristics.Size and structure of thoracic exopods are non-dimorphic when comparing equally sized subadults (adult females not available).Exopod of adult and subadult male pleopods 2-4 with short spine-like seta (besides other setae) at penultimate segment; pleopod 5 with group of three small, supernumerary setae in the respective position.
Description of the holotype.All female features within the limits of the species diagnosis.Female with 24.9 mm body length, marsupium empty (Fig. 3A).Rostrum contributing 0.6% to body length, carapace 23% (without rostrum), thorax 32%, pleon 49%, and (long) telson 19%.Parts of body cuticle ornamented with fields of minute, roughly ellipsoidal shallow depressions (Fig. 4C), some of which (accidentally) filled with external material.Tergites of thoracomere 8 and of adjoining pleomeres 1, 2 almost completely ornamented, carapace mainly on fields illustrated as shaded areas in Fig. 5D.Minor numbers of such depressions present on antennular trunk and antennal peduncle, again fewer on basal plates of thoracic exopods.Minute 'cils' (4-7 µm long slender scales) line outer margin and parts of dorsal face of eye rudiments.Carapace (Fig. 5D) with disto-lateral edges well rounded.Cervical sulcus distinct, no cardial sulcus visible.Posterior margin weakly concave.Carapace leaving 1.5 thoracic segments mid-dorsally exposed (as in other mysids the coverage may vary with swelling of the thorax due to yolk accumulation in the ovarian tubes).
Antenna (Figs. 3B, 5C).Sympod two-segmented, caudally in addition with end sac of antennal gland.Peduncle with 3 segments separated by transverse articulations.Basal segment contributing 30%, median segment 43%, and terminal segment 27% to total length.Basal segment smooth, median and terminal segments each with few barbed setae.
Labrum as in diagnosis and in Fig. 5E, damaged labium as in Fig. 5G, mounting of foregut failed.
Mandibles (Figs. 4A, B, 5F).Palp three-segmented with basal segment contributing 12-13%, median segment 62-63%, and terminal segment 24-26% to total palp length.Palp not hispid, its basal segment without setae, median segment with sparse setation on both lateral margins.Length of median segment 4 times maximum width, lateral margin slightly sigmoid while mesial margin sigmoid with near-median convex portion more strongly curved than proximal concave portion.Terminal segment 2.3-2.9 times as long as broad and 0.4 times as long as median segment.Terminal segment well setose, with longitudinal series of setae on distal 3/4 of rostral face.Right mandible (Fig. 4B) with 5 large teeth on pars incisiva; digitus mobilis reduced to slender, subapically slightly microserrated spine; pars centralis with large spinefree portion proximally followed by large lobe apically bearing seven long spines; these spines microserrated by acute bristles along their subbasal to subapical portions.Processus molaris of each mandible with strongly cuticularized masticatory lamellae.Left mandible (Figs.4A, 5F) normal, its pars incisiva with 4 large teeth; digitus mobilis with long, slender basis and with 2 large, plus more than 4 small teeth on apex; pars centralis all along with 12 slender spines bearing stiff bristles (only 4 spines roughly in focus in Fig. 4A, all 12 spines visualized in Fig. 5F).
Maxillula (Fig. 5H).Distal segment with 15 strong, bluntly serrated spines on transverse terminal margin.This segment subterminally with transverse series of 9 setae bearing long stiff barbs.Basal segment furnished with mainly longitudinal though basally bent, comparatively long series of densely set long hairs.Endite terminally with 3 large, distally spiny (by stiff bristles) setae flanked by shorter barbed setae.Two most proximal setae of endite much longer and more slender compared with their distally neighboring setae.
Maxilla (Fig. 5I) densely setose, without spines.Sympod with 3 mesial, only distally strongly setose lobes.Exopod extending to distal margin of basal segment of palp.Exopod with numerous plumose setae all along lateral margin; longest seta at tip, mesial margin with only 3 much shorter plumose setae in subapical position.Palp two-segmented, apical segment 1.1 times as long as basal segment.Apical segment 1.8 times longer than maximum width.Its transverse terminal margin densely setose, lateral margin sparsely setose, mesial margin with only 2 setae near disto-mesial edge.Basal segment basally broader, on rostral face (below drawing plane in Fig. 5I) near mesial margin with longitudinal series of 7 densely barbed, basally thick setae (visualized by dashed lines).
Etymology.The species name is an adjective with Latinized neutral ending formed by amalgamation of the Classic Greek adjective στενός (narrow) with the noun ουρά (tail) referring to the slender telson.
Type locality.E-Atlantic, Cape Verde Islands, about 42 km south of Santiago Island, DD 14.517 -023.55, on the sea floor in 3825-4025 m depth.

Definition of the genus Parapseudomma
The eye rudiment structure of the new species fits with the genus Parapseudomma and to a lesser extent also with Pseudomma, unlike the other 55 currently acknowledged genera of the Erythropinae, and also the remaining (171 minus 57) Recent genera of the Mysidae.Parapseudomma is distinguished from Pseudomma by (combinations of) a less deep cleft between the proximally fused eye rudiments, Wittmann: New deep-sea mysid from the East Atlantic Nauplius, 31: e2023013 comparatively large dorsal lobe from the disto-mesial edge of the basal segment of the antennular trunk, short rostral process of the labrum, thoracomeres with sternal processes only in non-adults and large hispid spines but no setae on the terminal margin of the telson.Molecular phylogenetic analysis by Kou et al. (2020) placed Parapseudomma and Pseudomma in different clusters.
The present finding of an eye-cyst (Figs.2C, 3E) connected by a duct with the bottom of the median cleft distally separating the incompletely fused eye rudiments both in Parapseudomma calloplura as well as P. stenurum n. sp.coincides with the evidence of such a cyst in Pseudomma kryotroglodytum Wittmann and Chevaldonné, 2021.These three species also share antennular bursae.Eye-cysts and bursae were probably generally overlooked before 2021 and could be expected for many more species of Erythropinae.Future findings could also lessen the overall morphological distance between Parapseudomma and Pseudomma.
Validity and affiliation of Parapseudomma stenurum n. sp.
With the reservation that adult males are unknown, the new species fits with all above-discussed features distinguishing Parapseudomma from Pseudomma and consequently it is attributed to the genus Parapseudomma.There are some important differences from the type species Parapseudomma calloplura: the new species differs by an unsegmented antennal scale, the carpus of thoracic endopods 3-8 separated from the propodus by a transverse rather than oblique articulation, and the digitus mobilis of the right mandible reduced to a slender spine (Fig. 4B) rather than bearing a number of well-developed teeth (Fig. 1C).Many additional minor differences are listed in the above diagnoses and descriptions.Regarding the major differences, it appears necessary to check other Erythropinae which also have a strongly reduced or a completely reduced digitus mobilis of the right mandible.
Abyssomysis cornuta Wittmann, 2020, from the abyssal plain in the Gulf of Guinea shows the digitus mobilis of the right mandible reduced to a slender spine in males and completely reduced in females.The digitus of the left mandible shows a long, slender basis and a well-toothed apex in both sexes of A. cornuta.Both sexes of the new species show mandibles (Fig. 4A, B) most similar to the male but not to the female of A. cornuta.The latter species differs from the new species among other features also by distantly set eyestalks, no median cleft on the eye-bar, and by a short triangular telson with bare lateral margins.
The only known male (holotype) of Xenomysis unicornis Kou, Meland and Li, 2020, from the hadal zone in the Mariana Trench shows a completely reduced right digitus mobilis as in the opposite sex (females) of A. cornuta.The monotypic genus Xenomysis Kou, Meland and Li, 2020 deserves special interest because according to Kou et al. (2020) its DNA sequences cluster with P. calloplura, although with low support.Xenomysis differs from P. stenurum n. sp. by eye-bar without median cleft, acute rostrum, carpus of thoracic endopods 3-8 separated from the propodus by an oblique articulation, and by acutely triangular, elongate telson.Several morphological features of X. unicornis remain unclear -loan of the type was hindered by the current pandemic situation and by legislative restrictions -only an unpublished photo of the right side of the cephalothorax was received (see Acknowledgements), which did not yield contrasting information.In the present interpretation the eponymous vertical alignment of the rostrum together with the cervical region of the carapace ruptured off the thorax as indicated in fig.6b, c by Kou et al. (2020) and the great height of the cervical region in fig. 1 (op.cit.) together with the exceptionally large distance between labrum and clypeus in figs.2d, 6a by Kou et al. (2020) might represent artefacts of accidental transient lateral compression of the cephalic region.This may have forced elongation of the cephalon with resulting stress on the cervical region of the carapace, which in response lifted up from the thorax and levered the rostrum vertically.The present author has observed comparable artefacts in the rostrum of a total of seven specimens of Euchaetomera typica G.O. Sars, 1883, and of three other species of Erythropinae with normally anteriorly directed rostrum.The unsegmented exopod of pleopod 3 and the incipient segmentation (as also here in Fig. 3H) of the endopod of pleopod 4 visible in fig.6f by Kou et al. (2020) could indicate an incomplete status of male maturity despite well-developed penes and appendix masculina in the holotype of X. unicornis.The distally setose lateral margin of the antennal scale

Family
Abbreviations# = station number of the DIVA-1 expedition BL = body length measured from tip of rostrum to terminal margin of telson DD = decimal degrees of geographic coordinates RMNH = Rijksmuseum van Natuurlijke Historie, Naturalis Biodiversity Center in Leiden ZMH = Zoological Museum Hamburg, also named Museum of Nature Hamburg e.n.= epinet of epibenthic sledge n.n.= net of epibenthic sledge not specified s.n.= supranet of epibenthic sledge

Figure 2 .
Figure 2. Parapseudomma calloplura (Holt and Tattersall, 1905), subadult male with BL 7.8 mm from the Bay of Biscay.A, Eye rudiments, dorsal; B, detail of panel (A) showing serrated portion of anterior margin of right eye rudiment; C, detail of panel (A) showing median eye-cyst; D, thoracic sternite 8 with median process and penis; E, telson.

Figure 3 .
Figure 3. Parapseudomma stenurum n. sp., holotype adult female with BL 24.9 mm (A, B, E) from the deep sea off Cape Verde Islands, and paratypes subadult female 14 mm (C), immature male 11.2 mm (D, G, H) and adult female 17.1 mm (F) from the Angola Basin.A, Female holotype in toto, lateral (most thoracic endopods broken); B, cephalic region of holotype, dorsal; C, rostrum (ro) and right eye rudiment (re), lateral; D, eye rudiments, dorsal; E, detail of panel (D) showing median eye-cyst in a larger specimen (holotype), dorsal.F, tail fan, dorsal; G, pleopods 3-5 in loco, lateral; H, detail of panel (G) showing tip of pleopod 4. A, two photos mounted together; A, C, objects artificially separated from background.

Figure 4 .
Figure 4. Parapseudomma stenurum n. sp., holotype adult female with BL 24.9 mm (A-E) from the deep sea off Cape Verde Islands and paratype subadult female 14 mm (F) from the Angola Basin.A, B, Masticatory portions of left (A) and right (B) mandibles with pars incisiva (pi), digitus mobilis (dm), spines of the pars centralis (pc) and pars molaris (pm), rostral; C, cuticular ornamentation on parts of the carapace indicated by shaded fields in Fig. 5D; D, distal portion of telson, dorsal; E, antennular bursa, dorsal; F, rostrum with respective portion of carapace, dorsal, dashed line enhances the left anterior margin of the rostrum.A, B, D, structures of the subsequent molt stage visible inside objects.