Discias forsskali, Anker, 2023

Anker, Arthur, 2023, A new deep-water species of Discias Rathbun, 1902 (Decapoda: Caridea: Disciadidae) from the Gulf of Aqaba, northern Red Sea, Zootaxa 5315 (2), pp. 187-194 : 188-193

publication ID	https://doi.org/ 10.11646/zootaxa.5315.2.7
publication LSID	lsid:zoobank.org:pub:7EACB58E-6B9D-4A8B-BB8D-4F4BF28CA776
DOI	https://doi.org/10.5281/zenodo.8139079
persistent identifier	https://treatment.plazi.org/id/039C87E6-E57F-A646-3CFC-2DF957D1FB35
treatment provided by	Plazi
scientific name	Discias forsskali
status	sp. nov.

Treatment

Discias forsskali sp. nov.

( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 )

Type material. Holotype: ovigerous female (pocl 2.7 mm, cl 3.8 mm), FLMNH UF 68639 , Saudi Arabia, Tabuk Province, Red Sea, Gulf of Aqaba, northwest of Ras Suwayhil al Saghir , OceanX sta. NTN0036, 28°51’15.5”N 34°49’19.1”E, depth 387.4 m, sediment core, Neptune (Triton 3300/3 submersible), coll. G Chimienti and T. Pensa, 19.10.2020 (fcn NTN0036 Core2 / OCX-030). GoogleMaps

Description. Carapace glabrous, conspicuously pitted on flanks, especially closer to ventral margin; orbit rounded; suborbital angle poorly delimited; antennal spine stout, submarginal, distally acute; branchiostegial spine absent; pterygostomial margin broadly rounded ( Fig. 1A, B View FIGURE 1 ). Rostrum reaching to distal end of second article of antennular peduncle, slightly descending, gently curved up distally, dorsoventrally slightly compressed, lanceolate, apically acute, with weak mid-dorsal carina; lateral margins of rostrum and mesial margin of orbit serrated with numerous (>30) minute teeth; distal-most portion of lateral margins of rostrum unarmed ( Fig. 1B, C View FIGURE 1 ).

Pleon dorsally patchily setose, laterally mostly glabrous; pleura of all pleonites with both anterior and posterior margins rounded; second pleonite without protruding mid-dorsal tooth on posterior margin; sixth pleonite about 1.2 times as long as fifth pleonite measured along mid-dorsal length ( Fig. 1A View FIGURE 1 ).

Telson 1.2 times as long as sixth pleonite, tapering posteriorly, about three times as long as proximal width; lateral margins straight; posterior margin about half-length of proximal telson width, broadly rounded, armed with nine spiniform setae of various length, two submedian setae noticeably longer than others; dorsolateral surface with two pairs of robust spines, proximal ones situated at short distance from lateral margin at about half-length of telson, distal ones submarginal and inserted at about 0.8 of telson length ( Fig. 2O, P View FIGURE 2 ).

Eyes well developed; cornea almost spherical, not noticeably wider than proximally tapering eyestalk; auxiliary eyespot (Nebenauge) visible in dorsal view ( Fig. 1B, C View FIGURE 1 ).

Antennular peduncle reaching distal 0.7 length of scaphocerite; first article about three times longer than second article, with distally acute stylocerite overreaching mid-length of first article, distolateral angle slightly produced; second article shorter than third article, widened distally, cup-shaped; third article slightly wider than long; dorsal (lateral) flagellum with swollen proximal section bearing at least six discernible units and nine clusters of aesthetascs ( Fig. 1B View FIGURE 1 ).

Antennal basicerite moderately robust, with two distal lobes; carpocerite falling short of mid-length of scaphocerite, unarmed; scaphocerite 2.5 times as long, widest at mid-length, lateral margins broadly convex; distal margin of blade somewhat truncate; distolateral tooth small, not exceeding distal margin of blade ( Fig. 1B, E View FIGURE 1 ).

Mandible with palp well-developed, composed of two articles (not including very short basal unit), proximal article bearing one subapical seta, distal article with six long stout setae; incisor process with four larger teeth and one small, isolated, dorsal tooth; molar process tapering distally, subtriangular, with serrated mesial margin and lateral margin armed with two small but stout, spine-like teeth ( Fig. 1F–H View FIGURE 1 ).

Maxillule with broad, distally bilobed palp, lower (ventral) lobe with one shorter dorsal seta and one longer and stouter ventral seta; distal margin of ventral (proximal) endite with long plumose setae; distal margin of dorsal (distal) endite with rows of stout spiniform setae interspersed with some slender setae ( Fig. 1I View FIGURE 1 ).

Maxilla with long, simple, distally curved palp; scaphognathite well developed, dorsal portion much broader than ventral portion; ventral (proximal) endite with long setae; dorsal (distal) endite bilobed, margins of both lobes furnished with long setae ( Fig. 1J View FIGURE 1 ).

First maxilliped with simple, moderately elongate, non-setose palp; exopod with short distal flagellum, reaching well beyond palp, and prominent caridean lobe; ventral (proximal) endite with few slender setae; epipod narrow, bilobed; dorsal (distal) endite furnished with long spiniform and slender setae distally ( Fig. 1K View FIGURE 1 ).

Second maxilliped with well-developed exopod, reaching beyond carpus of endopod; epipod narrow, bilobed; endopod with ischiomerus slightly longer than propodus; carpus small, very short, obliquely articulating with propodus; propodus long, wide, convex on both dorsal and ventral margins; dactylus semicircular, distally armed with 14 spiniform setae ( Fig. 1L View FIGURE 1 ).

Third maxilliped with well-developed exopod, reaching 0.8 length of antepenultimate article of endopod; all endopod articles robust; antepenultimate article elongate-subrectangular, almost four times as long as wide, slightly curved, armed with slender spiniform seta on distodorsal angle; penultimate article much slenderer than antepenultimate article, about 3.2 times as long as wide, armed with long stout spiniform seta on distodorsal angle; ultimate article about twice as long as penultimate and about as long as antepenultimate article, widening near mid-length, then tapering to apex, with flattened dorsal surface; distal margin armed with six stout spiniform setae, with two additional setae on distomesial margin, dorsomesial margin armed with five strong spiniform setae ( Fig. 1M, N View FIGURE 1 ).

All pereiopods with well-developed exopods, their distal thirds furnished with long plumose setae. First pereiopod robust, reaching to distal margin of scaphocerite in full extension; basis very stout; exopod not reaching distal end of ischiomerus of endopod; ischiomerus stout, widening distally, as long as propodus, with distal part deeply excavated to accommodate carpus, distodorsal margin with stout spiniform seta; carpus very short, compressed, set inside proximal third of propodus and partly fused to it, with one spiniform seta; propodus robust, subcylindrical, with dorsal surface somewhat flattened, twisted laterally relative to propodus; palm ovoid, about 2.1 times as long as wide; ventral (mesial) surface of palm with two comb-like rows of short setae in most proximal part, adjacent to carpus, and several long stout setae in distal half, including one near articulation with dactylus; pollex strongly arched, terminating in slightly recurved, acute, spiniform process; cutting edge (occlusal margin) bicarinate, excavated to accommodate cutting edge of dactylus; dactylus subcircular, with semi-hyaline, lamellar cutting edge, and one long spiniform seta on apex ( Figs. 1A View FIGURE 1 , 2A–E View FIGURE 2 ).

Second pereiopod much shorter and weaker than first pereiopod, extending to mid-length of scaphocerite; basis short, stout; exopod extending to 0.8 length of ischiomerus; ischiomerus robust, widening distally, longer than combined length of dactylus, propodus and carpus, ventrolateral surface with one stout subdistal spiniform seta lodged in deep pit; carpus short, squarish, about as wide as long; palm subcylindrical, about 2.6 times as long as than wide; pollex shorter than dactylus, with cutting edge armed with two stout sharp teeth, distal tooth much longer than proximal one; dactylus about half-length of propodus, with cutting edge armed with 10 stout sharp teeth increasing in size distally, distal two teeth by far strongest ( Figs. 1A View FIGURE 1 , 2F–H View FIGURE 2 ).

Third to fifth pereopods generally similar in shape and armature, decreasing in size posteriorly. Third pereiopod reaching to distal one-fourth of scaphocerite when fully extended; basis robust; exopod moderately long, not reaching half-length of merus; ischium short, about 0.35 length of merus, ventrolateral surface with two stout spiniform setae lodged in deep pits; merus subequal in length to combined length of distal three articles, slightly convex ventrally, ventrolateral surface armed with four evenly spaced, stout spiniform setae lodged in deep pits; carpus short, about 0.3 length of propodus, ventrolateral surface with strong spiniform seta lodged in deep pit, its tip overreaching distal margin of carpus; propodus 5.5 times longer than wide, feebly tapering distally, ventral margin with seven slender spiniform setae, distal longest; dactylus about 0.3 times as long as propodus, slightly curved, terminating in sharp unguis, ventral margin with four spinules increasing in size distally ( Figs. 1A View FIGURE 1 , 2I, J View FIGURE 2 ). Fourth pereiopod very similar to third, as illustrated; ventral margin of propodus with eight spiniform setae, including two longest distally ( Fig. 2K, L View FIGURE 2 ). Fifth pereiopod similar to third and fourth in most aspects, however, noticeably slenderer and with different article proportions; ischium shorter and less stout, with two stout spiniform setae; merus much slenderer and shorter than that of third pereiopod, with three stout spiniform setae; propodus also noticeably slenderer, with nine spiniform setae on ventral margin; propodus and dactylus together rotated about 90º from main appendage axis ( Fig. 2M, N View FIGURE 2 ).

Pleopods without specific features, typical for genus; endopod of second pleopod with slender appendix interna basally; male secondary sexual characters currently unknown. Uropods slightly shorter than telson; exopod with lateral margin straight, non-serrated, terminating in subacute subdistal tooth; subdistal spiniform seta robust, not reaching distal margin of exopod; transverse suture (diaeresis) slightly sinuous; endopod as long as exopod, slightly narrower ( Fig. 2O View FIGURE 2 ).

Eggs few (<15), large, diameter about 0.5–0.6 mm ( Fig. 1A View FIGURE 1 ).

Colour in life. Not recorded.

Etymology. The new species’ name honours one of the first explorers of the marine biodiversity of the Red Sea, and Carl von Linné’s (Linnaeus) disciple, Peter Forsskål (1732–1763).

Distribution. Presently known only from the type locality in the Gulf of Aqaba, northern Red Sea, off the coast of Saudi Arabia.

Ecology. Unknown, except for the depth of the sediment core sample taken at 387.4 m. No association with sponge, parchment worm or other possible hosts was recorded in the field. It is possible that the shrimp accidentally swam from the bottom or near the bottom into the core sampler. However, according to OceanX sampling data and photographs, station NTN0036 also featured octocorals and smaller pieces of rubble, which may easily have harboured a cryptic sponge, from which the shrimp was expulsed, and then collected and preserved.

Remarks. Discias forsskali sp. nov. belongs to a small group of species characterised by the smooth posterodorsal margins of all pleonites ( Fig. 1A View FIGURE 1 ) and the non-serrated lateral margin of the uropodal exopod ( Fig. 2O View FIGURE 2 ). This group includes, in addition to the new species, D. atlanticus Gurney, 1939 in the western and eastern Atlantic, with questionable records from the Indo-West Pacific (see below), and D. musicus Holthuis, 1981 in the Indo-West Pacific ( Gurney 1939; Holthuis 1951, 1981; Kensley 1983; Boothe & Heard 1987; Criales & Lemaitre 1997; Pachelle & De Grave 2015). In D. serrifer Rathbun, 1902 from the eastern Pacific and D. vernbergi Boothe & Heard, 1987 from the western Atlantic, the lateral margin of the uropod is conspicuously serrated ( Kensley 1983: figs. 2, 3i; Boothe & Heard 1987: fig. 3H). In addition, in the former species, as well as all the remaining species of the genus, the posterodorsal margin of the second pleonite is posteriorly produced into a sharp tooth overhanging the anterodorsal margin of the third pleonite (e.g., Bruce 1976: fig. 1; Wilson & Gore 1979: fig. 1A; Kensley 1983: figs. 2, 6; Pachelle & De Grave 2015: fig. 1B; Gan & Li 2022: fig. 1a). Using these two features combined, D. forsskali sp. nov. can be immediately distinguished from the aforementioned D. serrifer , D. vernbergi , D. exul (= D. mvitae ), but also from D. serratirostris Lebour, 1949 from the western Atlantic, D. brownae Kensley, 1983 from eastern Australia, D. pascuensis Fransen, 1987 from Rapa Nui (Easter Island), and D. nanhaiensis Gan & Li, 2022 from southern China ( Bruce 1976; Wilson & Gore 1979; Kensley 1983; Pachelle & De Grave 2015; Gan & Li 2022). On the other hand, D. forsskali sp. nov. can be easily separated from both D. atlanticus and D. musicus by the pointed, laterally finely serrated triangular rostrum, contrasting with the very differently shaped, distally truncate (with a minute apical tooth in D. musicus ) and laterally smooth rostrum in the latter two species ( Kensley 1983: figs. 4, 12; Criales & Lemaitre 1997: fig. 2a; Pachelle & De Grave 2015: fig. 2A). Furthermore, D. forsskali sp. nov. differs from D. atlanticus by the straight dorsal margin of the fifth pereiopod dactylus (strongly toothed in D. atlanticus ), and from D. musicus by the presence of a well-developed mandibular palp (absent in D. musicus ) ( Criales & Lemaitre 1997: fig. 2c, f; Pachelle & De Grave 2015: fig. 2D).

In the general shape and marginal serration of the rostrum, D. forsskali sp. nov. is most similar to D. exul , D. nanhaiensis , D. pascuensis and D. serratirostris , although the serration of D. forsskali sp. nov. extends onto the orbital margin, whereas in the other species, it is limited to the distal two-thirds or even only the distal half of the rostrum ( Fransen 1987: fig. 1B; Pachelle & De Grave 2015: fig. 1A, 4A; Gan & Li 2022: fig. 1c). The rostra of D. serrifer , D. brownae and the form described as D. mvitae (later synonymised with D. exul ) are noticeably more lanceolate in shape (cf. Bruce 1976: fig. 2A; Kensley 1983: figs. 3a, 8j), whereas D. vernbergi and D. nanhaiensis display two different intermediate configurations, however, with the former species sharing with D. forsskali sp. nov. the presence of a serration on both rostral and orbital margins ( Boothe & Heard 1987: fig. 2B; Gan & Li 2022: fig. 1c). In addition, the new species differs from D. exul , D. nanhaiensis , D. pascuensis , D. brownae , and D. serratirostris by the dactyli of the third to fifth pereiopod armed with row of spinules on ventral (flexor) margin, which is unarmed in the other species (cf. Kensley 1983: figs. 9a–c, 10m, n, 13g, h; Fransen 1987: fig. 3A–C; Pachelle & De Grave 2015: fig. 1F; Gan & Li 2022: fig. 3e–g), although in the species originally described as D. mvitae (now D. exul ), a row of spinules is present on the mesial surface of the dactylus of these appendages (cf. Bruce 1976: fig. 5). Additional differences between D. forsskali sp. nov. and some of these species can be observed in the proportions of the articles and armature of the merus of the third, fourth and fifth pereiopods, as well as in the armature of the fingers of the second pereiopod chela.

Gan & Li (2022) provided the most updated key to the nine species of Discias known at that time. Their identification key may not be the most practical since specimens need to be dissected to determine first, the presence or absence, and then the development of the mandibular palp (non-articulated vs. biarticulated). Nevertheless, following this key, D. forsskali sp. nov. would be positioned somewhere close to D. exul and D. brownae , two species with a second pleonite armed with a posterodorsal tooth, which immediately separates them from the new species (see above). On the other hand, based on most diagnostic characters of the rostrum, pleon, mandible, second to fifth pereiopods and telson (but not the uropods), D. forsskali sp. nov. appears to be closest to D. vernbergi . However, only a proper phylogenetic analysis would elucidate the intrageneric relationships within Discias .

Williamson’s (1970) record of D. atlanticus , based on a single male specimen (cl 3.2 mm based on scale in his fig. 1a) from a plankton sample off Sharm-el-Sheikh, Egypt, northern Red Sea, cannot be unambiguously attributed to D. forsskali sp. nov. The main argument for this hypothesis, in addition to the geographical proximity of Sharm-El-Sheikh to the type locality of D. forsskali sp. nov., is the presence of “small serrations” on the lateral margin of the rostrum ( Williamson 1970: 4; however, not illustrated). On the other hand, the differences in the shape of the scaphocerite (cf. Fig. 1E View FIGURE 1 ; Williamson 1970: fig. 2a) and telson (cf. Fig. 2O, P View FIGURE 2 ; Williamson 1970: fig. 1b) between D. forsskali sp. nov. and Williamson’s material are rather significant. Therefore, Williamson’s (1970) record is tentatively not included in the synonymy of D. forsskali sp. nov., awaiting collection of additional specimens and further lines of evidence. The subsequent Indo-West Pacific records of D. atlanticus by Bruce (1976) and Kensley (1983) remain problematic and may involve at least one additional undescribed species (S. De Grave, pers. comm.). The variation in the development of the mandibular palp and several features of the rostrum in the material reported as D. atlanticus by these authors appears to be well beyond intraspecific. Kensley (1983) himself concluded that given the degree of variation in the relative proportions of the rostrum, the Indo-West Pacific material assigned to D. atlanticus should be referred to as D. cf. atlanticus .

Discias forsskali sp. nov. is presently the deepest-recorded species in the genus Discias as most species of the genus were recorded from much shallower water, i.e., less than 100 m. The second-deepest record within the genus belongs to D. vernbergi , which was collected at depths of 54–74 m off Georgia and Florida, USA ( Boothe & Heard 1987). However, in the closely related monotypic genus Tridiscias Kensley, 1983 , T. transkei Kensley, 1983 was recorded from a depth range of 150–200 m off South Africa ( Kensley 1983). The members of the other two monotypic genera currently placed in the family Disciadidae , Lucaya bigelowi Chace, 1939 and Kirnasia nesisi Burukovsky, 1988 , which are morphologically more distant from Discias , were collected with pelagic trawls and nets, usually below 500 m, and may be considered as bathy- and mesopelagic shrimps ( Chace 1940; Burukovsky 1988; Burghart et al. 2007).