Abyssomysis cornuta, Wittmann, 2020

Wittmann, Karl J., 2020, Lophogastrida and Mysida (Crustacea) of the “ DIVA- 1 ” deep-sea expedition to the Angola Basin (SE-Atlantic), European Journal of Taxonomy 628, pp. 1-43 : 20-28

Abyssomysis cornuta gen. et sp. nov.

urn:lsid:zoobank.org:act:A4017620-6EAC-4567-A3EB-BDF006954C63

Figs 5–8

Diagnosis

Abyssomysis gen. nov. with eyeplate in the form of a narrow, transverse band with lateral, apically acute or narrowly rounded rudiments of the eyestalks resembling horns. Anterior margin of carapace evenly rounded, upturned on the edge, forming a weakly separated rim as a rostral plate. Only distal portions of the eye rudiments extend beyond the rostral plate. Antennular trunk longer than antennal scale in males, shorter than scale in females. Male trunk with comparatively large appendix masculina bearing a dense brush of long setae. Antennal peduncle not reaching beyond antennal scale in females, whereas reaching well beyond in males. Left mandible with lacinia mobilis well developed in both sexes. Right mandible without lacinia mobilis in females, but with lacinia reduced to a tiny, slender, apically toothed spine in males ( Fig. 6D ). Median segment of palpus of both mandibles with only three setae in terminal position, remaining portions of palpus smooth in females, whereas distal ¾ of the median segment showing dense series of at least 14 setae in males. Females with pleopods 1–5 continuously increasing in length caudally. Males with pleopods 1–3, 5 increasing in length caudally, pleopod 4 is not in series by being the longest; fourth pleopod 3-segmented, its basal segment contributes ⁴⁄₅ pleopod length, the two apical segments each 1⁄₁₀. Pleomere 6 longer than combined pleomeres 4–5. Telson 0.3–0.4 times length of pleomere 6; telson length 1.0–1.1 times maximum width near basis; terminal margin with two pairs of large, robust spines; outer robust spines shorter than inner ones.

Etymology

The species name is a Latin adjective with female ending, referring to the pronged eyeplate, well contrasting from that in the remaining Calyptommini so far known.

Type material examined (by present designation)

Holotype

ANGOLA BASIN • ♂ ad. (BL = 6.6 mm); 16°13.329′ S, 5°26.837′ E to 16°14.820′ S, 5°26.702′ E; bottom depth 5433–5434 m; 29 Jul. 2000; DIVA-1 exped., #350; supranet of epibenthic sledge; ZMH 58250.

Paratypes ANGOLA BASIN • 1 ♂ ad. (BL = 6.7 mm, on slides); same collection datas as for holotype; ZMH 58254 • 1 ♀ ad. (BL = 7.9 mm on slides); same collection datas as for holotype; ZMH 58256 • 1 ♂ ad. (BL = 6.1 mm, in vial), 1 ♀ imm. (damaged, BL = 6.7 mm, in vial); same collection datas as for holotype; ZMH 58255 • 1 ♀ ad. (BL = 8.0 mm in vial); 17°4.935′ S, 4°40.805′ E to 17°07.454′ S, 4°42.276′ E; bottom depth 5460– 5460 m; 25 Jul. 2000; DIVA-1 exped., #344; SMF 55189 • 1 ♂ imm. (BL = 3.4 mm, on slides); same collection data as for preceding; ZMH 58251 • 1 ♀ ad. (damaged, estimated BL = 7.0 mm, in vial); 16°16.989′ S, 5°27.279′ E to 16°19.280′ S, 5°27.205′ E; bottom depth 5430–5433 m; 28 Jul. 2000; DIVA-1 exped., #348; ZMH 58253 • 1 ♂ ad. (BL = 6.8 mm, on slides); same collection data as for preceding; SMF 55190 • 1 ♀ imm. (damaged, BL = 5.4 mm, in vial); same collection data as for preceding; ZMH 58252 .

Type locality

Station #350 ( Table 1 ) in the abyssal plain of the Angola Basin, 16°14′ S, 5°27′ E, depth 5433–5434 m.

Description of the holotype ( Fig. 5A )

All diagnostic features of males. Body length 6.6 mm, specimen not dissected. Cephalothorax without rostrum measures 40% body length, pleon without telson 51%. The great contribution by the pleon is largely due to the elongate sixth pleomere. Carapace including rostrum measures 38% body length. Carapace weakly inflated, with distinct cervical sulcus, anterior third of cephalic portion with knobbly cuticle. Carapace posteriorly emarginated, leaving half of ultimate thoracomere dorsally exposed. Eyes without visual elements. Small, horn-like rudiments of eyestalks widely set apart on a common eyeplate probably homologous to the ocular symphysis. Eyeplate dorsally covered by rostral plate, only part of eyestalk rudiments projecting from behind the rostrum in dorsal view.

ANTENNAE SENSU LATO ( Fig. 5A ). Antennular trunk is 15% body length. Trunk 1.1 times antennal scale length. Trunk conical with distally increasing width. Terminal segment is 1.4–1.5 times width of basal segment. Transverse articulations between the three segments of the trunk. Segments 1–3 contribute 35%, 20%, and 45% to total trunk length, respectively. Appendix masculina inserts ventrally behind inner flagellum, appendix ½ trunk length, its setae up to ¾. Basal segment of antennular trunk not produced at outer distal corner. Setose lobe dorsally in about median position shortly behind the anterior margin of each segment. Lobe of distal segment with four barbed setae and transverse series of four forward projecting teeth. Tooth size increases laterally. Antennal scale 1⁄₇ body length; its length four times maximum width. Apical lobe contributes 7% to total scale length, lobe wider than long, with seven plumose setae. Antennal peduncle ⁶⁄₅ scale length, segments 1–3 are 0.1, 0.4, and 0.5 times peduncle length, respectively. Sympod with long, linguiform, terminally rounded lobe on dorsal face.

MOUTHPARTS. Mandibular palpus slender. Its proximal segment, short, smooth all around. Median segment contributing ³⁄₅ to palpus length. Distal ¾ of median segment with dense series of obliquely

outward-backward projecting setae. Distal segment with dense comb of setae. Terminal segment three times as long as wide, ½ palp length.

THORACOPODS. Endopod 2 large, carpus and propodus fused, dactylus with medially projecting brush of barbed setae. Sympods 3–8 with anteriorly facing, terminally rounded lobe close to (broken) insertion of praeischium of endopod (basis fused with sympod). Paired penes short, knob-like.

PLEON ( Fig. 7D ). Sixth pleomere 1⁄6 body length. Pleomeres 1–5 are 0.3–0.4 times length of pleomere 6. Scutellum paracaudale sinusoid, evenly rounded. Pleopods with setose edge as found in most species of the subfamily Erythropinae . Pleopods 1–3 reduced to setose plates, pleopods 4–5 to setose rods. Pleopod 4, when stretched posteriorly, reaches to the distal seventh of pleonite 6. Tip of pleopod 4 with two setae. The larger seta about as long as the combined two distal segments. Median and basal segments each ending in a pair of shorter setae. Basal segment with two additional setae in subterminal position and other about eight setae ½ segment length from basis. Uropods setose all around, exopod terminally rounded, endopod about as long as exopod, each 1.2 times length of sixth pleomere, and extending 0.7 times their length beyond telson. Telson with smooth lateral margins. Potential presence of a minute median tooth (as in some paratypes, Fig. 7I ) on terminal margin could not be checked without dissection. The inner robust spines on terminal margin measure 45% telson length, the outer ones 37%. For additional details of telson see above Diagnosis and below Description of paratypes.

Description of paratypes

Body length of adult females 7.0–8.0 mm (n = 3), males 6.1–6.8 mm (n = 4, including holotype). Cephalothorax without rostrum measures 32–40% body length, pleon without telson 51–56%. Carapace including rostrum measures 31–38% body length. Carapace leaving posterior 0.5–1.0 thoracomere dorsally exposed. Carapace with distinct cervical sulcus in situ; sulcus not visible in detached carapaces. No pores (> 1 µm) on carapace.

ANTENNAE SENSU LATO ( Fig. 5 A–C, H). Antennular trunk strongly dimorphic, size is 10% body length in females, 15–17% in males. Trunk 0.7–0.8 times antennal scale length in females, 1.1–1.2 in males. Trunk almost cylindrical in females, whereas conical with distally increasing width in males. Terminal segment is 0.8–1.0 times width of basal segment in females, 1.3–1.4 in males. Segments 1–3 contribute 43–46%, 19–20%, and 35–39% to total trunk length in females; 32–38%, 18–20%, and 43–50% in males, respectively. Antenna ( Fig. 5H ) non-dimorphic, scale length 3–6 times maximum width. Apical lobe contributes 5–9% to total scale length, lobe with 6–8 plumose setae. Antennal peduncle 0.9–1.0 times scale length in females, 1.1–1.3 in males.

MOUTHPARTS ( Fig. 6 A–H). Labrum with frontal lobe bearing a small sinusoid rostral extension; the two posterior lobes terminally with median field of minute, stiff bristles, dorsally with cuticular ridges (dashed lines in Fig. 6A ). Median segment of mandibular palpus 5–6 times as long as wide. Pars incisiva of both mandibles with 5–6 moderately sized teeth, processus molaris strongly cuticularized. Lacinia mobilis of left mandible with six teeth; for right lacinia see Diagnosis. Pars centralis of left mandible with 6–7 ‘spines’ bearing stiff bristles or denticles. Right centralis with 8–9 ‘spines’ of that kind distally projecting from a large median lobe. Paired labia mostly smooth, inner terminal edge with small field of stiff setae, lacking spines or teeth. Lateral portions of labia locally with knobbly surface similar to that present in anterior portions of carapace. Distal segment of maxillula ( Fig. 6F ) ending in 10–14 slender, acute, smooth spines; subterminally with 4–6 small, basally barbed setae on caudal face. Mesial margin of endite with dense set of setae, 3–4 of which are strong, extending beyond the distal segment, the latter setae on their distal half barbed by stiff bristles. Maxilla ( Fig. 6 G–H) with well-developed exopod, 2-segmented palp and four strongly setose endites arising from trunk; exopod elliptical, extending only slightly beyond basal segment of palp, terminally with five plumose setae that decrease in size laterally; apical segment of palp is 1.6–1.8 times length of basal segment; terminal margin of apical segment weakly convex, almost truncate, distally with 4–5 modified spine-setae armed with various numbers of spinules and barbs ( Fig. 6H ); outer edge with a subbasally barbed seta which is longer than the apical segment; inner margin of this segment mostly smooth, with spines and setae only along its distal third, outer margin well setose; first (basal) endite with long, terminally barbed seta overreaching the palp (measured without setae and spines); endites 2–3 each with normal setae plus 3–6 modified setae whose barbs are to varying degree transformed to denticles; endite 4 with smooth or barbed setae only.

FOREGUT ( Fig. 8 E–H). Entire foregut with setae and moderately-sized spines only, no clavate spines or heavily armed spines or teeth. Primary cardiac filter formed ventrally by combs of stiff setae at inlet of oesophagus. Cardiac part of foregut with series of short, smooth spines with setae in between, on mesial and terminal margins of lateralia; with slender, microserrated spines ( Fig. 8F ) on disto-lateral portions of lateralia; and with apically pronged, proximally smooth, slender spines ( Fig. 8G ) on lateral portions. Stouter, centro-apically serrated spines ( Fig. 8H ) of various sizes on posterior part of lateralia, on dorso-lateral infolding, and on disto-lateral edges of superomedianum. Lateralia with weak and superomedianum with even weaker but distinct setation in addition to armature of spines. No setae present in pyloric part of foregut.

THORAX ( Figs 6 I–J, 7A–B). Thoracic exopods 1, 8 with terminally weakly widened basal plate rounded on outer edge. Exopods 2–7 with larger plate that is terminally more strongly widened, its edge more narrowly rounded. Exopods 1, 8 with 8-segmented, exopods 2–7 with 9-segmented, setose flagellum. Endopod 1 ( Fig. 6I ) only 10–13% body length measured from basis to dactylus; coxa with 0–3 setae close to segmental border with basis; basis with long, slender, all along setose endite; remaining segments without endite; dactylus with slender, smooth claw. First thoracic epipod foliaceous, linguiform, without seta. Endopod 2 ( Fig. 6J ) measures 28–37% body length; this is more than twice the size of endopod 1; basis with indistinct endite representing a rounded, weak prominence from inner margin; merus of both sexes longer than combined praeischium plus ischium, but shorter than combined carpopropodus plus dactylus; dactylus with brush of setae, without claw. Endopods 3–8 broken. Oostegite 1 ( Fig. 7A ) comparatively large, contributing to the anterior closure of the marsupium; oostegites 2–3 normal, even larger. Oostegites 1–3 proximally and dorsally with setae bearing short, acute barbs along distal 30–50%; ventral margin of oostegites 2–3 with plumose setae covering the slit between opposite, homologous oostegites. Penes ( Fig. 7B ) short, apically bilobate, with two large (related to penis size), smooth setae. No median processes visible on sternites 4–8 of both sexes.

PLEON ( Fig. 7 C–I). Sixth pleomere 1⁄5–1⁄6 body length. Width of pleopods, uropods and telson perspectively narrowed in Fig. 7C, D . Pleopod 4, when stretched posteriorly, reaches (without setae) to distal third of pleonite 5 in females, almost to the end of that pleonite in males. Tip of male pleopod 4 ( Fig. 7F ) with one smooth seta and one slightly longer, medially barbed seta. The smooth seta shorter up to about as long as the combined two apical segments. The median and the basal segments also end in a pair of such setae, but with both setae shorter and the smooth one more robust. The long basal segment subterminally with 0–2 additional, smooth, robust setae; additional 6–9 longer, barbed setae at ½ segment length from basis.

ENDOPOD OF UROPODS ( Fig. 7H ) with comparatively small statocyst containing a small statolith mineralized with fluorite; statoliths spherical in dorsal view, diameter 71–89 μm, this is 1.0–1.2% body length (n = 10 statoliths of 5 adults). Statolith formula not well established. Terminal margin of telson ( Fig. 7I ) with or without a minute median tooth. The inner robust spines of telson measure 41–45% telson length, the outer ones 31–37%.

Distribution

Recorded at stations #344, #348 and # 350 in the abyssal plain of the Angola Basin, range 16°13′ S to 16°17′ S, 4°40′ E to 5°27′ E, depth 5430–5460 m ( Table 1 ). All positive samples were taken with selfclosing epibenthic sledge. At least one sample was captured in the lower, another one in the upper net, suggesting that at least some specimens were positioned shortly above the sediment.

Eggs and larvae

Considering only non-damaged specimens, the female with 8.0 mm body length carried eight eggs with diameters of 0.47–0.50 mm, the female with 7.9 mm carried six nauplioid larvae at substage N2 with 1.3–1.5 mm body length. Nauplioids ( Fig. 8 A–D) with smooth cuticle all around, except for 5–10 slender spines at tip of antennula and a pair of cercopods in subapical position on abdomen. Each cercopod armed with a fan of numerous acute, slender spines.

Gut contents

Two dissected foreguts contained mainly fine, unidentifiable material, mineral particles and fragments of detritus; maximum size of particles 12 μm. These components also found in midgut and hindgut, with greater fraction of mineral particles also in oesophagus. There were no traces of meiofauna or macrofauna in oesophagus and foregut.

Figures

Fig. 5. Abyssomysis cornuta gen. et sp. nov., holotype, adult male with body length 6.6 mm (A: ZMH 58250), paratypes female 7.9 mm (B–C, E–F, H: ZMH 58256) and male 6.7 mm (D, G: ZMH 58254). A–B. Cephalic region of male (A) and female (B), dorsal; left antennula, right antenna and setae of antennal scale omitted. C. Detail of panel (B) showing lobe from terminal segment of antennula. D. Head in lateral view, setae of antennal scale omitted. E. Carapace expanded on slide. F–G. Eyeplate expanded on slide, for female (F) and male (G). H. Antenna, dorsal.

Fig. 6. Abyssomysis cornuta gen. et sp. nov., paratypes female with body length 7.9 mm (A–B, E–F, I: ZMH 58256) and males 6.7 mm (C–D, G–H: ZMH 58254), 6.8 mm (J: SMF 55190). A. Labrum. B–C. Mandibles with right palpus in female (B) and male (C), rostral. D. Detail of panel (C) showing reduced lacinia mobilis in right mandible of male. E. Labium. F. Maxillula. G. Maxilla. H. Detail of G showing modified spine-seta. I. First thoracic endopod. J. Second thoracopods.

Fig. 7. Abyssomysis cornuta gen. et sp. nov., holotype, adult male with body length 6.6 mm (D: ZMH 58250), paratypes female 7.9 mm (A, C, G: ZMH 58256) and males 6.7 mm (B, F: ZMH 58254), 6.8 mm (E, H–I: SMF 55190). A. Sixth thoracic sympod with oostegite. B. Eighth thoracic sympod with exopod and penis. C–D. Pleon in female (C) and male (D), lateral. E. First male pleopod. F. Fourth male pleopod. G. Fifth female pleopod. H. Uropods, ventral. I. Telson.

Fig. 8. Abyssomysis cornuta gen. et sp. nov., paratype female with body length 7.9 mm (E–H: ZMH 58256) and three of its nauplioid larvae at substage N2 (A–D). A. Nauplioid in toto, lateral. B. Tip of nauplioid abdomen in another specimen, obliquely dorsal. C. Detail of panel (A) showing tip of antennula. D. Tip of nauplioid antennula in another specimen. E. Cardiac portion of foregut, dorsal aspect, dorsal wall omitted. F–H. Details of E, showing modified spines.