Ischiomysis proincisa, Wittmann & Wirtz, 2024

Wittmann, Karl J. & Wirtz, Peter, 2024, An update on the genus Ischiomysis with a description of I. proincisa sp. nov. (Crustacea, Mysida) from a sublittoral marine cave in the Gulf of Guinea (tropical E-Atlantic), Zoosystematics and Evolution 100 (4), pp. 1431-1442 : 1431-1442

Ischiomysis proincisa sp. nov.

Figs 4 , 5 , 6 , 7 , 8 , 9

Type material.

São Tomé • Holotype adult ♂ (BL 4.2 mm, NHMW - CR-30447 ), paratypes, 1 ♂ ad. (BL 3.7 mm), 1 ♂ subad., 1 ♀ subad., 2 imm., 1 juv. ( NHMW - CR-30448 ); E-Atlantic, Gulf of Guinea, Ilhéu das Rolas (= small island crossed by the equator), off Ponta das Furnas (= cape); 0.0091 ° S, 6.5110 ° E; 19 m depth; 28 Aug. 2002, 9: 50–10: 10 local time; K. J. Wittmann leg.; V-shaped (i. e. two branches) semi-dark marine cave with three diveable entrances, from small mysid swarms in darker lateral recesses, diver operated hand net • Paratypes, 3 ♀♀ ad. (BL 4.0– 4.3 mm), 13 ♂♂ ad. (BL 3.8–4.6 mm), 4 ♀♀ subad., 7 imm., 7 juv. ( NHMW - CR-30448 ); 17 m depth; 22 Aug. 2002, 15: 23–15: 50 local time; remaining data as for holotype .

Etymology.

The species name is a Latin adjective with feminine ending, formed from the adjective incisa (incised) prefixed by the adverb pro (anteriorly), referring to the apically incised rostrum.

Type locality.

Off cape Ponta das Furnas, 0.0091 ° S, 6.5110 ° E, in 17–19 m depth inside semi-dark marine cave. Each branch of the V-shaped cave is about 30 m long and open on both ends.

Diagnosis.

Based on adults of both sexes. Large calotte-shaped cornea (Fig. 4 ) occupies distal 2 / 3 of eye surface (half eye length in dorsal view); cornea diameter is 28–35 % carapace length. Rostrum trapezoid, anteriorly incised (Fig. 5 G ), about as long as the terminal segment of the antennular trunk. Only the terminal segment of the antennular trunk near disto-mesial edge with obliquely forwards oriented subapically flagellate spine with handle ending in a tooth-like projection (Fig. 5 D ); this spine opposed by a large smooth whip seta. Appendix masculina short though well developed, densely setose. Antennal scale (Fig. 5 F ) with small terminal segment separated by a not always distinct suture; scale reaching to the terminal margin of the antennular trunk; scale overreaching the antennal peduncle by 1 / 2 up to 2 / 3 of its length. Scale weakly bent laterally, slender; length is 4–5 times maximum width. Labrum (Fig. 5 I ) with distally narrowly rounded, roughly triangular rostral protrusion. Both sexes with small median lobe on thoracic sternite 1, no median processes on sterna 2–8 (Fig. 6 E ). Carpopropodus of thoracic endopods 1–8 with 2, 2, 2, 2, 3, 3, 3, and 3 segments (Fig. 6 F, I, K , and Fig. 7 L, P ). Length of carpopropodus 3 is 4.0–4.5 times maximum width; carpus and propodus separated by a distinct suture (Fig. 7 A, C ). Carpus with 4 flagellate spines on distal fourth of its mesial margin in males (Fig. 7 B ) versus 5 spines on distal third in females (Fig. 7 D ). Propodus without spines. Carpopropodus of endopod 4 (Fig. 6 K ) with several simple smooth setae and 2–3 unilaterally barbed setae in two variants (Fig. 6 L, M ); claw almost straight, smooth, with bifid tip (Fig. 7 G ). Endopod 8 only in males with modified flagellate spine (Fig. 7 M ) on the outer margin of the ischium at one third ischium length from basis and an additional flagellate spine (Fig. 7 N ) basally on the inner margin; praeischium with a strong spiniform extension (Fig. 7 O ). Marsupium with large oostegite on thoracopods 7–8 plus a rudimentary plate on thoracopod 6. Penes tubular, terminally lobate (Fig. 6 E ); length without lobes is 3.5 times width; penes half as long as ischium of endopod 8. Pleopods (Fig. 8 E – K ) rudimentary, unsegmented, setose in both sexes, no spines. Uropods (Fig. 9 A ) setose all around, no spines. Exopod extends 14–27 % its length beyond endopod and 28–44 % beyond telson. Telson (Fig. 9 B ) length is 1.8–2.1 times maximum width and 1.0–1.2 times pleonite 6. Lateral margins with 13–18 spines only on distal half, not counting the apical spines. Telson with V-shaped narrow apical cleft penetrating 39–42 % telson length; cleft with total of 30–37 spine-like laminae along basal 45–53 % of its margins. Terminal lobes of telson each with two spines among which the disto-lateral spine is 14–15 % telson length; the disto-mesial spine 0.3–0.4 times length of the disto-lateral spine. Telson with total of 31–37 spines.

Description.

All features of the diagnosis. General appearance robust. Body red in vivo, length 3.7–4.6 mm in adult males (n = 15) and 4.0– 4.3 mm in adult females (n = 3). Cephalothorax represents 28–33 % body length, pleon without telson 51–56 %, telson 11–12 %, carapace without rostrum 20–26 %, and rostrum 4–5 %.

Eyes (Fig. 4 ). Eyestalks and cornea dorsoventrally compressed by a factor of 1.2. Cornea diameter twice the length of the apical segment of the antennular trunk. Eyestalks finely hispid by minute scales. Organ of Bellonci ellipsoidal, length 1 / 6 cornea diameter.

Carapace (Fig. 5 G, H ). The rostrum forms a distinct horizontal plate covering part of eyestalks. It reaches to 2 / 3 antero-posterior extension of normal-oriented eyestalks. Disto-lateral edges of the carapace well-rounded, anteriorly slightly produced. Posterior margin leaves the ultimate 1–1.5 thoracomere mid-dorsally exposed. As in many species of Mysidae , two characteristic groups of pores present on the midline of the carapace. The anterior group (Fig. 5 G ) is shortly in front of the cervical sulcus and comprises 6–10 pores with <1 μm diameter in symmetrical paramedian arrangement. The posterior pore group (Fig. 5 G, H ) is shortly in front of the posterior margin of the carapace; this group consists of 10–12 pores with about 2 μm diameter surrounding a larger but indistinct, rounded structure. Except for the here stated structures, carapace with smooth outer surface.

Antennulae (Fig. 5 A – E ). The trunk extends half its length beyond normal-oriented eyes. Measured along dorsal midline, the basal segment is 46–51 % trunk length, median 18–20 %, and terminal 30–34 %. Basal segment of the antennular trunk (Fig. 5 A, B ) with two setose dorsal lobes (apophyses) near distal margin, in addition with a disto-lateral lobe; median and terminal segments each with one setose dorsal lobe near distal margin. Basal segment on basal half of its lateral face with 3–5 minute setae. The large barbed seta arising from the inner distal corner of the median segment not extending beyond the terminal segment in both sexes. The flagellum of the spine at the terminal segment of the antennular trunk inserts more apically in the male (Fig. 5 D ) compared to the female (Fig. 5 E ); this dimorphism also found in the congener I. telmatactiphila . Epi-antennular process (solid line in Fig. 5 A ) subtriangular with small, rostral, acute projection; hypo-antennular process (dashed line in Fig. 5 A ) subcircular with blunt subtriangular rostral projection.

Antennae (Fig. 5 F ). Sympod dorsally with terminally rounded, tongue-like process. Disto-lateral edge of sympod with tooth-like process. Sympod caudally with bulbous lobe containing end sac of antennal gland. Antennal scale setose all around. Apical segment, if distinct, contributes 11–16 % to total scale length. Peduncle 0.7–0.8 times scale length but extending to only about half scale length due to its more caudal insertion. Basal segment is 24–29 % the length of the peduncle, second is 27–31 %, and third is 44–46 %.

Primary mouthparts (Fig. 5 I, J and Fig. 6 A – C ). Labrum (Fig. 5 I ) weakly cuticularized, densely covered by short, stiff bristles. Paragnaths (Fig. 5 J ) with moderately stiff bristles, no tooth-like bristles. Mandibular palp three-segmented (Fig. 6 A ). Its proximal segment without setae, 8–12 % length of palp. Median segment: 63–65 % palp length. Its length 3.0–3.7 times maximum width. Its lateral margin almost all along with 8–9 smooth setae, an additional basally barbed seta in distal-most position. Mesial margin with 4–6 smooth setae distributed with mostly large interspaces. Terminal segment 27–28 % palp length. Its mesial margin bare except for some setae near apex; distal half of lateral margin densely setose. Pars molaris with well-developed though comparatively small grinding surface in both mandibles. Left mandible (Fig. 6 C ) with pars incisiva bearing three teeth, digitus mobilis with two teeth, pars centralis with four basally thick, spiny teeth. Right mandible (Fig. 6 B ) with pars incisiva bearing 4–5 teeth, digitus mobilis with two teeth; pars centralis with 3–4 separate bases each bearing numerous slender spines that are bilaterally armed with stiff bristles. Bases decreasing in size and numbers of spines orally.

Foregut (Fig. 8 A – D ). Lateralia anteriorly with group of slender, apically coronate spines (Fig. 8 A ) with small denticles along distal <2 / 3 of the shaft. Lateralia more caudally with shorter, apically pronged spines (Fig. 8 B ) bearing small denticles along> 2 / 3. Lateralia on each side of the foregut more caudally with cluster of 6–7 weakly, in part indistinctly serrated spines (Fig. 8 C ) of varying size. Dorsolateral infoldings each with cluster of three strong rugose spines (Fig. 8 D ).

Maxillula (Fig. 5 K ). Distal segment terminally with series of 10–11 strong spines, among which the two most peripheral spines are largest and subapically serrated, whereas the spines in between are weakly or not serrated. This segment subterminally with two short setae bearing long barbs on their distal 2 / 3; no pores detected near these setae. These setae not reaching the basis of the spines. Only two short setae also found in the congener I. telmatactiphila , whereas most Mysidae species show more and longer setae of that kind. Endite of the maxillula terminally with three large spiny setae accompanied by three shorter setae of that kind, caudal face with two small barbed setae.

Maxilla (Fig. 6 D ) with terminal segment of palp 1.6–1.8 times longer than wide and 1.3–1.6 times length of the basal segment. Terminal segment with densely setose mesial margin; only 3–4 setae along distal 2 / 3 of the lateral margin, no spines. Basal segment with three basally thick, barbed setae.

Thoracic sternites (Fig. 6 E ). Sternite 1 with small, about linguiform anterior lobe. Sternites 2–8 without median processes in both sexes. Sternite 2 with 1–3 barbed setae on intersegmental joint with thoracopod 2. Penes distally ending in 4 lobes, no setae.

Thoracopods in general (Fig. 6 F – M and Fig. 7 ). Basal plate (Fig. 7 P ) of exopods expanded, length 1.5–1.9 times maximum width. Lateral margin of the plates ends in a broadly rounded corner. Flagellum of exopods 1–8 with 8, 9, 9, 9, 9, 9, 9, and 9 segments (Fig. 7 P ), respectively. Thoracopod 1 with large, leaf-like, smooth epipod. Basis (fused with sympod) of endopod 2 (Fig. 6 I ) with comparatively large lappet-like apophysis on rostral face; this apophysis small in endopods 3–8 (Fig. 6 K and Fig. 7 A, L, P ); no such apophysis in endopod 1. Ischium shorter than merus in endopods 1–4 (Fig. 6 F, I, K , and Fig. 7 A ), but longer than merus in endopods 5–8 (Fig. 7 L, P ). Thoracic endopods 1–2 each with dactylus (Fig. 6 G, J ) larger than that of endopods 3–8 (Fig. 7 F – K ); dactylus 3 (Fig. 7 F ) wider but not longer than dactyli 4–8 (Fig. 7 G – K ). Dactylus 1 with slender, weakly bent claw which is bilaterally “ serrated ” by stiff bristles along median portions (Fig. 6 G, H ). Claw 2 unilaterally rugose along proximal 2 / 3 (Fig. 6 J ). Claw 3 strong, somewhat bent, smooth (Fig. 7 F ). For claw 4, see “ Diagnosis ”. Claws 5–8 weakly bent, “ serrated ” in subapical portions (Fig. 7 H – K ). Combined praeischium plus ischium of endopod 2 (Fig. 6 I ) are 0.6 times merus length, carpopropodus plus dactylus 1.1 times merus. Dactylus 2 very large, with dense brush formed by large numbers of normal setae and 6–10 modified setae, the latter apically bent, bearing double series of stiff barbs along proximal 2 / 3 to 3 / 4.

Gnathopods (Fig. 7 A – F ). Thoracic endopod 3 forms a powerful subchela. Basis with much shorter endite (Fig. 7 A ), if any, compared to that (Fig. 6 I ) of endopod 2. Ischium of endopod 3 is 2.6–3.0 times as long as wide; merus 4.5–5.1 times as long as wide and 1.4–1.6 times length of ischium. Ischium and merus strong, with smooth setae only. Disto-mesial edge of merus with short longitudinal ridge bearing two setae (Fig. 7 E ). Carpopropodus as long as 0.8–0.9 times merus and 1.2–1.3 times ischium. Length of carpus 3.4–3.5 times maximum width.

Marsupium (Fig. 7 P, R ). Ultimate (third) oostegite rolled inwards to form two widely communicating sub-chambers. Its rostral and ventral margins with dense series of barbed (plumose) setae; inside with 5–9 microserrated setae close to the insertion on thoracic sympod 8; ventral half of oostegite 3 with about 7–10 small whip setae (Fig. 7 R ) loosely scattered over the outer surface.

Pleon (Fig. 8 E – K ). Pleomeres 1–5 measure 0.7–0.9, 0.6–0.8, 0.7–0.9, 0.6–0.9, and 0.7–0.8 times the length of pleomere 6, respectively; in other words, pleomere 6 is always shorter than combined pleomeres 4–5. Length without setae increases in series of pleopod 1, followed by subequal pleopods 2–4, and then by the much longer and slender pleopod 5. Pleopods 1–5 (Fig. 8 E – K ) mainly with plumose or barbed setae, but there are 1–3 smooth seta only in subbasal position on the outer margin; the latter setae clearly representing whip setae in pleopod 5 of both sexes. Pleopods 1–5 show no other types of setae, no extraordinarily long setae, no spines or teeth.

Tail fan (Fig. 9 A, B ). Scutellum paracaudale (labeled “ sc ” in Fig. 9 B ) forms a well-rounded shield covering the basolateral edge of the telson. Exopod of uropods (Fig. 9 A ): 4.9–5.6 times longer than broad, inner margin convex, outer margin very slightly concave, almost straight, 1.2–1.4 times the length of endopod. Statoliths composed of fluorite; shape discoid, diameter 60–136 µm, maximum thickness 27–60 µm, statolith formula 2 + 3 + (5–7) + (1–2) + (4–5) = 15–19 (n = 12). Telson (Fig. 9 B ) shaped as in I. telmatactiphila , length is 1.1 times endopod of uropod and 0.8–0.9 times exopod of uropod. Spines on lateral margins continuously increase in size from half-length to 3 / 4 - length of lateral margins and then decrease weakly in size distally. Laminae of the cleft are shorter than the pair of disto-mesial spines flanking the cleft. For additional details, see “ Diagnosis. ”

Nauplioid larvae (Fig. 9 C, D ) with three pairs of free naupliar appendages. Apical portions of antennulae, antennae, and abdomen with minute hairs.

Figures

Figure 4. Habitus of Ischiomysis proincisa sp. nov.; holotype adult ♂ (BL 3.9 mm, A, B, D) and paratype adult ♀ (4.2 mm, C); A, B. Holotype in toto, lateral (A) and dorsal (B); C. Paratype in toto, dorsal; D. Cephalic region of holotype, ventral. A – D. Photos of fixed specimens, objects artificially separated from background, green coloring of objects artificial.

Figure 5. Ischiomysis proincisa sp. nov., paratypes adult ♀ (BL 4.3 mm, A, E, I) and adult ♂ (BL 4.5 mm, B – D, F – H, J – K); A. Right female antennula with associated processes from the frons, dorsal; B. Left male antennula, dorsal; details of the terminal segment of the trunk show the disto-median lobe (C) and the disto-mesial flagellate spine (D); E. Disto-mesial flagellate spine from left female antennula, dorsal; F. Antenna with antennal gland, dorsal, many setae omitted from the antennal scale; G. Carapace expanded on slide; H. Detail of (G) showing posterior pore group, pore diameters not to scale; I. Labrum, aboral face; J. Labium, frontal face; K. Maxillula, caudal.

Figure 6. Ischiomysis proincisa sp. nov.; paratypes adult ♀ (BL 4.3 mm, A – D) and adult ♂ (BL 4.5 mm, E – M); A. Right mandible with palpus, caudal; B, C. Masticatory parts of right (B) and left (C) mandibles, caudal; D. Maxillary palp, rostral; E. Thoracic sternites 1–8 with penes, ventral; F. Thoracic endopod 1 with part of coxa, caudal; G. Detail of (F) showing dactylus 1 with claw, setae omitted; H. Secondary detail of (F) showing claw 1; I. Thoracic endopod 2, rostral; J. Detail of (I) showing dactylus 2 with claw, setae omitted; K. Thoracic endopod 4 with basis (sympod), rostral; L, M. Details of (K) showing modified setae of the carpus (L) and the propodus (M).

Figure 7. Thoracopods 3–8 in Ischiomysis proincisa sp. nov.; paratypes adult ♂ (BL 4.5 mm, A, B, G – O) and adult ♀ (BL 4.3 mm, C – F, P – R); A. Male thoracic endopod 3, rostral; details (B) show the flagellate spines of the carpus; C. Tarsus and distal portion of the merus in female thoracic endopod 3, rostral; details show the flagellate spines of the carpus (D) and the disto-mesial carina of the merus (E); F – K. Series of dactyli 3–8 with claw, setae omitted; L. Male thoracic endopod 8, caudal; details show the flagellate spines (M, N) of the ischium and the modified praeischium (O) with large tooth-like projection; P. Female thoracopod 8 with outer face of oostegite 3, lateral; details show a normal smooth seta (Q) of the ischium and a subapically flagellate whip seta (R) from the outer face of the oostegite.

Figure 8. Spines of the foregut and pleopods in Ischiomysis proincisa sp. nov.; paratypes adult ♂ (BL 4.5 mm, A – I) and adult ♀ (BL 4.3 mm, J, K); A – D. Modified spines of the foregut in dorsal view, from the anterior (A, B) and posterior (C) parts of the lateralia and from dorsolateral infolding (D); E – I. Series of right male pleopods 1–5, lateral = rostral face; J. Right female pleopod 1, lateral; K. Left female pleopod 5, lateral.

Figure 9. Ischiomysis proincisa sp. nov.; paratype adult ♂ (BL 4.5 mm, A, B) and nauplioid larvae (C, D); A. Uropods, ventral; B. Telson and right scutellum paracaudale (sc), ventral; C. Nauplioid larva at substage N 4, lateral; lower case labels indicate antennula (a 1) and antenna (a 2); D. Tip of pleon in another nauplioid specimen. Panel (A) combined from two photos; A – D. Green coloring of objects artificial; B – D. Objects artificially separated from background.