Mysidium (Orientomysidium) antillarum, sp. nov.

Mysidium (Orientomysidium) antillarum Wittmann sp. nov.

urn:lsid:zoobank.org:act: 7CCD97DD-6848-46AF-B806-82D93D3FF3BA

Figs 10–12, 13A–D

Etymology

The species name is a Latinized Spanish noun in the genitive plural, referring to the occurrence at islands of the Antilles.

Type material examined (by present designation)

Holotype CURAÇAO • ♂ ad. bl 7.3 mm; shallow marine waters of Curaçao, Reef Sint Marie; 12.2122° N, 69.0850° W; #C6; NHMW 26484.

Paratypes CURAÇAO • 48 ♀♀ ad. bl 6.2–8.0 mm, 37 ♂♂ ad. bl 5.1–7.5 mm, 11 subad.; same locality data as for holotype; #C6; MINGA MYS 433, NHMW 26485, ZMH K-55258.

Other material examined

CURAÇAO • 13 ♀♀ ad. bl 6.3–7.5 mm, 11 ♂♂ ad. bl 5.8–7.7 mm; #C1; MINGA MYS 429 • 20 ♀♀ ad. bl 6.2–8.2 mm, 6 ♂♂ ad. bl 4.9–6.3 mm, 11 subad.; #C2; ZMH K-55256 • 18 ♀♀ ad. bl 6.5–7.8 mm, 39 ♂♂ ad. bl 5.4–7.7 mm, 48 subad.; #C4; NHMW 26483 • 3 ♀♀ ad. bl 6.1–6.6 mm, 3 ♂♂ ad. bl 6.0– 6.3 mm, 109 subad.; #C7; MINGA MYS 435 • 1 ♀ subad. bl 4.5 mm; #C8 • 24 ♀♀ ad. bl 6.3-8.1 mm, 17 ♂♂ ad. bl 6.1-6.8 mm, 7 subad.; #C10; NHMW 26486.

BONAIRE • 65 ♀♀ ad. bl 5.8–8.0 mm, 83 ♂♂ ad. bl 5.8–7.6 mm, 9 subad.; #B11; ZMH K-55264.

Definition

All features diagnosed above for the genus Mysidium Dana, 1852, and its new subgenus Orientomysidium. Cornea globose in lateral view, calotte-shaped in dorsal view, with diameter 1.4–1.7 times as long as terminal segment of antennular trunk. Eyestalks smooth. Rostrum triangular, apically rounded or pointed, not extending beyond basis of eyestalks. Antero-lateral edges of carapace rounded. Only males with anterior margin of antennular trunk dorsally with rounded, shield-like, mediodistal extension (Fig. 10A–B) bearing 2–3 setae. The largest seta 0.1 times extension length. Appendix masculina 2.2– 3.9 times terminal segment of antennular trunk; its shorter, dorsal lobe with brush of long setae, its longer ventral lobe with less dense set of shorter setae. Length of antennal scale 7–11 times maximum width, scale clearly reaching beyond antennular trunk. Median segment of mandibular palp with setae on both margins. Distally angular hump on outer face of terminal segment of maxillula. Carpopropodus of thoracic endopods 1–8 with 2, 2, 3, 3, 2–3, 2–3, 2–3 or 2 segments, respectively. Basal segment occupies half total length of carpopropodus of endopod 3. Series of 2–7 serrated setae (Fig. 12B) along merus of third thoracic endopod. Pleopod 1 rod-like in both sexes, not considering the weak (sub)median widening at its vestigial endopodal portion (Fig. 12D, G). Sympod of male pleopod 4 with longitudinal series of 3–7 plumose setae on rostral face; without or with an indistinct medial widening (endite) at ¼ of sympod length from basis. Exopod with basal segment occupying 69–73% total length. Endopod reduced to a terminally rounded lobe with 29–41% sympod length; endopod with 5–12 mostly large, plumose setae of various sizes. Endopod of uropods 0.6–0.8 times as long as exopod. Telson subrectangular with apical cleft. Telson length 1.4–1.7 times maximum width near basis. Its lateral margins convex along proximal third and smooth along proximal 51–58%; distal portion of each margin with dense, continuous series of 12–17 acute spines. Subtriangular apical cleft penetrating 11–14% telson length; cleft separating two broadly rounded, apical lobes; cleft and terminal portion of lobes densely lined in continuous series by a total of 19–23 (sub)acutely pointed, stout laminae.

Description

SIZE. Adult females 5.8–8.2 mm (n = 191), males 4.9–7.7 mm (n = 197).

ANTENNAL APPENDAGES (Fig. 10A–B, F–G). Antennular trunk extends 35–56% its length beyond (artificially aligned) eyes. First to third segments occupy 47–52%, 14–18% or 30–37% total length of trunk, respectively. Trunk dorsally with forward-directed small, setose lobes near terminal margin of each segment: two lobes from the basal, one from the median, and one from the terminal segment (not counting the mediodistal extension of the anterior margin in males). Appendix masculina 0.7–1.4 times total trunk length, antennal scale 1.2–1.6 times trunk. Sympod of antenna (Fig. 10G) produced into spiniform extension on outer distal corner. Antennal scale with terminal segment occupying 10–14% total length and bearing five plumose setae.

MOUTH PARTS (Figs 10H, J, 11F–H). Mandibular palp (Fig. 10J) without spines; terminal segment with strong, modified, bent seta at apex, and barbed setae along inner and outer margins. Median segment of palp with angular, medially directed dilatation, both its margins setose. Proximal segment normal, with smooth margins. Distal segment of maxillula (Fig. 11G) terminally with strong spines, subterminally with one barbed seta; tip of endite of maxillula with apically modified setae (armed with stiff barbs) plus several setae with normal, fine barbs. Maxilla (Fig. 11H) with well-developed exopod, moderately large, two-segmented palp and three apically setose endites. Exopod with maximum width in submedian portions; its outer margin all along with continuous series of plumose setae. Basal segment of palp with three barbed setae. Apical segment about two times as long as basal segment. Length of apical segment 1.9–2.5 times maximum width. Apical segment densely setose on terminal margin but lined by small hairs along more than basal half of inner margin; apex with two strong, modified setae bearing strong, spine-like barbs along distal third of their inner margin.

FOREGUT (Fig. 11A–E). Setae, but no spines, close to inlet of oesophagus. Primary cardiac filter formed ventrally by dense combs of stiff setae behind this inlet. Lateralia and superomedianum densely covered by spines and/or setae. Apically coronate plus apically, weakly (Fig. 11B) to strongly (Fig. 11E) pronged, slender spines on anterior and median parts of lateralia. These spines with numerous minute denticles along distal 60–80% of their shaft. Two powerful, stout, apically pronged, serrated spines (Fig. 11C) on posterior part of lateralia; one additional, longer, more strongly serrated spine (Fig. 11D) of that type on the dorso-lateral infolding; yielding a total of six such powerful spines. Secondary filter formed by dense rows of setae in pyloric part of foregut.

THORAX OF BOTH SEXES (Figs 10C–D, 11J–K, 12A–C, 13A). Carapace (Fig. 10C) posteriorly emarginate, with well-rounded latero-terminal lobes; cervical sulcus strong; roughly V-shaped group of 10–16 pores (Fig. 10E) in front of cervical sulcus, transverse linear series of 16–23 pores (Fig. 10D) in cardial position (above heart). Sternites (Fig. 11J) smooth; a plumose seta accompanied by 1–2 shorter barbed setae on the joint between each sternite and its thoracopod. Endopod and exopod sizes increase from thoracopods 1 to 5 and decrease from 5 to 8. Basal plate of exopods with large, well-rounded laterodistal expansion in exopods 1–7. Flagellum 8-segmented in exopods 1 and 8, versus 9-segmented in remaining exopods. First thoracic epipod bilobate, with smooth margins, without seta. First endopod (Fig. 11J) with well-developed, setose endites from basis, ischium, merus. Endopods 1–2 with large dactylus and strong claw. Dactylus of endopod 2 with 4–6 modified and several smooth setae, no spine-like setae; modified setae each with two dense rows of strong, acute barbs along their median to subterminal portions. Numbers of serrated setae (Fig. 12B) along the merus of thoracic endopod 3 increase with increasing body size; also numbers of endopods with any setae of that type increase with body size, i.e., pertaining only to thoracopod 3 in smaller individuals and successively to thoracopods 4–8 as individuals become larger. Endopods 3–7 with less stout, well-developed dactylus bearing a long, needle-like claw; endopod 8 (Fig. 13A) with even smaller dactylus and short seta-like claw. Endopod 8 (when stretched) reaching backwards at most to middle of pleonite 4 and forwards to mandibles; its carpopropodus measures 0.3–0.5 times telson length.

THORACOPODS OF FEMALES (Fig. 12C). Basal plate of exopod 8 with short to indistinct latero-distal expansion. The oostegites from thoracopods 7–8 densely fringed with setae, together forming a large brood chamber. Thoracopod 6 with rudimentary oostegite bearing 2–4 setae at apex. These setae spinulose at least along their apical third. More such setae present in proximal portions of the oostegite from thoracopod 7.

THORACOPODS OF MALES AND PENIS (Fig. 13A). Basal plate of exopod 8 with distinct latero-distal expansion. Penes tubular, 0.7–0.9 times as long as merus of endopod 8. Penes apically bilobate, with row of 5–9 smooth, bent setae flanking the ejaculatory opening. One or two additional, smaller setae subapically on margin opposite bent setae.

PLEON (Figs 12D–L, 13B–D). Pleonites 1–5 are 0.5–0.6, 0.6, 0.6, 0.6 or 0.7 times the length of pleonite 6, respectively. Size increases from pleopods 1 to 4, with pleopod 5 ranging between 2 and 3 in both sexes. Pleopods 1–3 with a ventro-laterally directed fan of plumose setae. Setae of pleopod 1 (Fig. 12D, G) not larger than those of pleopods 2, 3, 5. Female pleopods 1–5 (Fig. 12D–F) and male pleopods 1–2, 5 (Fig. 12G–H, L) vestigial, rod-like, not considering the weak (sub)median widening, if any, at the vestigial endopodal portion. Male pleopod 3 (Fig. 12J) essentially rod-like as well, yet with hump-like endite at 60% pleopod length from basis. Male pleopod 4 reaching with its subapical seta to pleonite 6 or even up to apex of endopod of uropods. Its exopod with basal segment longest, second segment longer than third. Subterminal segment of exopod with a very long seta bearing dense series of minute bristles along its distal 20–53%; short terminal segment with shorter seta at tip, this seta with characteristic set of barbs (Fig. 12K) along its distal third. Endopod with its largest distal seta 1.2–2.0 times endopod length. Sympod with field of scales on its medial widening, or in analogous position upon missing widening. Scutellum paracaudale (Fig. 13B) subtriangular, apically pointed; upper margin concave, lower margin convex. Endopod of uropods (Fig. 13C) 1.0–1.3 times as long as pleonite 6, exopod 1.6–1.9 times pleonite 6. Length of exopod 8–10 times its maximum width. Exopod extends 0.3 times its length beyond endopod or 0.6–0.7 times beyond telson; endopod 0.4–0.6 times its length beyond telson. Telson (Fig. 13D) 0.4–0.5 times as long as exopod of uropods, 0.6–0.8 times endopod or 0.7–0.9 times pleonite 6.

STATOLITHS. Composed of fluorite; shape ellipsoidal to spherical in dorsal view (Fig. 13C), discoidal in lateral view, maximum diameter 126–188 µm, thickness 62–97 µm, measured in 10 adults. Tegmen moderately, though always distinctly convex; fundus weakly concave. Sagittal section very similar to that figured by Wittmann et al. (1993: Fig. 6P) for Heteromysis formosa. Statolith formula 2 + 3 + (0–1) + (4–13) + (6–12) = 19–25.

NAUPLIOID LARVAE. As in M. columbiae (Fig. 13H, J).

Distribution and habitat

In euhaline, sublittoral to littoral waters of Curaçao and Bonaire, both islands at 12° N. The new species forms swarms during the day, hovering closely above the sea floor at a depth of 0– 26 m. Also found near shore boulders, around and off piers.