Mysidium (Mysidium) triangulare Wittmann

Wittmann, Karl J. & Wirtz, Peter, 2019, Revision of the amphiamerican genus Mysidium Dana, 1852 (Crustacea: Mysida: Mysidae), with descriptions of two new species and the establishment of two new subgenera, European Journal of Taxonomy 495, pp. 1-48 : 16-23

publication ID	https://doi.org/ 10.5852/ejt.2019.495
publication LSID	lsid:zoobank.org:pub:65CC1141-E560-4979-97E5-F0701563C84B
DOI	https://doi.org/10.5281/zenodo.5695537
persistent identifier	https://treatment.plazi.org/id/B0EB7822-4D13-4D05-8C40-2A7F01297301
taxon LSID	lsid:zoobank.org:act:B0EB7822-4D13-4D05-8C40-2A7F01297301
treatment provided by	Plazi
scientific name	Mysidium (Mysidium) triangulare Wittmann
status	sp. nov.

Treatment

Mysidium (Mysidium) triangulare Wittmann sp. nov.

urn:lsid:zoobank.org:act:B0EB7822-4D13-4D05-8C40-2A7F01297301

Figs 5–8 View Fig. 5 View Fig. 6 View Fig. 7 View Fig. 8

Etymology

The species name is a Latin adjective with neutral ending, referring to the triangular terminal portion of the telson.

Type material examined

Holotype (by present designation) CURAÇAO • ♂ ad. bl 5.2 mm; sublittoral marine waters of Curaçao , Playa Lagun; 12.3181° N, 69.1511° S. #C8; NHMW 26487 View Materials .

Paratypes (by present designation) CURAÇAO • 57 ♂♂ ad. bl 3.8–5.7 mm, 44 ♀♀ ad. bl 4.8–5.9 mm, 8 subad.; same locality data as for holotype; #C8; MINGA MYS 436 , NHMW 26488 View Materials , ZMH K-55260 .

Other material examined

CURAÇAO • 2 ♀♀ ad. bl 5.3–5.5 mm, 3 ♂♂ ad. bl 4.0– 4.2 mm; #C3; MINGA MYS 430 • 1 ♀ subad. bl 3.5 mm; associated with swarm of M. integrum freely swimming in the entrance area of cave; #C10.

Definition

All features diagnosed above for genus and subgenus Mysidium Dana, 1852 . Cornea globose in lateral view; calotte-shaped in dorsal view, with diameter 1.7–2.5 times as long as terminal segment of antennular trunk. Eyestalks smooth. Rostrum triangular, apically pointed to well rounded, 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. 5A–B View Fig. 5 ); longitudinal series of 6–10 setae extending all over this extension plus a short proximal stretch. The largest seta 0.1–0.3 times extension length. Appendix masculina bilobate, densely setose; its length 1.5–2.0 times terminal segment of antennular trunk. Length of antennal scale 5–7 times maximum width, scale reaching far beyond antennular trunk. Median segment of mandibular palp with setae on both margins. Almost evenly rounded hump on outer face of distal segment of maxillula. Carpopropodus two-segmented in thoracic endopods 1–2, 8, or three-segmented in endopods 3–7, except that endopods 6, 7 may be twosegmented in some females. Basal segment occupies 0.4–0.5 times total length of carpopropodus of endopod 3. Pleopod 1 representing a stout, bilobate plate in both sexes. Sympod of male pleopod 4 with endite reduced to a weak medial hump or missing. Exopod with basal segment occupying 56–63% total length. Endopod reduced to lobe with 10–16% sympod length; lobe apically with one long, barbed seta and more proximally with additional 4–6 shorter, barbed setae. Endopod of uropods 0.6–0.8 times as long as exopod. Telson spatulate, length 1.9–2.1 times maximum width near basis; median portion with concave lateral margins, terminal portion triangular with rounded tip. Proximal 52–64% of lateral margins smooth; distal portion of each margin with continuous series of 5–11 acute spines, this series extending up to the corner with the triangular apical portion. Margin of the triangular portion densely furnished with a total of 21–24 strong, apically blunt laminae.

Description

BODY SIZE. Adult females 4.8–5.9 mm (n = 46), males 3.8–5.7 mm (n = 61).

ANTENNAL APPENDAGES ( Fig. 5A–B, F–G View Fig. 5 ). Antennular trunk extends 10–50% its length beyond (artificially aligned) eyes. First to third segments occupy 46–57%, 15–18% or 28–36% total trunk length, respectively. Trunk dorsally with forward directed small, setose lobes near terminal margin of each segment: two lobes from basal, one from median, and one from terminal segment (not counting the mediodistal extension of anterior margin in males). Appendix masculina 0.4–0.5 times total trunk length, antennal scale 1.1–1.5 times trunk. Sympod of antenna produced into spiniform extension on outer distal corner. Antennal scale with terminal segment occupying 13–24% total length and bearing five plumose setae.

MOUTH PARTS ( Figs 5H, J View Fig. 5 , 6A–C View Fig. 6 ). Mandibular palp 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 terminally with strong spines, subterminally with one barbed seta; tip of endite of maxillula with apically modified setae (armed with stiff barbs) plus several shorter setae with normal, fine barbs. Maxilla with well-developed exopod, moderately large, two-segmented palp, and three apically setose endites. Exopod with maximum width in median portions; its outer margin all along with 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 2.0–2.5 times maximum width, densely setose on terminal margin but lined by small hairs along more than basal half of inner margin. Apex of palp with two strong, modified setae bearing strong, spine-like barbs along distal third of their inner margin.

FOREGUT ( Fig. 8A–F View Fig. 8 ). Essentially as in M. antillarum sp. nov. ( Fig. 11A–E View Fig. 11 ). As main differences from that species, M. triangulare sp. nov. shows less strongly serrated, apically pronged, large spines: on each lateral half there are two spines ( Fig. 8E View Fig. 8 ) on posterior part of lateralia; and a longer one ( Fig. 8F View Fig. 8 ) on dorso-lateral infolding; the latter inserting in more median position than in M. antillarum sp. nov.

THORAX OF BOTH SEXES ( Figs 5C–D View Fig. 5 , 6D–G View Fig. 6 , 7A–C View Fig. 7 ). Carapace ( Fig. 5C–E View Fig. 5 ) posteriorly emarginate, with well-rounded latero-terminal lobes; cervical sulcus strong; roughly V-shaped group of 11–14 pores ( Fig. 5E View Fig. 5 ) in front of cervical sulcus, transverse linear series of 13–18 pores ( Fig. 5D View Fig. 5 ) in cardial position (above heart). Sternites smooth; a plumose seta accompanied by a shorter barbed seta on the joint between each sternite and the corresponding thoracopods ( Fig. 6D View Fig. 6 ). Sizes of endopods and exopods increase from thoracopod 1 to 4 or 5 and decrease from 5 to 8. Basal plate of exopods 1–7 with large, well rounded latero-distal expansion ( Figs 6D, F View Fig. 6 , 7A–B View Fig. 7 ). Flagellum 8-segmented in exopod 1 versus 9-segmented in exopods 2–7. First endopod ( Fig. 6D View Fig. 6 ) with well developed, setose endites from basis, ischium, merus. First thoracic epipod bilobate, with smooth margins, without seta. Endopods 1–2 with large dactylus and strong claw ( Fig. 6D–E View Fig. 6 ). Dactylus of thoracic endopod 2 with 3–6 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. Endopods 3–7 with less stout, well developed dactylus bearing a long, needle-like claw ( Fig. 6G View Fig. 6 ); endopod 8 with seta-like claw ( Fig. 7C View Fig. 7 ). Endopod 8 (when stretched) reaching backwards at most to end of pleonite 3 and forwards to maxillula; its carpopropodus measures 0.4–0.5 times telson length.

THORACOPODS OF FEMALES ( Figs 7A View Fig. 7 , 8G View Fig. 8 ). Basal plate of exopod 8 with short to indistinct latero-distal expansion; its flagellum only 8-segmented. Oostegites of thoracopods 7–8 densely fringed with setae, together forming a large brood chamber. Thoracopod 6 ( Fig. 7A View Fig. 7 ) with rudimentary oostegite bearing 1–2 long setae at apex. These setae spinulose at least along their apical third. More such setae present in proximal portions of oostegite from thoracopod 7 ( Fig. 8G View Fig. 8 ).

THORACOPODS OF MALES AND PENIS ( Fig. 7B–C View Fig. 7 ). Basal plate of exopod 8 with yet distinct latero-distal expansion; its flagellum 9-segmented. Penes pear-shaped, apically widening; 0.9–1.0 times as long as merus of endopod 8. Penes apically bilobate, with row of 4–5 smooth, bent setae flanking the ejaculatory opening. Two additional, smaller, straight setae subapically on margin opposite to bent setae.

PLEON ( Figs 7D–M View Fig. 7 , 8H–L View Fig. 8 ). Pleonites 1–5 are 0.6–0.7, 0.7–0.8, 0.7–0.9, 0.8–0.9 or 0.7–0.9 times as long as pleonite 6, respectively. Scutellum paracaudale ( Fig. 8H, J View Fig. 8 ) triangular, apically pointed to narrowly rounded; margins weakly concave to convex. Uropodal endopod ( Fig. 8K View Fig. 8 ) 1.0–1.3 times, exopod 1.4– 1.7 times as long as pleonite 6. Length of exopod 7–9 times maximum width. Exopod extends 0.2–0.3 times its length beyond endopod or 0.5–0.6 times beyond telson; endopod 0.3–0.5 times its length beyond telson. Telson ( Fig. 8L View Fig. 8 ) 0.5–0.6 times as long as exopod of uropods, 0.7–0.8 times endopod or 0.8–0.9 times pleonite 6.

PLEOPODS OF FEMALES ( Fig. 7D–G View Fig. 7 ). Pleopods 1–5 subequal in length. Pleopods 1–2 reduced to small bilobate plates, pleopods 3–5 to small, setose rods. Pleopods 1–4 with a ventro-laterally directed fan of plumose setae. Pleopod 1 with clearly the largest setae, although not as large as in males.

PLEOPODS OF MALES ( Fig. 7H–M View Fig. 7 ). Length increases from pleopod 1 to 4. Pleopod 5 is about the same length as pleopod 1, but less stout. Pleopods 1–3 with a ventro-laterally directed fan of plumose setae; pleopod 1 with the largest setae of that kind. Fourth pleopod reaching to terminal 50–80% of pleonite 6; its subapical seta reaching up to apex of telson. Its four-segmented exopod with basal segment longest, second segment longer than third, third segment longer than fourth. Subterminal segment of exopod with a very long seta bearing dense series of minute bristles along its distal half; short terminal segment with comparatively large but shorter seta at tip, this seta with characteristic set of barbs ( Fig. 7L View Fig. 7 ) along its distal half. Endopod with apical seta 2.0–2.8 times endopod length. Sympod with field of scales on its medial widening, or in analogous position upon missing widening.

STATOLITHS. Composed of fluorite; shape ellipsoidal to spherical in dorsal view ( Fig. 8K View Fig. 8 ); discoidal in lateral view, maximum diameter 89–107 µm, thickness 45–54 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 S.I. Smith, 1873 . Statolith formula 2 + 3 + 1 + (8–13) = 14–19.

NAUPLIOID LARVAE ( Fig. 8M–N View Fig. 8 ). Smooth cuticle all around, except for a pair of minute furcal processes and a number of minute setae on the blunt end of the abdomen.

Distribution and habitat

So far only known from euhaline, sublittoral waters of Curaçao (12° N), where the mysids occur in swarms hovering during daytime around and between corals.