Mysidium (Mysidium) gracile ( Dana, 1852 )

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 : 6-9

Mysidium (Mysidium) gracile ( Dana, 1852)

Fig. 1

Macromysis gracilis Dana, 1852: 653 –655 (senior synonym in combination with generic, subjective junior synonym).

Macromysis gracilis – Dana 1855: pl. 43, fig. 5a–m. — Smith 1873: 41 (in list). — Illig 1930: 599 (in synonymy).

Mysidium gracile – Czerniavsky 1887: 85 –87 (revised combination). — W.M. Tattersall 1951: 222 –223.

— Costa 1964: 8. — Brattegard 1969: 80; 1970: 127; 1974b: 102; 1975: 112. — Almeida Prado 1974: 53. — Quintero & Zoppi de Roa 1977: fig. 3. — Mauchline 1980: 338 (in list). — Băcescu & Ortiz 1984: 22 (in key). — Zoppi de Roa et al. 1989: fig. 2C. — Murano 1999: fig. 6.89. — Twining et al. 2000: 1845. — Price et al. 2002: 45. — Price & Heard 2004: 155; 2009: 938 (in list). — Ortiz et al. 2017a: table 1; 2017b: fig. 5. — Sorbe et al. 2007: table I. — Miyashita & Calliari 2014: 9, table 1. — Wittmann et al. 2014: 298. — Esteves-Astudillo et al. 2017: fig. 5. — Ortiz & Lalana 2018: 72.

Mysidia gracilis – Zimmer 1915b: 215 (subsequent combination with generic junior homonym); 1918: 24, figs 33–34.

Mysidia gracile – Illig 1930: 500 (as before, gender variant, in key).

Type material

Not examined.

Other material examined

BONAIRE • 7 ♀♀ ad. bl 4.4–5.2 mm, 21 ♂♂ ad. bl 3.4–5.1 mm, 1 subad.; #B11; ZMH K-55262 • 1 ♂ ad. bl 4.6 mm; #B12; MINGA MYS 439 .

CURAÇAO • 1 ♂ ad. bl 5.6 mm, 6 subad.; #C7; MINGA MYS 434 .

SE-BRAZIL • 19 ♀♀ ad. bl 4.8–6.3 mm, 15 ♂♂ ad. bl 5.2–6.6 mm, 13 subad.; #F13; NHMW 26482 .

Type locality

(Sub)tropical SW-Atlantic, harbour of Rio de Janeiro ( Dana 1852), about 22.90° S, 43.17° W. The present sample off Cabo Frio is at about 145 km east along the shoreline from this harbour.

Revised definition

All features as diagnosed for the genus and its subgenus Mysidium Dana, 1852 . Cornea globose in lateral view, calotte-shaped in dorsal view, with diameter 1.7–3.2 times as long as terminal segment of antennular trunk. Eyestalks smooth. Rostrum short, triangular, apically rounded to obtusely 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. 1A–B ); plumose seta at anterior margin of this extension showing 0.9–1.4 times extension length; longitudinal series of 6–8 shorter (partly minute) setae all over this extension plus a short proximal stretch. Appendix masculina bilobate, 1.6–2.7 times as long as terminal segment of antennular trunk. Length of antennal scale 4–6 times maximum width; scale clearly reaching beyond antennular trunk. Median segment of mandibular palp with setae on both margins. About evenly rounded hump on the outer face of the distal segment of the maxillula. Carpopropodus of first to eighth thoracic endopods with 2, 2, 3, 3, 3, 3, 3–2, and 2–3 segments, respectively. Basal segment occupies half total length of carpopropodus of endopod 3. Pleopod 1 rod-like to indistinctly bilobate in both sexes. Sympod of pleopod 4 with endite missing or reduced to weak medial hump. Basal segment occupies 56–69% total length of exopod. Endopod reduced to lobe with 12–18% sympod length; apically with one long seta and more proximally additional 4–7 shorter, barbed setae. Endopod of uropods 0.6–0.8 times length of exopod. Telson subrectangular, length 1.4–2.0 times maximum width near basis; lateral margins slightly tapering or parallel. Proximal 46–64% of lateral margins smooth, distal portion of each margin with dense, continuous series of 11–15 acute spines. Terminal margin concave, densely furnished with 15–21 apically blunt laminae.

Descriptive notes

Adult females of present material with 4.4–6.3 mm (n = 26) body length, males 3.4–6.6 mm (n = 38). Antennular trunk extends –5% up to +23% its length beyond (artificially aligned) eyes ( Fig. 1A ). Antennal scale 1.3–1.8 times trunk. Sympod of antenna produced into spiniform extension on outer distal corner. Thoracic endopod 8 (when stretched) reaching backwards at most to the middle of pleonite 4 and forwards to mandibles; its carpopropodus 83–94% length of merus or 29–53% of telson. Pleonites 1–5 are 0.5–0.7, 0.6–0.8, 0.6–0.8, 0.5–0.7 or 0.5–0.8 times as long as pleonite 6, respectively. Sizes increase from pleopods 1 to 4, while pleopod 5 ranges between pleopods 1 and 2 in both sexes; amplitude of this variation much stronger in males than females. Pleopod 1 ( Fig. 1D ) stouter compared to remaining pleopods. Pleopods 2–5 of females and 2, 5 of males are essentially rod-like. Pleopods 1–3 with a ventrolaterally directed fan of plumose setae in both sexes. Setae forming the fan of pleopod 1 larger compared to those of pleopods 2–5 in females and 2, 3, 5 in males. Male pleopod 4 ( Fig. 1F ) very long, exopod reaching at most to basal third of telson; its subapical seta up to the apex of telson. Endopod with apical seta 2.2–4.6 times endopod length. Sympod with field of scales on its medial hump, or in analogous position upon missing hump. Scutellum paracaudale triangular with acute apex; upper margin weakly convex, lower margin concave or S-shaped. Uropodal endopod 1.0–1.4 times, exopod 1.3–1.9 times as long as pleonite 6. Exopod extends 0.2–0.4 times its length beyond endopod, or 0.5–0.7 times beyond telson; endopod 0.2–0.6 times its length beyond telson. Telson ( Fig. 1G ) length 0.4–0.6 times uropodal exopod, 0.5–0.7 times endopod and 0.7–0.8 times pleonite 6. Statoliths composed of fluorite. Structure of foregut and nauplioid larvae essentially as in M. triangulare sp. nov. ( Fig. 8A–F, N–M ).

Distribution and habitat

Widely distributed, mainly in sublittoral, also in littoral waters of the West Atlantic in the range of 32° N to 24° S ( Bermuda to coast near São Sebastião, Brazil), including the Caribbean ( Price & Heard 2004; Ortiz et al. 2017b). Masses were recorded from the surface zone of a sandy beach in Venezuela (Esteves- Astudillo et al. 2017). Distribution mostly euhaline, also found in the metahaline reach of a lagoon by Zoppi de Roa et al. (1989). The species forms swarms close to the bottom in a variety of coral reef habitats ( Emery 1968; Brattegard 1969). Swarms may hover near the sea urchin Diadema antillarum Philippi, 1845 , where they withdraw to the safety of the interspaces between the spines when threatened ( Randall et al. 1964). They may also crowd in the nest caves of pomacentrid fish when a predator approaches ( Emery 1968). The swarms disperse during the night and the individuals show homing behaviour by regrouping at the same location in the morning ( Twining et al. 2000). Present records from Curaçao, Bonaire, and SE-Brazil; in swarms hovering during the daytime over the sea floor at a depth of 3–35 m, also found between corals.

Figures

Fig. 1. Mysidium gracile (Dana, 1852), off Cabo Frio in Brazil, non-types. A–B. ♂, 5.6 mm body length. C, G. ♂, 5.3 mm. D–F. ♂, 5.2 mm. A. Cephalic region, right antennula and setae of antennal scale omitted. B. Mediodistal extension of right antennular trunk, dorsal. C. Third thoracic endopod, caudal. D–E. Pleopods 1 and 3, rostral. F. Pleopod 4, caudal. G. Telson.

Fig. 8. Mysidium triangulare Wittmann sp. nov., from Curaçao, paratypes. A–F. ♀, 5.9 mm body length. G, J. ♀, 5.1 mm. H, K–L. ♂, 5.5 mm. A. Foregut; dorsal wall omitted. B–F. Details of panel (A) showing modified spines. G. Basal half of thoracopod 7 with right oostegite, lateral view. H, J. Variants of scutellum paracaudale, lateral view. K. Uropods, ventral view; setae omitted. L. Telson. M. Nauplioid larva at substage N2, lateral view. N. Detail showing tip of abdomen in another nauplioid specimen.