Siriella thompsonii Milne Edwards 1837

Siriella thompsonii (Milne Edwards, 1837)

(Figures 19, 20)

Cynthia sp. Thompson, 1829: 55, pl. vi.

Cynthia Thompsoni Milne-Edwards, 1837: 462.

Siriella vitrea Dana, 1852: 656, pl. 43, figs 6a–m. — Czerniavsky, 1887: 28 Siriella breviceps Dana, 1852: 658, pl. 44, fig. 1a–q.

Cynthia inermis Krøyer, 1861: 44, tab. 2, fig 6a–g.

Promysis galatheae Krøyer, 1861: 59, pl. 2, figs 8a–h. Siriella Edwardsii Claus, 1868: 271, pl. 18.

Siriella (Siriellides) indica Czerniavsky, 1882: 103, table XXXI, figs1–6.

? Siriella suluhensis Czerniavsky, 1882: 108. — Muller, 1993: 43. — Panampunnayil, 1995: 1945 (table). — Fukuoka & Murano, 2002: 89 (key).

Siriella thompsonii. — Filhol, 1885: 1. — Ortmann, 1893: 23. — Ortmann, 1894: 107. — Calman, 1901: 24. — Thiele, 1905: 4477, figs 7–8. —Ortmann, 1906: 971. — Hansen, 1910: 31. — Hansen, 1912: 192. —W.M. Tattersall, 1912: 122. — Hansen, 1913: 9. — W.M. Tattersall, 1914: 870. — Zimmer, 1914: 386. — Zimmer, 1916: 61. — Colosi, 1919: 5. — Colosi, 1922: 13. — W.M. Tattersall, 1923: 280. — W.M. Tattersall, 1926: 9. —Colosi, 930: 983. — Illig, 1930: 419. — Coifmann, 1936: 21, pl. VIII, fig. 12a–e, pl. IX, fig. 12f–g. — W.M. Tattersall, 1936a: 145. — W.M. Tattersall, 1936b: 279. — Coifmann, 1937: 2. — W.M. Tattersall, 1939: 234. — O.S. Tattersall, 1955: 84. — Gordan, 1957: 381. — O.S. Tattersall, 1962: 223. — Pillai, 1973: 38, figs 10,11. — Mauchline & Murano, 1977: 77. —McWillliam & Phillips, 1983: Plate 2. — Fenton, 1985: 37, 48 (key). — Lowry & Stoddart, 2003: 468. — Yerman & Lowry, 2007: interactive key.

Protosiriella thompsonii. — Czerniavsky, 1887: 27.

Heterosiriella galathae. — Czerniavsky, 1887: 39.

Siriella thompsoni. — Coifmann, 1936: 21, pl. VIII, figs 12a–e, pl. IX figs 12f–g. — W.M. Tattersall, 1951: 60. — O.S. Tattersall, 1961: 147. — Birstein& Tchindonova, 1962: 65. —da Costa, 1964: 3. — Ii, 1964: 62, figs 14, 15. — Pillai, 1965: 1693, fig. 19. — Carleton & Hamner, 1989: 464, table 1.

Type material. HOLOTYPE: Locality not known (assumed lost).

Type locality. Central Atlantic Ocean between Madeira and the West Indies.

Material examined. 6 specimens, plankton tow, near surface, 2–4 km E. of Yonge Reef, 1930 hrs Dec. 1 1978 (J.P. 78.3) AM P74063.

Size range: 2 males, 7.2 & 7.4 mm. 1 immature female, 5.2 mm. 3 juveniles, 2.9-3.2 mm.

Description. Head: eyes stalked, cornea hemispherical, large (diameter of adult male cornea 0.6 mm), colour dark red-brown. Rostrum triangular, acutely pointed, carapace with cervical groove (Fig. 19A). Antennal scale narrow (length 5.5x width), slightly shorter than antennular peduncle, apical suture present, apical lobe small, spine towards distal end of lateral margin (Fig. 19B). Labrum with prominent anterior medial spiniform process (about 0.5 length of labrum proper) (Fig. 19C). Mandibular palps with expanded proximal article (Fig. 19D). Palp of maxillary endopod fairly elongate and narrow (Fig. 19E). First thoracic somite fused with head, endopod of 1st thoracic limb broad, forming a gnathopod, dactylus terminating in a strong nail, epipodite leaf-like (Fig. 19F).

Pereon: second thoracic limb with moderately robust, setose endopod, functioning as a gnathopod, exopod natatory (Fig.19G), thoracic limbs 3–8 forming pereopods, endopods slender, exopods natatory, genital organ at base of 8th thoracic limb in males (Fig. 20A).

Pleon: male pleopods 2–5 all similar, biramous, with spirally coiled pseudobranchial rami, simple plumose setae, none forming a distal armature (Fig.20B). Tail fan with uropodal exopod shorter than endopod, proximal article of exopod with 3 or 4 graded robust setae at distal angle of outer margin, uropodal endopod with 76–78 robust setae on inner margin (Fig. 20C), telson fairly narrow, linguiform, with 2 stout l robust setae on each side of base, 18 graded robust setae on each lateral margin, distal border with 2 long robust setae on each side of 3 minute central apical setae and 2 delicate plumose setae (Fig. 20D).

Remarks. Pillai (1973) has discussed the considerable intraspecific variation observed in S. thompsonii and suggested that it might be evidence of subspeciation. The specimens taken in the Lizard Island region most closely resemble those examined by Ii (1964) from the waters off Japan and the East China Sea and Yellow Sea.

Associated mysid species. The 8 other mysid species taken in the same sample as S. thompsonii included 17 specimens of S. gracilis, a species generally found with S. thompsonii, as recorded by Pillai (1973).

Habitat. Pelagic in warm and temperate oceanic waters. Evidence discussed by Ii (1964) suggests that S. thompsonii moves into the surface waters at night and migrates into deeper layers during the day.

Distribution. S. thompsonii is an epi-pelagic species with a world-wide oceanic distribution in warm and temperate regions. According to Pillai (1973) it is the most widely distributed mysid in oceanic waters. It is known from the north-eastern region of Australia, in the Great Barrier Reef at Low Isles (W.M. Tattersall 1936a) and at Davies Reef (Carleton & Hamner 1989). It has also been taken in New South Wales coastal water (McWilliam & Phillips 1983) and off the west coast of Western Australia (Pillai 1973).

In the Lizard Island area, 6 specimens were caught offshore at night in a plankton haul in open water 2–4 km E of Yonge Reef.

FIGURE 19. Siriella thompsonii (Milne Edwards, 1837). A, cephalothorax (male, 7.4 mm). B, right antenna (male). C, labrum. D, left mandible. E, maxillules, with paragnath and left maxilla. F, left 1st thoracic endopod and epipodite. G, left 2nd thoracic limb. Scalebars = A: 0.5 mm; B–G: 0.2 mm.

FIGURE 20. Siriella thompsonii (Milne Edwards, 1837). A, left 8th thoracic limb (male). B, left 3rd pleopod (male). C, endopod of left uropod. D, telson and right uropod. Scalebars = A–D: 0.2 mm.