Siriella gracilis Dana, 1852

(Figures 10, 11)

Siriella gracilis Dana, 1852: 685, pl. 44. — Dana, 1855: pl. 44, figs 1a–g, 2a–c. — Filhol, 1885: 1. — Sars, 1885: 209, pl.36, figs25–28. — Czerniavsky, 1887: 29. — Ortmann, 1894: 107. — Calman, 1901: 602. — Hansen, 1910: 31. — Hansen, 1912: 193. —W.M. Tattersall, 1912:122. — Colosi, 1919: 6. — Colosi, 1920: 229. — Colosi, 1924: 3. — Illig, 1930: 419. — Coifmann, 1936: 25, pl. X, figs 14a–e, pl. XI, figs 14f–g. — Coifmann, 1937: 3. — W.M. Tattersall, 1939: 235. — W.M. Tattersall, 1943: 65. — W.M. Tattersall, 1951: 62. — O.S. Tattersall, 1955: 86. — Gordan, 1975: 379. — Ii, 1964: 72, fig.16. — Pillai, 1964: 6. — Pillai, 1965: 1693, fig.20. — Pillai, 1973: 41. — Mauchline & Murano, 1977: 76. — Fenton, 1985: 37, 46 (key). — Carleton & Hamner, 1989: 464, table 1. — Muller, 1993: 37. — Fukuoka & Murano, 2002: 58. — Lowry & Stoddart, 2003: 466. — Yerman & Lowry, 2007: interactive key.

Type material. SYNTYPES: location not known.

Type locality. Near St. Augustine Island and near Pitt Island, Kingsmill Group, Kiribati, Pacific Ocean.

Material examined. Two specimens, plankton tow, surface, open water between Lizard Is. & Eagle Cay, 2023 hrs June 2 1975 (STL-75-N8) (AM P73802). 17 specimens, plankton tow, surface, open water 2–4 km E. of Yonge Reef, 1930 hrs Dec. 1 1978 (J.P.-78-3) (AM P73803). Total: 19 specimens.

Size range: 3 males, 5.0– 6.2 mm. 3 immature males, 3.8–4.6 mm. 2 females, marsupium empty, 4.8, 4.9 mm. 4 immature females, 3.8–4.6 mm. 7 juveniles, 2.3–3.2 mm.

Description. Head: eyes stalked, hemispherical, golden-brown, small (diameter of adult male cornea 0.3 mm). Rostrum triangular, acute (Figs 10 A, B). Antennal scale shorter than antennular peduncle, broad (length 2.. 7 x maximum width), lateral spine not near distal end of scale (situated 0.75 of total length along lateral margin), apical suture present in Lizard Island specimens (Fig. 10 C). Labrum rounded with long anterior medial spiniform process, more than 0.5 length of labrum proper (Fig. 10 D). Basal article of mandibular palp broadly expanded (maximum width almost 0.5 x length) (Fig. 10 E). Maxilla moderately setose, palp on endopod prominent (Fig. 10 F). First thoracic somite fused with head, endopod of limb modified as a gnathopod, basis enlarged, exopod natatory, epipodite thin-walled (Fig. 11A).

Pereon: endopod of 2nd thoracic limb not markedly robust, dactylus reduced (Fig. 11B). Pereopods of moderate length, fairly slender, genital organ at base of 8th limb in males (Fig 11C).

Pleon: male pleopods 2–5 biramous, pseudobranchiae coiled, neither 3rd nor 4th pleopods with modified terminal setae (Fig 11D). Uropodal exopod with diaresis, 2–5 graded robust setae on distal part of outer margin of proximal article, distal article and inner margin of proximal article lined with plumose setae, uropodal endopod entire, slightly longer than exopod, margins fringed with plumose setae, inner margin with row of about 37 robust setae (11 long setae interspersed with smaller ones). Telson long, linguiform, extending beyond level of diaresis in uropodal exopod, 2 pairs of stout basal robust setae on proximal margins, followed by a slight marginal concavity, or ‘waist’ and 18–20 lateral robust setae increasing in size towards apex and 3 small terminal setae with a slender pair of long plumose apical setae (Fig. 11E).

FIGURE 11. Siriella gracilis Dana, 1852 . A, left 1st thoracic limb (male). B, left 2nd thoracic limb (male). C, right 8th thoracic limb (male). D, left 3rd pleopod (male). E, telson and left uropod (male). Scalebars = A–E: 0.2 mm.

Remarks. Features characteristic of this species are small golden-brown eyes, short broad antennal scale, natatory pleopods of males having unspecialised setae and small coiled pseudobranchiae and a tail fan in which the uropodal exopods are shorter than the endopods and the telson long and moderately broad, with the lateral robust setae terminating distally in a single long seta on each side. Pillai (1973) observed that S.

gracilis exhibits some intraspecific variations and he noted a slight sexual dimorphism, the rostrum being rounded in males and acutely pointed in females. This distinction is barely visible in the Lizard Island specimens, however (Figs 10 A, B). The uropodal exopod is recorded by Ii (1964) as having 2–3 robust setae on the distal outer margin of the proximal article, but the Lizard Island male examined had 5 graded robust setae on its margin (Fig. 11E), while the female had only 2. This may be a further instance of sexual dimorphism in S. gracilis .

Associated mysid species. The only other species caught in both samples with S. gracilis was Anchialina typica orientalis Nouvel, 1971, however there were 6 specimens of S. thompsonii in the second sample. This co-occurrence conforms with Ii’s (1964) observation on S. gracilis . He recorded it as being taken at the surface in offshore waters, usually with S. thompsonii .

Habitat. Pelagic, oceanic.

Distribution. Widespread in the tropical and temperate Indo-Pacific (W.M. Tattersall 1951), S. gracilis has previously been recorded from the Australian region by Pillai (1973), who found it off the Western Australian coast, by. Colosi (1919), who recorded it from the Torres Strait and Carleton and Hamner (1989), who took it on the Great Barrier Reef at Davies Reef.

During the Lizard Island survey, it was caught on 2 occasions, both at night, in plankton tows near the surface in open offshore waters. The first was in the passage between Lizard Island and Eagle Cay and the second 2–4 km east of Yonge Reef.