Siriella distinguenda Hansen, 1910

Talbot, Suzette, 2009, A survey of Mysida from the Lizard Island area, Great Barrier Reef, Australia, Subfamily Siriellinae (Crustacea, Mysida, Mysidae), Zootaxa 2114, pp. 1-49 : 13-17

Siriella distinguenda Hansen, 1910

( Figures 7 , 8, 9)

Siriella distinguenda Hansen, 1910: 42 , Pl. V, Figs 2 a–e. — Illig, 1930: 564 (key). — W.M. Tattersall, 1943: 66. — W.M. Tattersall, 1951: 79. — Gordan, 1957: 379. — Ii, 1964: 142. — Mauchline & Murano, 1977: 76. — Bacescu, 1979: 143. — Fenton, 1985: 37, 46 (key). — Muller, 1993: 36. — Lowry & Stoddart, 2003: 466. — Yerman & Lowry, 2007: interactive key.

Type material. SYNTYPES: 6 specimens, Zoological Museum, University of Amsterdam, Netherlands.

Type locality. Siboga Station 37, Sailus ketjil, Pulau Satengar (as Paternoster Islands), 27 m depth, Flores Sea; Siboga Station 41, 7°25’S 117°50.5’E, 10 m depth, Flores Sea; Siboga Station 125, anchorage off Sawan, Siau Island, Molucca Sea, Indonesia.

Material examined. Nine specimens, light trap, on living Porites (light on), Lizard Is. lagoon centre, 2228 hrs Jan. 4 1975 (STL-75-T11) (AM P73789). 4 specimens, plankton tow, surface, Lizard Is. lagoon centre, 1958 hrs May 29 1975 (STL-75-N5) (AM P73790). 2 specimens, plankton tow, surface, Lizard Is. lagoon centre, 2034 hrs May 16 1976 (STL-76-N10) (AM P73791). 1 male, light trap, on coral rubble (light on), passage between Palfrey & South Is., Lizard Is. lagoon, 2210 hrs Jan. 11 1977 (STL-77-L13) (AM P73792). 4 specimens, plankton tow, 5 m depth, Lizard Is. lagoon centre, 2305 hrs Jan. 8 1978 (STL-78-N5) (AM P73793). 7 specimens, plankton tow, 5 m depth, Lizard Is. lagoon centre, 2325 hrs Jan. 8 1978 (STL-78- N6) (AM P73794). 6 specimens, plankton tow, surface, Lizard Is. lagoon centre, 2342 hrs Jan. 8 1978 (STL- 78-N7) (AM P73795). 6 specimens, plankton tow, surface, Lizard Is. lagoon centre, 2357 hrs Jan. 8 1978 (STL-78-N8) (AM P73796). 23 specimens, fixed trap, above horizontal plate of favid coral, Echinopora lamellosa , reef S.of Research Point, overnight, Jan. 13–14 1978 (STL-78-F6) (AM P 73797). 19 specimens, plankton tow, surface, Lizard Is. lagoon, night, Feb. 2 1980 (J.M.L. 2.2.1. 2004) (AM P 73798). 6 specimens, plankton tow, 6 m. depth, off Osprey Is., night, Feb. 3 1980 (J.M.L. 3.2.2. 2134) (AM P73799). 1 male, plankton tow, surface, Lizard Is. lagoon, 0 130 hrs Dec. 2 1980 (Grindley #10) (AM P73800). 7 males, plankton tow, surface, Lizard Is. lagoon, 0 330 hrs Dec. 2 1980 (Grindley #11) (AM P73801). 1 male, light trap, Lizard Is. lagoon, 2000–0400 Dec. 21 1993 (Alexander). Total: 96 specimens.

Size range: 28 males, 8.5–12.5 mm. 5 immature males, 6.2–8.5 mm. 6 females, marsupium empty, 9.8–11.0 mm. 1 female, parasite in marsupium, 10.5 mm. 20 immature females, 3.5–9.2 mm. 35 juveniles, 1.7–4.5 mm.

Description. Head: eyes stalked, prominent, large, black globose (adult male cornea 0.9 mm in diameter). Rostrum triangular with small acute point ( Figs 7 A, B). Antennal scale as long as antennular peduncle, scale fairly narrow, (length about 4x maximum width), anterolateral spine towards distal end of outer margin, apical suture present ( Fig. 7 C). Labrum rounded with long anterior medial spiniform process, slightly more than 0.5x length of labrum proper ( Fig. 7 D). Mandibular palp with expanded basal article (length about 2.5x maximum width) ( Figs 7 E, F). Maxillule and maxilla typical of the genus (Figs 8A, B). Maxilla markedly setose, endopodal palp ovate. First thoracic somite fused with head, endopod of limb stout, modified as a gnathopod, exopod natatory, epipodite leaf-like (Fig. 8C).

Pereon: endopod of second thoracic limb moderately robust, functioning as a gnathopod (Fig. 8D), endopods of thoracic limbs 3–8 long and slender, dactylus ending in a long nail, exopods natatory, genital organ at base of 8th limb in males (Fig. 8E).

Pleon: male pleopods with straight pseudobranchial rami, pleopod 3 unmodified, exopod and endopod subequal, both terminating in plumose setae (Fig. 9A), pleopod 4 modified, endopod markedly larger and longer than exopod, exopod terminating in plumose setae, endopod terminating in 3 large acutely pointed robust setae (Figs 9B, C, D). Uropodal exopod with diaresis, 10–14 robust setae on outer margin of proximal article, distal article and inner margin of proximal article fringed with plumose setae, endopod slender, lacking diaresis, slightly shorter than exopod, margins fringed with plumose setae, row of robust setae on inner margin extending from statocyst to tip. Telson slender, linguiform, reaching slightly beyond level of diaresis in uropodal exopod, 3 stout robust setae on either side of base, more distally a slight marginal concavity, or ‘waist’, beyond which each margin is lined with 41 robust setae increasing in size towards tip, terminal margin with 3 small simple setae and a pair of long delicate plumose setae (Figs 9E, F).

FIGURE 8. Siriella distinguenda Hansen, 1910 . A, maxillule. B, maxilla. C, 1st thoracic limb. D, 2nd thoracic limb. E, 8th thoracic limb (male). Scalebars = A, B, C, D, E: 0.2 mm.

Remarks. Like S. anomala , S. distinguenda is large, with conspicuously dark and prominent eyes, but unlike the former species, it does not exhibit sexual dimorphism in its antennal scale, the lateral spine being near the distal end of the scale in both males ( Fig. 7 C) and females. Males of S. distinguenda can be recognised by the characteristic terminal armature of the modified endopods of the fourth pleopods together with their straight pleopodal pseudobranchiae. The Lizard Island specimens showed some variation from that described by Hansen (1910, Pl. V 2 c). Of the 5 males examined, 2 conformed to Hansen’s description, with a strong elongate outer seta and shorter curved inner seta extending from the terminal article and a long inner seta extending from the penultimate article (Fig. 9D). In the remaining 3, the shortest seta was on the penultimate article and the longer 2 on the terminal article. In all but 1 of the specimens, the terminal article was extremely small and the suture between it and the penultimate article ill defined or not visible (Fig. 9C).

FIGURE 9. Siriella distinguenda Hansen, 1910 . A, 3rd pleopod (male). B, 4th pleopod (male). C, 4th pleopod (male), terminal article of endopod, 1st configuration. D, 4th pleopod (male), terminal article of endopod, 2nd configuration. E, telson and right uropod (male). F, telson (male). Scalebars = A, B, E, F: 0.5 mm; C, D: 0.2 mm.

Females and juveniles are difficult to identify with certainty, but the structure of the tail fan is an aid in separating them from those of S. anomala . In S. distinguenda the moderately slender telson reaches slightly beyond the diaresis in the uropodal exopod (Fig. 9E), while in S. anomala it reaches the level of the diaresis (Fig. 6F). The uropodal endopod of S. distinguenda carries 70–80 robust setae in its inner marginal row, as opposed to that of S. anomala , which has about 60.

Parasites. A single S. distinguenda female contained a female choniostomatid copepod in her marsupium.

Associated mysid species. Of the 23 species found with S. distinguenda , only 4 co-occurred regularly. Anisomysis laticauda was present in 11 of the 14 samples, Pseudanchialina inermis in 10, Doxomysis acanthina Talbot, 1997 in 9 and Anisomysis incisa W.M. Tattersall, 1936 a in 8.

Habitat. Pelagic, sublittoral in coral reef areas.

Distribution. Described by Hansen from Indonesian waters, S. distinguenda has also been found in the Philippines ( W.M. Tattersall 1951). In Australia it is recorded from the vicinity of Heron Island in the Great Barrier Reef ( Bacescu 1979; Fenton 1982).

The Lizard Island specimens were caught at night. Apart from 6 netted near Osprey Island, they were all found in the lagoon, usually in plankton hauls, but also in traps set on or above living coral and in one instance, in a light trap on coral rubble.

Figures

FIGURE 7. Siriella distinguenda Hansen, 1910. A, anterior cephalothorax (male, 11 mm). B, lateral view (female, 10 mm). C, antenna, (male). D, labrum. E, left mandible. F, right mandible. Scalebars = A, B: 1.0 mm; C, D, E, F: 0.5 mm