Hemisiriella pulchra 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 : 41-45

Hemisiriella pulchra Hansen, 1910

( Figures 26 , 27, 28)

Hemisiriella pulchra Hansen, 1910: 46 , pl. V, fig. 1a–p. — Illig, 1930: 564. — W.M. Tattersall, 1936a: 147. — W.M. Tattersall, 1943: 66. — Gordan, 1957: 352. — Ii, 1964: 152, figs 40–41. — Ii, 1965: xxx. — Pillai, 1973: 50, figs 19–20. — Mauchline & Murano, 1977: 58. — Fenton, 1985c: 36 (key). — Muller, 1993: 31. — Lowry & Stoddart, 2003: 464. —Yerman & Lowry; 1977: interactive key.

Type material. SYNTYPES: numerous specimens, Zoological Museum, University of Copenhagen, Denmark and Zoological Museum University of Amsterdam, Netherlands.

Type locality. Indonesia: Sailus Ketjil, Pulau Satengar (as Paternoster Islands); between Kangeang and Pulau Satengar (as Paternoster Islands), 96 m depth, (7°25’S 117°50.5’E); off Pulu Sarassa, Postillon Islands, 36 m depth; west of Sulawesi (Celebes), 18 m depth (4°57.4’S 119°S 2.8’E); off Sawan, Siau Island; northeast of Sulawesi (Celebes) (0°17.6’S 129°14.5’E), 1000-0 m depth.

Material examined. One juvenile, plankton tow, surface, open water between Lizard Is. & Eagle Cay, 2010 hrs June 2 1975 (STL-75-N7) AM P74074. 1 juvenile, plankton tow, surface, Lizard Is. lagoon centre, 2022 hrs May 16 1976 (STL-76-N9). 3 specimens, plankton tow, surface, open water between Lizard Is. & Eagle Cay, 2235 hrs May 19 1976 (STL-76-N13) AM P74075. 5 specimens, plankton tow, 5–6 m depth, open water between Lizard Is. & Eagle Cay, 2250 hrs May 19 1976 (STL-76-N15) AM P74076. 2 juveniles, plankton tow, 5-6 m depth, open water between Lizard Is. & Eagle Cay, 2303 hrs May 19 1976 (STL-76-16) AM P74077. 1 male, plankton tow, 5–6 m depth, open water between Lizard Is. & Eagle Cay, 1938 hrs Jan. 8 1977 (STL-77-N13) AM P74078. 8 specimens, plankton tow, 5–6 m depth, open water between Lizard Is. & Eagle Cay, 1951 hrs Jan. 8 1977 (STL-77-N14) AM P74079. 1 juvenile, plankton tow, surface, open water between Lizard Is. & Eagle Cay, 2006 hrs Jan. 8 1977 (STL-77-N15) AM P47080. 3 specimens, plankton tow, surface, open water between Lizard Is. & Eagle Cay, 2017 hrs Jan. 8 1977 (STL-77-N16) AM P74081. 2 immature females, plankton tow, 5-6 m depth, Lizard Is. lagoon centre, 2325 hrs Jan. 8 1978 (STL-78-N6) AM P74082. 1 immature female, plankton tow, surface, Lizard Is. lagoon centre, 2342 hrs Jan. 8 1978 (STL-78-N7) AM P74083. 2 juveniles, plankton tow, surface, Lizard Is. lagoon centre, 0 0 33 hrs Jan. 14 1978 (STL-78-N10) AM P74084. Total: 30 specimens.

Size range: 2 males, 5.5 & 6.2 mm. 10 immature males, 3.0– 4.5 mm. 6 immature females, 3.5–5.7 mm. 12 juveniles, 1.3–3.2 mm.

Description. Head: eyes stalked, cornea hemispherical, colour orange-red. Rostrum short, obtusely triangular, apex bluntly pointed, cervical groove of carapace conspicuous ( Figs 26 A, B). Antennal scale shorter than antennular peduncle, broad (maximum width almost 0.5x total length), antero-lateral spine prominent, close to distal margin, apical lobe present with well-defined apical suture ( Fig. 26 C). Labrum with a long anteriorly directed medial spiniform process (length almost equal to that of the labrum proper). Mandibular palp slender, basal article not markedly expanded ( Fig. 26 D). Maxillule and maxilla typical of the genus, maxillary palp elongate and slender, moderately setose ( Figs 26 E, F). First thoracic somite fused with head, endopod of 1st thoracic limbs somewhat robust and setose, modified as a gnathopod, exopod natatory, epipodite leaf-like, fairly narrow (Fig. 27A).

Pereon: endopod of 2nd thoracic limb slightly less robust than that of 1st, dactylus setose, probably functions as a gnathopod (Fig. 27B). Endopod of 3rd thoracic limb enlarged, 2x length of endopods of thoracic legs 4–8, ischium markedly long and robust, dactylus reduced to a small blunt cone, surrounded by a cluster of closely set elongate setae forming a conspicuous brush at least equal in length to the propodus, each seta divided by a suture into a shorter smooth proximal part and a longer distal section lined on 1 side by a series of small rounded, denticles, (Figs 27C, D). Endopods of thoracic limbs 4–8 slender, dactylus terminating in a long curved nail.

FIGURE 27. Hemisiriella pulchra Hansen, 1910 . A, left 1st thoracic limb. B, left 2nd thoracic limb. C 3rd and 4th thoracic limbs. D. 3rd thoracic endopod, rounded dactylus and detail of a terminal seta. E, 3rd pleopod (male). F, 4th pleopod (male). Scalebars = A–C, E, F: 0.2 mm; D: 0.05mm.

Pleon: male pleopods 2–5 biramous, pseudobranchiae coiled, neither 3rd nor 4th pleopods with modified terminal setae (Figs 27E, F). Tail fan with uropodal exopod and endopod of approximately equal length, 4–5 distal robust setae on outer margin of proximal article of exopod in Lizard Island specimens, (7 recorded by Ii (1964) in large male), 36 robust setae on inner margin of endopod ( Fig. 28 A), telson linguiform, not extending beyond diaresis in uropodal exopod, 2 stout robust setae on each side of telson base, more distally a concave diastema in each lateral margin forming a distinct ‘waist’, followed by 9–10 lateral robust setae terminating distally in an outer long seta and an inner shorter seta flanking 3 small apical setules and a pair of long delicate plumose setae ( Figs 28 , B, C).

Remarks. Of the 2 other species currently recognised as belonging to the genus Hemisiriella , only H. parva is known to occur in Australian waters. In comparing it with H. pulchra, Pillai (1973) pointed out differences in the structure of the telson and in the relative lengths of the uropodal endopod and exopod. While the telson of H. pulchra has 2 pairs of basal robust setae, followed by a diastema in which the concave border forms a 'waist', the telson of H. parva has 3 pairs of basal robust setae and a straight lateral border lacking a 'waist'. In H. pulchra the uropodal exopod and endopod are the same length, or the exopod is slightly longer than the endopod, which has 36 robust setae in its inner marginal row, but in H. parva the endopod is distinctly longer than the exopod and has at least 40 robust setae in its inner row.

Associated mysid species. Anchialina typica orientalis Nouvel, 1971 , was present in 10 of the 12 hauls in which H. pulchra was taken. The 18 other species found with it all occurred in fewer than half the samples, those most frequently represented being Promysis orientalis Dana, 1852 , Anchialina penicillata and Anisomysis pelewensis , each of which were caught in 5 of the samples containing H. pulchra .

Habitat. Pelagic, oceanic.

Distribution. Hemisiriella pulchra was described from Indonesia ( Hansen 1910). It is also known from the South and East China Seas ( Ii 1964) and has a widespread distribution in the Indian Ocean extending as far as the west coast of Australia ( Pillai 1973). W.M. Tattersall (1936a) recorded it from eastern Australian waters in the region of the Great Barrier Reef, east of Low Isles. It was taken there in a series of tow-nettings at night, when it was found to be abundant, especially in the upper water layers.

The Lizard Island material consisted of young specimens, all taken in plankton hauls made at night, in dark conditions, without moonlight. Out of a total of 30, 24 were caught offshore in the open water between Lizard Island and Eagle Cay, mostly in deep hauls, and only 6 in the Lizard Island lagoon. This pattern of capture reflects the mainly oceanic distribution of H. pulchra .

Figures

FIGURE 26. Hemisiriella pulchra Hansen, 1910. A, cephalothorax with right antenna (male, 6.2 mm). B, lateral view (immature female, 5.7 mm). C, left antenna (male). D, labrum and mandible. E, left maxillule. F, right maxilla. Scalebars = A, B: 0.5 mm; C, D: 0.2 mm; E, F: 0.05 mm.

FIGURE 28. Hemisirella pulchra Hansen, 1910. A, left uropod (male) ventral view. B, telson and right uropod (male) dorsal view. C, terminal border of telson. Scalebars = A, B: 0.2 mm; C: 0.05 mm.