Siriella thompsonii H. Milne-Edwards 1837

Siriella thompsonii (H. Milne-Edwards, 1837)

(Figs. 2, 3)

Cynthia thompsonii H. Milne-Edwards 1837: 462.

Cynthia inermis— Kroyer 1861: 44, figs. 6a–g.

Siriella indica — Czerniavsky 1882: 103. — Taf. 31, figs. 1–16.

Siriella thompsonii — Sars 1883: 40 –41. — Thiele 1905: 447 –449, figs. 7–9. — Hansen 1910: 31. — Colosi 1922: 1 –22. — W. Tattersall 1913: 870 –871; 1923: 280; 1926: 9; 1939: 234; 1943: 63–64; 1951: 60–61. — Coifmann 1937: 21 –23, pl. 8, figs. 12a–e, pl. 9, figs. 12f–g. — Holmquist 1957: 44 –45. — O. Tattersall 1962: 223 –225, pl. 2. — Pillai 1973: 47, figs. 15–16. — Stuck et al. 1979: 234, figs. 2f, 3f, 4f, 5f. — Price et al. 1987: 47. — Escobar-Briones & Soto 1991: 80 – 89. — Wang & Liu 1994: 84 –86, fig. 11; 1997: 203. — Panampunnayil 2002: 371 –372, figs. 1A–C. — Murano & Fukuoka 2008: 41 –45.

Material examined. 1 female with eggs, Sta. 3 (35° 04'S, 44° 59'E), southern Indian Ocean, 0 3.02.2004, 22:33 hr., BN, sampling depth: surface waters. — 4 adult males, 2 immature males, 2 spent females, 1 female with eggs, 5 immature females, and 7 juveniles, Sta. 4 (36° 59'S, 45°E), southern Indian Ocean, 0 3.02.2004, 23:32 hr., BN, sampling depth: surface waters.

Diagnosis. Acute rostral projection. Large eyes. Antennal scale 4–5.6 times as long as broad. Uropodal exopod bears 2–8 spines, Lateral margin of telson bears 14–31 spines. Apex of telson subtruncate or slightly arched.

Description. Carapace anteriorly produced into triangular narrow rostrum with pointed apex extending near middle of first antennular segment (Figs. 2 A, B); antero-lateral corners rounded. Eyes large with expanding cornea, nearly extending too distal end of first segment of antennular peduncle.

Antennular peduncle of male much broader than female, second segment short, with 2 long setae on inner dorsal margin. First segment narrow, as long as third; outer margin concave, with 2–3 long and 2 small setae on distal corner (Fig. 2 C). Antennular peduncle of female slender, first segment narrow, as long as succeeding 2 segments combined, outer margin concave, with 2 long and 1 short setae on dorsal corner; second segment short, with 1 long setae on inner distal corner and 2 short setae on outer distal corner; third segment with 1 long seta at distal 1/3 of inner margin and 3 long setae at inner distal corner (Fig. 2 D).

In male, antennal scale more slender than in female, 5.6 times as long as broad, extending to distal end of third segment of antennular peduncle, terminal lobe as long as broad; outer margin naked, terminating in spine; distal suture present (Fig. 2 E). Antennal scale of female extending near distal end of third segment of antennular peduncle, 5 times as long as broad; terminal lobe slightly shorter than broad; distal suture present (Fig. 2 F). Antennal peduncle in female more slender than in male (Figs. 2 E, F), extending little more than half of the antennal scale and in male extending to distal 1/3 of antennal scale; second segment longest, 3.6 times as long as third in male and four times as long as third in female.

Labrum with long frontal process; tip slightly curved (Fig. 2 G). Second segment of mandibular palp expanded, 1.6 times as long as third segment in male (Fig. 2 H) and 1.8 times as long (Fig. 2 I), inner margin with 3 short setae in male and one short seta in female; outer margin with 16 long setae in male (Fig. 2 H) and 13 setae on corresponding part in female; third segment with 7 long setae in male and 3 long setae in female (Fig. 2 I) on proximal 2/3 part of outer margin.

Mandible, maxillule, and maxilla are of type common for the genus (Figs. 2 J–L).

Endopod of first thoracic limb short, robust, merus longer than carpopropodus terminating in strong claw (Fig. 3 A). Endopod of second thoracic limb, short, robust, merus longer than carpopropodus, terminal claw long and stout. Endopod of fourth to seventh thoracic limbs subequal, slightly longer than third (Fig. 3 B), becoming more slender towards posterior limbs, carpopropodus not divided into subsegments; terminal claw moderately slender (Figs. 3 B, C). Endopod of eighth thoracic limb more narrow than preceding limbs, carpoprodus slightly shorter than merus; not divided into subsegments (Fig. 3 D).

First male pleopod; exopod 10-segmented, endopod absent (Fig. 3 E). Second pleopod; endopod 11 and exopod 12 segmented (Fig. 3 F). In third to fifth pleopods, both rami with 12, 13 and 11 segments, respectively, and without any modified setae (Figs. 3 G–I). Pseudobrachial rami nearly straight in first and fifth pleopods and spirally coiled in second to fourth pleopods.

Uropodal exopod and endopod overreaching telson (Fig. 3 J). Proximal segment of exopod armed with 7– 8 spines in adult male (Fig. 3 J) and 4–5 in adult female (Fig. 3 M). Uropodal endopod slightly longer than exopod, armed on inner ventral margin with 64–68 spines showing alternate arrangement of 1 longer and 1 to 5 shorter ones (Figs. 3 J, L).

Telson 1.5 times longer than last abdominal segment, 2.7 times as long as broad in male (Fig. 3 J) and 2.8 times longer than broad in female (Fig. 3 N), extending beyond articulation of uropodal exopod. Lateral margin of telson armed with 2 stout spines on basal widened part, followed by comparatively narrow naked part, then posterior series of 27–29 spines in male (Fig. 3 J) and 20–22 spines in female, the spines gradually increasing length posteriorly (Fig. 3 N). Distal margin of telson slightly arched, with 2 pairs of long spines, outer slightly longer than inner, 3 median small spines and pair of plumose setae (Figs. 3 K, O).

Body length: adult male 10.2–10.4 mm, adult female 8.7–9.7 mm.

Remarks. The present specimens agree with previous descriptions, except for minor variations as follows: In the present specimens lateral margins of the telson bears 22-31 spines, while in earlier descriptions only 14-20 spines are reported (Sars 1885; Ii 1964; Pillai 1973). The present specimens measure up to 10.4 mm, which is the maximum size observed.

Population differences of S. thompsonii have been observed in morphological characters (Ii 1964; Pillai 1973; Panampunnyil 2002). Tattersall (1962) reported that in many species of the genus Siriella, changes in form take place with growth even after the animal has attained sexual maturity. This fact is particularly noticeable in S. thompsonii.

Distribution. Widely distributed in the tropical Atlantic, Pacific and Indian Oceans.