Siegius sp.

Kong, Chim Chee, 2024, A synopsis of the Tanaidacea (Crustacea: Peracarida) of Singapore, with a review of tanaidacean diversity in Southeast Asia and the South China Sea, Zootaxa 5451 (1), pp. 1-75 : 17-18

Siegius sp. SG#1

( Fig. 5 )

Material examined: Station SG69: 8 females (SUB-1193).

Remarks. The apseudid genus Bilobatus Sieg, 1993 was later replaced by Siegius Allsop, 2017 because it is a junior homonym of the scarab beetle genus Bilobatus Machatschke (see Allsopp 2017). The name “ Bilobatus ” refers to the bilobed lateral margins of pereonites 4–6, which is a character unique amongst apseudoidean genera ( Sieg 1993). Other characteristic features of Siegius include the rostrum bearing a short spine as well as the antennule peduncle article-1 bearing a large median spine and many denticles on its inner margin ( Bamber 1999). The bifid genital cone on pereonite-6 was initially considered an important taxonomic character of the genus (Guţu 2002) but its presence was later found to be inconsistent within the taxon (Guţu 2006). Siegius is currently represented by three species ( Anderson 2023), namely S. gallardoi ( Shiino, 1963) , S. djiboutiensis ( Băcescu, 1978b) and S. rostridentatus (Guţu, 2006) . Siegius gallardoi , the type species of the genus ( Allsopp 2017), was originally described from Vietnam ( Shiino 1963) but was subsequently found in many localities in the Indian and Pacific Oceans including Australia ( Sieg 1993; Bamber 2005), Brunei (( Bamber 1999), Hong Kong ( Bamber 2000) and Borneo, Malaysia ( Bamber & Sheader 2005). Siegius djiboutiensis also has a wide distribution, having been recorded from its type locality at Djibouti (Băcescu 1978b) as well as the Gulf of Thailand (Guţu & Angsupanich 2004b) and Brunei ( Bamber 2013b). Siegius rostridentatus is known only from its type locality at Queensland, Australia (Guţu 2006c).

The specimens from Singapore clearly belong to the genus Siegius based on the condition of the rostrum, antennule and the pereonites. Siegius sp. SG#1 is most similar to S. gallardoi as they share the following characters: (1) body length of about 4–6 cm; (2) cephalothorax with straight anterior margin; (3) antenna with nine outer and three inner flagellum articles; (4) pereopod-1 coxa spine with simple seta; (5) uropod basal article and exopod without plumose setae; (6) pleotelson much shorter than the combined length of pleonites, and with two proximal bulges, each with multiple setae on each side. However, Siegius sp. SG#1 differs from S. gallardoi by the (1) presence of ocular lobe apophysis; (2) more pronounced notches on pereonites 3–6; and (3) higher number of articles on the uropodal exopod. Siegius sp. SG#1 also has an epistomal spine, which was not mentioned in the three described Siegius species. Based on these morphological differences, Siegius sp. SG#1 is a potentially undescribed species. The specimens were collected only from station SG69 (see Fig. 2 ) in the Singapore Strait—a coral reef west of Pulau Pawai at 5–10 m depth, which is slightly shallower than the lower limit of the bathymetric range of 9–180 m recorded for the congeners.

Figures

FIGURE 2. Sampling stations in the West Johor (red), East Johor (green) and Singapore (blue) Straits, Singapore. Refer to Table 1 for details. This map was modified from nautical chart GSP1 ‘Anchorages and Aids to Navigation’ (Maritime Port Authority of Singapore 2012).

FIGURE 5. Siegius sp. SG#1, non-ovigerous female (SUB-1193), Singapore, dorsal habitus. Scale bar = 2 mm.