Actenos 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 : 25-26

Actenos sp. SG#1

( Figs. 11A–B )

Material examined. Station SG13: 6 incomplete and damaged specimens (002). Station SG22: 1 incomplete female with developing embryos (CR0513-P02-02), 8 May 2013; 1 incomplete specimen (CR1113-P02-01), 13

November 2013; 2 incomplete specimens, including 1 with missing posterior (CR0514-P02-03-03), 7 May 2014; 9 specimens including 8 ovigerous females (CR1114-P02-02-04), 11 November 2014; 1 incomplete female with oostegites (CR0717-P02-01), 19 July 2017; 1 male (CR1118-P02-01-01), 14 November 2018; 1 incomplete specimen (CR1219-P02-01-03) and 1 male (CR1219-P02-03-02), 4 December 2019. Station SG23: 1 incomplete female with empty brood pouch (CR0511-P06-02), 10 May 2011; 1 incomplete female with oostegites (CR1112-P06-02- 01), 6 November 2012; 1 incomplete female with empty brood pouch (CR1113-P06-03-03), 13 November 2013; 1 incomplete specimen (CR0514-P06-03), 8 May 2014; 1 incomplete specimen (CR1219-P06-01), 6 December 2019.

Remarks. Bamber (2013b) erected the genus Actenos and designated A. marshalli Bamber, 2013b from Brunei as the type species. This genus also comprises A. tenuicorporeus ( Shiino, 1963) and A. kalimantus Bamber, 2013b from Vietnam and Brunei, respectively ( Anderson 2023). Actenos is characterised by the (1) dorsally curved epistomal spine; (2) posterolateral expansion of the pleonites where the pleopods attach; (3) fused and bifurcate claws on the anterior pereopods; (4) absence of spiniform dorsal setae on the propodus of pereopod-1; and (5) lack of a ventrodistal row of spines on the propodus of pereopod-6 ( Bamber 2013b). All three Actenos species were recorded only from the shallow subtidal (i.e., 7.5–90 m) sandy habitats in Brunei and Vietnam.

These specimens from Singapore are most similar to A. tenuicorporeus , especially in terms of the slender body, but they differ from A. tenuicorporeus by the presence of (1) a large and several smaller apophyses on the ocular lobe; (2) a two-prong apophysis on antenna peduncle article-2; and (3) a larger number of articles on the uropodal endopod. Based on these morphological differences, Actenos sp. SG#1 is a potentially undescribed species. Actenos sp. SG#1 reaches up to about 7.5 mm in body length and is among the largest tanaids in Singaporean waters. This species was collected only from three stations, SG13, SG22 and SG23 (see Fig. 2 ), all located on shallow subtidal sandy silt bottoms along the eastern Singapore Strait at 20–54 m depth.

Genus Biropalostoma Guţu & Angsupanich, 2004

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 11. Actenos sp. SG#1, Singapore: (A) adult male, CR1118-P02-01-01, lateral habitus; (B) female with oostegites, CR0717-P02-01, dorsal habitus, with inset showing anterolateral spines (arrows) on pereonites 2–6. Scale bar = 2 mm.