Acetes indicus H. Milne Edwards, 1830

Alamri, Nasser & Alotaiby, Faraj, 2024, Taxonomic Assessment of Acetes indicus H. Milne Edwards, 1830 (Crustacea, Decapoda, Sergestoidea) as Revealed from Molecular and Morphological Analyses: Re-validation of A. spiniger Hansen, 1919 and Designation of a New Species, Bulletin of the National Museum of Nature and Science. Series A, Zoology 50 (2), pp. 49-68 : 57-60

Acetes indicus H. Milne Edwards, 1830

( Fig. 4 )

Acetes indicus H. Milne Edwards, 1830: 351 , pl. 11, figs 1–9.

? Acetes indicus : Kemp, 1917: 47, fig. 2a (in part)

Material examined. Bangladesh. Chiringa (= Chakari): Cox`s Market Bazar , 8 males (cl ca 3.5–5.0 mm), 19 females (cl ca 4.5–8.0 mm), 19 Aug 2018, coll. Kanak (formalin to 70% ethanol) (NSMT-Cr 31611); 5 males (cl ca 4.0– 4.5 mm), 29 females (cl ca 4.5–8.0 mm), data same as precedent (ethanol) (NSMT-Cr 31612); 2 specimens used for DNA analysis, data same as previous (NSMT-Cr 31613, 31614): Sitakunda Fish Market, Kumira-Sandwip Ship Ghat: 9 females (cl 6.0– 6.5 mm), 22°30.157′N, 91°41.324′E, Dec 2018, coll. Md Rakel-Ul Islam (NSMT-Cr 31615); 35 males (cl 4.3–5.6 mm), 37 females (cl 4.0–7.0 mm), Feb 2019, other data same as precedent (NSMT-Cr 31616); 3 specimens used for DNA analysis, data same as precedent (NSMT-Cr 31617–31619): 25 males (cl 4.6–6.0 mm), 9 females (cl 6.0– 7.6 mm), commercial catch, Feb 2019, coll. Md Rakel-UI Islam (NSMT-Cr 31620).

Description. Male. Rostrum ( Fig. 4a, h ) typical of Asian members of genus in having sub-triangular plate raised slightly from dorsal margin of carapace, having sharply pointed terminal tooth and small anterior tooth in addition to somewhat larger posterior tooth on its dorsal margin. Carapace smooth on surface, except for post-orbital and hepatic teeth.

Abdomen with first somite possessing procurved sharp tooth present at median part of ventral sternite between right and left pleopods. Sixth somite ( Fig. 4b ) twice length of fifth, postero-dorsal part ending in small postero-median tooth, no red spot on postero-ventral part. Telson ( Fig. 4b, c ) 0.7 times as long as sixth abdominal somite, with narrowly rounded or sub-triangular distal end.

Eye ( Fig. 4a ) with well pigmented cornea, latter wider than eye-stalk.

Antennule noticeably elongate in mature males, particularly in third segment of peduncle, 0.81–0.97 times as long as combined length of first and second segments; lower flagellum ( Fig. 4a, d ) modified into clasping organ, commonly composed of 12 segments, infrequently 13 or 14 segments; first segment of trunk rather long, noticeably narrowing at mid-length, three times or more as long as its narrowest part, forming distinct oblique conjunction with second one; major clasping spine moderately stout, reaching mid-length of fourth segment of main branch, bearing 2 incompletely aligned rows of minute serrated denticles, and also microscopically small clasping spine near base of major one; first segment of main branch with single basal setal-spine, and with or without single distal setal-spine, second and third segments each with single distal and/or sub-distal setal-spine, fourth segment 1.0–1.5 times as long as combined length of second and third segments, bearing usually 9 or 10 setal-spines on its entire margin, fifth segment with 1–3 setal spines.

Antennal scale ( Fig. 4a ) 0.75–0.80 times as long as carapace, slightly but distinctly extending beyond anterior margin of second segment of antennular peduncle.

External maxilliped and pereopods varying in length depending on body size. Third maxilliped in larger males extending as far as anterior end of antennular peduncle, distal segment three-fourths times as long as penultimate. First pereopod in larger males extending as far as cornea. Second pereopod in larger males reaching as far as anterior end of antennal scale. Third pereopod ( Fig. 4e ) in larger males extending anterior end of antennular peduncle, distinct sharp tooth present at disto-mesial part of basis and anteriorly curving projection on inner margin of coxa. Genital coxa ( Fig. 4e ) often with slender or sub-acute distal part in lateral aspect, while sub-rectangular or obtusely rounded in ventral aspect, with truncate or widely sinuous anterior part.

Petasma ( Fig. 4f, g ) with blunt and distally rounded capitulum bearing numerous hooks along lateral surfaces as well as distal part; processus ventralis slender, extending as far as distal margin of capitulum; no trace of pars astringen. Appendix masculina with 2 or 3 short hooks distally.

Endopod ( Fig. 4b ) of uropod with 1–6 irregularly-shaped red-spots in fresh materials: exopod with small tooth on lateral margin, demarcating non-ciliated and ciliated parts, latter about 0.6 times as long as former.

Female. Usually larger than males.

Antennule ( Fig. 4h ) not elongated as in males, first segment longest, and third segments 0.50– 0.55 times as long as combined length of first and second segments; no examined females with intact lower flagellum.

Antennal scale ( Fig. 4h ) 0.71–0.74 times as long as carapace, extending to mid-length to distal one-third of third segment of antennular peduncle.

Third and fourth thoracic sternite having a somewhat shallow and wide longitudinal groove, not conspicuously narrowing anteriorly, median swelling low and not pronounced ( Fig. 4i, j ).

Body size. Largest male recorded from Bangladesh cl 5.6 mm, female 8.0 mm.

Type locality. Ganges Delta.

Remarks. As mentioned above, our study evidently showed that the current concept of Acetes indicus is a species complex, which may have derived a consequence of cryptic speciation from a common ancestor, leading to the existence of three extant species (see Figs 1 , 2 ; Tables 2 , 3 ). Among them, the population collected from the coastal waters of Bangladesh was regarded the representative of A. indicus .

Besides, Pathan and Jalihal (1997) mentioned that the materials reported by Kemp (1917) contains both the large and small forms. We suspect that his materials may have included some specimens of the typical A. indicu s because the illustration of the male lower antennular flagellum ( Kemp 1917, fig. 2a) seems similar to this species rather than its South-East Asian congener.

Unfortunately, the sampling sites in this study were restricted to the eastern extremity of the Ganges Delta. Thus, a broad area of the northern coasts of the Gulf of Bengal was left un-surveyed, and future studies are strongly in need to clarify the nature of this species group in that region. The designation of a neotype, therefore, was not recommended until more detailed information are available from the region.

Distribution. Specimens attributable to Acetes indicus were found only from the coastal waters of Bangladesh ( Fig. 3 ).

Figures

Fig. 4. Acetes indicus H. Milne Edwards, 1830. (a–g) male (cl 5.6mm) from Kumia-Sandwip Ship Ghat Bangladesh, and (h–j) female (cl 7.3mm) from same locality (NSMT-Cr 31616): a, anterior part of cephalothorax, lateral; b, posterior part of abdomen, lateral; c, posterior part of telson, dorsal; d, left lower antennular flagel- lum; e, basal segments of third pereopod and genital coxa, ventral; f, right petasma, posterior; g, same, capitulum; h, anterior part of cephalothorax, lateral; i, basal segments of third pereopod and third and fourth abdominal sternites, ventral; j, lateral profile showing channels and sternites between third and fourth thoracic somites.Abbreviations: gc, genital coxa; iiist and ivst, third and fourth thoracic sternites, respectively.

Fig. 1. Bayesian inference trees of Acetes indicus, A. omorii sp. nov., and A. spiniger with the outgroups of other congeneric species based on the mitochondrial 16S (a) and mitochondrial COI (b) markers. Values at the nodes are Bayesian posterior probabilities/Maximum Likelihood/Neighbor Joining support values. For the haplotype abbreviations, see Table 1.

Fig. 2. Haplotype network (implemented in PopART) of the Acetes indicus, A. omorii sp. nov., and A. spiniger based on the mitochondrial 16S (a) and mitochondrial COI (b) markers using minimum spanning network (MSN) analysis with outgroups of congeneric species. Circles with different colours represent different species.

Fig. 3. Map showing sampling localities and distribution ranges of Acetes indicus, A. omorii sp. nov. and A. spiniger recognised in this study with certainty: blue solid circles (nos. 1–2) for A. indicus, green solid squares and open square (museum vouchered specimens) for A. spiniger, and red solid triangles for A. omorii sp. nov. Numerals indicating sampling locations: 1, Chiringa; 2, Kumira-Sandwip Ship Ghat; 3, Merbok Mangrove; 4; Matang Mangrove, 5; Kuala Selangor; 6, Malacca; 7, Samut Prakan; 8, Setarap Estuary, South Kalimantan; and 9, Goa.