Bioacoustics of Arcotheres tivelae

Remarks. The poorly described Pinnoteres (sic) placunae by Hornell & Southwell (1909) from Placuna (Placunidae) in Gujarat state in India and Pakistan has caused some uncertainty about its identity and, in addition the types are lost (cf. Schmitt et al. 1970). Similarities and suggestions that Pinnotheres tivelae Gordon, 1936 , and P. placunae are synonyms (Naderloo & Türkay 2012; Naderloo et al. 2013 ) may simply be due to the inaccuracies in the descriptions and figures of Hornell & Southwell (1909) . Naderloo (2017) later recognised the species of Gordon as a valid Arcotheres , but noted that no males of A. tivelae were available for more detailed comparisons. Trivedi et al. (2018) clarified the taxonomy of A. placunae and corrected the inaccuracies in the figures and description of Hornell & Southwell (1909) , confirming that the types are no longer extant and a neotype was selected from the original type locality in the Rann of Kutch (at present Gulf of Kachchh) in Gujarat state in India . Trivedi et al. (2018: 57) elaborated on the differences between the females of the two species, noting that in A. tivelae , the carapace is more subcircular, at 1.2 times wider than long (vs more transversely ovate and 1.4 times wider than long in A. placunae ; Trivedi et al. 2018 : figs. 1A, C, F, 2A); the posterior carapace margin is usually gently convex (vs strongly concave in A. placunae ; Trivedi et al. 2018 : figs. 1A, C, F, 2A); the MXP3 ischiomerus is 2.2 times as long as wide (vs 2.6 times as long as wide in A. placunae ; Trivedi et al. 2018 : figs. 2F, G); the female dactylus is about half the length of the cheliped palm in adults (vs dactylus slightly shorter than the palm in A. placunae ; Trivedi et al. 2018 : figs. 2D, E); the longer female P4 merus is 1.7 times longer than the P5 merus (vs 1.3 times in A. placunae ; Trivedi et al. 2018 : fig. 4C, D, G, H); the shorter P4 dactylus is almost twice the length of the P3 dactylus (vs 1.2 times in A. placunae ; Trivedi et al. 2018 : fig. 4B, C, F, G); the P5 dactylus is about 0.75 times longer than the shorter P4 dactylus (vs 0.84 times longer in A. placunae ; Trivedi et al. 2018 : fig. 4C, D, G, H); the dorsodistal half of the P5 dactylus is glabrous (vs covered with short setae in A. placunae ; Trivedi et al. 2018 : fig. 4D, H); the P4 and P5 propodi are lined with setae along the flexor margin (vs almost glabrous in A. placunae ; Trivedi et al. 2018 : fig. 4A–H); and there are no spinules on the P5 flexor margin (vs with 2 short rows of spinules present on the flexor margin; Trivedi et al. 2018 : fig. 4H’, L’). The present female specimens of A. tivelae confirm these differences (cf. Figs. 1A, D, G–I , 2A, D, E, H, I ). The males of A. tivelae and A. placunae are also distinct. The male carapace of A. tivelae is more circular in shape with the anterolateral margins strongly convex and the dorsal surface inflated; Fig. 3A, B (vs less circular in shape with the dorsal surface less convex and the anterolateral margins gently convex in A. placunae ; Trivedi et al. 2018 : figs. 1D, 3A), and the G1 has the distal part bent obliquely inwards towards the sternum; Figs. 3H, I , 4F–I (vs G1 gently curved inwards with a slender tapering distal part in A. placunae ; Ng & Ngo 2022 : fig. 9F, G). As noted by Ng & Ngo (2022) , the figures of the male pleon and G1 tip of A. placunae are somewhat inaccurate in Trivedi et al. (2018 : fig. 3B, D). Despite previous comparisons with A. placunae , the morphology of A. tivelae is actually most similar to A. exiguus ( Bürger, 1895 ) and A. rayi Ahyong & Ng, 2007 , two species that also inhabit venerid bivalves. Ng & Ahyong (2022) revised the taxonomy of these two species, showing that Pinnotheres winckworthi Gordon, 1936 , P. vicajii Chhapgar, 1957 , P. casta Antony & Kutyamma, 1971 , and P. obscuridentata Dai & Song, 1986 , were all junior synonyms of A. exiguus (see also Trivedi et al. 2020 ). Arcotheres exiguus has a wide distribution in the Indo-West Pacific, occurring from the western Indian Ocean to southern China , while A. rayi is known thus far only from the Philippines and Peninsular Malaysia ( Ng & Ahyong 2022 ). Females of A. tivelae can be distinguished from those of A. exiguus and A. rayi by the P4 dactylus being slightly shorter than that of P5 (vs slightly or much longer in A. exiguus and A. rayi ). The carapace of female A. exiguus and A. rayi (e.g., Ng & Ahyong 2022 : figs. 37A, C, 39A, D, 46A, 47A, C) is generally wider than that of A. tivelae ( Fig. 1A, D ); but as Ng & Ahyong (2022: 5) argued, carapace shape is not always reliable, and there are some specimens of A. exiguus that also have a more rounded carapace (e.g., Ng & Ahyong 2022 : fig. 38A, B). The carapace of A. tivelae is also more inflated and firmer than that of A. exiguus and A. rayi , which are flatter and more membranous; unfortunately, these aspects are difficult to quantify and best observed by direct comparison of specimens of each species. The inner margin of the ischiomerus of the MXP3 at the widest point is always rounded in all the specimens of A. tivelae examined ( Figs. 2A , 4A ) but in A. exiguus , it is usually angular ( Ng & Ahyong 2022 : figs. 41B, 42A, 45A) but this character is unreliable as in some specimens, it is also rounded ( Ng & Ahyong 2022 : figs. 41Q, 42L). In A. rayi , the angle of the inner angle of the ischiomerus is rounded ( Ng & Ahyong 2022 : figs. 49B, 52A); but it can easily be separated from A. tivelae by its proportionately shorter P5 dactylus ( Ng & Ahyong 2022 : figs. 48D, E, 49I, M). The most unambiguous character that separates the species is actually the form of the G1. Compared to A. exiguus and A. rayi , the G1 of A. tivelae is relatively less curved with the distal part gently tapering and without a subdistal projection; Figs. 3H, I , 4F–I (vs distinctly C-shaped in A. exiguus with a dorsal subdistal projection and the tip is rounded; Ng & Ahyong 2022 : figs. 44K, L, 45H, I, 52N, O). Arcotheres tivelae is also superficially similar to another congener also found in venerid bivalves, A. obesus ( Dana, 1852 ) , which is known only from Gafrarium from Fiji and Malaysia ( Ng & Ahyong 2022 ). The carapace of A. obesus , however, is soft and poorly chitinised, the dactylus of MXP3 is relatively longer, reaching the tip of the propodus, and P2–P5 are proportionately much longer; and in addition, the G1 has a low dorsal subdistal subtruncate angle before the sharp tip (cf. Komai et al. 2020 : figs. 2A–C, 3E–H, 4F, H, J, L, N; Ng & Ahyong 2022 : figs. 55A, B, 56B–I, K, L).