Bioacoustics of Arcotheres ocularius

Remarks. Arcotheres ocularius n. sp. is most similar to A. arcophilus and a group of four nominal species closely allied to A. palaensis . This group includes A. rhombifer ( Bürger, 1895 ) , A. guinotae Campos, 2001 , and two species currently in Pinnotheres but clearly also referable to Arcotheres , A. onychodactylus ( Tesch, 1918 ) comb. nov. and A. paralatissimus ( Dai & Song, 1986 ) comb. nov. (PKLN & S.T. Ahyong, unpublished data). All differ from A. ocularius in the same features as discussed for A. palaensis , below. In addition, Japanese specimens identified as “ Arcotheres alcocki ” by Takeda & Konishi (1988) actually represent A. palaensis after re-examination of the material (NSMT-Cr 9363). Additional specimens of this taxon from Japan were also obtained for comparisons (see Material and methods). In A. ocularius n. sp. , the carapace is distinctly more strongly chitinised and calcified, with the cuticle stiff and relatively rigid (versus cuticle distinctly less chitinised and softer, even in fresh specimens of A. palaensis ); eyes are clearly visible in the dorsal view in A. ocularius n. sp. ( Fig. 7 A–C, 8A) (versus eyes are not visible or occasionally, only the edge of the eye is visible in A. palaensis ; Ahyong & Ng 2007 : fig. 9A; PKLN & S.T. Ahyong, unpublished data); the maxilliped 3 dactylus reaches to or extends beyond the tip of the propodus in A. ocularius n. sp. ( Fig. 8C, F, G , 16A ) (versus the maxilliped 3 dactylus is always shorter, never reaching the tip of the propodus in A. palaensis ; cf. Ahyong & Ng 2007 : fig. 9C); the female P4 merus and propodus are proportionately longer in A. ocularius n. sp. ( Fig. 9L ) (versus proportionately shorter in A. palaensis ; Ahyong & Ng 2007 : fig. 9A); the male telson is subpentagonal or subhexagonal in A. ocularius n. sp. ( Figs. 13H , 15F ) (versus semicircular in A. palaensis ; cf. Takeda & Konishi 1988 : fig. 3C, as P. alcocki ); and the G1 has the tip subtruncate ( Figs. 13I, J , 15G, H ) (versus the tip of the G1 has an elongated slender projection in A. palaensis ; cf. Takeda & Konishi 1988 : fig. 3D, E, as P. alcocki ). Arcotheres ocularius n. sp. and A. palaensis are both associated with Anadara spp., but A. palaensis is also reliably known from Barbatia species (also Arcidae ) ( Takeda & Konishi 1988 ; Campos 2001; Watanabe & Henmi 2008 ; Watanabe 2013 ; PKLN & S.T. Ahyong, unpublished data). Schmitt et al . (1973) also listed Mactra (Mactridae) and Placuna (Placunidae) as host bivalves for A. palaensis but these records are probably incorrect and the pinnotherid species was probably misidentified. For Placuna from Singapore , at least, Ng (2018) showed that the species present is A. placunicola Ng, 2018 . Compared to A. arcophilus , in which the eyes are also distinctly visible in the dorsal view, in A. ocularius n. sp. the maxilliped 3 dactylus reaches to or extends beyond the tip of the propodus ( Figs. 8C, F, G , 16A ) (versus never reaching the tip of the propodus in A. arcophilus ; cf. Ahyong & Ng 2007 : fig. 2C), the female P4 merus is proportionately longer in A. ocularius n. sp. ( Fig. 9L ) (versus proportionately shorter in A. arcophilus ; cf. Ahyong & Ng 2007 : fig. 2A), the male telson is subpentagonal or subhexagonal in A. ocularius n. sp. ( Figs. 13H , 15F ) (versus semicircular in A. arcophilus ; Ahyong & Ng 2007 : fig. 2E), and the G1 is C-shaped with the tip subtruncate in A. ocularius n. sp. ( Figs. 13I, J , 15G, H ) (versus G1 more slender and elongate, being only broadly arcuate with the tip possessing an elongate projection in A. arcophilus ; PKLN & S.T. Ahyong, unpublished data). In an ongoing genetic study of Asian pinnotherids, the Fijian and present Lombok specimens cluster closely with each other, distinct from what is here defined as A. palaensis s. str. on the basis of the 16S mitochondrial gene (Tsang Ling-Ming, pers. comm.). Lombok actually has both species, with another Lombok female specimen (ZRC 2018.762) clustering with A. palaensis s. str. instead.