Bioacoustics of Gopkittisak ngankeeae

Remarks. Gopkittisak ngankeeae n. sp. possesses two diagnostic gaeticine characters, which are the presence of long setae on the Mxp3 palp and the fused male pleonites 3–5 ( Davie & Ng 2007 ). Gopkittisak ngankeeae n. sp. does not possess a longitudinal sternal sulcus to accommodate the Mxp3 filtering setae, but instead has an anterior sternal depression for this purpose (visible in Fig. 9D ); this character is also present in G. gallardoi and G. angustum ( Naruse & Clark 2009 ; Komai 2011 ). All three species of Gopkittisak have a laterally ovate carapace, with a posterior, transverse ridge, and share three important features on Mxp3: (1) presence of deep, longitudinal depressions on the ischium and merus (not illustrated by Serène & Soh 1976 : fig. 28A); (2) oblique articulation between the ischium and merus; and 3) carpus proximally articulating along lateral half of the distal margin of the merus ( Figs. 9A, C , 10C ; Serène & Soh 1976 : fig. 28B, pl. VIII, fig. A; Ghani & Tirmizi 1991 : fig. 1C; Naruse & Clark 2009 : figs. 1a, c, 3; Komai 2011 : figs. 1, 2A, E, 5A). Importantly, the first two of the above-listed Mxp3 features are also present in Brankocleistostoma fossulum ( Fig. 6C ; Ng 2012 : fig. 3D, E). Gopkittisak ngankeeae n. sp. can be separated from its two congeners by the outline of the carapace, in which the convex anterolateral margin meets the posterolateral margin at a wide angle ( Fig. 9A ). In contrast, the lateral margins of the carapace are subparallel and less divergent posteriorly in G. gallardoi and G. angustum ( Serène & Soh 1976 : fig. 28B, pl. VII, fig. A; Ghani & Tirmizi 1991 : fig. 1A; Naruse & Clark 2009 : figs. 1A, 3; Komai 2011 : figs. 1, 2A). Male specimens of G. ngankeeae n. sp. are also distinguished by the presence of a flange lateral to the orbits, followed by several lobes, on the suborbital crista ( vs. without such flange in G. gallardoi and G. angustum ) ( Fig. 9B, C ; Naruse & Clark 2009 : fig. 1b; Komai, 2011 : fig. 2D). Additionally, males of G. ngankeeae n. sp. have a keeled lobe located proximally on the anterior margin of the glabrous cheliped merus ( vs . absent in G. gallardoi and G. angustum ; Fig. 9A, B ; proximal part of merus not illustrated in Naruse & Clark 2009 and Komai 2011 , but this was confirmed by our examination of comparative material of the two species). The flange on the suborbital crista and a glabrous cheliped merus with a keeled lobe on the anterior margin may be involved in stridulation. In addition, the P3 and P4 propodi are glabrous or have a row of relatively few, short setae in G. ngankeeae n. sp. ( vs . setose in G. gallardoi and G. angustum ; Fig. 9A ; Komai 2011 : figs. 3E–G, 5B, C). This character, however, should be used with caution, as setation may be lost due to abrasion or vary with age, and the degree of setation in G. ngankeeae n. sp. sometimes approaches that of G. angustum . Whereas G. ngankeeae n. sp. is readily distinguishable from its two congeners by the aforementioned characters, G. angustum and G. gallardoi are remarkably similar and differentiated based on characters that can vary among conspecifics, for example, cw to cl ratio due to age and setation due to abrasion. Colour patterns generally appear to be variable within each species of Gopkittisak ( Figs. 11 , 12 ) and therefore have rather limited value as an interspecific diagnostic character (but see above). The type material of G. gallardoi , deposited in the Phuket Marine Biological Center, Thailand , was unavailable for this study, although topotypic material collected from Phuket in 2018 (ZRC 2008.1053) was examined and compared with the material of G. angustum from Japan and the Philippines (see above). Examination of more material of these two species, as well as molecular analyses, will eventually clarify the geographical distributions and species limits of G. gallardoi and G. angustum and their relationship with G. ngankeeae n. sp.