Pselactus spadix tunnels wood in the mid intertidal to high intertidal zones and is submersed for several hours with each tide. The submergence and subsequent recovery behaviour, together with sub elytral morphology of P. spadix was investigated and compared to a terrestrial wood borer of the same subfamily Euophryum confine. An investigation of adult activity following submersion in seawater showed an akineses developed within 28 and 11 minutes in P. spadix and E. confine respectively. Following submersion for varying periods (0-82hrs), P. spadix adults were always found to recover more quickly than E. confine, while mortality of E. confine was significantly higher than that of P. spadix, with death points recorded as 6 and 98 hours respectively. The sub elytral morphology of P. spadix was similar to that of marine beetles. Folds in the dorsal body surface, a high density of setae on the abdominal tergites and the posterior underside of the elytra, together with spiracles opening into a sub elytral cavity indicate a plastron function in P. spadix. In contrast, E confine did not show any morphological adaptations towards plastron development. Behavioural and morphological adaptations indicate P. spadix is adapted to inhabit the intertidal zone.

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