New features of Peristediidae fishes in the South China Sea

Deep-diving surveys enable observations of new features of behavioural and functional dimensions of the fish that are typically missing from specimen-based taxonomy. This study documents three rare armoured searobins (Peristediidae: Scalicus engyceros, Paraheminodus murrayi, Peristedion liorhynchus) in Xianbei Seamount, Zhongsha Islands and Shenhu Canyon in the South China Sea. Not seen in the dead specimen, the fish in life exhibits bizarre shrimp-like (in common sense) morphology: the lip barbels formed a laterally rake-like structure, and the pectoral fins functioned as legs that literally 'walked' on the sediment. The fish S. engyceros showed both sideways and backward walking. The paired pectoral fins extrude outwards laterally like flat plates, supporting body balance during walking and swimming. When disturbed, individuals escaped rapidly using both pectoral appendages and tail fins, combining shrimp-like jerky movements with typical fish swimming. The same species have been reported to occur in both the North and South Pacific oceans, suggesting that their oceanic distribution is by larval dispersal via oceanic currents such as equatorial and Indonesian throughflow currents. The co-occurrence of three congeners in the same sea may hint local geographic speciation at work after settlement in a seamount habitat.