Has the main habitat of Potamocaris species been overlooked until now? (Harpacticoida, Parastenocarididae)

Distribution and habitat preference of Potamocaris bidens and Forficatocaris sp. from a Brazilian river are discussed. The majority of the known species of both genera have been described from nearshore but emerged sands of South American rivers and lakes. Compared to Forficatocaris, the descriptions of the Potamocaris species were done from very few specimens. In this study, quantitative sampling surprisingly yielded up to about 1000 adults and copepodids of P. bidens per 70 cm3 of sand, whereas the abundance of Forficatocaris sp., the co-occurring species at the sampling location, was about 10 to 50 individuals per 70 cm3 of sand. P. bidens was restricted to samples from submerged sands, and Forficatocaris sp. was found in samples from emerged sands only. Observations of live animals revealed a special behaviour of P. bidens: when exposed to water currents or other mechanical disturbance, these animals quickly adhere to sand grains with the caudal rami and roll up their body ventrally until disturbance ceases. This can be viewed as a special adaption to prevent dislocation to unsuitable environmental conditions. The sampling data and live observations suggest that submerged sands exposed to water currents or wave action might represent the main habitat of Potamocaris species.