Planes on a Snake? On the Identities of Crab Larvae Rafting on Sea Snakes

Rafting on surface-drifting flotsam or pelagic animals may represent an important behavioral strategy for larval transport and recruitment among brachyuran crabs. For some, like crabs of the genus Planes (Grapsidae), rafting has even become an adult lifestyle. After a 2012 study reported high numbers of unidentified grapsid megalopae rafting on pelagic sea snakes (Hydrophis platurus) off Pacific Costa Rica, we hypothesized their identity to be Planes minutus, which are commonly found rafting on flotsam and sea turtles in the area. To test the "Planes on a snake" hypothesis, we sequenced barcoding genes (CO1 and/or 16S) for 106 individual megalopae collected from sea snakes and compared with those of known species in the region. Three distinct species were detected, and all were in the predominately inter- to supratidal family Grapsidae: Pachygrapsus socius, Goniopsis pulchra, and Grapsus grapsus. No Planes were detected. While the "Planes on a snake" hypothesis was refuted, our results suggest that larval rafting may be a successful strategy for recruitment in inter- and supratidal grapsid crabs, but not other inter- to supratidal crabs nor Planes. The prevalence of larval rafting only in grapsid crabs suggests that the behavior may have provided the necessary precursor for the evolution of an obligate rafting species, like Planes.