In situ observations of Dungeness crab megalopae used to estimate transport distances by internal waves

Abstract

Larvae of many coastal organisms develop offshore and must migrate to shore to set- tle. Two mechanisms of migration are proposed for crustacean postlarvae: onshore swimming and transport by internal waves. In the Northeast Pacific, to swim ashore organisms must swim east. Additionally, internal wave transport distance increases when organisms swim in the direction of wave propagation. Thus, larval behavior may strongly influence onshore migration, though few studies have examined larval behavior in situ. Megalopae of the Dungeness crab Metacarcinus magister (formerly Cancer magister) were observed in situ to determine if they orient the direction of their swimming and to determine swimming speed over ground. They were not oriented to the east, rather they swam at the surface in the direction of the current at speeds of ~9.8 cm s −1 which previous work suggests should increase transport distance by internal waves. We tested the influ- ence of these behaviors on transport by deploying a thermistor mooring for 24 d and calculating transport distances for all observed waves. We calculated transport potential for both passive par- ticles and particles that swam with the surface current at speeds ranging from 0 to 10 cm s −1 . Of the 69 waves observed none would have transported passive particles, but as swimming speed of organisms increased the number of transporting waves increased dramatically. At swimming speeds of 10 cm s −1 , all waves would have transported larvae. Megalopae of M. magister do not migrate ashore by swimming east rather they swim with surface currents, which may allow them to better exploit internal waves as a transport mechanism.