ORIENTATED SWIMMING BY MEGALOPAE OF SEVERAL EASTERN NORTH PACIFIC CRAB SPECIES AND ITS POTENTIAL ROLE IN THEIR ONSHORE MIGRATION

The megalopal stage of most near shore and intertidal crabs must return to the coast to complete their development. Crab megalopae are strong swimmers and if they swam consistently shoreward they could conceivably swim back to shore. This hypothesis was tested for megalopae of Pachygrapsus crassipes (Randall), Lophopanopeus bellus bellus (Stimpson, 1860), Cancer oregonensis (Dana), and C. gracilis (Dana) by investigating their swimming orientation when housed in a transparent container with a view of the underwater illumination or swimming freely in the sea. In the transparent container, megalopae tended to swim in the direction of the sun's bearing. Free swimming megalopae swam straight courses and displayed significant preferred swimming directions. Pachygrapsus crassipes and L. bellus bellus megalopae swam at the sea surface and parallel to the current direction. Free swimming C. oregonensis and C. gracilis swam at about 3 to 5 m depth and in the direction of the sun's bearing. Megalopae of Pachygrapsus crassipes and C. oregonensis were observed on 4 and 3 days respectively and they did not preferentially swim in a shoreward direction. These results suggest that they do not migrate back to shore by swimming. The orientated swimming may, however, assist the megalopae in returning to shore or locating a settlement site. Swimming with the surface current might help megalopae to migrate shoreward in the convergence zone over internal waves or Langmuir circulation cells. Swimming in the direction of the sun's bearing might represent a search behavior for a benthic settlement site.