Al. Shanks, ORIENTATED SWIMMING BY MEGALOPAE OF SEVERAL EASTERN NORTH PACIFIC CRAB SPECIES AND ITS POTENTIAL ROLE IN THEIR ONSHORE MIGRATION, Journal of experimental marine biology and ecology, 186(1), 1995, pp. 1-16
The megalopal stage of most near shore and intertidal crabs must retur
n to the coast to complete their development. Crab megalopae are stron
g swimmers and if they swam consistently shoreward they could conceiva
bly swim back to shore. This hypothesis was tested for megalopae of Pa
chygrapsus crassipes (Randall), Lophopanopeus bellus bellus (Stimpson,
1860), Cancer oregonensis (Dana), and C. gracilis (Dana) by investiga
ting 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 dir
ection of the sun's bearing. Free swimming megalopae swam straight cou
rses and displayed significant preferred swimming directions. Pachygra
psus crassipes and L. bellus bellus megalopae swam at the sea surface
and parallel to the current direction. Free swimming C. oregonensis an
d 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 preferent
ially swim in a shoreward direction. These results suggest that they d
o not migrate back to shore by swimming. The orientated swimming may,
however, assist the megalopae in returning to shore or locating a sett
lement site. Swimming with the surface current might help megalopae to
migrate shoreward in the convergence zone over internal waves or Lang
muir circulation cells. Swimming in the direction of the sun's bearing
might represent a search behavior for a benthic settlement site.