Clams of the species Donax variabilis migrate shoreward during rising
tides and seaward during falling tides. These clams spend most of the
time in the sand, emerging several times per tidal cycle to ride waves
. Migration is not merely a passive result of waves eroding clams out
of the sand; rather clams actively jump out of the sand and ride speci
fic waves. Such active migration is experimentally demonstrated during
a falling tide by comparing the motion of dead and live clams; live c
lams emerge from the sand and move seaward even when dead ones do not.
As low tide approaches, live clams become progressively less active.
They cease migrating for 2 hours around low tide and resume jumping to
migrate shoreward after the tide has turned. During the rising tide,
far from being passive, the clams jump out to ride only the largest 20
% of waves. Specifically, they choose swash that have the largest excu
rsion, i.e., those swash that move furthest on the beach.