The simplest explanation of the aggregation behaviour of O. struma
during its reproductive season is that the animal endeavours to
enhance its reproductive rate at the minimum cost to biochemical
expenditure. Onchidium cannot be easily found on sunny days even
though the temperature is appropriate. It begins to exit its hole and
feed when the light intensity is below 25 lux, and it will spread its
body and become more active with the declining intensity of light. The
optimal time for watching is from dusk to night. The sea slugs shows
aggregated spatial dispersion during the daytimes of their prereproductive
and reproductive periods, which appears to be closely
related to propagated requirement. O. verruculatum has been
observed feeding at night (McFarlane, 1979), which is very similar
to our results in this experiment.