It is usually only the large whelks that
follow after Leptasterias polaris while it is eating, smaller whelks
are probably more vunerable. The violent movements of the shell which is
part of its defence are always seen amongst adults, but very seldom amongst
the young. This can be because defensive behaviour developes as the whelk
gets older or because those individuals that defend themselves survive to
became adults. The high rate of mortality amongst the young (shell length
= 3-5cm) would support this hypothesis. When the starfish is not eating
or in motion, whelks flee from this predator, whelks that have lived together
with starfish have learned when it is best to keep their distance and when
to approach. Snails that have not lived close to Leptasterias polaris
do not show this defensive behaviour.
During evolution, whelks that have drawn
closer to eating starfish have recieved relatively more food than those
that have avoided starfish and probably have greater luck when reproducing.
The whelk that is "brave and smart" has also spread its genes to a greater
extent than those that are "cowardly and stupid". They have adapted and
developed a defence against those predators that exist in their surroundings.
This development occurs quite rapidly as the whelk does not roam over large
areas and mix its genetic structure with individuals from areas with other
types of predators. The ability to adjust its behaviour according to the
presence of predators and food suggest that the whelks nervous system can
coordinate information to a greater extent than what is normally believed
to exist amongst other invertebrates.