Only a few individuals survive
Over 99% of the eggs that are produced became
food for about 30 embryos. Some of these 30 embryos, which grow more quickly
and eat their "siblings" result in only 3-10 individuals being
born when hatching takes place. Before hatching, which occurs after about
3-9 months, the larvae go through several shell-less stages within the capsule.
When the snail hatches from its egg, an approx. 3mm miniture replica of
the parent with a shell appears. The whelk grows in size continuously throughout
its life, but the growth rate decreases as the snail becames older and also
varies during the year. For example, it has a low rate of growth during
the summer when it is inactive and under stress when the water temperature
is relatively high. Variations in the growth rate give rise to denser and
sparse rings on the snails operculum, much in the same way as rings in a
tree trunk. It has been shown though, that these rings can be very vague
and therefore difficult to use when determining the age of a whelk. A whelk
can be between 10-15 years old.
Poorly distributed genes
The whelk becames sexually mature after 4-9
years when the snail is between 3-8cm in length. The male, when compared
with the female is smaller when it becames sexually mature. The differences
in size and age at sexual maturity can be great, even when compared to snails
from neighbouring areas. This is probably due to the fact that the mixing
of genes between the different areas is very limited. Young whelks that
hatch directly on the bottom are distributed over a limited area compared
to other specie that have a free swimming larva stage in their development.
As they get older and became sexually mature, they mate in roughly the same
areas as did their ancestors and therefore do not mix their genes with other
whelks that "live" further away.
Whelks within the same area adjust to local conditions and
therefore can be genetically different to whelks that live in an another
area. Snails that wander out of areas which they normally inhabit, can for
example meet predators which they have not developed a defence mechanism
against. One example of such a sitution is that whelks that have not lived
close to lobsters have thinner shells in comparison with those that have.
A thinner shell makes the whelk that has wandered into a foreign
area more vunerable to attack from a lobster. If whelks within a certain
area are exterminated, it can take a while before whelks became established
in the area again, therefore whelks are very sensitive to over-fishing.
If whelks are subjected to intensive fishing and/or predation, it would
be better for the survival of the species if they became sexually mature
at an earlier age. These genes have been widely spread and it is therefore
common with smaller shells when they reach sexual maturity.