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.


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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson