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The Seashore

Dogwhelk (Nucella lapillus)

A Dog Whelk

The dogwhelk can be separated from other seashore snails by the distinct groove along the shell lip. There is a wide variation in shell size, shape, thickness and even colour although they are often a pale cream. Sometimes they have bands of blue-black. They grow up to 3-4 cm.

Dogwhelk colour variation

Dogwhelk colour variation

Dogwhelks are carnivores, eating barnacles, limpets and mussels. The radula modified for shell boring is assisted by chemical means. The groove in the shell lip allows water in for breathing as this boring takes many hours: e.g. 48 hours for a barnacle. They are very adaptable feeders, changing diet according to availability. Hundreds are laid in capsules although on 6% are fertile. Only these give rise to mini-adults whose first meal is the unfertile eggs in their capsule. The young migrate to the lower shore to feed, moving up as they mature.
They are found on all rocky shores with a wide variation in shell size, shape and thickness. For more information on shell variation consult John Crothers paper in Field Studies (1974) Volume 4. This forms the basis of many student projects and the effect of wave action on shell morphology. Essentially, by measuring the aperture (operculum), where the animal's foot emerges to hold on to the substrate, and comparing this with the spire height of the shell as a ratio the aperture is, typically, much wider in exposed conditions. The theory being that it requires a stronger muscular foot to hang on. This can also be tested by using a Newton Meter (spring balance) to pull them from the substrate and measuring the force required. It has little effect removing them in this way but do return them to the rock afterwards. There may also be a reduction in the length of shell, with a rounding off under exposed conditions. They are much more "pointy" on sheltered shores. Note that a thicker shell gives protection from crabs on sheltered shores.

Laying eggs

Dogwhelk numbers were decimated in the 1970's as a side effect of tributyltin (TBT). TBT is a highly toxic chemical found in antifouling paint used on boats. With the increase in marinas around Europe there has been a steady leaching of TBT into the waters. It makes female dog-whelks develop a penis and sperm duct, a condition called imposex. Female dogwhelks begin to develop male sex organs when exposed to levels of TBT below 1 part per trillion. When levels reach 5 parts per trillion the male organs are so large that they block the vulva. The females are unable to release their eggs for fertilisation, and the population is doomed. Imposex in the dogwhelk is a sensitive indicator of pollution by TBT although it is now banned from being used in antifouling paints on small craft. "Dumpton's syndrome" is the name given to a dogwhelk mutation found near to Dumpton, Ramsgate UK in the early 1990's. It causes a reduction in penis size. The population of dogwhelks in the area had almost disappeared because of the TBT coming from the ships in the area. This mutation appears to protect the female from imposex and has now started to reverse the trend and the population is on the mend. However, with no planktonic larva the dispersal of the mutation will be slow.

For students working in the Milford Haven area, a recent survey showed that almost 50% were of the Dumpton's Gap form. Recovery here has been amazing with a steady increasing density along the enclosed shores of the Haven. The more sheltered shores north of Dale towards the Gann have still not recovered whereas recovery, generally around Dale, is spectacular.

Dogwhelks are widely distributed from the far north of Scandinavia to the Atlantic coast of Spain. Mainly rocky seashores in the middle and lower shore.

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