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The Seashore


Common Limpet (Patella vulgata)

Patella

Limpets are dome-shaped molluscs with no spiralling of the shell. Patella is the genus name for the most abundant group. They can live up to 16 years and may achieve a maximum diameter of 7 centimetres. The black lichen, Verrucaria mucosa, can often be seen growing on the shell.

These are very important seashore herbivores, feeding on microscopic algae which covers the rocks like a fine carpet. Due to the fact that they graze so efficiently they remove young seaweeds and may, therefore prevent the establishment of larger seaweeds, like bladderwrack. They feed by scraping the radula ("tongue") across the rock surface. The radula has teeth (toughened with iron) on it and acts like a file. Each sweep of the radula removes fine algae and leaves a grazing mark on the rock. Recolonisation by the algae soon takes place. Often the only algae to be found is that growing on the shell where they cannot reach.

Limpet radula marks on the rock Limpet radula marks on the rock

The attachment to rock is legendary. This adhesion to the surface is by the muscular foot and the secretion of a chemical. They can hold on to rock with a force of 75 lbs/sq.in. Clamping down at low tide will prevent drying out. As limpets settle down they rotate the shell and grind it into rock which produces a good fit but also, on death, leaves a scar on the rock surface. To breathe they remove oxygen from the water. This is drawn in to the gills via a hole above the head. When the tide goes out they have a problem with the lack of water. They clamp down and reduce their metabolism which in turn reduces the need for water. The shell has a high degree of waterproofing to conserve water.

The shell shape varies with wave action, time spent emersed and possibly food supply (Ballantine, 2005, pers.comm.). A limpet whose muscles are tensed as shell is secreted produces a domed result, relaxed limpets produce flatter shells. Well fed limpets, whose tissue pushes out to the shell margin, produce flatter shells than hungry limpets. Increased wave action and/or time spent emersed should produce taller limpets but the pattern (especially on exposed shores) is complicated by local topography. The sexes are separate but as in many molluscs they often begin life as males changing to a female as they get older. For the first year of P. vulgata's life they are all neuter, then change into males. After 4-7 years approximately 34% change into females. Gametes fertilise externally in the seawater. The release of gametes from the adults is triggered by a lowering of the temperature in autumn, with 11 degrees C being the critical value. This, coupled with rough weather, stimulates the realise. The eggs hatch into planktonic larvae which live in the plankton for barely 2 weeks before settling out on to the shore. All this sex stuff means that they are protandrous hermaphrodites.

Limpets with two scars left on the rocks

Limpets with two scars left on the rocks
Limpets are very susceptible to oil pollution and their death results in a marked increase in the growth of weed. In the period following a spill of oil there is a "green flush" caused by the unlimited growth of green algae such as Enteromorpha and Ulva.

Punk Limpet:

Punk Limpet: limpet in a rockpool with Enteromorpha growing on it

A very abundant mollusc across the entire tidal region of a rocky seashore, not being limited to anyone zone. This is because it can avoid as well as tolerate most of the problems of the seashore. It can be found throughout most of the Atlantic coast although rarer in the south and extreme north.

Here is an exciting and rare photo:

Spawning Limpet

When the temperature dips below 11 degrees in October it stimulates the male limpets to release sperm into the water. The photo shows a limpet in a rock pool lifting up its shell (pale foot bottom right) and squirting sperm out into the pool. We know it is a male as sperm is white and eggs are pale green in the limpet. Fertilisation is external, with the temperature also triggering the female to release the eggs. In this way the precise temperature change has synchronised the release of gametes.

See also Key-hole Limpet


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