Limpets attached to rocks are a familiar sight around our coastline. Their shells are exposed to the relentless pull of the tides. But the animals inside clamp themselves to the rock surface, safely secured against the stormy seas.
Limpets are algae-eating sea snails and belong to a major group of invertebrates, the molluscs. The strongly constructed, cone-shaped shell fits over the whole animal, protecting its soft body from seabird attacks, and alleviating the pressure of pounding waves as they break over the rocks.
Life on the exposed shoreline is hazardous for limpets, particularly because of the tidal cycle which makes shore dwellers vulnerable to sharply contrasting conditions. When the tide is in, limpets are immersed in seawater at fairly constant temperatures.
The tide is out they are exposed to desiccation and rapid temperature changes, as well as the occasional shower of rain. To combat this, limpets can clamp their shells hard against the rocks, sealing themselves off from the hostile elements. This also restricts water loss and invasion by freshwater until the tide returns.
Inside the shell
The body of a limpet is made up of three basic parts. The abdominal (visceral) mass, which houses the digestive, reproductive and excretory organs and is covered by a special area of skin called the mantle. The moderately developed head bears the mouth and sensory organs, hence the foot is located below the abdominal mass.
Each limpet has a flat, well-developed, sucker-like foot which plays two important roles. First, it is a powerful attachment organ. Many limpets live on exposed shores where, in stormy weather, heavy seas exert immense force. The limpet uses its foot to anchor itself to smooth rocks.
The muscles of the body can pull the shell down hard against the rock and effectively seal the animal inside. The limpet may clamp down so tightly that the rocks are often marked in rings where the shell regularly wears against them. The foot also enables the limpet to crawl forward, with alternating phases of contraction and relaxation. These pass across the foot like minute ripples, thrusting it against the rocks and giving it forward locomotion.
The tentacles on the head, and the other sense organs of the body, pass information to the simple brain about the presence of food. The limpet then gathers food by using a special feeding organ known as the radula. As in many molluscs, including snails, this takes the form of a rasping tongue-like organ.
Under the microscope, it looks like a ribbon that carries hook-like teeth. The radula, situated inside the mouth, is constantly being renewed at one end while being worn away by everyday use at the other. As the limpet feeds-on weeds and the fine film of microscopic algae and detritus which cover the rocks-it presses the radula against the rock and pulls it back and forth so that pieces of algae are filed off together with particles of the rock itself.
This rasping activity leaves characteristic marks on the rock known as limpet trails. Adult limpets remain in one place on the shore. They normally feed when they are covered by the sea at high tide, though they sometimes forage on most days when the tide is out.
After each feeding expedition, they return to the same ‘home’ spot. By their regular grazing activities, limpets keep the rock where they live clear of developing algae and small encrusting animals. In this way, they are an important factor in controlling the development and abundance of some other shore species.
From Egg to Larvae
There are male and female limpets. They usually reproduce in early spring, though this depends on the species. When the female releases her eggs into the sea, the male releases his sperm, and fertilization takes place. Microscopic, drifting larvae hatch and soon develop as members of the planktonic community.
At first, the larvae feed on stored food material from their egg but after a while, they take microscopic algae drifting in the sea. After several months they are ready to settle on the shoreline and change into juveniles. They are particularly vulnerable to predators, such as rock pool fish, at this time.
Examining a Limpet
Telling the common limpet (Patella vulgata) from its relatives P. aspera and P. depressa is difficult; you have to knock the limpet off the rock to examine the underside of the animal with a lens. You have around the tip of the mantle, just inside the shell, runs a ring of delicate tentacles. If these are transparent, and invisible except through a lens, then it is the common limpet.
If the tentacles are opaque, and visible to the naked eye, then it is one of the other two. In P. Aspera the feet are orange, and the shell interior is orange-grey, with brownish markings running only a short way inside. In P. depressa the foot is cream to brown, but the brown markings inside the shell extend a long way in from the lip.
It is also the harder of these two to find, as it occurs only on exposed middle shore rocks, while its relative is generally restricted to lower shore rocks. The common limpet is more widespread around the middle shore zone.
The common limpet (Patella vulgata) may reach 7 cm (3in) in length and is one of our largest and best-known limpets. It lives on rocky shores-except the most exposed and the most sheltered and is found in a belt along the middle shore.
The common limpet also attaches itself to piers and pilings at the same tidal level. The scars, or ring marks, where the animal’s shell has worn against the rock surface, are clearly evident here.
Read More – The Amazing Rock Drilling Shellfish
Cross-Section View Inside a Limpet
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