Why do limpets live together?

The fact that many animal species, such as herds of antelope or shoals of fish, live together in herds is well understood: there is safety in numbers. Why limpets live together is a mystery, but ecologist Tim Theobalds thinks he has come up with the answer, and his findings could have important implications for their conservation and commercial collection.

Speaking at the British Ecological Society’s Winter Meeting, being held at the University of York on 18–20 December 2002, Theobalds of the University of Plymouth will say that like higher animals, limpets stick together to protect themselves against bird predation.

“The occurrence of animals living in groups is common, for example, shoaling fish, or antelope herds on the African plain, and for such animals it is easy to see why they would want to exist in a group: a group provides greater vigilance and protection from predator attack than would be afforded to an individual on their own; similarly, by existing in a group there is a reduced chance of a particular individual being selected for attack. The grouping behaviour of limpets however, is less easy to understand. After all, they are seemingly simple creatures without the complex social structures of other grouping animals,” Theobalds explains.

Unlike more mobile species, limpets defend themselves from bird attacks by clamping themselves tightly onto rocks when they are disturbed. And it is the odd way in which oystercatchers tackle limpets that gave Theobalds a clue as to why limpets live in groups.

According to Theobalds: “Oystercatchers prey on limpets by pecking at them, which dislodges the limpet from the rock. They then turn the limpet over and eat the flesh. But oystercatchers appear to attack only one limpet within a group before going on to attack another limpet in a separate group. As limpets are able to move only extremely slowly, it is puzzling why an oystercatcher doesn’t just stand in one group and attack all the limpets, one-by-one. It would save the oystercatcher a lot of time and energy in hopping from one group to the next – after all, the limpets have no chance of fleeing. I wanted to test whether this is because when oystercatchers attack a limpet, its neighbours in the same group detect the attack through vibrations and clamp down onto the rock, so making them much harder for the bird to dislodge.”

To test this theory, Theobalds examined the clamping response of limpets when their neighbours come under a simulated oystercatcher attack. “I simulated the sharp peck that an oystercatcher delivers to a limpet by rolling a marble down a steel tube aimed at the front end of the limpet. Five seconds later, I measured the neighbouring limpets’ clamping response by attaching a digital spring scale to them and pulling them until they detached from the rock,” he says.

Theobalds found that compared with limpets whose neighbours had not been attacked, these limpets were significantly harder to remove from the rocks. “The study strongly suggests that limpets exist in groups to protect themselves against birds. The early-warning signal received, in the form of vibrations from attack of a neighbouring limpet, gives them a clue that a predator is in their midst and that they must clamp down to counteract the predatory threat. Thus, the motivations for grouping behaviour appear to be as valid for the simple limpet as they are for the more complex fish or antelope.”

Theobalds’ results suggest that to ensure oystercatchers have enough limpets to eat, commercial collection of limpets should ape the birds’ behaviour – collecting limpets from many different groups rather collecting all the limpets from the same group. “Commercial collection of limpets should ensure that the same number of groups of limpets remains. If half the number of groups of limpets along a shoreline are commercially removed, this will halve the number of limpet groups that oystercatchers can potentially prey upon, which could have serious implications for their food supply,” he says.

Tim Theobalds will present his full findings at 12:30 on Thursday 19 December 2002.