When an enemy approaches, striped sea urchins fire a volley of poisonous pedicellariae, which bite into the enemy and drive him away from the urchin.
On the body of echinoderms – that is, sea urchins, starfish and others – you can see special structures called pedicellaria. Externally, pedicellariae look like miniature claws or tweezers, and when they were discovered at the beginning of the 19th century, they thought that these were some kind of external parasites – the “tweezers” moved as if on their own.
Striped sea urchin. (Photo: jonas_sandager / Flickr.com) The tip of a striped sea urchin’s pedicellaria with three denticles and a venom receptacle. (Photo: Hannah Sheppard Brennand/Southern Cross University) ‹ › View full size
With the help of flexible pedicellariae, echinoderms cleanse themselves of debris and real parasites; some species use them to capture food floating by; and no other animal has such devices.
In an article in The American Naturalist, zoologists from Southern Cross University and the University of New South Wales report that the striped sea urchin (Tripneustes gratilla) has found another use for its tweezers. Sea urchins, despite their spines and calcareous shell, often become prey for sea predators. And so, in order to scare away some predatory fish, T. gratilla, sensing danger, shoots a whole cloud of its pedicellariae into the water. The “mini-claws” of the striped sea urchin are equipped with three teeth and a container with poison: the teeth pierce the enemy’s body and remain in it, irritating the fish with the toxin, even if it has long moved away from the urchin.
The fact that it’s all about the pedicellaria toxin was established in experiments with two species of fish that prey on sea urchins: when they were offered food with the “tweezers” of a striped sea urchin, the fish often simply refused such food, but if the poison was removed from the pedicellaria , then the fish stopped paying attention to them and absorbed the food with hedgehog “claws”.
T. gratilla shoot pedicellariae even before a predator touches them. Water carries the poisonous weapon to a fairly large distance from the hedgehog itself, and, as observations have shown, fish generally avoid getting into the currents that come from the T. gratilla colony. Among animals, there are a number of species that use remote means of scaring: for example, some tarantula spiders use their paws to clean off the lightest hairs, which fly into the air and greatly irritate the nose and throat of the one who is interested in the spider; and bombardier beetles generally shoot a hot chemical mixture at the enemy – and striped sea urchins with their volleys of poisonous pedicellaria fall into the same company.
Based on materials from LiveScience.