Natures wonders: sea urchin defense (Introduction)

by David Turell , Tuesday, April 25, 2017, 01:20 (2768 days ago) @ David Turell

They fire toxic barbs:

http://www.livescience.com/58794-sea-urchins-launch-mobile-jaws-at-predators.html?utm_s...

"A common and colorful sea urchin has some truly bizarre appendages that seem to move independently from its body, and now scientists know why: It shoots these tiny, venomous jaws into the water to deter predators.

"These teensy, toothy jaws are called pedicellariae, and when scientists discovered them in the early 1800s, they thought the jaws were parasites because they seemed to move independently from the urchin. Now, researchers find that urchins use their pedicellariae not only to defend themselves when attacked, but also as a warning to fish and other sea creatures to "stay away!"

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"Pedicellariae are found only in echinoderms, particularly sea stars and sea urchins. The type found on collector urchins are known as globiferous, meaning they have a three-pronged jaw and a venom sac at the end of a long stalk. When disturbed, the urchins shoot a cloud of pedicellariae into the water around their bodies. Those that meet their mark sink their tiny, venomous teeth into the predator's skin. Even if a predator fish tears away the structure in its haste to flee, the jaws remain embedded, and the venom sac keeps pumping irritating toxins into the fish's flesh.

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"What Sheppard Brennand and her colleagues discovered was that fish don't have to make direct contact with sea urchins to be shot with pedicellariae. To prompt T. gratilla to shoot off these structures, the researchers poked the sea urchins with forceps in a lab for 30 seconds, to simulate predation. Then, they incorporated pedicellariae into squid snacks and offered them to two species of fish that prey on urchins: the black axil chromis (Chromis atripectoralis) and the stocky anthias (Pseudanthias hypselosoma). In an aquarium setting, the fish ate 50 percent fewer treats containing venomous pedicellariae compared with treats containing no pedicellariae. When the researchers washed the pedicellariae of their venom, the fish readily accepted between 80 percent and 90 percent of the squid snacks embedded with tiny jaws, compared with fewer than 20 percent of the treats if the venom wasn't rinsed.

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"Discovering that the pedicellariae cloud deterred fish was the most exciting finding," Sheppard Brennand said. "We had hypothesized that this might be the case, but until you actually do the research and examine the data, you don't know what the outcome will be."

"Deterring predators with a long-range defense may save the urchins a lot of wear and tear, since they don't necessarily have to be bitten by every fish that needs to learn to stay away, the researchers wrote. Lots of animals have "pursuit-deterrent" signals like this that don't require contact with predators. Porcupines have their quills, for example, and some species of spider kick off tiny, irritating hairs. Bombardier beetles spray hot, irritating chemicals. And urchins, it seems, have their mobile bite."

Comment: Defense at a distance is a great concept. This is a simple early form of an animal with a very complex defense. Hard to imagine it evolved by chance.