Natures wonders: nudibranches eat/steal their defence (Introduction)

by David Turell , Tuesday, September 18, 2018, 14:38 (2256 days ago) @ David Turell

Soft colorful sea slugs eat hydroids without difficulty to defend themselves with the hydroids own defences:

https://www.kqed.org/science/1929993/this-adorable-sea-slug-is-a-sneaky-little-thief

"The summer months bring low morning tides along the California coast, providing an opportunity to see one of the state’s most unusual inhabitants, sea slugs.

"Also called nudibranchs, many of these relatives of snails are brightly colored and stand out among the seaweed and anemones living next to them in tidepools.

“'Some of them are bright red, blue, yellow -- you name it,” said Terry Gosliner, senior curator of invertebrate zoology and geology at the California Academy of Sciences in San Francisco. “They're kind of designer slugs.”

"But without a protective shell, big jaws or sharp claws, how do these squishy little creatures get away with such flamboyant colors in a habitat full of predators?

***

"Nudibranchs come in a staggering variety of shapes and sizes. Many accumulate toxic or bad-tasting chemicals from their prey, causing predators like fish and crabs to learn that the flashy colors mean the nudibranch wouldn’t make a good meal.

"But Gosliner and his graduate student assistants are particularly interested in a group of nudibranchs that sport dozens of long outgrowths on their backs called cerata, which resemble colorful dreadlocks. These species take stealing defenses from their prey to a whole new level.

"Many of these nudibranchs feed on hydroids, smaller relatives of jellies that stay attached to the rocky seafloor.

"According to Gosliner, most hydroids are about the size of half of your little finger, some a bit larger. “Some of them look like seaweed, while others have a branching pattern that resembles a bird’s feather,” he said.

"Like their free-swimming cousins, hydroids have tentacles armed with stinging cells to catch tiny plankton out of the water.

"Each one of those stinging cells contains a structure called a nematocyst that resembles a microscopic harpoon, tethered to the tentacle by a long hollow tube. It’s what gives jellies their sting.

“'If anything tries to nibble on the hydroids, they shoot out their nematocysts,” said Gosliner. “So the hydroids are able to capture their prey or defend themselves using the same structures.”

"But they’re not enough to stop nudibranchs from devouring the hydroids -- stinging tentacles and all. They seem unfazed, even as the nematocysts fire off in their mouths.

"But not all of the stinging nematocysts fire right away. Some that are not yet fully mature stay intact and travel through the nudibranch’s complex digestive tract to become a fearsome weapon.

"The nudibranch’s gut has fingerlike branches that extend up into the long cerata on its back. The unfired stingers travel up into the cerata and concentrate in little sacs at the tips, where they continue to develop.

"If a fish or crab tries to bite the nudibranch, it squeezes those sacs and shoots out the stingers, which immediately pop in the predator's mouth. It doesn’t take long for predators to avoid the brightly colored nudibranchs."

Comment: This is a complex arrangement which, if you think about it, cannot have developed by chance. The sea slugs had to develop a quick form of tolerance. Its gut has to extend into cerata which had to be evolved to provide a spot for the nematocysts to attack an enemy.