One beastie’s pigment is another’s poison. The marine-dwelling sea hare converts pigment from its food into a chemical weapon, a new study shows.
It is the first description of an animal taking photosynthetic pigment from its diet and turning it into a molecule that can wield off would-be attackers, researchers report in an upcoming issue of Animal Behaviour.
Some animals create noxious chemicals by building them in-house from bits of protein, others order toxins to-go, such as monarch caterpillars that eat poisonous milkweed plants, becoming poisonous to predatory birds in the process. But performing slight alterations to turn a benign algae pigment into a toxic compound is a new level of chemical ingenuity, says marine chemical ecologist James McClintock of the University of Alabama at Birmingham.
“It’s pretty neat,” says McClintock, who was not involved with the work.
Like their snail cousins, sea hares are soft, plump and slow. These creatures — so named because their tentacles resemble rabbit ears — graze on algae and sea grass in coastal waters. But like slugs, sea hares have no shell to hide in. When faced with a predator such as a crab or spiny lobster, some sea hares blast the attacker with a defensive spray, a combination of dark purple ink and a shimmery whitish substance called opaline.
To clear up which of the ink’s compounds conferred its bite, Michiya Kamio of the Tokyo University of Marine Science and Technology chemically analyzed the ink of the California sea hare, Aplysia californica, and conducted several experiments exposing blue crabs to the secreted ink’s separate ingredients.
One compound found in the ink portion of the sea hare spray stood out as especially offensive to the crabs: aplysioviolin. Crabs shunned shrimp juice (usually a treat) when it was accompanied by a squirt of APV, the team reports.
The team also showed how the sea hares make APV. The chemical precursor to APV is a mild-mannered photosynthesis pigment called phycoerythrobilin that’s found in red algae and a few other organisms. Sea hares graze on the algae, loading up on phycoerythrobilin. Just two small chemical tweaks — snipping off a protein and adding a chemical decoration known as a methyl group — by the sea hare produces the offensive APV, says Kamio. The sea hare’s ink gland serves as its chemistry lab, Kamio and colleagues report in the May Chemistry & Biodiversity.
Unlike ink released by a defensive squid, sea hare ink doesn’t blind or visually distract. It interferes with other senses, says Kamio, who did the work while at Georgia State University in Atlanta. When the researchers put temporary blinders on the crabs, the animals had the same response to the offensive ink as when they could see.
That the ink is more than a blind is in line with previous work. When sprayed with the sea hare’s inky concoction, spiny lobsters fall into a sort of sensory trap that leads them to begin grooming and eating behaviors. But the compounds that spur such behaviors in lobsters dominate the opaline portion of the sea hare’s spray, not the purple ink component.
Variations in ink spray and in how predators react to it reveal the breadth of the sea hare’s chemical defenses, says McClintock. “If you’re particularly vulnerable,” he says, “you have to have an arsenal.”
