Sea slugs, also called nudibranchs or butterflies of the ocean, ingest toxic chemicals from the sponges they eat and store away the single most noxious compound to use against their predators later on. By preventing certain cellular processes, latrunculin A is highly toxic to fish, fungus, and even some cancer cell lines. The findings are published in PLOS ONE this week.
Like poison dart frogs and vividly colored butterflies, many nudibranch gastropods with chemical defenses have bright colors that signal their toxicity to predators. They’re also known to derive defensive metabolites from their diets. Latrunculin A has previously been isolated from sponges living in the Red Sea and the Pacific, as well as from the gut contents of sponge-feeding nudibranchs from the family Chromodorididae. They might be actively sequestering the compound, but not much is known about its distribution in different sea slugs body parts. Defensive compounds are often localized in places where predators most frequently attack, while other compounds are eliminated through the gut.
A team led by Karen Cheney from the University of Queensland studied five closely-related nudibranch molluscs (pictured below) collected by scuba divers in the Great Barrier Reef and from South East Queensland in Australia. The team dissected each nudibranch into two or three parts: internal organs, the mantle (the back surface), and in some cases, the mantle rim, where the defensive mantle dermal formations are located. (The mantle rim is that yellow, orange, or pink border.)
Only latrunculin A was present in the storage reservoirs of the mantle rim; a variety of other compounds were found in the internal organs. That means the sea slugs selected just one toxic chemical weapon to sequester and accumulate in a part of their mantle that’s the most exposed to would-be predators.
When the team conducted comparative toxicity and cytotoxicity tests using purified nudibranch compounds, they found that latrunculin A is several times more potent than any other compounds present. It can even inhibit cancer cell growth. And feeding tests revealed that latrunculin A was very unpalatable to rock pool shrimps; small amounts killed them. Nudibranchs must have evolved a way to protect themselves from the toxicity of latrunculin A, though exactly how they sequester it from sponges without causing damage to themselves remains a mystery for now. Their digestive system might be specially adapted to detoxify the compound.
Many products used to make drugs are molecules that plants and animals use to protect themselves. Studying marine molluscs has already led to the discovery of many biologically potent chemicals with analgesic, anti-inflammatory, antiviral, and anticancer activity.
From K.L. Cheney et al., 2016 PLOS ONE
