Quick Dips

Swimming with Slugs

On a trip to any rocky shoreline you're likely to encounter a variety of marine snails and other shelled molluscs. Within the tide pools at low tide you may see periwinkles, nerites and a variety of limpets. If you are very lucky, or slip on an air cylinder and dive below the tide line, you may also encounter one of the delicate and beautiful marine slugs or nudibranchs, which are providing scientists with some fascinating research opportunities.

While most marine gastropods go through life carrying a self-built defensive fortress to escape predation, nudibranchs (commonly referred to as sea slugs) have managed to escape the necessity of toting around such heavy protection by evolving various and intriguing alternative strategies. Aeolid nudibranchs use tiny stinging cells called nematocysts, located in the tips of the dorsal appendages called cerata, as their means of defence, in much the same way as corals, anemones and jellyfish. In fact, these nudibranchs get their nematocysts from the cnidarians they eat, although how they keep these foreign cells alive or transport them to the site of action is still a mystery.

Other types of nudibranchs can either shed non-vital parts, like geckos shedding their tails, hide in nooks or crannies or employ colouring which makes them so well camouflaged they're nearly impossible to see. Some partake in elaborate swimming displays that are thought to confuse potential predators. Perhaps one of the most intriguing defensive strategies, however, is the ability of certain nudibranchs to utilise the toxic compounds in their food sources as a means of chemical defence.

In a manner akin to monarch butterflies, some nudibranchs have developed the natural ability to deal with toxic chemicals in their food and turn them into their own protection. Unlike their insect counterparts which store the toxins throughout their bodies, these animals somehow manage to transport the compounds to glands near their gills and other vital organs on their backs and release them as a deterrent when predators get too close. Some of these compounds are so toxic that certain nudibranchs cannot be maintained in the same aquaria with fish. This defence is also a means of protecting the next generation of sea slugs, with toxic compounds having been found in the egg masses of some nudibranchs.

Part of the research undertaken in the laboratories of marine natural products chemists involves the investigation of the toxic compounds found in nudibranchs and other marine invertebrates (isolation and structure determination), as well trying to determine the roles these compounds play within the natural ecosystems of the animal or plant investigated (chemical ecology).

Chemists in New Zealand and elsewhere have isolated a variety of complex compounds from these organisms which have potent biological effects. Recent research has shown that some nudibranchs are in fact getting compounds from the organisms they eat for use as a chemical defence.

This defensive strategy is not foolproof however. One genus of nudibranchs, called Tambje, preys upon a bryozoan from which it gets a series of compounds to use as a defence against fish predators. Unfortunately for the poor little Tambje, another nudibranch, Roboastra tigris, has developed a taste for the smaller slug and has the same tolerance of the toxins as its prey. In fact, the larger nudibranch is actually able to detect trace amounts of the "defensive" compounds in the slime trail of Tambje, follow the trail like a bloodhound to its victim, consume the smaller gastropod and then salvage the compounds for use as its own protection against predators.

So next time you encounter one of these beautiful marine molluscs, take a moment to reflect on how something which looks so delicate and helpless has actually traded the fortifications of its cousins for a well equipped arsenal. Sometimes, even in the lives of small and fragile animals, the best defence is a strong defence.

Eric J. Dumdei, Department of Chemistry, University of Canterbury