SLUGS. TO ANYONE ACCUSTOMED TO THE GARDEN VARIETY, THEY´RE SLIMY PESTS. BUT SEA SLUGS ARE A SOURCE OF GREAT BEAUTY AND WONDER FOR MANY DIVERS.
They're blind hermaphrodite pirates, who live on their chemosensory wits. They evolved to live without a shell, having split off from marine snails many millions of years ago. The commonly used technical term for the group is nudibranch (pronounced to rhyme with 'tank'), because in the main, nudibranchs are characterised by naked (nudi-) gills (-branch). Correctly, the term for the group is opisthobranch (which also rhymes with 'tank').
Living organisms all have to solve the various challenges of being alive. The most critical is respiration, or breathing to us landlubbers, and oxygen uptake to the technically minded. Most nudibranchs have gills, gorgeous feathery structures with a huge surface area for oxygen to dissolve across and into the animal. Some are so small they don't need gills, instead absorbing oxygen directly through their skins. Even these small animals, however, tend to have cerata, gorgeously coloured projections which extend from their bodies in clumps or else a dense mass. Beautiful though they are, cerata increase skin surface area for oxygen absorption.
Once respiration is conquered, an animal needs to beaware of its environment. Although some group members (sea hares for example) have eyespots which are capable of sensing light and dark, in general nudibranchs use chemical senses to navigate their worlds. They have chemosensory tentacles at the front of their bodies known as rhinophores (rhino- means nose) and they could be said to smell their way around. These rhinophores come in different shapes and form part of the classification of the group.
Living without, or nearly without a shell means that sea slug defences cannot be passive or mechanical. They can't simply retreat into their shells, as snails do. Instead, their defences are active and chemical. In practice, this means poison. Some of the most ancient sea slugs generate their own toxins, but the vast bulk of the group pirate their defensive chemicals from their prey. It's a pretty cunning trick. Some sea slugs specialise in eating sponges. Far from being the inert lumps only good for use in a bath, sponges have existed for well over 600 million years and are generally very good at dissuading other animals from feeding on them. They do this with a combination of spiky skeletal structures (think of biting into a cactus with the spikes on the inside) and toxins. And in general, they have been very successful. But not against nudibranchs. These specialists not only use the sponges' flesh for food, but out of a typical cocktail of tens of toxins that the sponges produce, will concentrate five or six of the most toxic poisons and sequester them in their own tissues for their own defences. Other nudibranchs, the aeolids, concentrate on things which sting, eating hydroids, anemones or bluebottles and taking their stinging cells for their own use.
It's a marvel of chemical ingenuity. The aeolid will feed on its specific prey and take its unripened and unfired stinging cells for itself. These cells pass through the digestive system and into the tips of the cerata, where there is a special sac for them. Once in the sac, the stinging cell is caused to ripen, and then is available for the aeolid's defence. Other nudibranchs use the toxins of moss animals or sea fans for defence.
So successful have they been that only very few specialist predators (mostly other nudibranchs) will try to tackle a nudibranch. And this forms the basis for the protection of some species of flatworm, which save themselves the bother of eating all the toxic stuff and rather mimic nudibranchs. The idea is that that predators will likely confuse them with a nudibranch and thus avoid them.
Like snails, nudibranchs tend to rasp away at their food with a serrated tongue or radula. The structure of the radula is another tool used when classifying nudibranchs. As to what they eat, as mentioned, toxic animals are generally high on the list. Sponges, sea fans, hydroids, anemones, bluebottles and moss animals are all contenders. Some nudibranchs will only feed on a specific species of prey, while others are less fussy. One group, the sap-sucking slugs, are specialist feeders on seaweeds. Much like other nudibranchs use their food for their own defences, sap-sucking slugs get their food to generate their food for them. What they do is take the photosynthetic cells from the seaweed and keep them in their own tissues. The chloroplasts then use sunlight to generate sugars in the same way they do in the seaweed. Some of these slugs don't need to feed any more once they have taken up sufficient chloroplasts, and continue their lives in much the same way as plants, letting the chloroplasts absorb sunlight and generate energy for their lives.
With respiration, environmental awareness, defences and food sorted, what's left is ensuring there are more sea slugs for the next generation. Because sea slugs are slow-moving usually bottom-dwelling creatures, it makes sense to maximise the chances of mating success should they meet another member of their own species. Nudibranchs do this by being hermaphrodites. Not only that, they also start out lives with developed sperm production so that even a newly metamorphosed nudibranch can mate with a mature one, storing sperm until its eggs are ready for fertilisation.
When nudibranchs of the same species meet, they generally arrange themselves head to tail with right sides facing. The genital pores and the coyly named intromittent organs line up and sperm is transferred. It's a bit complicated considering dealing with two sets of sperm, particularly if self-fertilisation must be avoided. For convenience, sperm coming from the individual is known as autosperm. Sperm from the partner is called allosperm. So the autosperm goes out of the individual and into the partner, and the allosperm comes into the nudibranch in question and is stored in a special chamber until the eggs are ready for fertilisation. When the eggs are ready, they are passed through the allosperm chamber and fertilised, and then laid in a ribbon spiked with toxic chemicals.
The time the eggs take to develop and hatch depends on the species. Though some species will hatch directly into the adult form, most nudibranchs hatch into planktonic larvae which must drift and find a food source before they metamorphose into the adult form. Then it's all about breathing, smelling, feeding, casual piracy and finding a mate.
Each egg ribbon is specific to the species laying it and may be a tangled string of tiny eggs, a rosette of various colours or a simple spiral.
All this and beauty too. It's hard to believe sea slugs and garden slugs are part of the same evolutionary group.
text: Georgina Jones
photo: Jenny Strömvoll
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