We Love Slugs
Posted on Jun 01, 2008 by bruce_moore
This is an article that was originally published in the "What's Happening" tourist information magazine which is published by Safari Tours and distributed in North Sulawesi.
As divers learn to notice and appreciate the smaller marine inhabitants, the sea slugs known as nudibranchs have become one of the most popular "critters" encountered underwater. Known as the butterflies of the sea, these graceful creatures exhibit great diversification in their shape, coloration and ornamentation.
Some advertise their distastefulness with wild colors while others blend in discreetly with their surroundings - usually their food source - and one can have a difficult task trying to figure out which end one is looking at. Since they lack the protection of shells as used by some of their relatives, nudibranchs have developed a variety of effective defense mechanisms. Some secrete unpleasant acids or even powerful toxins to deter predators while some "borrow" stinging cells from the hydroids which they eat and pass them to the brightly-colored tips of their cerata (fleshy projections that resemble spines or fingers) which is the most likely part of their body to be noticed and attacked. The predator, on receiving a mouthful of stinging cells, will quickly learn that this creature is to be avoided.
In the arena of procreation, nudibranchs have evolved an interesting adaptation to maximize their chances of species survival. Being hermaphroditic, every nudibranch possesses both male and female sex organs and is thus able to mate with any other member of their own species (and may even try with other species) as well as lay eggs. The eggs are usually laid in a ribbon on or near their preferred food source and for some species the egg mass will even be the same color as the food in order to better blend in. They are so well-adapted that species which feed on slow-growing food such as sponges, sea pens and sea fans live for around a year while species that feed on briefly-blooming species such as hydroids and bryozoans have short life cycles as well, living for up to only six weeks.
Nudibranchs feed on a wide variety of creatures, but many species are picky eaters, preferring a single species of sponge, ascidian (sea squirt) or whatever, which makes finding them less difficult if you first search for the food source in your hunt for a particular nudi.
But even without refined "critter eyes" nudibranchs are often so colorful that if a diver stops and inspects any particular patch of reef, in most cases one will be found.
In sand habitats, where inhabitants must better protect themselves as they are more exposed than on a reef, nudibranchs often have hitchhikers - imperial shrimp. The shrimp not only has the luxury of protection, living on a non-edible host, but it can pick up tasty morsels disturbed from the sand as the nudibranch travels on its own search for sustenance. One variety of imperial shrimp has even altered its pattern to better blend in with the Spanish dancer nudibranch which is its host.
Much is still to be learned about nudibranchs, with new species and variations discovered with regularity. North Sulawesi, especially Lembeh Strait, is one of the very best destinations on the planet to see a wide variety of these attractive and fascinating creatures, so on your next dive, keep a sharp eye out for a flash of intense color in a small package and you will have probably found one of the hundreds of nudibranch species that make our underwater world a far more interesting place.
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