“Beautiful” and “Slug”. The two words hardly seem compatible. Certainly slug, an unattractive word when applied to terrestrial creatures, conjures up visions of slimy, drab invertebrates crawling through dank, rotting, organic waste, leaving a trail of slime in their wake. Enthusiastic gardeners will perceive them as arch-enemies, destroying prized plants and produce. The garden slug can have few champions. By contrast there are truly beautiful slugs in the coral gardens of tropical oceans, as well as being found in temperate and even polar seas.
Sea slugs, otherwise known as Nudibranchs (literally “naked gills”) are represented by numerous species with some 2,500 presently recorded, although it is thought that up to 5,000 may possibly exist. They can be so small as to be almost invisible to the naked eye, whilst at the other extreme they may be huge, capable of tipping the scales at 3 lbs. The species of sea slug most likely to be found on offer to the aquarium hobbyist will range from about one to three inches in length. They will be brightly coloured and be from the genera of Chromodoris or Notodoris. Phyllidia species are also often collected. Unfortunately they will also be short-lived as their feeding requirements are highly specialised and rarely to be found in marine aquaria. However, nudibranchs are short lived animals by nature anyway and captive breeding would be necessary to adequately justify maintaining them as popular aquarium animals. Perhaps a very large reef aquarium, populated by a wealth of sessile invertebrate life, would present the best chance of keeping a few of the more amenable species. Various coelanterates could provide a suitable selection from which one particular species might be accepted as an exclusive source for nutrition and sustenance.
The bright colours so many nudibranchs brashly display indicate an animal with no need of protective camouflage. Indeed, they send a message to would-be predators that they are creatures are not to be messed with! They are unpalatable at best, and fatally toxic at their worst. Some species are armed with stinging cells (nematocysts) which they have acquired, ingesting them from anemones or hydroids to be relocated in the gills of the nudibranch. Exuding a repulsive toxic slime is another potent defence. The leathery Phyllidia species of nudibranch have glands atop the peaks of their warty skin. These glands readily secrete a foul toxin when needed, and is the main defence mechanism of these species. While most nudibranchs are found crawling purposefully across areas of substrate or feeding casually from a chosen sessile invertebrate host, others are capable of swimming effectively, and able to distract and escape a would-be predator with this ability.
The naked gills of Chromodorid nudibranchs are often set high on the rear of the body, like an ostentatious display of feather plumes. The Phyllidia species lack these ornamental gills, absorbing their oxygen requirements through the tough skin. The Chromodorids will differ from the Phyllidias in another visual aspect. They sport prominent projections from the head like two horns. These are sensory organs known as Rhinophores, and may be quite ornamentally sculptured. Both gills and “horns” can be swiftly retracted when danger threatens. The Phyllidia species also have rhinophores, but in general they tend to be less obvious due to being somewhat smaller, thinner, and partially masked by the knobbly nature of the body texture.