Moridilla brockii Bergh, 1888. Darwin Harbour, Northern Territory, Australia. May 1986. PHOTO: Bill Rudman.
In most snails, bivalves and other molluscs, the shell is the primary organ of defence. It is a “house” into which the animal can retreat from attack from predators and from harsh environmental conditions. Although the loss of the shell in sea slugs has given them the freedom to evolve spectacularly shaped bodies and brilliant colour patterns, it has come at a price. Their soft juicy bodies are a tempting available item of food for other hungry animals.
Nudibranchs, and to some extent other sea slugs, have evolved a series of strategies to protect themselves from predation. Many, but not all of these strategies, involve colour. In vertebrate animals with good eye sight, colour patterns have often evolved as signalling displays between individuals of a species or between different species. For example the great tail of the male peacock is an advertising sign to females, while eyespots in many moths and other insects are there to trick birds into believing they are dealing with a much larger prey than they though.
Nudibranchs however have very poor eyesight, in fact it is unlikely that they can sense much more than light and dark and passing shadows. Their colour patterns therefore have evolved entirely for defense against predators, mainly fish, which have well developed vision. Here are some of the main ways colour is used amongst sea slugs.
The first line of defence is to try and become invisible. Anyone who has attempted to collect nudibranchs knows how hard it is to find many species. There are many spectacular examples of camouflage. In some cases both the texture and the colour of the body match the colour and the texture of the food they eat. Examples include the nudibranchs:
Trippa osseosa (Dorididae)
Rostanga arbutus (Dorididae)
Jorunna sp. (Dorididae)
Corambe sp. 1 (Corambidae)
Favorinus japonicus (Glaucidae)
Cuthona kuiteri (Tergipedidae)
Marionia sp (Tritoniidae)
A few opisthobranchs have the ability to change their colour to better match their surroundings. In one well-studied example, the bubble shell Haminoea navicula, has colour pigments in its skin contained in ramifying epithelial and subepithelial cells. By the extension and contraction of parts of these "melanophores" colour pigments can be moved or obscured so that the skin colour can change from black or dark brown to white in four or five hours. (See Edlinger,1982).
A few nudibranchs have bright colour patterns which are usually hidden but can be displayed rapidly as a means of startling and frightening off a potential predator. The classic example is the Spanish Dancer, Hexabranchus sanguineus, which only displays the brilliant red and white colour pattern on the dorsal side of it mantle when it is disturbed and beginning to swim.
A shelled sacoglossan example is:
Lobiger viridis (Oxynoidae)
WARNING DISPLAY (APOSEMATIC COLOURATION) and MIMICRY
Although the startling deimatic displays are clearly warning displays, they involve an element of surprise. In one nudibranch family, the Chromodorididae, the colour patterns of many species are spectacular and obvious. Research in recent years has shown that colour in chromodorids is often used to warn potential predators that these animals are full of nasty distasteful chemicals and not worth attempting to eat. This is known as APOSEMATIC COLOURATION.
In a development of this we often find geographic areas where groups of unrelated chromodorids have evolved very similar colour patterns, so that they share the load of teaching fish to leave the colour pattern alone. One example of this MIMICRY in southeastern Australia are a group of about ten red spotted species, some of which are very difficult to tell apart. Most chromodorids have these mantle glands.
Many fishes that swim in open water are dark above and light, often coloured silver, below. This adaptation results in the fish being difficult to see from above because the dark colour of the dorsal surface blends into the dark colour of the water below. It also helps to make the fish less visible from below because the light colour of the underside of the fish is less noticeable against the light shining from above. Two pelagic nudibranchs are spectacular non-fish examples of this very special form of colour camouflage.
• Edlinger,K.,1982. Colour adaption in Haminoea navicula (Da Costa) (Mollusca - Opisthobranchia). Malacologia, 22: 593-600.
• Rudman, W.B. (1981b) The anatomy and biology of alcyonarian feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society, 72: 219-262.
• Rudman, W.B. (1981c) Polyp mimicry in a new species of aeolid nudibranch mollusc. Journal of Zoology, London, 193: 421-427.
• Rudman, W.B. (1982a) The taxonomy and biology of further aeolidacean and arminacean nudibranch molluscs with symbiotic zooxanthellae. Zoological Journal of the Linnean Society 74: 147-196.
• Rudman, W.B. (1985) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris aureomarginata, C. verrieri and C. fidelis colour groups. Zoological Journal of the Linnean Society 83: 241-299.
• Rudman, W.B. (1986b) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Noumea purpurea and Chromodoris decora colour groups. Zoological Journal of the Linnean Society 86(4): 309-353.
• Rudman, W.B. (1986c) Nudibranchs: Nature's thieves. Australian Natural History, 22(1): 2-6.
• Rudman, W.B., Avern, G. (1989a) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. Zoological Journal of the Linnean Society 96(3): 281-338.
• Rudman, W.B. (1991) Purpose in Pattern: the evolution of colour in chromodorid nudibranchs. Journal of Molluscan Studies, 57, (T.E. Thompson Memorial Issue):5-21.
Rudman, W.B., 1998 (October 14) Opisthobranch defence. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/defcol
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