Nudibranchs and most of their relatives are devoid of protective external shells and, as such, these delicate soft-bodied creatures appear vulnerable and defenceless.
Nothing could be further from the truth.
During their 200 million years of evolution opisthobranchs have developed an amazing number of survival systems. Chemical and biological weapons, swimming escape responses, aggressive displays, mimicry, warning colouration, patterns, ornamentation, camouflage and secondary armaments are just some of the techniques used by this very successful group. Some species of Plocamophorous are capable of biochemical luminescent light flashes when provoked; the how, or why they do it, remains a mystery.
Deceptive camouflage defence
Lemon Aegires Aegires citrina is almost invisible when amongst its food sponge Leucetta lemon. The nudibranch uses a range of applications for its defence, including camouflage, chemistry from the sponge and body spicules.
What is even more incredible is that all these adaptations evolved without any application to the function of sight. Opisthobranchs have no eyes in some cases, only light receptors so that they have no perception that they may be pink and their food sponge is also pink. Everything relies on chemo-reception and evolutionary adaptation. However, despite all their many survival tactics some opisthobranchs are preyed upon by swimming crabs, sea stars, anemones, flatworms, other opisthobranchs, univalves and fish.
While the majority of the more well known nudibranchs are brightly coloured and patterned and have noxious defence chemistry, some of the less colourful and obscure species rely on their camouflage, mimicking the colour, shape and/or texture of their prey.
Postural behaviour mimicry is the first line of defence for Souverbys Lobiger Lobiger souverbii which uses its modified parapodia to mimic the branch pattern of its host algae Caulerpa sp. From even a short distance it remains almost invisible.
Some species of the genera, Aegires, Phyllodesmium, Jorunna, Rostanga, Discodoris, Doris, Verconia, Doriopsilla, Marionia, Tritoniopsis, Phestilla and others mimic their food sources. Some were so successful that they took us 30 years of searching for us to find them and there are a lot more out there awaiting discovery.
Many opisthobranchs obtain their chemical repellents from their food sources. Sessile invertebrates such as sponges have a well developed arsenal of chemistry developed by various species of bacteria living in their tissues. Nudibranchs have evolved methods to not only devour sponges as a food source but are able to restructure the preys chemistry to produce their own chemical compounds which are stored in their skin and reproductive organs which in turn is secreted into their egg ribbons.
At the slightest disturbance the Mimic Paradoris Paradoris liturata squirts out volumes of toxic white secretions from its white pustuled mantle glands.
In some cases (Chromodorids) the chemical compounds are stored in glands (seen as pustules or pigmented masses) along the mantle edging. Acidic and toxic chemicals called sequiterpene isocyanides are found in many species of Phillidid nudibranchs while the genus Ceratosoma store their nasties in the fleshy hump formed over their gills.
Sponges are not the only forms of sessile invertebrate chemistry used by nudibranchs, they also utilise the deterrents of various species of soft corals, gorgonians and other cnidarians.
Highly visible warning colours of bright yellow and black markings, a hard body full of spicules and toxic chemistry derived from its food sponge Pericharax heteroraphis means that the Minor Notodoris Notodoris minor gets few hassles from predators.
The chemical diterpene trocheliophoral is a fish repellent produced by the leather coral Sarcophyton sp. By feeding on Sarcophyton sp. the long-cirri Phyllodesmium Phyllodesmium longicirrum ingests this non-toxic repulsing chemistry which is stored in its body as protection from fish and other predators. Many sea hares release a purple dye as a defence mechanism, but the dye does not have any noxious chemistry and only appears to be a visual deterrent.
Kune's Chromodoris Chromodoris kunei feeds on sponges and is brightly coloured and quite toxic. It would appear that it also brings attention to itself by continually flapping its mantle as it moves along. Quite extraordinary behaviour.
Stinging defence mechanisms
In the process of feeding on cnidarians a number of aeolid nudibranchs devour the preys stinging cells, called nematocysts. Even as the aeolids are immune to the stinging cells they devour immature stinging cells which are stored in the tips of the nudibranchs cerata. The nematocysts continue to develop and mature and eventually become the aeolids defensive system. The mere fact that aeolids feed on sessile stinging cnidarians such as hydroids also protects them from visual predators who know well enough (from experience) to steer clear of hydroids.
The hydroid feeding Red- backed Doto Doto sp. consumes hydroid polyps and tissue and in the process devours under-developed stinging cells (Nematocysts).
However, unlike other hydroid- feeding Nudibranchs ( such as Pteraeolidia ianthina ) and ( Flabellina rubrolineata ) it does not store the undischarged nematocysts in it s cerata for defence.
Some aeolids cast off, or shed cerata when attacked. The detached cerata often move independently (similar to the cast tail of a lizard). These movements often attract the predator while the nudibranch gets away.
Nudibranchs of the genus Melibe are also able to detach the large gill flaps on their backs at will (much to the dismay of nudibranch photographers) while opisthobranch sapsuckers from the family Calyphyllidae, especially Cyerce and Polybranchia are prone to casting off their cerata when stressed.
When attacked, the Black and Gold Cyerce Cyerce nigricans casts off its sticky cerata; these polyps continue moving of their own volition. The moving cerata act as decoys while the slug escapes.
The sap sucking genus Oxynoe is able to exude a white noxious substance and if that fails to work it will autotomise its long tail. Most opisthobranchs have the ability to regrow their cast off parts.
It seems that most defences are mainly aimed at visual predators such as fish and those opisthobranchs with chemical defences, bright colours and patterns to match, appear to rely on the fishs memory of bad taste in the first instance of mouthing to avoid further attacks.
Some wrasses and pufferfish have learnt to suck opisthobranchs in and spit them out very quickly up to a dozen times (washing). (See Australian Marine Fish, Neville Coleman, 1992). This behaviour effectively reduces the strength of the chemical discharge each time the animal is sucked in.
Once the chemistry is depleted, the fish swallows the slug. (So much for the past scientific theories that because fish have small brains and no cerebral cortex, they are unable to recall past events or initiate a learning response.) Someone should really enlighten such learned scientific theorists, that in nature, anything is possible, even learning!
Swimming escape response
Various species of nudibranchs react to imminent danger or interference by rising off the bottom and swimming away from the point of disturbance. It is also thought that some use their swimming abilities to rise up into the currents to be transported to other locations, even though these swims may only be over short distances.
A few nudibranchs employ swimming tactics for retreating from threat presence. The most famous of these is the Spanish dancer Hexabranchus sanguineus which swims as a regular escape response to any sense of danger.
The Spanish dancer Hexabranchus sanguineus is well known for its swimming escape response and as such is often targeted by photographers who wish to capture its colourful movements. Other swimming genera include Melibe and Bornella though they use quite different movements to gain momentum. Species of sea hares Aplysia spp. are able to swim along by flapping their extensive parapodia and the ornate Sagminopteron can also swim using its mantle flaps.
There are nudibranchs with luminescence defences and side-gilled slugs which break off parts of their mantles as autotomatic defence and no doubt there are other methods we have yet to discover. That is what makes the life of a nudibrancher so exciting, there is still so much out there, waiting to be discovered.
Not all bad tasting nudibranchs are safe from predators. Here a Red-papulose Gymnodoris Gymnodoris rubropapillosa is happily gobbling up the toxic tasting Obscure Hypselodoris Hypselodoris obscura.