Nudi Notes

Illusory – Delusory – Elusory

Apr 9, 2024

Illusory – Delusory – Elusory

– The Art Of Blending In –

Sea Slugs that mimic their food or other invertebrates in the habitat.

In other NudiNotes we have looked at the many ways that sea slugs mimic other sea slugs, sea slugs that advertise their colourful presence as a toxic warning and other sea creatures that mimic sea slugs for protection. In this NudiNote we review sea slugs that look like their food whether that be plant or animal and even some that mimic unrelated life forms in their habitat. By definition this phenomenon is considered a form of camouflage rather than mimicry but I feel, to the general reader, a better sense is gained of what is actually happening if the word mimicry is thrown into the story as well.
(In the featured image above a specimen of Jorunna hervei matches fairly closely in texture and colour its sponge host. It also wraps 
itself to the sponge contour.)

Above: Three images that show spongivore nudibranchs presenting with an appearance that mimic sponges. Left: Aldisa pikokai, often found feeding on a red sponge, exhibiting three depressions down its centreline that mimic the oscula of sponges. Centre: A specimen of Hallaxa translucens that has just crawled off its sponge food. The resemblance of this nudibranch in colour and texture to the sponge is remarkable. Feeding scars are visible on the sponge. Right: Although the shade of lilac is darker than the sponge the markings and texture of this Jorunna daoulasi very closely match its sponge food. Even the cupping of the gill creates an illusion of a sponge oscula.

 

Not Cognitive
There aren’t too many pretty pictures of sea slugs in this article because they are all “trying to be” inconspicuous. Having said that we must remember that in reality they are not “trying” at all. All the changes that are wrought to their appearance are random mutations that have proved to be advantageous to the sea slug through selection pressure in the habitat and passed on to offspring.

Disruptive Colouration and Patterns
Of the sea slugs that aren’t brightly coloured or patterned many are drably coloured and yet others exhibit disruptive colouration – disruptive that is, against the background of their habitat. The colours and pattern break up their outline on the substrate making them less conspicuous. Removed from that background for whatever reason, they become incongruously obvious. Underwater photographers are often tempted to do this with certain subjects to obtain better definition and create a more “appealing” image or definition of the isolated subject.

 

Above: A sap-sucking sea slug and its food. Left: A species of Sacoproteus sacoglossan (most likely S. smaragdinus) with the spherical nature of the cerata on its notum mimicking the shape and form of the food it is found upon, the sea-grape algae Caulpera, Right.

 

Background Imitation
When you can’t hide under a rock because your food is growing out in the open and you spend the greater proportion of your life exposed upon that food source, it makes sense to be able to blend in and be inconspicuous to potential predators by impersonating the appearance of that food because it is not that food the predator is looking for. Many and varied are the textures, colours, patterns and notal appendages possessed by the sea slugs to achieve that. Mutation and selection pressure in the habitat has caused many species to evolve an appearance whereby those textures, colours, patterns and appendages cause them to be camouflaged upon their background food source, that is, mimicking it, with some species being nigh impossible to discern thereon, unless you know what you are looking for.

 

Above, Left: The sacoglossan Elysia maoria lives and feeds upon the algae Codium sp. It is most difficult to discern in situ with its dark colouration matching that of the algae, and often only the spawn laid by the sea slug alerts the searcher to its presence. The white rhinophores and the white tufts on the parapodial edges match debris often lying on the surface of the algae. Right: Even some sea hares are mimics. Here a specimen of Phyllaplysia lafonti blends into the greenish-brown Padina sp. algae. The algae itself is not the food source but rather it grazes on epiphytic diatoms found there. The longitudinal striping on the surface of its notum serve to increase the illusion.

 

Form and Colour
Colour is not always everything. Sometimes the form presented by the mimic is so good that the colour hardly matters. On the other hand some are so plain as to be transparent and therein is the key to their particular form of camouflage – the substrate shows right through their body making them almost invisible against that background.

 

Above: Some nudibranchs mimic life forms that they don’t prey upon. Left: Miamira alleni is a sponge eating dorid nudibranch, however it possesses large prolonged knobbly protuberances that create an impression of a xeniid soft coral found in the same habitat. Right: A colour form of an undescribed species of aeolid nudibranch, Noumeaella sp., believed to feed on hydroids but often found on this pink compound ascidian. There is also a light brown colour form.

 

Behaviour
Naturally, behaviour too plays an important role in making the camouflage process function most efficiently whereby, for example, they wrap themselves over and around the contours of their food or hold aloft and even move their cerata to resemble the branches of their soft coral or hydroid food through which they are moving and preying upon. Another aspect that works in favour of the sea slugs, apart from those that are dedicated hunters and necessarily swift movers, is the fact that sea slugs really don’t move around too much. An almost static lifestyle, the lack of discernable movement while feeding, surely aids detection avoidance.

 

Above: A soft coral-eating aeolid nudibranch and its food. Left: The octocoral or soft coral Xenia sp. Its feeding process has the polyps located at the ends of tall stalks pulsing – opening and closing. Right: Phyllodesmium rudmani a predator on the Xenia sp. coral. The cerata of this nudibranch mimic the polyps and their stems in the closed position. Even the zooxanthellae taken up by the nudibranch are located in the same position as they are found in the soft coral. The only tell-tale give aways are the non-pulsing of the ceratal tips and the rhinophores.

 

Spawn Tip-off
The one thing that often betrays their presence, to us as sea slug searchers anyway, is the spawn they lay on their food, although I doubt this aspect denounces them to a predator. The usually different coloured spawn arranged geometrically as a spiral is most obvious to the human eye.

 

Above, Left: The small nudibranch Idaliadoris maugeansis feeds on the zooids of encrusting bryozoans. While its form does not mimic its food directly the transparent nature of its wide mantle allows the nature of the bryozoan colony to show through making it nearly impossible to spot. Often it is only the nearby spawn spirals that betray its presence. Right: These two large adult specimens of the aeolid nudibranch Pleurolidia juliae can be seen on the stems of its food, the black hydroid Solenderia sp. hydroid, down near its base. Smaller specimens are difficult to spot but the orange to pink spawn spirals laid on the branches stripped of the white polyps are a tip-off.

 

Reasonable Resemblance Sufficient
To revisit a point made in the earlier articles on mimicry, the mimic does not have to be perfect to get by. A reasonable resemblance by colour and/or shape would often be sufficient to bestow some degree of protection upon the sea slug especially when immobile and situated upon its food.

 

Above: The sacoglossan Elysia pusilla feeding on the alga Halimeda, lies flat on the surface, presents as green in colour and bears markings that serve to break up its outline and replicate the markings on the Halimeda surface.

 

Regarding the images illustrating this NudiNote, in some cases, the sea slugs are shown separate to their food source background, otherwise they would not be readily observable to the reader.

David A. Mullins – April 2024

References:
– Rudman, W. B. (2004 July 22) Camouflage. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/camouflage and associated messages.

– Behrens, D. W. (2005). Nudibranch Behaviour. New World Publications, Florida, USA.

– Coleman, N. (2008) Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld.

– This NudiNote has been modified from a previously published article in Dive Log Australasia Magazine – NudiNotes Column, Issue: #404 (February 2024): pp. 27-29 by David A. Mullins.