Hitchin’ a Ride on the Sea Slug Train

It is often said there is no such thing as a free meal, but what about a free ride to collect your meal?

One sees sea slugs on many sessile invertebrates either feeding or traversing but only occasionally does one see invertebrates upon sea slugs. In general sea slugs are very good at keeping foreign growth and other species – epibionts – from taking up residence upon them. Their bodies are covered in many types of surface glands. Some of these contain toxic compounds for defensive use against a direct physical attack whilst others release acidic compounds to keep their surface clean of constantly arriving opportunistic settlers. Other glands secrete mucus that serves to virtually wash their epidermis free of foreign particles. The mucus may contain other substances that also encourages unwanted hitchhikers to release their hold. Not all sea slugs have the same glands, with number, type, size, location, content and target varying among the orders, families and even to genus level, however, regardless of the type or extent of their armoury, most sea slugs encountered appear to maintain an overall clean surface free of bacteria, protists and other fouling organisms.

A clean skin – Chromodoris lochi exhibits a totally clean appearance with no fouling organisms – a product of the operation of many surface glands.

The larger of the dorids act as host, from time to time, to certain shrimps that are fascinating to watch as they lightly move over, from side to side, forwards and backwards and under the sea slug, at times hanging over the side to sample and feed from the passing substrate as the sea slug glides across it, a bit like the reverse of a sushi train. Hexabranchus species (the Spanish Dancers), Hypselodoris tryoni and some of the Ceratosoma are perhaps the best known of these hosts, although any species large enough may become a convenient taxi. Certain species of shrimp seem most inclined to take up this hitchhiking way of life. As well as leaning over the side and feeding on the substrate they are also known to consume the mucus produced by the sea slug and even the faecal pellets ejected through the slug’s anal papilla, located in the centre of the dorsal gill circle. Juveniles of the shrimp are most likely to be found within the gill circle taking advantage of the protective cover it provides as well the ready supply of food. If the gill is large enough the adult shrimp will also hide therein as well as slipping underneath the slug if threatened. The large enrolled edges of the mantle of Hexabranchus species also provide a convenient haven for retreat. The sea slugs never seem antagonised by the presence of these shrimps moving over their body, not even retracting a rhinophore when the shrimp blunders over it.

The commensal shrimp Zenopontonia rex hitchhiking on and hanging over the side of the nudibranch Ceratosoma tenue. The shrimp takes advantage of the nudibranch’s travel to sift through the passing substrate for items of food.

Another colour form of the shrimp Zenopontonia rex. This colour form is a good match to the mantle of the a Hexabranchus nudibranch. Here, it too, is sampling the passing substrate.

The shrimp can sample and eat from the ever changing substrate but quickly retreat into the protection of the rolled edge of the nudibranch’s mantle if threatened.

Juvenile shrimps use the protection afforded by the ring of gills of the nudibranch to avoid unwanted attention of predators.

A faecal pellet being ejected through the anal papilla in the centre of the dorsal gill circle of a Hexabranchus nudibranch. Faecal material and mucus produced by the nudibranch are a food source of the commensal shrimp.

Those sea slugs that carry a shell also manage to keep it clean, the exception being the umbrella-shell sea slugs of the Umbraculoidae. Their limpet like shell has a protective periostracum, the outermost and organic layer of the shell – thin in Umbraculum and thick in Tylodina – nevertheless there can be heavy fouling growths upon the shell from a wide range of epibionts including algae, hydroids, bryozoans, anemones, sponges, ascidians – actually, just about any marine sessile lifeforms. This can almost appear to become a self-sufficient community, on the move, even hosting other small sea slugs. These umbrella shells have no way of keeping their shelled surface clean and as they age the periostracum becomes eroded and abraded allowing attachment by fouling organisms. (The periostracum is thought to mostly provide a layer of protection from attack by boring organisms.)

The limpet-like shell of the umbrella-shell sea slug Tylodina corticalis can be fouled by all manner of sessile lifeforms. Here it is encrusted with bryozoans and coralline algae.

The large umbrella-shell sea slug Umbraculum umbraculum exhibiting a very heavy covering of fouling growth including, a number of different algae, sponges, ascidians and perhaps hydroids because a small aeolid nudibranch (arrowed) is also feeding there.

A close up of the small aeolid feeding in the habitat growing on the shell.

One hitchhiker that is rather more sinister is the copepod. This small crustacean is a parasite, rather than just looking for a free ride. Their tenancy is betrayed by the presence of paired egg sacs, of sausage shape, belonging to the female, and most usually seen protruding from the gill pocket of dorids. The gill pocket is a handy access point for feeding upon the haemolymph of the slug but it is not the only point of entry, with many records of them entering via the body wall as well. Aeolids are also parasitised by copepods with their egg sacs visible among the cerata but most usually observed anteriorly on the head and pericardial hump. The egg sacs are exposed like this to readily allow dispersal of the hatching larvae of the copepod.

The orange egg sacs of a parasitic female copepod protruding from the gill pocket of the dorid nudibranch Goniobranchus hintuanensis.

A close-up of the egg sacs. The gill pocket is a convenient place of copepod penetration.

Pink parasitic copepod egg sacs (arrowed) visible on the dorsum, just behind the rhinophores of the dendronotinid nudibranch Marionia sp.

Don’t believe everything you see …… especially if you have a couple of dive guides with a wicked sense of humour. This little set up was prepared for me at Milne Bay by my dive guides who were otherwise scrupulous about moving things. After spotting it and taking a pic for “prosperity” I swam around the other side of the bommie to see the guides, regulators out of their mouths, laughing away. It was at the end of the final dive of the trip. (Flatworm Thysanozoon nigropapillosum, as redescribed, on the nudibranch Phyllidia elegans.)

So whether feeding on, or feeding from, a sea slug the rewards are there either as a meal ticket or a means to vary one’s feeding grounds with little effort – by taking the “sea slug train”.

David A. Mullins – March 2021 (Revised July 2023)

References:

– Rudman, W. B. (2004 July 24). Symbiosis, commensalism, mutualism and parasitism. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/symbio and associated messages.

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

– Wagele, H., Ballesteros, M. & Avila, C. (2006). Defensive Glandular Structures in Opisthobranch Molluscs – From Histology to Ecology. Oceanography and Marine Biology: An Annual Review, 44, 197-276.

– Burn, R. (2015). Nudibranchs and related molluscs. Museum Victoria.

– Ponder, W. F. & Lindberg, D. R., with illustrations by Ponder, J. M., (2020). Biology and Evolution of the Mollusca, Volume One & Two. CRC Press, Taylor & Francis Group.