NUDIBRANCH GILL GUARDING
Shell-less but not defenceless
The sea is full of hungry mouths, many many hungry mouths either lying patiently in ambush or constantly on the move, in all directions, hunting for any opportunity to take a bite and feed. The last thing any animal needs is to have some attractive piece of their anatomy waving about in the water column looking like an easy meal. But many of the sea slugs, the nudibranchs in particular, do just that. The word nudibranch means naked gill of course, and early workers realized this exposed, naked gill was an important difference to warrant their own grouping within the gastropods hence the order Nudibranchia. So how do they prevent wholesale predation on those attractive and exposed “feathery” appendages that are so necessary for their survival?
Loss of shell – Exposure of gill
The sea slugs in detorsioning over evolutionary time and losing the shell of their sea snail forebears (secondary loss), or at least reducing its enveloping coverage, caused the gill, in those that retained it, to become exposed and seemingly vulnerable. There is a progression of change apparent in the different lineages related to shell reduction or loss, mantle cavity reduction or loss (where the gill had been located) and the resulting necessary development of alternative means to protect this vulnerable organ. There are quite a number of adaptations for this purpose many not immediately appreciated.
These alternative methods utilised by the true nudibranchs include:
– The ability of the gill to retract by withdrawing fully into a protective pocket.
– Protection of the gill behind a physical barrier.
– Concealment of the gill among nearby appendages.
– The positioning of sacrificial decoy appendages.
– Repugnant chemical glands situated near the gill.
– Embedding of spicules in the integument and nearby appendages.
– Replacing the single almost centrally located (and easily targeted) gill with a series of secondary “gill” structures dispersed down the length of the body.
Often, more than one of these methods are used in combination to achieve a higher level of protection.
Among the dorid nudibranchs:
The cryptobranchs (hidden gills) have a pocket into which the gill can retract, below the surface of the mantle, for protection. In many, the region adjacent to the gill pocket carries defensive glands or spicules. A very few of the cryptobranchs have also developed protective appendages around the gill pocket (Ceratosoma and Miamira). Some of these appendages also carry defensive glands.
The phanerobranchs (visible gills) do not possess a pocket for full retraction, however the gills can contract somewhat and many have developed extra-branchial processes around the gill. These can take many forms, from simple, short or long appendages either side to a complete ring around the gill. There is no doubt that some are sacrificial in acting as a decoy to draw away the attacker’s attention from the gill itself. A good example of this is the light emitting globes situated on the larger processes surrounding the gill of the Plocampherous species. In others there is a shield or significant barrier lying across the front of the gills for protection. Some also have a quite sturdy gill-supporting base extending up each gill branch acting somewhat as a physical barrier for the delicate filaments located on the inner surface.
The phyllidiids belonging to the porostome group (suctorial feeders) are quite different. They do not have a true gill but rather, secondary gills in the form of triangular leaflets concealed and protected in the hyponotal groove, that region where the underside of the mantle overhang joins the main body wall.
Among the cladobranch nudibranchs:
The other major group of true nudibranchs, the cladobranchs (branching gills), have evolved a different strategy. In this group the true gill is lost. Most have structures that are referred to as secondary gills. These are either arranged longitudinally down the notal edge as branched “feathery” appendages as in the dendronotids or as cylindrical cerata as in the aeolids. Either way the large surface area provided enables sufficient gaseous diffusion and the usually larger number of these secondary gills and their dispersion provides a higher level of functional survival from an attack than the – “all the eggs in one basket” – single gill of the dorids.
Across the third subgroup of the cladobranchs, the arminids, (not a natural grouping) several methods are utilised. Some examples include: the Armina, that have secondary gill leaflets under the mantle in the hyponotum just like the phyllidiids except they are only located anteriorly but with additional “folds” medially and posteriorly; the Dermatobranchus, that have no secondary gill structures relying instead upon simple diffusion through the large surface area of their mantle and body wall with large haemolymph sinuses located just below the integument; and Janolus and Madrella for example, that possess numerous cerata on the notum.
As with all things in nature there are many ways to “achieve an outcome” everyone of which has evolved through changes wrought by selection pressure in their habitat.
David A. Mullins – September 2022
– Rudman, W. B., Willan, R. C. & Burn, R., (1998). Opisthobranchia. Pp. 915-1035 in Beesley, P. L., Ross, G. J. B.and Wells, A. (eds.), Mollusca: The Southern Synthesis. Fauna of Australia. 5, Part B. CSIRO Publishing, Melbourne.
– Behrens, D. W., (2005). Nudibranch Behaviour. New World Publications, Florida, USA.
– 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.
– This NudiNote has been modified from a previously published article in DIVE LOG Australiasia Magazine’s – NudiNotes Column, Issue: #394 (June 2022): 10-12 by David A. Mullins.