CERATA: The Multifunctional “Fingers” of Sea Slugs
Part 1 – Introduction
The term cerata (singular: ceras) is most often associated with the processes that arise from the dorsum of the Aeolidina – the aeolid nudibranchs. However, it is also commonly applied to the processes found on the dorsum of some of the Sacoglossa sea slugs such as those of the Caliphyllidae, Hermaeidae and Limapontiidae families (everyone knows ”Shaun the Sheep”) even though they have evolved independently through a different lineage. The literature also, from time to time, refers to the dorsal processes of the Dendronotina and many of the Arminina as cerata although in the Dendronotina the terms branchial plumes or secondary “gills” are used more frequently.
Interestingly, among the Doridoidea – the dorid nudibranchs, there are some genera, namely Cadlinella and Ceratophyllidia for example, whose members have distinctive cerata-like processes on their dorsum, but these are referred to as tubercles or papillae rather than cerata. In the former the processes are not autotomised but in the latter they are, and have also been reported to expand and contract. Other dorid nudibranchs with distinctive “non-ceratal” dorso-lateral processes include Okenia – Goniodorididae family and Kaloplocamus, Plocamopherus, Limacia and Crimora – Polycerididae family. There is a paucity of information in the literature on the internal anatomy of these non-ceratal processes, but none receive branching from the digestive system and neither are they believed to play any major role in respiratory function. Some however are known to possess concentrations of spicules or glands containing noxious or repugnant compounds and some can be autotomised, both indicating they may have developed for defensive purposes. Not included here, of course, are all of those species that have specialised processes for the protection of rhinophores (extrarhinophoral processes) or gills (extrabranchial processes).
Early descriptions of the aeolids (eolids) referred to their dorsal appendages as dorsal or branchial papillae. According to Herdman, 1890, the term cerata was introduced for them by Lankester, 1875 in his Mollusca article in the Encyclopaedia Britannica. Lankester did not invent the word, which is of Greek derivation meaning “horn”, the word having long been in use to describe the antlers (horns) of deer etc. Lankester however did not reserve the term for those processes that we today call cerata but rather used it broadly to encompass all “…papillae or ‘cerata’ of the dorsal integument….”.
Generally speaking most cerata are thin-walled elongate (finger-like or otherwise) outgrowths of the dorsum, are haemolymph (blood and lymph fluid) filled and also receive a branch from the digestive system. Branches of the digestive gland may also penetrate the cerata in certain species of Sacoglossa its presence or absence there varying even within families. In those Sacoglossa, Dendronotina and Arminina species that do possess cerata however, some only have the branches of the digestive system penetrating as far as the base of each ceras while in others it penetrates the entire length.
The cerata of the Aeolidina could be considered the apogee of ceratal development. The various functions attributed to them far surpass any other anatomical feature with regards to diversity of roles. These are: gaseous exchange, receiving a branch of the digestive system, hosting and “farming” of zooxanthellae, storage of digested products, modified for reproductive assistance (though rarely so), acting as a “heart” (also rarely so) and a number of defensive applications, including autotomy, deimatic display, nematocyst storage and deployment, a number of defensive glandular secretions and host mimicry.
The functions discussed in the following parts are those ascribed generally to the Aeolidina. Rudman, 1981 comments: “The cerata are the hallmark of an aeolid. Although similar dorsal outgrowths of the body are found in other opisthobranch groups such as the sacoglossans, arminids and dendronotaceans, in no other group have they developed to such a degree. They have a major role to play in the animal’s defence and also in the efficiency of its metabolism.”
(Note: where an sp. number is used in this NudiNote it refers to a species shown on this site.)
David A. Mullins – January, 2022
– Lankester, E. R., (1883). Mollusca, Encyclopaedia Britannica, ninth edition, Vol: xvi, p. 655.
– Herdman, W. A., (1890). On the Structure and Functions of the Cerata or Dorsal Papillae in some Nudibranchiate. Mollusca. Quarterly Journal of Microscopial Science 31: 41-63.
– Rudman, W. B., (1981). 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., (1999, July 1). Cerata (ceras) in aeolids. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/ceras
– 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.