Breathing, cerata and gills

Sea slugs breathe using a variety of anatomical structures that differ according to the group to which they belong. As aquatic animals, they obtain oxygen dissolved in the water and eliminate carbon dioxide through surfaces adapted for gas exchange. In some groups, this function is performed mainly by specialised gills; in others, the cerata or a substantial part of the body surface are involved.

The gills of dorid nudibranchs

One of the most characteristic respiratory structures in dorid nudibranchs is the branchial plume, located on the posterior dorsal surface of the body, usually surrounding the anus. It consists of branched projections that increase the surface area available for gas exchange with the water.

The shape and degree of branching of the gills vary among species. In some dorids, the branchial plume can retract into a protective cavity when the animal feels threatened, whereas in others the gills remain exposed. Traditionally, this distinction has been used to differentiate dorid nudibranchs with retractile gills, known as cryptobranchs, from those that cannot completely conceal them, known as phanerobranchs.

Phyllidiids are a notable exception: they lack the typical perianal branchial plume found in many other dorid nudibranchs and possess specialised lateral respiratory structures.

Cerata: respiration, digestion and defence

Cerata are projections of the body surface that extend from the dorsum and, in many cases, are arranged in rows or groups along the sides of the body. They are particularly characteristic of aeolid nudibranchs, although they also occur in other sea slug lineages.

In aeolids, cerata play an important role in respiration. Their relatively thin walls and large surface area facilitate the diffusion of oxygen from the water into the haemolymph, the circulatory fluid of molluscs, and the removal of carbon dioxide. Because of this function, they are often described as secondary gills, although they are not gills in the strict anatomical sense.

Cerata also perform other functions. In many aeolids, each ceras contains an extension of the digestive gland, which is involved in processing and distributing nutrients. In addition, in numerous species, the tip of each ceras contains a cnidosac, a specialised structure where nematocysts obtained from cnidarian prey are stored. These stinging capsules may retain their functionality and be used as a defence mechanism.

Some sea slugs can voluntarily shed part of their cerata when disturbed or attacked. This phenomenon, known as autotomy, may act as a distraction strategy against predators. Lost cerata can subsequently regenerate.

Other forms of respiration

Not all sea slugs have a dorsal branchial plume or cerata. In other groups, the gills may be located inside the mantle cavity, beneath the mantle or along the sides of the body. Their arrangement reflects the anatomical diversity of the different lineages of marine gastropods.

Some sacoglossans, for example, have lateral body extensions known as parapodia. These structures can increase the surface area exposed to the water and contribute to gas exchange. In other species, the flattened body shape and the body surface play an important role in respiration.

Thus, gills, cerata, parapodia and the general body surface represent different anatomical solutions that enable sea slugs to obtain oxygen dissolved in water. Their diversity provides a good example of how the anatomy of these gastropods has adapted to different marine lifestyles.

References

• Dean, L. J., & Prinsep, M. R. (2017). The chemistry and chemical ecology of nudibranchs. Natural Product Reports, 34(12), 1359–1390. https://doi.org/10.1039/C7NP00041C
• Potts, G. W. (1981). The anatomy of respiratory structures in the dorid nudibranchs, Onchidoris bilamellata and Archidoris pseudoargus, with details of the epidermal glands. Journal of the Marine Biological Association of the United Kingdom, 61(4), 959–982. https://doi.org/10.1017/S0025315400023080
• Wägele, H., & Klussmann-Kolb, A. (2005). Opisthobranchia (Mollusca, Gastropoda)—More than just slimy slugs. Shell reduction and its implications on defence and foraging. Frontiers in Zoology, 2, Article 3. https://doi.org/10.1186/1742-9994-2-3

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