Many people are surprised to learn that some nudibranch sea slugs are actually corallivorous, that is they feed upon corals. Those that keep corals in aquariums however know only too well the havoc certain nudibranchs can cause particular corals. Corals don’t look to be particularly appetising but a little explanation of their anatomy will help.
There are two basic coral types. To generalise, these are commonly referred to as soft corals (octocorals – eight tentacles to their polyps) and hard or stony corals (six, or multiples of six, tentacles to their polyps), both types being preyed upon by certain nudibranchs. Regardless of type, all corals have soft tissue, whether that be the thin layer of polyps all connected together by the coenosarc tissue that covers the corallite skeleton of the hard corals or, the polyps and the coenenchyme (a thin layer in the polyps but otherwise forms the bulk of the soft coral mass as supporting tissue) in the soft corals. The form and composition of structure is indeed diverse but to generalise again, the hard corals have that corallite skeleton for support that is secreted by the polyps whereas the soft corals generally have sclerites (small spiny internal spicules of various shape and size) within their coenenchyme tissue, in varying densities, for support. The soft corals include the gorgonian sea fans that have a fleshy covering over a horny axis that develops into long stems or a fan-like shape.
These corallivorous nudibranchs have evolved to feed upon that soft tissue and polyps. All belong to the nudibranch suborder Cladobranchia and all are obligate coral feeders. These include some species from each of the major groups within the Cladobranchia: Arminina, Dendronotina and Aeolidina. Not all corals though are subject to such predation with most corallivorous nudibranchs demonstrating a distinct preference in their choice of prey, although there have been some reports of host switching if the preferred species is unavailable.
In the Arminia it is the Arminidae family, principally the Armina and Dermatobranchus, that prey upon the soft corals (including gorgonians and also the related sea pens) but generally speaking the specific diet of many is not well known.
Among the Dendronotina, those cladobranchs with raised rhinophore pockets and well developed dorso-lateral secondary gills down each side of the body, it is the members of the Tritoniidae family that feed on the soft corals (including gorgonians) with Tritonia, Tritoniopsis and Marionia being the best known.
The Aeolidina has a number of corallivorous species. These include some members of the Tenellia/Phestilla (specifically those formerly/currently known as Phestilla depending on taxonomy followed), that prey on hard corals, and the Phyllodesmium, that prey on soft corals (including gorgonians). Unlike their other relatives in the Aeolidina, the Phyllodesmium and Phestilla/Tenellia do not sequester the nematocysts (stinging cells) from their prey for defence purposes, even if they have cnidosacs, it being surmised that the nematocysts of their prey corals are not powerful enough for that defensive purpose. Many though have evolved toxic glands on their cerata plus colour and morphology (structural form) to match/mimic their host becoming cryptic in situ to provide protection. The cerata of most can also be autotomized to distract predators as a further defensive attribute. Many of these aeolids are also known to take up zooxanthellae from their host and farm them (for varying periods) symbiotically in their cerata such that they have often been referred to as “solar-powered” nudibranchs.
David A. Mullins – February, 2021
I’m indebted to Terry Farr for guidance on the octocoral identifications.
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– This NudiNote has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #384 (October 2020): 17 by David A. Mullins.