More Colonial Consumers
Sea Slugs that graze upon Ascidians, Bryozoans and Entoprocts
It’s been mentioned before that the feeding preferences of sea slugs, across the entire group, encompass an extremely diverse range of food types, but conversely, each species usually has an extremely specific diet. In this NudiNote, three other phyla from the Animal Kingdom, that sea slugs prey upon, are discussed. Sponges and cnidarians, such as corals, hydroids and anemones, are well known colonial animals that sea slugs consume and those food sources have been covered in previous NudiNotes. However, there are other, not so well-known, colonial animals that certain sea slugs feed upon. These are the ascidians, bryozoans and entoprocts. (Although there are some ascidian species eaten by sea slugs that are solitary forms.)
Ascidians, belying appearance, belong to the chordates (phylum Chordata) albeit a primitive form compared to the vertebrates that also comprise the phylum. In the adult form they are simple filter-feeders with an outer “tunic” attached to the substrate and may be solitary or colonial (compound) in presentation. It is the brief, non-feeding larval stage that provides their affinity to the chordates with a notochord in the tail that is lost, as such, when it settles, attaches and metamorphoses into the adult.
Of the Nudibranchia it is among a couple of the phanerobranch families (those that lack a gill pocket) that the ascidian predators are found e.g. the Nembrotha of the Polyceridae family and also some of the Goniodoris and Okenia of the Goniodorididae family.
Some of the larger Nembrotha appear to graze through a field of ascidians consuming the animals entirely e.g. the large Nembrotha milleri, while others e.g. Nembrotha lineolata, prefer to climb up the ascidian in order to plunge their long feeding tube down through the ascidian’s open siphon and consume the inner organs. The larger ascidians with a tough outer layer or “tunic” are drilled into by some species of Nembrotha to gain access to the inner tissues. Certain species of Goniodoris can be found situated in craters they have eaten out of compound ascidians, often with their egg ribbon laid around the edge of the crater.
Some of the side-gilled slugs too, e.g. those of the Pleurobranchus genus, prey on ascidians, most usually being active and feeding nocturnally.
Bryozoans are minute colonial animals. Most are sessile, highly variable in shape, living attached to the substrate as encrusting sheets or standing more erect not unlike coral growths or algae (arborescent) and feeding via a tentacled lophophore protruded from their surrounding exoskeleton.
The Bryozoa, both encrusting and arborescent forms, are a major food source for phanerobranch nudibranchs. Many, if not the greater proportion of the phanerobranchs are bryozoan predators notably the Polyceridae family (e.g. Polycera, Tambja, Plocamopherus, Kaloplocamus, Thecacera, Crimora to name a few of the more well-known genera), Goniodorididae family (Goniodoris, Goniodoridella, Okenia, withTrapania being a significant exception) and Onchidorididae family (Diaphorodoris, Idaliadoris and Onchidoris for example). The presence of the nudibranch on the bryozoan, especially the broad encrusting type, is often noticed by the change in colour of the predated portion where the living animals have been removed from their exoskeleton, not unlike the polyps of a calcareous coral. Some nudibranchs are translucent such that the underlying features of the encrusting bryozoans show through clearly, thus providing a form of camouflage for the predator as it feeds. The presence of these species may be often betrayed by the circular egg mass they lay upon the bryozoan colony nearby. Previously, many of the species that feed on encrusting bryozoans that grow on algal fronds were thought to be feeding on the algae as they were always found crawling over it.
Also in the Arminina of the cladobranchs, the family Janolidae (e.g. Janolus), family Proctonotidae (e.g. Caldukia) and family Madrellidae (e.g. Madrella) are well known consumers of bryozoans. Of those mentioned, the Janolus feed upon arborescent forms whilst the Madrella feed on the encrusting types.
It was long thought that species of Trapania fed on sponges, having always been sighted thereon, however closer investigation revealed that what they were actually consuming was a totally different organism that lives in association with the sponges – the entoprocts (you might sometimes see them called kamptozoans). Entoprocts are tiny filter-feeding animals that resemble hydroids, superficially anyway, and because of their perceived similarity both as adult and larvae to the bryozoans were originally classified as such. They are now classified in their own phylum the Entoprocta. The adults are sessile dwellers attached to the substrate or living epizooically on other invertebrates and most are colonial. Reproduction is sexual and the eggs hatch into planktonic larvae but all can also reproduce by cloning. This brings us to those most often seen in association with sponges. Unless you are looking for them they can be difficult to see. A fuzzy appearance to the surface of a sponge can be an indication of their presence.
Species of entoprocts are believed to be the sole prey of the phanerobranch genus Trapania, relatively small members of the family Goniodorididae, of the Nudibranchia.
David A. Mullins – September 2021
– Willan, R. C. (1984). A review of diets in the Notaspidea (Mollusca: Opisthobranchia), Journal of the Malacological Society of Australia, 6(3-4): 125-142
– Rudman, W. (1987). The genus Trapania (Nudibranchia: Goniodorididae) in the Indo-West Pacific. Journal of Molluscan Studies, 53, 189–212.
– Rudman, W. B., (2000, May 25). Ascidians (Sea squirts, Tunicates). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/ascidian
– Picton, B., (2001, Feb 12). The food of Trapania pallida !!. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3767
– Picton, B., (2001, Feb 12). More Trapania food!!. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3770
– Rudman, W. B., (2001, Feb 12). Comment on The food of Trapania pallida !! by Bernard Picton . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3767
– Behrens, D. W. (2005). Nudibranch Behaviour. New World Publications, Florida, USA.
– Rudman, W. B.,(2006, December 21). Kamptozoa (Entoprocta). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/kamptozoa
– Rudman, W. B., (2006, December 23. Bryozoa (Ectoprocta, Lace corals). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/bryozoa
– Gosliner, T. M. & Fahey, S. J. (2008). Systematics of Trapania (Mollusca: Nudibranchia: Goniodorididae) with descriptions of 16 new species. Systematics and Biodiversity 6 (1): 53–98.
– Burn, R. (2015). Nudibranchs and related molluscs. Museum Victoria.
– Willan R. C. & Chang Y. -W. [Yen-Wei]. (2017). Description of three new species of Tambja (Gastropoda, Nudibranchia, Polyceridae) from the western Pacific Ocean reveals morphological characters with taxonomic and phylogenetic significance for traditional Polyceridae and related ‘phaneorobranch’ nudibranchs. Basteria. 81(1-3): 1-23.
– Gosliner, T. M., Valdés, Á. & Behrens, D. W. (2018). Nudibranch and Sea Slug Identification: Indo-Pacific – 2nd Ed. New World Publications, Jacksonville, Florida.
– 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 Magazine’s – NudiNotes Column, Issue: #389 (August 2021): 44-45 by David A. Mullins.