Author: Abraham, 1876

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 120 mm

Sightings: Sunshine Coast

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Ceratosoma tenue Abraham, 1876

Within the Ceratosoma genus there are three species with a considerable similarity of appearance. These are: Ceratosoma tenue, together with Ceratosoma trilobatum and Ceratosoma gracillimum the first two creating the most uncertainty. These three have long confused taxonomists with their overlapping external likeness and considerable intraspecific variation of colour and pattern. One taxonomist in the late 19th century even created three new names for C. trilobatum and three new names for C. tenue, each name reliant upon a single preserved museum specimen. There are at least five synonyms for C. tenue currently recognized.

Ceratosoma tenue is a large cryptobranch nudibranch of unusual shape and recorded up to 120 mm in size. It has an elongate body that is firm and rigid with a high profile and long tail. The mantle skirt is much reduced but is expanded into lobes at three places on both sides – on the head just anterior to the rhinophores, a wing-like expansion just anterior to the gills and the third in-between those two that is joined posteriorly to the gill lobe by a reduced notal ridge. The most posterior portion of the mantle is raised up behind and recurves anteriorly over the gills in the manner of a protective “horn” (however it can be held in the horizontal position) at approximately the midpoint of the animal. This “horn” contains large and closely packed defensive mantle glands around the edges. These glands contain noxious substances obtained from their sponge prey that may be released as a defensive plume when irritated or delivered as a nasty bursting mouthful should the attractive horn, acting as a lure and obviously sacrificial, be bitten. The anterior rim of the head region also contains some of these glands but there are fewer and they are not so tightly packed. Colouration is quite variable and ranges from orange/brown to yellow/green sometimes on a white background with yellow, orange and purple spots. The much reduced edge of the mantle carries a broken (sometimes unbroken) line of purple spots. The rhinophores are brown/orange/yellow in colour but all seem to carry a small purple tip.

The orange/red spawn is laid in a spiral of about three whorls on edge. The unattached or free edge is much longer than the attached edge causing the ribbon to have an undulating or “frilly” appearance.

Ceratosoma tenue is a sponge feeder thought to prey upon species of Dysidea. Interestingly when feeding the everted oral tube can be seen to be coloured in a similar manner to the body rather than the plain white or uncoloured oral tube of most chromodorids.

C. trilobatum was the first to be described, in 1827. The specific epithet of trilobatum refers to the three lobes around the gills – a large lateral each side and the posterior horn. C. tenue, our subject here, was described almost 50 years later and much of the confusion experienced by the uninformed, apart from similar colours and patterns, relates to C. tenue possessing three expanded lobes down each side often leading to the use of trilobatum for this species instead. Colour and pattern should not be relied upon for differentiation of the two but rather the number of side lobes, C. tenue = three, C. trilobatum = 2.

Both also have a similar geographic distribution, Red Sea, Indian Ocean, Indonesia, Philippines, Melanesia, Australia and the Western Pacific Ocean generally.

References:
– Abraham, P.S. (1876) Notes on some genera of nudibranchiate Mollusca, with notices of a new genus and some hitherto undescribed species, in the collection of the British Museum. Annals & Magazine of Natural History (4)18: 132-146, pls. 6-7.

– Rudman, W.B. 1984. The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society, 81: 115-273.

– Rudman, W. B. (1988). The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: the genus Ceratosoma J.E. Gray. Zoological Journal of the Linnean Society. 93(1): 133-185.

– Rudman, W.B. (1991). Purpose in Pattern: the evolution of colour in chromodorid nudibranchs. Journal of Molluscan Studies, 57: 5-21.

– Rudman, W.B., 2003 (Jun 5). Comment on Ceratosoma tenue with egg ribbon by Danny Van Belle. [Message in] Sea Slug Forum. Australian Museum, Sydney.

– Rudman, W.B., Bergquist (2007) A review of feeding specificity in the sponge-feeding Chromodorididae (Nudibranchia: Mollusca). Molluscan Research, 27(2): 60-88.

– Rudman, W.B., 2007 (Oct 11). Comment on Ceratosoma tenue feeding [1] by Bruce Wilkie. [Message in] Sea Slug Forum. Australian Museum, Sydney.

– Johnson, R.F., Gosliner, T.M. (2012). Traditional taxonomic groupings mask evolutionary history: A molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS One 7 (4): e33479.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #357 (April 2018): 12 by David A. Mullins

David A. Mullins, December 2019