Named for it’s ‘t-bar’ shape, C. trilobatum’s most distinguishing feature is its mantle. Covering the whole anterior dorsal side of the individual, the mantle extends from the distinctly cephalised head, culminating in the ‘lure’ like protuberance above the anal gill. It is margined be a thick, unbroken dark purple line. There are two sets of paired lateral lobes, one cephalic and one in line with the gills. At the head, there are paired lamellate rhinophores. As in all dorid nudibranchs, the gills are clustered circularly around the dorsal anus, with twenty branches featuring reduced lamellae (Rudman), that can be retracted when disturbed. The foot extends from below the mantle, and continues posterior to the gills at a length greater than the mantle, giving the body en elongated shape. When handled the body is firm and has the common slimy feel noted with many soft bodied marine species.

Most likely due to the confusion caused by the varied colour morphs, size estimates for this animal vary. General consensus and personal and community field observations indicate that it is quite large compared to most other nudibranch species. Individuals used for Rudman's observations were approximately 80-115mm, and Marshal & Willan noted a maximum length of 130mm.

As previously stated, there is a vast variety of colour morphs within this species, so diverse that they were thought to be many separate species instead of one. As the nudibranch eye is rather basic, these bright and varied colour schemes are likely to be a form of aposomatic warning colouration to alert predators that they are unpalateable (Behrens).

 

In the current specimen, body colouration is a mottled mixture of oranges, brown and cream, overlaid with orange spots. The gill structures are orange and cream, and with bright orange rhinophores tipped in purple. The mantle is bordered completely by a thick, unbroken purple line. The foot is bordered by a non-uniform shaded purple to blue line.  

 

One of the best examples of the diversity of colour morphs is the difference between Western Australian C. trilobatum, compared to east coast specimens, which occur at similar latitudes on opposing ends of the continent. Additionally, a specimen found in the Northern Territory and initially identified in print as C. corallinum (Marshall & Willan), a name no longer accepted, featured a bright magenta colouration with a white mantle ridge.

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It is worth noting that this species, particularily the colouration found in south-east Queensland, is frequently confused with C. tenue, due to their similar shape and appearance. However they can be distinguished by the lack of a third lateral lobe, whoch is present in C. tenue, occurring posterior to the head and anterior to the gills. Additionally, in C. tenue, the purple border around the mantle has a ‘dashed’ line appearance, whereas in C. trilobatum it is solid.

The species is found in a variety of habitats and depths. Around North Stradbroke Island, the population appears most abundant at Amity Point, a shallow(<8m), sandy bottomed coastal gradient with patches of rocks and artificial substrates such as jetty pilings and debris. Individuals have also been noted at the island's north eastern offshore rocky reefs, at approximately 16m in depth. At Heron Island, a few individuals were found at a subtidal region with patches of course sand, and plentiful living coral, between 14-22m in depth (Marshall & Willan). Habitat choice is likely driven by location of prey species.

C. trilobatum, like many cryptobranch dorid nudibranchs, are carnivorous and eat sponges, most likely the siliceous variety (Marshall & Willan). They are likely to prey on a limited number of species and unless dislodged by an outside force, individuals likely remain in locations where preferred prey is found (Willan & Coleman). The photographic evidence provided for C. trilobatum in eastern Australia, as most are found on or nearby sponges, supports this.

Not many animals appear to target nudibranch species as a prey item (Marshall & Willan), however they have been observed being consumed by fish. It is possible that there is a heavy predation pressure on Ceratosoma nudibranchs is some regions, as many specimens, including some C. trilobatum, have been found missing the ‘lure’ like acid gland protuberance (Rudman).

The only apparent defence in this species is it’s toxic ‘lure’ and accompanying aposomatic colouration. The term ‘lure’ refers to the high, curved dorsal protuberance present behind the gills, that acts as a physical shield and terminates in a concentrated acid gland (Behrens). These acids are produced using chemicals found in the animals prey sponges, which are extracted and modified and moved to the storage area without damaging the nudibranch. By advertising this high appendage, would-be fish predators will attack it first, resulting in minimal losses for the individual. Additionally, the acids likely have an unpleasant taste and may directly damage the predator’s mouth, resulting in avoidance of the species and similarly coloured individuals. The success of this defence can be noted in healthy individuals collected missing this protuberance (Rudman).

Unfortunately, no reproduction was observed during this study.

Nudibranchs are hermaphroditic, and have fully functioning organs of both sexes, however they do not self fertilise. Mating behavior consists of two individuals lining up right side to right side, or 'top to toe', and both sets of organs are utilised. Both individuals will then lay spawn, which in C. trilobatum is orange (Marshall & Willan).

Specific larval information for this species is not readily available. As in all nudibranchs, there is likely a planktotrophic larval stage, before settlement occurs when appropriate prey is located.  

Locomotion is achieved as in other dorid species, by muscular waves through the foot. As a larger individual, it is unlikely that cilia are used much, if at all, in movement (Behrens). This species has been observed in captivity to leave a slime trail behind on the tank glass (community observation), so mucus secreted by the foot likely assists in forward movement. A form of 'rearing' was also observed in the field.

As is common amongst most gastropods, there is distinct cephalisation resulting in a clear anterior head. A basic eye, likely only capable of sensing changes in light, direct observations to the brain via an unrefined optic nerve (Behrens). Rhinophores with a lamellate (plated) structure are used for detecting chemicals dissolved in the environmental medium, comparable to terrestrial smell, and for sensing vibrations. These can be retracted when stressed or harassed. The mantle in this species is reduced, with the complete loss of a mantle cavity and the ‘firmness’ experienced to the touch is attributed to a thickening of the body wall (Rudman). All mantle glands are concentrated to the dorsal protuberance (Rudman; Behrens). Reproductive organs are located on the right side, due to partial detorsion common in many nudibranchs.

"A sponge - nudibranch interaction", 1985, Toxicon, vol. 23, no. 1, pp. 33-33.

Behrens, D.W., Petrinos, C. & Schrurs, C. 2005, Nudibranch behavior, New World Publications, Jacksonville, Fla.

Karuso, P. & Scheuer, P.J. 2002, "Natural Products from Three Nudibranchs: Nembrotha kubaryana, Hypselodoris infucata and Chromodoris petechialis", Molecules, vol. 7, no. 1, pp. 1-6.

Marshall, J.G. & Willan, R.C. 1999, Nudibranchs of Heron Island, Great Barrier Reef: a survey of the Opisthobranchia (sea slugs) of Heron and Wistari Reefs, Backhuys, Leiden.

 

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 (2/3): 115-27

Willan, R.C. & Coleman, N. 1984, Nudibranchs of Australasia, Australasian Marine Photographic Index, Sydney.

This compilation benefited greatly from the assistance of the greater Brisbane and Gold Coast diving community, including "Scuba Buddies Brisbane" members. Many thanks must go to Peter Grigg, Anna Toma and Loren Mariani for their generous photo contributions. Special thanks must go to Ryan Goldfinch for his help, time and patience in answering my questions.

Additional thanks go to Manta Lodge and Scuba Center at Point Lookout, for their help on the initial dive where I first found my study species.