Author: (Risbec, 1956)

Order: Nudibranchia  Family: Chromodorididae

Maximum Size: 70 mm

Sightings: Sunshine Coast

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Goniobranchus coi  (Risbec, 1956)

The expanse of the dorsal mantle possessed of the cryptobranch dorid nudibranchs readily lends itself as a broad canvas for an incredible diversity of presentations. Whether it be striking, gaudy or flamboyant as a warning statement to predators, or plain and drab so as to be cryptic against the substrate attempting to avoid detection, this diversity seems to know no limit of colour, pattern and combinations thereof. One species that advertises its unpalatable nature, but in a manner that can only be described as a most elegant expression, is Goniobranchus coi.

Goniobranchus coi is a moderately large species up to 70 mm in length, but most usually found in the 20 to 50 mm range, having a generous mantle overhang that covers the whole body and the foot, apart from a small section of the posterior foot being sometimes exposed. It is elongate oval in shape with slight undulations around the periphery and is essentially flat but with some arching or fullness across the midline. The smooth mantle carries a thin dark purple line to the margin followed by a white band (sometimes absent) and then an even wider band of tan to purplish pink. Inside of that again the colour transforms to yellow or cream that may possess several dark spots, sometimes rings. This band is intruded upon by the large semicircular to finger-shaped scallops of a bilaterally-symmetrical “wavy ring” that encompasses the rhinophores, gills and the dorsal midline region located in-between, this region being tan to purple in colour, sometimes mottled. The ring is comprised of a dark purple to black band bordered with a narrow white line to its outside. To generalise, there are four pairs of scallops laterally plus single anterior and posterior scallops. Defensive mantle glands form a submarginal band around the mantle perimeter. The foot is translucent white to cream with an opaque white band to its border. The narrow dorsal purple edge to the mantle is repeated ventrally, often with an associated much broader but paler purplish band. There is a broad purple region located anteriorly on the underside of the mantle. The junction of the body and the mantle ventrally is marked by a dark purple line. Many subtle variations to these colours and arrangements have been recorded.

The rhinophore clubs are a translucent light brown colour often matching the encircled region of the mid-dorsum in colour. The gills, quadrangular in section, have both of the lamellate faces a translucent brown but the smooth inner and outer faces are lighter, sometimes white.

Goniobranchus coi is known to rhythmically raise and lower its mantle edge usually in a single movement for the entire periphery exposing the whole foot, sides of the body and oral tentacles.

The spawn is laid upon the substrate, attached along the length by one edge, as an upright spiral ribbon of two to three whorls. It is cream to yellow in colour with darker yellow spots of extra-capsular yolk scattered throughout. Goniobranchus coi is a sponge predator known to feed upon Chelonaplysilla violacea of the Darwinellidae Family. It separates and concentrates the noxious distasteful antifeedant chemicals produced by the sponge and contained in its tissues, into the previously mentioned mantle glands for its own defensive purposes.

Distribution is western and central Pacific and possibly even the eastern Indian Ocean off the tropical coast of Western Australia.

Originally described as Glossodoris coi from a specimen collected off the coast of Vietnam in the South China Sea it was later placed in Chromodoris before molecular sequencing in 2012.

Goniobranchus gleniei (Kelaart,1858) is a similar species thought to be restricted to the Indian Ocean. (The Sea Slug Forum has recorded a sighting of it in Solomon Islands, western Pacific Ocean.) It has different presentation to the central region of the mantle that is a dull brown colour with darker patterning that may form circles or other odd shapes.

Above: Goniobranchus coi exhibiting mantle flapping behaviour.

David A. Mullins – April 2021

References:

– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI: 26-27 (Species #69)

– Rudman, W. B. (1987) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris epicuria, C. aureopurpurea, C. annulata, C. coi and Risbecia tryoni colour groups. Zoological Journal of the Linnean Society 90: 305-407.

– 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, The Netherlands.

– Rudman, W. B., (1999 October 17) Chromodoris coi (Risbec, 1956). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chrocoi and associated messages.

– Rudman, W. B., (2001 Mar 19). Comment on Chromodoris gleniei from the Solomon Islands by Erwin Koehler. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/3964

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

– 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)

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #386 (February 2021): 16 by David A. Mullins.