Author: (Bergh, 1896)

Order: Nudibranchia  Family: Myrrhinidae

Maximum Size: 30 mm

Sightings: Whitsundays, Anilao Philippines, Timor Leste

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Phyllodesmium briareum (Bergh,1896)

Phyllodesmium briareum has a long narrow body with a small head. The oral tentacles are long and tapering whilst the rhinophores are shorter and smooth. 

The cerata are long (some almost as long as the body) and tubular in shape with some distal tapering, terminating in a blunt tip. They are marshalled in single rows, each row elevated on a ridge. The arrangement of cerata is distinctive and usually consistent. Anterior to the pericardial gap are two rows of cerata on both sides of the notum that form an inverted vee as they slope towards each other. Posterior to the pericardial gap the cerata are arranged in single rows paired each side down the length of the notum. Each row contains four to five cerata with the largest ceras closest to the midline and the rest graduating smaller in size laterally. The first two post-pericardial rows present with a gap on the notum between left and right rows but the balance of rows, of up to four pairs, have no gap between left and right. The arrangement is completed posteriorly by a further two or three single cerata. 

The body, cerata, rhinophores and oral tentacles are translucent white allowing regular rows of minute brown specks to show through, presenting an overall brown appearance. These brown rows are arranged longitudinally on cerata, rhinophores and oral tentacles but transversely on the body. The brown rows are comprised of zooxanthellae sequestered from their soft coral prey and utilised for their photosynthetic by-products. The tips of all the appendages – the cerata, the rhinophores and the oral tentacles carry a soft yellow pigmentation.

The genital pore is located on the right side, just below the vee opening formed by the two sloping pre-pericardial rows. The anus is also on the right side but more posteriorly, just behind the base of the medial-most ceras of the first post-pericardial row. 

The foot is quite narrow, however at the anterior corners it flares out to form angular propodial extensions.

Size has been recorded to 30 mm.

Phyllodesmium briareum feeds on soft corals of the Briareidae family of which some species of Briareum are on record as prey. It is thought that the author of the species, Bergh, gave it the name briareus due to its feeding association with this soft coral. As previously mentioned it has a symbiotic relationship with the zooxanthella obtained through its consumption of the soft coral and benefits from the by-products of the zooxanthellae’s photosynthesis. It also benefits from the camouflage afforded when on its prey by the colour created by the zooxanthellae within its tissues.

The type locality is Ambon (Amboina at the time), Indonesia from where the holotype was collected in 1890 the description being published in 1896, but it has a wide western Pacific distribution with other records from Indonesia; Philippines; Vietnam; Japan; Vanuatu; Malaysia; PNG and Queensland, Australia.

Originally described as Ennoia briareus. It was redescribed for the first time by Rudman, 1991, who placed it in Phyllodesmium (Ennoia being a synonym of Phyllodesmium) that necessarily required a change to the ending of the specific epithet briareus to briareum to match the gender of Phyllodesmium.

David A. Mullins – August 2023

References:
– Bergh, L. S. R. (1896). Eolidiens d’Amboine. Revue Suisse de Zoologie. 4(2): 385-394.

– Rudman, W. B. (1991). Further studies on the taxonomy and biology of the octocoral feeding genus Phyllosdesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies 57:167–203.

– Rudman, W. B. (1999 November 25). Phyllodesmium briareum (Bergh, 1896). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/phylbria and associated messages.

– Yonow, N. (2017). Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 16. The Nudibranchia – Dendronotina, Arminina, Aeolidina, and Doridina (Mollusca: Gastropoda: Heterobranchia). Archiv für Molluskenkunde. 146(1): 135-172.

– Gosliner, T. M., Valde ́s, A ́. & Behrens, D. W. (2018). Nudibranch & Sea Slug Identification – Indo-Pacific, 2nd Edition. New World Publications, Jacksonville, Florida, USA.