Author: Rudman, 1991

Order: Nudibranchia  Family: Myrrhinidae

Maximum Size: 130 mm

Sightings: Sunshine Coast

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Phyllodesmium magnum  Rudman, 1991

Phyllodesmium magnum is a large (up to 130 mm) aeolid nudibranch that feeds upon corals, not the hard reef building species but a few of the soft octocoral types.

Elongate in shape it tapers from a broad anterior down to a narrow tail posteriorly. The rhinophores are smooth as are the oral tentacles that are twice as long as the rhinophores. Both have rounded tips. The large foot is twice the body width and bears blunt propodial tentacles (anterior foot corners). This nudibranch is characterised by the distinctive densely packed cerata it bears upon its dorsum the largest located medially and becoming smaller towards the edge of the dorsum. These cerata are large and smooth, changing from a cylindrical shape proximally to broad and flattened finally finishing in a bluntly tapering tip. They are curved and may curl up, often quite tightly, completely obscuring the dorsum, especially as it feeds. All the cerata contain many fine branches of the digestive gland. Each of these many branches bears a terminal chamber within which the nudibranch farms live zooxanthellae that it has obtained from the tissues of its prey. These appear as brown patches. Very fine branches of the digestive gland also extend into the body wall, foot and even the rhinophores and oral tentacles, and these too house zooxanthellae. All of these features contribute to this species being especially well adapted to provide optimum conditions for zooxanthellae symbiosis. This relationship provides nutrients and oxygen, byproducts of the zooxanthellae photosynthesis, for use by the nudibranch.

The colour of Phyllodesmium magnum is greatly influenced by the presence of the zooxanthellae. The body colour is translucent white or cream however, depending upon the concentration of zooxanthellae, it can appear a quite dark brown. The same may be said of the cerata that are yellow ventrally but may be dark brown dorsally. Rhinophores and oral tentacles may be translucent white or yellow but depending upon the penetration of farmed zooxanthellae may be dark brown for any length dorsally. Often a lilac colour may appear on portions of the head, rhinophores, oral tentacles, tail and even the dorsal surface of the cerata. Rudman suggests this may be a result of the incidence of light upon the zooxanthellae. There is a consistent white stripe presenting on the head running from just anterior to the rhinophores to between the base of the oral tentacles. In some specimens yellow is obvious on the tips of the rhinophores, oral tentacles, cerata and as a line around the margin of the foot. Juveniles of the species are sometimes white to cream indicating a lack of resident zooxanthellae at that stage of their development.

Zooxanthellae are microscopic singled-celled plants (dinoflagellates) of the genus Symbiodinium that live in the tissues of certain animals in a mutualistic relationship. The best known hosts are the cnidarians such as anemones, corals and jellyfish but are also seen in the giant clams, the Tridacna. Certain species of nudibranchs and flatworms derive their population of zooxanthellae secondarily through their cnidarian diet.

As mentioned, the presence of these zooxanthellae in the nudibranch Phyllodesmium magnum can be observed from the brown mottled appearance (sometimes lilac) of the cerata and even the skin of the body and rhinophores. The nudibranch obtains nutritional benefits from the byproducts of the zooxanthellae photosynthetic activity that are released into the digestive gland. The expanded flat dorsal surface of the nudibranch’s cerata with its profusely branched digestive gland is always directed outwards/upwards towards the sunlight for maximum exposure increasing the efficiency of the zooxanthellae that are concentrated nearer the outer sun-directed face. Because of this capability of obtaining nutrients from the photosynthesising zooxanthellae it is often referred to as a “solar-powered” sea slug.

Unlike many aeolid nudibranchs, species of Phyllodesmium, including Phyllodesmium magnum, do not carry the defensive cnidosac full of stinging nematocysts, obtained from the coral, in its cerata tips. Generally speaking most soft corals and especially the ones that Phyllodesmium magnum feed upon have nematocysts that are less powerful than those of the anemones and hydroids. It is thought this might make those nematocysts unsuitable to collect and store in cnidosacs for defensive use. There is however a terminal sac that may or may not be homologous with the cnidosac.  For defensive purposes it engages instead in a type of behaviour called autotomy whereby if threatened it deliberately casts off its cerata, quite readily, to distract and confuse potential predators. The cast off or autotomised cerata wriggle energetically for some time (hours) and produce a sticky secretion from epithelial glands such that they strongly adhere to the substrate or even to the predator thereby preventing dispersal in currents or surge.

The octocoral species that Phyllodesmium magnum has been observed feeding upon all belong to the family Alcyoniidae and include Sinularia and Sarcophyton species and upon which, at times, it may be quite cryptic and difficult to discern with its cerata tightly coiled up and held close to the body. If it has been feeding upon a particular specimen of coral for sometime its presence may be betrayed by a pile of indigestible, discarded coral sclerites at the base of its host.

This species has a wide distribution in the tropical & subtropical western Indo-Pacific. It has been recorded from South Africa and even the Red Sea. Yonow suggests that a species called Eolida bella, by Rüppell & Leuckart in 1830, may be this species and should perhaps take precedence.

David A. Mullins – September 2020

References:

– Rudman, W. B. (1981). The anatomy and biology of alcyonarian feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society 72: 219-262.

– Rudman, W. B. (1991). Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies, 57(2): 167-203.

– Rudman, W. B., (1999 November 22) Phyllodesmium magnum Rudman, 1991. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/phylmagn & associated messages.

– Fabricius, K. & Alderslade, P. (2001). Soft Corals and Sea Fans. A comprehensive guide to the tropical shallow-water genera of the Central-West Pacific, the Indian Ocean and the Red Sea. Australian Institute of Marine Science: Townsville. 1-264.

– Yonow, N., (2008) Sea slugs of the Red Sea. Pensoft Publishers, Sofia

– Moore, E. & Gosliner, T. (2014). Additions to the Genus Phyllodesmium, with a Phylogenetic Analysis and its Implications to the Evolution of Symbiosis. The Veliger 51(4):237–251.

– Martynov, A., Mehrotra, R., Chavanich, S., Nakano, R., Kashio, S., Lundin, K., Picton, B. & Korshunova, T. (2019). The extraordinary genus Myja is not a tergipedid, but related to the Facelinidae s. str. with the addition of two new species from Japan (Mollusca, Nudibranchia). ZooKeys 818: 89–116.

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – Critter ID with NudiNotes Column, Issue: #375 (October 2019):12 by David A. Mullins.