Author: Moore &Gosliner, 2014

Order: Nudibranchia  Family: Myrrhinidae

Maximum Size: 50 mm

Sightings: Sunshine Coast

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Phyllodesmium undulatum Moore & Gosliner, 2014

This species of aeolid nudibranch makes a stunning presentation against the red and yellow backdrop of the gorgonian coral upon which it preys.

Phyllodesmium undulatum is narrow and elongate with a transparent body allowing the internal organs to show through. The dorsum carries a thin white mid-line running longitudinally the fully length of the animal. Some specimens present with an additional white line laterally on both sides of the dorsum that extends from the base of the oral tentacle to each successive ceratal cluster, surrounding the base of each and even arching across to the opposite cluster when they are closely situated. The mostly transparent cerata are cylindrical and each contains a highly visible, exceptionally undulating, but unbranched extension of the digestive duct – hence the name undulatum. The colour of the digestive ducts changes from a dark cream or brown to orange at the distal end. Sub-apically the long cerata bear a light blue/purple colour with the yellow tips being tapered and curled and carrying non-functional cnidosacs. The oral tentacles are longer than the rhinophores however both are long, tapered to a point and smooth with yellow extremities sometimes with blue colouration evident towards the base. Some specimens present without blue pigment on any of the appendages. The narrow foot broadens anteriorly having developed corners that are moderately tentacular and with yellow edging.

Phyllodesmium undulatum preys on red and yellow gorgonian corals of the Melithaea genus such as Melithaea flabellifera.

Size is recorded up to 46 mm in length.

Distribution is broadly termed as Western Pacific Ocean but is only rarely sighted north and south of the Philippines and Malaysia. The Phyllodesmium as a genus are only known from the tropical regions of the Indo-Pacific.

Generally speaking, the Phyllodesmium are known for their well-developed cerata that often mimic the polyps of their very specific octocoral food source making them cryptic in situ. The cnidosacs of many aeolids, that contain nematocysts (stinging cells) sequestered from their prey, are either lacking or non-functional in the Phyllodesmium it being suggested that the nematocysts of their soft coral prey are not powerful enough to be useful in a recycled defensive capacity. While most species of Phyllodesmium are called solar-powered slugs because they host symbiotic zooxanthellae in their tissues, mainly in the cerata, Phyllodesmium undulatum is one of only a few that do not do so. This species is considered the most primitive of the Phyllodesmium due to the cylindrical cerata, unbranched ceratal digestive ducts, lack of symbiotic zooxanthellae in its tissues and presence of cnidosacs (though non-functional).

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

– Gosliner, T.M., Valdés, Á., Behrens, D.W. (2018). Nudibranch and Sea Slug Identification: Indo-Pacific – 2nd Ed. New World Publications, Jacksonville, Florida.

– 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: #359 (June 2018): 12 by David A. Mullins

David A. Mullins, July 2019