Author: (Eschscholtz, 1831)

Order: Nudibranchia  Family: Fionidae

Maximum Size: 25 mm

Sightings: Sunshine Coast

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Fiona pinnata (Eschscholtz, 1831)

The image above is of a moribund and dehydrated Fiona pinnata specimen collected from a large piece of driftwood on the beach, above the tideline at the time. The animal was placed back in sea water to try and “hydrate” it somewhat in an attempt to secure an image. This, as expected, was only partially successful, however, rhinophores, mouth, oral tentacles, foot and cerata are all observable. This species is the only member of the aeolid genus Fiona.

The body is elongate with the foot extending laterally past the width of the body. Rhinophores and oral tentacles are both long and smooth and translucent white. The notum carries patches of opaque pigment. Colour varies according to diet: white/cream/brown with a gooseneck barnacle diet and blue/purple with a siphonophore/chondrophore diet. The numerous cerata are irregularly sited laterally down the length of the notum leaving the midline region of the dorsum free. The cerata do not possess cnidosacs and interestingly carry an undulating (though immobile) fin-like membrane on their posterior-dorsal edge giving the cerata a sail-like appearance. The digestive gland is visible within. The white anal papilla is most obvious, being located midway along the right side dorsolaterally just within the line of cerata.

Distribution is worldwide in all oceans on floating items that have gooseneck barnacles (Lepas spp.) attached upon which they feed. Cuttlefish (Sepia) bones are a common habitat. Siphonophores and chondrophores are also believed to be part of their diet. It does not swim itself and lacks the buoyancy mechanism of the Glaucus. Life is limited to the amount of food available on their raft upon which they lay spawn. The hatched planktonic larvae drift in the ocean current until they are able to metamorphose on a suitable food source and repeat the cycle.

David A. Mullins – March 2023

References:
– Lalli, C. M. & Gilmer, R. W. (1989) Pelagic Snails – The Biology of holoplanktonic Gastropod Mollusks. Stanford University Press Stanford, California.

– Rudman, W.  B. (1998). Suborder Aeolidina Pp 1011-1017 in P.L. Beesley, G.J.B. Ross, and A. Wells, eds., Mollusca: The Southern Synthesis. Fauna of Australia. 5, Part B. CSIRO Publishing, Melbourne.

– Behrens, D., image by Spalding, G. Opisthobranch of the Week, Week: 26. Mike Miller’s Slug Site. Available at: http://slugsite.us/bow/nudiwkps/nudiwk26.html

– Rudman, W. B. (2002 June 18). Fiona pinnata (Eschscholtz, 1831). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/fionpinn and associated messages.

– Burn, R. (2015). Nudibranchs and related molluscs. Museum Victoria.

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