Author: Gosliner, 1995

Order: Sacoglossa  Family: Plakobranchidae

Maximum Size: 30 mm

Sightings: Sunshine Coast, Whitsundays, Capricorn/Bunker Group GBR

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Thuridilla carlsoni Gosliner, 1995

The genus Thuridilla was raised by Bergh back in 1872. Since then it has had a mixed existence, often synonymized with Elysia, until Jensen, 1992 confirmed its validity. In 1995 Gosliner described 9 new species for the genus including our subject here – Thuridilla carlsoni. Today, in 2021, we have 23 described species in the genus plus a handful of undescribed candidates awaiting taxonomist attention, as well. The greater majority are of Indo-Pacific distribution.

Thuridilla carlsoni is not as flashy as some of its sister species having muted colours and a nebulous patterning rather than bold splashes of contrasting colouration and pattern. It is well camouflaged, almost cryptic, on the turfing algae it is most generally found in association with. Its shape is limaciform in line with all of the Thuridilla species.The background colour is cream, covered in deep green spots, or patches consisting of clustered spots, such that the impression given is often of a cream reticulation over the parapodia. The parapodial margins are undulating but are most usually carried tightly closed where they meet on the midline as a wavy line extending posteriorly to the tail. The parapodial margins are pale yellow (sometimes pink), then white before turning pale green prior to the small green spots that then merge into the larger spots. Submarginally, in the region of the small green spots, there is a line of plain cream patches from the commencement of the parapodia down to the tail end. The large enrolled rhinophores, slit down the latero-posterior side, are essentially cream, the basal portion receiving a run up of the green spotting from the notum often followed by a finger of pale yellow. Distally they carry a burnt orange band, darkest on the tip. Eye spots are visible where the rhinophores join the head posteriorly. A view inside of the parapodia is not often afforded, but when visible, reveals a white submarginal band followed by diffuse black spotting that coalesces into a broad black band. The majority of the internal surface is a greenish yellow with darker diffuse green longitudinal lines. These lines are indicative of the veins servicing the parapodia.

There is disagreement in description of the composition of the green spots on the external parapodial surfaces. Gosliner, 1995, refers to them thus: “Thuridilla carlsoni has scattered green pigment granules….” whereas Rudman, 2006, states: “They are not pigment spots but ‘gardens’ of chloroplasts obtained from the green algae this animal feeds on.” Nearly every other Thuridilla though has bright and or solid colouration on the parapodia that in no way resembles a chloroplast “farm”.

Being a sacoglossan Thuridilla carlsoni is a sap-sucking slug feeding on the cell contents of algae. Their uniserrate radula pierces the cell wall and the cellular contents are sucked out for digestion.

The spawn is laid as a tight flat spiral of at least two whorls and is most usually a pale cream colour.

Distribution is wide in the Into-Pacific in tropical and sub-tropical waters.

Thuridilla carlsoni is named after Clay Carlson, one of the first to record the species from Guam.

From his systematics, Gosliner referred to Thuridilla carlsoni as the least derived member of the genus due to its features being almost entirely plesiomorphic.

David A. Mullins – August 2021

References:

– Jensen, K. R. (1992). Anatomy of some indo-pacific Elysiidae (Opisthobranchia: Sacoglossa (=Ascoglossa)), with a discussion of the generic division and phylogeny. Journal of Molluscan Studies, Volume 58, 3: 257–296.

– Gosliner, T. M. (1995). The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of their phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences, 49(1): 1-54.

– Rudman, W. B. (2006 Dec 30). Comment on Thuridilla carlsoni from Sulawesi by Mike Krampf. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http:www.seaslugforum.net/find/18628

– Pittman, C. & Fiene, P. (2020). Thuridilla carlsoni, Sea Slugs of Hawaii, Available at: http://seaslugsofhawaii.com/species/Thuridilla-carlsoni-a.html

– MolluscaBase eds. (2021). MolluscaBase. Thuridilla Bergh, 1872. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=137929 on 2021-08-03.