405_April_24

Thuridilla carlsoni is one of the few in this genus of sea slugs that has a colouration that could be considered camouflage.

A peak inside the parapodia of Thuridilla carlsoni , a view that is not often afforded as the genus in general keeps them tightly closed.

of the Thuridilla that exhibit the stunning colours, especially along their margins, but it often extends forwards onto the head and out along the rhinophores. In this way the colourful Thuridilla , crawling around out in the open substrate, are different from the mostly cryptic Elysia species that are generally a uniform green colour, often of a dull hue and therefore camouflaged upon their host algae. The inside surface of Thuridilla parapodia, rarely visible, may also have quite interesting colouration. Although the hypothesis remains to be tested, it is thought that these bright colours and free movement out on the open

substrate of the Thuridilla might be evidence of aposematic (warning) colouration of noxious chemicals, within their tissues, as a defensive method, much like many of the colourful chromodorid nudibranchs (some of these chemicals are actually called thuridillins). In no way could they be described as camouflaged or cryptic. There are notable exceptions to this generalisation though. The parapodia of Thuridilla are essentially smooth although some do present with low smooth longitudinal ridges that in some specimens are broken up into spots that are raised and smooth. The auriculate rhinophores are long and smooth with a

Thuridilla bayeri is one of a group (the Thuridilla gracilis species-complex), of more than a dozen now recognised and putative species, that have very similar colouration.

Thuridilla sp. (sp. 10). A juvenile specimen of a putative species within the Thuridilla gracilis species-complex.

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DIVE LOG Australasia #405 - April ‘24

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