Nudi Notes

THURIDILLA THRILLS – Sea Slug Eye Candies That Aren’t Nudibranchs

Jun 4, 2024

THURIDILLA THRILLS
Sea Slug Eye Candies That Aren’t Nudibranchs

Thuridilla is a tropical and warm temperate genus of sea slug species belonging to the diverse sap-sucking order Sacoglossa. If you are looking for out-of-this-world colour contrasts and combinations to photograph, you need look no further to satisfy that appetite. 

The Plakobranchidae family of sea slugs holds the Thuridilla along with Elysia, Bosellia (lack parapodia) and Plakobranchus (broad and flattened body) genera. So the Thuridilla and Elysia all mostly have a similar body form – long and narrow, lacking a shell, high in profile in relation to width and possessing flap-like parapodia (extensions of the foot) that extend along the body for most of its length and wrap up both sides to usually meet on the dorsal midline with sinuous margins.

 

Above: The marginal and submarginal coloured bands of the parapodia are often a valuable characteristic for species separation. Clockwise from Upper left: Thuridilla hoffae; Thuridilla livida; Thuridilla multimarginata and Thuridilla sp. 08.

 

Without reference to the colours and their arrangement it would be impossible to separate species of Thuridilla based on external features so it is not surprising that colour patterns had been the basis of species differentiation. It is those parapodia 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 as well. 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, (some of these chemicals are actually called thuridillins), much like many of the colourful chromodorid nudibranchs. In no way could Thuridilla species be described as camouflaged or cryptic. There are a couple of notable exceptions to this generalisation though. 

 

Above: Clockwise from Upper left: Thuridilla lineolata; Thuridilla neona; Thuridilla sp. 07 (Image courtesy of Terry Farr) and Thuridilla undula (this last image leaves a lot to be desired).

 

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 often colourful auriculate rhinophores are long and smooth with a vertical slit, that together form a vee and are directed anteriorly.

 

Above: The majority of Thuridilla have a smooth surface to the parapodia but those shown here have raised coloured spots that sometimes are formed into ridges. Clockwise from Upper left: Thuridilla albopustulosa; Thuridilla vataae; Thuridilla sp. 02 and Thuridilla sp. 01. Compared to all the other brightly coloured and patterned species, the stark all white Thuridilla sp. 02 seems a bit odd, however, against the substrate it too is highly visible.

 

Crypsis, although infrequently expressed through camouflage against the habitat by this group, is however now well-known among certain clades of Thuridilla – the “Thuridilla gracilis species-complex” in particular – where as many as four currently described species (three of them resurrected from synonymy) and as many as a further ten putative species have now been identified. This is the other form of crypsis whereby unrecognised species lie hidden within already well-known species. This is known as taxonomic crypsis. This species complex contains a number of species that have long confounded taxonomists. Their shared features of a dark body with white or creamish longitudinal lines and an orange/red inner parapodial margin had been problematic. The molecular sequencing has allowed accurate differentiation between the species and therefore the ability to know which external colour features are important and consistent enough to be applied for that purpose. The colours and their arrangement on the head and rhinophores, when closely examined, have proven most helpful.

 

Above: This composite image serves to demonstrate the difficulty in separating some of the species within the “Thuridilla gracilis complex” and why they remained cryptic for so long. Molecular sequencing identified separate species but there is often very little by way of observable differences to make that separation. In many cases it is just the arrangement of the coloured bands on the rhinophore tips that provide the clue. Martín‐Hervás et al., 2021, have revealed there are more than a dozen now recognized and putative species, that have very similar colouration.
Clockwise from Upper left: Thuridilla sp. 11 (Thuridilla sp. 6 of Martín‐Hervás et al., 2021)(Image courtesy of Terry Farr); Thuridilla sp. 12 (Thuridilla sp. 7 of Martín‐Hervás et al., 2021)(Image courtesy of Terry Farr); Thuridilla sp. 13 (There is no match in Martín‐Hervás et al., 2021) and Thuridilla sp. 14 (Thuridilla sp. 2 of Martín‐Hervás et al., 2021).

 

Above: Clockwise from Upper left: Thuridilla bayeri; Thuridilla sp. 10 (Thuridilla sp. 8 of Martín‐Hervás et al., 2021); Thuridilla ratna and Thuridilla sp. 04. These species too are considered to be in the “Thuridilla gracilis” complex.

 

Whereas the Elysia are often found on their food such as the readily identifiable algae including, but not limited to, Caulpera, Codium and Halimeda, (and searching through those can be rewarding), the Thuridilla are not. They seemingly appear to be just wandering across the open substrate. This is where their food is often found, thought to be areas of various types of filamentous algae, covering the substrate.

 

Above: Upper row: Left: Thuridilla cf. splendens, a common species on the Sunshine Coast with some similarities to Thuridilla splendens; Right: Thuridilla cf. splendens seen here feeding on turfing filamentous algae. The diet of Thuridilla species is not well documented but are believed to feed on filamentous algae;
Lower row: Left: Thuridilla carlsoni, one of the few cryptically camouflaged species of the genus; Right: A look inside the parapodia of Thuridilla carlsoni. Many of the Thuridilla have interesting colouration inside their parapodia that are most usually kept tightly closed.

 

As far as reproduction is concerned like all the sea slugs Thuridilla species are simultaneous hermaphrodites with their reproductive pores located on the right side just below and posterior to the right rhinophore. See the NudiNote: Thuridilla Entanglements – Some observations on Thuridilla mating, for an overview and also the recording of hypodermic insemination that seems not to have been seriously reported or studied previously.

(Note: Names and sp. numbers in the captions refer to species on this website: NudibranchDomain. Those sp. numbers that may be following in brackets have their reference attached.)

David A. Mullins – May 2024

References:
– 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.

– Handeler, K. & Wagele, H. (2007). Preliminary study on molecular phylogeny of Sacoglossa and a compilation of their food organisms. Bonner Zoologische Beitrage. 55, 231–254.

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

– Martín‐Hervás, M. D. R., Carmona, L., Malaquias, M. A. E., Krug, P. J., Gosliner, T. M. & Cervera, J. L. (2021). A molecular phylogeny of Thuridilla Bergh, 1872 sea slugs (Gastropoda, Sacoglossa) reveals a case of flamboyant and cryptic radiation in the marine realm. Cladistics. DOI: 10.1111/cla.12465.

– Mullins, D. A. (2022) Thuridilla Entanglements – Some observations on Thuridilla mating. A NudiNote available at nudibranchdomain.org (and appended references)

– This NudiNote has been modified from a previously published article in Dive Log Australasia Magazine – NudiNotes Column, Issue: #405 (April 2024): pp. 25-27 by David A. Mullins.