Elysia translucens

Elysia translucens (Pruvot-Fol, 1957)

Taxonomic Note: Regarding the validity of Elysia translucens, Martín-Hervás et al. (2024) reviewed the literature on this species. The original description by Pruvot-Fol (1957) was based on a specimen from Banyuls-sur-Mer, France (Mediterranean Sea), preserved in formalin. This specimen exhibited a completely translucent appearance, which led the author to tentatively propose it as a new species, albeit without full certainty. However, the distinct appearance of this individual—compared to other E. viridis specimens that the author had also fixed in formalin—ultimately resulted in the designation of E. translucens as a separate species. Other studies have shown that E. viridis specimens can exhibit a wide range of colors, including dark green, light green, brown, yellow, or nearly white, depending on their most recent diet and feeding time (Rauch et al. 2018). The type specimen may have undergone prolonged starvation before being collected, which, combined with the fixative effect of formalin, could explain its translucent appearance. Martín-Hervás et al. (2024) also examined published radular illustrations from other authors (Bouchet 1984; Thompson & Jaklin 1988), but the results were inconclusive, despite certain radular differences. They concluded that these variations in radular morphology could be diet-related, as previously reported for E. viridis (Jensen 1993; Rauch et al. 2018). Additionally, externally, the E. translucens specimen illustrated in Bouchet (1984) closely resembles E. rubeni and E. margaritae, and it is depicted on Flabellia petiolata, a substrate also associated with E. rubeni (Martín-Hervás et al. 2020). The drawing of E. translucens in Thompson & Jaklin (1988) does not provide diagnostic features, as it is identical to E. viridis in Thompson (1981). Moreover, based on their descriptions, specimens identified as E. translucens from the Adriatic and Aegean Seas could belong to almost any Elysia species, and the illustration of a juvenile specimen in Thompson & Jaklin (1988) more closely resembles E. rubeni. Unfortunately, no external or radular morphological description is available for the specimen identified as E. translucens in the molecular analysis by Christa et al. (2014), which also was collected from Banyuls-sur-Mer. However, all specimens from this locality analyzed by Martín-Hervás et al. (2024)—including the GenBank sequence—clustered with E. gordanae specimens from other Mediterranean locations. Given the current information on E. translucens, it remains unclear whether it represents a distinct species. Consequently, it should be considered a taxon inquirendum.

Description
Body elongate, soft, and dorsoventrally flattened. Entirely colorless and translucent under magnification, including the pericardial hump, rhinophores, tail, foot, and digestive diverticula. Only two pigmented features are visible: the black, pyriform eyes and a thin, snow-white marginal line along the parapodial edges. Rhinophores are short, rolled laterally, and unpigmented. Parapodia are wide and clear, displaying only the white edge. The foot is broad, smooth, and completely transparent. The digestive gland, typically greenish in other Elysia species due to retained chloroplasts, lacks any pigmentation. Due to the transparency, some internal structures are faintly visible: The buccal bulb is very small (it is much bigger in E.viridis). The radular morphology is distinctive: the teeth are much smaller and more pointed, with a differently shaped base, a more prominent lateral “spur” (épéron), and a stronger, coarser central cusp serration visible under lower magnification.

Biology
No live observations of Elysia translucens have been properly confirmed so far. The true species is known only from the preserved type specimen. Based on related Elysia species, it is likely benthic and associated with shallow-water macroalgal habitats. Feeding is presumed to be suctorial on algal cells; however, no signs of kleptoplasty (retention of chloroplasts) were observed, which may be due to species-specific traits, or the effects of starvation, or preservation. Pruvot-Fol rejected the hypothesis of postmortem depigmentation, noting that other E. viridis specimens from the same batch retained their coloration and that the E. translucens specimen appeared fully developed. This suggests the lack of pigmentation is a natural trait and not an artifact.

Etymology

• Elysia. It is a name of Latin origin, derived from the word Elysium, which in mythology is the home of the blessed, known as the “Elysian fields”. The name Elysia also means “God’s oath” in Hebrew. Frieder Sauer comments that Elysia means heavenly.
• Translucens. From Latin “Translucentia”, transparency.

Distribution
Known only from the original specimen collected in Banyuls-sur-Mer, western Mediterranean Sea (France). No confirmed records have been reported since. Some sources list its presence in various Mediterranean basins (Alboran Sea, Adriatic, Ionian, Aegean, Levantine Sea, and Tunisian shelf), though always with caution due to the lack of live observations and the extreme rarity of the species.

Known georeferenced records of the species: Elysia translucens

Sources:
OBIS GROC 2010-2011 Enric Madrenas João Pedro Silva Bernard Picton GBIF	OPK VIMAR Manuel Ballesteros M@re Nostrum Altres fonts Marine Regions

More pictures

Elysia translucens

Elysia translucens

Elysia translucens by Enric Madrenas

Elysia translucens by Enric Madrenas

Elysia translucens by Enric Madrenas

Elysia translucens @ L’Escala by Enric Madrenas

Elysia translucens @ L’Escala by Enric Madrenas

Elysia translucens @ L’Escala by Enric Madrenas

Bibliography

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Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

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