Cadlina pellucida

Cadlina pellucida (Risso, 1826)

Cadlina pellucida by Luis Ángel Díaz Álvarez

Taxonomic note: The genus Cadlina constitutes a distinct lineage within dorid nudibranchs. It was traditionally included in the family Chromodorididae; however, morphological and molecular studies have led to its reassignment to the family Cadlinidae (World Register of Marine Species, 2024). This change is supported by distinctive anatomical characters, including radular structure, the presence of specific dermal glands, and differences in the reproductive system, all of which clearly separate Cadlina from typical chromodoridids (Johnson, 2011; Valdés, 2002). A relevant point is that the genus has been revised using molecular phylogenetics. The work of  Johnson (2011), based on mitochondrial and nuclear DNA sequences, demonstrated that Cadlina forms a well-supported clade, independent within Doridoidea, reinforcing its status as a separate family (Johnson, 2011). Recent studies suggest that the genus Cadlina exhibits significant cryptic diversity, particularly in temperate and cold regions of the Northern Hemisphere, where morphologically similar species can only be distinguished using molecular characters (Johnson, 2011). In this context, identification based solely on external features -as often occurs in fieldwork- may lead to errors. As for Cadlina pellucida, it is currently regarded as a valid and accepted species, originally described as Doris pellucida by Antoine Risso in 1826. Its diagnostic combination -a translucent mantle with visible glands and dark rhinophores- has been considered stable in the literature, although species delimitation within the genus has historically been complex due to morphological variability and the existence of cryptic species (Johnson, 2011).

Synonyms

• Cadlina clarae Ihering, 1880
• Doris pellucida Risso, 1826
• Echinochila pellucida (Risso, 1826)

Description
A small dorid nudibranch that rarely exceeds 20–25 mm in total length, with an oval and relatively flattened body. The mantle is broad, convex, and translucent white or semi-transparent, giving the animal a characteristic “milky” appearance. The dorsal surface bears tubercles or small, more opaque protuberances of variable size, which may be partially arranged in parallel lines towards the mantle margin. Within the mantle, whitish or yellowish glands are visible through the translucent tissue. The rhinophores are short, lamellate, and retractile, dark in colour (brown to blackish), constituting a key diagnostic feature distinguishing it from related species. The branchial region consists of a plume of 5–7 tripinnate, dark-coloured gills arranged in a circle around the anal papilla at the posterior part of the dorsum.

Biology
It feeds on sponges, including species such as Spongia agaricina, Halisarca dujardini, Stylotella columella, and others of the genus Sycon. Egg masses are small and are typically deposited in cavities, crevices, or low-light areas of the habitat, usually close to the food source. It is frequently found in cryptic microhabitats (holes, shaded areas), associated with the distribution of its prey.

Etymology

• Pellucida. From Latin “pellūcida”, transparent.

Distribution
It has a wide distribution in temperate and cold waters of the Northern Hemisphere. It has been recorded in the Arctic Ocean, the northeastern Atlantic (including the coasts of the Iberian Peninsula and France), and the western Mediterranean, although it is generally considered an uncommon or rare species.

Similar species
It may be confused with Cadlina laevis, from which it differs in having dark rhinophores (brown to blackish), whereas C. laevis has white or pale rhinophores. In addition, C. pellucida tends to have a more translucent mantle and a less uniform tubercle pattern.

Videos

Vídeos by Luís Sánchez Tocino (Universidad de Granada)

 

More pictures

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

• AsturNatura
• Biodiversity Heritage Library
• El Litoral de Granada
• European Sea Slugs by Morddyn
• Flickr pictures
• GROC
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slug Forum
• The Conchological Society of Great Britain and Ireland
• World Register of Marine Species

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