Pleurehdera stellata

Pleurehdera stellata (Risso, 1826)

Pleurehdera stellata by Enric Madrenas

Taxonomic Note: The systematics of Pleurobranchida (side-gilled sea slugs) have long been hindered by persistent incongruences between morphological and molecular data. Traditionally, morphological assessments divided the single superfamily Pleurobranchoidea into two families: the shell-less Pleurobranchaeidae and the internally shelled Pleurobranchidae. Ghanimi et al. (2020) demonstrated that both morphological and molecular evidence support the view that the widely distributed species Berthella stellata (Risso, 1826) is, in fact, a species complex comprising no less than eight distinct species. Furthermore, they found that the B. stellata complex forms a monophyletic group with the Eastern Pacific species Berthella strongi (MacFarland, 1966). A critical review of the literature led to the retention of Berthella stellata for the Eastern Atlantic and Mediterranean species, while the name Berthella pellucida (Pease, 1860) was resurrected for the species found in the Hawaiian Islands. However, molecular evidence has so far yielded conflicting hypotheses. To address this, Moles et al. (2023) conducted a multilocus phylogenetic analysis that included type species from all known genera within this group. By employing various masking algorithms to enhance phylogenetic resolution, their results supported the recognition of a new superfamily, Tomthompsonioidea, characterized by the absence of an acidic oral gland and the presence of a helicoid shell. This superfamily was found to be the sister group to all other Pleurobranchida. Their study also demonstrated that the genus Berthella is paraphyletic, now comprising four clades: Berthella sensu stricto, Boreoberthella, Pleurehdera, and Tomoberthella. Their findings revealed a distinct Berthella clade largely corresponding to the B. stellata complex as described by Ghanimi et al. (2020), while also adding further complexity. The B. stellata group was identified in tropical and temperate waters of the Eastern Pacific, Caribbean, and Mediterranean, including species such as B. andromeda Ghanimi et al., 2020, B. strongi (MacFarland, 1966), B. nebula Ghanimi et al., 2020, and B. vialactea Ghanimi et al., 2020. This B. stellata group was found to be sister to a Pacific clade comprising B. pellucida (Pease, 1860) from Hawaii, B. cf. postrema from New Caledonia, a newly sequenced B. cf. ‘stellata’ from Japan, and Pleurehdera haraldi Ev. Marcus & Er. Marcus, 1970 from Palmyra Atoll in the central Pacific. Since Pleurehdera haraldi was the type and only species of its genus, and it nests within this Berthella clade, Moles et al. (2023) proposed transferring all the aforementioned species to Pleurehdera. Members of Pleurehdera share a reticulate mantle with a pale, gelatinous appearance, a small internal shell, a vestigial or absent median buccal gland, and paw-shaped jaw platelets bearing regularly sized, relatively large denticles on both sides of the platelets’ shaft (Marcus & Marcus, 1970), and are found in temperate latitudes, ranging from the Mediterranean to the Atlantic Ocean.

Synonyms

• Pleurobranchus stellatus Risso, 1826 (original)
• Berthella albocrossata Heller & Thompson T., 1983 
• Berthella postrema Burn, 1962
• Berthella stellata albocrossata Heller & Thompson, 1983
• Berthella tupala Er. Marcus, 1957
• Pleurobranchus pellucidus Pease, 1860
• Berthella stellata (Risso, 1826)

Description
This is a species that can reach up to 30 mm in length. In large specimens the mantle is very flat, with a rounded contour and that completely covers the foot. The main feature of this species is the whitish “Y” shaped spot on the back, that could be somewhat irregular. The coloration of this species is very variable but the most widespread large specimens usually show different shades of brown with small irregular whitish spots. Smaller specimens can be whitish, yellowish or reddish. This coloration is due to the many color spots embedded in the mantle. The Y-shaped stain can be absent in bigger specimens. The inner shell is not seen by transparency below the mantle. The cephalic veil is colored light brown, as are the rhinophores, gill and hiponotum, and the underside of the foot is whitish. The gill is bipinnate and has up to 19 slides to either side of the spine.

Biology
In the Catalan coast appears frequently at shallow depth (20-30 cm), under rocks with abundant colonial tunicates that can be its food. In the breeding season it is common to see groups of multiple specimens (up to 10-12 in Es Caials) copulating and laying eggs. The spawn is a gelatinous spiral ribbon of about two turns, with numerous little white eggs. This species also has glands in the foot that expel a very acidic secretion (pH 0-1) for defensive purposes.

Etymology

• Pleurehdera. Despite Marcus & Marcus (1970) didn’t explain the origin of the name when proposing the genus, its name seems derived from the Greek “pleurá” (πλευρά), meaning “side”, and “dera” (δέρα) meaning “skin”, a word related to “derma” (δέρμα) also meaning “skin”. It might refer to a distinctive skin-like or mantle texture of the genus.
• Stellata. From Latin stellatus meaning “starred”, “with a star”.

Distribution
This species is found in the Mediterranean and the northeastern Atlantic. It was previously thought to be circumtropical, but there were many other species cryptic with P. stellata. In the Iberian Peninsula it has been reported in all its coastal areas, also in the Canary Islands, Madeira and Azores. In Catalan waters it is frequent in Es Caials (N of the Costa Brava).

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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
• BiologiaMarina.org
• DORIS
• El Litoral de Granada
• Flickr pictures
• GROC
• Interim Register of Marine and Nonmarine Genera
• M@re Nostrum
• MedSlugs (Atl.E)
• MedSlugs (Med)
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slug Forum
• World Register of Marine Species

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