Occurrence of the sea slug Thecacera pennigera (Order Nudibranchia; Suborder Doridina; Family Polyceridae) at Walberswick, Suffolk
J. R. Ellis
The sea slug Thecacera pennigera (Montagu, 1813) was first described from a specimen collected from rocks exposed at low water at Milton (south Devon). It was subsequently recorded from elsewhere in the English Channel, including Weymouth (Thompson, 1854) and Cornwall (Alder & Hancock, 1845–1910). It is now known to occur in shallow waters (usually <20 m deep) the southern and western coasts of the British Isles and along the Atlantic seaboards of France, Spain and Portugal (Picton and Morrow, 2023). Thecacera pennigera has also been reported from elsewhere in the world, including the Mediterranean Sea, South Africa, Brazil, Pakistan, India, Japan, Australia and New Zealand (see Ravinesh et al., 2017 and references therein) and it has been suggested that this cosmopolitan distribution may reflect ship-based dispersal (Thompson & Brown, 1984).
Whilst widespread along the coasts of the English Channel, the distribution of T. pennigera also extends into the southern North Sea (Seaward, 1990), with occasional published reports from the Netherlands and East Anglia. The published records from the Netherlands have come from Vlissingen, where specimens were collected from a sea dyke near the Oranje windmill in October 1954 (ca. 51.44°N, 3.58°E; Swennen, 1961) and from near Plompe Toren, on the Oosterschelde, in October 1985 (ca.51.68° N, 3.77°E; Dekker, 1986). There has also been a record from ‘off Lowestoft’ (Swennen, 1961; Hamond, 1972), although other details are seemingly unavailable.
This sea slug has several characteristic features, with Montagu (1813) describing it as having a “singular and gaudy” appearance. The background colouration is a translucent white colour, and this is adorned with orange flecks and smaller black flecks. Morphologically, the anterior end has wing-like processes, and there is a pair of dorso-lateral ceratal processes behind the gills. The paired rhinophores are protected by complex sheaths, with these open along the inner lateral margins that face each other. Despite the characteristic appearance and shallow-water habitat of T. pennigera, it is viewed as an uncommon species (Alder and Hancock, 1845–1910; Dekker, 1986, Thompson, 1988), although it has been suggested that it can be locally abundant (Thompson & Brown, 1984). In terms of maximum size, earlier accounts reported it to reach lengths up to 15 mm (Alder & Hancock, 1845–1910; Pruvot-Fol, 1954) with more recent studies reporting maximum sizes of about 30 mm (Dekker, 1986; Thompson, 1988).
Given the paucity of records of this species from the Suffolk coast, it is interesting to note that multiple specimens of T. pennigera were observed in the summer of 2023. All the observed specimens were caught on the seaward side of the harbour wall at Walberswick (ca. 52.3139°N, 1.6728°E) at low water during spring low tides. Specimens were up to about 23 mm in length (Figure 1). All were found close to the base of the harbour wall in shallow water, with this section of the wall generally covered with hydroids, various tube-forming polychaetes and bryozoans. Indeed, T. pennigera is known to predate on the bryozoan Bugula, including B. plumosa (Picton & Morrow, 2023).
Figure 1. Specimen of the nudibranch Thecacera pennigera from Walberswick, including scale (inset).
Specimens were first noted on 2 August and 4 August 2023, with three individuals recorded on the latter day. A single individual was also observed on 16 August 2023, eight individuals across the low tides on 31 August and 1 September, and five individuals on 13 September. No specimens were observed during the spring tides of 28–29 September 2023.
Other data available on the National Biodiversity Network Atlas (https:// species.nbnatlas.org/; accessed 7 August 2023) show that most UK records of T. pennigera are from the coasts of the English Channel, Bristol Channel and Irish Sea through to the northern coast of Northern Ireland, with only two records for the southern North Sea. These comprised one dive record from the River Blackwater, Essex (51.7351°N, 0.8554°E) in September 2014, and one record from Felixstowe seafront (51.9580°N, 1.3525°E) in September 2015. Whilst there is a nominal record from ‘off Lowestoft’ (Swennen, 1961), the present records are some of the more northerly records for this species in the North Sea. Interestingly, all the North Sea records appear to have been observed in the months from August to October. Further sampling of hard substrata in the area could usefully be undertaken in order to better inform on the distribution of this species in the area.
References
Alder, J. & Hancock, A. (1845–1910). A monograph of the British nudibranchiate Mollusca. London: Ray Society.
Dekker, R. (1986). Second record of Thecacera pennigera (Gastropoda: Opisthobranchia) from the Dutch coast. Basteria, 50: 45–46.
Hamond, R. (1972). The marine mollusca of Norfolk. Transactions of the Norfolk and Norwich Naturalists’ Society, 22: 271–306.
Montagu, G. (1813). Descriptions of several new or rare animals, principally marine, discovered on the south coast of Devonshire. Transactions of the Linnean Society of London, 11: 1–26 + 5 plates.
Picton, B. & Morrow, C. (2023). Nudibranchs of Britain, Ireland and Northwest Europe. Second Edition. Wild Nature Press / Princeton University Press, 360 pp.
Pruvot-Fol, A. (1954). Mollusques opisthobranches. Faune de France, 58: 460 pp.
Ravinesh, R., Jabir, T., Sneha Chandran, B. K., Biju Kumar, A. & Mohamed Hatha, A. A. (2017). First record of the alien sea slug Thecacera pennigera (Montagu, 1813) (Mollusca: Gastropoda: Nudibranchia: Polyceridae) from southwest coast of India. Journal of Aquatic Biology and Fisheries, 5: 222–224.
Seaward, D. R. (1990). Distribution of the marine molluscs of north west Europe. Nature Conservancy Council (Peterborough, UK), 114 pp. Swennen, C. (1961). Data on distribution, reproduction and ecology of the nudibranchiate molluscs occurring in the Netherlands. Netherlands Journal of Sea Research, 1: 191–240.
Thompson, T. E. (1988). Molluscs: Benthic opisthobranchs: Mollusca, Gastropoda: keys and notes for the identification of the species. Synopses of the British Fauna (New Series), 8: 1–356 + 8 plates.
Thompson, T. E. & Brown, G. H. (1984). Biology of opisthobranch molluscs. Volume 2. London: Ray Society, 229 pp.
Thompson, W. (1854). Thecacera pennigera. Annals And Magazine of Natural History (Second Series), 14 (No. 81): 237.
J. R. Ellis
CEFAS, Pakefield Road, Lowestoft, Suffolk, NR33 0HT