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Suffolk Natural History, Vol. 59
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). Trans. Suffolk Nat. Soc. 59 (2023)