DESCRIPTION OF THE SEA ANEMONE ANTHOPLEURA THALLIA (GOSSE 1854) Marymegan Daly and Bernard Picton ABSTRACT The inconspicuous intertidal sea anemone Anthopleura thallia is redescribed based on recent collections in England and Ireland. The redescription includes a complete account of the types and sizes of cnidae, and documents variability in colouration and pattern within and among populations. This is the first record of the species in Ireland. Anthopleura thallia is sympatric with the better-known and widespread species Aulactinia verrucosa, and the two may co-occur at a single site; they differ in colouration, reproductive biology, internal anatomy and the types of cnidae in the body. INTRODUCTION Although the sea anemone fauna of the British Isles is historically the best-studied in the world (e.g. Gosse 1860; Stephenson 1928; 1935; Manuel 1981; 1988), many of the species found there are inadequately known. Anthopleura thallia (Gosse 1854) is among the most obscure species in the intertidal zone; the most recent record of speci- mens in Britain is that of Manuel (1981). The cited range of An. thallia includes the Isle of Man (Moore 1937), the south-west coast of Britain (e.g. Gosse 1854; 1860; Stephenson 1935; Manuel 1981; 1988) and the Mediterranean (Williams 1998; Ocan ˜a and den Hartog 2002; Ates and den Hartog 2011). Manuel (1981; 1988) includes the Atlantic coast of France in its range but provides no precise locality or citation; Fauvel (1905) cites specimens from Vauville, Normandy; and Tessier (1965 reports specimens from Roscoff, Brittany. Additional records in the North Atlantic include Madeira Island and Canary Islands (Ocan ˜a and den Hartog 2002) and Galicia, Spain (Ramil 1985; den Hartog and Ates 2011) The species is reported from Israel, Red Sea, Gulf of Aqaba (Schmidt 1970) and the Croatian coast of the Adriatic (Kruz ˇic 2002), but these are doubtful (den Hartog and Ates 2011). Carlgren (1949) includes Ireland in the range for An. thallia but provides no citation. There are no type specimens for this species, and none of the major museum collections of Actiniaria (i.e. The Natural History Museum (London), California Academy of Sciences, Swedish Museum of Natural History, U.S. National Museum of Natural History, Yale Peabody Museum) contain identified representatives of this species (M. Daly, pers. obs.; Fautin 2011). In addition to being relatively rare, An. thallia is poorly known. Most records of this species are citations of other works (e.g. Gosse 1858; Verrill 1899; Cornelius et al. 1990; Loukmidou et al. 1996; Chintiroglou et al. 1997), classifications (e.g. Gosse 1858; Andres 1883; Carlgren 1949) or faunal guides (e.g. Moore 1937; Manuel 1981; 1988; Cornelius et al. 1990). The photographs in Wood (2005) are not this species but Sagartia ornata (den Hartog and Ates 2011). Dixon and Dixon (1889) provide the first and most complete account of the inter- nal anatomy and musculature of An. thallia; Stephenson (1935) cites this work in his treatment of the species, adding some details about variation in the number of tentacles and mesenteries, and providing a thorough discussion of colour varia- bility. Although these treatments are accurate, they lack many of the details now used to differentiate species and genera in the family Actiniidae. Most conspicuously absent from these accounts is any discussion of the types and sizes of cnidae in various regions of the body. Stephenson (1935) includes measurements of the nematocysts of the acrorhagi, but does not precisely identify their type, or characterize the cnidae of any other part of the body. In his study of the statistical properties of cnidae sizes in actiniarians, Williams (1998; 2000) reports size ranges of basitrichs from the tentacle tip of specimens of An. thallia but does not characterize or measure any other cnidae. Anthopleura Duchassaing de Fonbressin and Michelotti, 1860 is distinguished from Gyractis Boveri, 1893 solely in having marginal projections that bear acrorhagi, structures that contain holo- trichous nematocysts (England 1987; Daly 2003). Thus, without information on the types and distribution of cnidae, it is impossible to determine whether An. thallia actually belongs in Anthopleura. As part of this redescription, we provide a complete account of the anatomy and the distribu- tion and size ranges of cnidae for An. thallia. The marginal projections of A. thallia bear acror- hagi, confirming that it belongs in Anthopleura. Received 19 July 2011. Accepted 24 October 2011. Published 30 April 2012. Marymegan Daly (Corresponding author: daly.66@ osu.edu), Department of Evolution, Ecology & Organismal Biology, The Ohio State University, 1315 Kinnear Rd, Columbus, OH 43212, USA; Bernard Picton, Zoology Department, Ulster Museum, Belfast, Northern Ireland. Cite as follows: Daly, M. and Picton, B. 2012 Description of the sea anemone Anthopleura thallia (Gosse 1854). Biology and Environment: Proceedings of the Royal Irish Academy 2012. DOI: 10.3318/ BIOE.2012.10. DOI: 10.3318/BIOE.2012.10. BIOLOGY AND ENVIRONMENT: PROCEEDINGS OF THE ROYAL IRISH ACADEMY, VOL. 112B, 16 (2012). # ROYAL IRISH ACADEMY 1