Ceratothoa steindachneri Isopoda: Cymothoidae) new to British waters with a key to north-east Atlantic and Mediterranean Ceratothoa T. Horton The University of Reading,Whiteknights, PO Box 228, Reading, RG6 6AJ. E-mail: t.horton@reading.ac.uk This paper presents the ¢rst record of Ceratothoa steindachneri from Cornwall, making it the ¢rst resident species of the ¢sh^parasitic isopod family Cymothoidae in Britain. The host is Echiichthys vipera Trachinidae) the lesser weever ¢sh, the isopod attaching to the host tongue. A complete redescription and reillustration for C. steindachneri is given, and a neotype is selected for the species Ceratothoa parallela. A key to the north-east Atlantic and Mediterranean species of Ceratothoa is included, with a brief illustrated account for each species. INTRODUCTION Cymothoid isopods are ectoparasites of marine, fresh, and brackish water teleost ¢sh. The records of cymothoids from elasmobranchs probably represent trawl transfers Brusca, 1981). Cymothoids parasitize numerous families and species of ¢sh, including many of commercial importance. Cymothoids attach externally, occur as more intimate ectoparasites in the buccal-cavity/gill-chamber, or burrow inside the ¢sh to develop in a pouch. Species of the family Cymothoidae are rarely found in the cool and cold temperate latitudes Brusca, 1981). Thus the recent discovery of cymothoids attached in the buccal cavity of lesser weever ¢sh Echiichthys vipera Cuvier), Trachinidae) in Whitsand Bay, Cornwall, is unusual. This discovery reveals the ¢rst established population of any cymothoid in Britain and represents a new host and range extension of Ceratothoa steindachneri Koelbel, 1878. Identifying known species of cymothoids is highly problematic and, as a result, the taxonomy of the group is far from clear. Original illustrations of species of cymothoids are frequently inadequate and super¢cial, often consisting of a single dorsal view of a single adult female. Original descriptions are also often insu¤cient, describing characters known to be highly variable or polymorphic, whilst the more important taxonomic char- acters, such as mouthparts and pleopods, are omitted. Thus, although descriptions and illustrations of most of the seven Mediterranean and north-east Atlantic species exist, they are in need of revision. Furthermore, an identi¢cation key to the Ceratothoa in these regions is lacking. This paper contains a thorough redescription of the female, male and pullus secundus of C. steindachneri, and includes a key to the Ceratothoa in the north-east Atlantic and Mediterranean. A full synonymy and information on the geographical range and host speci- ¢city for each of the species featured in the key is also provided. MATERIALS AND METHODS The majority of specimens of Ceratothoa steindachneri were obtained in 1996 by B. Okamura from lesser weever ¢sh Echiichthys vipera) in Whitsand Bay, Cornwall NHM reg. nos. 2000.1823^1828). Comparisons were made with specimens identi¢ed by J.P. Trilles at the MNHN, Paris reg. nos MNHN-Is28-33), and specimens taken from E. vipera in the collections of the Paris Museum reg. nos MNHN-Is5696 & MNHN-Is5697) and Natural History Museum reg. nos NHM 1999.1295-1301). Holotypes were examined of all species in the following key. Segments of the pereon pereonites), coxae coxal plates) and pereopods are numbered with Roman numerals. Segments of the pleon pleonites) and pleopods are numbered with Arabic numerals. The anterior or ventral) lamella of the pleopods is taken to be the exopod, the posterior or dorsal) lamella, the endopod. Sex and life stage of the specimen is given. Sizes are indicated both by the scale bars on the illustrations and in the text where necessary. Complete synonymy, and information on geographic range and host preferences are provided. GEOGRAPHIC RANGE The geographic range covered by this work extends from as far south as Senegal in north-west Africa, around the coast of the Mediterranean, and as far north as Britain. Genus Ceratothoa Dana, 1852 Ceratothoa Dana, 1852: 303; Schioedte & Meinert, 1883: 332; Bowman, 1978: 217; Brusca, 1981: 177; Bruce & Bowman, 1989: 2. Codonophilus Haswell, 1881: 471. Rhexana Schioedte & Meinert, 1883: 289. Cteatessa Schioedte & Meinert, 1883: 296. Meinertia Stebbing, 1893: 354. Rhexanella Stebbing, 1911: 179. J. Mar. Biol. Ass. U.K. 2000), 80, 1041^1052 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom 2000)