© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/06/205 – 148 NANCY MERCADO 1 , EDUARDO SUÁREZ-MORALES* , 2 and M. SILVA-BRIANO 1 1 Universidad Autónoma de Aguascalientes. Centro de Ciencias Básicas, Biología. Aguascalientes, Mexico 2 El Colegio de la Frontera Sur (ECOSUR), Unidad Chetumal, A.P. 424, Chetumal, Quintana Roo 77000, Mexico; e-mail: esuarez@ecosur-qroo.mx. A New Acanthocyclops KIEFER, 1927 (Copepoda: Cyclopoida) from Central Mexico with Comments on the Distribution of the Genus in Middle America key words: limnology, Cyclopidae, diversity, crustacean zooplankton Abstract A new species of Acanthocyclops KIEFER is described from central Mexico. It differs from its con- geners by a combination of characters including mainly: 11–13 antennular segments, a spine formula of 3444 and modified setae on legs 2–4. The presence of a compound distal antennular segment is aber- rant within the Cyclopoida. The new species seems to be related to Nearctic forms of the vernalis–robustus clade. Ancestors of this lineage probably reached central Mexico as a result of glacia- tion events and the new species is a remain of stranded postglacial populations; some of these forms were succesful in colonizing tropical lands. A key for the identification of the species of Acanthocy- clops recorded in Mexico is included. 1. Introduction The genus Acanthocyclops KIEFER, 1927 is among the five most speciose of the cyclopoid copepod subfamily Cyclopinae: there are more than 48 nominal species and subspecies of this genus known to date (DUSSART and DEFAYE, 1985; BOXSHALL and HALSEY, 2004). The knowledge of Acanthocyclops in the Americas is still scarce and biased. The number of species known in North America is the highest in the continent (12 species) (YEATMAN, 1959; REID et al., 1991; REID, 1998; WILLIAMSON and REID, 2001), versus only 4 in South America (REID, 1985; ROCHA and BOTELHO, 1998). In Mexico, Central America, and the insular Caribbean, REID (1990a) recognized only two species of Acanthocyclops: A. robus- tus (SARS, 1863) and A. vernalis (FISCHER, 1853). More recently, two new species of this genus were described from material collected in southeastern Mexico (REID and SUÁREZ- MORALES, 1999; FIERS et al., 2000). It is widely recognized that some species groups show a high intraspecific morphological (DODSON, 1994; DODSON et al., 2003) and genetic (GRISHANIN et al., 2005) variability. This, together with incomplete descriptions, resulted in many species with an uncertain status (see REID et al., 1991; EINSLE, 1996) and a progressively complex taxonomy that relies on only a few relatively stable characters. This tendency is shared by other genera in the subfamily Cyclopinae (ROCHA, 1998). One of the most relevant cases of taxonomical confusion includes A. vernalis and A. robustus, each of them probably representing a complex of cryp- tic species (DODSON et al., 2003). Some of these are being described from different geo- graphical areas (MIRABDULLAYEV and DEFAYE, 2002, 2004). Internat. Rev. Hydrobiol. 91 2006 2 148–163 DOI: 10.1002/iroh.200510856 * Corresponding author