The Eurasian Thermocyclops crassus (Fischer, 1853) (Copepoda, Cyclopoida) found in southeastern Mexico

A limnological survey carried out during January 1998 in the state of Tabas co, southeastern Mexico, established the occurrence of the Eurasian freshwa ter copepod Thermocyclops crassus (Fischer, 1853) at several sites. This sp ecies can be distinguished from the morphologically close T. decipiens (Kie fer, 1929) by its seminal receptacle, with a clear "T" shape, female genita l somite length/width ratio exceeding 1.2, furcal length/width ratio below 2.5, and the ornamentation on the connecting lamellae of legs 1 and 4. The introduction of this eurytopic species in freshwater systems of Tabasco is probably related to aquacultural activities. This record increases the numb er of species of Thermocyclops known in Mexico, and is the first documented record of an introduced freshwater copepod species in this country. A key for the identification of the species known in Mexico and Central America i s presented.