Species of Ascocotyle Looss, 1899 (Digenea: Heterophyidae) of the Yucatan Peninsula, Mexico, and notes on their life-cycles.

Heterophyid trematodes of the genus Ascocotyle Looss, 1899, metacercariae of which were found in fish from freshwater (cenotes [sinkholes], lakes and the Rio Hondo River) and brackish coastal lagoons of the Peninsula of Yucatan, Mexico, are reviewed. The following species were found: Ascocotyle (Ascocotyle) tenuicollis Price, 1935; A. (A.) nunezae Scholz, Vargas-Vazquez, Vidal-Martinez & Aguirre-Macedo, 1996; A. (A.) chandleri Lumsden, 1963; A. (Phagicola) nana Ransom, 1920; A. (P.) diminuta Stunkard & Haviland, 1924; A. (P.) macrostoma (Robinson, 1956); and A. (Leighia) megalocephala Price, 1932. Descriptions of metacercariae and adults from naturally and experimentally infected hosts are provided. Considerable variation was found in the number (18–22) and arrangement of circumoral spines of A. (P.) nana: most specimens (70%) had 20 spines arranged in one complete row of 16 spines and four additional spines dorsally, which differs from the arrangement of circumoral spines in the type-series of this species. In contrast, spination patterns of A. (A.) tenuicollis (16 + 16 spines in two rows) and A. (P.) diminuta (16 + 2) were constant. The validity of A. (P.) diminuta and A. (P.) nana, species recently resurrected from synonymy with A. (P.) angrense Travassos, 1916, was confirmed. Except for A. (A.) tenuicollis, all species are reported from Mexico for the first time. The heron Casmerodius albus represents a new definitive host of adults of all the above-mentioned species excluding A. (A.) chandleri. A key to the identification of Ascocotyle metacercariae occurring in fishes of the Yucatan Peninsula is also provided.