Two new species of Aristocleidus (Monogenea) from the gills of the Mexican mojarra Eugerres mexicanus (Perciformes, Gerreidae) from southwestern Mexico

Aristocleidus mexicanus n. sp. and Aristocleidus lacantuni n. sp. are described from the gills of the Mexican mojarra Eugerres mexicanus (Gerreidae, Perciformes) from the Rio Lacantún basin, Chiapas State, Mexico. These new species differ from previously described congeneric species in the characteristics of several structures, including: (a) ventral anchors, with differences in length (i.e. 46–50 µm in A. mexicanus vs. 38–43 µm, 34–37 µm, and 26–33 µm in Aristocleidus hastatus Mueller, 1936, Aristocleidus sp. of Mendoza-Franco, Violante-González & Roche 2009, and Aristocleidus lamothei Kritsky & Mendoza-Franco, 2008, respectively) and shape (i.e. slightly angular union of elongate arcing shaft and point in A. mexicanus vs. point and shaft united at a conspicuous angular bend in A. hastatus and Aristocleidus sp., and evenly curved shaft and point in A. lamothei); (b) male copulatory organ, i.e. a coiled tube with less than one ring in A. mexicanus and A. lacantuni (vs. a coiled tube of about 1½ in Aristocleidus sp.); (c) distal end of the accessory piece (ornate in A. mexicanus vs. distally flattened and trifid in A. hastatus and A. lamothei, respectively); (d) vaginal tube (moderately long in A. mexicanus vs. short in A. lamothei and looping in Aristocleidus sp.); and (e) ventral bar (anteromedial process with terminal horn-like ornamentation in A. lacantuni vs. ornamentation absent in the other species). This study reports for the first time species of Aristocleidus from freshwater environments in Mexico.