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As part of a biological inventory of the Rio Lacantún basin in the Biosphere Reserve of Montes Azules from Chiapas State (southeastern, Mexico), the following monogenoid ectoparasites infecting the external surface of the anal opening and the gill lamellae of the freshwater fish Astyanax aeneus (Characidae) in ten streams were found: Cacatuocotyle chajuli sp. nov. (anus), Cacatuocotyle exiguum sp. nov., and Cacatuocotyle sp. (gill lamellae). C. chajuli is differentiated from its single congener, Cacatuocotyle paranaensis (Boeger et al. Syst Parasitol 36:75-78, 1997), from the gills of Characidium lanei (Characidae) from Paraná Brazil, in having a noticeably V-shaped haptoral bar and larger hooks and anchors. C. exiguum differs from these two latter species in the size of its anchors. Illustrations and data on morphological and biometric variability of individual specimens of C. chajuli and C. exiguum from different streams are provided. The present data support the statement about three species of Astyanax, which harbor the richest monogenoidean diversity in the Neotropics with a total of 18 species reported up to now. Occurrences of species of Cacatuocotyle on different sites of infection on three distant host species [including a Neotropical cichlid (Cichlidae) species] suggest that these monogenoideans switching to new hosts can result in the availability of potential hosts within the same habitat instead of showing signs of preferential switching between closely related hosts or on their respective microhabitats.

Phalloidin fluorescence technique, enzyme cytochemistry and immunocytochemistry in conjunction with confocal scanning laser microscopy have been used for the first time to describe the nervous and muscle systems of the viviparous monogenean gill parasite, Macrogyrodactylus clarii. The gross spatial arrangement of muscle and associated cholinergic, peptidergic and aminergic innervations has been examined. The central nervous system (CNS) consists of paired cerebral ganglia from which emanate three pairs of longitudinal ventral, lateral and dorsal nerve cords, connected at intervals by transverse connectives. The CNS is better developed ventrally than dorsally or laterally, and has the strongest reactivity for all neuroactive substances examined. Structural and functional correlates of the neuromusculature of the pharynx, haptor and reproductive tracts have been examined. Results implicate acetylcholine, FMRFamide-related peptides (FaRPs) and serotonin in sensory and motor function in this monogenean, although confirmatory physiological data are obviously required.

Immune responses in ectothermic vertebrates and the life-processes of their parasites are profoundly linked to ambient temperature, but the functional effect of thermal environment on infectivity and host-specificity in helminths from ectotherms is poorly known. Primary infection establishment of Protopolystoma xenopodis (Monogenea) in compatible hosts ( Xenopus laevis: Anura) is strongly modulated by ambient temperature. Significantly fewer worms survived to the urinary bladder stage at 25 degrees C than at 15 degrees C (relatively high and low temperatures for this system in nature). Pre-infection exposure of X. laevis to cold (10 degrees C) did not significantly affect parasite establishment in the urinary bladder (at 15 or 25 degrees C p.i.), nor shorter-term postlarval survival in the kidneys (at 20 degrees C p.i.), suggesting that residual immunosuppressive effects on host susceptibility are not important. Low temperatures had no permissive effect on the establishment of P. xenopodis in incompatible hosts (Xenopus wittei). The link between thermal conditions and parasitic infection of ectotherms is discussed.