The aggressive display in Betta splendens is particularly prominent, and vital to its adaptation to the environment. Methylmercury is an organic variation of Hg that presents particularly pronounced neuro-behavioral effects. The present experiments aim to test the effect of acute and chronic poisoning with methylmercury on the display in Bettas. The animals were poisoned by trophic means in both experiments (16 ug/kg in acute poisoning; 16 ug/kg/day for chronic poisoning), and tested in agonistic pairs. The total frequency of the display was recorded, analyzing the topography of the agonistic response. The methylmercury seems to present a dose-and detoxification-dependent effect on these responses, with a more pronounced effect on motivity in acute poisoning and on emotionality in the chronic poisoning. It is possible that this effect could be mediated by alteration in the mono-amino-oxidase systems. Keywords: methylmercury, aggression, emotional behavior, Betta splendensEl despliegue agresivo en la Betta splendens es especialmente prominente y es vital para su adaptación al medio ambiente. Metil-mercurio es una variación orgánica de Hg que presenta efectos neuro-conductuales especialmente pronunciados. Los experimentos actuales intentan poner aprueba el efecto de envenenamiento agudo y crónico con metil-mercurio sobre el despliegue en Bettas. Los animales fueron envenenados tróficamente en ambos experimentos (16 ug/kg e el envenenamiento agudo) y probados en parejas agonistas. Se registró la frecuencia total del despliegue, analizando la topografía de la respuesta agonista. El metil-mercurio parece presentar un efecto dependiente de la dosis y de la detoxificación sobre estas respuestas, con un efecto más pronunciado sobre la motilidad en el envenenamiento agudo y sobre la emotividad en el envenenamiento crónico. Posiblemente, este efecto podría mediarse por la alteración en los sistemas de mono-amino-oxidasa.

The present article tried to establish dark/light preference in five different species of teleosts. We proposed, using the data obtained with this method in zebrafishes (Danio rerio), Cardinal-tetras (Paracheirodon axelrodi), lambaris (Astyanax altiparanae), Nile tilapias (Oreochromis niloticus), guppies (Poecilia reticulata) and banded-knife fishes (Gymnotus carapo), that preference for dark environments is a reliable and low-cost index of anxiety/fear in those species. A scototactic pattern of exploration was found in all species, and the pattern of locomotion in the white environment suggests its aversiveness for those species, with the exception of G. carapo and O. niloticus. A comparative analysis uncovered species differences in approach-avoidance dimensions of the task. The data are discussed in terms of the behavioral ecology of the animals and prey-predator relationships, suggesting a link with predator defense strategies in teleost.The dark/light preference model is already established as an "ethoexperimental" anxiety model in rodents (cf. Bourin & Hascöet, 2003). It is based on the natural aversive quality of brightly-lit environments for mice, shaping -352 -a conflict situation in which the animal must deal with its natural tendency to explore in face of the aversiveness of the environment. The rodent dark/light preference model is an exploration model, in the sense that it measures locomotor activity in both environments as an index of anxiety (Green & Hodges 1991; Prut & Belzung 2001; Belzung & Griebel 2003; Hascöet, Bourin, & Dhonnchadha, 2001); there are other, non-locomotor, models of anxiety (eg., inhibitory avoidance), but those are not of concern for the objectives in this article. Locomotor models of anxiety use exploratory behavior (defined as "a speciesspecific behaviour pattern concerned with the gathering of information" concerning the environment: O' Keefe & Nadel, 1978, p. 242) as an index of anxiety or anxiety-like states, relating it to foraging behavior or to appraisal of novel environmental stimuli (Belzung & Griebel 2001;File, 2001). The main rationale is that exploratory behavior would correlate with neophobia, a tendency to avoid new environments (Misslin & Cigrang 1986), forming a mixed pattern of behavior that consists in gradual approaching and exploration of the new environment associated with "scanning" and "risk-assessment" behaviors.Ethoexperimental models use variables that are akin to the concept of "antipredator apprehension" from behavioral ecology (risk assessment, defensive distance, predatory imminence continuum, risk associated suppression of competing motivational systems; Kavaliers & Choleris, 2001). Apprehension is considered to reflect a motivational state, and is defined as "any reduction in attention to other activities (e. g., foraging, mate seeking) as a result of increasing the allocation of attention to detecting and/or responding to potential predators" (Kavaliers & Choleris, 2001, p. 579). Exploratory apprehensive behavior (denoting the pattern of ex...