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Introduction
The 'lambari-do-rabo-amarelo', Astyanax bimaculatus (Linnaeus, 1758), is a small-sized species that is widely distributed, being found in several South American basins (Lima et al., 2003). This species does not migrate for reproduction, it reproduces both in lentic and lotic water environments and has a long reproductive season and multiple spawning (Godinho et al., 2010). The genus Astyanax previously belonged to the Tetragonopterinae sub-family but is currently included in the incertae sedis group in the Characidae family (Lima et al., 2003). This situation is due to a lack of monophyly among the sub-families that was evident from phylogenetic studies using osteologic, biometric and meristic parameters (Weitzman & Malabarba et al., 1998). The species of the incertae sedis group are small in size and are very important for energy flow along the food chain as they are food for piscivore species (Martins et al., 2011).
Eggs of different fish species may be distinct in size, shape, colour, specific density and degree of adhesiveness (Rizzo et al., 2002) and may be classified as adhesive, semi-adhesive or non-adhesive. However, the presence of a jelly coat is not directly related to egg adhesiveness as the jelly coat is present frequently in Siluriformes eggs, but not all fish of this order have adhesive eggs (Rizzo et al., 2002). Some species of Characiformes, such as Schizodon knerii, Triportheus guentheri, Bryconops affinis, Hoplias spp., Hoplerythrinus unitaeniatus, Acestrorhynchus spp. and Curimatella lepidura present adhesive eggs but do not have a jelly coat (Rizzo et al., 2002; Gomes et al., 2007). In some fish groups, adhesion of the egg to the substrate is accomplished via an adhesive disc around the micropyle (Honorato-Sampaio et al., 2009; Perini et al., 2010). Although the jelly coat is a feature observed commonly in Siluriformes eggs, to our knowledge the occurrence of this coat in Characiformes eggs has not been reported previously.
Embryonic development of fish consists of a series of morphological events and cellular movements that starts with the penetration of the spermatozoid into the oocyte through the micropyle and culminates with the hatching of the larvae to the external environment (Solnika-Krezel, 2005). After...