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The Brazilian basslet (Gramma brasiliensis) is an ornamental fish species of significance to the fish market, and the development of a breeding protocol is of interest to the aquarium trade. There is little information regarding the behaviour of G. brasiliensis in captivity; thus, the aims of this study were to: (1) investigate pair and harem formation in different-sized aquaria (70 and 110 L) and tanks (150 and 310 L); (2) investigate the choice of different macroalgae species and synthetic materials by the fish for nest building in captivity; and (3) describe the courtship behaviour of captive fish and compare it with available literature for G. brasiliensis and Gramma loreto in natural environments. Based on the results, we suggest that G. brasiliensis should be kept in pairs because of their aggressive behaviour. Macroalgae of the genus Chaetomorpha and all synthetic materials were used by the fish to build nests. The courtship behaviour of G. brasiliensis in captivity is described for the first time and is similar to that reported for G. loreto in a natural habitat, namely ‘U-turn with body quivering’.

In the tropical Atlantic, the ichthyofauna between the coast of Brazil and the Caribbean regions, divided by the Amazon barrier, is very similar presenting several geminate species, including Gramma brasiliensis, endemic in Brazil, and its Caribbean counterpart Gramma loreto. Morphological and molecular studies have helped establish evolutionary patterns that sister-species of these two marine habitats are subjected to. However, their chromosomal characteristics are only beginning to be better characterized. Accordingly, a comparative cytogenetic analysis was carried out in G. brasiliensis and G. loreto, seeking evidence of cytotaxonomic markers implicated in the karyotypic diversification of these species and likely associated with speciation events. Heterochromatic regions and their affinity to fluorochromes GC- or AT-specific were identified, as well as the distribution of ribosomal DNA sites in chromosomes, either by silver nitrate impregnation (Ag-NORs) or dual-color FISH mapping with 18S and 5S rDNA probes. While displaying the same diploid number, 2n = 48 chromosomes, considered basal for Perciformes, the two species differed in karyotype structure, showing karyotypic formulas and species-specific heterochromatin pattern. The cytological characters found support the differentiating status of these species, possibly achieved under the conditions of allopatry due to the Amazon/Orinoco barrier, showing chromosomal peculiarities in Grammatidae species when compared to other groups of Perciformes.

Nesting is a common behaviour associated with reproduction in several taxa. Nevertheless, this important parental care behaviour is rarely reported for reef-associated fish. The present study provides the first description of bird-like complex nesting behaviour of the endangered basslet (Gramma brasiliensis). Males were observed building nests using macroalgae thalli around holes and depressions, in coralline substrate. Algae were used to camouflage the nest entrance and to form a cushioned bed for egg laying. Nesting is a critical aspect of the reproductive strategy of G. brasiliensis, so, this behaviour is a key factor for the management measures implications on the conservation of the species.