Absorption and accumulation of bioavailable cyanobacterial metabolites (including cyanotoxins) are likely in fish after senescence and the rupturing of cells during bloom episodes. We determined the toxicity of cyanopeptides identified from two strains of Microcystis (M. panniformis MIRS-04 and M. aeruginosa NPDC-01) in a freshwater tropical fish, Astyanax altiparanae (yellowtail tetra, lambari). Aqueous extracts of both Microcystis strains were prepared in order to simulate realistic fish exposure to these substances in a freshwater environment. Both strains were selected because previous assays evidenced the presence of microcystins (MCs) in MIRS-04 and lack of cyanotoxins in NPDC-01. Identification of cyanobacterial secondary metabolites was performed by LC-HR-QTOF-MS and quantification of the MC-LR was carried out by LC-QqQ-MS/MS. MIRS-04 produces the MCs MC-LR, MC-LY and MC-HilR as well as micropeptins B, 973, 959 and k139. NPCD-01 biosynthetizes microginins FR1, FR2/FR4 and SD-755, but does not produce MCs. Larval fish survival and changes in morphology were assessed for 96 h exposure to aqueous extracts of both strains at environmentally relevant concentrations from 0.1 to 0.5 mg (dry weight)/mL, corresponding to 0.15 to 0.74 μg/mL of MC-LR (considering dried amounts of MIRS-04 for comparison). Fish mortality increased with concentration and time of exposure for both strains of Microcystis. The frequencies of morphological abnormalities increased with concentration in both strains, and included abdominal and pericardial oedema, and spinal curvature. Results demonstrate that toxicity was not solely caused by MCs, other classes of cyanobacterial secondary metabolites contributed to the observed toxicity.

Fish of the family Gerreidae, mainly species of the genera Diapterus and Eugerres, have high potential for cultivation, because of their saline tolerance. A detailed cytogenetic analysis of Diapterus auratus, Diapterus rhombeus and Eugerres brasilianus was conducted using conventional staining, C-banding, Ag-NOR, AT/GC-specific fluorochrome staining and mapping of ribosomal sequences with 5S and 18S rDNA probes. All the species exhibited symmetrical karyotype, 2n = 48 acrocentric chromosomes. Ag-NORs and 18S rDNA are present in the interstitial position on pair 1 (genus Diapterus) and pair 6 in Eugerres brasilianus. The 5S rDNA sites, located in the interstitial position (pair 11), are conserved in the three species. Heterochromatic regions are similar in the Diapterus species, showing a pattern of reduced and centromeric bands, differing from E. brasilianus, where, in addition to these, more prominent interstitial bands were observed. GC-rich regions are located at ribosomal sites. Karyotypic comparison between Diapterus and Eugerres reveals similarity in chromosomal macrostructure, differing in C-positive heterochromatin distribution and position of 18S sites, indicating the occurrence of structural microrearrangements. Although complementary analyses are needed, the similarities observed for these and other species suggest the possibility of breaking postzygotic barriers and their potential use, through induced interspecific or intergeneric hybridizations.

The cobia, Rachycentron canadum, a species of marine fish, has been increasingly used in aquaculture worldwide. It is the only member of the family Rachycentridae (Perciformes) showing wide geographic distribution and phylogenetic patterns still not fully understood. In this study, the species was cytogenetically analyzed by different methodologies, including Ag‐NOR and chromomycin A₃ (CMA₃)/DAPI staining, C‐banding, early replication banding (RGB), and in situ fluorescent hybridization with probes for 18S and 5S ribosomal genes and for telomeric sequences (TTAGGG)_n. The results obtained allow a detailed chromosomal characterization of the Atlantic population. The chromosome diversification found in the karyotype of the cobia is apparently related to pericentric inversions, the main mechanism associated to the karyotypic evolution of Perciformes. The differential heterochromatin replication patterns found were in part associated to functional genes. Despite maintaining conservative chromosomal characteristics in relation to the basal pattern established for Perciformes, some chromosome pairs in the analyzed population exhibit markers that may be important for cytotaxonomic, population, and biodiversity studies as well as for monitoring the species in question.

Several chromosomal features of Gerreidae fish have been found to be conserved. In this group, it is unclear whether the high degree of chromosomal stasis is maintained when analyzing more dynamic regions of chromosomes, such as rDNA sites that generally show a higher level of variability. Thus, cytogenetic analyses were performed on 3 Atlantic species of the genus Eucinostomus using conventional banding (C-banding, Ag-NOR), AT- and GC-specific fluorochromes, and fluorescence in situ hybridization mapping of telomeric sequences and 5S and 18S rDNA sites. The results showed that although the karyotypical macrostructure of these species is similar (2n = 48 chromosomes, simple Ag-NORs seemingly located on homeologous chromosomes and centromeric heterochromatin pattern), there are differences in the positions of rDNA subunits 5S and 18S. Thus, the ribosomal sites have demonstrated to be effective cytotaxonomic markers in Eucinostomus, presenting a different evolutionary dynamics in relation to other chromosomal regions and allowing access to important evolutionary changes in this group.