Otto Gadig

Sharks of the genus Rhizoprionodon can be considered some of the most important predators along the trophic coastal marine ecosystems and represent an important economic resource for the small-scale fisheries, especially on the Brazilian coastline. In order to analyze the population structure of the shark Rhizoprionodon lalandii of São Paulo, Southeastern coast of Brazil, levels of genetic diversity were identified by nucleotide sequence analyses of the mitochondrial DNA control region. The results obtained from this study present moderate values of haplotype diversity and low nucleotide diversity. Although the AMOVA tests (ΦST = 0.08394, P < 0.01) had shown slightly differences among the studied samples, evidence for the occurrence of population structuring was not found, which may be a general feature of sharks living in coastal areas.

Sharks are suffering from intensive exploitation by worldwide fisheries leading to a severe decline in several populations in the last decades. The lack of biological data on a species-specific basis, associated with a k-strategist life history make it difficult to correctly manage and conserve these animals. The aim of the present study was to develop a DNA-based procedure to discriminate shark species by means of a rapid, low cost and easily applicable PCR analysis based on 5S rDNA repeat units amplification, in order to contribute conservation management of these animals. The generated agarose electrophoresis band patterns allowed to unequivocally distinguish eight shark species. The data showed for the first time that a simple PCR is able to discriminate elasmobranch species. The described 5S rDNA PCR approach generated species-specific genetic markers that should find broad application in fishery management and trade of sharks and their subproducts.

Although the Brazilian sharpnose shark, Rhizoprionodon lalandii (Muller and Henle, 1839), is an inshore species widely distributed in the Western Atlantic from Panama to Uruguay, there is little available information on its biology. During a long-term study of small coastal sharks caught by gill net fisheries in southeastern Brazil (PROJETO CAÇÃO), 3643 specimens of R. lalandii were examined, comprising 61.3% of the total sharks, and including all sizes classes, from 30 to 78.5 cm TL, and weights from 100 to 2950 g. The length–weight relationships were not significantly different between sexes. Overall sex ratio favoured the males slightly at the rate of 1.3:1. Sex ratios, however, did differ significantly between season and size classes. This species occurred in this area all year long. Three seasonal size-class occurrence patterns were recognized: (1) between October and March, the juveniles were more frequent; (2) from April to July, adults were most common; and (3) from August to September, neonates were most numerically abundant. Such patterns were associated with reproductive tactics that may reduce intra-specific and inter-specific competition with hammerhead shark neonates (Sphyrna lewini), probably result in reduced natural mortality of the offspring during their first few months.