BULL SHARK PHYLOGENETIC RELATIONSHIPS

Bull Shark Phylogenetic Relationships



Bull Shark Phylogenetic Relationships

Introduction

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.

Overview

The evolutionary history of elasmobranchs has almost certainly featured the convergent evolution of adaptations related to their reproductive biology. For example, the occurrence of limited histotrophy (where developing embryos obtain some nourishment from absorption or ingestion of uterine secretions) among the distantly related squaliform and carcharhiniform lineages is best explained by independent acquisition of that trait. Other instances where convergence or parallelism are suspected include the loss of placental viviparity in the carcharhinid Galeocerdo cuvieri and in some members of the family Triakidae. However, to refine our understanding of the mode and tempo of the evolution of reproductive strategies in sharks it is first necessary to critically examine existing hypothesis of phylogeny using modern methodology. In the present report, we focus on the family Triakidae because based on current understanding of carcharhiniform phylogeny and reproductive biology, the study of triakid lineages promises insights into the evolution of placental viviparity.

The family Triakidae (houndsharks, smooth-hounds, tope, and whiskery sharks) is one of eight families that compose one of the most species-rich orders of sharks, the Carcharhiniformes. Triakids are generally small- to medium-sized sharks that most frequently inhabit coastal regions in tropical and temperate seas throughout the world and feed primarily on benthic crustaceans, cephalopods and bony fish. Many triakid species (e.g. Mustelus spp., Galeorhinus galeus) are targets of commercial fisheries of local to regional significance (. Triakids exhibit placental viviparity and limited-histotroph viviparity (aplacental viviparity), however the lack of a well-corroborated phylogenetic hypothesis for the family and for the order hinders inferences regarding the frequency, timing and consequences of changes between those two reproductive modes in the evolutionary history of these sharks.

As currently defined, the Triakidae is thought to include the living representatives of an “intermediate evolutionary phase” between the basal carcharhiniforms (e.g., Scyliorhinidae and Proscyllidae) and the so-called “higher carcharhinids” (e.g., Carcharhinidae and Sphyrnidae; Compagno, 1988). The patterns of variability in mode of reproduction seem to reflect that putative evolutionary trend. Oviparity and yolk-sac viviparity are the prevailing reproductive modes among basal carcharhiniforms; among triakids there are species with limited histotrophy and species with placental viviparity; ...