Meeting Abstract
In animals, mechanical defenses can take various shapes, from the protective plates of the armadillo, the venomous spurs of the male platypus, and the quills of the porcupine. Stingrays choose an aggressive way to defend themselves with a whip-like motion of the tail, which drive a serrated spine, called a barb, into predators. Barbs are highly-modified dermal denticles and vary considerably in shape from from the tip to the base, and in terms of serrations shape and number. We used micro-CT scanning to visualize fine-scale barb morphology across 70 species of stingrays, representing around 30% of total taxon diversity, including freshwater potamotrygonids (Potamotrygon, Plesiotrygon), tropical dasyatid whiprays (Dasyatis, Neotrygon, Fluvitrygon), mollusk-crushing myliobatids (Myliobatis, Rhinoptera), New and Old World round rays (Urotrygonidae: Urotrygon, Urobatis; Urolophidae), and gymnurid butterfly rays. We then examined the evolution and morphological disparity of barb shape across a time-calibrated molecular phylogeny for all stingrays. We find that barb shape is highly variable, in particular the serrated length of the barb, length of the barb base, and overall number of serrations. Variability is also evident in barb cross-sectional shape, which varies from a flattened blade (Urotrygon aspidura) to a more complex, T-shaped bayonet (Rhinoptera bonasus). We find no overt morphological distinctions between the barbs of marine vs. freshwater taxa, instead finding more nuanced differences between taxa inhabiting pelagic, reef, deep riverine, and coastal habitats. We discuss using barb morphology as a taxonomic character, especially relevant given their prevalence in chondrichthyan fossil beds, and the ecological significance of the barb as a defensive structure.
