Biology and seasonal replacement of caudal spines in the endangered spiny butterfly ray Gymnura altavela (Elasmobranchii, Gymnuridae) from the southwestern Atlantic

Abstract Although caudal spines are present in most Myliobatiformes species, their biology and ecological function, beyond self-defense, are still poorly understood. Novel information on the development and replacement of the species’ caudal spines was obtained herein by examining the caudal spines of 63 Gymnura altavela specimens. Our data suggest that G. altavela in the southwestern Atlantic undergoes an annual caudal spine replacement cycle, with new spines forming in spring and summer. This replacement may be anticipated by spine breakage, as double-spined stingrays with broken lower spines were observed in winter. Unlike previously studied stingrays, new spines in G. altavela developed on the dorsal region of old spines. Furthermore, this study is the first to investigate the possible use of caudal spines for assessing stingray age. Although spine width growth follows body morphometry, growth bands were not observed along the length of the caudal spines or in their interior. Therefore, these calcified structures do not appear to be suitable for age assessment, at least when considering the classical approach of counting annuli composed of growth bands.