Spine Replacement in a Freshwater Population of the Atlantic Stingray, Dasyatis sabina

Abstract

Most rays in the order Myliobatiformes possess a hardened spine or barb near the base of the tail (Bigelow and Schroeder, 1953; Halstead et al., 1955; Halstead, 1970). The spine is composed of an inner core of vasodentine and a thin outer layer of enamel-like material. Major structural features of the spine are retrorse serrations along the lateral margins, a pungent tip, longitudinal grooves on both the dorsal and ventral surfaces, and a raised longitudinal ridge along the ventral surface. Many species are known to have venom-secreting cells, either scattered in the integumentary sheath covering the spine or in glands associated with the grooves lateral to the midventral ridge (Ocampo et al., 1953; Halstead et al., 1955; Smith et al., 1974). The spine and associated venom apparatus are especially well developed in the whip-tail stingrays of the family Dasyatidae. Although most species carry only one spine at a time, multiple spines are reported in some species of rays (see review in Thorson et al., 1988). Usually there are only two spines when multiple spines are present. However, some species occasionally exhibit three or more spines simultaneously. These are referred to as supernumerary spines. Supernumerary spines have been reported for Dasyatis centroura (Bigelow and Schroeder, 1953; Wallace, 1967), Potamotrygon yepezi (Thorson et al., 1988), Aetobatus narnnanr (Gudger, 1914), and a few other species (Thorson et al., 1988). There has been confusion as to whether a re-