Embryonic development timeline in skates (Chondrichthyes: Rajiformes): Sympterygia acuta as the first case study in the family Arhynchobatidae

Oviparous elasmobranch embryos (Chondrichthyes) have been the focus of several embryological studies; they are useful models for studying early ontogeny in vertebrates, as can help explore the existence of common developmental patterns among species. Skates (Rajiformes) are the most speciose order of oviparous elasmobranchs, however, few studies are focused on embryo development and only based on one skate family: Rajidae. Here, we extended the study of embryo development to other skate family, Arhynchobatidae, which represent about 1/3 of all skate species. Three adult female bignose fanskates (Sympterygia acuta) were held in captivity in order to provide the first complete embryonic development timeline for any species within the Arhynchobatidae family. Our results allowed further comparisons at the embryonic scale of different oviparous elasmobranch families, providing an updated cross-species overview of the early ontogeny. Incubation in S. acuta lasted 97 ± 1.4 days at 11–21.7 °C, and hatching size was 93.2 ± 0.2 mm in total length and 49.2 ± 0.3 mm in disc width. Early embryos of S. acuta were anatomically similar to other oviparous elasmobranch embryos, with several structures appearing at the same time, but late embryonic development was comparatively delayed. The late resorption of both the external yolk sac and the external gill filaments, and also the delay in the slit opening could indicate a low metabolic demand in S. acuta, which would probably be coupled with its seasonal reproductive cycle. Some structures such as external gill filaments and claspers appeared at a similar time in some species of Rajidae and also in Arhynchobatidae, but at different times in species of the same family, showing an inconsistency also found within shark families. Although the sequential scheme remained relatively constant, small heterochronies would be present within skates, within sharks, and also between skates and sharks.