Comparative morphology of cotylephores in Platystacus and Solenostomus: modifications of the integument for egg attachment in skin-brooding fishes

Abstract

External skin brooding evolved independently in several groups of fishes. Cotylephores, sites for the attachment of developing embryos, occur within the fused pelvic fins of the ghost pipefishes, Solenostomus, on the ventral surface of the South American catfish, Platystacus, and on other aspredinid cattfishes of the tribe Aspredini. Cotylephores are transient outgrowths of tissue that occur only on brooding fish. They consist of a pedicle that extends from the abdomen or fin and terminates in an apical calyx. The calyx supports and adheres to the egg envelope that encloses the developing embryo. The pedicle of the catfish cotylephore is a single, large structure (1615 ± 23.25 µm), while those of ghost pipefishes are small and branched (687 ± 3.89 µm; number of branches: 26 ± 0.63). In both instances, a simple cuboidal epithelium, whose cells bear microplicae, encloses an extensive connective tissue core that contains large blood vessels that ramify into a capillary plexus in the calyx. Cotylephores of Platystacus are more heavily vascularized than those of Solenostomus and contain 34.77% more blood vessel surface area at the calyx, to which the egg envelope adheres. In addition to their role in attachment, cotylephores may function in embryonic maintenance through the exchange of gas across the egg envelope, mediated by female circulatory vessels within the cotylephore calyx. Comparisons of morphology and development suggest that cotylephores are the result of convergent evolution of a reproductive adaptation.