Adaptations for reproduction and development in the skin-brooding ghost pipefishes,Solenostomus

Synopsis

Ghost pipefishes comprise a small family (Solenostomidae) of skin-brooding fishes related to true pipefishes and seahorses (Syngnathidae).Solenostomus embryos develop within the fused pelvic fins of the female, unlike syngnathids in which males brood the eggs. Embryos, enclosed in egg envelopes, are attached to epidermal stalks, termed cotylephores, that occur only in brooding females. Cotylephores are cellular outgrowths of the epithelium on the inside surface of the pelvic fins. They attain a mean length of 687 ± 3.89 μm and diameter of 105 ± 3.38 μm. Cotylephores originate on the epithelial surface that lies over the lepidotrichia and they develop into multi-headed cylindrical branches approximately 125 ± 3.65 μm in length and 78 ± 2.19 μm in diameter. A mean of 26 ± 0.63 lateral branches are found on fully developed cotylephores. Each branch terminates in a wide apical calyx, approximately 112 ± 4.16 μm in diameter, to which the egg envelope adheres. Adjacent calyces of the same cotylephore establish attachments with the envelope of a single egg. Cotylephores are composed of a surface epithelium that is continuous with the skin and a fibrous connective tissue core that contains blood vessels that ramify into an apical capillary plexus. The plexus may function in maternal-embryonic metabolic exchange. The cotylephores ofSolenostomus closely resemble the epidermal stalks (cotylephores) that are the sites of egg attachment in the skin-brooding South American catfish,Platystacus cotylephorus. Based on similarity in structure and probable function, cotylephores in the two groups of fishes are an example of evolutionary convergence.