Osmoregulatory role of the paternal brood pouch for two Syngnathus species.

Even though sea horses, sea dragons, and pipefishes of the teleost family Syngnathidae have been studied for over a century, the physiological significance of the paternal brood pouch is not well defined. Here we document the regulation of brood pouch osmolality during embryonic development for Syngnathus floridae and Syngnathus fuscus, particularly during the middle brood stages or around hatching. S. fuscus brood pouch osmolality was significantly lower than non-brooding conspecifics for all brood stages, but early and late stage S. floridae displayed pouch osmolality comparable to non-brooding males, suggesting brood pouch osmoregulation at these stages may be important for S. fuscus embryonic development. Quantification of Na(+), K(+), Ca(2+), and Mg(2+) in paternal blood plasma, pouch fluid, and embryos indicated that regulation of these ions contributes to pouch osmoregulation and furthermore, that ions in the brood pouch are most likely derived from the environment and not paternal blood. While both species displayed significant increases in dry mass and changes in embryonic ion concentrations during development, net uptake of paternally-derived ions was not documented. Overall, this examination furthers our understanding of syngnathid brood pouch physiology and offers insight into the evolution of paternal care in these fishes.