Seahorses: Osteogenesis and Caudal Fin Loss

Abstract

Seahorses (Family: Syngnathidae) are well known for their highly derived head shape, prehensile tail and armoured body, but their osteogenesis is poorly described. Here we compare the osteogenesis of Hippocampus reidi over an ontogenetic series spanning the first 93 days after release from the brood pouch to that of a smaller series of Hippocampus; namely H. subelongatus. We also compared our results to those published for a dwarf species, H. zosterae. We show that ossification onset in H. subelongatus is earlier than in H. reidi, despite similar sizes at parturition. Interestingly, osteogenesis in H. zosterae is similar to that of the larger species, H. subelongatus. Also, the growth rate of all three species is similar initially but at one month of age, the size of the dwarf species H. zosterae remains relatively constant whilst the other two species continue growing with an accelerated growth phase. Putting these observations within a phylogenetic context suggests that there has been a heterochronic shift in the timing of osteogenesis in H. reidi and H. zosterae. We conclude that H. zosterae is a proportioned dwarf rather than a developmentally truncated dwarf species. Furthermore, we show that caudal fin loss is incomplete in Hippocampus seahorses.