Embryogenesis in the Dwarf Seahorse, Hippocampus zosterae (Syngnathidae)

Abstract

Embryogenesis of the dwarf seahorse, Hippocampus zosterae, was studied by scanning electron microscopy of a series of developmental stages. Stages ranged from initial cleavage of the egg through term embryos. Embryos hatch from their egg envelopes about midway through development, yet remain nestled in stromal chambers of vascularized epithelium within the male brood pouch until their yolk reserves are consumed. The difference in body shape between the pipefish and seahorse first becomes discernible during mid-development, just before hatching. At this stage, embryos begin to develop their characteristic prehensile tail in contrast to the straight body and typical caudal fin of most species of pipefish. Post-hatching, ‘yolk-sac’ larvae have a well developed head, that is set at a right angle to the body axis, and fully formed fins. As seahorse embryos approach term, lepidotrichia calcify, and the prehensile tail is capable of muscular contraction. Dermal scutes first appear at this stage and ossify in the term embryo. The dermal armor is then fully formed and functional. At term, the yolk reserves have been depleted, and the young are released from the brood pouch as free-swimming, free-feeding miniature versions of the adults.