Information about early development after male release lags behind studies of juveniles and adult seahorses, and newborn seahorses, similar in shape to adults, are considered juveniles or fry. During early life, Hippocampus hippocampus present behavioural (shift in habitat, from planktonic to benthic) and morphological changes; for this reasons, the aims of this study are to define the stage of development of H. hippocampus after they are expelled from the male brood pouch and to establish direct or indirect development through an osteological analysis. The ossification process was studied in 120 individuals, from their release to 30 days after birth. To analyse the osteological development, Alcian Blue-Alizarin Red double staining technique for bone and cartilage was adapted to this species. At birth, H. hippocampus presents a mainly cartilaginous structure that ossifies in approximately 1 month. The bony armour composed of bony rings and plates develops in 10 days. The caudal fin, a structure absent in juveniles and adult seahorses, is present at birth and progressively disappears with age. The absence of adult osteological structure in newborns, like coronet, bony rings and plates, head spines and components allowing tail prehensile abilities, suggests a metamorphosis before the juvenile stage. During the indirect development, the metamorphic stage started inside brood pouch and followed outside and leads up to reconsider the status of H. hippocampus newborns.
Keywords: Coronet; Indirect development; Metamorphosis; Newborn seahorse; Osteological development; Seahorse larvae.