Early life history of the <scp>Korean</scp> seahorse, <i>Hippocampus haema</i> : Evidence for direct settlement and ontogenetic shifts in snout morphology

Early life-history traits of seahorses are closely linked to juvenile survival, dispersal potential and the onset of substrate attachment; however, detailed stage-resolved descriptions remain limited for many Hippocampus species. This study examined reproductive behaviour and early ontogenetic development of the Korean seahorse, Hippocampus haema, during a 40-day laboratory rearing period. Newborns were released in a relatively advanced skeletal condition, particularly in the caudal skeleton. Skeletal staining showed that the caudal bony plates were already ossified and connected at birth, whereas the trunk plates were still only partially connected. Consistent with this condition, newborns bent the prehensile tail and attached to nearby structures immediately after release, indicating functional competence for substrate attachment at birth and suggesting a very short or indistinct planktonic phase under the observed conditions. Coronet morphology changed markedly during juvenile development, with the anterior coronet spine progressively fusing with the coronet. Morphometric model comparison did not strongly support a continuous power function allometry across measured traits. However, segmented regression identified a significant breakpoint in snout depth at 21.3 mm standard length, after which snout depth increased approximately 2.6-fold more steeply. These findings indicate that H. haema is characterised by advanced caudal skeletal development at birth, immediate tail attachment, dynamic coronet development and a discrete ontogenetic shift in snout depth growth. This stage-resolved description provides baseline information for identifying early developmental stages and interpreting early life history variation in this species.