The microanatomy of the central nervous system and brain of the Indo-Pacific seahorse, Hippocampus barbouri, during development

The central nervous system (CNS) of Teleostei is a complex system of self-governance and its morphology is reflected in the physiological and reproductive behaviors. The Indo-Pacific seahorse, Hippocampus barbouri Jordan & Richardson, 1908, is a new candidate species for aquaculture in Thailand. In this study, we investigated the brain morphology of H. barbouri across various developmental windows. Light microscopic observations of adult brains revealed a large optic tectum in the mesencephalon, whereas the cerebral hemispheres and the cerebellum are of medium size. The detailed brain structures were generally similar to those of other teleosts; however, only five distinct layers were present in the optic tectum, including the stratum marginale, stratum opticum, stratum album central, stratum griseum central, and stratum periventriculae, versus six layers observed in other fish. One day after birth (1 DAB) the brain was a packed structure without any clear sub-structures. The number of capillaries in the optic tectum began to increase at 6 DAB, and at 14 DAB several features, including small blood vessels in the optic tectum and Purkinje cells, became noticeable. By 35 DAB, the optic tectum became highly vascularized and included five layers. Additionally, large Purkinje cells were developed in the cerebellum. Based on the brain development pattern, we speculate that the predatory ability of this fish starts to develop from 6 to 14 days after birth.