Over the last thirty years, the California sea lion (Zalophus californianus) has been a focal point for sensory, communication, cognition, and neurological disease studies in marine mammals. However, as a scientific community, we lack a non-invasive approach to investigate the anatomy and size of brain structures in this species and other free-ranging, live marine mammals. In this presentation, we provide the first anatomically labeled magnetic resonance imaging (MRI)-based atlas derived from a live marine mammal, the California sea lion. The brain of the California sea lion contained more secondary gyri and sulci than the brains of terrestrial relatives, the canids and mustelids. The olfactory bulb was small. The hippocampus of the California sea lion was found mostly in the ventral position with very little extension dorsally, quite unlike the canids and the mustelids, in which the hippocampus is present in the ventral position but extends dorsally above the thalamus. Additionally, in contrast to the canids and the mustelids, the pineal gland of the California sea lion was strikingly larger. In addition, we report three-dimensional reconstructions and volumes of cerebrospinal fluid (CSF), cerebral ventricles, total white matter (WM), total gray matter (GM), cerebral hemispheres (WM and GM), cerebellum and brainstem (WM and GM), and hippocampal structures all derived from MR images (Figure 1). These measurements are the first to be determined for any pinniped species. In California sea lions, this approach can be used to investigate the neurological effects of acute, chronic, and developmental exposure to marine neurotoxins associated with harmful algal blooms (e.g., domoic acid).
Three-dimensional (3D) reconstructions of brain structures of a live California sea lion using magnetic resonance images. A.) Rostral view of the 3D reconstruction of the brain. Surface of left hemisphere = dark pink; surface of right hemisphere = light pink; left cerebrospinal fluid (CSF) = dark purple; right CSF = light purple. B.) Rostral view of the three-dimensional reconstruction of the white matter (WM), with the gray matter (GM) and exterior CSF removed. The CSF of the third and lateral ventricles are visible (aqua blue), as well as the left (green) and right (red) hippocampi. WM of left hemisphere = orange; WM of right hemisphere = tan; WM of cerebellum and brainstem = yellow. C.) Rostral view of the lateral ventricles and hippocampi, with the white matter stripped away. Lateral ventricles = aqua blue; green = left hippocampus; red = right hippocampus; white = fimbria; brown = alveus; yellow = fornix; bright pink = mammillary bodies. D.) Rostral view of the hippocampi, with the lateral ventricles removed. Green = left hippocampus; red = right hippocampus; white = fimbria; brown = alveus; yellow = fornix; aqua blue = septal nucleus; bright pink = mammillary bodies. Scale bars = 8 cm.