Histology and microstructure of the brain and eye in four fish species across different depths

Vision is an important sensory system for most fish species. The eye structure is related to the ecology of the respective fish species and depends on the depth of life of that species. Depending on the type of lifestyle, feeding, behavior, and habitat of the fish, the different lobes of the fish's brain have different sizes. Information about eye and brain structure can help in ecology modeling, aquaculture and fisheries industries, and fish histopathology studies.

In this study, the structures of the eyes and brains of four different fish species (Scarus ghobban and Scarus persicus, Sphyraena obtusata, and Pseudorhombus elevatus) in various habitats were investigated. Fishes were collected from three areas of surface waters (S. obtusata), bottom waters (P. elevatus), and around coral reefs (Scarus species). After photographing and identifying the fish, the samples were biometrically measured, and some morphometric characteristics were measured. After dissecting the fish and extracting the brain and eyes, the samples are immediately fixed in Buen's solution. After fixation, the eye and brain samples were dehydrated with an ethanol series and embedded in paraffin. Serial sections were prepared with a thickness of 5 µm. Sections stained with Hematoxylin-eosin (H&E) were photographed. The prepared slides were examined under a microscope, and the number of cone and rod eye cells, as well as the percentage of brain neurons, were estimated.

The results of this study showed that the percentage of cone cells in the eyes of S. obtusata was higher than that of other fish species, while the percentage of rod cells was higher in the eyes of P. elevatus. The percentage of cone and rod cells in the eye of parrotfish had intermediate values. On the other hand, the rate of front and middle brain neurons in Sphyraena obtusata was higher than in different species. In contrast, the percentage of posterior brain neurons was higher in P. elevatus than in others. Our findings showed that the predatory fish often uses its sense of sight and smell to hunt, which is due to the well-developed front brain and middle brain of this fish. Our results showed that the development of all parts of the brain in both types of parrotfish is nearly identical, and therefore, it can be concluded that parrotfish use all their senses equally to survive.

Based on the available data, the percentage of neurons in the posterior cingulate and the forebrain of the benthic fish P. elevatus was high, which can reflect the strength of olfactory and motor senses in this demersal fish.