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Mudskippers are members of the Oxudercidae family and are amphibious fishes that are able to spend long periods out of water. In the present study, Boleophthalmus dussumieri was sampled from Petroshimi and Jafari stations. In order to perform histological studies, the samples (3 males and females from each station) were anesthetized and fixed in Boyne's fixative solution in accordance with biological ethical principles. Then, tissue sectioning and H&E staining were done. Observations by light microscope showed that the epidermis consists of a corresponding epithelium consisting of three cell layers. The dermis of the skin consists of thick collagen fibers and many blood capillaries. In some areas, protrusions from the dermis enter the epidermis, which are called dermal protrusions. There was a significant difference in the thickness of the epidermis layer in males and females. It was 87.97 microns in females and 74.32 microns in males. Based on the amphibian way of life of slugs, in species that are more hydrophilic, the upper and middle parts of the body, which are often exposed to air, have smaller amounts of diffusion distance. Therefore, it can be expected that air absorption through the skin in B. dussumieri takes place more in the upper part of the body than in the lower part or appendages.IntroductionThe term mudskippers refers to four genera, namely Periophthalmus, Periophthalmodon, Boleophthalmus, and Scartelaos, which are known as fish-amphibians and have different degrees of adaptation to terrestrial conditions. Boleophthalmus dussumieri from the Oxudercidae family is distributed along the coasts of the Oman Sea and the Persian Gulf. Mudskippers are able to breathe through the skin, the lining of the mouth (mucous or mucous membrane), and the mouth (throat). This action will be possible when their skin is moist and wet. This type of breathing, which is used in a similar way in amphibians, is called skin or skin breathing. So far, various studies have been done on this group of fish in East Asia. In order to further investigate thistopic, in the present study, the histological investigation of the dorsal and ventral surfaces of B. dussumieri and its role in respiration was done. Materials and MethodsIn this research, some specimens of B. dussumieri were caught alive by hand during the spring season of 2020-2021 from Khor Mahshahr, located in the northwest of the Persian Gulf. The checking of morphological index traits was done with the help of a valid identification key. A 1 x 1 cm skin incision was made from the skin of the back and abdomen. The usual method of preparing tissue sections was used. Then, 5 μm thick sections were prepared and subjected to general hematoxylin-eosin (H&E) staining. To perform histometric studies, an optical microscope equipped with a Dino-Lite digital lens and Image J software was used. The measurement results were analyzed using SPSS 20 software. Results and Conclusions Microscopic observations showed that the epidermis consists of a corresponding epithelium consisting of three cell layers, including the surface layer, the middle layer, and the reproductive layer. The surface layer of the epidermis consists of several layers of flat and squamous cells along with a large number of blood capillaries (intraepithelial blood capillaries) that are located near the body surface. Nucleated red blood cells in these capillaries are located near the surface of the fish body. The middle layer of the epidermis consists of several layers of spherical to polyhedral cells, which have a central nucleus and are called vesicular cells. The basal or germ layer also consists of several rows of cubic cells with a spherical nucleus. Also, the histological study showed that the dermis of the skin consists of thick collagen fibers.Comparing the thickness of the epidermis layer of the skin showed that there is a significant difference between the male and female sexes in terms of the thickness of the epidermis. Also, the thickness of the epidermis was measured in females more than in males. There was a significant difference in the thickness of the epidermis between the dorsal-ventral surfaces in each of the male and female sexes. There was no significant difference between the male and female sexes in terms of diffusion distance. However, the average distance of diffusion in the female sex compared to the male sex included smaller values. In males, the diffusion distance between the dorsal and ventral surfaces was very different. But in females, it was significant only at the abdominal level. The difference in basal layer thickness was significant in both areas. The comparison of the middle layer in males and females had a significant difference and its thickness was measured more in females than in males. Finally, the comparison of the external layer showed that the thickness of this layer in the abdominal part of males has a significant difference compared to the abdominal surface.The comparison of the tissue sections of the skin in B. dussumieri populations showed that the adaptation of this group of fish to dry conditions had an effect on the thickness and structure of the epidermis, the distribution, abundance, and function of the capillaries in the epidermis. The presence of skin protrusions in B. dussumieri reduces the outer layer of the epidermis. This is consistent with the results of other studies in which dermal protrusions are observed in all Boleophthalmus and Scartelaos genera in the structure of the epidermis. Based on the amphibious lifestyle of the gill-feeding fishes, it seems that in species that are more hydrophilic, the upper and middle parts of the body are often exposed to the air leading to smaller values ​​of the diffusion distance in these areas. The two genera Periophthalmus and Periophthalmodon perform skin breathing directly through the air and are known as air-breathing fish. Alternatively, adaptations such as a thicker epidermis, well-developed vascularization, fewer blood capillaries, and reduced diffusion distance may enable dermal respiration to survive in poorly oxygenated water during the periodic dry season. Funding Funding was provided by the Department of Biology, Faculty of Sciences, Razi University, Kermanshah, Iran.

Mudskippers are members of the Oxudercidae family and are amphibious fishes that are able to spend long periods out of water. In the present study, Boleophthalmus dussumieri was sampled from Petroshimi and Jafari stations. In order to perform histological studies, the samples (3 males and females from each station) were anesthetized and fixed in Boyne's fixative solution in accordance with biological ethical principles. Then, tissue sectioning and H&E staining were done. Observations by light microscope showed that the epidermis consists of a corresponding epithelium consisting of three cell layers. The dermis of the skin consists of thick collagen fibers and many blood capillaries. In some areas, protrusions from the dermis enter the epidermis, which are called dermal protrusions. There was a significant difference in the thickness of the epidermis layer in males and females. It was 87.97 microns in females and 74.32 microns in males. Based on the amphibian way of life of slugs, in species that are more hydrophilic, the upper and middle parts of the body, which are often exposed to air, have smaller amounts of diffusion distance. Therefore, it can be expected that air absorption through the skin in B. dussumieri takes place more in the upper part of the body than in the lower part or appendages. [ABSTRACT FROM AUTHOR]