The skin of mudskipper fishes is adapted for terrestrial life primarily through the distribution of the mucous cells, epidermal vascularization and the presence of the dermal bulges and the middle cell layer composed of swollen cells (Zhang et al., 2000). The present study showed that the thickness of the epidermis in males and females of P. waltoni was lower than in B. dussumeri. Zhang et al. (2003) and Park et al. (2006) reported that the thickness of the epidermis depends mainly on the thickness of the middle layer, i.e the size and number of the swollen cells. This study showed that in P. waltoni and B. dussumeri the thinnest epidermis is in the abdominal region. The mucous cells were lacking in the epidermis of P. waltoni whereas found in B. dussumeri. Like other species of Boleophthalmus such as B. Boddarti and B. pectinirostris, B. dussumeri had dermal bulges, whose function appears to be the prevention of the desiccation on exposed tidal flats (Zhang et al., 2000), however such a structure was not developed in P. waltoni. Although the dermal bulge is not developed in P. waltoni as in other species of Periophthalmus, their entire bodies are covered by swollen middle cells. The middle cell layer likely plays an important role in defending against desiccation (Zhang et al., 2000). We found the swollen middle cells in both P. waltoni and B. dussumeri. The swollen middle cells appear to be a common skin feature in mudskippers and are recognized to have a significant function not just as a barrier to water loss, but also in water storage (Yokoya and Tamura, 1992; Zhang et al., 2003). Several mudskipper species, including Periophthalmus modestus (Yokoya and Tamura, 1992), Periophthalmus magnuspinnatus (Park et al., 2003), Periophthalmodon septemradiatus, and Periophthalmodon Schlosseri, have swollen middle cells (Zhang et al., 2003). On the contrary, the swollen middle cell was not found in other air-breathing fishes (Mittal and Munshi, 1971; Whitear, 1986; Yokoya and Tamura, 1992; Graham, 1997) and amphibians (Whitear, 1986) and plays an important role not only as a barrier to water loss but also stored a large amount of water (Yokoya and Tamura, 1992).
Our results also showed that the diffusion distance in P. waltoni was lower than in B. dussumeri. The vascularization of the epidermis is one of the similar structural features in mudskippers (Zhang et al., 2003). The presence of blood capillaries in the epidermis, which is close to the outer surface, makes it possible for the gas exchange to occur between the blood cells inside capillaries and the environment. The distance of the capillaries from the skin surface is within the range of 500 µm, so the process of diffusion is quite adequate for the exchange of gases. The diffusion distance in P. magnuspinnatus is about 1.5 µm on average (Park et al., 2000) and on the dorsal body of B. Boddarti and B. pectinirostris range between 2–6 µm (Zhang et al., 2000). This means that the skin is an efficient organ for respiration in mudskippers (Mittal and Munshi, 1971). However, the diffusion distance is variable and is closely related to the life modes of mudskippers. In B. dussumeri which often stays in more aquatic environments, the diffusion distance is more than in P. waltoni. B. dussumeri stays in their burrows during the high tide, at which time the burrows are submerged. They emerge from their burrows when the mudflat is exposed to the air by the ebb tide, and move about on the mudflat, exposing all or a part of their bodies to the air. While in the water, they expose the head and a part of the dorsum above the water (Park et al., 2003).
In P. waltoni, which is highly terrestrial, the capillaries in the dorsal and ventral areas of the body are more closely distributed to the surface of the epidermis rather than in B. dussumeri. This means that the cutaneous gas exchange in P. waltoni occurs more closely to the surface of the epidermis.
Moreover, the diffusion distance is different between the ventral and dorsal epidermis (Al-Kadhomiy and Hughes 1988). In B. dussumeri the diffusion distance in the ventral epidermis was more than in the dorsal epidermis. The ventral skin which is frequently immersed in the water or soft mud showed a greater diffusion distance. This is while in P. waltoni the diffusion distance in the ventral epidermis was less than in the dorsal epidermis. Tamura et al. (1976) showed that the proportions of the oxygen uptake via the skin in water were 48% for Periophthalmus Cantonensis and 36% for B. Chinensis while in the air the corresponding figures were 76 and 43%.