Fish skin is a multi-functional envelope that acts as physical barrier to potential bacterial infections (Zhang et al., 2021), abrasion (Lv et al., 2023), sensory system (Mogdans, 2019), color expression (Vissio et al., 2021), much more freedom motion (Clark et al., 2016), acid-base regulation (Perry and Gilmour, 2006), excretion of nitrogenous compounds (Wood, 1993), and osmoregulation (Marshall, 2012). Such physiologies are well-supported by the ED with SPCs, CCs, MCs, and SCCs confirmed in this study. Among them, two unicellular secretary glands, CCs and MCs, not only produce alarm-clue chemicals (proteins and pheromones such as serotonin and 5-HT) with cytoplasmic membrane breakage for antipredator response in conspecifics (Zaccone et al., 1990; Carreau-Green et al., 2008; Manek et al., 2013) but also engage in defense against pathogens that can penetrate the skin (Pollock, 2011) and repair damaged tissues with chondroitin and keratin (Damasceno et al., 2012). They can help oxygen penetrate deeper toward the dermal matrix of connective tissue due an abundance of water and acidophilic proteins of a positive ion (Jakubowski, 1958; Mittal and Munshi, 1971; Park, 2002). These reports indicate that CCs and MCs of M. raitaborua may constitute a cytological delivery system for efficient oxygen diffusion or storage in cutaneous respiration, and act as a skin protector against harmful substances a fish migrating can expect in encounter in contaminated habitats M. raitaborua also had two types of MCs: elongated (MCⅠ, ii) and oval (MCⅡ). The elongated MCⅠ is a goblet mucous cell due to its nucleus position, cell morphology, and histochemistry, and has been reported in the skin of other teleosts (Rakers et al., 2011; Mohamed et al., 2020; Abolfathi et al., 2022). Fishelson (1996) noted that abundant goblet cells of the skin of the marine eel Siderea grisea skin are relevant to skin-damage reduction during movement on a hard substrate and the initiation of digging into the substrate. Elsheikh (2012) confirmed that goblet cell secretion of Oreochromis niloticus protects the epidermis of the buccal cavity from physical abrasion during feeding. These findings support the presence of more MCⅠs of the operculum of M. raitaborua, which feeds on burrowing fish or invertebrates living in the sand and dig into bottom substrate using its head as ecology, at least in genus Moringua (Smith, 1997).
Many amphibious fishes exhibiting cutaneous respiration contain a thicker ED produced by large secretary cells as follows: 38.4–156.8 µm thick, a freshwater goby Rhinogobius brunneus (Kim et al., 2022); 35.4–150 µm, a trident goby Tridentiger brevispinis (Kim, 2022); 136.3–195.5 µm, a mud loach Misgurnus mizolepis (Park et al., 2001); 146–495 µm, a torrent catfish Liobagrus mediadiposalis (Park et al., 2003a); 59.0 µm 297.0 µm, an eel goby Odontamblyopus lacepedii (Park et al., 2003b), M. raitaborua (246.8 ± 51.5 µm, 139.1–358.0; mean ± SD, range) with CCs and MCs. Reduced diffusion distance of the skin also is strong evidence that confirms more rapid gas-exchange, as measured by an ascending vascularization that represents two histological categories in its occurrence position (Glover et al., 2013): i) intraepidermal blood capillaries of the outermost surface layer (Mastcembellus pancalus with a mean diffusion distance of 34.0 µm; Mittal and Munshi, 1971; Periophthalmus modestus with a mean of 1.4 µm; Park et al., 2000), the middle layer (Liobagrus mediadiposalis with a mean of 169 µm; Park et al., 2003a), and the stratum germinativum (Rhinogobius brunneus with a range of 35.0–202.6 µm; Kim et al., 2002), and ii) well-developed dermal vascularization among collagen fibers of SL just below the basement membrane (Pseudobagrus brevicorpus, with a range of 19.9–399.4 µm, Park et al., 2010; abd Tridentiger brevispinis, with a range of 51.4–216.9, Kim, 2022). In this study, M. raitaborua showed reduced diffusion distance (268.8 ± 58.7 µm), which was similar to and affected strongly by ET (covariance, P < 0.001) in all skin regions, indicating that capillaries of M. raitaborua can get close to the basement membrane of the SL. For such a histological character, Park et al. (2003a) suggest that a reduced diffusion distance (mean = 169 µm, range = 22.5–220) by dermal vascularization as well as intraepidermal blood capillaries in L. mediadiposalis are meaningful histological modifications for fish that enable them to survive in frequently hypoxic habitats. Ba-Omar and AI-Riyami (2009) reported that rich dermal vascularization below the epidermis and in the dermis of an amphibious benny, Istiblennius edentulous, facilitates efficient gas exchange for cutaneous respiration. Thinner dorsal, lateral, and ventral bodies of M. raitaborua with reduced diffusion distance by ascending blood capillaries and well-developed vascularization may be collectively represent major skin region for gas-exchange and the supply of deficient oxygen through cutaneous respiration.