ABSTRACT
32The skin of a fish is a multifunctional organ and may serve important roles in communication, sensory perception, locomotion, respiration, excretion, osmoregulation, and thermal regulation. The skin is self-active secretory organ that their cellular components provide many useful products. Goblet cells secrete mucus that keep the body surfaces moist and protect it from stressors and club cells produce the alarm substances that initiate the alarm reaction. The dermal chromatophore units provide patterns of coloration and absorb or reflect radiations. They include melanophores, xanthophores, erythrophores, and iridophores. The skin of fish shows various inter-species differences, as some species have scales and others have special cells such as sacciform cells, eosinophilic granular cells (EGCs), and rodlet cells (RCs). The skin is also a vehicle for coetaneous sense organs allowed the fish for detection of predators and foods. Among them are taste buds and lateral line system. The lateral line is sensory system that allows fish to sense objects and motion in their surrounding aquatic environment. The lateral line system included into two subsystems: mechanoreceptive neuromasts and electroreceptive ampullary and tuberous organs. Two types of neuromasts occurred in fish species: superficial and canal neuromasts. The tuberous organ is more frequently occurring in the head region and contains 4–5 sensory receptor cells that play a significant role in sensation of weak electrical stimuli. Canal neuromasts are embedded in the dermis in form of tunnel-like canals. Large number of superficial neuromasts located at the lower lips and the head of redtail shark. The ampullary organ is localized in the head region behind the eye and is formed of specialized receptors specialized in detecting the electromagnetic fields as well as temperature gradients. Ampullary organs have been classified into two different types based on the size and the length of the canals.
