In aquatic environment, fishes are the earliest vertebrates that have a well–developed immune system. The main lymphoid organs of fishes are thymus, spleen and head–kidney. These organs consist of mixed lymphoid and myeloid elements. Investigations on the lymphoid tissues and development of cell-mediated and humoral immunity in several vertebrate classes have shown that the immune mechanism is generally similar in fishes. However, fishes do not have bone marrow and lymph nodes and, instead, possess well-developed pronephric head-kidney consisting of lymphoid and interrenal tissue (Deivasigamani, 2008).
The head-kidney (HK) is a unique, important lymphoid organ in fish. It contains more lymphocytes than spleen and has been shown to be actively involved in antibody production. The thymus is well developed in both cartilagenous fish and bony fish, but is absent in the jawless fish. In elasmobranches it has been shown that the thymus is the source of lymphoid cells which are ultimately, distributed to other peripheral lymphoid organs. The structure of the teleosts, thymus has been studied in several fish species, but there is considerable controversy as to degree of heterogeneity of the thymic epithelial and non-epithelial components and functional properties.
Many marine species of teleosts use estuaries as nursery areas, thereby benefiting from such features as the presence of a greater productivity, a reduced incidence of piscivorous predators and a lower salinity than in their natural environment. Although the juveniles of some of these species also frequently exploit protected inshore marine environments, this important group of marine fish has often been referred to as estuarine-dependent.
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