COMPARATIVE-STUDY OF THE ONTOGENY OF THE LYMPHOID ORGANS IN 3 SPECIES OF MARINE FISH

A histological examination was made of the ontogenetic development of lymphoid organs from hatching through 3 months of age in three marine fishes: the yellowtail, Seriola quinqueradiata, red sea bream, Pagrus major, and Japanese flounder, Paralichthys olivaceus. The structure and location of the lymphoid organs (thymus, kidney and spleen) were similar to those of other teleosts previously documented. The kidney, which consists of undifferentiated stem cells, was first detected within a week after hatching; the spleen and thymus were subsequently observed during the early phase of the postlarval period. Small lymphocytes appeared during the later phase of the postlarval period in the sequence thymus, kidney and spleen. The appearance of these lymphoid organs during early ontogeny was significantly later than in freshwater fishes hitherto reported. This diagnostic feature of lymphoid organ development suggests that no specific defence mechanism is developed during the larval period in the present species. Possible defence mechanism of larval fish are discussed.