Effect of moulting and lunar rhythms on the lymphoid organ spheroid (LOS) cells of the black tiger prawn (Penaeus monodon)

Studies on the effect of moult and lunar cycles on the lymphoid organ spheroid (LOS) cells of Penaeus monodon were conducted at two different times both in laboratory and field trials. The spheroid to total tissue (STT) ratio of the lymphoid organ varied during the life span of the prawns. Animal size (weight; total length) and sex had no significant effect on the STT ratio (P > 0.05). The moult cycle also had no significant effect on the STT ratio (P > 0.05) indicating that the fluctuation in the number of spheroid cells during the life of the prawns was not related to the stage of moult. Significant differences in STT ratio appeared within the lunar rhythms both in laboratory held and farmed animals (P < 0.05). The presence of lunar related patterns on the abundance of LOS cells in laboratory held animals suggested that this effect was endogenous and hereditary, without environmental cues. In farmed animals, the effect of light intensity during the lunar cycles and the hydrostatical effect of spring neap tide may provide environmental cues absent in the laboratory animals.