Molt cycle related changes and effect of short term starvation on the biochemical constituents of the blue swimmer crab Portunus pelagicus

Abstract

Synthesis and hardening of a new exoskeleton are essential to the arthropod molting process. The present study emphasizes the variations in the levels of hemolymph total free sugars, hepatopancreas glycogen and cuticular proteins during the molting stages of Portunus pelagicus. It also reports the effect of short-term starvation conditions on the biochemical constituents of the hemolymph. Intermolt crabs were subjected to 6days of starvation and hemolymph samples were taken. Standard biochemical procedures were followed toward the quantification of total proteins, total free sugars and total lipids. The total free sugar level in the hemolymph of P. pelagicus was observed to increase during early premolt D0 (3.108±0.032g/ml) and a gradual decrease till late postmolt B stage (0.552±0.124g/ml), suggesting the need for total free sugars to provide energy for the apolysis process. Increase in the levels of hepatopancreas glycogen was observed from 1225±0.04μg/mg in early premolt D0 to 1700±0.3μg/mg in late premolt D2–3. This is in correlation with the decreased levels of free sugars during premolt stages, suggesting an increase in the storage of glycogen reserves in the hepatopancreas. Cuticular proteins increased during stage B (2.702±0.093g/ml) and stage C (3.065±0.012g/ml), indicating exoskeleton hardening and mineralization. Results of the starvation studies clearly showed a steady decline in the level of total free sugars till day 6 (0.099±0.00g/ml) when compared to the control (8.646±0.08g/ml). Gradual decrease of total lipids was also observed from the first day of the experiment (6.088±2.44g/ml) to the last day of the study (0.401±0.20g/ml) which was 85% lesser than the control (8.450±0.49g/ml)suggesting the efficient usage of total sugars to consolidate the loss of energy reserves during starvation. The knowledge of Molt-cycle events can be used as a tool for the evaluation of the developmental state providing a morphological reference system for physiological and biochemical studies related to crab aquaculture. Starvation studies enlightens that increasing carbohydrate levels in crab feed together with good protein content could alleviate the natural effects of starvation, improve farm productivity and reduce the deleterious impact of nitrogen pollution generated by rich-protein feeds used in crab farming.