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Odor-mediated behavioral responses of the blue swimming crab Portunus pelagicus L. instars

The blue swimming crab Portunus pelagicus is an important fishery commodity in the Philippines. The fishery however is getting overexploited in many fishing grounds in the country and stock enhancement activities have been conducted purportedly to improve wild stock population. In addition, efforts to initiate and improve culture techniques for blue swimming crabs developed, yet little is known about its ecology and behavior during its early stages of development, particularly during the instar stage. In this study, sensitivity and behavioral responses of P. pelagicus instars (~0.5 cm CW) to risk odor was investigated using a Y-maze aquarium. Test animals were introduced to different chemical sues such as 1) mussel extract as food odor; 2) snapper odor as predator odor and; 3) seawater as control. Control tests showed that crabs did not indicate preference to either left or right chamber (P>0.05). Higher proportion of crabs (both settling and tracking individuals) were drawn towards the mussel extract treatment (P<0.05) which suggests that crabs were attracted to the food odors. When introduced with snapper odor-conditioned seawater, crab instars preferred to stay in the untreated water (P<0.05). These results suggest that even during their early stages of development, P. pelagicus instars were highly sensitive to chemical cues and considered the snapper odor as potential predatory threat as evidenced by risk aversion and less amount of movement in the Y-maze tank. Results are discussed on possible applications, such as inhibition of cannibalism especially during molting stage of juvenile crabs.