Natural beds and cultures of the scallop Argopecten purpuratus are exposed to hypoxia periodically because they are located near upwelling zones. We evaluated the effects of environmental hypoxia on juvenile and adult scallops' escape responses and recovery capacities using 3 different hypoxic cycle (i.e., a gradual decrease and increase of dissolved oxygen) trials. Trials 1 and 2 consisted of single (1-day) hypoxic cycles; trial 3 included exposure to several consecutive daily hypoxic cycles over 7 days, which occurs in the natural environment. Trial 1 evaluated the combined effects of energy demands from escape responses and metabolism on escape responses. during environmental hypoxia. Trial 2 evaluated only the effect of environmental hypoxia on the escape responses. Before and after the hypoxic cycles, the scallops were exposed to their main predator, the sea star Meyenaster gelatinosus. We evaluated 6 indicators of the scallops' escape response: reaction time, the total number of claps, duration of the clapping response until exhaustion, clapping speed, the proportion of claps, and recovery clapping rate after a recuperation of 5 min or 10 min (for juveniles and adults, respectively). The combined effect of functional and environmental hypoxia (trial 1) affected both juvenile and adult escape responses, with a reduction in their number of claps (15% and 25%, respectively), the clapping rate in juveniles (17%), and the clapping time in adults (19%). However, environmental hypoxia alone (trial 2) affected juvenile escape responses only, with a reduction in their number of claps (18%) and clapping rate (17%). After hypoxic exposure in trial 3, adults only showed a reduced escape capacity (clap number and clapping time of 16% and 17%, respectively), although both adult and juvenile scallops had a reduced capacity to recuperate their initial clap number (18% and 23%, respectively) and clapping rate (21% and 17%, respectively). Recovery capacity was associated with a strong reduction in phasic muscle carbohydrates during this period. Overall, hypoxia reduced the escape capacity of A. purpuratus, which may implicate a higher vulnerability to predation (for natural populations) and a decreased physiological status to support other stress factors (for cultured or natural populations).
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Vol. 32 • No. 2