Effects of food deprivation states on behavioral and physiological responses to hypoxia in rock crabs (Cancer irroratus)

Jiang, Qiwu (2022) Effects of food deprivation states on behavioral and physiological responses to hypoxia in rock crabs (Cancer irroratus). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Hypoxia has become increasingly prevalent in benthic marine environments. Despite this fact most experiments have investigated the effects of hypoxia alone, without taking into account the routine activities of animals. Crustaceans are among the most sensitive taxa to changes in environmental oxygen. Here, I used the Atlantic rock crab (Cancer irroratus) to determine the interactive effects of hypoxia and feeding/food deprivation state on aerobic metabolism, hemolymph biochemistry, feeding and gastric processing as well as the behavioral responses of this shallow water decapod. The critical oxygen partial pressure (Pcrit) was related to the food deprivation state of the crabs. Starved crabs exhibited the lowest (Pcrit), while fed crabs had the highest Pcrit. Interestingly, below the Pcrit level (fasted crabs), an elevated oxygen consumption (MO2) after feeding and increased locomotor activity indicated that the Pcrit calculated by piecewise linear regression might represent a hypometabolic shift, which allowed some aerobic scope reserved for critical activities. Hypoxia retarded the specific dynamic action (SDA) and gastric processing of both fasted and starved crabs, resulting in lower peak MO₂ and more prolonged duration that MO₂ remained elevated. The starved crabs exhibited a lower peak MO2, prolonged duration and higher energy expenditure of SDA, and slightly longer transit times for digesta compared with fasted crabs. It was postulated that starvation may have triggered a cross-tolerance to hypoxia, since both long-term starvation and severe hypoxia elicited similar hypometabolic responses. Changes in oxygen consumption were paralleled by changes in hemolymph biochemistry. Following feeding, only fasted crabs exhibited a significant increase in hemolymph L-lactate concentration. This hypoxia-induced alkalosis and elevated L-lactate may have improved the hemocyanin-oxygen affinity and thus oxygen transport, below the Pcrit level. During exposure to severe hypoxia (20% oxygen), rock crabs reduced food intake, and some crabs refused to eat. Although food deprivation state did not significantly affect food intake, more starved crabs fed in 20% oxygen than their fasted conspecifics. Although the rock crabs did not appear to exhibit a preference when offered a choice of oxygen levels, the level of hypoxia played an important role in regulating their activity levels, which was also influenced to a lesser degree by the food deprivation state. Overall, the fed crabs were less active than their fasted conspecifics in 20% oxygen (below Pcrit of both fed and fasted crabs), likely because they could not balance the simultaneous demands of digestion and increased activity. The results of this thesis suggest that although responses to hypoxia are modulated by food deprivation states, they are unlikely to severe enough to affect the distribution or survival of rock crabs.

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