All organisms must feed in order to survive. The actual process of feeding and subsequent digestion of food requires energy and also causes significant increases in metabolism. Because this general effect of feeding is well known, scientists usually take this into account and monitor animals under controlled experimental conditions where feeding is carefully regimented. This ensures that all animals are in a similar physiological state. Organisms are able to tolerate changes in their environment (e.g. oxygen, temperature, salinity) by channeling energy towards necessary physiological mechanisms. However, if the energetic costs of digestion are large then they may have profound effects on these physiological systems. Thus the digestive state of an animal may hamper its ability to cope with changes in environmental parameters. Little is known how animals balance the simultaneous demands of several systems and how the physiological requirements of digestion, impact other physiological processes. The project will examine aspects of both feeding behavior and digestive physiology of a variety of decapod crustaceans (crabs shrimps, lobsters) in response to low salinity exposure. Despite extensive knowledge of many aspects of crustacean biology, few reports focus on how environmental stress affects feeding, or how digestive processes compete with or compensate other physiological processes. Five crab species of varying ability to tolerate low salinity (and also of economic and ecological importance) will be studied. They are, the blue crab Callinectes sapidus, and the green crab Carcinus maenas, both of which have efficient physiological mechanisms that allow them to migrate between marine and freshwater habitats; the spider crab Libinia emarginata, which is largely confined to fully marine environments; and the Dungeness crab Cancer magister, and red rock crab Cancer productus, both possessing physiological mechanisms that allow them to tolerate marine to brackish water environments. There are three objectives of this research. First, to determine the trade-offs or gains associated with acquiring food versus expending energy on physiological mechanisms required for survival in low salinity. Salinity choice chamber experiments, in conjunction with remote tracking of animals in the field, will be used to determine the salinity preference range of each species. The underlying theme will be to determine whether this preference range is altered by food availability or the hunger level of the animal. What are the effects of salinity change on digestion rates of crabs? The second objective of the project is to establish if salinity alters the time taken for digestion and movement of food through the gut system, and if this differs with the salinity tolerance of each species. This objective will be achieved by using barium tracer in food provided to the crabs exposed to different salinities, and tracking the movement of food through the gut with X-ray analysis. Dynamic X-ray techniques will be used to study how low salinity changes gut contraction and food passage through regions of the digestive system. The energy expenditures involved with digestion and survival in low salinity regimes are compensated for, in part, by adjustments in the cardiovascular and respiratory physiology. The third goal of the project is to establish how respiratory and cardiovascular function is modulated by the physiological changes associated with digestion. A pulsed Doppler flow-meter will be used to measure cardiac parameters, and oxygen uptake will be monitored in a flow through respirometer. While species of crabs exhibit differential cardiovascular and respiratory responses to low salinity, it is unknown if and how these are affected during feeding and digestion. Most scientists starve experimental animals prior to and during experiments. Because these control conditions rarely occur in nature, and crabs may encounter low salinity at different times of the digestive cycle, this work will demonstrate that, in addition to changes in the environment, the nutritional state of the animal is important in modulating physiological mechanisms. This work will expand our knowledge on the currently limited topic of crustacean digestive physiology, and will increase our understanding of invertebrate physiology in general.
