Award Date

2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological Sciences

Department

Life Sciences

Advisor 1

Iain McGaw, Committee Chair

First Committee Member

Allen Gibbs

Second Committee Member

Carl Reiber

Third Committee Member

Frank van Breukelen

Graduate Faculty Representative

Shawn Gerstenberger

Number of Pages

197

Abstract

Estuaries are highly productive and serve as vital habitats for numerous decapod crustacean species. However, the environmental conditions within estuaries are often highly dynamic and subject to large changes in salinity and temperature that occur on seasonal and tidal scales. Not all of the species occupying these habitats are adept in coping with changes in these environmental conditions. This dissertation describes the influence of low salinity conditions on the 1) habitat preference, 2) feeding behaviour and 3) digestive physiology of crustaceans inhabiting estuaries. I have primarily focussed on a weak osmoregulator, the Dungeness crab, Cancer magister , but have also compared some aspects with the blue crab, Callinectes sapidus , an efficient osmoregulator. Recordings of temperature, salinity and depth using archival data tags affixed to crabs in the field showed that adult Cancer magister spent the majority of time in deep water where they were not subjected to stressful salinity or temperature conditions. When crabs did enter into the estuary, these forays often corresponded to times of increased food abundance and crabs avoided challenging temperature and salinity conditions by exploiting the estuary during nocturnal high tides when salinities were higher and temperatures lower. In the laboratory, experiments in temperature and salinity gradients showed that Cancer magister alters thermal and salinity preference behaviour in response to a food stimulus. Thus, while Cancer magister primarily avoids challenging environmental conditions, they may forage in these areas during times of increased food abundance. Since crabs may be foraging in low salinity, the effects of low salinity and starvation on feeding behaviour of Cancer magister were examined. The likelihood of feeding, the amount of food consumed and the time spent feeding were all reduced in low salinity. However, these responses were partially overridden by starvation. Removal of the sinus gland (the potential source of inhibitory hormones that regulate feeding) revealed that changes in feeding behaviour result from hormonal regulation rather than physiological limitation. Although crabs regulate food intake in response to hyposaline exposure, they may be exposed to low salinity conditions at any point in the digestive cycle. Therefore the effects that low salinity has on digestive physiology and how these effects were influenced by osmoregulatory ability were examined. In Cancer magister , exposure to low salinity post-feeding resulted in a prioritization towards the responses to low salinity, resulting in a reduction in oxygen uptake that corresponded to a reduced rate of gut contraction and an increase in gastric evacuation time. These reductions also corresponded with a delay in digestive enzyme secretion and a subsequent reduction in the post feeding increase in circulating free amino acids in the hemolymph. In contrast, Callinectes sapidus , an efficient osmoregulator, displayed a summation of the metabolic responses to low salinity exposure and digestion. Accordingly, digestive processes continued unabated in low salinity, resulting in a build up of free amino acids in the hemolymph.

Keywords

Blue crabs; Callinectes sapidus; Cancer magister; Digestion; Dungeness crabs; Environmental conditions; Estuaries; Foraging; Hormonal regulation; Osmoregulation; Salinity

Disciplines

Ecology and Evolutionary Biology | Physiology | Zoology

Language

English


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