Glucose is transported in crustacean hepatopancreas and intestine by Na+-dependent co-transport, while Na+-dependent D-fructose influx has only been described for the hepatopancreas. It is still unclear if the two sugars are independently transported by two distinct cotransporter carrier systems. In this study lobster (Homarus americanus) hepatopancreas brush border membrane vesicles (BBMV) were used to characterize, in detail, the cation-dependency of both D-[3H] glucose and D-[3H] fructose influxes, while in vitro perfused intestines were employed to determine the nature of cation-dependent sugar transport in this organ. Over the sodium concentration range of 0-100 mM, both 3H-D-glucose and 3H-D-fructose influxes (0.1 mM; 1 min uptakes) by hepatopancreatic BBMV were hyperbolic functions of [Na+], exhibiting Km values of 2.30 ± 0.59 and 2.58 ± 0.95 mM, respectively. D-[3H] glucose and fructose influxes by hepatopancreatic BBMV over a potassium concentration range of 15-100 mM were hyperbolic functions of [K+], exhibiting Km values of 9.85 ± 0.41 and 12.6 ± 0.80 mM respectively. Both sugars displayed significant (p < 0.01) Na+/K+-dependent and Na+-independent uptake processes. Transepithelial 25 μM D-[3H] glucose and D-[3H] fructose fluxes across lobster intestine over a luminal sodium and potassium concentration range of 0 – 50 mM and 5-100 mM, respectively, were hyperbolic functions of luminal [Na+] and [K+]. As with hepatopancreatic sugar transport, transepithelial intestinal sugar transport exhibited both significant (p < 0.01) Na+/K+-dependent and Na+-independent processes. Results suggest that both D-glucose and D-fructose are transported by a single carrier process in each organ with sodium being the preferred cation for both sugars in the hepatopancreas, and potassium being the preferred cation for both sugars in the intestine.

In this thesis I assessed the causes of long-term changes in two large, commercially important decapod crustacean populations, American lobster (Homarus americanus) and snow crab (Chionoecetes opilio), in the northwest (NW) Atlantic Ocean. By combining available time-series data, including commercial landings, research surveys, and local ecological knowledge (LEK), I explored the causes of an observed ecosystem shift in the NW Atlantic (~1950–2009) which entailed a region-wide decline of groundfish and an increase in benthic invertebrates, including these decapods. Three hypotheses were examined to explain the increase in decapod abundance: (1) the predation hypothesis, whereby a decrease in predatory groundfish led to an increase in their decapod prey (top-down effects); (2) the climate hypothesis, whereby changes in temperature or other climatic variables helped to increase decapod numbers (bottom-up effects); and (3) the anthropogenic hypothesis, whereby changes in fishing pressure drove decapod population dynamics. I explored these hypotheses separately for lobster and snow crab, which may experience different ecological and commercial pressures. First, I investigated the interactions between predatory groundfish and lobster in the inshore region of southwest Nova Scotia. Long-term fisheries-independent abundance indices for lobsters and their predators are available for Gulf of Maine (GOM) waters in the USA, but not in Canada. To address research gaps I designed and executed a survey to collect the LEK of lobster fishermen fishing in the Canadian GOM. Forty-two fishermen were interviewed. Corresponding survey results from the USA were compared to the LEK results. Both sources provided evidence for a top-down effect (predation release), contributing to observed increases in GOM lobster abundance and landings. Second, I explored relationships between lobster abundance and landings in the NW Atlantic as they may relate to temporal changes in predators, temperature, climate (North Atlantic Oscillation Index, NAOI), and fishing. Available landings data and fisheries-independent abundance estimates were collated to investigate trends in lobster abundance and catch. Links between lobster, groundfish, temperature and climate indices were explored using mixed effects models. Results offered partial support for the predation hypothesis, namely in the waters off Newfoundland, Nova Scotia, and southern New England as well as broad support for a climate effect on early life stages. This effect appeared related to a region-wide climate signal, the NAOI, but was independent of changes in water temperature. Fishing effort appeared to be following lobster abundance, rather than regulating abundance in a consistent way. Third, variation in snow crab abundance was examined through meta-analysis of time-series data of cod and crab abundance and temperature. Temperature had opposing effects on the two species: snow crab abundance was negatively correlated with temperature whereas cod and temperature were positively related. Controlling for the effect of temperature, the analysis revealed significant negative interactions between snow crab and cod abundance, with cod leading snow crab up to a five-year lag. Results indicate that snow crab is largely influenced by temperature during early post-settlement years and becomes increasingly regulated by top-down mechanisms as they approach fishery recruitment. Overall, I conclude that both climate and predation can act as population controls on large decapod populations, but these variables affect decapods at different life stages.

Previous nutritional physiology research using L-histidine and zinc in American lobster intestine (Homarus americanus) has suggested that these solutes can be co-transported as complexes (Histidine-Zinc-Histidine) across the intestine using a peptide transporter. Furthermore, transport of L-leucine was shown to be inhibited by high calcium concentrations. Dipeptide and bis-complex transport and the role of calcium were investigated in the perfused intestines of lobster and Atlantic white shrimp (Litopenaeus setiferus). Following trans-intestinal transport, serosal medium was analyzed for amino acid composition by gas chromatography. In lobster, the transport of glycylsarcosine (Gly-Sar) from mucosa to serosa was stimulated two-fold with luminal pH 8.5, compared to the pH 5.5 control. Mucosa to serosa and serosa to mucosa fluxes of Gly-Sar were measured; the dipeptide was transported intact in both directions, but the net flux was from mucosa to serosa. The use of 0.5mM calcium chloride stimulated Gly-Sar transport two-fold, compared to 25 mM. In shrimp, the addition of 50 µM zinc chloride increased the rate of L-histidine transport, while Gly-Sar inhibited histidine transport in the presence of zinc. The rate of histidine transport was significantly higher with 1mM calcium chloride than with 25mM. These results suggest that shrimp transport bis-complexes in a manner similar to lobster. High calcium concentration had an inhibitory effect on both amino acid and dipeptide transport. Proposed mechanisms accounting for the effects of metals and calcium on trans-intestinal transports of both amino acids and dipeptides by lobster and shrimp digestive tracts are discussed.