Physiological responses of the crabs Carcinus maenas and Cancer irroratus to the combined challenges of digestion and low salinity

Penney, Chantelle M. (2015) Physiological responses of the crabs Carcinus maenas and Cancer irroratus to the combined challenges of digestion and low salinity. Masters thesis, Memorial University of Newfoundland.

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Abstract

Osmoregulation and digestion are energetically demanding processes that typically result in an increase in metabolic rate. Crabs that move into low salinity environments to feed must be able to balance the demands of both processes. However, achieving this balance may pose greater challenges for weak osmoregulators and conformers than for stronger regulators. In this study, the rate of oxygen consumption (MO₂) of green crabs (Carcinus maenas, an efficient osmoregulator) and rock crabs (Cancer irroratus, a weak osmoregulator) was examined as a function of feeding and salinity. MO₂ increased over 2-fold in both species after feeding in 32‰ and returned to pre-fed values in 17-20 hours. When fasted crabs were given an acute hyposaline exposure (24 and 16‰ for C. irroratus; 16 and 8‰ for C. maenas), MO₂ increased and remained elevated with lower salinities resulting in higher peak values for MO₂. When challenged with low salinity after feeding, C. maenas responded with an immediate addition of the MO₂ due to feeding upon that due to salinity stress, whereas C. irroratus reacted with a delayed increase in MO₂ following the salinity drop. Several aspects of their physiology and behaviour were investigated in an attempt to explain these differences. Although protein synthesis is reported to account for the majority of the postprandial increase in MO₂, this did not appear to be the case in this study. There was no effect of feeding or low salinity on protein synthesis rates in the leg, gills, or hepatopancreas of either species, and this suggests that protein synthesis can continue in low salinity as long as substrates are available. Further, the behaviour (i.e. activity) of the crabs and aspects of their mechanical digestion could not explain the differences in MO₂. C. irroratus became more active than C. maenas in low salinity, and C. irroratus exhibited a trend towards a decrease in gut contraction rate which resulted in longer transit times of the meal through the gut in this species. This reduction in gastric processing may have stemmed from an inability of C. irroratus to regulate osmotic water onload as efficiently as C. maenas, and this likely delayed the additive effect of the metabolic demands of digestion and salinity stress.

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