Feeding and digestive processes

The functional morphology of the gastric system of Carcinus maenas follows a similar pattern to other decapod crustaceans. Ingested food is initially processed in the foregut; here, the ossicles of the gastric mill crush and grind food and mix it with digestive enzymes secreted from the hepatopancreas. The resultant digesta is transferred to the midgut region by rhythmic contractions of the pyloric region of the foregut. From here, the liquid phase of the digesta enters the hepatopancreas tubules, where intracellular digestion begins. Larger material may remain in the midgut, and some absorption may occur directly across the midgut walls. The cells of the midgut wall secrete a peritrophic membrane that encloses waste material. From here, the wastes enter the hindgut region which regulates the expulsion of faeces by peristaltic muscular waves. These digestive processes in C. maenas can be altered by both environmental and biotic factors. An increase in temperature increases contraction rates of the foregut, leading to more rapid gastric evacuation rates. In contrast, a decrease in salinity tends to lead to an increase in gastric processing times. The processing of a meal results in a twofold to fourfold increase in metabolic rate in C. maenas. This is termed the specific dynamic action and represents the energy required for the mechanical and chemical breakdown of the meal, as well as the subsequent transport and intracellular protein synthesis. The characteristics of this post-prandial metabolic response are influenced to various degrees by temperature, salinity, and oxygen and food type in C. maenas.