This article reviews the mechanical processes associated with digestion in decapod crustaceans. The decapod crustacean gut is essentially an internal tube that is divided into three functional areas, the foregut, midgut, and hindgut. The foregut houses the gastric mill apparatus which functions in mastication (cutting and grinding) of the ingested food. The processed food passes into the pyloric region of the foregut which controls movement of digesta into the midgut region and hepatopancreas where intracellular digestion takes place. The movements of the foregut muscles and gastric mill are controlled via nerves from the stomatogastric ganglion. Contraction rates of the gastric mill and foregut muscles can be influenced by environmental factors such as salinity, temperature, and oxygen levels. Gut contraction rates depend on the magnitude of the environmental perturbation and the physiological ability of each species. The subsequent transit of the digesta from the foregut into the midgut and through the hindgut has been followed in a wide variety of crustaceans. Transit rates are commonly used as a measure of food processing rates and are keys in understanding strategies of adaptation to trophic conditions. Transit times vary from as little as 30 min in small copepods to over 150 h in larger lobsters. Transit times can be influenced by the size and the type of the meal, the size and activity level of an animal and changes in environmental temperature, salinity and oxygen tension. Ultimately, changes in transit times influence digestive efficiency (the amount of nutrients absorbed across the gut wall). Digestive efficiencies tend to be high for carnivorous crustaceans, but somewhat lower for those that consume plant material. A slowing of the transit rate allows more time for nutrient absorption but this may be confounded by changes in the environment, which may reduce the energy available for active transport processes. Given the large number of articles already published on the stomatogastric ganglion and its control mechanisms, this area will continue to be of interest to scientists. There is also a push towards studying animals in a more natural environment or even in the field and investigation of the energetic costs of the components of digestion under varying biotic and environmental conditions will undoubtedly be an area that expands in the future.
Historically, the decapod crustacean circulatory system has been classed as open. However, recent work on the blue crab, Callinectes sapidus, suggests the circulatory system may be more complex than previously described. Corrosion casting techniques were refined and used to map the circulatory system of a variety of crab species (order: Decapoda; family: Cancridae) to determine if the complexity observed in the blue crab was present in other species. Seven arteries arose from the single chambered heart. The anterior aorta, the paired anterolateral arteries, and the paired hepatic arteries exited from the anterior aspect of the heart. The small-diameter posterior aorta exited posteriorly from the heart. Exiting from the ventral surface of the heart, the sternal artery branched to supply the legs and mouthparts of the crab. These arteries were more complex than previously described, with arterioles perfusing all areas of the body. The arterioles split into fine capillary-like vessels. Most of these capillaries were blind ending. However, in several areas (antennal gland, supraesophageal ganglion) complete capillary beds were present. After passing through the capillary-like vessels, blood drained into a series of sinuses. However, rather than being arbitrary spaces as previously described, scanning electron micrographs showed the sinuses to be distinct units. Most of the sinuses formed a series of flattened membrane-bound lacunae. This complexity may qualify the decapod crustacean circulatory system as one that is "partially closed" rather than open.
The circulatory system of adult blue crabs, Callinectes sapidus, was mapped by either injecting barium sulfate into intact animals followed by radiography or by resin corrosion casts (Batsons Monomer). Seven arteries arise from the heart. The anterior aorta exits from the anterior dorsal surface of the heart and gives rise to the optic arteries; these arteries supply hemolymph to the supraesophageal ganglion and eyestalks. The paired anterolateral arteries also exit from the anterior dorsal surface of the heart and supply hemolymph to the gonads, hepatopancreas, stomach, antennal gland, mandibular muscles, and the hypodermis of the anterior cephalothorax. The paired hepatic arteries exit the heart anteriorly and ventrally and branch profusely within the hepatopancreas. A smaller side branch, the pyloric hepatic artery, supplies hemolymph to the pyloric stomach and midgut. The smallest artery, the posterior aorta, branches off the posterior ventral surface of the heart; it joins with the inferior abdominal artery in the region of the second abdominal segment and these arteries supply hemolymph to the hindgut and abdomen. The largest artery is the sternal artery, which exits from the ventral surface of the heart; the ventral thoracic artery branches off the sternal artery and supplies hemolymph to the chelae, the mouthparts, and to each pereiopod. The present study shows that the circulatory system is highly developed, with arteries dividing into smaller capillary-like vessels that ramify profusely within individual organs. The return vessels, the sinuses, are discrete channels rather than random open spaces, as previously described. The present study refines and advances descriptions of the circulatory system and is discussed in relation to recent work on hemolymph flow in crustaceans.
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