The ultrastructure of the posterior midgut caecum of Pachygrapsus crassipes (Decapoda, Brachyura) adapted to low salinity

Morphology and ultrastructure of digestive system in pre-zoea and zoea I larvae of red king crab, Paralithodes camtschaticus (Tilesius, 1815)

The digestive system structure in pre-zoea and zoea I larvae of the red king crab Paralithodes camtschaticus has been examined. During this development period, the digestive system consists of an esophagus, a stomach, a midgut (where the hepatopancreas ducts open), and a hindgut. The esophagus begins from the oral slit on the animal's ventral side and extends vertically up to the junction with the cardiac stomach. The latter is followed by the pyloric stomach. At the stages under study, crabs have a cardiac-pyloric valve and a pyloric filter in the stomach already developed. The midgut begins with an expansion in the cephalothorax, enters the pleon, grows narrower there, and extends to somite 3 of pleon. The hepatopancreas is represented by a symmetrical paired gland which occupies almost the entire cephalothorax space and opens with its ducts at the junction of the pyloric stomach with the midgut. The hepatopancreas is divided into the anterior and posterior lobes. At the pre-zoea stage, the anterior lobes are large and filled with yolk. At the zoea I stage, the anterior lobes are smaller relative to the entire hepatopancreas, and the posterior lobes increase and form tubular outgrowths. It has been shown that during the transition from pre-zoea to zoea I, the number of mitochondria in enterocytes increases and a peritrophic membrane forms in the midgut. These changes are probably associated with the transition to independent living and feeding.

Morphological and histological description of the midgut caeca in true crabs (Malacostraca: Decapoda: Brachyura): origin, development and potential role

Background

The decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs. Several studies focused on the study of the digestive system of the decapods, constituted by the oesophagus, stomach, midgut tract, midgut gland, and hindgut. Nevertheless, in the midgut tract there are associated a set of organs called “midgut caeca”, which are among the most controversial and less studied digestive organs of this group. This work used the common spider crab Maja brachydactyla Balss, 1922 as a model to resolve the origin, development, and potential role of the midgut caeca. Such organs were studied in the larvae (zoea I, zoea II, megalopa), first juveniles, and adult phases, being employed traditional and modern techniques: dissection, micro-computed tomography (Micro-CT), and light and electron microscopical analyses (TEM and SEM).

Results

The common spider crab has a pair of anterior midgut caeca and a single posterior caecum that originate from the endoderm germ layer: they develop from the midgut tract, and their epithelium is composed by secretory cells while lacking a cuticle lining. The midgut caeca are small buds in the newly hatched larvae, enlarge linearly during the larval development, and then continue growing until became elongated and coiled blind-tubules in adults. The adult midgut caeca are internally folded to increase their inner surface. The electron microscopy observations showed that the midgut caeca are highly active organs with important macroapocrine and microapocrine secretory activity. Our results suggest that the role of the caeca might be related to the digestive enzyme secretion. The secretory activity should increase as the animal grows in size.

Conclusion

The present study resolves the embryonic origin of the midgut caeca (endoderm derived organs), development (general lengthening starting from small buds), and role (active secretory organs). The secretory activity of the midgut caeca should be incorporated in the current models of the digestive physiology in different decapod taxa.

Synthesis of digestive enzymes, food processing, and nutrient absorption in decapod crustaceans: a comparison to the mammalian model of digestion

The ∼15.000 decapod crustaceans that are mostly omnivorous have evolved a structurally and functionally complex digestive system. They have highly effective cuticular chewing and filtering structures in the stomach, which are regularly renewed by moulting. Decapods produce a broad range of digestive enzymes including chitinases, cellulases, and collagenases with unique properties. These enzymes are synthesized in the F-cells of the hepatopancreas and are encoded in the genome as pre-pro-proteins. In contrast to mammals, they are stored in a mature form in the lumen of the stomach to await the next meal, and therefore, the enzymes are particularly stable. The fat emulsifiers are fatty acyl-dipeptides rather than bile salts. After mechanical and chemical processing of the food in the cardiac stomach, the chyme is filtered by two unique filter systems of different mesh-size. The filtrate is then transferred to the hepatopancreas where the nutrients are absorbed by the R-cells, mostly via carriers, resembling nutrient absorption in the small intestine of mammals. The absorbed nutrients are used to fuel the metabolism of the hepatopancreas, are supplied to other organs, and are stored in the R-cells as glycogen and lipid reserves. Export lipids are secreted from the R-cells into the haemolymph as high density lipoproteins that mainly consist of phospholipids. In contrast to mammals, the midgut tube and hindgut contribute only little to food processing and nutrient absorption. The oesophagus, stomach and hindgut are well innervated but the hepatopancreas lacks nerves. Hormone cells are abundant in the midgut and hepatopancreas epithelia. Microorganisms are often present in the intestine of decapods, but they are apparently not essential for digestion and nutrition.

Morphology of the midgut trunk in the penaeid shrimp,Sicyonia ingentis, highlighting novel nuclear pore particles and fixed hemocytes

The morphology of the midgut trunk (MGT) in the penaeid shrimp Sicyonia ingentis was examined by light and scanning and transmission electron microscopy. Although the function of the MGT is poorly understood, it is not involved with the digestion and absorption of nutrients, and it appears to be the surface of a shrimp least protected from penetration by potential pathogens. As described for other decapod crustaceans, the MGT in shrimp is composed of a simple columnar epithelium separated from a layer of connective tissue by a thick basal lamina. Beneath the basal lamina is a previously unreported layer of hemocytes, exclusively of the granulocyte variety, embedded in a matrix continuous with the basal lamina and extending into the connective tissue. This layer was observed in four other species of penaeid shrimp. Granulocytes in circulation can phagocytose and encapsulate foreign material and the granules contain antibacterial molecules, lysosomal enzymes, and prophenoloxidase. We suggest that the granulocytes associated with the basal lamina have matured at this site and are well positioned to fight potential pathogens that have penetrated the epithelial layer of the MGT. A second observation is the presence of clusters of cylinders bound to the nuclear pores of the epithelial cells. The possibility that these clusters are viruses, organelles, or abnormal organelles induced by disease or toxic materials is discussed. These unique particles were observed in S. ingentis but none of the other penaeid shrimp we examined.

On the morphology and fine structure of the alimentary canal ofCorophium volutator(Pallas) (Crustacea: Amphipoda)

In the mud-dwelling amphipod,Corophium volutatorthe foregut is lined with cuticle and consists of an oesophagus and a stomach, with the latter divided into cardiac, pyloric and funnel regions. The midgut comprises an intestine that is enlarged considerably by three pairs of diverticula: the small anterior dorsal and posterior caeca and the massive ventral caeca. Anteriorly, the intestine encompasses the funnel region and the ventral caeca open into the floor of the stomach at the posterior end of the pyloric region. The hindgut is essentially a simple tube connnecting the intestine with the anus. Particles of food pass along the oesophagus and enter the stomach through a valve. Rows of setae, or folds of cuticle, divide the stomach longitudinally into food, circulation and filtration channels. Ingested particles with a diameter greater than 2 pm are confined to the food channel and supplied with fluids and enzymes from the circulation channels. The digestive enzymes are produced primarily by the ventral caeca and are supplied to the circulation channels through a valve at the entrance of each ventral caecum. Any fine particles and soluble materials extracted from the food channel in the cardiac region are transported into the filtration channels through the first filter of a two part system. Digestible material continues to be extracted in the pyloric region where the volume of the lumen of the food channel is reduced by the intrusion of the vertex of the ventral pyloric ridge. The basis of this ridge supports the second filter which produces a filtrate with particles less than 0.06 pm in diameter. Material retained on the filter membrane is returned to the food channel by brush-like setae facing the membrane. The final filtrate is transported to the ventral caeca. A valve at the entrance to each ventral caecum prevents contamination of the filtrate by material in the food channel. All indigestible food is passed sequentially along the funnel, intestine and, finally, the hindgut from which it is voided as a faecal pellet. Most digestion and absorption occur in the ventral caeca where the epithelium is differentiated into the R /F and B cells. The R /F cells have a much thicker and denser microvillous border than the B cells. Each R /F cell also has numerous mitochondria located mainly ventral to the nucleus in the mid-region. Rough and smooth endoplasmic reticula are sited primarily in the apical and basal regions of the cell, respectively. Furthermore, most of the rough endoplasmic reticulum is confined to cells in the distal region of the caecum which probably forms the main site for the production of digestive enzymes. The proximal region of the caecum contains numerous lipid droplets and is probably involved in the absorption, transport and storage of the products of digestion. Each B cell has a single large, fluid-filled vacuole, distal to which are mitochondria and numerous smaller vacuoles of varying size forming an ‘apical complex’. The nucleus is located proximal to the vacuole together with free ribosomes and rough endoplasmic reticulum. Material from the lumen of the caecum is taken by pinocy tosis into the ‘apical complex’. The large vacuole develops at the expense of the ‘apical complex’ and the microvillous border. The vacuole is eventually liberated into the lumen of the caecum and the cell disintegrates. These discharges may supply enzymes to other regions of the gut, or they could be waste products derived from intracellular digestion. The anterior dorsal caeca and most of the intestine contain cells with a normal complement of organelles. These cells probably make a minor contribution to the processes of digestion and absorption. However, the cells of the posterior caeca and those at the posterior end of the intestine have an extensive development of smooth endoplasmic reticulum. In some cells the mitochondria have a dense matrix and there are only a few free ribosomes and cisternae of rough endoplasmic reticulum. The fine structure of the epithelium in the posterior caeca is typical of tissue that transports fluids and ions. The hindgut has a microvillous border which abuts its cuticular lining. In addition, some cells have numerous mitochondria which are often associated with infolds of the basal cell membrane. The fine structure of this tissue is similar to the ‘ion pumps’ described in the gut of insects which serve to maintain the normal ionic concentration of the blood. The posterior region of the hindgut has no structural specializations.