|Gut structure and digestive cellular processes in marine Crustacea|
Brunet, M.; Arnaud, J.; Mazza, J. (1994). Gut structure and digestive cellular processes in marine Crustacea. Oceanogr. Mar. Biol. Ann. Rev. 32: 335-367
In: Oceanography and Marine Biology: An Annual Review. Aberdeen University Press/Allen & Unwin: London. ISSN 0078-3218, more
Anatomical structures; Digestive system; Crustacea [WoRMS]; Marine
|Authors|| || Top |
- Brunet, M.
- Arnaud, J.
- Mazza, J.
In all crustaceans the digestive tract is divided into three distinct regions the relative importance of which varies greatly in different groups. The foregut and hindgut have a chitinous lining and do not play an important role in digestive processes which essentially arise in the midgut and in the associated caeca, i.e. hepatopancreas, and diverticula. In decapods, the digestive cellular processes develop in hepatopancreatic tubules which contain several cell types. R-cells have long microvilli and store large amounts of lipids and glycogen. F-cells display an abundant rough endoplasmic reticulum as well as a supranuclear vacuole. B-cells develop a very large vacuolar apparatus before being extruded into the lumen. E-cells are undifferentiated cells arising from mitosis that develop into R-, F-, or B-cells. There are two opposing concepts concerning the functions of these cells. Some investigators consider that F-cells can synthesize digestive enzymes and store them in a supranuclear vacuole before transformation into B-cells. The B-cells enlarge considerably following the development of the vacuolar apparatus, arising from intensive endocytosis, and then discharge their contents, especially digestive enzymes, into the lumen. Other investigators feel that F-cells synthesize and secrete digestive enzymes contained in zymogen-like granules. Then they undergo transformation into B-cells which probably associate an intracellular digestive role, to substances entering by endocytosis, with an excretion role, of waste products from digestive metabolism. Concerning R-cells, most of the expressed opinions attribute as the major function the absorption of small molecules from the lumen through the apical plasma membrane and their metabolization into lipids and glycogen. In addition, according to some opinions, they possibly carry out contact digestion and detoxification by storing metals in an insoluble form. Amongst the other Malacostraca studied, mysids and amphipods, the three main cell types (R, F and B) are also present and the digestive cellular processes develop nearly as in decapods. In isopods, however, the caecal epithelium is entirely devoid of B-cells and shows two main cell types, large and small, with several common features, which are generally both involved in secretion and absorption. In the lower Crustacea, studies on digestive cytophysiology are relatively rare except in calanoid copepods. In this group the digestive tract consists of a single tube devoid of any caecum, but the midgut epithelium contains several cell types which are comparable with those described in decapods and display similar functions. Yet in copepods an affiliation is suggested between R- and F-cells while, in contrast to most decapods, B-cells have no direct relation with F-cells and probably arise from undifferentiated E-cells. In other Entomostraca studied, the structure of the digestive gut is more complex with different caeca and glands associated with the midgut, but the cellular organization is simpler as in branchiopods (a single cell type) or in thoracic cirripedes (two fairly alike cell types). In some Entomostraca, and particularly in copepods, glandular formations located in the labrum release their secretions into the stomodeum. These secretions may contain both mucosubstances causing agglutination of food particles and glycoproteins corresponding to enzymes involved in the preliminary digestive phase. In all Crustacea, the faeces arise in the midgut from indigestible substances in the lumen and cellular components resulting from disintegration of epithelial cells. The waste products are surrounded by one or more peritrophic membranes arising from successive delaminations of cellular material located at the apex of the cells. This material forms a matrix in which is included a network of chitinous microfibrilIs arranged in different textures.