The structure and function of the digestive system of terrestrial isopods

Abstract

With 7 plates and 4 figures in the text)The functional morphology of the digestive system of terrestrial isopods (woodlice) has been investigated. Observations have been made on live juveniles of Oniscus usellus and Porcellio scuber in which some digestive processes can be observed directly through the cuticle. The fine structure of the foregut of juveniles and adults of these two species has been studied by light microscopy and scanning and transmission electron microscopy. The passage of coloured dyes and colloidal gold through the digestive system has also been followed.The digestive system of Oniscus asellus and Porcellio scuber is divided into five regions. The foregut (l), and the anterior chamber (2), papillate region (3) and rectum (4) of the hindgut, are ectodermal in origin and lined with cuticle. The hepatopancreas (5), which opens into the foregut, consists of four blind-ending tubules which are endodermal in origin and not lined with cuticle. A powerful muscular sphincter is present between the papillate region and the rectum. There is no peritrophic membrane in the lumen of either the hepatopancreas or the hindgut.Fragments of food pass via the oesophagus to the proventriculus of the foregut where they are briefly masticated before passing rapidly into the anterior chamber of the hindgut. During this process, the food is mixed with secretions derived from the hepatopancreas. The anterior chamber possesses a pair of dorsal channels formed from a typhlosole and lateral infoldings of the gut wall. When the hindgut is full of food, contraction of muscles surrounding the gut forces liquids and fine food particles back into the foregut via the typhlosole channels which open at the junction of the anterior chamber and papillate region. This material, which contains fluids derived from the hepatopancreas, and products of digestion, is filtered in the foregut and passed into the lumen of the hepatopancreas where further digestion, and absorption of nutrients, takes place. The hepatopancreas may secrete these fluids back on to the food in the anterior chamber and receive products of digestion uiu the typhlosole channels several times during a digestive cycle. The lumen of the foregut, hindgut and hepatopancreas often contains a dense population of microorganisms which may be involved in digestion. Residual material in the hindgut passes eventually into the rectum where faecal pellets are compacted before they are voided.The ability of terrestrial isopods to recycle and retain fluids within the digestive system, by the development of the typhlosole channels (which are not present in marine isopods), may have been a major contributory factor in their successful transition from water to land.

“CandidatusBacilloplasma,” a Novel Lineage ofMollicutesAssociated with the Hindgut Wall of the Terrestrial IsopodPorcellio scaber(Crustacea: Isopoda)

“…Due to the role of isopods as decomposers of plant material, it is tempting to speculate that these adapted bacteria could play some role in the digestion process of the host. However, their occasional absence in some of the inspected animals, as well as their association with the papillate region of the hindgut, where most of the absorption of water and ions takes place (17), apparently diminishes their role in the digestive process. It is therefore most likely that these bacteria are commensals, which are well adapted to the isopod gut environment.…”

“…Like other terrestrial isopods, P. scaber is a herbivorous scavenger, feeding predominantly on decayed plant material and thus contributing to nutrient and energy cycling in terrestrial ecosystems (43). Due to its ecological importance, easy handling, breeding capability under laboratory conditions, and tolerance to polluted environments as well as the considerable body of knowledge about its biology, this crustacean is commonly used as a test organism in terrestrial ecotoxicological and ecophysiological studies (12,17).The tripartite digestive system of P. scaber consists of a short foregut comprising an esophagus and stomach, a midgut consisting of two pairs of blind-ended tubular digestive glands, and a long, tube-like hindgut. The last of these comprises two functionally different parts, an anterior chamber and a papillate region with a rectum (17).…”

“…Like other terrestrial isopods, P. scaber is a herbivorous scavenger, feeding predominantly on decayed plant material and thus contributing to nutrient and energy cycling in terrestrial ecosystems (43). Due to its ecological importance, easy handling, breeding capability under laboratory conditions, and tolerance to polluted environments as well as the considerable body of knowledge about its biology, this crustacean is commonly used as a test organism in terrestrial ecotoxicological and ecophysiological studies (12,17).…”

“…The last of these comprises two functionally different parts, an anterior chamber and a papillate region with a rectum (17). The foregut of terrestrial isopods is generally poorly inhabited by microorganisms.…”

Toxicokinetics of Ag in the terrestrial isopod Porcellionides pruinosus exposed to Ag NPs and AgNO3 via soil and food

“…During food digestion, fluids and fine particles are separated from coarse particles in the primary and secondary filters in the proventriculus, going to the hepatopancreas afterwards. The coarse particles are voided and eliminated as fecal pellets after the digestive process (Hames and Hopkin, 1989). It is known that metals taken up with food may enter the body and reach the hepatopancreas without reaching other parts of the body (Vijver et al, 2006).…”