Feinstruktur peritrophischer membranen

When Fluorescence Is not a Particle: The Tissue Translocation of Microplastics in Daphnia magna Seems an Artifact

Previous research reported the translocation of nano- and microplastics from the gastrointestinal tract to tissues in Daphnia magna, most prominently of fluorescent polystyrene beads to lipid droplets. For particles >300 nm, such transfer is biologically implausible as the peritrophic membrane retains these in the daphnid gut. We used confocal laser scanning microscopy to study tissue transfer applying the setup from a previous study (neonates exposed to 20 and 1000 nm polystyrene beads at 2 µg L^-1 for 4 and 24 h), the same setup with a fructose-based clearing, and a setup with a 1000-fold higher concentration (2 mg L^-1 ). We used passive sampling to investigate whether the beads leach the fluorescent dye. Although the 1000 nm beads were visible in the gut at both exposure concentrations, the 20 nm beads were detectable at 2 mg L^-1 only. At this concentration, we observed fluorescence in lipid droplets in daphnids exposed to both particle types. However, this did not colocalize with the 1000 nm beads, which remained visible in the gut. We further confirmed the leaching of the fluorescent dye using a passive sampler, a method that can also be applied in future studies. In summary, we cannot replicate the original study but demonstrate that the fluorescence in the lipid droplets of D. magna results from leaching of the dye. Thus, the use of fluorescence as a surrogate for particles can lead to artifacts in uptake and translocation studies. This highlights the need to confirm the stability of the fluorescence label or to localize particles using alternative methods. Environ Toxicol Chem 2019;38:1495-1503. © 2019 SETAC OPEN PRACTICES: The present study has earned Open Data/Materials badges for making publicly available the digitally shareable data necessary to reproduce the reported results. Learn more about the Open Practices badges from the Center for Open Science: https://osf.io/tvyxz/wiki.

Nutrition in terrestrial isopods (Isopoda: Oniscidea): an evolutionary-ecological approach

The nutritional morphology, physiology and ecology of terrestrial isopods (Isopoda: Oniscidea) is significant in two respects. (1) Most oniscid isopods are truly terrestrial in terms of being totally independent of the aquatic environment. Thus, they have evolved adaptations to terrestrial food sources. (2) In many terrestrial ecosystems, isopods play an important role in decomposition processes through mechanical and chemical breakdown of plant litter and by enhancing microbial activity. While the latter aspect of nutrition is discussed only briefly in this review, I focus on the evolutionary ecology of feeding in terrestrial isopods. Due to their possessing chewing mouthparts, leaf litter is comminuted prior to being ingested, facilitating both enzymatic degradation during gut passage and microbial colonization of egested faeces. Digestion of food through endogenous enzymes produced in the caeca of the midgut glands (hepatopancreas) and through microbial enzymes, either ingested along with microbially colonized food or secreted by microbial endosymbionts, mainly takes place in the anterior part of the hindgut. Digestive processes include the activity of carbohydrases, proteases, dehydrogenases, esterases, lipases, arylamidases and oxidases, as well as the nutritional utilization of microbial cells. Absorption of nutrients is brought about by the hepatopancreas and/or the hindgut epithelium, the latter being also involved in osmoregulation and water balance. Minerals and metal cations are effectively extracted from the food, while overall assimilation efficiencies may be low. Heavy metals are stored in special organelles of the hepatopancreatic tissue. Nitrogenous waste products are excreted via ammonia in its gaseous form, with only little egested along with the faeces. Nonetheless, faeces are characterized by high nitrogen content and provide a favourable substrate for microbial colonization and growth. The presence of a dense microbial population on faecal material is one reason for the coprophagous behaviour of terrestrial isopods. For the same reason, terrestrial isopods prefer feeding on decaying rather than fresh leaf litter, the former also being more palatable and easier to digest. Acceptable food sources are detected through distance and contact chemoreceptors. The 'quality' of the food source determines individual growth, fecundity and mortality, and thus maintenance at the population level. Due to their physiological adaptations to feeding on and digesting leaf litter, terrestrial isopods contribute strongly to nutrient recycling during decomposition processes. Yet, many of these adaptations are still not well understood.

Comparative structure and ultrastructure of the midgut and hepatopancreas of five species of Mysidacea (Crustacea): functional implications

Comparative structural, ultrastructural, and cytochemical studies were made of the midgut and hepato pancreas of five species belonging to the two suborders of the order Mysidacea, the Lophogastrida and Mysida, under different feeding conditions to investigate their role in digestive processes. Nutritional experiments were also carried out to complete the study. The midgut shows structural differences between the two suborders. The epithelium is composed of a single cell type that is implicated in absorption and production of peritrophic membranes. The intestinal epithelium lies on a characteristic three-layered basal lamina, the thickness of which would increase the strength of the intestinal wall for peristaltis and for transport of fecal pellets. The hepatopancreas displays marked structural differences between the suborders, and the cell distribution is better ordered in Mysida. Absorptive R-cells play an important role in lipoprotein metabolism. B-cells are implicated in intracellular digestion, as is attested to by the presence of a typical vacuolar apparatus. The total absence of the F-cell, considered to secrete enzymes in the Decapoda, raises the question of the origin of the enzymes necessary for extracellular digestion. A review of the cell affiliations of the hepatopancreas in the Mysidacea is required.

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.

The fine structure of the digestive system of Daphnia pulex (Crustacea: Cladocera)

The alimentary canal of Daphnia pulex consists of a tube-shaped foregut, a midgut (mesenteron) with an anterior pair of small diverticula, and a short hindgut. The foregut and hindgut are structurally similar. Each is formed by a low cuboidal epithelium 5 mum tall and lined with a chitinous intima. The midgut wall consists of a simple epithelium resting on a thick beaded basal lamina which is surrounded by a spiraling muscularis. Anteriorly the midgut cells are columnar in shape being 30 mum in height each having a basal nucleus, anteriorly concentrated mitochondria and in apical border of long thin microvilli. Posteriorly the midgut cells become progressively shorter so that in the posteriormost region of the midgut the cells are 5 mum tall and cuboidal in shape. The microvilli concomitantly become shorter and thicker. All mesenteron cells contain the usual cytoplasmic organelles. The paired digestive diverticula are simple evaginations of the midgut. The wall of each consists of a simple epithelium of cuboidal cells 25 mum in height, each with a brushed border of long thin microvilli. Enzyme secretion appears to be holocrine in mode and not confined to any one region of the mesenteron though definitely polarized anteriorly. The thin gut muscularis encircles the entire length of the midgut and caeca. Thick and thin filaments appear to be in a 6:1 ratio.