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

Abstract

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.

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

“…Not much is known about the reason why isopods ingest soil particles, but it seems to be important for the digestive process in these organisms (Zimmer, 2002).…”

The short- and long-term effects of changes of vegetation structure on isopod (Oniscidea) diversity and composition in Mátra Mountains

“…Thus, the correct selection of target groups for interventions is worth considering. It is true that isopods are not important arthropods from a nature conservation point of view, but their contribution to ecosystem processes may be significant [55]. Therefore, it is necessary to study the ecology of isopods.…”

A new species and first record of Pulmoniscus Leistikow, 2001 (Isopoda, Oniscidea, Philosciidae) from Colombia

“…Some specimens were observed feeding on faeces of A. seniculus. Terrestrial isopods contribute significantly to the organic matter decomposition cycle, and are influenced by physico-chemical aspects of the leaf litter and abiotic characteristics (Zimmer and Topp, 1997;2000;Zimmer, 2002;2004;Zimmer et al, 2002;Quadros and Araujo, 2008). Pulmoniscus turbanaensis n. sp.…”