ABSTRACT
Harpacticoid copepods, many isopods such as Microcharon, Microcerberus, stenasellids, and the amphipods Bogidiellidae and Ingolfiellidae exhibit a thread-like body with elongated posterior legs and uropods (see Figure). The water mites Wandesia and Stygolimnochares display a long posterior body. Long claws, legs, and uropods are an advantage for pushing the animal through the labyrinth of narrow channels. A high number of specialized sensory setae such as aesthetascs and plumose setae on antennae and other appendages allows the detection of chemical gradients, prey, or the opposite sex. Mucus glands along the body allow slipping while a thin and transparent integument, a strong muscular system, greater exoskeleton articulation providing body flexibility, and thigmotactism (keeping in contact with the substrate particles), represent other adaptations to interstitial life. Anatomical reductions include the absence of one ovary, a reduced number of testicular utricula and digestive caeca, and simple and linear cerebron and gut structures. Such reductions probably require developmental gene modifications. The relatively stable and oligotrophic interstitial environment has selected Kreproductive strategies and limited energy loss: few large eggs are produced, development is slow, spawning events are widely spaced in time, and reproduction is more or less continuous; longevity is increased, from 7-10 months (asellids) to about 1520 years (amphipod Niphargus and isopod stenasellids). Late hatching; protection of gametes, eggs, and embryos; precocious fertilization; viviparity; and dormancy during dry or cold periods (tardigrades, nematodes) are other features enhancing energy economy. In addition, some organisms are able to regulate their relatively low oxygen consumption.
