The tanaids are a group of around 1,500 known species of mostly small (0.5 to 2 cm long) Peracarida
that occur worldwide. They are mainly marine and benthic (bottom-dwelling), but a handful of species live in brackish or nearly fresh water. They often live in burrows or tubes and are known from ocean depths ranging from coastal (1 to 2 m) to around 9000 m. Many are suspension feeders, but some are detritivores or predators. Although most species in this group are free-living or tube-dwelling, Exspina typica
have been found in the intestine and body cavity of deep sea holothuroid echinoderms ("sea cucumbers"). The tanaids were found to actively "dig" into the host's skin, grasping tissue with their claws and producing tunnels in the body wall.
(Brusca and Brusca 2003; Alvaro et al. 2011; Kakui et al. 2011 and references therein)
Kakui et al. (2011) analyzed phylogenetic relationships within Tanaidacea using data from the 18S rRNA gene.
An excellent online source on the Tanaidacea is Gary Anderson's Peracarida Taxa and Literature