excavate burrows or fan shaped depressions, in Lake Rotoiti at depths of . In streams, koura take cover on the bottom beneath fallen leaf litter, fallen logs, and tree roots and undercut banks. Tree fern roots that project into the stream are thought to provide excellent cover for juveniles. Fossil evidence of Paranephrops in Pleistocene sediment demonstrate occurrence in or near marginal-marine habitat, unfortunately there is little published literature about occurrences of fossilized Paranephrops. Predators, cannibalism and disease Eels, perch, catfish, and trout are the major aquatic predators of koura. Terrestrial predators include rats, kingfishers, shags, scaup, stoats, and kiwi. Shag populations in the Rotorua lakes district in the North Island of New Zealand have been shown to feed on koura as the bulk of their diet. Predation on koura by trout is thought to be restricted to larger adult trout. Streams and lakes with established populations of trout have been shown to affect koura abundance. Cannibalism in koura is most likely to occur when koura are sick or moulting. Cannibalism can be a greater problem in high-density situations where competition for shelter and territory is greatest. Juvenile koura can be consumed whole by larger koura, and this presents problems for aquaculture in ensuring continuity of intergenerational growth. Koura use their chelae for both attack and defence, and when one limb is lost, the koura will divert energy for overall growth to restoring the lost limb. The only disease known to seriously affect koura is "white tail disease" caused by the microsporidian parasite Thelohania contejeani. This parasite causes degeneration of striated muscle in the tail area, which turns the tail a pale white colour and correspondingly leads to death soon after. Growth Koura, like all crustaceans, moult their exoskeletons to increase in size. During moulting, the carapace becomes soft with calcium being resorbed and the remaining outer shell shed. The new carapace forms underneath, where it takes a number of days to harden. Calcium for this new outer shell comes from gastroliths that line the stomach wall of the koura, and these produce around 10–20% of the calcium needs for exoskeleton production. The gastroliths drop into the koura's foregut, where they are broken down to allow the adsorption of calcium. After moulting, the demand for calcium to harden the exoskeleton is high, and this demand is met in part by the koura eating its discarded exoskeleton. The remaining calcium required