Growth
Koura like all crustaceans moult their exoskeletons in order 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. Post moulting the demand for calcium in order to harden the exoskeleton is high and this demand is met in part by the koura eating its discarded exoskeleton. The remaining calcium required to completely harden the exoskeleton is achieved by absorption from the water. Lower limits of 5 mg/L of calcium (Ca) in water for temperate species of koura have been suggested as sufficient to support exoskeleton hardening.
Water temperature and calcium concentrations are thought to be the key variables determining koura growth rates. P. zealandicus have high survivability (>80%) rates below 16 °C (61 °F) but temperatures above this correlate with lower rates of survivability. Higher death rates are thought to be associated with increased activity of koura at higher temperatures. Greater activity by koura increases cannibalistic behaviour and increased activity may also impact on water quality with the greater production of ammonia as a waste product. Survival of koura also increases with higher calcium concentrations in the water and this is thought to be in part due to a lower incidence of moulting related deaths and decreased risk from predation. A calcium concentration value of 20–30 mg/L Ca in water is thought to be ideal for maintaining koura growth and survivability in aquaculture setups.
Read more about this topic: Paranephrops
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