New Zealand's only freshwater shrimp... New Zealand has only a single species of decapod freshwater shrimp; Family Atyidae: Paratya curvirostris, in the entire country. It is the southernmost member of its family the Atyidae whose distribution includes the western edge of the Pacific Ocean from Siberia, through Asia, Indonesia and Australasia. P. curvirostris is found only in New Zealand (i.e. it is endemic) and has been found all the way from Northland to Stewart Island, and on the Chatham Islands. As its distribution moves south its upper altitudinal limit decreases suggesting temperature is an important factor limiting this distribution.

Adult (therefore probably female) freshwater shrimp. Note how the transparent body makes for an excellent camouflage.

This species is a characteristic occupant of lowland streams, where it is usually found amongst vegetation. Population densities are highest when the stream margin has both a well developed floating fringe in addition to well developed weed beds. Adults may be found all the way downstream to the transition between fresh and brackish water, often migrating with the flow and ebb of the tide to maintain this balance. They can reasonably survive up to around 50% seawater strength. P. curvirostris is the only member of its family to exhibit protandry; a remarkable life history in which sex is defined by the age and maturity of the individual rather than at development. All individuals undergo a male phase of development before transforming to adult female forms. Therefore small females are rare, and almost all large individuals are likely to be female rather than the more immature males. It is thought that protandry with larger females may be advantageous for production of more energetically expensive eggs, as opposed to sperm. The eggs of the female are carried under the tail until hatching where the young are then released to float downstream in the current to the sea to develop. Details on the habitat of shrimp larvae are limited, although they are known to occupy estuaries: evidence for yet another species which utilises these frequently undervalued habitat as nurseries. These juveniles migrate upstream peaking around late summer/ autumn in a similar way to the well-known annual whitebait migrations. Movement is by swimming by flapping movements of the fin-like structures under the tail (abdomen) (pleopods) when the water velocity is low, and by walking gripping the substrate with the pincers of their walking legs when the water velocity is higher. Native freshwater shrimp are detritivores feeding on detritus, algal films on rocks, and wood etc., and on bacteria in decreasing order of importance. These are scraped off and picked up by the 2 pairs of pincers held out front which possess large curved hairs. Larger particles are further fragmented by the mouthparts (maxillipeds). These shrimp have been found to be our most sensitive freshwater species to dissolved ammonia, and third most sensitive crustacean and fish species to high temperature after common smelt (Retropinna retropinna) and koura (Paranephrops planifrons). They also have a relatively limited range of tolerance to pH variation to either acidic or basic conditions. This may be largely due to associated dissolved metals in much the same way that acid affects fish rather than excessive pH levels per se. Preferred temperatures are around 20 oC, with circum-neutral pHs also preferred (around pH 7.0). When acclimated to warmer temperatures native freshwater shrimps, as do most species, exhibit higher preferred and lethal temperatures than previously. These are due to slow, finite, but reversible adaptive changes in body chemistry and physiology. Lethal oxygen concentrations are very low (only around 10% saturations at 20oC) being higher in warmer temperatures where there is both an increased oxygen demand by the cold-blooded animal and also a decreased total amount of dissolved oxygen available than at cooler water temperatures. Larger shrimps will generally reach a lethal oxygen concentration faster than smaller shrimps, as will moulting individuals over those that are not moulting. The larger female shrimps are therefore the most susceptible of a given population to oxygen stress, their biased mortalities comprising the reproductive potential of the population. Although visible signs of distress may be absent, stress levels may be quite severe in these situations, affecting fundamental factors such as metabolism. Back to CRUSTACEA FURTHER READING Carpenter, A. 1983: Population biology of the freshwater shrimp Paratya curvirostris (Heller, 1862) (Decapoda: Atyidae). New Zealand Journal of Marine and Freshwater Research 17: 147-158. Chapman M. A.; Lewis, M. H. 1976: An introduction to the freshwater crustacea of New Zealand. Collins. Auckland, New Zealand. 261p. Winterbourn, M. J.; Mason, K. 1983: Freshwater life: streams, ponds, swamps, lakes and rivers. Mobil New Zealand Nature Series. Reed. Wellington, New Zealand. 76p.