Some decades ago I was travelling around Australia and was lucky enough to climb Uluru. Pools on top of the rock had been produced by recent rain and, curiously, in many of them strange aquatic invertebrates were present (see photo). This specimen is sitting in my camera lens cap, which has a diameter of 62 millimetres. It looks like an ancient trilobite. Why and how was it on top of the famous, massive rock, and what is it? What happens to the creatures when the short-lived puddles dry up?
鈥� The animal pictured is a , Triops australiensis. They are crustaceans in the class Branchiopoda 鈥� meaning 鈥済ill-legged鈥� 鈥� and this term reflects the fact that they use their legs for breathing as well as for motility.
Their external morphology appears to have remained unchanged for 220 million years or more, and one species, Triops cancriformis, has been claimed by some to be the oldest extant animal species. They occur in bodies of fresh or slightly salty water that periodically dry out, such as ephemeral lakes, farm dams, ditches and even puddles left after rain.
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The eggs of these animals have a very strong shell and are resistant to drying out. In some species, a period of desiccation is actually necessary for the creature鈥檚 development. The eggs can tolerate freezing and temperatures up to 80 掳C, and may remain viable for 25 years. In some species, hatching may take up to a year following exposure to suitable conditions, but in T. australiensis it usually takes several weeks at most. Once hatched, development from egg to adult may take only a further few weeks in summer temperatures. The animals have a lifespan of up to three months, and adults reach about 35 millimetres in length.
The shrimps feed on microscopic organisms, aquatic worms, other shrimp species, frogs鈥� eggs and larvae, decomposing vegetation and other detritus, and sometimes even moulting individuals of their own species. The small size and the robustness of the eggs allow them to be carried on the wind for hundreds of kilometres from their pools of origin, and it is probably this mode of transport that would have delivered the eggs to the top of Uluru.
It is also possible that the eggs might have been carried up in mud caked on a visitor鈥檚 boots. Although in this instance such a method of transport is essentially innocuous, it is nevertheless a salient reminder of the need to ensure that all clothing and equipment is cleaned before moving from one ecosystem to another.
Harko Werkman, Woodbridge, Tasmania, Australia
鈥� I bought a packet of desiccated shield shrimp eggs (Triops australiensis) on the internet for my boyfriend鈥檚 30th birthday. As the species name suggests, shield shrimps have three eyes: two compound eyes and one 鈥� a simple median eye, first appearing in the larval stage. They closely resemble their Triassic ancestors, which existed around 220 million years ago.
鈥�Triops has three eyes and closely resembles its Triassic ancestors from 220 million years ago鈥�
Blown around with the red dust, eggs eventually settle in crevices and grooves 鈥� even on the top of the great rock 鈥� where they may remain viable for up to 10 years. I guess that means my boyfriend has an excuse for not hatching them yet.
Kate Hutson, School of Earth and Environmental Sciences, University of Adelaide, Australia
鈥� Triops species are found on most continents but are rare in the UK, where the tadpole shrimp (Triops cancriformis) is currently known to exist in only two locations, the New Forest and the Solway Firth. Visitors to the , Dumfriesshire, can view this species in the visitor centre.
Triops is typical of ephemeral, or temporary, wetlands, and can survive drying to persist for up to 30 years as eggs or cysts. The eggs at Caerlaverock were collected to provide a safety net for the population in Scotland, where they persist in one temporary pond which has been created by cattle trampling around a fence post.
Emma Hutchins, Head of Reserves Management, Sally Cordwell, Head of Public Relations, Wildfowl and Wetlands Trust, Slimbridge, Gloucestershire