Shield Shrimp

by Ade Foster


The Tadpole or Shield shrimps belong to the Class Crustacea, in the sub-class Branchiopoda (gill-legged), and the order Notostraca. (flat-back) There are two genera world-wide, Triops and Lepidura, each represented by a single species in Australia, T. australiensis australiensis and L. apus viridis.

The most prominent and easily recognisable feature is the dorsal carapace or shield which covers the majority of the animal. It is attached at the front only, and can be lifted clear of the body. There are a pair of sessile (fixed) compound eyes at the front of the shield. The body itself is made up of a large and variable number of segments or body-rings, numbering up to 70. The last segment has a pair of caudal rami or tails and inLepidurus a supra-anal plate which is not found in Triops. The first eleven segments make up the thorax, and each bears a pair of foliaceous (leaf-like) appendages called phyllopods, the last of these bear the ovisacs. These legs are beaten back and forward and propel the animal very quickly through the water, and also act as the breathing apparatus. The number of legs behind the thorax varies greatly, and may be up to sixty. It is peculiar to Notostraca that each segment may bear more than one pair of legs. There are a number of the hind most segments are free of legs, forming a long and very mobile tail.

Notostraca have been found in a variety of habitats including temporary rain-pools, slightly salty lakes, freshwater lakes, farm dams, drainage ditches and the like. They are never associated with marine, saline or estuarine environments. They seem to be most commonly associated with freshwater habitats which dry up periodically, and in one species at least, it is necessary for the eggs to undergo a period of desiccation before they will develop. In all species, eggs are very resistant to long periods of dry conditions.

Notostraca are found on all continents except Antarctica, and in most climates where ephemeral pools occur. In Australia they are found wherever there is suitable habitat. In the North and the drier inland areas, only Triops is found. It hatches at any time of year, given suitable pools of water. In the Southeast, where both Triops and Lepidurus occur the adults have only been recorded in winter and early spring. The range of the two species is mostly distinct, with some overlap in NSW and N. Victoria.

Populations from around the world vary greatly in the manner of their breeding. There appear to be males and females in most populations, although all individuals are to some degree hermaphrodite. Parthenogenesis exists in some populations, but they cannot self fertilise. In Northern Europe and the USA males occur rarely, with populations being female or hermaphrodite. In warmer climes the sexes are balanced. Eggs are carried in the egg sac until mature then laid in the silt at the bottom of the pool. Some hatch immediately, others form oocytes and remain in the soil after the pool dries, remaining viable for periods of up to ten years without water. The microscopic young grow quickly and after several moults resemble the adult. This rapid growth may take as little as twenty four hours. Eggs appear after seven days and shrimps are fully mature at three weeks. Desiccated eggs begin to hatch within three days of the pools filling again.

Shield shrimp filter feed on microscopic animals in the mud at the bottom of a pool, may catch small animals or graze on water plants. Even cannibalism of recently moulted fellows has been reported. The phyllopods have a channel between them and the beating forms a current which carries the food animals or particles to the mouth.

Several members told me that these creatures were common in their youth, but not seen so often now. Could this be a product of our environment, or just that as we get older, we don’t have as much time to play around in puddles?