Salt water sun lovers

29th December 2000 at 00:00
Brine shrimps make tiny but fascinating guides to big biological concepts such as habitat and population, promises Stephen Tomkins.

Think of an animal that can live in large numbers on the sunny side of your classroom, with minimal care requirements. Will it be one that will engage all your pupils in their powers of observation, imagination and hypothesis-making and at the same time provide good learning in key concepts in science? If you have never tried brine shrimps have a go with them and see if they fit the bill.

Brine shrimps are almost costless to manage and easy for pupils to watch and handle. They are useful for teaching ecology and animal behaviour and pupils can easily take them home and use them for project assignments. These aquatic invertebrates are ideal subjects for science, from primary classes all the way through secondary years to the sixth-form. You might wonder why you have not met these characters before, or perhaps you have - they are sometimes nicknamed "sea monkeys", a complete misnomer because not only are they far from being primates, they do not come from a marine environment, although their gymnastics in their inland salty underwater habitats are impressive.

What are brine shrimps?

In the wild, brine shrimps, which are species of the crustacean Artemia, live in salt lakes, in many subtropical areas of the world. A famous example of their habitat is the Great Salt Lake in Utah, US; one nearer Britain is the Camargue in the South of France. In such environments, after the winter and spring rains, the lake teems with brine shrimps right through the summer.

The water may dry up completely in some salt pans so the brine shrimps are adapted to survive in a dry egg-cyst stage, but after the dry season is over they hatch out from these tiny egg-cysts in the water and grow to adult size, in as little as a couple of weeks. Artemia franciscana adults in the Great Salt Lake are about a centimetre long. Millions of brine shrimps live in such habitats, at densities of 20 or more to the litre, but there are few fish in the shallow salt lakes, which, because they dry up, are a transitory aquatic environment. Perhaps this is why the brine shrimps are seasonally so numerous. It also means they have few predators except avocets and flamingoes, which wade in the shallow water and filter them out with their beaks.

Brine shrimps filter-feed on minute unicellular algae in the water, consuming these micro-organisms as fast as they multiply, so that the shrimp populations may boom or bust. As the animals grow they pair up and mate (even going around together in male-female pairs).

Males and females are easily distinguishable. The males are a pale blue-green and have long grasping antennae; the females are often orange and carry a large brood pouch of eggs. Females produce live young or dormant egg-cysts. Their life cycle is a few months long, and when they die their remains rot in the water, releasing their nutrients back into the environment. The nutrients provided in this way become food for microbial decomposers and the algae supply grows afresh. At the end of the summer the waters of their salt-lake habitat can appear red with floating brine shrimp egg-cysts. These are collected and used commercially throughout the world for hatching into small brine shrimp larvae that may be used to feed fish fry, both for the aquarium trade and in commercial fisheries.

How to keep brine shrimps in school

Brine shrimps are not difficult to rear in continuous culture in the classroom. All you need is an aquarium tank of about 30 litres, filled with salt water at 3.5 per cent and a suitable microbial community (of algae and decomposers) to supply them with food. It is most important to provide an ecosystem that will develop the physical conditions available in their natural habitat. Bright light is essential - a south-facing window is ideal or an added artificial light will provide for enough algal photosynthesis and so stimulate shrimp population growth. As long as the tank is above 20xC (25xC is ideal) they will breed, but they are able to tolerate all classroom temperatures.

Suitable sea salt (you need 35g per litre of water) can be bought at your supermarket. There should be sand and finely ground shell in the bottom of the tank. Shrimp eggs are obtainable from most petshop aquarists, and both a supply of brine shrimp eggs and a source of a suitable microbial community are obtainable from the Brine Shrimp Ecology Project (see box left) .

How to use brine shrimps in teaching

Once your school has a model salt-water ecosystem for brine shrimps in the classroom, both the animals and many of the ecological and behavioural processes of life can be easily observed.

A larger number of small ecosystems can be set up in plastic lemonade or cola bottles. Such "shrimp bottles" are costless, almost unbreakable, and are easily taken home by children for study. You can hand bottles out in the class and a small group of pupils may observe the animals closely with or without a magnifier. In such a setting, children readily identify with brine shrimps, seeing in their world a mirror to their own lives and the life processes of their own environment. Watching brine shrimps will call forth from young pupils all sorts of affective feelings as well as promoting intelligent observation.

The brine shrimp tank or bottle makes it easy to observe reproduction, growth and population dynamics. Pupils not only learn this science but are involved in animal care, observation, hypothesis-making, prediction, and experimental design.

At key stages 2 and 3 we have found that pupils keeping a brine shrimp diary in a "shrimp bottle", kept at school or at home, develop a good understanding of the animals themselves and develop skills of observation, drawing and reporting on events in their bottle.

Consistently, we have found that young shrimp watchers will not only learn about the form and the way of life of the animals, but will also describe animal behaviour in detail, and even identify imaginatively and empathetically with individual animals.

The courtship and mate-guarding of the tandem pairs is fascinating (see drawing, left). Tandem pairs are made when a second pair of antennae in the males forms claspers that hold the female during mate-guarding. For several days the male and female move around together in this tandem position. Children have made written observations in their brine shrimp diaries about romance and "blind-date" in the brine shrimp world.

Some key areas of ecology may be investigated at KS3. At the outset, it is not at all obvious to pupils what these animals eat. Children will notice that brine shrimps aggregate in the sunniest corner of their tank or bottle. It is clear that these animals love being in the light - and photosynthesis by small green plants powers their food chain. The brine shrimps swim around in their bottle in constant motion, some filter-feeding algae from the water. Pupils can discover this feeding relationship by experiment. Shrimps can be removed from the water by gentle use of a tea-strainer or large open-mouthed plastic pipette. If pupils sieve the shrimps out of the water in the bottle, the water will soon go green, showing that the algae are being removed by the brine shrimps. Enrich this water with some mineral nutrients and the algae grow faster (and so do the shrimps if they are in there feeding). Look at a brine shrimp with a lens or microscope and you can see the filter-feeding action and observe that their tiny, transparent gut is full of the green algae.

Brine Shrimp Ecology Project

A fascinating study of population growth can be made, putting just one shrimp pair in a single bottle and watching the subsequent population explosion. In a large tank the effect of a predator, such as a flamingo, can be modelled by sieving the brine shrimps off at a steady rate and seeing how their population alters.

At any key stage, pupils enjoy discovering, using a shallow dish of brine shrimps on an overhead projector, how the animals turn over if the light is coming to them from below. Clearly brine shrimps understand up from down in a very different way from us. Using an overhead projector the finer details of their swimming can be watched, enlarged for the whole class, as their image is projected on the wall. This is entrancing. Pupils can investigate how the animals choose between different shades and colours of light on the screen, a welcome change from "choice chambers" using woodlice.

At KS4 and in the sixth-form appropriate studies of brine shrimps can focus on such issues as pollution and the subtleties of animal behaviour. It is easy to build model ecosystems with differing levels of nitrate in the water to observe the effects of organic enrichment. I have found that sixth-formers have been able to develop their skills in practical investigation by looking at foraging behaviour, swimming speeds and reproductive behaviour. Students can observe sexual strategies by giving single shrimps a choice between two different-sized mates, to find whether they select larger or smaller partners.

Stephen Tomkins is a senior lecturer in science education and director of studies for biology teaching at Homerton College, Cambridge, and also the Faculty of Education lecturer in biological education, for the PGCE, at the University of CambridgeE-mail:

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