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The world of the water-flea.

Humans have surely known of the existence of water-fleas since they first scooped up water into bowls or cups, although study of these fascinating creatures had to wait until microscopes were invented. This is because although female water-fleas can grow up to five millimetres long, half as long as a little fingernail, they can be as small as 0.2 millimetres and so virtually invisible to the naked eye.

Probably named after their hopping or skipping movements, water-fleas are, in fact, not related to true fleas, which are insects. They belong to the Crustacea group, and so are related to crabs, lobsters, shrimps and prawns. They are placed in a group now termed Anomopoda (formerly Cladocera), a sub-division of the Branchiopoda. Alongside their huge variety in size, different species of water-fleas also have very diverse habitats. Members of the families Daphnidae and Bosminidae tend to be found in open water and are easily visible when collected in a jar or bottle. Those in the family Chydoridae spend their time among weeds and bottom debris and are often smaller and less noticeable.

Daphnia were one of the very first organisms to set up home in the Museum's Wildlife Garden, appearing only a few weeks after rainwater had begun to collect in the newly lined excavations. They might have been introduced via fertilised eggs resting on the feet or plumage of some mallard ducks that were seen in the garden around the time. Exactly which species of Daphnia these early arrivals were is not known, but four species have been spotted during the past 12 years: D. longispina, D. hyaline, D. pulex and D. atkinsoni.

Despite being small, the tiny creatures have a fairly complex structure. Like the other members of their group, water-fleas have a kind of shell. In fact, they are almost entirely enclosed by a carapace, the anterior of which encloses their head, like a helmet. They absorb oxygen through the inner shell of the carapace, as well as through their internal limbs. Below this, the carapace divides into two equal parts, like an open coat, down the front, or ventral, surface. In Daphnia, the carapace ends in a spine of varying length, absent in the families Chydoridae and Bosminidae. The open ventral sruface in all water-fleas allows for the protrusion of the end, or posterior, part of the body. This is known as the post abdomen and ends in a single foot that bears two curved claws. The bottom-dwelling and weed-haunting Chydorids can push themselves through tangled algae and debris with the help of these claws.

Water-fleas have two pairs of antennae. Depending on the species, the first pair are used for various things, from swimming in open water by Daphnia or to propel Chyroid at high speed as they scuttle through the bottom debris of the pond. The second pair of antennae is much smaller and thought to provide a sense of smell.

Water-flea eyes are particularly fascinating. The large, compound eye has lenses arranged around a very dark area of pigment. These lenses help concentrate light on to the area of pigment, transmitting impulses along the optic nerve to the brain, which, in turn, controls the swimming antennae, so guiding the water-flea towards its food. Below this eye, and further forward, is another eye that is usually much smaller and very simple in structure.

Because water-fleas are transparent, if you look at them under the microscope you can easily see their rapidly beating heart near the dorsal, or back, surface of the carapace. You can also spot their more steadily beating internal limbs. They usually have five or six pairs of limbs and each one has large combs of very fine bristles. The constant beating of the limbs causes a current of water to flow through the carapace, bringing with it food particles. These are filtered out by the combs and passed to the mouth parts in the base of the head, where they enter the long digestive canal, or gut. In Daphnia, the gut is fairly straight with a curve at each end. In the Chydorid water-fleas it may be coiled or looped. In both groups it ends in the post abdomen from where faecal matter is discharged. The current of water caused by the action of the internal limbs also brings dissolved oxygen, which is absorbed through the surface of the limbs and the inner surface of the carapace. This is why water-fleas have the name Branchiopoda, which means gill foot.

A large number of blood corpuscles may be seen flowing through the internal structures of the water-flea. Although there is no system of blood vessels, the blood flows through a complex series of passages to all parts of the body. Daphnia species that live in poorly oxygenated waters, for example polluted farmyard ponds, tend to have a large amount of haemoglobin in their blood corpuscles. This enables them to make the best use of the reduced amount of oxygen available, with the result that these Daphnia look quite pink. When congregating in vast numbers near the surface of the pond the swarm can look like a pink football and is easy to spot from several metres away.

So how do these tiny creatures reproduce? They have two methods, depending on the seasons. In the summer, when food is plentiful, females lay eggs that quickly develop into other females-replicas of their mothers, and hatch out in the female's brood pouch before swimming away.

However in leaner months, or when conditions become unfavourable, such as a shortage of food, drying up of the pond or rapidly falling temperatures, females produce eggs that hatch into males. These males fertilise other females and this results in fertilised eggs that only hatch when suitable conditions are restored, possibly the following spring. These fertilised, resting eggs remain enclosed in a specially thickened part of the female's brood pouch and, at the mother's next moult they are discarded. In the autumn, these brood pouches still containing the fertilized eggs can often be found floating at the water's surface in large numbers.

Some 90 species of water-flea have been recorded in the British Isles, 12 of which I have found, to date, in the Museum's Wildlife Garden ponds during my collections. They may be small, but there is far more to these tiny creatures than meets the eye.

This article first appeared in Nature First, the magazine of the Natural History Museum, London

Eric D. Hollowday

Wildlife Garden volunteer at the

Natural History Museum, London
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Title Annotation:Water Insect study
Author:Hollowday, Eric D.
Publication:Environmental Education
Geographic Code:4EUUK
Date:Jun 22, 2013
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