Water Flea (Daphnia obtusa)

 

Photograph by Peter Parks

Water fleas, or Daphnia, are small freshwater crustaceans ranging from 0.2 - 10 mm in length. They get their name from their short jerky hopping movement through the water. They swim using their prominent second antennae, which are branched and armed with bristles. This picture shows a female water flea incubting eggs.

Water fleas range from 0.2-10mm in length. They have both sexual and asexual phases within their life cycles. A population normally consists entirely of females that reproduce asexually. Under optimum conditions, a female may produce more than 100 eggs per brood, repeating every 3 days. A female may have as many as 25 broods in its lifetime, but the average is about 8. When conditions are more difficult and cold sexual reproduction occurs as males are produced.

Daphnia are typically freshwater organisms, but, some are found in slightly salty water. Some species have been observed in salinities up to 4 ppt. They are generally tolerant of poor oxygen levels within water but are very sensitive to bubbles, which can kill them. They are also not very tolerant of changes in the ionic compostion of the water that they are living in. Addition of some salts (for example potassium and sodium) can immobilise the fleas and kills them.

Cladocerans are an important component to any freshwater ecosystem. Many species occur seasonally in vast numbers, providing a valuable food supply to many other creatures and they in turn consume large quantities of planktonic algae. They are found in all kinds of standing waters - lakes, ponds, ditches, marshes - as well as canals and slow-flowing rivers. Some species are planktonic, others clamber about on vegetation or amongst the sediment.

 

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