Ward's World Working with Guides

Page 3 Daphnia magna and other cladocerans are of great importance in the aquatic food chain. They are a food staple for young and adult fish; stomach contents of a variety of fish can contain up to 95% cladocerans. Hydra and both immature and mature insects also eat Daphnia. Due to its varied habitats and its complex yet easily studied anatomy, the water flea is an ideal organism for study. Although the complex muscular system obscures some of Daphnia's smaller anatomical features, the essential parts of most organ systems can be easily distinguished. The most visible feature is the long, bent, dark-colored intestine. The simple football-shaped heart is readily visible behind the head on the dorsal side of the animal. Daphnia possesses no actual blood vessels, its colorless blood plasma being guided throughout the animal by a series of min- ute mesenteries. Its heart rate varies with water temperature, making it easy to alter the Daphnia's heart rate and observe the changes. Environment Cladocerans are common in almost all aquatic environments, with the exception of fast-moving streams and brooks, and heavily polluted waters. The two most common inhabitants of ponds, permanent pools, and temporary pools are D. magna (usually supplied) and D. pulex. Daphnia move through the water in a series of "hops" produced by rapid strokes of its feathery paired antennae. Reproduction and Growth In cladocerans, reproduction is parthenogenic, which means that eggs develop without fertiliza- tion in the brood chamber and hatch there as fully developed young. They develop during the year, in most habitats, and only females are produced. The number of eggs per clutch (group of eggs) varies among species; D. magna carries ten to fifteen. When the young hatch, the adult releases them by moving its post abdomen downward. Normally one clutch is released during each adult instar, or molt. Daphnia populations peak in spring and autumn, beginning when the water temperature rises to approximately 12°C. During these times, special "sexual" males and females may be produced, usually in response to a variety of environmental circumstances such as excessive crowding of females, a decrease in the food supply, impending harmful change in environmental conditions such as a pond drying up, or an increase in water temperature. Under these conditions, males copulate with specialized females who produce haploid eggs. These haploid eggs are housed in the brood chamber, the walls of which thicken and darken to form an ephippium. The brood chamber, which houses one or two "winter eggs", separates from the rest of the carapace during the next molt. When released, ephippia either sink to the bottom of the body of water or float on the surface, depending on the currents, wind, and even other animals to distribute them. Ephippia and their eggs are capable of withstanding the rigors of winter and summer droughts, and can survive in temporary ponds that dry up in the summer and fill up again in the fall. Ephippia eggs hatch parthenogenic females. The length of Daphnia's life cycle ranges from 10 – 40 days, depending on water temperature. The number of instars for D. magna is between six and 22, their duration lasting from one day to several weeks, depending on environmental conditions. Most cladocerans grow to a length of 0.2 mm to 3 mm; mature D. magna are usually 3 mm long. Working with Daphnia

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