Photo credit: Manuel Cano Alfaro (Guatemala)
Review: Abdel Rahman El Gamal
This specimen of Daphnia belonged to the fauna of Lake Amatitlan, Guatemala
Introduction: Daphnia is a large genus comprising over 200 species of small freshwater crustaceans that belong to the family Daphniidae and the order “Cladocera”. These small crustaceans of 1–5 millimeters in length are commonly called water fleas due to their jerky swimming motions resembling the movements of fleas. Daphnias live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers. Some types of Daphnia can be seen with the human eye, while others must are microscopic. This is represented by Daphnia magna and Daphnia pulex for the large and microscopic species respectively.
Key characteristics: Depending on the species, Daphnia ranges from 0.5mm to 1cm long. In most Daphnia species, the body has a ventral gap in which the pairs of legs lie. Their outer carapace is transparent, so many internal organs can be seen, even the beating heart. On the head there is a compound eye and a pair of antennae, which are used for swimming. Females are usually larger than males and have a brood chamber under their outer carapace where eggs are carried.
Feeding: Daphnia uses its third pair legs to filter the water to keep the large unabsorable particles out, while the other sets of legs beat the water and create the current which brings in the appropriate Daphnia’s food of unicellular algae, protists and bacteria into their digestive tract.
Reproduction: Most Daphnia species have a life cycle alternates between asexual (Parthenogenic) reproduction and sexual reproduction. Under favorable conditions, Daphnia females reproduce asexually in which the laid eggs hatch into exact clonal copies of their mother. Following each molt, the mature female lays number of eggs which vary according to species from as few as 2 to as much as 20 eggs or more; these eggs develop without fertilization into females. On the other hand, when food turns scarce, Daphnia reproduce sexually in order to produce genetic variation and increase the chance of species survival. Under typical conditions, eggs hatch after about a day, and remain in the female’s brood pouch for around three days before being released into the water, and pass through a further 4–6 instars over 5–10 days before reaching an age where they are able to reproduce.
Daphnia and food chain: Daphnia are an extremely important part of aquatic food chains. They eat variety of primary producers such as algae, yeast, and bacteria. Many animals eat the Daphnia to survive. Those include tadpoles, salamanders, aquatic insects, and many of small fish species. Hence, the decline in Daphnia populations can lead to algae overgrowth and vice versa which in turn could affect fish production.
Reactions to environmental stress: Daphnias are very fragile animals and are extremely sensitive to water toxicity to which they exhibit number of natural responses leading to the use of Daphnia as a test or indicator organism in the environmental monitoring program and for ecotoxicological studies especially some of their response such as their heartbeat could be seen through their transparent outer carapace. Daphnias are known to be very sensitive to chlorine and various heavy metals.
Daphnia and hemoglobin production: Daphnia do not need to maintain a high level of hemoglobin to survive. However, they are exposed to low oxygen conditions (hypoxia), they can increase hemoglobin production turning them red as seen through their transparent carapace. Their ability to produce hemoglobin provides them a survival advantage in hypoxic water conditions. It is interesting to note that Daphnia with high hemoglobin content have access to food sources found in low oxygen areas giving them another advantage.