Daphnias - Some of the smallest and most common crustaceans

We all know some of the bigger crustaceans like shrimps, lobsters and crabs. But far from all crustaceans are as large as these. Daphnias are some of the smallest crustaceans in the world and can be found in almost all ponds, streams, lakes and other freshwater habitats. Their size ranges from a few hundred microns to around 5 mm for the largest species. Like other crustaceans, daphnias are covered in a hard exoskeleton for protection. Underneath this tough exterior are organs like a heart, a digestive tract, reproductive organs, an eye and a primitive brain. And because daphnias are mostly transparent, it's easy to observe their anatomy on your own.

In the back of the animal is a cavity used for protection of their embryos as they develop. After a few days the babies are fully developed, able to swim and feed, and ready to enter the real world. At some point the mother will lift her foot creating a small opening for the babies to swim out of. In a matter of days, the newborn are ready to reproduce themselves and these animals are therefore able to multiply rapidly when conditions are right.

The embryos on the left are fairly new, but the embryos on the right 

have developed eyes, are able to move and ready to be born

Daphnias are filter feeders and use several sets of fast moving appendages located in the front of the animal to filter algae and bacteria out of the water. A big population is therefore able to help control algae growth. Together with other small crustaceans like copepods they are a very important food source for small fish and other animals.

Reproduction mostly happens asexually, but when temperature decreases in fall and conditions start to decline they will switch to sexual reproduction. This will lead to the development of a different type of egg called an ephippium. These resting eggs are able to tolerate the freezing temperatures of winter and temporary desiccation. When conditions improve once again in spring the egg will hatch. These resting eggs can also be bought as an easily transportable and easy to store source of “live” fish food. All you have to do is add water, and in a couple of days you will have a nice crustacean snack for your aquarium fish.

A narrated video with Daphnias at different magnifications with different illumination techniques.