Daphnia’s maneuvers in the dark

Like many other crustaceans, water fleas are very sensitive to light. They have a light-sensitive organ, also called Ocellus, with which they can swim in the direction of the light.

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

Some of the smallest crustaceans in the world

Crustaceans are a huge group of animals living all over the world. There are more than 67.000 known species which dramatically vary in size from around 0.1 mm to almost 4 m. Most are aquatic animals, like crabs and shrimp but some are terrestrial like the woodlouse. All crustaceans have a tough exterior called an exoskeleton used to support and protect their body. This armor is mainly made up of long polymers called chitin combined with calcium carbonate to make it hard. Here we will look at a few of the smallest crustaceans you might encounter in a water sample. The first one we take a closer look at is a copepod.

It is covered in symbiotic algae. The copepod is surrounded by 

younger individuals at different stages of development.

Mysterious crystal growth on tiny crustaceans

Crustaceans form a large group in the animal kingdom, and most are familiar with some of the larger members like crabs, lobsters, and shrimps. But many crustaceans are a lot smaller than those, and make up an important part of the food chain as food for fish and other larger animals. One of these planktonic species is daphnia magna, and with a size of 1.5 to 5 millimeters it is one of the largest in its family and just visible with the naked eye. However, it is not the daphnia itself we will focus on today. But a strange phenomenon that occurs on its body, as well as on a few other tiny crustaceans like it.

Female Daphnia magna carrying the next generation in a cavity.