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.

Copepods are present in almost all bodies of marine- and freshwater. Most are around 1-2 mm in size but some species can be significantly larger with a size of 1 cm. Copepods make up a huge part of plankton, and are a major food source for larger animals such as fish. Copepods are also very important in the global carbon cycle. As these animals grow they moult several times, thereby shedding their carbon rich exoskeletons which sink to the bottom reducing CO2 levels in the atmosphere.

Notice the big heart just above the embryos.

The next crustacean is a bit larger with a size of up to 5 mm. Daphnias live in all kinds of freshwater habitats from small temporary puddles to huge lakes. They feed on algae and therefore contribute to managing algal growth. These animals are also an important food source for larger animals as fish. And because they are relatively easy to culture, many fish breeders buy daphnia eggs to hatch live food for their fish. Female Daphnias have a cavity in the back of their exoskeletons which they use to protect their offspring during development. When the babies are ready to enter the “real world”, the mother contracts her body creating an opening for the young to swim out.

Embryos at different stages of development.

The last crustaceans are Simocephalus. These look a lot like Daphnias, and can also be found in the same places. However, Simocephalus spp. tends to be a bit smaller than Daphnias. Other differentiating characteristics are the lack of a “tail” which most Daphnias have. Another more subtle way to tell them apart is their head shape. Daphnias have pointy noses whereas Simocephalus looks like it has been punched in the face.

Both Daphnias, simocephalus and other crustaceans can be covered in some mysterious crystal we don't know much about. You can read a lot more about this mystery and see images and video of the phenomenon here.