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.

When we look at these crustaceans with regular brightfield- or darkfield microscopy, like in the image above, nothing seems out of the ordinary. But if we illuminate them with polarized light, it is an entirely different story. This reveals beautiful crystalline patterns on the exoskeleton of the animals that light up like luminescent paint in the polarized light. This is because some of these crustaceans are covered in birefringent crystals, however, no one really knows what these crystals are made of and information about them is incredibly hard to find.

Female Daphnia magna covered in crystals on one side.

These strange crystals do not appear on all of the crustaceans and some only have a tiny amount, even though they are from the same sample and of the same species and age. And to make them even more mysterious, if the animals are cultured in a lab, the crystals sometimes doesn’t appear at all. So, they are likely made of something dissolved in the water in their natural environment. Most suggests that they are made up of carbohydrates or minerals like calcium carbonate. What we do know, is that the crystals grow on their exoskeletons where the mysterious substance finds a nucleation site and starts to grow and expand, sometimes covering the entire body surface. However, the crystals do not seem to harm the animals, and are just incredibly beautiful to look at.

A lot of crustaceans illuminated with polarized light, showing a lot of crystals 

on some animals, but only a tiny amount, or non at all on others.