By using a plankton net in a river, it becomes easy to collect an incredible things. Luckily, we got a lot of Daphnia, this small planktonic crustacean was omnipresent in our sample.
This organism have been well studied as model in biology since its transparency is giving the possibility to see its different organs. From 1–5 millimetres long, Daphnia appears very nicely under a foldscope.
On this video we can clearly see its compound eye, its heart betting super fast, the whole structure of its gut and even a smaller Daphnia that was present inside this bigger one a minute before. It’s interesting to learn that :
Most Daphnia species have a life cycle based on “cyclical parthenogenesis”, alternating between parthenogenetic (asexual) reproduction and sexual reproduction. For most of the growth season, females reproduce asexually.
This organism is a filter feeders because capable of creating a current by beating their legs to bring material into their digestive tract and ingesting mainly unicellular algae that are even visible inside its gut in this video.
The nervous system is characterized by the cerebral ganglion, which is located close to the gut and near the eye. Juvenile and adult Daphnia have one large compound eye, whereas embryos show two brownish eye spots that fuse during the last part of the development. The compound eye helps to orient the animal while swimming. A small structure called a naupliar eye is located between the mouth and the compound eye on top of the cerebral ganglion.
The naupliar eye is generally a single median eye comprising three- or four-pigment cup ocelli, sometimes with a lens, probably enabling the organism to determine the direction of a source of light for orientation. The median eye is typical of crustacean nauplius larvae. It may degenerate as the larva develops, or it may persist into the adult form.
It’s then easy to understand why this eye is capable of a small spinning orienting the visible pigment cup ocelli in the intermediate position to capture the direction of the light more accurately.
Ecology, Epidemiology, and Evolution of Parasitism in Daphnia https://www.ncbi.nlm.nih.gov/books/NBK2042/