Have you ever seen, while looking at a small body of water, tiny jumping organisms? Chances are you were looking at cladocerans, microscopic crustaceans commonly named water fleas! With around 700 species that have been identified, cladocerans can be found all over the world in freshwater and marine habitats. Unlike larger crustaceans such as shrimps, lobsters and crabs, cladocerans are a little less known to the general public. Yet, they occupy a key position in aquatic food webs by grazing on algae and bacteria but also by being preyed upon themselves by larger animals such as fishes, birds, frogs and other aquatic predators.
Cladocerans have been around for a long time, the first cladocera fossil record goes back to the late Permian (around 240 million years ago) where Daphnia-like and chydorid-like organisms have been described. A few Daphnia-like fossils from the Jurassic period (225 million years ago) in good condition have also been retrieved. By the early Triassic, cladocerans made their official appearance and started to proliferate. Nevertheless, abundant and well-preserved fossils were only found starting from the Oligocene period, which was around 38 million years ago.
General Biology
Members of the Cladocera phylum are characterized by a transparent bivalve carapace and 10 pairs of appendages. Their flattened leaf-shaped limbs called phyllopods, are used for respiration and create a water current that attracts nearby food particles into the filtering apparatus and ultimately, into the mouth. They use their large branched-shaped antennas to swim and to direct themselves through water. Their unique way of swimming resembles the jumping movements of a flea, thus where their common name “water fleas” comes from!
Depending on the species, adults can measure from 0.2 mm to 18 mm and can be spotted with the naked eye. Although, to be able to see their most obvious features, a microscope is required! All cladocerans possess a single central compound dark eye excepted some chydorid species that possess two. At a magnification of 40X, it’s possible to see the transparent muscles that are attached to the single eye which enable it to move.
While most cladoceran’s shells are transparent, some species of Scapholeberis have a black shell! This phenomenon is due to melanin pigment deposits present on the exoskeleton of the animal, making them the darkest of cladocerans. Melanin is also found in a wide range of different animals and plants. In humans, melanin is present in the epidermis, hair and iris!
Since most carapaces are transparent, it’s usually easy to observe different organ systems that compose an individual such as the circulatory system. Cladocerans possess an open blood circulation with a heart located dorsally that beats around 200 to 350 times per minute. As us, they possess hemoglobin proteins to support oxygen transport around their body cavity and those blood cells can even be observed. Cladoceran’s blood can usually be seen as pale yellow but can also be red due to the presence of hemoglobin proteins. When oxygen is reduced in their environment, some cladocerans such as Daphnia can increase their number of hemoglobin which allows them to boost the level of oxygen in their blood, enabling them to survive long periods of time when oxygen is scarce.
Feeding
There are three different ways of obtaining food in cladocera crustaceans and all of them rely on the movement of thoracic limbs. Some cladocerans such as chydorids and macrothricids, which mainly live at the bottom of the water column, collect food from the surface of various substrata by scraping on it with their strong second set of limbs. They can also use their carapace as a suction cup to create a negative pressure that attracts food particles into the filtering apparatus.
Together with planktonic cladocerans such as Daphnia, bottom feeders can collect food by filtering suspended particles in the water with the help of their phyllopods. Intake of food in suspension feeders is almost continuous and these particles usually transit quickly through the digestive system. Within around 7 minutes, food enters and exits the body and all nutrients necessary for growth, reproduction and metabolism have been extracted.
Most suspension feeders are planktonic instead of bottom dwellers and eat a wide range of microorganisms such as bacteria, algae, ciliates, small rotifers and copepod larvae. Before entering the mouth, food particles are crushed by the mandibles for optimal digestion.
Some cladoceran species are known to be carnivorous and predatory and feed on the carcasses of other cladocerans or even on living Hydra. In contrast, cladocerans are a source of food for many freshwater predators such as pond insects and larvae, flatworms, hydras, salamanders, predacious copepods, birds and fishes. You might’ve even fed your own aquarium fishes with Daphnia since they can be bought in most pet stores!
Reproduction
Depending of environmental conditions, cladocerans are able to reproduce sexually or asexually. Even if males can be produced through sexual reproduction, cladocerans principally reproduce asexually through parthenogenesis. Only few animals such as bdelloid rotifers, aphids, bees and ants are able to reproduce this way. By reproducing parthenogenetically, cladocerans can lay eggs that won’t need to be fertilized by spermatozoids to develop into a fully living female embryos. This method is especially helpful to maximize reproduction rates when environmental conditions are harsh. Despite being really convenient for rapid growth of a population, deleterious mutations often appear through asexual reproduction, and individuals aren’t able to adapt to abrupt environmental changes since their genomes are almost identical.
Adult female cladocerans within the Daphnia genus produce clutches of eggs every 3 to 4 days until their death. Those eggs are deposited into the brood chamber which is positioned dorsally, underneath the carapace. Within approximately 24 hours, embryos hatch from their eggs but need to stay 3 more days in the brood pouch for further development before being ready to be set free. Under optimal conditions, females can live more than two months and in large species, lay over a hundred eggs per clutches!
How to View Them?
Cladocerans such as chytrids and Daphnia can be found at your local pond or lake and look absolutely beautiful under darkfield, oblique illumination and polarized light! The birefringent properties of their muscles and shells make them look like shining stars under crossed polarized illumination.
The best way to observe them is with a compound microscope and a 4X, 10X or 20X objective. The heart is particularly spectacular to look at when magnified 200 times with the 20X objective! Even if stunning under the microscope, their shells are fragile and can be easily broken under the weight of a cover slip. It’s always better to use well slides with a cover slip or a normal slide without any cover slip and just enough water for them to swim in.
The videos and photographs were made using the Motic BA310 Elite compound microscope with oblique illumination and an iPhone 11 Pro mounted on a specialized adaptor.
References
- Dodson, S. L., Cáceres, C. E., & Rogers, D. C. (2010). Cladocera and other Branchiopoda. In Ecology and classification of North American freshwater invertebrates (pp. 773-827). Academic Press.
- Ebert, D. (2005). Ecology, epidemiology, and evolution of parasitism in Daphnia. National Library of Medicine. 5-9.
- Fryer, G. (1974). Evolution and adaptive radiation in the Macrothricidae (Crustacea: Cladocera): a study in comparative functional morphology and ecology. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 269(898), 137-274.
- Smirnov, N. N. (2017). Physiology of the Cladocera. Academic Press.
- Wininger, J. D. (2004). Parthenogenetic Stem Cells. Handbook of Stem Cells, 635–637.
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