Daphnia
A genus of small crustaceans used as live food and model organisms.
Daphnia is a genus of small planktonic crustaceans, 0.2–6.0 mm in length, commonly called water fleas due to their saltatory swimming style. They are notable as a popular live food in fishkeeping and as a model organism in scientific studies, particularly for observing heart rate and the effects of stimulants and depressants due to their translucent carapace.
- Size range mm
- 0.2–6.0
- Typical body length mm
- 1–5
- Heart rate bpm
- 180
- Lifespan months
- up to 13–14 in cold oligotrophic lakes; typically 5–6
- Number of species
- over 200
- Brood size
- 1–2 eggs in small species to over 100 in large species
Lore & Background
Daphnia are members of the order Anomopoda and live in various aquatic environments from acidic swamps to freshwater lakes and ponds. The two most common species are D. pulex (small and most common) and D. magna (large). They are often associated with the related genus Moina, which is much smaller. The body is divided into segments, though not visibly, and is covered by a translucent carapace with a ventral gap for the legs. The compound eyes, second antennae, and abdominal setae are prominent features. The heart is located at the top of the back, just behind the head, and beats at about 180 bpm under normal conditions.
Daphnia are filter feeders, ingesting unicellular algae, organic detritus, protists, and bacteria. Beating legs create a current through the carapace, and the second and third pairs of legs filter out large particles. Swimming is powered by the second set of antennae, producing a jumping motion. They show behavioral and morphological changes in response to predator kairomones, such as developing neck-teeth in the presence of Chaoborus larvae. Chitin-related genes are involved in these defenses.
Most Daphnia species reproduce via cyclical parthenogenesis, alternating between asexual and sexual reproduction. Asexual broods of diploid eggs hatch after about a day and remain in the brood pouch for three days. Toward the end of the growing season, males are produced, and females produce haploid sexual eggs that form resting eggs protected by an ephippium. These ephippia can withstand extreme conditions and hatch into females when conditions improve.
Reader's Guide
Daphnia are a popular live food in tropical and marine fishkeeping, often fed to tadpoles or small amphibians such as the African dwarf frog. Their significance extends to scientific research as a model organism: their translucent exoskeleton allows direct observation of the beating heart, blood cells, and feeding mechanism under a microscope. They are used to study the effects of alcohol, caffeine, nicotine, and adrenaline on heart rate, as they are tolerant of being observed live under a coverslip. Daphnia also serve in environmental studies, as they produce hemoglobin in low-oxygen environments and show genetic responses to toxins like microcystins. The genus comprises over 200 species, with several considered threatened, including Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi. Some species are halophiles, found in hypersaline lakes such as the Makgadikgadi Pan. The understanding of species boundaries is hindered by phenotypic plasticity, hybridization, and poor taxonomic descriptions.
Did You Know?
- Daphnia are called water fleas because their jumping motion resembles that of fleas.
- The heart rate of Daphnia is about 180 bpm under normal conditions and can be altered by alcohol, caffeine, nicotine, or adrenaline.
- Resting eggs of Daphnia are protected by a hardened coat called an ephippium and can withstand extreme cold, drought, or poor food availability.
- Some Daphnia species develop neck-teeth at the back of the head in response to predator kairomones from Chaoborus larvae.
Frequently Asked Questions
Who is Daphnia?
Daphnia is a genus of tiny planktonic crustaceans, often nicknamed 'water fleas' because of their characteristic jumping swim. They range from about 0.2 to 6.0 mm in length, with most individuals falling in the 1–5 mm band.
What are Daphnia's powers/role in the fishkeeping world?
They serve as a beloved live food source for aquarium fish and invertebrates. Beyond the tank, they double as model organisms in labs, where their see-through carapace lets researchers watch heartbeats and drug responses in real time.
Why is Daphnia important to the hobby and to science?
Their translucent body makes them ideal for observing cardiac activity at roughly 180 beats per minute, which is why they're go-to subjects for stimulant and depressant studies. In the hobby, they're a staple live feed that's easy to culture and highly nutritious for small fish and shrimp.
How many Daphnia species are there, and how big do they get?
There are over 200 recognized species in the genus. Most adults sit between one and five millimetres, with the full size range spanning 0.2 to 6.0 mm depending on the species and environmental conditions.
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