Water Flea

Water fleas are small planktonic crustaceans in the genus Daphnia, belonging to the class Branchiopoda rather than the insects. Despite their common name and flea-like jerky swimming motion, they are more closely related to shrimp and other crustaceans than to any true insect. Their bodies are enclosed in a transparent, often faintly greenish or yellowish shell-like carapace, through which internal organs, developing eggs, and a single large compound eye can often be seen with a hand lens or microscope.

Water fleas propel themselves through the water in short, hopping bursts using their large, branched second antennae, which give the characteristic flea-like movement. As filter feeders that strain algae, bacteria, and fine organic particles from the water, they form a critical link in freshwater food webs, converting microscopic plant material into food for larger invertebrates, fish fry, and other predators.

• Tiny, translucent to pale gray or greenish body enclosed in a rounded, shell-like carapace
• A single large, dark compound eye visible near the front of the head
• Long, branched second antennae used for swimming in short hopping bursts
• Visible internal structures, including a curved gut and, in females, a brood pouch that may contain developing eggs or young
• Best observed with a hand lens or microscope, though large swarms can appear as a cloudy haze in still water

Water fleas are found as free-floating plankton in ponds, lakes, slow streams, and other standing freshwater bodies nearly worldwide, often most abundant in nutrient-rich water with plentiful algae. Populations can fluctuate seasonally, often peaking in spring and again in autumn when algae is abundant and water temperatures are moderate.

Water fleas are filter feeders, using specialized limbs inside the carapace to create a current that draws in water and strains out algae, bacteria, and fine organic particles for food. They are a foundational food source for many aquatic predators, including small fish, aquatic insect larvae, and other invertebrates, making them a key component of freshwater food webs. Some species can shift their body shape or produce defensive spines in response to chemical cues from predators, a phenomenon well studied by ecologists.

Under favorable conditions, female water fleas reproduce asexually, carrying unfertilized eggs in a brood pouch that develop directly into miniature copies of the adult without a distinct larval stage, allowing populations to grow rapidly. When conditions worsen, such as when food becomes scarce or temperatures change, some individuals switch to sexual reproduction, producing hardy resting eggs that can survive drying, freezing, or other harsh conditions until favorable conditions return. This flexible reproductive strategy allows water fleas to rapidly colonize new or fluctuating water bodies.