Underwater Ballet

What am I looking at?

These are two freshwater water fleas (Polyphemus pediculus) under darkfield illumination. Their large, fused compound eyes are the dark domes at the top of the image (1). Their legs are below their heads (2). Their long tail spikes protrude from their back ends (3). You can see the contents of their stomachs in brown and orange (4) through their translucent bodies.

Biology in the background

Water fleas are small crustaceans that live in both fresh and salt water. The species pictured here live in bodies of fresh water throughout the Northern Hemisphere. While most water fleas are filter-feeders – feasting on algae, bacteria, protozoa, and detritus – these are predatory and eat other small zooplankton (small invertebrates that live in the water) by grasping and holding them with their four legs.

This is one of the few species of plankton that hunts visually. They use their huge, compound eyes to detect light and shadow as prey passes them by. The compound, or faceted, eyes are fused in the middle to form a dome. During the day, large groups of these fleas can be seen clustered in the water close to the shore of rivers and lakes. At night they disperse, moving into open water to hunt.

They can grow up to 1.5 millimeters long, or roughly two-thirds the size of a human thumbnail.

Technique

These images were created using a form of light microscopy known as darkfield microscopy.