Seed shrimp, though microscopic, accomplish a lot. They carry tiny algae on their backs as they drift, and they even provide food for popular marsh residents like mallards and egrets.
They do this even as they spend their lives cemented in between two shells that fit together like the halves of seed that never bursts into a plant. Inside is a simple two-part body structure: head and thorax. From the head extend four pairs of legs, the majority of the seven pairs; the thorax only has three pairs. Why are these legs so important that they have to sprout from the creature's head? They are critical for feeding. With its legs, the seed shrimp can filter even smaller particles than itself out of the surrounding water.
Seed shrimp drift in the middle layer of cooler water of vernal pools and freshwater marshes. Enough light must still reach this layer for it to be an adequate home for algae. The tiny green plants sometimes hitchhike on the seed-like shells of the brown, pebble-like crustaceans.
We were fortunate to attend a plankton information event at a local nature center on the edge of the delta where the Sacramento and San Joaquin rivers meet in Northern California. On a rainy day, scientific children followed a ranger right out to the edge of the marsh, and boots sinking into sticking mud, threw a fine-mesh plankton net right out into the water. After we arrived back inside, the ranger first used a microscope that projected the whole plankton scene on a large screen on the wall. Amidst all the green trash items (random algae bits and so forth), the water buzzed with life. I immediately liked the brown ovals zipping by, and I soon learned they were seed shrimp. One-eyed copepods were abundant (the planktonic Sponge Bob character), and we even saw segmented worm-like plankton.
The experience got even better as one ranger was able to isolate a seed shrimp and get better magnification of its body, which gleamed like a tiger eye stone. The kids went on to explore how to combine cork and clay to make a plankter (which is the word for one instance of plankton) that would not float directly on top of the water but also would not sink to the bottom. Plankton hydrodynamics turned out to be trickier than we thought as we tried to get our creations to stay in that middle zone. This experience corrected my misconception that plankton float on top of the water; actually that zone can be too warm and subject to fluctuations in temperature. Drifting in the middle is the real plankton zone!