Amphipod
(Gammarus sp.)

These particular amphipods, of the genus Gammarus, go by many common names, including "scuds" and "side-swimmers". This is because they are strongly compressed laterally. They are crustaceans, like lobsters, crabs, and shrimp, although a lot smaller. A single scud is about the width of my (and possibly your) index fingernail in length. They tend to live in large groups along the coast. If you've ever lifted a rock or especially disturbed a large bunch of seaweed, you may have seen these "shrimps" teeming in the water. They are so small that identifying the species is difficult without a microscope or at least a magnifying glass. They breed by the male holding the female with his legs and swimming around for a few days, after which he fertilizes the eggs in her brood pouch.