Harpacticoid bodies, which rarely exceed 0.07 in (2 mm) in length, are not clearly divided into three distinct regions, and they vary dramatically in shape. Some harpacticoids are long and snake-like, while others are flat. Their antennae are very short and forked.
Harpacticoids are planktonic, generally living in the muddy or sandy areas. Instead of swimming, these cope-pods hop, using the appendages on their thoraxes in combination with a rocking motion of their bodies. While most harpacticoids feed on organic waste or algae, some species are predators, swarming over small fish and immobilizing them by eating their fins.