Copepods, barely two millimetres long, may outweigh every other animal group
Copepods are tiny crustaceans, usually 1 to 2 millimetres long and nearly transparent, yet some scientists think they form the largest animal biomass on the planet. Each night many rise to graze near the surface, then sink by day, carrying carbon into the deep sea. They may matter more to the ocean carbon sink than krill.
Some 13,000 species are known, living in almost every body of water, from open ocean and caves to puddles, damp moss and the water trapped inside pitcher plants. The name means oar-feet, after the head appendages that row them along. Most have a single bright red eye in the middle of a see-through head, though a few genera have paired eyes with two lenses arranged like a telescope. They are so small that most need no heart or gills, taking oxygen straight through the body surface.
Their appetite is enormous for their size. A single copepod can eat up to 373,000 phytoplankton cells a day, filtering roughly a million times its own body volume of water daily. Species in cold, rich seas store surplus energy as oil droplets that can fill over half the body in polar forms, and they sink by converting those oils into denser fats. Moulted shells, droppings and breathing at depth all move carbon downward, part of a surface ocean sink absorbing around 2 billion tons of carbon a year.
Speed is their defence. Many have nerve fibres sheathed in myelin, a rarity among invertebrates, and in some cases the wrapping is organised much like a vertebrate's, enabling explosive escape jumps. The slow seahorse outwits them anyway, drifting close so gently that no turbulence betrays it before it sucks the copepod in.
The larval nauplius looks so unlike the adult that it was once treated as a separate species, and until 1832 copepods were confused with insects or zoophytes. Growth to adulthood can take a week or a year; one species needs about 7 days at 25 degrees Celsius but 19 at 15. About half of all species are parasites, while copepods themselves host gut parasites that can halt their feeding entirely.
Source: Copepod
