Eat and be eaten
Animal plankton or zooplankton are the link between plant plankton (phytoplankton, the food producers) and the larger animals of the sea. In the deadly contest for survival in the ocean, size matters. Single-celled zooplankton graze on phytoplankton or ingest each other if they get the chance. They are eaten by larger, many-celled zooplankton such as jellyfish, crustaceans and arrow worms, which are in turn eaten by fish, squid, marine mammals and sea birds.
Foraminifera and radiolaria are predatory, single-celled creatures with shells made of calcium carbonate (foraminifera) and silica (radiolaria). They come in a variety of shapes and sizes, and may form colonies. About 1,000 species of foraminifera and 150 species of radiolaria have been found in New Zealand waters. They feed by extending sticky parts of their body through pores in their shells, engulfing passing victims. These predators are important to the study of fossils, as their shells are often found preserved in ocean sediments.
Vast areas of the seabed around New Zealand consist of plankton ooze, sometimes hundreds of metres thick. It is made from the tiny lime skeletons of foraminifera (calcareous ooze) or the glass skeletons of diatoms and radiolarians (siliceous ooze). It takes about 100 years for a millimetre of ooze to form.
Copepods, the tiny relatives of crabs and crayfish, are the most abundant animals in the plankton. They are important food for young fish, especially hoki (Macruronus novaezelandiae). The adults look like miniature shrimps, and most are about the size of a pinhead. They have a complicated life cycle involving six larval and five juvenile moults; there is little similarity between early larval forms and the adult.
The plant-eating copepods are very energetic, and each day they need to consume their own weight in phytoplankton. Using comb-like feeding organs to sweep food into its mouth, one copepod can eat 130,000 cells of phytoplankton in a day. Bacteria digest copepod faeces, and by doing so, release nutrients back into the water that help sustain the phytoplankton.