The Maxillopoda is a class of crustaceans that includes barnacles, which are immobile, and copepods, which can move about. These unlikely classmates are placed together by scientists because of their similar body segmentation.
Among the best known and widely encountered members of this group are the filter-feeding barnacles. The barnacle zone is a familiar site on many rocky shores – a white zone of little shelly pyramids occupying the highest reaches of the tide. Goose barnacles are stalked and can attach to driftwood and other floating objects in the sea. Rocky-shore barnacles are encased in a set of calcium carbonate plates and, externally, lack the appearance of typical crustaceans. However, their jointed limbs and shrimp-like bodies reveal their crustacean nature.
Two pounds of flesh
Few, if any, New Zealanders eat barnacles nowadays, but they once featured in the diet of some Māori. The species Epopella plicata was found in coastal middens around the Aotea Harbour in south Auckland, along with pipi, cockles and tuatua shells. It takes a lot of barnacles to get a good meal: 10,000 Epopella yield about 1 kilogram (two pounds) of meat.
Many barnacles are symbionts: animals that live with, on, or in other animals. Some barnacle parasites of crabs cause feminisation of their host. These parasites can be identified as barnacles only when in the larval stage. Others live on the heads of whales. These form large aggregations but do not apparently harm their host. Some are host specific: the barnacle Coronula diadema is a typical associate of humpback whales.
The copepods are a highly diverse group, and they occur in nearly all aquatic habitats, including water trapped in the bracts of rain forest plants and the leaf litter of forest floors. There is no widely used common name for this group of generally small (less than 5 millimetres) crustaceans, although the pelagic (ocean swimmers) are occasionally referred to as oar-footed bugs. These animals can occur in vast numbers and are important in the oceans’ food chains, being a major source of food for commercial fish species such as sardines and herrings.
Another large group lives on the sea floor. These benthic or bottom-dwelling copepods are often flat and roughly circular. Along with benthic isopods, they are highly sensitive to habitat disturbance and adversely affected by pollution. They are probably important food for the larval and juvenile stages of fish. When these tiny crustaceans disappear, so too do the adult fish whose larval stages feed on them: small does not necessarily mean insignificant.