Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Maxillopoda
Subclass Copepoda

Common name: copepods

Overview

Copepods are a diverse group of minute crustaceans widespread in marine and freshwater habitats. Copepods usually have short, cylindrical body that is wider at the anterior end. The head has a single, central eye (at least in the larval stage), which is usually bright red and elongate and conspicuous antennules with bristles that are the main swimming organs. Behind these is the much smaller second pair of antennae. The front of the head often juts forward like a small beak. The thorax has six segments with the first legs (maxillipeds), the second, third, and fourth legs (swimming legs), and the fifth legs. The last pair are small and may be greatly modified in males for the transfer of sperm packets (spermatophores) during mating and are important in family and generic identification. Mature females can be distinguished by a swollen genital segment (1st urosomal segment). The abdominal segments usually are without appendages, except the caudal (terminal) segment which bears two rami (tails) that can end in long setae. Copepods are typically very small, 1�2 mm long and their exoskeleton is typically transparent. Identification to species level can be difficult, particularly for early developmental stages and many parasitic species that are so modified that they are barely recognisable as crustaceans except as larvae.

Distribution and diversity

The copepods are the most abundant and species-rich group of crustaceans and are found worldwide including in the polar regions. Globally there are over 24,000 species inhabiting all oceans, seas, estuaries, rivers and lakes with a handful of species found in moist forest litter. They are considered to be the most numerous metazoans on earth and are the dominant form of marine plankton. There are many hundreds of Australian species including numerous endemic species from inland waters and groundwater.

Life cycle

Parthenogenesis and sexual reproduction are both known in copepods. During mating, the typically smaller male copepod grasps the female with his modified first antennae and deposits the spermatophores into seminal receptacle openings. Females of some species carry their eggs for a short time in a brood pouch on the front part of the thorax until ready to hatch, while other species release theirs freely into the water once fertilised. They hatch to nauplius larva which are small, oval and unsegmented with three pairs of appendages; antennules, antennae and mandibles. The larval phase consists of 6 naupliar stages and 5 copepodid stages before the adult stage is reached. Copepodids moult between each stage and resemble the adult but are smaller and lack the full number of biramous swimming legs. The development may take from less than one week to as long as one year and the life span of a copepod ranging from six months to a year. Under adverse conditions, and in response to factors such as population density, temperature and photoperiod, some copepod species can produce thick-shelled dormant or resting eggs (cysts) that can withstand desiccation. Most cysts remain in the sediment until hatching, although a small percent of the diapausing individuals may stay in the plankton, a so-called �active diapause�. Cysts can be dispersed by water, wind or by birds and other animals.

Feeding

Most free-living copepods are filter-feeders that generate a water current by rapidly moving their legs over their second maxillae (part of their mouthparts) that actively collects food particles such phytoplankton, nauplii, rotifers and other small invertebrates. Several larger species of copepods are predators of small invertebrates using maxillipeds armed with sharp spines. A large number of copepod species are endo- or ectoparasites on almost every phylum of aquatic animal, attaching themselves to fish, sharks, marine mammals, and many kinds of invertebrates such as molluscs, tunicates and corals in a range of marine and fresh water environments.

Ecology

Copepods may be free-living, symbiotic, or endo- or ectoparasitic and occur in a wide range of salinity and temperatures levels in just about every available aquatic habitat on earth. Species can be found in subterranean caves and other groundwater, water-filled bromeliads and pitcher plants, wet leaf litter and damp moss on the ground and the water and sediments of swamps, streams, rivers, ephemeral water-bodies, lakes and the open ocean. Free-living copepods are active swimmers, beating the first antennae and swimming legs to propel themselves through the water. Their abundance and diversity make copepods an ecologically important group in the food webs of both surface and underground fresh waters. Freshwater species have been used successfully in the biological control of disease-bearing mosquitoes (Insecta: Diptera) in water containers where they eat the1st and 2nd instar larvae. In untreated water in some countries, a correlation has been found between copepod abundance and cholera, because the cholera bacteria attach to the surfaces of planktonic animals, including copepods. Parasitic species that damage fish skin and gills are commercially significant pests in marine and freshwater fish farms.