Siphonostomatoid copepods: (1) Fish lice – caligids

Members of the Crustacea, which includes the copepods, have a modular construction like leeches, their bodies consisting of a repetitive series of segments. However, crustaceans are arthropods, characterised by possession of an external skeleton and jointed limbs. Each segment possesses one pair of limbs, and in crustaceans each limb basically consists of two branches (biramous). The inner branch is termed the endopod and the outer branch the exopod, and the separate units of a jointed limb are referred to as ‘podomeres’ or as ‘articles’ (the former term will be used in this book). No modern crustacean can be found in which individual segments and their biramous limbs can be identified from one end of the body to the other, because fusion of segments has occurred to varying extents, together with modification or loss of segmental limbs. Typically arthropods have segments fused together or regionally grouped to form head, thorax and abdomen, but the divisions between these regions may be obscure. Groups of fused segments are referred to as tagmata (singular: tagma). Copepoda is a relatively small group of arthropods with currently about 11,500 valid species worldwide (Boxshall & Halsey, 2004). However, in terms of numbers of individuals, free-living copepods are enormously abundant. In marine muds and sands they are a major component of the interstitial fauna living between sedimentary particles, but it is in the marine and freshwater pelagic realms that they dominate. A single sweep with a fine net, either in the sea or in freshwater, is likely to collect large numbers of planktonic copepods. In the shallow waters of the North Sea, for example, they may reach 70,000 per cubic metre and even at depths of 4000 m in the North Atlantic may persist at densities of 100 or so per cubic metre (references in Huys & Boxshall, 1991). In fact, based on his studies of marine plankton, Sir Alistair Hardy (1970) claimed that in terms of absolute numbers of individuals, the copepods are the most numerous multicellular animals on the planet. In his estimation they are likely to outnumber the insects and the nematodes, both of which have been nominated for this position.