The systematics and ecology of free-living marine nematodes

Nematodes are the most abundant metazoans in marine (littoral, estuarine, coastal and oceanic) sediments, extending from the high-water mark into the deepest oceanic trenches (Nicholas, 1975). All marine free-living nematodes are considered to be members of the meiobenthos; small organisms, mainly metazoans, which are separated from the larger macrobenthos either on a methodological basis (i.e. as all animals passing a 1 mm or a 0.5 mm sieve) or a taxonomic basis (i.e. particular animal groups such as Nematoda, Harpacticoida, Gastrotricha, Kinorhyncha, Tardigrada, Foraminifera etc. which consist exclusively or mainly of small species living in sediments). In most circumstances, nematodes are numerically the dominant group of the meiofauna. In fact, they usually comprise more than 90% of the metazoan fauna (McIntyre, 197 1). That nematodes are ecologically a very successful group is also demonstrated by their high species diversity. The number of species present in any one habitat is usually an order or magnitude greater than for any other major taxon (Platt & Warwick, 1980). In all, about 4000 species of free-living marine nematodes, belonging to some 450 genera, have been described to date, but many more remain to be discovered. The ecology of marine meiofauna has been reviewed by Swedmark (1964), McIntyre (1969), Coull (1973) and Fenchel (1978). Nicholas (1975) and Platt & Warwick (1980) have considered some general aspects of the biology of marine free-living nematodes. Ecological research involving nematodes was restricted at first to treating nematodes as a single taxonomic unit of the meiofauna, also considered to be a functional unit. We know now that nematodes are ecologically very heterogeneous and that they occupy different trophic positions in benthic food webs. These differences may be as large between families of marine nematodes as they are between orders in macrobenthic groups. This trophic diversity was first examined by Wieser (1953a) who divided nematodes into four groups according to the structure of the buccal cavity, by postulating that morphological differences are linked to different feeding mechanisms: selective deposit-feeders; non-selective deposit-feeders; epigrowth feeders; predators and omnivores. The validity of this classification has been questioned since several important exceptions exist (Wieser, 1960, Boucher; 1972; Ward, 1975), but Wieser's proposition has remained an important tool in the interpretation of nematode assemblages. However, more direct methods have become available and in the last decade important ecological and systematic advances have been made. Experimental work involving the cultivation of species in the laboratory started in the late sixties and permitted much new information to be obtained on different aspects of nematode ecology. Field work has progressed both in methodology and in the number of habitats and geographical areas which have been investigated. In systematics, the number of people involved was higher in the seventies than ever before and about 42 papers were published each year in the period 1971-1975 by about 50 marine nematologists, describing about 11 1 new taxa yearly (Gerlach, 1980). In this paper we attempt to synthesize research on the systematics and ecology of marine free-living nematodes. It is not our aim to evaluate all papers published in the last decades but rather to give an overview of current research and problems