ECOLOGY OF MARINE MEIOBENTHOS

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Abstract
Summary: 1. The term ‘meiobenthos’ (or ‘meiofauna’) has been used in the literature in a broad sense to designate collectively small individuals, mainly metazoans, which could be separated from the larger animals by fine sieves of about i mm. or 0.5 mm. mesh. It has been variously defined in terms of the sampling or sorting technique employed. Thus used it indicates a purely statistical category including temporary members which are juvenile stages of the macrofauna, as well as permanent members –species of small adult size.2. The term has also been used in a narrower sense, referring mainly to the permanent members and restricted to particular animal groups such as Nematoda, Harpacticoida, Gastrotricha, Kinorhyncha, Tardigrada, etc. This usage designates a more natural grouping of small organisms with certain biological characteristics as well as sampling requirements in common, which distinguish them from both larger and smaller organisms.3. Special collecting and sorting methods are required for meiofauna. Small core tubes are most convenient for collecting, while sorting, often with stained samples, is usually done by elutriation, decanting or repeated sieving. Other techniques are discussed.4. In the intertidal zone total population numbers range from IIXIO3 to more than 16 × 106 per m.2 On the majority of grounds nematodes are the most numerous group, with harpacticoids second. The highest densities are usually on the softer deposits in sheltered areas.5. Subtidally the numbers on the shelf range from 4 × 103 to 3.2 × 106 per m.2 and again nematodes and copepods are the main animals, with soft deposits richest. Density declines towards deep water but even in the abyssal zone numbers range from 1.6 to 17.0 × 104 per m.2.6. Intertidal distribution of meiofauna is determined by temperature and salinity and also by the grain size of the deposit which affects the interstitial space, water content, and availability of food and oxygen.7. On intertidal muddy deposits the fauna is confined mainly to the upper few centimetres. The main controlling factor on some sediments seems to be reduced oxygen due to poor drainage, and on others the close packing of the particles leading to reduced interstitial space. This restriction to surface layers exposes the fauna to extremes of environmental conditions which in turn limit the intertidal distribution.8. On intertidal sand, with interstitial space extensive and drainage better, life can be found deeper than 1 m. below the surface and the fauna can migrate vertically to aggregate in areas of optimum conditions. Oxygen is again of prime importance, but migrations are also caused by salinity changes and by interaction between species.9. On subtidal deposits the fauna is much restricted to the superficial layers, but data on sandy grounds are sparse.10. On beaches, marked seasonal fluctuations have been observed in the meiofauna, usually correlated with temperature and reproduction. In subtidal areas these changes are much less obvious.11. The main predators on meiofauna are small fish and certain meiofauna groups such as Hydrozoa, Turbellaria and Nematoda. The meiofauna, apart from these predatory groups, feeds largely on algae and organic debris, with bacteria playing an important part in its nutrition.12. Many meiofauna species have generation times, from egg to egg production of about 1 month, and at least 2–4 generations are produced annually.13. In subtidal grounds the meiofauna is from 30–190 times greater numerically than the macrofauna but intertidally the ratios are much more varied. Interaction between the two size categories may partly explain the ratios, but the apparently greater ability of meiofauna to flourish in areas of environmental stress is perhaps also relevant.14. Certain species may be associated with particular types of deposit, but because of their small size, the distribution of meiofauna individuals should perhaps bethought of in terms of microhabitats rather than communities in the macrofauna sense. However, it may be possible to increase the precision of benthos community descriptions by including selected meiofauna species.15. A large proportion of meiofauna individuals appears to be free from predation by animals of higher trophic levels. The meiofauna thus seems to be at the top of a food chain. It will play an important part in recycling nutrients, and in some areas may compete with larger animals for food.