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.

Ecology of marine meiobenthos

St. Lucia Estuary on the Zululand coast, which once had a rich fauna of prawns and fish, has recently deteriorated. An ecological survey has shown that this is due to silting at the mouth and very saline water in the upper lakes. These conditions are traced back to droughts, erosion and man's interference with the filtering action of swamps. The present fauna and flora are described and plans for improving the estuary are discussed.

The ecology of south african estuaries

Specimens of Chlamys opercularis, Modiolus modiolus, Mytilus edulis, Crassostrea gigas, Scrobicularia plana and Mya arenaria were exposed to both gradual (sinusoidal) and abrupt (square-wave) salinity fluctuations and measurements made of osmotic, Na+, Mg2+ and Ca2+ concentrations in the hemolymph and where applicable in the mantle fluid. In both sinusoidal and square-wave regimes fluctuating between 100 and 50% seawater (100%=ca. 32‰ S), the hemolymph Na+, Mg2+, Ca2+ and osmotic concentrations followed the concentrations of the external medium in Chlamys opercularis. The hemolymph and mantle fluid osmotic Na+, Mg2+ and Ca2+ concentrations of Modiolus modiolus, Mytilus edulis, Crassostrea gigas and S. plana followed those of the external medium as long as the molluscs' shell valves remained open. There were no changes in the ionic or osmotic concentrations of the hemolymph or mantle fluid of any of these species during periods of shell-valve closure. The hemolymph osmotic, Na+ and Mg2+ concentrations of wedged-open Modiolus modiolus, Mytilus edulis, C. gigas and S. plana followed those of the external medium. Hemolymph Ca2+ concentrations showed a damped response in C. gigas and Mytilus edulis. The hemolymph osmotic, Na+, Ca2+ and Mg2+ concentrations of Mya arenaria fluctuated in a similar manner to the external medium, but were damped. Wedged-open Mytilus edulis exposed to fluctuating salinity and supplied with a constant supply of 10 mM Ca2+ showed greater changes in hemolymph ionic and osmotic concentrations than M. edulis exposed to the same salinity fluctuation without a constant Ca2+ supply. Chlamys opercularis and Modiolus modiolus survived in a 50% seawater minimum sinusoidal salinity fluctuation for 10 days; wedged-open M. modiolus survived only 3 days. Burrowing had no effect on the osmotic, Na+, Mg2+ or Ca2+ concentrations of the hemolymph of Mya arenaria or S. plana exposed to fluctuating salinities. All of the species studied were shown to be osmoconformers.

Effect of salinity fluctuation on the osmotic pressure and Na + , Ca 2+ and Mg 2+ ion concentrations in the hemolymph of bivalve molluscs

Thirteen estuaries in the Eastern Cape Province, South Africa, were assigned to one of four types of estuaries (river-dominated permanently open, marine-dominated permanently open, medium-sized temporarily open/closed and small temporarily open/closed), and macrobenthic zonation patterns were compared using multivariate statistics. Three major habitat zones were identified: two relating to substrate (a sand zone and a mud zone), and a third zone characterised by water of low salinity (≤2.2). These zones are inhabited by four major groups of macrobenthos. The distribution of two of these (estuarine endemic sand fauna and estuarine endemic mud fauna) is limited by the nature of the substratum, whereas the distribution of the other two (fauna originating from the marine habitat and oligohaline fauna) is limited by salinity. Estuarine endemics were present in all four types of estuaries. Marine fauna was numerically important only in permanently open systems, while oligohaline fauna was abundant only in the upper reaches of river dominated open systems. Medium-sized temporarily open/closed estuaries lacked both marine and oligohaline forms, but these were present to a limited extent in small temporarily open/closed estuaries. Hence, salinity is not the primary environmental variable determining zonation patterns in Eastern Cape estuaries, and particularly in temporarily open/closed estuaries, it is of minor importance.

What limits the distribution of subtidal macrobenthos in permanently open and temporarily open/closed South African estuaries? Salinity vs. sediment particle size

R. Bassindale

Papers

A Comparison of the Varying Salinity Conditions of the Tees and Severn Estuaries