Relationship between sediment texture,species richness and volume of sediment sampled by a grab

This paper discusses the problems involved in obtaining grab samples for direct comparison of the respective benthic fauna, using information from a survey conducted across the South African Continental Shelf below the Benguela Current. Many factors influence the depth of grab penetration into the sediment and, hence, the grab sample volume. One of the most important of these factors is sediment texture. While this fact has been long recognised, most workers have attached little significance to it. It is shown here that an exponential relationship exists between the grab sample volume and sediment texture, until the minimum percentage of silt plus clay that will give a maximum grab sample volume is reached. This relationship only extends to a certain depth, in this case to 280 m. There are more species per unit number of specimens (“species richness”) in association with sand or muddy-sand than with mud. A linear relationship is given between the grab sample volume and species richness between the depths of 280 and 440 m, inclusive.