Predicted effects of a hypothetical oil spill on the saltmarsh crab Sesarma catenata

The effect of Qatar Light crude oil on the saltmarsh crab Sesarma catenata was investigated. These crabs are well adapted to the estuarine environment and mainly occur in the zone dominated by the saltmarsh grass Spartina maritima. They have an excellent aerial respiration and water-saving system and their burrows do not penetrate to water level. Static bioassays conducted showed that adult crabs are fairly resistant to the water-soluble fraction (WSF) of the crude oil, but that larval survival is affected at a much lower WSF concentration. Many of the crabs were, however, affected during the bioassays and may be considered as ‘ecologically’ dead.

In the event of spilled oil entering the estuary it is predicted that oil would form a layer over the water contained in the burrows. Although oil initially evaporates quickly, the layer of oil would drastically retard the diffusion of oxygen into the water, while some of the oil would dissolve into the water. It is estimated that the concentration reached at the end of a tidal cycle would not produce mortality, but would affect the crabs' respiration. Because of the small burrow volume, the crabs would be expected to utilize all the oxygen in the burrows before the next high tide. However, before that occurred, and at a critical oxygen saturation, the crabs would be expected to leave the water and climb through the layer of oil. Most crabs that return to clean water immediately, would probably not be further affected, except that the oil in their branchial chambers would reduce the area available for oxygen exchange. In some crabs, carapace contamination induces moulting, which in turn would make them more sensitive to oil pollution. The crabs would avoid polluted sediment, but might be attracted to very low and very high concentrations. Crabs remaining in the oil would clearly soon die of the combined effect of toxicity and oxygen depletion. Oil occurring on the saltmarsh surface would be degraded relatively fast, but once it seeps into the anaerobic mud, degradation virtually ceases. Experimentally, the hatching success ofgravid females, normal development and size of larvae spawned is not affected by Qatar Light crude oil, but their photopositive response is affected. The dangers of transferring oil from damaged tankers in Algoa Bay is discussed and recommendations made.