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Salinity tolerance of the burrowing ocypodid crab, Paratylodiplax blephariskios, in the St. Lucia and Mhlathuze estuaries, KwaZulu-Natal, South Africa
Abstract
The ocypodid crab, Paratylodiplax blephariskios, is endemic to the southeast coast of southern Africa where it forms an important component of the estuarine benthic fauna in muddy substrata. The St. Lucia and Mhlathuze estuaries are two of the largest estuaries in which P. blephariskios occurs, and probably support the largest populations of the crab. While there is relatively little information from the Mhlathuze estuary, the maintenance of an open mouth through dredging at St. Lucia between 1970 and 1992 prevented excessive salinities from occurring in the tidal portion of this system, called the Narrows. However, the mouth was allowed to close in December 1992, after which salinities rose to 51ppt before the mouth was breached in September 1993. This period constitutes the only record of hypersaline conditions in the Narrows and coincides with a marked decline in the P. blephariskios population. Given that there are no published data on the salinity tolerance limits of species in the genus, it was decided to investigate the salinity tolerance of the crab from the St. Lucia and Mhlathuze estuaries.
Multiple regression of salinity and temperature on crab counts at St. Lucia showed significant differences between crab abundance and temperature in both hypersaline and fresh conditions, and between crab abundance and salinity in fresh conditions. Ninety-six-hour LC50 testing on adult crabs determined that they could tolerate salinities up to approximately 55ppt. Significant decreases in LC50 values were recorded at temperatures between 30ºC and 35ºC, and between 20ºC and 17.5ºC compared with the control at 25ºC. However, the near collapse of the St. Lucia crab population could not be explained simply by their acute upper salinity tolerance limit being exceeded. It was found that the onset of torpor occurred at approximately 5ppt lower than the upper acute limit and it was suggested that cessation of feeding may occur before the onset of torpor. If this was the case, it could reduce the effective lethal concentration as much as 15ppt lower than the acute LC50 and would account for the general decline in the crab population.
There was a difference of approximately 5ppt between the salinity tolerance limits of the two populations, with the Mhlathuze population having an upper limit of approximately 60ppt compared with 55ppt in the St. Lucia population. This difference was ascribed to the exposure of the Mhlathuze population to wider salinity fluctuations arising from greater tidal exchange volumes and freshwater input compared with St. Lucia, which has no major rivers entering it.
Keywords: Paratylodiplax blephariskios; Ocypodidae; Brachyura; salinity; temperature; salinity tolerance; St. Lucia estuary; Mhlathuze estuary; KwaZulu-Natal
(Afr J Aqua Sci: 2002 27(1): 21-29)
Multiple regression of salinity and temperature on crab counts at St. Lucia showed significant differences between crab abundance and temperature in both hypersaline and fresh conditions, and between crab abundance and salinity in fresh conditions. Ninety-six-hour LC50 testing on adult crabs determined that they could tolerate salinities up to approximately 55ppt. Significant decreases in LC50 values were recorded at temperatures between 30ºC and 35ºC, and between 20ºC and 17.5ºC compared with the control at 25ºC. However, the near collapse of the St. Lucia crab population could not be explained simply by their acute upper salinity tolerance limit being exceeded. It was found that the onset of torpor occurred at approximately 5ppt lower than the upper acute limit and it was suggested that cessation of feeding may occur before the onset of torpor. If this was the case, it could reduce the effective lethal concentration as much as 15ppt lower than the acute LC50 and would account for the general decline in the crab population.
There was a difference of approximately 5ppt between the salinity tolerance limits of the two populations, with the Mhlathuze population having an upper limit of approximately 60ppt compared with 55ppt in the St. Lucia population. This difference was ascribed to the exposure of the Mhlathuze population to wider salinity fluctuations arising from greater tidal exchange volumes and freshwater input compared with St. Lucia, which has no major rivers entering it.
Keywords: Paratylodiplax blephariskios; Ocypodidae; Brachyura; salinity; temperature; salinity tolerance; St. Lucia estuary; Mhlathuze estuary; KwaZulu-Natal
(Afr J Aqua Sci: 2002 27(1): 21-29)
