Comparative salinity tolerances of four siphonariid limpets in relation to habitat restriction of the rare and endangered Siphonaria compressa

  • G Wilson
  • GM Branch
  • P de Coito

Abstract

The salinity tolerances of four South African species of limpet in the genus Siphonaria were experimentally tested, in the context that one of them, S. compressa, is South Africa’s most endangered marine mollusc and is restricted to two lagoonal localities that normally experience little variation in salinity. Its habitat is additionally restricted in that it occurs only on the eelgrass Zostera capensis. The other three species, S. capensis, S. concinna and S. serrata, are abundant and widespread on temperate rocky shores. We hypothesised that S. compressa would have limited tolerance to low salinities, contributing to its absence from otherwise suitable estuaries that experience regular fluctuations of salinity. The percentage survival of the limpets was recorded in salinities ranging from 2 to 35 over that period. Both salinity and species significantly affected the survival of the limpets over the 96 hours. Survival was highest in salinities close to those of normal seawater, and greatly reduced in the hypo saline waters. S. compressa had the lowest salinity tolerance of the four species, its time-to-50% mortality in hyposaline waters being significantly less than that for the other three species, which differed in a manner that could be correlated with their zonation patterns. Restriction to eelgrass, coupled with low salinity tolerance, probably explains why S. compressa is found at only two localities, both of which normally experience salinities close to that of seawater. Even there, mass mortalities of S. compressa have been recorded in association with die-backs of eelgrass after abnormal freshwater flooding or sedimentation — further emphasising the vulnerability of S. compressa to extinction.

Keywords: conservation; range restriction; rarity; salinity; Siphonariidae

African Journal of Marine Science 2009, 31(3): 311–318

Author Biographies

G Wilson
Marine Biology Research Centre, Department of Zoology, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
GM Branch
Marine Biology Research Centre, Department of Zoology, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
P de Coito
Marine Biology Research Centre, Department of Zoology, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X