Reproduction, recruitment, growth and mortality of co-occurring species of the intertidal limpet Siphonaria with contrasting reproductive modes

Abstract

The pulmonate limpets Siphonaria concinna and S. serrata co-exist intertidally on many South African rocky shores. The two species are similar morphologically and in terms of their intertidal habitat and distribution; both lay gelatinous egg masses on the shore, but have different reproductive strategies. Siphonaria concinna has planktotrophic development involving high fecundity and small eggs. Field observations over 2 years showed that the majority of spawning occurs during summer. A discrete recruitment event follows an estimated planktonic period of 2 months. Siphonaria serrata has direct development with large eggs and lower fecundity. Spawning is seasonal, although egg masses can be found throughout the year. Newly hatched juveniles can only be detected after ∼2 months of growth. A single juvenile cohort follows ∼4 months after spawning. A comparison of the life-history traits of the two species revealed that S. concinna reproduces at a smaller size and juveniles grow faster but have a higher mortality rate than S. serrata juveniles. Adult growth rates for S. concinna are slower and likewise show higher mortality; maximum adult size is smaller and the lifespan is estimated to be shorter than that of S. serrata. Accordingly, the life history of S. serrata is characterised by reproduction at a larger size, and juveniles that grow slower and have a lower mortality rate than S. concinna juveniles. Adults grow to a larger size, suffer lower mortality and probably live longer. These life-history attributes correspond to the classic r- and K-selected traits.