Comparative Demography of Littorina rudis, L. Nigrolineata and L. Neritoides on Three Contrasted Shores in North Wales

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Abstract
A possible relationship between reproductive mode and demography was sought between species of Littorina that brood young, lay benthic egg masses and broadcast planktonic larvae. Population size-frequency and density and cohort survivorship were monitored for L. rudis on a sheltered saltmarsh, L. rudis and L. nigrolineata on a boulder shore and for L. rudis and L. neritoides on an exposed cliff face. Adult size classes formed a single broad peak and, except in L. neritoides, young recruits formed a 2nd peak each summer. In the saltmarsh, L. rudis population density fell from a maximum after summer recruitment to a minimum in spring; this was less evident in L. rudis and L. nigrolineata on the boulder shore and undetectable in the cliff populations of L. rudis and L. neritoides. Spring densities fluctuated between years by maximum factors of 7, 6, 9, 9 and 2 in saltmarsh L. rudis, boulder shore L. rudis, cliff L. rudis, L. neritoides and L. nigrolineata, respectively. There was no evidence of density dependence in any of the population trajectories. Artificial crevices increased the local density of L. rudis and L. neritoides on the cliff by attracting snails seeking shelter, but did not affect the total population size. Mortality rate diminished progressively through the juvenile phase to a constant rate in adults. Generation times were 24, 18, 18, 25 and 41 mo. for the saltmarsh L. rudis, boulder shore L. rudis, L. nigrolineata, cliff L. rudis and L. neritoides, respectively. Age-specific reproductive value increased beyond the age of 1st sexual maturity and declined only when growth had almost ceased. All populations except that of L. nigrolineata had values of R0 [net reproductive rate] < 1. Isoclines of juvenile vs. adult survivorship for R0 = 1 were used to assess the reliability of the observed survivorship values. Apart from a possible advantage of oviparity vs. ovoviviparity during phases of population expansion, no relationship between demography and reproductive method could be found. Oviparity may be advantageous where egg survivorship is high and ovoviviparity is low. Broadcasting of planktonic larvae by L. neritoides may be correlated with the temporal and spatial unpredictability of microhabitat quality between shores.