Population dynamics, growth and reproductive rates of Littorina nigrolineata Gray from a moderately sheltered locality in North Wales

Abstract

The population dynamics of Littorina nigrolineata Gray on a moderately sheltered boulder shore was characterized by an almost constant mortality rate of post-juvenile snails both within and between years. Cohorts of post-juvenile snails had a “half life” of about 5 months. Juvenile snails had higher mortality rates than post-juveniles. Mortality rate was independent of size in post-juveniles. The population of post-juvenile snails fluctuated only about two-fold during the 3 yr of study. Maxima of population size occurred each September as a result of an autumnal pulse of recruitment into the post-juvenile size classes. These recruits originated from eggs laid in the summer of the previous year. The size-frequency structure of the population lacked well defined modes except during the spring and summer when the pulse of recruits formed a distinct mode, which later disappeared due to growth and mortality of the recruits. Oviposition continued throughout the year with a pulse in summer. Newly laid eggs took 4–7 wk to hatch. The hatchlings had a mean shell height of 0.5 mm and were estimated to take 6–9 months to grow to a shell height of about 4 mm. Growth increments over periods of 3 months. estimated from tagged cohorts of snails ranging from 3 to 16 mm, released on a stone jetty, revealed seasonal and size-specific changes in growth rate. Growth rate was significantly lower between December and March than at other times of the year. Seasonal differences in growth rate did not, however, significantly affect the yearly size increments of snails hatching in summer or winter. Growth increments did not decline linearly with increasing body size, as is assumed by the von Bertalanffy growth model, but increased to a maximum at about 6–7 mm shell height. Beyond the maximum, the growth increments decreased linearly with increasing shell height. This non-linearity caused an inflexion in the growth curve, so that the shell height. This non-linearity caused an inflexion in the and 12.4 mm by the von Bertalanffy growth curve. The deviation from the von Bertalanffy growth model was possibly due to departures from the assumptions that the assimilation rate is proportional to the square of a linear size dimension and that the metabolic rate is proportional to the cube of the same dimension. A von Bertalanffy growth curve, fitted to growth increments measured from “growth checks” on tagged snails on the boulder shore, predicted slightly faster growth rates than the von Bertalanffy growth curve fitted to increments of mean cohort size on the stone jetty. The closeness of the two curves, however, suggested that both methods of measuring growth increments were tolerably accurate.