Age, Growth and Shape of the Intertidal BrachiopodWaltonia inconspicuaSowerby, from New Zealand

Abstract

Using large samples from six localities in New Zealand, the population structure and longevity of the intertidal brachiopod Wahonia inconspicua Sowerby has been established. The population is one containing two modes. The first year group extends up to 3.0 mm shell length and together with the second year group constitutes the first mode of the length distribution histogram. Reproduction commences at approximately 10 mm length or during the fourth year and at this stage the second mode is formed by the overlap of year groups of mature animals. In Otago Harbour W. inconspicua was found to live up to eight years and occasionally survive for as long as 15 years. Growth is slow and unevenly distributed through life, being fastest during the first four years and decreasing after reproduction commences. Two years later, at approximately 15 mm shell length there is a further reduction in growth rate, and when the animal reaches 17 mm length it slows yet again and becomes very small in senescence. However, egg production at this stage is very high being greater than 18,000 per year. The rate and type of growth varies considerably between localities. Rough water conditions cause slower growth and more gibbous form, and phenotypic stunting appears to have occurred.