Age, Growth and Shape of the Intertidal Brachiopod Waltonia inconspicua Sowerby, from New Zealand

Using large samples from six localities in New Zealand, thepopulation structure and longevity of the intertidal brachiopodWahonia inconspicua Sowerby has been established. The populationis one containing two modes. The first year group extends upto 3.0 mm shell length and together with the second year groupconstitutes the first mode of the length distribution histogram.Reproduction commences at approximately 10 mm length or duringthe fourth year and at this stage the second mode is formedby the overlap of year groups of mature animals. In Otago HarbourW. inconspicua was found to live up to eight years and occasionallysurvive for as long as 15 years. Growth is slow and unevenly distributed through life, beingfastest during the first four years and decreasing after reproductioncommences. Two years later, at approximately 15 mm shell lengththere is a further reduction in growth rate, and when the animalreaches 17 mm length it slows yet again and becomes very smallin senescence. However, egg production at this stage is veryhigh 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 gibbousform, and phenotypic stunting appears to have occurred.