GROWTH-RATE AND SUBSTRATE-RELATED MORTALITY OF A BENTHIC BRACHIOPOD POPULATION

Vital staining and careful examination of potential substrates enabled an accurate census of Terebratulina retusa to be made and prompted a study of their population dynamics. Seasonal samples of T. retusa from a deep water Modiolus-brachiopod assemblage were always dominated by small (< 1 mm) individuals as growth rate of the post-larvae, estimated from changes in mean cohort length, was unexpectedly slow. Six months after settlement the animals had barely doubled in length, a rate of increase consistent with laboratory studies, but an order of magnitude less than conventional estimates. As the attainment of a size refuge is the only documented strategy by which articulate brachiopods counter overgrowth or disturbance this observation has profound implications for survival. Mortality of T. retusa in the Fifth of Lorn, from different substrates, followed an unexpected pattern. Virtually the only substrate on which adult T. retusa were recorded was the surface of M. modiolus shells, although juvenile T. retusa attached to this substrate suffered enhanced levels of mortality. Grazing pressures and spatial competition, believed to be reduced on complex surfaces, may account for the elevated mortality levels of M. modiolus-attached post-larvae prior to the apparent size refuge at a length of 2 mm.