Article: Ecology and population structure of the Recent brachiopod Terebratulina from Scotland

The ecology and population structure of the Recent articulate brachiopod Terebratulina retusa (Linnaeus) are described. The population studied occurs around the margins of a depression of more than 220 m in the Firth of Lome, Scotland, and is predominantly attached to the horse-mussel Modiolus modiolus (Linnaeus). Spawning occurs regularly in late spring and late autumn, and is initiated at temperatures of 10-11°C. The highly synchronized reproductive cycle, from spawning to spatfall, occurs within 3 weeks in nature. Length-frequency histograms prepared from large representative samples collected at regular intervals during 1977-1979 are unimodal and right-skewed due to the predominance of juveniles. Regularly spaced subsidiary peaks in the histograms correspond to biannual settlement cohorts; in later life successive peaks merge to form a single annual peak. This pattern is identical to that predicted by computer-based simulations. Recently settled specimens grow rapidly to an average length of 2.75 mm within 3 months during both spring and autumn; thereafter the animals grow (initially by 4 mm per year) throughout life, although at a progressively reducing rate from the third year of life onwards. Growth slows or ceases in winter in all but recently settled specimens. The maximum life span is 7 years. The mortality rate remains constant, although the causes of death are not apparent. The growth-lines form biannually, at times of pronounced environmental and physiological disturbance.