Skeletal growth bands in brittle stars: microstructure and significance as age markers

Abstract

Skeletal growth banding has been studied in the vertebral arm ossicles of the shallowwater brittle stars Ophiothrix fragilis, Amphiura filiformis, A. chiajei, Ophiura ophiura and O.albida collected from near Oban. Scanning electron microscopy of ossicle microstructure show the bands reflect differences in stereom porosity and surface relief on the surface of the wing-like fossae of the ossicle. The evidence supports the interpretation of the bands as annual growth markers, with coarse-pored stereom laid down during periods of active growth in summer/autumn whilst finer-pored stereom, that often forms ‘breaking wave’- like ridges, is formed during periods of slow or negligible skeletal growth during the rest of the year. Growth curves fitted to growth ring measurements indicate three contrasting growth strategies amongst the species examined. These data generally support recent interpretations of demographic structure reached from studies of size frequencies in samples collected from other areas in the NE Atlantic range of these species.