Mechanisms of size-specific growth in the sea urchin, Stronyylocentrotus droebachiensis

by Laura Anne Windecker

Understanding of size-specific growth in the urchin Strongylocentrotus droebachiensis contributes to understanding basic mechanisms of growth and to maintenance of the urchin fishery. For studies of size-specific growth rates, a size range of urchins (diameter: 6.5-66.2mm) was collected from a Maine urchin barren in the summer. For two-week growth experiments, urchins were assigned to either an initial control, unfed (in filtered seawater or in unfiltered flow-through seawater) or fed (mussel flesh or salmon feed pellets) group. Size-specific fractional weight change was greater for fed urchins (mean- 14.6%) than for unfed urchins (mean= 2.0%), but did not differ between feeding treatments. Skeletal plates were loosely attached in the well fed urchins but tightly attached in the unfed groups and in the initial group. All skeletons of fed urchins fluoresced brightly, indicating that skeletal growth had occurred. In contrast, in the unfed group 31% of the skeletons fluoresced brightly, 54% fluoresced faintly, and the rest did not fluoresce. For urchins in which gonad index (GI) was independent of size (those over 45mm diameter), we observed a significant 300% increase in GI from 4% in an initial sample to 12% in fed urchins. For the 38-week growth (long-term) experiment, a size range of urchins were randomly divided into either a fed (salmon feed pellets) or minimally fed group. Growth was greater for fed urchins than minimally fed urchins. Percent and absolute weight gain was greater for smaller urchins. The von Bertalanffy growth model fits this data better than the Tanaka model, indicating that this population of urchins experiences asymptotic growth. Rapid short-term growth results can be sustained for smaller urchins, but not for larger urchins. Finally, preliminary investigations into mechanisms of growth through materials testing of spinal ligaments showed that Triton-X stiffened the ligaments of S. droebachiensis indicating the presence of mutable collagenous tissue (MCT) in ligments.