WINDECKER, L*; JOHNSON, AS; ELLERS, O; Bowdoin College; Bowdoin College; Bowdoin College: Size-specific effects of diet during rapid growth in urchins.

Understanding size-specific growth in the urchin Strongylocentrotus droebachiensis contributes to understanding basic mechanisms of growth and to maintaining the urchin fishery. A size range of urchins (6.5-66.2 mm diameter) were collected from a Maine urchin barren in summer. Urchins were assigned at random to 1 of 5 treatments: an initial group, unfed (in filtered sea water or in unfiltered flow-through sea water) or fed (mussel flesh or salmon feed pellets). To determine whether there was skeletal growth, all urchins were injected with tetracycline. For urchins in which gonad index (GI) was independent of size (those over 45 mm diameter), we observed a significant 300% increase in the gonad index from 4% in an initial sample to 12% in urchins fed mussels or salmon feed. There was no difference in GI between feeding treatments. In contrast, unfed control groups did not experience an increase in GI. ANCOVA revealed that 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. For fed urchins, smaller urchins grew much faster than did larger urchins. For example, the fastest growing urchin grew by 76% by weight per 2 weeks and was 8.8 mm diameter . The larger urchins (>55 mm) typically grew about 1-3% by weight . Skeletal plates were loosely attached in the well fed urchins but tightly attached in the unfed groups and in an initial control 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% percent fluoresced faintly, and the rest did not fluoresce. These well-fed urchins grew much faster than do field populations probably due to food limitation in the wild.