Graduate Thesis Or Dissertation
 

Growth of a sea urchin, Allocentrotus fragilis, at different depths off the Oregon coast

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s7526g11h

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  • Allocentrotus fragilis (Jackson) was obtained from six stations at depths of 100 to 1260 m on the continental shelf and upper slope off Newport, Oregon. Ages and growth rates of A. fragilis were determined by two methods: 1) A procedure was developed to make growth zones of the skeletal test plates visible. Alternating light and dark growth zones were found to be formed semi-annually. The total number of growth zones was used to indicate the urchin's age. 2) Age and growth rate values were also determined from analyses of size-frequency distributions of trawl collections from 200 m. Collections from other depths were not adequate for size-frequency analyses. Gonad indices of A. fragilis from 200 m were used to determine spawning periodicity and frequency. A semi-annual frequency was found, with spawning occurring in early spring and early autumn. No A. fragilis specimens collected below 400 m were reproductively mature. The growth curve of A. fragilis from 200 m, which was plotted from the mean test diameter of age groups defined by test plate growth zones, shows a good least-squares fit with von Bertalanffy's growth equation. Growth rates were similar for A. fragilis from 100-600 m, but decreased for specimens from 800 and 1260 m. The maximum test diameter decreased with increasing depth below 200 m. Variation of magnesium content of the calcareous skeletal plates was due largely to variation with age. Little skeletal Mg variation was found at different depths for specimens of the same age. Greater Mg content differences occurred between young and old specimens from the same depth, and between young and old test plates of the same individual. The effects of several environmental factors on the growth rate and maximum size of A. fragi1is are discussed. Of these factors, food availability, water temperature, and oxygen tension form gradients with depth or distance offshore; and were considered to be important in affecting the growth of A. fragilis.
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