Growth of a Sea Urchin, Allocentrotus fragilis, off the Oregon Coast

Abstract

Allocentrotus fragilis (Jackson) was obtained from six stations at
\ndepths of 100 to 1,260 m on the continental shelf and upper slope off Newport,
\nOregon.
\nAges and growth rates of A. fragilis were determined by two methods: (1) from
\nsize-frequency distributions of trawl collections from 200 m, and (2) from growth
\nzones on skeletal test plates. Collections from other depths were not adequate for
\nsize-frequency analyses. Gonad indices of A. fragilis from 200 m were used to
\ndetermine spawning periodicity and frequency. A semiannual frequency was suggested,
\nwith spawning occurring in early spring and early autumn. No individuals
\ncollected below 400 m were reproductively mature. A procedure was developed to
\nmake growth zones of the skeletal test plates visible. Dark growth zones are
\nthought to correspond to semiannual periods of growth, one-half the number of
\ndark growth zones indicating the urchin's age.
\nThe growth curve of A. fragilis from 200 m, which was plotted from the mean
\ntest diameter of age groups defined by test plate growth zones, shows a good least-squares
\nfit to von Bertalanffy's growth equation. Growth rates determined from plate
\ngrowth zones appeared to be similar for A.fragilis from 100 to 600 m, but decreased
\nfor specimens from 800 to 1,260 m. The asymptotic size decreased with increasing
\ndepth below 200 m.