Red sea urchins can live over 100 years: confirmation with A-bomb [14.sup]carbon - Strongylocentrotus franciscanus

Fishery Bulletin, Oct, 2003 by Thomas A. Ebert, John R. Southon

Results

Of the 1582 tag recoveries from all sites, 739jaws showed a growth increment, [DELTA]J, of [less than or equal to]0.02 cm and of these only 13 had a labial measurement >0, which is at the end of the jaw at the mouth opening. The smallest nonzero measurements were 0.001 cm and therefore growth less that this was recorded as 0; 54 sea urchins in the sample had clear tetracycline marks but 0 measurable growth. For large jaws, the measured labial component was too small to be measured and therefore all of the calculated [DELTA]J since the late 1950s was milled from the esophageal end of the jaw only.

Tetracycline tagging indicated that annual jaw growth (Fig. 1) was very slow for large sea urchins and many individuals showed annual increments of less than 0.01 cm. The resulting growth curve of jaw length as a function of age (Fig. 2A) showed that at least some large individuals would be expected to have ages in excess of 100 years. If this age estimate is correct, a drop in [14.sup]C should be found in successive small slices removed from large jaws, which would first show current [14.sup]C levels and then drop to pre-bomb levels. Because the Halftide Rocks samples were collected in 1992 we used [DELTA]t = 35 years, which would go back to 1957. Using Equations 1-3, growth parameters given in Fig. 1, and [DELTA]t = 35 years, we estimated the increment to be between 1 and 2 mm for jaws between 2.5 and 2.6 cm (Fig. 2B).

[FIGURES 1-2 OMITTED]

Successive milled samples from the esophageal ends of large jaws (Fig. 3, A-D) showed a precipitous drop in radiocarbon to prebomb levels over 1-2 millimeters, in agreement with predictions. Variations across replicates and samples probably are the result of differences in the width of milled samples and an inability to remove all recently deposited calcite or to follow the exact growing edge of the jaw with the milling machine. Smaller jaws (Fig. 3, E-G) were not expected to show a prebomb signature, and indeed they did not. They do, however, indicate the [14.sup]C level to be expected in recent skeletal material and emphasize the rapid change in radiocarbon shown in large jaws. Changes in 14C in successive milled samples in jaws E-G are similar to changes shown in coral samples from the Galapagos (Guilderson and Schrag, 1998) and may indicate that [14.sup]C levels in surface waters in regions of strong upwelling were still rising when sea urchin were collected in 1992. The conclusion is that [14.sup]C analysis supports the age estimates based on tetracycline tagging and use of the Tanaka function: large red sea urchins are old and may have ages of 100 years or more

[FIGURE 3 OMITTED]

Discussion

The largest reported red sea urchins, with body diameters over 19 cm, are from British Columbia, Canada, (Bureau, 1996) and with estimated jaw lengths of about 2.8 cm would be expected to be around 200 years old (Fig. 2A). Age estimates of 100 years far exceed estimates of life span for other sea urchins (Table 1) based on growth lines in ossicles. Natural growth lines, however, tend to underestimate ages of old individuals because very small increments will have alternations of dark and light areas that are difficult or impossible to resolve and hence counts underestimate age (Ebert, 1988). For example, the maximum age estimate for Strongylocentrotus droebachiensis, the commercial species of the U.S. east coast, is 25 years by counts of growth lines (Robinson and MacIntyre, 1997) but at least twice this if tagging and size structure (Russell et al., 1998) are used. Similarly, tagging and size structure of Evechinus chloroticus (Lamare and Mladenov, 2000) have indicated survival rates similar to S. franciscanus but the maximum number of growth lines reported was only 10 (Dix, 1972). Survival rate, however, is not a fixed parameter for a species and there is local variation, as well as geographic patterns, evident in the survival rate for S. franciscanus (Ebert et al. 1999).