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

Red sea urchins (Strongylocentrotus franciscanus) along the west coast of North America, like most large sea urchins in temperate waters worldwide, are the focus of a commercially important fishery. In a review of biological data for purposes of fishery management, the life span of red sea urchins was suggested to be 7-10 years (Sloan, 1986) and they have been included with much shorter-lived species for illustrating complex population dynamics (Hastings and Higgins, 1994). Recent work with tetracycline and calcein tagging (Ebert, 1998; Ebert et al., 1999), however, has shown that individuals continue to grow throughout life, although at a very slow rate, and large individuals are estimated to be in excess of 100 years old. A potential problem with the studies using tetracycline and calcein is that one-year time intervals were used between tagging and recapture and therefore it is possible that occasionally there may have been very good years for growth that were missed. If occasional growth spurts occurred, largest sizes would have been attained in much less than 100 years. The potential problem of missed good years for growth could be resolved with a marker that captures a longer period of time. The accuracy of age estimates has consequences for resource management where size limits may need adjustment in order to protect older individuals (Hilborn and Waiters, 1992; Congdon et al., 1994 Ebert, 1998). There is also the need to understand the evolution of life histories of species where long life tends to be an indicator of uncertainty in individual reproductive success (Murphy 1968; Roff, 1992; Stearns, 1992).

Enhanced radiocarbon in the oceans due to atmospheric testing of nuclear weapons that began in the 1950s (Nydal and Lovseth, 1983; Broecker et al., 1985, Duffy et al., 1995) provides a permanent marker in carbonate-based skeletal elements that are no reworked by resorption and deposition during growth and hence has a long time period between mark and recovery. The enhanced radiocarbon marker has been used in various studies to validate the periodic (usually annual) nature of growth zones in fish (Kalish, 1993, 1995; Campana, 1997; Campana et al., 2002) and invertebrates (Turekian et al., 1982; Witbaard et al., 1994 Peck and Brey, 1996) where validation by chemical tags such as tetracycline has been impractical. Red sea urchin lack interpretable growth zones (Breen and Adkins, 1976) and therefore there is no natural feature to serve as a cross check for studies using chemical tags. In the present study we present a test and confirmation of age in red sea urchins estimated from tetracycline tagging using an enhanced [14.sup]C signal in the ocean from nuclear weapon testing.

Materials and methods

Red sea urchins were tagged with tetracycline from 1989 to 1992 in northern California, Oregon, and Washington and collected after time intervals of approximately one year (details presented in Ebert et al., 1999). It is not possible to determine whether a live sea urchin has a tetracycline mark and therefore large collections had to be made. Skeletal elements were cleaned with sodium hypochlorite bleach to remove all organic material not bound in the calcite of the skeleton, and then skeletal ossicles were examined by using UV illumination to detect the tetracycline marks, which fluoresce yellow. Growth increments were measured in jaws of Aristotle's lantern of 1582 tagged-recovered red sea urchins and used to estimate growth parameters. Jaw ossicles, the demipyramids of Aristotle's lantern, are internal skeletal elements that grow around all surfaces but not equally in all directions so that a change in jaw length, [DELTA]J, is mostly at the end closest to the esophagus and there is little growth closest to the mouth, the labial end, where the teeth extend from the jaw.

The Tanaka function (Eq. 1) was used to describe growth (Tanaka, 1982, 1988) because it can model data that show an initial lag, an exponential phase with a maximum, and can include continuing growth throughout life. This function is described in greater detail elsewhere (Tanaka, 1982, 1988; Ebert et al., 1999). The usual formulation of the Tanaka model is [DELTA]size as a function of size at time t and [DELTA]t is assumed to be fixed for all individuals in the sample, usually at [DELTA]t = 1 year (Tanaka, 1982, 1988; Ebert, 1998; Ebert et al., 1999) and not included explicitly in the equation. In the present study we estimated the amount of jaw that would have to be removed to represent the time span from the time of collection in the 1990s with relatively high [14.sup]C levels to the time before atmospheric testing of atomic bombs (relatively low [14.sup]C) and therefore the Tanaka model was modified from previous uses to make [DELTA]J a function of [J.sub.j]+[DELTA]t, the size on the date of recapture rather the date of marking, which is the usual way of estimating growth parameters. Also, [DELTA]t was explicitly included as a variable (Eq. 2),