Growth model of the reared sea urchin Paracentrotus ... - SciViews

General introduction
method used. In addition, Levitan (1988) demonstrated that interactions
exist between adult Diadema antillarum as maximal size is densitydependent.
The second method to estimate age uses the natural growth bands. The
trabecules within the sea urchin skeleton are more or less densely packed
depending on growth rate (Pearse & Pearse, 1975). A succession of fast
and slow growth stages results in light and dark bands, respectively, in the
stereom of the ossicles (Jensen, 1969b; Pearse & Pearse, 1975; Sime,
1981; Gage, 1991, 1992; Lumingas & Guillou, 1994). It is postulated that
there is only one period of fast growth and another period of slow growth
per year. If this is true, counting these growth bands allows determining
the ages of the echinoids. If there is a single recruitment in a narrow time
window during the year (Ebert, 1983), precision is even better. Not all
authors agree with the validity of this method. Ebert (1986) questioned it
and Russell & Meredith (2000) experimentally demonstrated it is not valid
for Strongylocentrotus droebachiensis. However, Gage (1992) validated it
for Echinus esculentus with an experiment using echinoids kept in cages in
the sea during two years.
Many authors consider that if they use both methods simultaneously –
cohort separation and growth rings analysis– and get the same result, each
method is validated by the other one (Duineveld & Jenness, 1984;
Lumingas & Guillou, 1994; Gebauer & Moreno, 1995; Turon et al, 1995;
Jordana et al, 1997). Yet, if the number of growth rings is correlated with
the size, not the age, one would interpret a group of fast-growing
individuals as being older, and a group of slow-growing ones as being
younger and eventually mix animals of different age in a single cohort.
This would result in an agreement between both methods although
conclusions on size at age are incorrect.
Measuring relative growth (without knowing age) is an alternative to
calculating growth rate of individuals in the field. Animals are tagged,
field-released and captured again one year later (Ebert, 1977, 1988a;
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