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

c. Material
metamorphosis) or hypotheses that can be formulated about changes in
individual growth (such as intra- or interspecific competition).
The aim of this paper is to propose a growth model taking into account
usually neglected aspects such as individual variations or intraspecific
competition. This means we will question the concept of "mean
individual" and tentatively build up a growth function with parameters
carrying high biological meaning.
The dataset we used results from a growth study of a single cohort of
sea urchins, Paracentrotus lividus, reared over a period of 7 years in a
controlled environment (see Grosjean et al, 1998, see Part I, for a detailed
description of the rearing protocol). Echinoids were never size-sorted, nor
individually tagged. All sea urchins in the cohort were measured every 3
months starting at 6 months old (younger echinoids are too fragile to be
measured alive) until 4.5 years old, and then every 6 months until 7 years
old (Fig. 28A, see also Annex II). Due to mortality, the total number of
individuals in the cohort dropped from 725 at 6 months old to 221 at 4.5
years old and to 67 at 7 years old (Fig. 29). Size is expressed by the
ambital test diameter D which corresponds to the external diameter of the
test at its largest region (the ambitus) excluding spines. D is measured with
an electronic sliding caliper at the nearest 0.1 mm (Grosjean et al, 1999,
see Part II) and recorded into 1 mm-wide size classes. Note that between
400 and 1200 days, size distributions were heavily skewed, or even
multimodal. This is the effect of an intraspecific competition (Grosjean et
al, 1996, see Part III).
Part IV: A growth model with intraspecific competition
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