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

Intraspecific competition: an additional experiment
To investigate intraspecific competition in batches with a large initial
dispersion of sizes, two series of 6 replicates of 60 'small' sea urchins (7.5
to 9 mm test diameter; 720 individuals in the total) coming from the
heading group of a single fertilization (Ff), and two series of 6 replicates of
10 'large' sea urchins (20 to 24 mm test diameter; 120 individuals in the
total) extracted also from the heading group of another single fertilization
(Fg) were set up at the beginning of the experiment (Fig. 25). Small sea
urchins of Ff were 5 months old at the beginning of the experiment while
those of Fg were 13 months old. One series of small and one series of
large individuals were reared separately. The two remaining series of small
and large individuals were reared together ('mixed' series). The surface of
the rearing baskets used was the same for all batches (20 x 30 cm).
Figure 26 presents the change of size distributions in the three
experimental series. Table 11 presents corresponding statistics. The six
replicates from the same series are pooled after verifying that the
differences between them are not significant (Kruskal-Wallis test, p >>
0.05, except for the small individuals in the mixed batch at 2 months were
0.01 < p < 0.05, see Table 11). The presence or absence of smaller sea
urchins did not influence very significantly the growth of larger
individuals. The latter were thus not inhibited whatsoever. It is, however,
interesting to note that size distribution of larger echinoids did not spread
much with time, which means their density was low enough to avoid
competition among them and allowed a homogeneous growth of the whole
set. On the other hand, growth of smaller sea urchins was strongly affected
by the presence of larger individuals: their growth was limited and, as a
consequence, they remained more grouped than batches of small
individuals alone. In batches where small sea urchins occurred alone,
competition took place and size distribution spread. This is an effect
already identified in the previous experiments.
Part III: Experimental studies of the intraspecific competition
127