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

to general growth) that explains 55.0% of the whole variance (Fig. 21).
Integument and skeleton measurements are also well correlated with the
first axis. The second axis explains 30.2% of the total variance. Gonad
measurements are highly correlated with this axis that represents change
with the reproductive cycle. Note that body size measurements are not
much correlated with gonads measurements in any axis. Indeed, due to the
rigidity of the test, diameter, height and total weights are not affected by
the size of the gonads (when the sea urchin has small gonads, its general
cavity is filled with coelomic fluid with about the same density than the
gonads). This is convenient because body size measurements can be
considered as soma measurements, independently of the size of the
gonads. The third axis amounts for 12.9% of the total variance and is
slightly represented by digestive tract measurements. It should be due to its
contents, as a consequence of the feeding activity during the last day
before dissection. 98.2% of the variance is explained by the first three axes
that are kept (Table 6).
Table 6. Principal components analysis: contribution of the parameters to the three first axes.
Parameter Axis 1 Axis 2 Axis 3
Height 0.895 0.311 0.246
Diameter 0.869 0.369 0.269
Weight of Aristotle's lantern 0.852 0.365 0.323
Weight of spines 0.833 0.450 0.248
Weight of test 0.804 0.450 0.347
Immersed weight 0.799 0.510 0.298
Fresh weight of integuments 0.789 0.507 0.339
Dry weight of integuments 0.769 0.574 0.291
Total fresh weight 0.729 0.552 0.390
Drained weight 0.729 0.575 0.371
Dry weight of digestive tract and its content 0.699 0.407 0.567
Fresh weight of digestive tract and its content 0.629 0.454 0.626
Dry weight of gonads 0.360 0.900 0.231
Fresh weight of gonads 0.392 0.891 0.215
We are now confident that a body size measurement, e.g., the diameter
of the test, is an adequate representation of the growth achieved by all the
somatic organs. Sea urchin appears to be a good experimental subject for
studying growth because size is easy to measure with accuracy thanks to
the rigidity of its skeleton that constraints its shape; also because it exhibits
Part II: Measurement for size in the sea urchin
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