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

Choice of measurement
Immersed weight is definitely the most accurate and less stressing
measurement of sea urchin body size. However, it is not possible, using
this method, to measure hundreds of echinoids in a reasonable period of
time. We thus used the test diameter, as the optimal compromise between
accuracy and speed.
The diameter has another advantage: it is the only parameter that can
be measured on very small animals of a few hundreds of microns (that is,
the size of P. lividus just after metamorphosis, see Fig. 2B, p. 37). To do
this, we have to kill and fix the individuals (3% glutaraldehyde) in order to
manipulate them and transfer them on a millimeter-graduated support to be
photographed. The picture is digitalized and analyzed with a custom image
analysis software (ShellAxis, available at http://www.sciviews.org). For a
description of the program, see Van Osselaer & Grosjean (2000) and for
extensive tests of accuracy and reproducibility of measurement with this
software, see Van Osselaer (2001).
Measuring body size with good accuracy is one aspect, knowing what
it really means in terms of relative sizes of the different organs during
growth is another one. Indeed, successive measurements would be really
representative of somatic growth if they were strongly correlated with
growth of all somatic organs.
To determine how different compartments of the sea urchin vary with
body size, a stratified sample (from 5 to 60 mm every 5 mm) of 440
animals was analyzed. 220 sea urchins were dissected in spring and 220 in
autumn, which correspond to two contrasting reproductive stages in the
field: empty or full gonads, in order to make sure maximum variance is
included in the dataset. Measurements done on each individual were:
immersed weight; two perpendicular diameters at the ambitus; height; total
fresh weight; fresh "drained" weight (that is, the test is opened by cutting
around the ambitus and the two resulting parts are left upside down on
Part II: Measurement for size in the sea urchin
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