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

output. The quality of gametes is often a little bit lower from December till
February, though still usable most of the time.
Measurements of reared sea urchins
Essentially two criteria are used to quantify the performances of the
rearing method: (1) the survival rate with time and (2) the growth rate, that
is, the change of test diameter of the urchins with time (gonadal size and
quality are taken into account only after the minimal market size has been
reached). The first is determined by counting survivals in a single batch
(issued from a single fertilization and a single larval rearing tank) at
various times. The counting of eggs, embryos and larvae is performed on
at least five samples of the homogenized batch (the volume chosen to
count each time is at least one hundred individuals) and the total amount is
estimated by extrapolating the mean concentration found to the whole
volume. The survival rate of competent larvae, postlarvae and juveniles is
determined by rearing subsamples of 50 to 100 individuals in SCT sieves.
Several replicates (at least five) are sacrificed and counted at each time.
All subadults and adults of a batch are counted and measured every 3
months (typically between a few hundred to a few thousand individuals in
each batch) during size sorting. Measurements of subadults and adults do
not induce additional stress or mortality other than those occurring during
the normal size grading operation (no additional manipulations). Mortality
caused by manipulations could thus be attributed to the rearing method
itself.
Size is evaluated by means of the diameter, which is measured to the
ambitus of the test (its largest part) considered without spines. To prevent
errors caused by a possible slightly oval shape, we measure two
perpendicular diameters, both to the ambitus, and only the average is
considered. The diameter of juveniles, after fixing them (glutaraldehyde
3%), is measured on digitized microphotographs using a specific image
analysis software (Grosjean et al, 1996, see Part III). The diameter of
subadults and adults is measured with a sliding caliper. Fresh weight, used
Part I: Set up of an experimental rearing procedure for echinoids
77