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

Fenaux (1990) for the same species in cultivation and by Ebert & Russell
(1993) for wild populations of Strongylocentrotus franciscanus. When
growth initiates in term of test diameter, size distribution expands. This
individual variability is not genetic but is attributable to a reversible sizebased
intraspecific competition (Grosjean et al, 1996, see Part III) that
takes place rapidly, even in size-sorted batches, although sorting reduces
its effect. Presently, the exact impact of this competition on productivity
and the best way to avoid it (if it should be avoided at all) are still
unknown.
A third problem that will probably occur when considering further
intensification of echiniculture in closed or semiclosed systems is the
depletion of dissolved carbonates and the accumulation of CO2 in
seawater. In growing, the sea urchin builds a magnesium-calcite skeleton.
This skeleton represents an important fraction of the body weight: between
28% and 31% of the total fresh weight for P. lividus (measured on animals
issued from the reared strain, after digestion of organic tissues with a
12°Chl bleaching agent under gentle agitation, n = 356). Thus, for each kg
of fresh weight produced, about one-third has to be supplied in one or the
other form of calcium carbonate. However, P. lividus is unable to
assimilate efficiently carbonates provided as a solid substrate (calcareous
rocks, algae or cuttlefish bones for instance) because the pH of its
digestive tract is too high to dissolve large amounts of solid calcite
(between six and eight, for a review see Lawrence, 1982; for data
concerning P. lividus see Claerebout & Jangoux, 1985). The main usable
source of magnesium/calcium carbonates is thus present under a dissolved
form in seawater. If calcium and magnesium ions (respectively 400 mg
and 1,350 mg per kg seawater at a salinity of 35‰, Spotte, 1991) are not
limiting, the quantity of dissolved carbonates available could be consumed
very quickly in intensive closed or semiclosed systems (unpublished data).
Most of the carbonate alkalinity (about 2.3 - 2.4 meq / kg seawater,
corresponding to 140 mg of HCO3 - ) remains unavailable for
skeletogenesis, the pH dropping too much when sea urchins consume it
Part I: Set up of an experimental rearing procedure for echinoids
87