S. Pouvreau et al., Filtration by the pearl oyster, Pinctada margaritifera, under conditions of low seston load and small particle size in a tropical lagoon habitat, AQUACULTURE, 176(3-4), 1999, pp. 295-314
Tn the oligotrophic waters of tropical atoll lagoons where Pinctada margari
tifera lives and is farmed, primary production is mainly due to picoplankte
rs (< 1 mu m) and total particulate matter > 1 mu m never exceeds 1.5 mg l(
-1). To understand the paradoxical growth of pearl oysters in such poor tro
phic conditions, retention efficiency (RE), clearance rate (CR) and gill st
ructure of P. margaritifera were investigated. Absolute retention efficienc
ies (in percent) of several algae were measured by simultaneous sampling of
water in inhalant and exhalant pearl oyster currents. RE varied from 15% f
or 1 mu m particles (cyanobacteria Aphanocapsa sp.) to 98% for 5 mu m parti
cles (algae Chaetoceros gracilis and Tahitian Isochrysis aff. galbana) and
was independent of dry tissue weight (W, in grams). Measurements of clearan
ce rate (CR, in litres per hour) of several algae in relation to oyster siz
e were also carried out by monitoring the removal rate of particles in clos
ed and flowing systems. Both systems gave the same results: the lowest CR w
ere obtained with Aphanocapsa sp. suspension and the higher values with the
best retained algae (C. gracilis and Tahitian Isochrysis aff. galbana). Wh
en well-retained algae are used, CR becomes equivalent to pumping rate (PR,
in litres per hour) and was then related to dry tissue weight as follows P
R similar to CR = 25.88W(0.57). Th, proportionality coefficient of this equ
ation, 25.88 1 h(-1) g(-1), is the highest ever published for bivalve mollu
scs. Gill area (GA, in square millimetre) is high, from 1000 to 12000 mm(2)
, and follows the equation GA = 3502W(0.67). The pumping rate per unit gill
area (5 ml mm(-2) h(-1)) is one of the highest reported values. Clearance
activity in undisturbed pearl oysters and in conditions close to pearl fann
ing sites is very high because of (1) a high relative size of gill and (2)
a maximal use of the pumping capacity per unit gill area. Microscopic obser
vation of gill filaments showed the lack of eu-latero-frontal cirri, which
implies that pearl oysters an not adapted to retain pico-sized particles. C
onsequently, amounts of particulate matter directly available to the pearl
oyster are low in lagoon environment. However, Pinctada clearance activity
is very high, in such a way that even inefficient capture of small particle
s provide sufficient nutrients. This trophic strategy gives an explanation
to the rapid growth of this non-symbiotic bivalve in poor waters. (C) 1999
Elsevier Science B.V. All rights reserved.