Filtration by the pearl oyster, Pinctada margaritifera, under conditions of low seston load and small particle size in a tropical lagoon habitat

Abstract

In the oligotrophic waters of tropical atoll lagoons where Pinctada margaritifera lives and is farmed, primary production is mainly due to picoplankters (<1 μm) and total particulate matter >1 μm never exceeds 1.5 mg l −1. To understand the paradoxical growth of pearl oysters in such poor trophic conditions, retention efficiency (RE), clearance rate (CR) and gill structure of P. margaritifera were investigated. Absolute retention efficiencies (in percent) of several algae were measured by simultaneous sampling of water in inhalant and exhalant pearl oyster currents. RE varied from 15% for 1 μm particles (cyanobacteria Aphanocapsa sp.) to 98% for 5 μm particles (algae Chaetoceros gracilis and Tahitian Isochrysis aff. galbana) and was independent of dry tissue weight ( W, in grams). Measurements of clearance rate (CR, in litres per hour) of several algae in relation to oyster size were also carried out by monitoring the removal rate of particles in closed and flowing systems. Both systems gave the same results: the lowest CR were obtained with Aphanocapsa sp. suspension and the higher values with the best retained algae ( C. gracilis and Tahitian Isochrysis aff. galbana). When 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 PR∼CR=25.88 W 0.57. The proportionality coefficient of this equation, 25.88 l h −1 g −1, is the highest ever published for bivalve molluscs. Gill area (GA, in square millimetre) is high, from 1000 to 12 000 mm 2, and follows the equation GA=3502 W 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 farming 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 observation of gill filaments showed the lack of eu-latero-frontal cirri, which implies that pearl oysters are not adapted to retain pico-sized particles. Consequently, 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 particles provide sufficient nutrients. This trophic strategy gives an explanation to the rapid growth of this non-symbiotic bivalve in poor waters.