The application of a simple, fast and reliable volumetric microrespirometric
method to assess respiration of bivalve larvae is discussed. As a model, C. gigas
larvae of various sizes were used. Metabolic activity of veliger larvae was
assessed by measuring respiratory rate for use in ecophysiological
modelling. As an example of the application of this approach, additional
measurements of veliger respiratory rates were carried out to assess the
effect on larval metabolism of different concentrations of leachate from
wood treated with chromated copper-arsenate (CCA).
Veligers of Crassostrea gigas (length from 95 to 331 µm) were fed with a mixture of
Isochrysis galbana and Chaetoceros pumilum cells. Experiments were performed in a 20 °C constant seawater
temperature. The wet and dry weight of four length (L, in µm) classes
of larvae were obtained from which the relationship between total dry weight
and veliger length was derived {DW = e(3.27+L×0.0154)} (R2 = 99%). Moreover, tissue dry weight (TDW; in ng)
was calculated according to Gerdes (1983). Since the mathematical model
between larval length and respiratory rate explained 88% of the total
variability, a more conservative approach using oyster larval dry meat
weight (in ng) and respiratory rate (in µl O2 h−1) was
developed to establish a linear model explaining 94.5% of the
variability: Resp. = -3.849 × 10-4 + 5.211 × 10-6 × TDW. These
experiments provided updated figures of C. gigas larval respiratory rates for use in
ecophysiological models. The relationship between tissue dry weight and
respiratory rate was close to previous estimates obtained by Gerdes (1983)
and Hoegh-Guldberg and Manahan (1995) at 25 °C and 20 °C respectively.
Our experiments demonstrate that volumetric microrespirometry is suitable
for assessing larval respiratory rate and therefore can be used to assess
impacts of pollutants on an early larval stage. Oysters exposed to leachates
from chromated copper arsenate (CCA) treated timber at 5 kg m−3 showed
initially highly variable respiratory rates while those rates decreased
drastically for a 15 kg m−3 CCA treatment exposure. Among bioindicators
using physiological response to assess pollutant effects, swimming activity
and respiratory rates can be compared, the later showing a significant
response at a lower pollutant concentration.