J .M. Navarro et al. J. Exp. Mar. Biol. Ecol. 247 2000 67 –83
69
lanicus, species which increases its clearance and ingestion rates in response to specific metabolites from the diatom Chaetoceros muelleri. This knowledge on the chemorecep-
tory capacity of bivalves is important to understand the feeding behaviour of bivalves in nature as well as under laboratory conditions.
The main objective of the present study was to evaluate the interactive effect of diet and temperature by measuring the scope for growth and gonad maturation of the scallop
Argopecten purpuratus during reproductive conditioning, under the hypothesis that both diet and temperature are key factors influencing potential for production and gameto-
genesis in this species.
2. Material and methods
2.1. Experimental design Mature scallops obtained from an aquaculture site at Tongoy Bay 308 S were
induced to spawn by raising the temperature and adding microalgae. Spent animals were distributed in 18 tanks which were divided into two groups and placed in separate
controlled temperature laboratories, one at 168C and the other at 208C. Three groups in each laboratory were fed with a mixture of microalgae 50 Isochrysis galbana 1 50
of Chaetoceros gracilis, three other groups received 70 of the same mixture of microalgae plus 30 by weight of carbohydrates commercial potato’s starch, particle
size: 10.7765.82 mm, and the other three tanks received 70 microalgae plus 30 by weight of a lipid emulsion particle size: 2.1560.54 mm. The experimental ICES
International Council for the Exploration of the Sea lipid emulsion was provided by Artemia Reference Center in Belgium and correspond to the EmDHA one whose
composition is described in Caers et al. 1999. Direct observation on the lipid emulsion showed that this compound is very stable in seawater. It was verified by counting the
particles every hour during 5 h, and a very small reduction in particle number was detected less than 7. The food, supplied continuously by a peristaltic pump,
amounted to 3 daily of the animal dry weight.
The physiological measurements were carried out during the third week of the conditioning of the scallops. The different physiological variables were measured on 18
scallops three replicates per temperature diet combination, nine scallops coming from the group conditioned at 168C and the remainder from the group conditioned at 208C
each scallop coming from a different tank. As in the conditioning experiments, the physiological measurements were done in the controlled temperature laboratories with
the same three experimental diets described above. Physiological rates were standardised to 3 g dry tissue weight according to the formula given by Bayne and Newell 1983:
b
Y 5 W W ? Y
s s
e e
where: Y 5physiological rate for an animal of standard weight, W 5standard weight of
s s
the animal, W 5observed weight of the animal, Y 5uncorrected measured physiologi-
e e
cal rate, b 5weight exponent for the physiological rate function.
70 J
.M. Navarro et al. J. Exp. Mar. Biol. Ecol. 247 2000 67 –83
2.2. Physiological measurements 2.2.1. Clearance and ingestion rates
These rates were measured using a static system, in which the decrease in particle density in the experimental aquarium was monitored in relation to time Widdows,
1985. These measurements were carried out using a particle counter Elzone 180XY, Particle Data, fitted with a 120-mm orifice tube. The experimental medium was
maintained homogeneous by aeration in each experimental aquarium. The measurements were carried out over a period of 6 h in 8-l aquaria filled with seawater, each aquarium
containing a single scallop. One additional aquarium, but with no scallop, was used as a control. The experiments were carried out at 16 and 208C. The experimental food
6 21
concentration was 30310 particles l
at each experimental diet, using the same mixtures of diets as during the conditioning process: a mixture of microalgae, b
mixture of microalgae plus lipids, and c mixture of microalgae plus carbohydrates. Ingestion rate represents the amount of food ingested per unit of time. It was
estimated as the product of the clearance rate and the total weight of a given experimental diet. The organic ingestion rate was calculated as the product of clearance
rate and the organic fraction of the diets.
2.2.2. Net absorption efficiency Absorption efficiency was estimated by determining the organic and inorganic content
of the food ingested and the faeces, following the ratio method of Conover 1966. Representative samples of each diet mixture were collected during the experiments, as
well as faeces from each experimental aquarium. Samples were filtered through pre- ashed, pre-weighed 47-mm glass fiber filters under low vacuum. Filters were rinsed with
isotonic ammonium formate 3.4, dried to a constant weight at 808C, weighed, combusted at 4508C for 3 h and weighed again to estimate the organic and inorganic
fraction contained in the food and faeces. Absorption rate was calculated as the product
21
of the organic ingestion rate mg h and absorption efficiency .
2.2.3. Oxygen uptake Rates of oxygen uptake for individual scallops were measured in sealed chambers of
2.5 l volume, using a polarographic analyser Model YSI 5351. To ensure steady oxygen consumption, the scallops were allowed 30 min of acclimation until their siphons
appeared to be open. Two measurements were made on each occasion for each scallop and oxygen concentration was not allowed to fall below 70 of saturation. The water in
the respirometer was mixed with a magnetic stirrer and the output signal from the O
2
analyser was monitored continuously on a chart recorder. Considering that the experimental scallops were well fed before the experiments, the measured rates of
oxygen uptake are assumed to represent the routine metabolism of A . purpuratus. Values
21
of oxygen uptake were expressed as ml O h and transformed to energy equivalents
2
using the conversion factor 1 ml O 519.9 Joules Elliott and Davison, 1975.
2
J .M. Navarro et al. J. Exp. Mar. Biol. Ecol. 247 2000 67 –83
71
2.2.4. Ammonia excretion Ammonia excretion was determined by the phenol–hypochlorite method of Solorzano
1969. Scallops were well fed and then placed individually in glass beakers containing 0.5 l of filtered 0.45 mm seawater. One additional beaker containing filtered seawater,
but with no animals, served as a control. Following an incubation period of 2 h, samples from the water containing the scallops and from the control were analysed. Values for
21
excretion rate were expressed in mg NH –N h and transformed to Joules using the
4
conversion factor: 1 mg NH –N524.8 J Elliott and Davison, 1975.
4
2.2.5. Scope for growth This physiological index represents the energy available for growth and reproduction
after all metabolic demands have been met from the absorbed ration. Scope for growth was calculated by the equation given by Widdows 1985, after converting all the
21
physiological rates to energy equivalents J h .
2.2.6. Gonad maturation Individual scallops from each conditioning tank were sampled every 6–8 days to
analyse gonad development and to induce spawning at the end of the conditioning experiment. A visual criterion was used to define ripe scallops, characterised by turgid
and highly pigmented gonads. In the present study we consider only gonadal develop- ment and the spawning capacity of scallops conditioned at the different temperatures and
diets. Biochemical analyses of the gonad will be described elsewhere.
2.3. Statistical analysis Physiological rates were related to dry meat weight by linear regression analysis, after
log-transformation of all variables. Analysis of variance followed by a Tukey test of significance was carried out to compare the total weight and organic content of the
different diets and also the effects of the temperature diet treatments on each physiological variable. In all statistical tests differences are considered significant when
P ,0.05. All analyses were carried out with the statistical package Statistica for Windows, v. 4.2.
3. Results
