Effect of microalgal concentration and water temperature on the physiology of the Caribbean scallops Argopecten nucleus and Nodipecten nodosus.

ABSTRACT Argopecten nucleus and Nodipecten nodosus are two Caribbean scallops occurring in Colombia, which have recently been selected for artificial culture based on their high commercial value. As part of an effort to develop culture technology for these species, we studied the effects of food concentration and temperature on feeding rates (filtration, ingestion, and absorption), oxygen consumption, ammonium excretion and scope for growth in adults of the two scallop species. We tested the effects of four concentrations of the microalga Isochrysis galbana (10, 20, 40 and 60 cells [micro][L.sup.-1]) at three water temperatures (20, 25 and 28[degrees]C), at a constant salinity of 36[per thousand]. The results showed that increases in food concentration induced increases in feeding rate, oxygen consumption and growth potential, but at values of 60 cells [micro][L.sup.-1] these variables decreased, indicating saturation of the digestive tract. The excretion rate increased at low food concentrations, particularly at the middle (25[degrees]C) and/or at the highest temperature tested (28[degrees]C). This suggested utilization of endogenous proteins as a supplementary energy source under these conditions. The increase in temperature had no significant effect on the feeding variables or on the scope for growth of A. nucleus, but raised the N. nodosus ones. All the physiological rates for the two species were similar except for oxygen consumption, which was greater in N. nodosus than in A. nucleus. Using values obtained for the algae concentrations, which produced the greatest growth potential, the optimal value for L galbana for both the scallops was 40 cells [micro][L.sup.-1], whereas the optimal temperature for N. nodosus was 25[degrees]C. There was no single optimal temperature for A. nucleus, which functioned equally well at between 20[degrees]C and 28[degrees]C.

KEY WORDS: scallop physiology, scallop culture, Argopecten nucleus, Nodipecten nodosus, Colombia, Caribbean.

INTRODUCTION

Temperature and concentration of particulate food are two of the main factors that affect bivalve filter feeders in relation to their growth (Wilson 1987), survival (Paul 1980), gonadal conditioning (Martinez et al. 2000, Martinez & Perez, 2003) and physiology (Bricelj, et al. 1987, Navarro & Iglesias 1995, Navarro et al. 2000). Studies carried out on the physiology of bivalve filter feeders in response to broad ranges of microalgal concentrations (Griffiths & King 1979, Bacon et al. 1998, MacDonald et al. 1998, Velasco & Navarro 2002, 2003) and/or water temperature (Ali 1970, Winter 1970, Bougrier et al. 1995, Sicard et al. 1999, Laing 2000) have shown that the filtration rates, ingestion, absorption, oxygen consumption and/or excretion become modified with changes in these parameters, and allow maintenance of high values in scope for growth within given ranges of the parameters depending on the species observed.

Argopecten nucleus (Born, 1780) and Nodipecten nodosus (Linne, 1758) are two scallops of commercial importance from the Caribbean coast of Colombia. These species are epibenthic filter feeders and may coexist on sandy bottoms from 10-100 m depth. A. nucleus is a moderately-sized, unattached species (length = 50 mm), whereas N. nodosus is a larger (length = 150 mm) species, which remains attached to hard substrates. No naturally occurring beds of these scallops have been encountered in the sea off Colombia; population aggregates have been maintained in artificial cultures, produced from the capture of (scarce) naturally occurring seed obtained in spat collectors (maximal of 6-77 spats [collector.sup.-1]) (Urban, 1999). These two scallop species have shown high growth rates in suspended cultures, reaching commercial sizes within 11 mo (Urban 1999, INVEMAR 2003). It is known that growth and survival of N. nodosus is negatively affected by extreme temperatures (<20[degrees]C and >29[degrees]C), low salinities (<29[per thousand]) and high turbidity (Rupp & Parsons 2004, Rupp et al. 2005).

Because the scarce availability of wild "seed" does not support commercial cultures of these scallops, it was of interest to develop the technology for seed production under hatchery conditions. Success in seed production depends on the identification of optimal environmental parameters for maintenance of the adults, larvae and postlarvae in the laboratory. Because the two species under study occur naturally in relatively high and constant salinity, whereas the temperature and environmental food offerings may vary considerably, we selected the latter two variables of interest for the present study. In preliminary studies we determined that the chrysophyte Isochrysis galbana produced the best scope for growth in the two scallop species (Velasco, in press). This study was carried out to determine the effect of different concentrations of L galbana and different temperatures on physiological variables related to feeding (filtration rates, ingestion and absorption), oxygen consumption, excretion and scope for growth of Argopecten nucleus and Nodipecten nodosus adults.

MATERIALS AND METHODS

Collection and Maintenance of Scallops

Specimens of Argopecten nucleus (mean length 47 [+ or -] 5.3 mm and dry weight 1.25 [+ or -] 0.35 g) and Nodipecten nodosus (mean length 71 [+ or -] 9.5 mm and dry weight 2.26 [+ or -] 0.85 g) were collected at the Bahia Neguanje bivalve culture station (Lat. 11[degrees] 20' N., Long. 74.05" W.), in Santa Marta, Colombia. The water temperature in this area ranges between about 22[degrees]C and 30[degrees]C, with salinities between 33 and 37 [per thousand] and environmental seston concentrations between 0.2 and 4.7 mg [L.sup.-1]; the organic content of the seston ranged between 15% and 60% (Urban 1999).

The scallops were transported wet, at a temperature similar to that of the environment, to the Mollusc and Microalgae Laboratory of the Universidad del Magdalena at Taganga (Lat. 11[degrees]16'N, Long. 74[degrees]11'W), where they were cleaned of epibionts, and individually marked. Acclimation to laboratory tanks was achieved by maintaining the scallops for one week in static seawater in a 250 L tank at a salinity of 36 [per thousand] and temperature of 25[degrees]C. The water was aerated with a compressed air bubbler, and a daily food ration of Isochrysis galbana equivalent to 3% of the dry weight of the scallops. The water was renewed every 24 h.

Experimental Design

A factorial design was used in which physiological parameters of the scallops were measured at three different water temperatures (20, 25[degrees]C, and 28[degrees]C) and four different feeding regimes with Isochrysis galbana at 2, 4, 8 and 12 mg [L.sup.-1] (10, 20, 40 and 60 cells [micro][L.sup.-1] respectively). Each of these 12 treatments was applied to seven individual scallops of each species selected haphazardly from the original groups of specimens obtained. The salinity was maintained at 36 [per thousand] in all the experiments; complete oxygen saturation was maintained in the water over a period of 15 h, of which 12 h represented an acclimation period in the test system with a three hour period for taking the measurements.

The experimental diets were administered using a flow-through system consisting of 16 chambers (0.8 L for A. nucleus and 1.6 L for N. nodosus), designed following Riisgard (1977). A constant flow (150 [+ or -] 10 mL [min.sup.-1]) of the experimental diet was directed by gravity from a mixing tank into each chamber; 14 chambers were used for individual bivalves (7 individuals of each species) and 2 chambers contained empty valves, which served as controls. Normal valve opening by test specimens was monitored and individuals that failed to open normally were eliminated from the experiment.

The experimental diets were prepared by mixing appropriate volumes of 1-[micro]m microfiltered seawater and the microalgae Isochrysis galbana, which had been cultured in F/2 medium (Guillard 1974), after direct cell counts of cell densities by hemacytometer.

Characterization of Feeding Ration

Each experimental diet was analyzed in terms of total particulate matter (TPM) and particulate organic matter (POM). Water was sampled from the outflow of both control chambers every hour. Each sample was divided into three subsamples, which were filtered onto tared Millipore glass fiber filters ([empty set] = 0.45 [micro]m), which had been prewashed in distilled water and preashed at 450[degrees]C for 4 h. Filters with samples were rinsed with isotonic ammonium formate to remove salt and prevent cell lysis, and were dried at 70[degrees]C for 48 h to determine the TPM (mg [L.sup.-l]). POM (mg [L.sup.-1]) was determined as the loss in weight of the samples after ashing at 450[degrees]C for 4 h.

Determination of Physiological Feeding Variables

These variables were determined by the method of biodeposition described by Iglesias et al. (1998), as validated by Navarro and Velasco (2003) using the flow-through chamber method described by Riisgard (2001). Feces and pseudofeces were quantitatively collected every 3 h using Pasteur pipettes. The mass and organic content of the material deposited by each test bivalve were estimated separately using the method described above for the diet analysis. The equations for the physiological variables were the same as those of Iglesias et al. (1998).

Filtration Rate

Filtration rate (FR) represents the amount of particulate material removed from the water per unit time.

Inorganic filtration rate IFR (mg [h.sup.-1]) = IRR + IER (1)

Organic filtration rate OFR (mg [h.sup.-1]) = IFR * (POM/PIM) (2)

Total filtration rate FR (mg [h.sup.-1]) = IFR + OFR (3)

where: IFR = inorganic filtration rate (mg [h.sup.-1]); IRR = rate of production of pseudofeces inorganic matter (mg [h.sup.-1]), IER = rate of production of feces inorganic matter (mg [h.sup.-1]), POM = particulate organic matter (mg [L.sup.-1]) and PIM = particulate inorganic matter (mg [L.sup.-1]).

Ingestion Rate

When no pseudofeces is produced the total filtration and organic matter filtration rates are represented, respectively, by the total (IR) ingestion rate and organic ingestion rates (OIR). When pseudofeces is produced, the IR is calculated by subtraction of the rate of pseudofeces production from the FR (Bayne et al. 1993).

IR (mg [h.sup.-l]) = FR - RR OIR (mg [h.sup.-1]) = OFR - ORR (4)

where: RR = rate of production of total pseudofeces (mg [h.sup.-1]) and

ORR = rate of production of pseudofeces organic matter (mg [h.sup.-l]).

Efficiency (AE) and rate of absorption (AR): represent material ingested, which is absorbed per unit time.

AE(%) = AR/(OFR - ORR) * 100 (5)

AR (mg [h.sup.-1]) = OIR - OER (6)

where: OER = rate of production of feces organic matter (rag [h.sup.-1)].

Oxygen consumption (CO: mL O2 [h.sup.-1]) and ammonia excretion (UR: [micro]g N[H.sub.4]-N [h.sup.-1]): were determined on fed animals by placing them in individual chambers (0.8 and 3 L) after rinsing the chambers with 25% HCl and filling with <1-[micro]m filtered seawater. Chambers were sealed and incubated for 2 h at the same temperature in which they has been fed, alongside a control chamber devoid of specimens. Two water samples were then taken from each experimental chamber to determine oxygen consumption rates and rates of excretion. Oxygen consumption was never measured at ambient oxygen tension lower than 70% saturation. Oxygen concentration was estimated by the Winkler method, modified after Carritt and Carpenter (Strickland & Parsons 1972) and ammonia excretion was determined by the phenol-hypochlorite method (Solorzano 1969).

Scope for growth (SFG): is a physiological index of energy balance to estimate production (growth + reproduction) by an individual animal. It was calculated from the equation given by Widdows (1985) after converting all the physiological rates to energy equivalents (J [h.sup.-1]):

SFG = A - (R + U) (7)

where: A = Energy absorbed (J [h.sup.-1]) = AR mg [h.sup.-1] x 11.40 J [mg.sup.-1] (Velasco, in press), R = Oxygen consumption (J [h.sup.-1]) = OC mL[O.sub.2] [h.sup.-1] x 20.08 J (Gnaiger 1983), U = Ammonia excretion (J [h.sup.-1]) = UR mg [NH.sub.4]-N [h.sup.-1] x 24.8 J (Elliot & Davison 1975).

Standardization of Variables

The physiological rates were converted to a standard individual of 1-g dry tissue weight and with a macroscopic gonadal stage of I; for this, the soft tissues were dried at 70[degrees]C for 48 h, and then individually weighed. Standardization used the equation of Bayne et al. (1987).

Yts = [(1 g/We).sup.bl] x Ye (8)

Yms = [(l/E).sup.b2] x Yts (9)

where Yts = physiological rate of an individual of standard size (1 g), Ye = uncorrected rate, We = weight of the experimental individual, b1 = dependence of the physiological rates on the sizes of the individuals, Yms = physiological rate of the individual at a standard stage of maturity (1), E = stage of maturity of the experimental individual (between 1 and 4) and b2 = dependence of the physiological rates on the stage of maturity of the individuals. The "b" values used for each physiological rate were determined from exponential regressions between the physiological measurements, the dry weights, and stage of maturity of specimens of different sizes and degrees of maturity of the two scallop species. For the rate of production of biodeposits, the bl values were 0.98 and 0.41 and for b2 they were -0.67 and -0.44 for Argopecten nucleus and Nodipecten nodosus, respectively. For the oxygen consumption rates, the bl values were 1.52 and 0.43 and for b2 they were 0.32 and 0.52 for A. nucleus and N. nodosus, respectively. Finally, for the rates of excretion, the b1 values were 1.16 and 0.97 for A. nucleus and N. nodosus, respectively, and no standardization was carried out related to the stage of maturity, because this factor did not affect the excretion rate.

Statistical Analyses

Tests were carried out on the normality and the homogeneity of the variances of all the physiological variables, and transformed to comply with these requisites. Clearance, filtration, ingestion, absorption and SFG were transformed to square, oxygen consumption was transformed to square roots, and a rank transformation was applied to the absorption efficiency and excretion rate data. Factorial analyses of variance were carried out to establish the degree of influence of the food concentration and temperature, and the interactions of these two factors on each of the physiological variables. One-way analyses of variance were carded out to make specific comparisons between treatments, and to determine significant differences among them using a Bonferroni multiple range test. Correlation analyses were carried out to establish associations among the physiological variables. Finally, analyses of covariance were carried out to compare the physiological variables between the two species. All the statistical analyses were carried out using Statgraphics-plus 5.0 software, with an [alpha] of 0.05 for all tests of significance.

RESULTS

Feeding Rates

The highest rates of filtration, ingestion and absorption by Argopecten nucleus were 85.2, 55.5 and 52.4 mg [h.sup.-1], respectively, whereas the lowest rates, respectively, were 7.1, 4.6 and 4.1 mg [h.sup.-1]. In Nodipecten nodosus the filtration rates varied between 5.8 and 83.5 mg [h.sup.-1]; ingestion rates between 3.7 and 56.7 mg [h.sup.-1] and the absorption rates between 3.2 and 53.3 mg [h.sup.-1] (Fig. 1). The food concentration significantly affected the rates of filtration, ingestion and absorption of A. nucleus (df = 3, P < 0.001) and N. nodosus (df = 3, P < 0.0001). Increase in physiological rates occurred with increases in the food concentration, with the highest values observed at 40 cells [micro][L.sup.-1]. At food concentrations of 60 cells [micro][L.sup.-1] all the rates decreased, except in N. nodosus at 20[degrees]C, where they continued to increase.

[FIGURE 1 OMITTED]

Temperature did not significantly affect the feeding rates of Argopecten nucleus (df = 2, F = 1.13, P = 0.3316) whereas the contrary was true of Nodipecten nodosus (df = 2, F = 22.22, P = 0.0001), although only when the scallops were presented with intermediate food concentrations (20 and 40 cells [micro][L.sup.-1]); at these food concentrations, the low temperature (20[degrees]C) and the high temperature (28[degrees]C) produced a significant decrease in the physiological variables related to feeding. The analysis of covariance showed that the feeding rates of both scallop species were statistically similar (df = 1, P < 0.0828). There was production of pseudofeces in all the treatments; in N. nodosus this varied between 14% and 52% of the material filtered, whereas in A. nucleus this range was between 18% and 47%, with the highest values observed in the treatments containing the highest food concentrations and at extremes of temperature.

Absorption Efficiency

The absorption efficiency of Argopecten nucleus varied between 60.9% and 96.3%, whereas that of Nodipecten nodosus was between 74.5% and 97.5% (Fig. 2). The concentration of microalgae had a statistically significant influence on the absorption efficiency of both scallop species (df = 3, P < 0.0020), but a direct relation was not verified. The values obtained at 40 cells [micro][L.sup.-1] were significantly higher than those at 20 cells [micro][L.sup.-1]. Temperature affected the efficiency of absorption of A. nucleus (df = 2, F = 6.33, P = 0.0034), with higher values obtained at 25[degrees]C than at the other temperatures. In N. nodosus the absorption efficiency was not significantly affected by the temperature (df = 2, F = 2.83, P = 0.0666). The analysis of variance did not detect significant differences between the absorption efficiencies of the two species (df = 1, F = 0.01, P = 0.8289).

[FIGURE 2 OMITTED]

Oxygen Consumption Rate

The oxygen consumption of Argopecten nucleus varied between 0.18 and 1.05 mL [O.sub.2] [h.sup.-1] and that of Nodipecten nodosus was between

0.5 and 1.63 mL [O.sub.2] [h.sup.-1] (Fig. 3A). The factorial analysis showed that the oxygen consumption by the scallops was significantly affected by the food concentration (df = 3, P < 0.0001) and temperature (df = 2, P < 0.0001). In A. nucleus the oxygen consumption rates were significantly lower at the lowest algal concentration (10 cells [micro][L.sup.-1]), whereas in N. nodosus the highest values were obtained at 40 cells [micro][L.sup.-1]. Increase in temperature increased oxygen consumption, with statistically higher values at 25[degrees]C and 28[degrees]C in A. nucleus and at 28[degrees]C in N. nodosus. The analysis of covariance showed that the oxygen consumption by N. nodosus was significantly higher than that of A. nucleus (df = 1, F = 35.13, P = 0.0001).

[FIGURE 3 OMITTED]

Excretion Rate

Ammonium production by Argopecten nucleus varied between 91.9 and 326.6 [micro]g [h.sup.-1], whereas that of Nodipecten nodosus ranged from 84.2-340.4 [micro]g [h.sup.-1] (Fig. 3B). The factorial analysis of variance showed that the rate of excretion by the two scallop species was significantly affected by the microalgal concentration (df = 3, P < 0.0016) and by the water temperature (df = 2, P < 0.0030). The excretion rates measured at the lowest food concentration (10 cells [micro][L.sup.-1]) were significantly higher than those obtained at higher food concentrations (40 cells [micro][L.sup.-1]). The temperature affected the excretion rate of these scallops only at low food concentrations (10 and 20 cells [micro][L.sup.-1]), and produced significantly higher values at 25[degrees]C than at 20[degrees]C (df = 2, P < 0.0300). At higher food concentrations, the excretion rate was not influenced by temperature (df = 2, P > 0.1275). No statistical differences were detected between the rates of ammonium release by the two scallop species (df = 1, F = 0.08, P = 0.7841) using the analysis of covariance.

Scope for Growth

The scope for growth of Argopecten nucleus fluctuated between 33.2 and 580.2 J [h.sup.-1] and that of Nodipecten nodosus between 9.6 and 582.2 J [h.sup.-l] (Fig. 4). The correlation analysis showed a positive association between the scope for growth and the feeding variables, which was highly significant (r > 0.9784, P < 0.0001), whereas no association was noted between the oxygen consumption rates (r = -0.08, P = 0.3579). Correlation of the scope for growth with excretion rates was negative (r = -0.2422, P = 0.005). The factorial analysis of variance showed that the growth potentials of the two scallops was significantly influenced by the microalgal concentration (df = 3, P < 0.0001), whereas the water temperature affected N. nodosus (df = 2, F = 16.01, P = 0.0001) and not A. nucleus (df = 2, F = 0.86, P = 0.4289). The scope for growth increased together with food concentration up to 40 cells [micro]L and thereafter decreased at 60 cells [micro][L.sup.-1], except in N. nodosus at the low temperature (20[degrees]C), at which the scope for growth continued to increase. Temperature affected the scope for growth of N. nodosus only at the intermediate food concentrations of 20 y 40 cells [micro][L.sup.-l] (df = 3, P < 0.0277), with an increase in scope for growth stimulated at 25[degrees]C. The analysis of covariance showed that the scope for growth of the two scallops was statistically similar (df = 1, F = 3.2, P = 0.0761).

DISCUSSION

Feeding Rates

The highest rates of filtration, ingestion, and absorption recorded for the two scallop species in this study were obtained at intermediate concentrations of microalgae offered as food. These results were in agreement with results of previous studies on the feeding of bivalves exposed to broad ranges of algal concentration as in the case of Mytilus chilensis (Velasco & Navarro 2002, 2003). The decrease in feeding rates at high concentrations of microalgae has not been noted in other studies, such as those of Aulacomya ater (Griffiths & King 1979), Mytilus chilensis (Navarro & Winter 1982), Placopecten magellanicus (Bacon et al. 1998) and Musculista senhousia (Inoue & Yamamuro 2000). Alternatively, stabilization of feeding rates may occur at high food concentrations, as shown for Tapes philippinarum (Coutteau et al. 1994); Mya arenaria (Bacon et al. 1998) and Mulinia edulis (Velasco & Navarro 2002, 2003). It may be that the latter two results were because of the narrow range of microalgal concentrations tested. The reduced feeding rates of the bivalves at low food concentrations has been explained as the direct, and uncontrolled scarcity of food, whereas at high concentrations it may represent a reduction in the pumping of water, and to the increase in production of pseudofeces, which allows the bivalve to regulate the ingestion rate and avoid saturation of the alimentary system (Iglesias et al. 1996, Velasco & Navarro 2002). The concentration of Isochrysis galbana at which the ingestion rate of Argopecten nucleus and Nodipecten nodosus was at a maximum, inducing regulation, or lowering of ingestion ("saturation concentration"), was at 40 cells [micro][L.sup.-1] (8 mg [L.sup.-1]); this value coincided with that observed in other bivalves fed on microalgal diets including Meretrix meretriz (8 mg [L.sup.-1], Zhuang & Wang 2004) and P. magellanicus (7 mg [L.sup.-1], Bacon et al. 1998).

[FIGURE 4 OMITTED]

The lack of influence of the temperature on the feeding rates of Argopecten nucleus showed that this species had a high capacity for rapid acclimation within the temperature range tested. Similar results had previously been presented for Argopecten purpuratus exposed to 16[degrees]C and 20[degrees]C for three weeks (Navarro et al. 2000). The increase in feeding rates in Nodipecten nodosus with increase in temperature and its decrease at temperatures as high as 28[degrees]C, indicated that this species did not become acclimated to different temperatures in a short time (15 h). This result coincided with the responses described for other bivalves, including Argopecten ventricosus (Sicard et al. 1999) and Pecten maximus (Laing 2000). Other studies only described increases in feeding rates with increases in temperature, as with Arctica islandica, Modiolus modiolus (Winter 1970), Venerupis decussata, Mercenaria mercenaria (Walne 1972), Ostrea edulis (Beiras et al. 1995) and Pecten fumatus (Heasman et al. 1996) possibly because of the fact that the cited authors worked within a narrower temperature range. The presently described effect of temperature on the feeding rates of N. nodosus was observed only at intermediate food concentrations (20 and 40 cells [micro][L.sup.-1]). Extreme concentrations apparently produce a limiting effect, which annulled the positive effect obtained at intermediate temperature (25[degrees]C).

The similarity between feeding rates in Argopecten nucleus and Nodipecten nodosus contrast with the higher capacity for retaining particulate in the bigger branchial area per unit dry weight of N. nodosus (Velasco, in press). It suggests that A. nucleus has higher pump activity than N. nodosus. Other studies, which have compared interspecific feeding rates have demonstrated greater particle retention and feeding rates in species having greater branchial surface areas (Foster-Smith 1976, Hawkins et al. 1990, Velasco & Navarro 2002).

Absorption Efficiency

Absorption efficiency generally decreases when the concentration of microalgae increases, as in the cases of Mytilus edulis (Widdows 1978), Aulacomya ater (Griffiths & King 1979), Mytilus chilensis (Navarro & Winter 1982), Argopecten purpuratus (Fernandez-Reiriz et al. 2005) and Hiatella arctica (Sejr et al. 2004). This has been attributed to the decrease in residence time of the food in the stomach when the ingestion rate is high (Bayne et al. 1989). The absorption efficiencies of Argopecten nucleus and Nodipecten nodosus were relatively high, and its variation showed no relation with the concentration of microalgae, nor with the ingestion rate, in agreement with the results obtained with Mya arenaria, Placopecten magellanicus (MacDonald et al. 1998) and in Meretrix meretriz (Zhuang & Wang 2004). The maintenance of high absorption efficiencies in A. nucleus and N. nodosus at the highest food concentration tested could be related to the strong regulation of the ingestion rate, which avoids oversaturation of the digestive tract and affects the efficiency of the digestive process.

The lack of influence of temperature on the absorption efficiency of Nodipecten nodosus coincides with that found for Argopecten purpuratus after an acclimation period (Navarro et al. 2000), indicating a high degree of plasticity in this variable with regard to temperature changes. In Argopecten nucleus the increase in temperature from 20[degrees]C to 25[degrees]C produced an increase in the absorption efficiency as noted for Ostrea edulis (Beiras et al. 1995) and Meretrix meretriz (Zhuang & Wang 2004). Low temperatures produce a decrease in kinetic energy of molecules, and a decrease in the probability that they will react upon colliding (Eckert et al. 1990), which should retard hydrolysis of different substrates and reduce the efficiency of their digestion.

The similarity of the absorption efficiencies of Argopecten nucleus and Nodipecten nodosus agree with results obtained in comparisons of absorption efficiencies among bivalve species, including Mulinia edulis and Mytilus chilensis (Velasco & Navarro 2003, 2005) and Mya arenaria and Placopecten magellanicus (MacDonald et al. 1998). This indirectly suggests a similarity in the composition and/or activity of the digestive enzymes of the two species, possibly induced by being exposed to similar environmental conditions. It was demonstrated in M. edulis and M. chilensis that although the composition and activity of the digestive enzymes in their digestive glands and crystalline styles were different (Labarta et al. 2002), they had comparable absorption efficiencies (Navarro et al. 2003, Velasco & Navarro 2003).

Oxygen Consumption

Oxygen consumption by Argopecten nucleus and Nodipecten nodosus increased with the increase in microalgae concentration, and at the highest concentration tested, this rate decreased, demonstrating a similar response to that of the feeding rate. This showed that the costs of feeding activities, digestion and absorption were variable and depended on the food concentration. Similar results have been described for other bivalves such as Donax vittatus (Ansell 1973), Aulacomya ater (Griffiths & King 1979), Mytilus edulis (Thompson & Bayne 1972, Bayne et al. 1984,1989), Mya arenaria (MacDonald et al. 1998), Musculista senhousia (Inoue & Yamamuro 2000), Mulinia edulis, Mytilus chilensis (Velasco & Navarro 2003), Cerastoderma edule (Navarro et al. 1992, 1994) and Hiatella arctica (Sejr et al. 2004). It is in contrast, however, with the finding that there was no influence of food concentration on oxygen consumption in M. arenaria and Placopecten magellanicus (Mac Donald et al. 1998).

The increase in oxygen consumption by Argopecten nucleus and Nodipecten nodosus with increase in water temperature is explained by the positive effect of increase in temperature on the speeds of metabolic reactions (Eckert et al. 1990) and is in agreement with results from other bivalves such as Ostrea edulis (Beiras et al. 1995) Crassostrea gigas (Bougrier et al. 1995) and Argopecten ventricosus (Sicard et al. 1999). This behavior differs from that found for O. edulis (Beiras et al. 1995) and Argopecten purpuratus (Navarro et al. 2000) after being experimentally held at a different temperature for a relatively long period (3 wk); under these conditions the oxygen consumption did not vary with temperature, showing an acclimation of the organisms to manipulation of this parameter. According to Eckert et al. (1990), acclimation of tissues and organisms to different temperatures could occur because of the modification of sensitivity of enzymatic activity, changes in molecular structure of one or more enzymes, or to changes in the quantities of enzymes.

In spite of the similarities among feeding rates of the scallops, Nodipecten nodosus had greater oxygen consumption than Argopecten nucleus, suggesting that the latter species was more efficient than the former in utilization of this resource in carrying on its metabolic functions. In studies comparing oxygen consumption among bivalve species (Mac Donald et al. 1998, Velasco & Navarro 2003), it was noted that the existence of interspecific differences in oxygen consumption coincided with parallel differences in feeding rates.

Excretion Rate

The decrease in excretion rate in Argopecten nucleus and Nodipecten nodosus with increases in food concentration contrasts with most comparable studies, as the more typical response has been a positive relation between these variables (Mulinia edulis, Mytilus chilensis, Velasco & Navarro 2003, Hiatella arcaica, Sejr et al. 2004), or even no relation at all (Mya arenaria and Placopecten magellanicus, MacDonald et al. 1998). It is known that bivalves use proteins stored in body tissues as energy sources when the food supply is incapable of satisfying metabolic requirements, resulting in an increase in the excretion rate (Bayne & Newell 1983). Therefore the lower ranges of food concentration tested in this study (10 and 20 cells [micro][L.sup.-1]) may have been insufficient to supply the energetic demands of the two scallop species, which then had to rely on the catabolism of endogenous proteins.

The reduced excretion rates of Argopecten nucleus and Nodipecten nodosus at low water temperatures, particularly when the food concentration was 20 cells [micro][L.sup.-1] or less, could be explained by a decrease in metabolism under these conditions, which could in turn, lower the metabolic demands of the organisms to a point at which they were not required to resort to the use of stored proteins. The same tendency has been observed in other bivalves such as Ostrea edulis (Beiras et al. 1995).

The excretion rate of Argopecten nucleus was similar to that of Nodipecten nodosus, similar to findings in interspecific comparisons under the same conditions in Mya arenaria and Placopecten magellanicus (MacDonald et al. 1998) and Mulinia edulis and Mytilus chilensis (Velasco & Navarro 2003). The similarity in excretion rates of the bivalves, together with the similarities between absorption rates, suggested that the utilization of proteins in the two species comparisons were quite similar.

Scope for Growth

The positive high correlation between scope for growth and feeding rates, and the low values of oxygen consumption and excretion indicated that the energy acquisition by feeding was more important than the energy output in determining the values of scope for growth. Because values for ingestion and absorption depend on the filtration rate, it can be assumed that the filtration rate is the main determinant of physiological condition in Argopecten nucleus and Nodipecten nodosus under the conditions tested in the present study. Similar conclusions have been derived for Argopecten purpuratus (Navarro et al. 2000), Mya arenaria and Placopecten magellanicus (Bacon et al. 1998, MacDonald et al. 1998), and Mulinia edulis and Mytilus chilensis (Velasco & Navarro 2002; 2003). Nevertheless, in species such as Ostrea edulis (Ansell & Sivadas 1973) and M. edulis (Velasco & Navarro 2002), under certain conditions the oxygen consumption may constitute an important portion of the energy absorbed, and may have an important effect on the growth potential.

The high growth potentials of Argopecten nucleus and Nodipecten nodosus when fed intermediate microalgal concentrations (40 cel [micro][L.sup.-1]), as well as the low values obtained at the extremes of the algal concentration (10 and 60 cel [micro][L.sup.-1]), are in accord with results obtained for Aulacomya ater (Griffiths & King 1979), and Mytilus chilensis (Velasco & Navarro, 2003). They differ, however, with findings from other studies, where scope for growth and algae concentration showed an inverse relation, as in M. chilensis (Navarro & Winter 1982), or a direct relation as in Cerastoderma edule, Tapes decussatus (Navarro & Iglesias, 1995), Mulinia edulis (Velasco & Navarro 2003), Mya arenaria and Placopecten magellanicus (MacDonald et al. 1998). These differences in the responses of the scope for growth to food concentration may be caused by the utilization of more narrow ranges of food concentration in the laboratory than those to which the test organisms are accustomed in their natural habitats.

The scope for growth of A. nucleus was not influenced by temperature, showing that this species had great plasticity of physiological rates in relation to this factor. On the other hand, the scope for growth of N nodosus was greater at the intermediate temperature in our study (25[degrees]C), where there was a high degree of sensitivity of the feeding rates in relation to temperature. The lowering of scope for growth at lower temperature was probably related to decrease in metabolic rate, and at high temperatures to the denaturation of related enzymes (Eckert et al. 1990). The direct relation between scope for growth and temperature coincide with that observed in other species such as Cerastoderma edule, Tapes decussatus (Navarro & Iglesias 1995), Argopecten purpuratus (Navarro et al. 2000) and Pecten maximus (Laing 2000). At extremes of food concentration, the scope for growth of Nodipecten nodosus was low, and independent of temperature, therefore under these conditions the negative effects of either the absence, or excess of food on the physiological condition of the organism nullified the positive effect of providing an optimal temperature of 25[degrees]C. Navarro and Iglesias (1995) found that at very low concentrations of food that the scope for growth of Cerastoderma edule and Tapes decussatus became negative and decreased with increase in temperature. In this study, the food concentrations were not so extreme as to produce negative scope for growth at which we might have observed a similar phenomenon.

It has been found that the optimal food concentration increases with the water temperature for Arctica islandica, Modiolus modiolus (Winter 1970) and Pecten maximus (Laing 2000). This relation was not observed in this study with either scallop species. In Argopecten nucleus the optimal food concentration was independent of temperature, because the optimal food concentration was found to be the same (40 cel [micro][L.sup.-1]) at all the temperatures tested. Conversely, in Nodipecten nodosus the optimal food concentration increased with decrease in temperature. This response was probably because saturation of the alimentary system did not occur under low temperature conditions and high food concentrations, thus permitting continuance of high ingestion and absorption rates.

Similarities between the scope for growth in Argopecten nucleus and Nodipecten nodosus indicate that the two species may compete with equal efficiency under the same conditions of food concentration and temperature, based on the environmental conditions occurring in each of their natural habitats. Studies that have compared scope for growth between species with differing habitats (epifaunal and infaunal) have found interspecific similarities under certain shared feeding conditions, which may not occur under other conditions in which interspecific differences become manifest (MacDonald et al. 1998, Velasco & Navarro 2002, Savina & Pouvreau 2004).

In conclusion, this study found that: (1) the scope for growth and the feeding rates of Argopecten nucleus and Nodipecten nodosus increased with increased food concentration but decreased at higher and lower extremes of food concentration; (2) the scope for growth and feeding rates of A. nucleus were not affected by temperature, with the reverse true for N. nodosus, increasing at 25[degrees]C and decreasing at higher temperature (28[degrees]C); (3) A. nucleus and N. nodosus showed similar responses and values in all their physi ological rates, except for oxygen consumption, where A. nucleus displayed lower rates.

ACKNOWLEDGMENTS

The author gratefully acknowledges W. Barbosa, J. Barros and the staff of the Moluscos y Microalgas Laboratory of the Universidad del Magdalena for their help during the experiments and also thanks ASOPLAM and INVEMAR for providing the experimental animals. This study was supported by a research project CONCIENCIAS-SENA-UNIVERSIDAD DEL MAGDALENA 111709-12394 and the grant International Foundation for Science (IFS) A/3363-1.

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L. A. VELASCO * Instituto de Investigaciones Tropicales (INTROPIC), Universidad del Magdalena, Carrera 2 No 18-27, Taganga, Santa Marta, Colombia

* E-mail: [e-mail]