Effects of salinity on sand burrowing activity, feeding and growth of the clams Mactra veneriformis, Ruditapes philippinarum and Meretrix lusoria.

ABSTRACT Mactra veneriformis, Ruditapes philippinarum, and Meretrix lusoria are dominant clams in Japanese tidal flats. Juveniles and adults of these species were reared in the laboratory and the effects of salinity on their sand burrowing activity and growth- and clearance-rates were examined. Juveniles of each species (shell length 10-16 mm) filtered water actively in the salinity range of 11.8-34.6 psu, with clearance rates not affected by salinity. The clams showed positive growth in the above salinity range, but growth rates were retarded at 11.8 psu, probably because of increased respiration at low salinity levels. In 6.1-psu seawater, adult Ma. veneriformis dug into the sand after a few days acclimation, Me. lusoria did not and R. philippinarum died. At 10.8 psu, adult Ma. veneriformis filtered water actively, Me. lusoria dug into the sand but did not filter water, and R. philippinarum neither dug into sand nor filtered water. These results indicate that the clam species examined are euryhaline but that the response to low- salinity water ([less than or equal to] 11 psu) by adult clams (shell length 31-37 mm) differed among species: Ma. veneriformis is the most adaptable to low- salinity water, followed (in order) by Me. lusoria and R. philippinarum. These differences, however, are not consistent with the distribution patterns of these clam species in the Shirakawa tidal flat (Japan), where salinity varies spatially and temporally.

KEY WORDS: clearance rates, growth, Mactra veneriformis, Meretrix lusoria, Ruditapes philippinarum, salinity

INTRODUCTION

Salinity affects the behavioral and physiological processes of bivalves (e.g., Kautsky 1982) and is considered to potentially control species distribution in the field (Shriver et al. 2002, Marsden 2004). Thus, understanding behavioral and physiological processes (e.g., feeding, respiration, energetics and growth) as a function of salinity will help explain the distribution and dominance of bivalve species in the environment, where salinity varies spatially and temporally. To date, however, studies on these processes are limited (Bohre 1972, Stickle & Sabourin 1979, Numaguchi & Tanaka 1987, Navarro 1988, Navarro & Gonzalez 1998, Gardner & Thompson 2001, Kim et al. 2001).

Shirakawa tidal flat (32[degrees]47'N, 130[degrees]36'E) is one of the largest sandy tidal flats in Ariake Sound, western Kyushu, Japan, with an area of 4.1 [km.sup.2]. It is located at the mouth of the Shirakawa River; the salinity on the flat fluctuates spatially and temporally and decreases to <10 psu after a heavy rainfall (Tamaki et al. in prep.).

Infaunal clams Mactra veneriformis Deshayes in Reeve, 1854 (Mactridae), Ruditapes philippinarum (Adams & Reeve, 1850) (Veneridae) and Meretrix lusoria (Roeding, 1798) (Veneridae) are abundant and show different distribution patterns on the flat. Ma. veneriformis and R. philippinarum dominate the middle to upper and lower to middle zones of the flat, respectively, and the main distribution area of Me. lusoria is close to the river mouth (Tamaki 2004, Tamaki et al. in prep.). Thus, it is reasonable to anticipate that salinity affects the distribution/dominance pattern of the bivalve species on the Shirakawa flat. In this study, burrowing activity, feeding and growth of these species as a function of salinity were examined in laboratory experiments. The aim was to clarify the role of salinity as a controlling factor in determining the abundance pattern of these bivalves in a sandy tidal flat environment.

MATERIALS AND METHODS

Materials and Analytical Procedures 

Clams were collected primarily from the Shirakawa flat. In some cases, they were obtained from a tidal flat in Tokyo Bay (35[degrees]39'N, 139[degrees]59'E; Nakamura et al. 2002) or from the Kitsuki tidal flat in the Seto Inland Sea (33[degrees]25'N, 131[degrees]38'E; Table 1). Diatoms (Thalassiosira sp., cell diameter ~5 [micro]m) were used as the algal diet for the clams and were cultured in a modified f/2 medium at 20[degrees]C (N[O.sub.3.sup.-] concentration, 200 [micro]M; salinity, 25-30 psu; Nakamura et al. 1995). Rearing water for clams was prepared by adding 50 mL of late log-phase dense algal culture to 1000 mL of filtered and diluted seawater, salinity was adjusted by the addition of deionized water to the original seawater (~35 psu). All experiments were conducted at 20[degrees]C under dim light.

The length (1), height (h) and width (d) of the shells were measured to the nearest 0.1 mm with a digital caliper. The soft-body dry weight of each animal (w) was measured with an electric balance (detection limit 0.1 mg) after the soft body had been dried for 2-3 d at 80[degrees]C. The condition index (CI) was defined as the ratio of w to the "volume" of the shell (v = 1 x h x d):

CI = w/v. (1)

The ash-free dry weight (AFDW) of the soft body (b) was measured by heating the dried soft body at 450[degrees]C for 6 h. The ratio of b/w was 0.80 [+ or -] 0.02, 0.83 [+ or -] 0.03 and 0.82 [+ or -] 0.03 (mean [+ or -] standard deviation) for juvenile Ma. veneriformis (n = 19; l = 12-13 mm), R. philippinarum (n = 20; l = 11-12 mm) and Me. lusoria (n = 16; l = 10-13 mm), respectively.

The chlorophyll-a (Chl-a) concentration in the rearing water was measured fluorometrically after filtration of the seawater sample through a glass fiber filter (Whatman GF/C).

Sand Burrowing Activities of Juvenile Clams

Clams were reared in covered chambers (inner diameter 8.5 cm; height 3.5 cm) containing sand (depth ~1 cm) and 55 mL of rearing water of varying salinity (6.7, 11.2 or 25.0 psu) (Table 1, runs A-1 and A-2). Juvenile Ma. veneriformis or R. philippinarum, preconditioned at 30 psu for 1 d, were placed on the surface of the sand (10 animals per chamber; day 1). The animals were reared in the salinity-adjusted rearing water without aeration for 1 d and were then transferred to a beaker along with the rearing water. The sand was washed with filtered and diluted seawater (salinity within 2 psu of the rearing water). Fresh rearing water (55 mL) was added to the chamber, and the animals were returned to the surface of the sand as mentioned earlier. These procedures were continued daily until day 4 for Ma. veneriformis and day 10 for R. philippinarum. The number of animals that dug into the sand within 30 min after placement was recorded.

Effects of Salinity on Growth and Clearance Rates of the Juvenile Clams

Clam Rearing

Clams brought to the laboratory were placed in plastic chambers (inner diameter 11 cm; height 20 cm) that contained sand (depth ~4 cm) and 1,000 mL of GF/C-filtered seawater (30 psu), and preconditioned with aeration until the next day (day 1). On day 1, the animals in each chamber were transferred to a beaker containing ~100 mL of the seawater from the chamber. The remaining seawater was discarded, and the sand was washed with filtered seawater (salinity within 2 psu of the rearing water). Then, 1,050 mL of the rearing water (salinity = 11.8-34.6 psu) was added to the chamber and the animals (1-3 animals per chamber) were returned to the chamber; the incubations continued with aeration. The top of each chamber was covered with plastic wrap. These procedures were repeated daily and clams were reared for 19-21 d (Table 1, Runs B-1-a, B-1-b, B-2, B-3). Animals were labeled with numbers using a felt-tip pen, and their l, h and d values measured before they were placed in the rearing water on day 1. In addition to estimate the initial CI values, several acclimated animals were euthanized on day 1 and their l, h, d and w values measured.

Chl-a concentrations in the fresh rearing water ([C.sup.IN]) were monitored daily by measuring the Chl-a concentration of the original algal culture and the degree of dilution; [C.sup.IN] was ~40 [micro]g [1.sup.-1] throughout the experiments. The Chl-a concentration in the rearing water just before replacement of the culture ([C.sup.OUT]) was also measured. At the end of the incubation period, l, h, d, and w of all animals were measured.

Clam Growth Assessment

Clam growth was assessed based on changes in the total soft-body AFDW in a chamber (B) during the rearing period (T). B is defined as:

B = [summation][b.sub.i] = [alpha] x [summation][w.sub.i], (2)

where [alpha] is the b/w ratio for each species and the subscript i denotes each individual. The weight-specific daily increment of B (G; % [d.sup.-1]) was used as an index of growth:

G = 100 x ([DELTA]B/<B>)/T, (3)

where [DELTA]B is the increment of B during the rearing period ([DELTA]B = [B.sub.fin] - [B.sub.init]) and <B> is the time-averaged B over T. (B) is approximated as ([B.sub.init] + [B.sub.fin])/2, where [B.sub.init] and [B.sub.fin] are the initial and final values of B, respectively, during the rearing period. [B.sub.fin] was calculated from Eq. (2) with direct measurements of w after incubation. [B.sub.init] was estimated from Eq. (1) and initial estimates of CI: B = [alpha] x [summation][w.sub.i] = [alpha] x [summation] ([CI.sub.i] x [v.sub.i]). Because CI on day 1 can be approximated as a normal distribution of N([CI.sub.0], [[sigma].sub.0.sup.2] (Table 1; Kolmogorov-Smirnov test; P > 0.2), the 95% confidence interval of [B.sub.init] is given as:

[B.sub.init] = [alpha] x [[CI.sub.0] x [summation][v.sub.i] [+ or -] 1.96 [[sigma].sub.0] [square root of [summation]][v.sub.i.sup.2]. (4)

G values were assessed as a function of salinity.

Weight-specific daily rations during the rearing period in terms of AFDW (% [d.sup.-1]) were calculated as:

Q = 100 x 40([summation]V x ([C.sub.j.sup.IN] - [C.sub.j+1.sup.OUT]))/(T x <B>), (5)

where V is the volume of the rearing water in a chamber; subscript j denotes day j of the experimental culture; the factor 40 is the ratio of AFDW/Chl-a (w/w) for the algal diet (Nakamura 2004).

Feeding Activity

Feeding activities were measured on the final day of the incubation. After the replacement of the rearing water, seawater (50 mL) was sampled just after all animals in the chamber had dug into the sand (usually within 10 min after replacement of the medium) and 45 min to 1 h later. Feeding activity of the clams was assessed in terms of the weight-specific clearance rates (CR):

CR = [(V/[t.sub.s]) x ln ([C.sub.0]/[C.sub.1]) - [CR.sup.*]]/([summation][w.sub.i]), (6)

where [C.sub.0] and [C.sub.1] are the Chl-a concentration at the beginning (time 0) of the feeding experiment and at time [t.sub.s], [summation][w.sub.i] is the total soft-body dry weight in a chamber; and [CR.sup.*] is an apparent clearance rate without animals (control):

[CR.sup.*] = (V/[t.sub.s]) x ln ([C.sub.0.sup.*]/[C.sub.1.sup.*]), (6')

where [C.sub.0.sup.*] and [C.sub.1.sup.*] are the Chl-a concentration in the control at the start and time [t.sub.s]. [CR.sup.*] was measured the day after each experimental culture had finished. [CR.sup.*] was [less than or equal to] 60 mL [h.sup.-1] throughout the experiments and had little if any affect on the CR values.

Feeding Activity of Adult Clams

Adult Ma. veneriformis and R. philippinarum were reared as outlined in the previous section with the exception of the following point (Table 1, Runs C-1 and C-2): the rearing water that was replaced on a daily basis (fresh seawater, 1,000 mL) did not contain the algal food. Instead, dense algal culture (50 mL, 25 psu) was supplied 60 min after placement of the animals. Seawater was sampled just after the introduction of the algal food and 30 min later for Chl-a measurements. Clearance rates were measured daily. In addition, R. philippinarum was reared at 6.1 psu for 6 d (Table 1, Run D-1) to check survivorship.

Me. lusoria were reared at 6.1 and 25.0 psu with two animals per chamber from day 1-4. In the rearing-water replacement process on day 5, one of the two animals in each chamber was transferred to a new chamber, and salinity was changed: from 6.1-25.0 and from 25.0-10.8 psu. The remaining animal was reared at the original salinity level. Rearing was continued through day 8, and clearance rates were measured from day 5-8 (Table 1, Run C-3). Other processes were the same as in the preceding paragraph.

RESULTS

Sand Burrowing Activities of Juveniles

On day 1 when Ma. veneriformis were newly transferred to the seawater with salinities of 6.7, 11.2 and 25.0 psu, 3, 9 and 10 animals out of 10, respectively, dug into the sand within 30 min (Fig. 1a). On or after day 2, [greater than or equal to] 90% of animals dug into sand quickly, indicating their rapid adaptation to the new salinity level. In contrast, of the 10 R. philippinarum placed in 11.2 psu seawater, only one animal dug into the sand on day 1. The number of animals that dug into the sand at this salinity increased gradually with time; all animals dug into the sand by day 9 (Fig. 1b).

[FIGURE 1 OMITTED]

Growth and Clearance Rates of Juvenile Clams

Ma. veneriformis were reared at the daily ration of ~6% [d.sup.-1] except for animals reared at 11.8 and 16.2 psu in Run B-1-a, where rations were 7% and 8% [d.sup.-1] respectively. Animals at all salinity levels dug into sand quickly after the daily replacement of the rearing water and cleared water actively ([C.sub.j+1.sup.OUT]/[C.sub.j.sup.IN]) [less than or equal to] 0.1) throughout the rearing period. The shell length increased by 0.2-0.9 mm in Run B-1-a and by 0.7-1.5 mm in Run B-1-b during the rearing period; they showed a tendency to retard the shell increment at lower salinity levels (data not shown). The growth of Ma. veneriformis in terms of soft-body AFDW (G value; Eq. 3), was positive at all salinity levels, with values of 1.0%-2.8% [d.sup.-1]. The G value at 11.8 psu was slightly but significantly lower than those at [greater than or equal to] 16.4 psu (Fig. 2a; Mann-Whitney U-test, P < 0.025, 1-sided).

[FIGURE 2 OMITTED]

As CR of bivalve species (including R. philippinarum) usually follows a power function of w with an exponent of about -1/3 (Bayne & Newell 1983, Nakamura 2004), the CR values obtained by Eq. 6 were standardized to <w> of 15 mg ([CR.sub.15]) as:

[CR.sub.15] = CR (15/<w>).sup.-1/3], (7)

where <w> is the averaged soft-body dry weight (<w> = [summation][w.sub.i]/n; n = number of clams in a chamber). [CR.sub.15]-values were 11.9-20x41 [g.sup.-1] [h.sup.-1] and not affected by salinity (Fig. 2b; [r.sup.2] = 0.087, P > 0.2).

R. philippinarum were reared at the daily ration of ~6% [d.sup.-1]. In contrast to the result for Run A-2, animals reared at 11.8 psu dug into sand within 1 h and cleared water even on day 1 ([c.sub.2.sup.OUT]/[C.sub.1.sup.IN] = 0.16). Shell length increased by 0.2-1.5 mm during incubation and the increment was retarded at 11.8 psu (data not shown). G value was positive (1.4-2.6% [d.sup.-1]), but was significantly lower at and below 16.4 psu compared with clams at salinities [greater than or equal to] 20.9 psu (Fig. 3a; U-test, P < 0.05, 1-sided). CR-values standardized to <w> of 10 mg ([CR.sub.10]) were in the range of 20.6-30.1 1 [g.sup.-1] [h.sup.-1] and not affected by salinity (Fig. 3b; [r.sup.2] = 0.289; P > 0.2)

[FIGURE 3 OMITTED]

Me. lusoria were reared at the daily ration of ~5% [d.sup.-1]. All animals dug into sand quickly and cleared water actively ([C.sub.j+1.sup.OUT]/ [C.sub.j.sup.IN] [less than or equal to] 0.1). Increase in shell length during the culture was in the range of 0.2-0.5 mm. G values were positive (0.5-2.0% [d.sup.-1]) but significantly lower at and below l6.4 psu compared with those at [greater than or equal to] 20.9 psu (Fig. 4a; U-test, P < 0.05, one-sided). [CR.sub.15] was in the range of 9.0-17.0 1 [g.sup.-1] [h.sup.-1] and not affected by salinity (Fig. 4b; [r.sup.2] = 0.110; P > 0.2).

[FIGURE 4 OMITTED]

Burrowing Activities and Clearance Rates of Adult Clams

Individuals of Ma. veneriformis placed in [greater than or equal to] 15.6 psu seawater dug into the sand quickly on day 1. Although the clearance rate (standardized to <w> of 300 mg; [CR.sub.300]) at 15.6 psu was less than the half of that at 20.3 and 25.0 psu on day 1, it rebounded by day 2 (Fig. 5a). Animals in 10.8 psu seawater did not dig into the sand on day 1, but they did so on and after day 2; clearance rates rebounded by day 3. Animals in 6.1-psu seawater did not burrow from day 1-3. One of the animals started burrowing on day 4, and both of them dug into the sand on and after day 5. However, [CR.sub.300]-values at 6.1 psu were much lower than those observed at higher salinity levels.

[FIGURE 5 OMITTED]

On day 1, R. philippinarum did not dig into the sand except for the animal placed in 25.0 psu seawater. [CR.sub.300] values at 25.0 psu on day 1-2 were low, but rebounded by day 3 (Fig. 5b). The animal placed in 20.3 psu seawater dug into sand on and after day 2, and [CR.sub.300] values reached a plateau by day 2. At 10.8 psu, animals opened their siphons on and after day 4 but did not dig into sand throughout the rearing period. In addition, [CR.sub.300] values were much lower than those at [greater than or equal to] 20.3 psu. Experiments were conducted in duplicates at 15.6 psu, and results differed between chambers. In one experimental chamber, one of the animals dug into sand on day 2 and both of them dug on and after day 3; [CR.sub.300] values rebounded by day 3. In the other chamber, animals did not dig into sand throughout the rearing period, and [CR.sub.300] values were low. Animals placed in 6.1 psu seawater did not dig into the sand and died by day 6 (Run D-1; clearance rates were not measured).

Me. lusoria reared at 6.1 psu did not dig into the sand throughout the experiment; no feeding activities were detected from day 5-8 (Fig. 6a; CRs were not measured from day 1-4.). The animal reared at 6.1 psu and returned to the seawater with salinity of 25.0 psu on day 5 dug into the sand within 30 min, and filtration resumed on the same day (day 5, Fig. 6a). The animal reared at 25.0 psu and returned to 10.8 psu on day 5 dug into the sand within 1 h after the placement. However, its feeding activity was low from day 5-8 (Fig. 6b).

[FIGURE 6 OMITTED]

DISCUSSION

The clam species examined in this study are abundant in areas with large salinity fluctuations (e.g., tidal flats at a river mouth), and are consequently expected to be euryhaline. The positive growth and high feeding activity shown by these species in a wide range of salinities (Figs. 2-4) support this expectation. The response of each species to low-salinity water ([less than or equal to] 11 psu) differed among species, however; this will be discussed further in later sections.

Comparisons With Previous Studies

Although the burrowing activity and clearance rates of Ma. veneriformis were not affected by salinity at [greater than or equal to] 11 psu (Fig. 1a, 2b, 5a), they are retarded at 6.1 psu (Fig. 5a). These results were consistent with a previous observation of adult Ma. veneriformis (Tanaka 1956): the crawling activity of dissected gill was not affected at salinity [greater than or equal to] 9 psu but was retarded at 7 psu.

Survivorship of R. philippinarum in low salinity water was examined by Kurashige (1942) and Ikematsu & Matsumoto (1956). In both cases, the clams (shell length ~1-35 mm) died at 7 psu within 9 d, but survived at 14 psu. These results are consistent with our data that the clams with shell size of ~35 mm died within 6 d when reared at 6.1 psu.

Numaguchi & Tanaka (1987) examined the survival and shell growth of juvenile Me. lusoria as a function of salinity. Shell growth was positive with no mortality at [greater than or equal to] 12.8 psu; survival rate at 6.4 psu was <10% after 28 d. Although the growth of Me. lusoria at ~6 psu was not examined in this study, our observation that the adult animal neither dug into the sand nor showed positive filtration at 6.1 psu (Fig. 6a) also indicates that the salinity level of ~6 psu was unfavorable for Me. lusoria. In addition, the positive growth at 11.8 psu (Fig. 4a) was consistent with the results of the previous study.

Effects of the Growth Stage on the Responses to Low Salinity Water

When R. philippinarum juveniles (shell length = 4-6 mm; Run A-2) were placed into low-salinity water of 11.2 psu, it took 8 days for all animals to dig into the sand (Fig. 1b). In addition, adult R. philippinarum placed in 10.8-psu water (Run C-2) neither dug into the sand nor filtered water for 9 d (Fig. 5b). On the other hand, late-stage juveniles (shell length = 11-12 mm; Run B-2) dug into the sand and filtered water on the same day they were placed into seawater of 11.8 psu. These observations may indicate that adaptability to low-salinity water differs with growth stage, being highest for late-stage juveniles. It should be noted, however, that the late-stage juveniles used in this study were collected from Tokyo Bay (a distance of ~1000 km apart from Shirakawa-flat); the other clams were from the Shirakawa-tidal flat. Geographic differences in salinity tolerance have been suggested for some bivalve species (Newell 1991); consequently, the results indicating a correlation between salinity adaptation and growth stage should be treated cautiously.

Growth and Feeding as a Function of Salinity

In a previous study, R. philippinarum were reared in the same system as in this study with growth (G) as a function of daily ration (Q) examined at a salinity of 30 psu (Nakamura 2004). A linear relationship between Q and G was observed, which is reasonable from the standpoints of assimilation efficiency and weight loss by respiration for bivalve species. In this study, G-values of R. philippinarum at a salinity of [greater than or equal to] 20.9 psu (Fig. 3a) were comparable to those expected from the earlier mentioned relationship between Q and G. This, together with the reproducibility in the growth pattern of Ma. veneriformis in Runs B-1-a and B-1-b (Fig. 2a), indicates that our rearing system worked well for assessing clam growth.

Previous studies indicated that retardation of bivalve growth in low-salinity water is coupled with a decrease in feeding rates (Navarro 1988, Navarro & Gonzalez 1998). In this study, however, G values in Runs B-1-B-3 were retarded at 11 psu without decreasing the clearance or feeding rates (Figs. 2-4). The daily ration did not differ greatly among salinity levels examined. In addition, exposure of Thalassiosira sp. cells (the algal diet) to low-salinity water (11 psu) for 24 h caused neither cell mortality nor change in cell morphology (Y. Nakamura unpublished data). Retardation of growth at low-salinity levels could be attributable to the decreased assimilation efficiency or increased energy loss by respiration. An increase in respiration rates at low salinity was observed in R. philippinarum and other species (Stickle & Sabourin 1979, Kim et al. 2001), probably reflecting increased energy demands for osmoregulation (Davenport 1979). Consequently, the growth retardation was most likely caused by an increase in the respiration rate at low salinity.

Ecological Considerations

The responses of adult clams to low-salinity waters were compared among species: at ~6 psu, Ma. veneriformis dug into the sand and filtered water after acclimation (Fig. 5a). In contrast, Me. lusoria neither dug into the sand nor filtered the water (Fig. 6a), and R. philippinarum died at this salinity (Run D-1). The activity of Ma. veneriformis at 11 psu after acclimation was almost the same as that at 25 psu (Fig. 5a). At 11 psu, Me. lusoria dug into the sand but did not filter the water (Fig. 6b); R. philippinarum neither dug into the sand nor filtered the water at 11 psu (Fig. 5b). These observations indicate that Ma. veneriformis is the species most adapted to low-salinity water, followed in order by Me. lusoria and R. philippinarum.

Heavy rainfall and freshwater floods on the Shirakawa flat often induce mass mortality of Ma. veneriformis and R. philippinarum (Tamaki 2004, Tamaki et al. in prep). In such cases, mortality rates of Ma. veneriformis are higher than those of R. philippinarum along the intertidal gradient where their distribution overlaps. Although the mortality pattern of the less abundant Me. lusoria is not as clearly detected, its main distribution area is close to the mouth of the Shirakawa River. These field observations suggest that Ma. veneriformis is the most vulnerable, and Me. lusoria the most tolerant to low-salinity water. This order is the reverse of that obtained in the present laboratory experiments. Such inconsistency between field and laboratory results with respect to salinity has also been reported in mussels (Gardner & Thompson 2001). More information on the effects of salinity on larval development and settlement, and impact of other factors (such as turbidity and heavy sedimentation associated with floods) is probably required to explain the different distribution patterns of the three bivalve species in the field.

ACKNOWLEDGMENTS

The authors thank the members of Oshima Fishery Association and S. Watasue for their help in the field and K. Kusube for technical assistance in the laboratory. This study was partially supported by Grant-in-Aid Nos. 13854006 and 15570018 from Japan Society for the Promotion of Science.

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Y. NAKAMURA, (1) * K. HASHIZUME, (1) K. KOYAMA (2) AND A. TAMAKI (2)

(1) National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japan; (2) Marine Research Institute, Nagasaki University, Taira-machi 1551-7, Nagasaki 851-2213, Japan

* Corresponding author. E-mail: [e-mail]

TABLE 1.
Summary of experimental conditions.

                                                  Origin of
Experimental                                        Sample
    Run        Species     Objective Variables   (tidal flat)

  A-1          Ma. v *     Burrowing activity     Shirakawa
  A-2          R. p **     Burrowing activity     Shirakawa
  B-1-a        Ma. v       Growth and feeding     Shirakawa
  B-1-b        Ma. v       Growth and feeding     Shirakawa
  B-2          R. p        Growth and feeding     Tokyo Bay
  B-3          Me. l ***   Growth and feeding     Kitsuki
  C-1          Ma. v       Feeding                Shirakawa

  C-2          R. p        Feeding                Shirakawa

  C-3          Me. l       Feeding                Shirakawa

  D-1          R. p        Survivorship           Shirakawa

                            Shell        Salinity       Rearing
Experimental    Sampling    Length         Range        Duration
    Run           Date       (mm)          (psu)          (d)

  A-1           1 July 04    5-7      6.7, 11.2, 25.0      4
  A-2          19 July 04    4-6        11.2, 25.0        10
  B-1-a        12 July 03   15-16       11.8-34.6         21
  B-1-b        28 Aug 03    12-13       11.8-34.6         20
  B-2          25 Sept 03   11-12       11.8-34.6         19
  B-3          15 June 03   10-13       11.8-34.6         19
  C-1          19 July 04   31-33        6.1-25.0          8

  C-2           1 July 04   32-35       10.8-25.0          8

  C-3          19 July 04   33-35     6.1, 10.8, 2.50      8

  D-1          19 July 04   35-37        6.1, 25.0         6

Experimental             Number of Animals
    Run                    in a Chamber

  A-1                          10
  A-2                          10
  B-1-a                         1
  B-1-b                         2
  B-2                           3
  B-3                           3
  C-1          1 ([greater than or equal to] 10.8 psu)
                 or 2 (6.1 psu)
  C-2          1 (>20.3 psu)
                 or 2 ([less than or equal to] 15.6 psu)
  C-3          1 (day 5-8)
                 or 2 (day 1-4)
  D-1          2

Experimental   Initial Condition Index
    Run        Mean [+ or- ] SD, N

  A-1
  A-2
  B-1-a        11.3 [+ or -] 1.5, N = 19
  B-1-b        19.0 [+ or -] 2.7, N = 44
  B-2          21.8 [+ or -] 2.3, N = 45
  B-3          16.2 [+ or -] 1.3, N = 16
  C-1
  C-2
  C-3
  D-1

Ma. v *: Mactra veneriformis

R. p **: Ruditapes philippinarum

Me. L ***: Meretrix lusoria