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Bedload transport of newly-settled juveniles of the Manila clam Ruditapes philippinarum observed in situ at Banzu tidal flat, Tokyo Bay.

ABSTRACT There have been numerous attempts to create and/or develop harvestable tidal areas where juveniles of the natural Manila clam Ruditapes philippinarum may be abundantly stocked. To prevent erosion and/or promote the colonization of natural juveniles through improving the physical stability of bottom sediment, it is important to understand the mechanisms that regulate dispersal and recolonization of benthic juveniles. The physical transport of bottom sediment is known to have a substantial effect on the spatial distribution patterns of infaunal bivalves in intertidal soft-bottom habitats. During the summers of 2004 and 2005, we conducted field experiments to identify the physical transport mechanisms for newly-settled postlarval Manila clams until growth to a size of I mm shell length at the Banzu tidal flat of Tokyo Bay in Japan. We used sediment traps to collect natural and released hatchery-reared juveniles, in parallel to acquiring measurements of seawater flow and observations of the spatial distribution of newly-settled natural juveniles. There was a sharp increase in the number of clams that were collected in the traps placed on the soft-bottom surface when [[tau].sub.w] (wave shear stress) exceeded 0.3 N [m.sup.-2]. Because the [[tau].sub.c] (advection shear stress) was far lower than [[tau].sub.w] ([[tau].sub.c]/[[tau].sub.w] =1/60-1/130), the initiation of juvenile transport appeared to depend primarily on wave-generated oscillatory flow. The number of trapped juveniles regressed linearly to the weight of the sediment that was simultaneously collected in the trap ([R.sup.2] = 0.99, 0.81), which indicated that clams were transported in a similar way to that of sediment grains, despite juveniles and sand particles exhibiting different physical properties (size and specific gravity). Hence bedload transport may have resulted from the biologically induced adhesion of juvenile clams to sediment grains and/or their burrowing behavior. In the release- recovery experiment of marked juveniles, a larger number of clams were recovered from traps that had been placed downstream of the water current from the release point. A denser distribution of the natural Manila clam population settled in mid-July 2004, and subsequently moved several hundreds of meters inshore within a one month period. The concurrent monitoring of bottom flow during a total 4 wk period in the summer of 2004 indicated that [[tau].sub.w] frequently exceeded the incipient threshold of bedload transport (assumed to be 0.3 N [m.sup.-2]). Consequently, the bedload movement of Manila clam juveniles in the study area was expected to be initiated at a wave shear stress that was greater than the incipient threshold, and in a downstream direction of the advection current. Because juvenile clams in the summer population appeared to be frequently subjected to hydrodynamic stress, which forces juveniles to move and halt incidentally in the early benthic stages, physical transport is likely to contribute to the changing pattern of juvenile distribution at the Banzu tidal flat of Tokyo Bay in Japan.

KEY WORDS: Ruditapes philippinarum, bedload transport, shear stress, post-larval distribution, intertidal sandflat


For decades, there have been numerous attempts to create and/or to develop a harvestable tidal area where juveniles of the natural Manila clam Ruditapes philippinarum (Adams and Reeve, 1850) may be abundantly stocked. Examples include the arranged deployment of bamboo poles, sand bags, or concrete blocks (Kobe et al., 1985, Fisheries Agency 1980, 1995), as well as the spreading of sand, crushed gravel, or shells over the sea-bottom surface (Toba et al., 1992, Thompson 1995, Nasu et al., 2002), and the modification of bottom elevation or the creation of dredged channels by engineering works (Fisheries Agency 1979, 1988a, 1988b). These methods were intended to prevent juveniles from being subject to erosion and to improve recolonization rates, by stabilizing the physical fluctuations of the bottom level. In addition, these methods aimed to increase the protective effect against predators by changing bottom sediment characteristics and to enhance the planktonic food supply by promoting water exchange. Whereas expected positive results were detected in some cases (Thompson 1995, Naito and Tsukushi 2004), an increase in the number of natural juveniles continued for few years (Ishida et al., 1994) or was far lower than expected (Tateishi et al., 1995) in other cases. Ultimately, the effect of some of these techniques remains inconsistent and unfeasible for clam culturists, particularly with respect to controlling hydrodynamic environment.

Inconsistent results primarily arise due to the inherent population dynamics of natural stocks of infaunal bivalves, such as Manila clam, which involves spatial and temporal fluctuations in recruitment abundance (Sekiguchi et al., 1995, Miyawaki and Sekiguchi 2000). These fluctuations depend on the spatial scale and abundance of larval supply (Nanbu et al., 2006, Toba et al., 2007), mortality caused by the deterioration in water quality and sediment characteristics (Fujii 2007), and on the predators and competitors that are present (Spencer et al., 1992, Saito and Imabayashi 1997, Mistri 2004a, 2004b). Hence, the natural abundance of Manila clam juveniles is inevitably characteristically unstable.

In addition, several of the methods which used to enhance natural clam stocks were designed to simply follow past examples of abundant natural spat-fall, which occurred in the vicinity of artificial constructions or under specific local conditions in aquaculture environments. However, the acquisition of expected satisfactory effects, or even the assessment of the precise causes of resultant poor effects, is difficult to achieve, except for understanding the mechanism that influences the stock dynamics of benthic juveniles. Hence, our knowledge remains limited about how the artificial manipulation of the hydrodynamic environment affects the dispersal and recolonization of Manila clam juveniles.

The transport of juveniles after settlement has been observed in many species of bivalves, and found to be an important process that determines the pattern and abundance of distribution of bivalve populations (Emerson and Grant 1991, Hunt and Scheibling 1997, Hiddink and Wolff 2002). The factors that control the transport of juvenile bivalves comprise hydrodynamic stress as a driving force, size and burrowing behavior of the bivalves as a resistant force, and the characteristics of the bottom sediment, which influence both forces (Norkko et al., 2001, Hunt 2004, Lundquist et al., 2004, Hunt 2005). In shallow or intertidal habitats, hydrodynamic force is the primary factor that generates the transport of bivalves (reviewed by Palmer et al., 1996). Hydrodynamic stress often changes over short periods, in response to changes in tidal and wind-driven waves and currents (Olivier et al., 1996, Hunt and Mullineaux, 2002), which are associated with tidal cycles and local weather conditions, particularly in intertidal areas that are commonly inhabited by Manila clams. However. previous studies, which examined the effects of hydrodynamic stress in relation to physical measurements and the rate of bivalve erosion, made observations at time intervals of a minimum of 1 tidal cycle or 1 day (Commito et al., 1995, Bouma et al., 2001, Norkko et al., 2001, Lundquist et al., 2006). Furthermore, studies of Manila clams have only investigated hydrodynamic transport in relation to variable spatial densities and physical parameters at intervals of several days or weeks (Kakino 2000. Kakino et al., 2010). As physical parameters may change every minute or hour, this detailed time scale is required to understand the mechanism of transport for the early life stages of bivalves in unsettled hydrodynamic environments.


For this purpose, we conducted field experiments to identify the physical transport mechanisms of newly-settled postlarval Manila clams until growth to a size of 1 mm shell length at the Banzu tidal flat of Tokyo Bay in Japan. In addition, we investigated spatial changes in the distribution patterns of newly-settled individuals. The objective of this study is to develop hypotheses about the mechanism of physical transport by clarifying the pattern of bedload traveling in clams and to discuss the influence of transport on the spatial distribution of juveniles.


Study site

The study site was located at the Banzu tidal flat on the eastern coast of Tokyo Bay (Figure 1) in Japan. The Banzu tidal flat stretches 8 km along the shore and extends 1.0-1.5 km offshore, forming a shallow area within an arc-shaped estuary system at the mouth of Obitsu River. The level height of the intertidal sandy bottom ranges from [+ or -]0 to +2.0 m [MLWS (mean low water in spring tide) = [+ or -]0 m]. The maximum tidal amplitude attains 2.2 m, with 1.0-1.5 km of the seaward length of the bottom surface being exposed to the air in the spring ebb tide. The seasonal temperature and salinity of the offshore sea water has a range of 8-28[degrees]C and 25-31 PSU, respectively (Chiba Prefectural Fisheries Research Center 2005). Juveniles of Manila clams were transplanted into culture plots by culturists every spring. The density of Manila clams, including transplanted individuals was 20-500 individuals [m.sub.-2] = (Chiba Prefectural Fisheries Research Center 2004). Infaunal bivalves were mainly dominated by Manila clams, in addition to naturally recruited shiofuki-gai Mactra veneriformis (Reeve, 1854), Chinese mactra Mactra chinensis (Philippi, 1846), and Jackknife clam Solen strictus (Gould, 1861).

The rectangular study area (900 m x 500 m) was established to the north of the mouth of the Obitsu River mouth in July 2004 (Figure 1-4). To analyze the spatial distribution of clams and sediments, the area was divided into 4 sections (A-D) based on bottom height and topography: an offshore subtidal section (bottom height <[+ or -]0 m) A, the margin of the tidal flat where a surf zone is generated in the ebb tide ([+ or -]0- +0.4 m) B, an extensive flat bottom section of planar topography (+0.2 - +0.6 m) C, and an nearshore section where the bottom height gradually elevates toward shore (>+0.6 m) D. The sampling stations of the clams and sediments were arranged on 13 along-shore lines that transected the study area. The trap collection experiments of juvenile clams and the monitoring of bottom flow were conducted at P.

Distribution of clams and sediments

Clams were repeatedly sampled at a total of 59 stations at 2 wk intervals from July 1 to August 16, 2004. At each sampling station, a cylindrical corer (41 mm in diameter) was inserted into the sediment to sample the benthic Manila clam juveniles together with surface sediment of 5-rem thickness. Three replicated core samples at each station were mixed and fixed in 1% seawater formalin with 0.1% rose bengal. We followed the methodologies of Toba et al., 2007 for the protocols to sort clams from the sediment, identify clam species, and measure the shell length of sampled clams.

For sediment grain analysis, 3 replicated core samples were collected at 35 stations using the same corer at the same time as the clam samples were collected. At each station, surface sediment samples of 5-mm thickness were mixed and transferred to the laboratory. Grain size analysis was performed by wet sieving, using x2-step mesh size sieves of 0.063-2 mm in size (JIS A1204, Japanese Standards Association 1990). The median grain size, [D.sub.50], was determined graphically from the cumulative curve of sediment dry weight. The silt + clay content was determined as the percentage dry weight of the <0.063 mm size fraction of sediment.

Polymodal size frequency distributions of the clams sampled on each date were divided into plural normal distributions following Gorie (2002). All identified normal distributions were regarded as cohort populations, each of which consisted of individuals from a single spawning event.

Time-series measurement of bottom current

A two-dimensional electro-magnetic current recorder (Compact-EM, JFE Advantech Co., Ltd.) was installed at site P to monitor seawater flow. The body of the recorder was buried vertically into the bottom substrate, with the center of the spherical sensor being positioned outside of the bottom boundary layer (20 cm above the bottom surface). The height at which the sensor was positioned ensured that the appropriate conditions to measure flow velocity at the outside of bottom boundary layer. Because that water flow at the study site was dominated by waves of<50 cm in height of 2-5 s cycles (Kakino 2000), bottom boundary layer at the site P was not considered to develop higher than 20 cm above the bottom surface excluding stormy conditions. Water current data were sampled for 5 rain at 2 Hz, resulting in 600 data points per burst, at 90 rain intervals. Data were recorded from 12:00 on July 17-06:00 on July 31 and from 15:00 on August 2-07:30 on August 16, 2004. Water temperature was simultaneously monitored from a temperature sensor that was equipped on the recorder.

Trap collection experiments of bedload

The collection of sediments and clams in the bedload was conducted using a cylindrical sediment trap (Emerson 1991) at site P on July 7 (ET1) and August 17 (ET2), 2004. The sediment trap was composed of an inner trap that was made of plastic pipe and an outer cylindrical holder. The inner trap was used to accumulate the sediments and clams. The outer holder was a blind-ended plastic cylinder (56 mm inner diameter x 500 mm length) that had sponge fittings on the inside. The holder was buried vertically into the bottom sediment, and the top of the pipe was flush (i.e., at the same level) with sediment surface. At the onset of collection, the inner trap (41 mm inner diameter x 400 mm length) was inserted into the holder, with the open top-end being located at the same vertical position as the holder. Emerson (1991) noted that an aspect ratio of >20 is required to avoid resuspension of minute and light substances such as organic detritus that accumulated in the trap. We used an aspect ratio of 9.8 (=400/41 mm) because, during the short period of collection (15 min) in the current study, the amount of clam juveniles and mineral sediments that were expected to accumulate was not large enough to saturate the available space within the trap. The holder was positioned at the site one day prior to the experiments to prevent the effects of artificial disturbance on ambient bottom sediment, and a rubber cap was temporarily placed over the top and left in place until the initiation of experiments.

Sediments and clams were collected from 06:00-17:55 (ET1) and 06:00-16:55 (ET2) at intervals of 20 min using a single trap. Each collection period lasted 15 rain. Eight sets of triple replicates of sediment cores were sampled around the site at low tide to estimate ambient clam size and sediment grain size. The fixation and staining of samples, extraction and identification of Manila clam juveniles, and analysis of grain size of sediments that were collected in traps and cores, followed the same methodology as the distribution analysis. Water current and temperature were recorded at 20-min intervals concurrently with trap collection. Current data were sampled for 5 rain at 2 Hz at each record.

Release-recovery experiment of hatchery-reared clams

Release-recovery experiments were carried out on September 19, 2005, to estimate the transport direction of released hatchery-reared clams under field hydrodynamic conditions. Three circular plots were established l0 m apart from one another near the site P. Twelve sediment traps were set at regular intervals in a circular arrangement, with a radius of I m from the release point at each plot (see Figure 8C). In the 3 plots, traps were numbered clockwise from 1 (north of release point) to 12, respectively. The preparation of traps and the manner of setting up the traps followed the same method as for ET1 and ET2.

Hatchery-reared Manila clam juveniles (mean shell length, 1.4 [+ or -] 0.0 ram) were derived from induced spawning by using naturally conditioned adults. Spawner clams were collected at coastal shallow waters in Tokyo Bay in the spring of 2005. The obtained larvae and settled juveniles were reared for 4 mo in an indoor tank, and were fed with cultured Pavlova lutheri or Chaetoceros gracilis. Prior to the experiments, clams were maintained in 50 ppm seawater solution of alizarin red for 2 days to stain the shell surface for later reidentification (Hidu and Hanks 1968, Tanaka 1980). Under the microscope, stained clams could be easily distinguished from ambient natural juveniles already inhabiting the plots.

Each plot was assigned 24,500 individuals of clams (numbers of individuals were estimated using wet weight). Clams were placed on the release point under the protective cover of a plastic cup (16 cm diameter x 12 cm height) for 1 h to allow the clams to burrow into the sediment. The experiment was initiated when the plastic cup was removed, which was alter most of the clams seemed to have completed burrowing. The traps were kept open for 90 rain after the initiation of experiments to collect clams that flushed over the trap entrance. The experiment was repeated 3 times, with overlapping timeframes (EL1-EL3: 13:15 14:45, 13:45 15:15, and 14:1-15:45). Water current and temperature were monitored at 15 rain intervals during the experiments, following the same method used for ET1 and ET2. The experiments were conducted during the flooding spring tide.

Hydrodynamic analysis

In shallow or tidal areas, waves coexist with currents as the hydrodynamic forces. In addition, sediments are often consisted with various sizes of sand grain particles. In the analysis of the relationship between movement of the clam juveniles and hydrodynamic forces, we applied the model for the sediment transport being used in coastal engineering. In this model, movement of the mixed-sized sediment grains is assessed using the physical forces of currents and waves as the drivers.

The velocity [u.sub.c] and direction [[theta].sub.c] (north = 0[degrees], increasing clockwise) of the advection current at each burst were calculated using eqs. (1) and (2):


[[theta].sub.c] = arctan ([u.sub.EW/[u.sub.NS]) (2)

[u.sub.NS] = 1/n [n.summation over (i=1)] [u.sub.i]

[u.sub.EW] = 1/n [n.summation over (i=1)] [v.sub.i]

where [u.sub.NS] is the mean current velocity of the north-south component, [u.sub.i] is the recorded north-south component of the advection current, [u.sub.EW] is the mean current velocity of the east-west component, [v.sub.i] is the recorded east-west component of the advection current, and n is the number of data points per burst (=600).

The flow velocity of the waves [u.sub.W] was the total amplitude of flow velocity (difference between the maximum and minimum of flow velocity), which was calculated by eq. (3), with significant (1/3 maximum) waves being extracted using zero-crossing method:

[u.sub.w] = a 2[pi] / T (3)

a = H / 2 1 / sinh (2[pi]h / L)

where T is the wave cycle, H is the wave height, L is the wave length, h is the water depth.

Subsequently, the shear stress of the advection current [[tau].sub.c] (N [m.sup.-2]) (4) and waves [[tau].sub.w] (N [m.sup.-2]) (5) were obtained following the equations of Kleinhans (2005):

[[tau].sub.c] = 1/8 [sigma] [f.sub.c] [u.sub.c.sup.2] (4)

[f.sub.c] = 0.24 [(log 12h / [k.sub.s]).sup.-2]

[[tau].sub.w] = 1/2 [sigma] [f.sub.w] [u.sub.w.sup.2] (5)

[f.sub.w] = exp [5.216 [([2.5D.sub.50] / [A.sub.orb]).sup.0.194] -5.977]

[A.sub.orb] = [u.sub.w] T / 2[pi]

where [sigma] is the density of sea water (=1,025 kg [m.sup.-3]), [k.sub.s] is the roughness length of Nikuradse (=0.00019), [D.sub.50] is the median grain size of the sediments (=0.00023 m).

The time-series changes of [u.sub.EW], [u.sub.NS], [u.sub.C], and [u.sub.W] were computed using TS-Master (Version 6.5g) software, which is freely available on the web ( home_page.html, or by Dr. Kawamata (National Research Institute of Fisheries Engineering, Fisheries Research Agency, Japan). Calculations of [[tau].sub.c] and [[tau].sub.w] were performed using an "MS-Excel sheet for the calculation of threshold of bivalve transport" (National Research Institute of Fisheries Engineering 2009).

Statistical analysis

Data for clam density in the 4 sections (A D) of the study area were log-transformed before the homogeneity of variance was examined using Bartlett's test. When homogeneity of variance was detected, the difference among the sections was examined using 1-way ANOVA followed by Tukey's HSD post hoc test. When homogeneity of variance was not detected, the significant difference was examined using the Steel-Dwass test. The same procedure, without log-transformation, was applied to identify the significant difference for comparisons of [D.sub.50] and silt + clay content in the sediment in the 4 study area sections (A-D), as well as for the number of trapped clams between classified shear stresses during ET1 and ET2, and among the groups of traps during EL1-EL3.


Distribution changes of clams and sediments

The shell length of the Manila clams that were sampled from July 1 to August 16, 2004, ranged from 0.24.0 ram. Four cohorts were identified from polymodal size-frequency distributions of shell length for the clams that were collected on each sampling date (Figure 2). Of these cohorts, the population with the highest density, C1, was selected for further analysis. The shell length of C1 was 0.52 [+ or -] 0.11 mm (mean [+ or -] SD) and 0.77 [+ or -] 0.13 mm on the initial (July 1) and final sampling dates (August 16), respectively.

On July 1,2004, higher densities of the newly-settled C 1 were mainly located at an offshore subtidal area (Figure 3). Statistical tests showed that the density of sampled clams at the stations in section A differed significantly from those in the other sections (P < 0.05, Tukey-HSD; Table 1). There was an increase in the total density of el benthic individuals from July 1 to July 16 (Figure 2), which indicated that larval settlement was in progress during this period. When recruitment appeared to have nearly finished in mid-July, the distribution of early juveniles extended shoreward. No significant difference was found among sections A-D on July 16 (P > 0.05). However, clam densities in the subtidal area decreased noticeably until August 2, resulting in the densities being significantly different between sections A and C, as well as A and D (P < 0.05, Steel-Dwass). A statistical difference was not found among sections B, C, and D on August 2: however, the stations with the higher densities were located at section C. In early August, the densest distribution of clams was primarily limited to the intertidal area, where the bottom height was +0.2-+0.6 m. Whereas an overall decline of clam density was observed on August 16, the stations in section D continued to have the highest density. Temporal changes in clam density from early July to mid-August 2004 resulted in dense distributions of newly-settled juveniles shifting shoreward within the study area.


The median grain sizes D so of the bottom surface sediment in sections A D on July 1 were 0.24 [+ or -] 0.02, 0.23 [+ or -] 0.01, 0.22 [+ or -] 0.01, and 0.24 [+ or -] 0.01 mm, respectively, with no significant difference being found among the 4 sections (P > 0.05, Tukey-HSD; Table 2). Similarly, on July 16, August 2, and August 16, the mean Ds0 in sections A-D ranged from 0.19-0.24 mm, with no significant difference among the sampling dates, except between C and D on August 16 (P < 0.05, Steel-Dwass).

The mean silt + clay contents of the surface sediment in sections A-D on July 1 were 2.0 [+ or -] 0.55, 1.8 [+ or -] 0.10, 2.1 [+ or -] 0.16, and 2.0 [+ or -] 0.30%, respectively, with no significant difference among the 4 sections (P > 0.05: Table 2). On July 16, August 2, and August 16, the mean Ds0 in sections A-D ranged from 0.19-0.24 mm, with no significant difference among the sampling dates.

Trap collection of clams and sediments

During ET1, the number of clams collected in a 15-min trapping period changed noticeably with bottom shear stress (Figure 4C). The number of trapped clams was 0,19 [indiv.trap.sup.-1] 15 [min.sup.-1] from 06:00-15:55, followed by a sharp increase to 37-148 [indiv.trap.sup.-1] 15 [min.sup.-1] after 16:20. Water depth at site P rose from 18 cm to 49 cm with the flooding tide after 16:20. The northeast wind speed increased to 10.5-11.2 m [s.sup.-1] (maximum velocity in a 1 h period) in the same period (Figure 4A). The bottom shear stress of the current [[tau].sub.c] was low, remaining nearly unchanged (at 0.0027-0.0050 N [m.sup.-2]) during the experiment, whereas the wave shear stress [[tau].sub.w] rose quickly to 0.2g-0.39 N [m.sup.-2] after 16:20. The water temperature during ET1 was 22.8-32.2[degrees]C.

Similarly, during ET2, a large number of clams were trapped at 14:20 16:55, when the [[tau].sub.w] increased to 0.26-0.37 N [m.sup.-2] (Figure 5C). An increase of [[tau].sub.w] after 14:20 was accompanied with a rise in water depth (45-127 cm) and a strong north wind (11.9-12.8 m [s.sup.-1]; Figure 5A). The [[tau].sub.c] was maintained at 0.0027-0.0050 N [m.sup.-2]. The monitored water temperature during ET2 was 22.8-32.2[degrees]C, excluding the period of aerial exposure.

The shell length of trapped clams and core-sampled ambient clams was 0.38 [+ or -] 0.01 mm and 0.39 [+ or -] 0.01 mm for ET1, and 1.11 [+ or -] 0.04 mm and 1.35 [+ or -] 0.06 mm for ET2, respectively. No statistical difference in mean shell length was detected between trapped and ambient clams during either ET or ET2 (P > 0.05, t-test).


In the relationship between increasing [[tau].sub.w] and the number of trapped clams during ET1, a slight augment of trapped clams began above 0.1 N [m.sup.-2] (Figure 6A). However, most of the trapping samples remained at <10 indiv, [trap.sup.-1] 15 [min.sup.-1] with a [[tau].sub.w] below 0.3 N [m.sup.-2]. The number of trapped clams increased rapidly to 90 148 indiv, [trap.sup.-1] 15 [min.sup.-1] when the [[tau].sub.w]. was >0.3 N [m.sup.-2] Likewise, during ET2, the lower number of trapped clams increased noticeably to 12-108 indiv, [trap.sup.-1] l5 [min.sup.-1] when [[tau].sub.w] was >0.3 N [m.sup.-2] (Figure 6B). For both ET1 and ET2, comparison of the number of trapped clams at different [[tau].sub.w] classes of intensity showed significant differences for the 0.31-0.40 N [m.sup.-2] class versus the other 3 classes of below 0.30 N [m.sup.-2] (ETl: P < 0.05, Steel-Dwass: ET2: P < 0.05, Tukey-HSD; Table 3). However, the number of trapped clams in the 0.31-0.40 N m 2 class during ET2 did not differ statistically from the 0.210.30 N [m.sup.-2] class, but did differ against those in the 0.11-0.20 N [m.sup.-2] and 0.01-0.10 N [m.sup.-2] classes.

During both ET1 and ET2, the number of trapped clams was closely correlated with the dry weight of the sediment that was trapped during the same time period (ETI: [R.sup.2] = 0.99, ET2: [R.sup.2] = 0.81; Figure 7). During ET1, the [D.sub.50] of trapped sediments that were collected during the period when [[tau].sub.w] reached 0.31-0.40 N [m.sup.-2] (n = 3, 43.6-62.1 g [trap.sup.-1] 15 [min.sup.-1]) was 0.20 [+ or -] 0.00 mm. The mean [D.sub.50] of the ambient surface sediments (n = 8) that were sampled during the experiment (0.20 [+ or -] 0.00 mm) did not differ significantly from that of the trapped sediment (P > 0.05, t-test). During both ETI and ET2, the [D.sub.50] of the trapped (0.20 [+ or -] 0.00 mm, n = 6) and ambient sediments (0.19 [+ or -] 0.00 mm, n = 8) exhibited no significant difference (P > 0.05).

The velocity and direction of the bottom advection current changed gradually with the tidal cycle during ETI (Figure 4B). Greater velocities of advection current were observed during the flooding tide (8.1-12.3 cm [s.sup.-1] at 06:00-07:40 and 15:20-17:40). The direction of the advection current at 17:00-17:55, when [[tau].sub.w] exceeded 0.31 N [m.sup.-2] was generally shoreward (i.e., northeast). Similarly during ET2, the advection current was in a shoreward direction when [[tau].sub.w] was >0.31 N [m.sup.-2] (Figure 5B).

Release-recovery of clams

The observed ranges of [[tau].sub.w] during ELl-EL3 were 0.13-0.23, 0.20-0.26, and 0.19-0.26 N [m.sup.-2] respectively (Figure 8A). Simultaneously, the [[tau].sub.c] during EL1-EL3 remained at 0.0020.006, 0.002-0.004, and 0.002-0.004 N [m.sup.-2], respectively. The direction of the advection current was consistently north to northeast during EL1-EL3 (Figure 8B). The velocity and direction of the advection current when the maximum [[tau].sub.w] was recorded (at 15:00, 0.26 N [m.sup.-2]) were 4.2 cm [s.sup.-1] and northeast, respectively. The water temperature during the experiment was 24.6-27.7[degrees]C.


Whereas the number of clams recovered from the traps was 0-2 indiv, [trap.sup.-1] during EL1, the number during EL2 and EL3 increased to 0-29 and 0-33 indiv, [trap.sup.-1], respectively (Figure 8C). Traps that recovered the largest number of clams were located to the northeast of the release point (trap numbers 3 and 4) during EL2 and EL3. These directions coincided with the downstream direction of the advection current from release point in each experiment. With respect to the water current direction when [[tau].sub.w] reached the maximum threshold, traps were divided into 4 groups: E trap no. 2-4, F trap no. 5-7, G trap no. 8-10, and H trap no. 11-1. Comparison of the number of recovered clams among the groups of traps resulted in a significant difference being found between E trap and the other trap groups (P < 0.05, Tukey-HSD) during EL3 (Table 4).

Time-series change of shear stress at P

The minimum maximum value of [[tau].sub.w] at site P during July 17-31 and August 2-16, 2004, was 0.04-0.68 and 0.03-0.64 N [m.sup.-2], respectively (Figure 9). The time-series change of [[tau].sub.w] was irregular, intermittently exceeding 0.3 N [m.sup.-2] in 8 out of the 15 monitored days in July, and 9 out of the 15 monitored days in August 2004. The ranges of [[tau].sub.c] during July 17-31 and August 2-16 were 0.00-0.05 and 0.00-0.03 N [m.sup.-2] respectively. The recorded water temperatures during these periods were 21.530.2[degrees]C and 24.9-33.0[degrees]C, respectively.



Bottom shear stress and bedload movement of clam juveniles

The hydrodynamic force that erodes bottom surface substrates is expressed as bottom shear stress. Shear stress is separated into 2 components based on the driving force: wave shear stress [[tau].sub.w] and current shear stress [[tau].sub.c]. In this study, a close relationship was found between [[tau].sub.w] and the number of trapped clams. These results indicate that the bedload movement of newly-settled Manila clams was initiated by an increase in the oscillatory force of waves in the study area. The movement of juvenile clams should be examined by the integration of the shear stress of both the sea current and waves [[tau]] (for example, see Swart (in Soulsby 1997), [[tau]] = [[tau].sub.c] + [[tau].sub.w]/2). However, in the current study, [[tau]] was dominated by [[tau].sub.w] when clam movement occurred during ET1 and ET2 ([[tau].sub.c]/[[tau].sub.w] =1/60-1/130). Hence, wave-generated oscillatory flow is an important driver for the initiation of bivalve transport (Turner et al., 1997, Lundquist et al., 2006).


Generally, the initiation of sediment movement occurs when the hydrodynamic force becomes larger than gravity and frictional forces. The physical conditions that facilitate the incipient movement of sediments are usually expressed as threshold shear stress, above which bedload movement begins (Southard 2006). During ET1 and ET2, the number of the trapped clams did not increase linearly with increasing [[tau].sub.w], but did increase sharply at a [[tau].sub.w] of >0.3 N [m.sup.-2]. In addition, the number of clams and the weight of sediment collected in concurrent trapping samples were strongly correlated. Thus, newly-settled Manila clam juveniles appeared to start moving passively like sediment grains. Based on our observation, we may assume that the critical threshold of [[tau].sub.w] is approximately 0.3 N [m.sup.-2] for Manila clams that have recently settled at this site.


The size of trapped clams was 0.38 [+ or -] 0.01 mm and 1.11 [+ or -] 0.04 mm during ET1 and ET2, respectively. In the current study, the measured and calculated specific gravity of Manila clam juveniles were reported to be 1.53 (SL 5.0-7.0 mm, Takemura 1972) and 1.06 (0.6-1.0 mm, Montaudouin 1997), respectively. Based on these parameters, the critical thresholds for the incipient movement of Manila clam juveniles of both size groups was calculated to be 0.006-0.076 and 0.010-0.082 N [m.sup.-2] according to Kleinhans (2005). In comparison, the critical threshold of ambient sediment grain was determined to be 0.157 N [m.sup.-2] [[D.sub.50] = 0.20 mm, sg = 2.65 (=quartz)]. Thus, the physical calculations predict that newly-settled clams may begin to move at a lower hydrodynamic force compared with sediment grains, assuming that both particles are inert and exposed to seawater flow on the substrate. This prediction does not support the in situ observations of the present study. This is because the juveniles of infaunal bivalves, including Manila clams, exhibit burrowing and/or anchoring of the byssus thread soon after metamorphosis (Baker and Mann 1997, Carriker 2001). In addition, the foot of small Manila clam juveniles often adheres strongly to bottom substrates (Montaudouin 1997). Hence, the simultaneous initiation of bedload movement by clams and sediments may be explained by the burrowing and/or adhering behavior of clams with the sediment.

In contrast, larger juveniles of other bivalve species that are eroded from the sediments show different patterns to that of the sediment due to the behavior of these clams. For example, previous studies observed that Mya arenaria (L., 1758), Mercenaria mercenaria (L., 1758), and Macomona liliana (Iredale, 1915) often reduce their erosional vulnerability by active burrowing (Hunt 2004, Lundquist et al., 2004). Furthermore, Macoma balthica (L., 1758) and Cerastoderma edule use hydrodynamic entrainment by drifting the byssus thread (Beukema and de Vlas 1989, Montaudouin 1997). Considering that the physical characteristics of recently settled Manila clams make them susceptible to erosion, the similar pattern of movement that was observed for clams and sediment grains in the current study may have also resulted from clam behavior. Roegner et al. (1995) documented that freshly dead juveniles (0.24-0.27 mm) of Mya arenaria, which had been killed by immersion in formalin solution, were eroded with sediment grains in an indoor flume experiment, despite that the dead clams did not burrow or adhere to the sediment grains. However, in their experiments, the intervals of selected velocity in the flow regime may not have been sufficient to identify the difference in erosion rates for inert clams versus sediment grains. Overall, a number of physical and biological factors control the various patterns of spatial movement of infaunal bivalve species during the postsettlement stage (Bouma et al., 2001, Negrello Filho et al., 2006). We suggest that biologically induced adhesion and/or burrowing behavior may represent one such factor in newly-settled Manila clams.



Physical transport of juvenile clams

The released Manila clam juveniles tended to be transported in a downstream direction of the advection current during EL2 and EL3, although this trend was less statistically clear for EL2. The transport of released clams cannot only arise as a result of [[tau].sub.c] because the force was far smaller than the assumed critical threshold (0.3 N [m.sup.-2]). Basically, clam movement was initiated once % exceeded the critical threshold, after which the directional transport of Manila clam juveniles seemed to be influenced by the additional effect of the advective current, even if [[tau].sub.c] was smaller than the critical threshold of incipient movement. The observed maximum [[tau].sub.w] during EL2 and EL3 was 0.26 N [m.sup.-2] which was slightly less than the assumed critical threshold. This low value may imply that the clams were unable to complete settling into the substrate due to the short time period that was allowed for burrowing prior to the initiation of the experiment.

During ET1 and ET2, a greater [[tau].sub.w] was provided by the complex effect of rising water depth and increasing wind velocity. Wind-generated waves are one of the most common and important factors that cause bivalve transport (Turner et al., 1997, Lundquist et al., 2006), particularly in intertidal sandflats (Commito et al., 1995). In some of the land-based flume experiments, the hydrodynamic effect on the rate of erosion or distance of transport of bivalve species has been examined using a single directional steady stream (Roegner et al., 1995, Montaudouin 1997, Hunt 2004, Lundquist et al., 2004). As a result, the physical force that drives bivalve transport in steady stream conditions is overwhelmed by [[tau].sub.c]. In such conditions, once increasing [[tau].sub.c] has exceeded the critical threshold for bivalve movement, the bivalves are expected to be transported a long distance in a short time period, due to the high advection velocity. However, in wave-dominated flow regimes, even if [[tau].sub.w] exceeds the threshold for bivalve movement, long-distance transport may not occur in a short time period if [[tau].sub.c] is not sufficiently large enough.

As recorded in the time-series observation at site P, the shear stress [[tau].sub.w] was subject to large variation in a short time period, i.e., less than an hour in some cases, and exceeded the assumed critical threshold frequently. Bedload transport is time-variable, due to fluctuations in tidally changing water depth and local wind parameters, and is supposed to be an incidental event for newly-settled Manila clam juveniles in this area. Several studies investigating the influence of physical factors on bivalve transport focused on parameters that were observed at time intervals of longer than 1 tidal cycle or 1 day (Commito et al., 1995, Grant et al., 1997, Bouma et al., 2001, Norkko et al., 2001, Lundquist et al., 2006). Likewise, in Manila clams, only a few studies discuss hydrodynamic transport based on differences in spatial densities and estimated physical parameters, using timeframes of several days or weeks apart (Kakino 2000, Kakino et al., 2010). However, the bedload transport of Manila clam juveniles in situ indicates that clams repeatedly move and halt in shorter periods as a result of fluctuating hydrodynamic forces. This study demonstrated that the factors that determine the initiation and direction of clam transport are the hydrodynamic forces of waves and current, which were expressed as [[tau].sub.w] and [[tau].sub.c] respectively. The complex effect of both functions, in combination with tide and weather variations, is the fundamental mechanism driving the transport of Manila clams in the early benthic stages in this area.

Changes in clam distribution

The locations of the stations at which large number of clams was sampled changed spatially in the study area during the summer of 2004. Excluding subtidal section A, where the hydrodynamic conditions and macrofauna (including predators) were probably different from those of the intertidal fiat, a denser distribution of Manila clam juveniles was recorded to move shoreward by over several hundred meters within a one month period from mid July to mid August in the other 3 sections.

Spatial changes in the postlarval distribution of other infaunal bivalves have been commonly recorded at intertidal soft bottom habitats (reviewed by Hunt and Scheibling 1997). The major factors that cause this change in distribution are transport (Emerson and Grant 1991, Norkko et al., 2001) and mortality (Hunt and Scheibling 1997, Hunt and Mullineaux, 2002, Hunt 2004). However, the pattern of spatial distribution is ultimately determined by the spatial pattern of larval supply, mortality, and transport in the benthic stage. In this study, we have no data on the mortality of clam juveniles. However, the [D.sub.50] of sediment grain size and silt + clay content did not differ greatly among sections A D throughout the study period, which was inconsistent with the differences in recorded clam density among these sections. In addition, the influence of silt + clay content on the mortality of Manila clams (Hayashi et al., 1992) was also not observed in this study.

The distance and rate of transport of natural clam inhabitants could not be estimated during this study. During both ET1 and ET2, the direction of the advection current was shoreward when [[tau].sub.w] exceeded the critical threshold of clam movement. Thus, the shoreward transport of clam juveniles, which coincided with the shifting direction of the densest distributions, occurred at the site. Natural clam inhabitants are supposed to be subject to frequent transportation events in normal tidal and weather conditions. Therefore, hydrodynamically induced transport may contribute to the observed changes in the spatial distribution of newly-settled Manila clams in Tokyo Bay, Japan.


The authors express sincere gratitude to Dr. H. Kuwahara, Dr. H. Saito, Dr. S. Kawamata, Dr. R. Nanbu (National Research Institute of Fisheries Engineering), Dr. J. Higano (National Research Institute of Aquaculture), and Dr. H. Yamakawa (Tokyo University of Marine Science and Technology) for helpful suggestions for the physical analysis and inspiring discussion and comments about this study and Mr. K. Hasegawa, Ms. H. Hama, and Ms. Y. Watanabe for their careful assistance in the preparation of field operation and sample analysis. This study was accomplished through the research program "Investigation on the Environmental Condition for Abundant Clam Distribution" (Chiba Prefectural Fisheries Research Center), which was financially supported by the Fisheries Agency.


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(1) Tokyo Bay Fisheries Laboratory, Chiba Prefectural Fisheries Research Center, Futtsu, Chiba 293-0036, Japan; (2) Graduate School of Tokyo University of Marine Science and Technology, Konan, Minato-Ku, Tokyo 108-8477, Japan

* Corresponding author: E-mail:

DOI: 10.2983/035.030.0318
Statistical differences of clam densities at the sampling
stations within the 4 study sections (A-D). The division of the
study sections is described in the text and shown in Figure 1.

              Sampling station divisions

Date              A    B    C

Jul. 1 2004   B   *
              C   **   --
              D   **   --   --
Jul-16        B   --
              C   --   --
              D   --   --   --
Aug. 2        B   --
              C   **   --
              D   **   --   --
Aug-16        B   --
              C   --   --
              D   **   --   --

**, P<0.01; *, P<0.05; --, not significant (Tukey-HSD posthoc or
Steel-Dwass test).

Changes of D50 and silt + clay content of the bottom surface
sediment at the sampling stations within the 4 study
sections. The division of the 4 sections is described
in the text and shown in Figure 1.

Character     Date                   Sampling station groups

                                     A                    B

[D.sub.50]    Jul. 1 2004    0.24 [+ or -] .02 *  0.23 [+ or -] .01
(mm)          Jul-16         0.19 [+ or -] .00    0.22 [+ or -] .01
              Aug. 2         0.19 [+ or -] .00    0.21 [+ or -] .00
              Aug. 16        0.20 [+ or -] .01    0.24 [+ or -] .03
Silt + clay   Jul. 1 2004     2.0 [+ or -] .55     1.8 [+ or -] .10
(%)           Jul-16          2.1 [+ or -] .48     2.0 [+ or -] .73
              Aug. 2          2.7 [+ or -] .35     1.7 [+ or -] .20
              Aug. 16         2.6 [+ or -] .31     2.0 [+ or -] .07

Character     Date                  Sampling station groups

                                    C                   D

[D.sub.50]    Jul. 1 2004   0.22 [+ or -] .01   0.24 [+ or -] .01
(mm)          Jul-16        0.20 [+ or -] .00   0.22 [+ or -] .01
              Aug. 2        0.19 [+ or -] .00   0.20 [+ or -] .00
              Aug. 16       0.19 [+ or -] .00   0.22 [+ or -] .01
Silt + clay   Jul. 1 2004    2.1 [+ or -] .16    2.0 [+ or -] .30
(%)           Jul-16         2.1 [+ or -] .20    1.8 [+ or -] .38
              Aug. 2         2.9 [+ or -] .24    2.3 [+ or -] .18
              Aug. 16        2.6 [+ or -] .31    2.1 [+ or -] .23

* Mean  [+ or -]  SE.

Statistical differences in the number of trapped clams with
respect to the classified wave shear stress during ET1 and ET2.

                           Wave shear stress | [sub.w] (N [m.sup.-2])
                           0.01-0.10   0.11-0.20   0.21-0.30

Jul. 7 2004    0.11-0.20      --
(ET1)          0.21-0.30      --          --
               0.31-0.40      *           *            *
Aug. 17 2004   0.11-0.20      --
(ET2)          0.21-0.30      --          --
               0.31-0.40      **          **          **

**, P < 0.01; *, P < 0.05; --, not significant (Tukey-HSD posthoc
and Steel-Dwass test).

Statistical differences in the number of trapped clams among
the 4 groups of traps (E-H), which were deployed around
the clam-releasing point. The division of the stations
is described in the text and shown in Figure 8.

Date                 Group of traps

                     E (2-4)   F    G

EL1    F (5-7)         --
       G (8-10)        --      --
       H (11,12,1)     --      --   --
EL2         F          --
            G          --      --
            H          --      --   --
EL3         F           *
            G           *      --
            H           *      --   --

Numerals in parentheses represent trap no.

*, P < 0.05; --, not significant (Tukey-HSD posthoc test).
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Author:Toba, Mitsuharu; Ito, Makoto; Kobayashi, Yutaka
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:9JAPA
Date:Dec 1, 2011
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