Spatio-temporal variations in density of different life stages of a brackish water clam Corbicula japonica in the Kiso estuaries, central Japan.

ABSTRACT: By frequent sampling from May 2001 to October 2002, we examined spatio-temporal variations in densities of different life stages (planktonic larvae, new settlers and small, large and commercial individuals) of Corbicula japonica in the Kiso estuaries (the Ibi-Nagara Estuary and the Kiso Estuary). Planktonic larvae were found for a much longer period (mainly in June to December 2001 and in April to October 2002) than reported in previous studies. Densities of planktonic larvae were significantly higher in the Kiso Estuary than in the Ibi-Nagara Estuary, whereas the reverse was true for new settlers and commercial individuals, indicating that larval settlement processes may be critical in establishing significant differences in the density of benthic stages between these estuaries. Ontogenetic habitat shift of the clam was detected in the Ibi-Nagara Estuary but not in the Kiso Estuary: the main habitats for new settlers and small individuals were located in the upper parts of the Ibi-Nagara Estuary and large and commercial individuals were located in the middle part. On the other hand, in the Kiso Estuary, all life stages from new settlers to commercial individuals were found mainly in the upper part. The above distribution patterns of C. japonica were much different from those of 3 other bivalves (Ruditapes philippinarum, Musculista senhousia and Mactra veneriformis) that are also common and abundant in these estuaries: high densities of these 3 bivalves were always found around the river mouths throughout their ontogeny.

KEY WORDS: clam, Corbicula, larval recruitment, larval settlement

INTRODUCTION

Three Corbicula species have been recognized in Japan (Sakai et al. 1994, Harada & Nishino 1995): C sandai and C leana are found in freshwater, and C japonica in estuarine waters. The former two species are endemic to Japan, whereas the latter is endemic to estuarine waters in eastern Asia (from Sakhalin Is. in eastern Russian to Kyushu in the south, including Korea). In Japan, C japonica is commonly found in estuarine waters along the Japan Sea and the Pacific coast from Hokkaido in the north to Kyushu in the south (Harada & Nishino 1995), although the clam has not yet been reported from the Ryukyu Archipelago south of Kyushu, which geographically belongs to the subtropical to tropical zone. Interestingly, the 2 freshwater Corbicula species have no planktonic larvae, whereas C. japonica has (Kimura et al. 2004).

The brackish water clam C. japonica is very common and abundant in Japanese estuarine waters, and is one of target species for clam fisheries in Japan, particularly in the Tone Estuary, central Japan, the brackish Lake Shinji along the Japan Sea coast, and the Kiso estuaries (the Ibi-Nagara Estuary and Kiso Estuary), central Japan (see Mizuno et al. 2005; Fig. 1). Despite several regulations imposed for Corbicula fisheries management in Japan, the total annual catch yields of Corbicula species (ca. 99% of the yield is C. japonica) have decreased drastically over the last 3 decades (Fig. 1). Traditionally in Japan, including in the Kiso estuaries, the hard-shell clam Meretrix lusoria has been commercially more important than C. japonica, being much bigger in shell size and preferring more saline and sandy beds around river mouths. Drastic decreases of Mer. lusoria yield in the Kiso estuaries occurred in the late 1970s, when C. japonica yield abruptly increased in these estuaries (Fig. 1). Thus, fishing efforts became concentrated on the C. japonica fishery (Fig. 1). Unfortunately, the causes of these drastic decreases have not yet been identified.

[FIGURE 1 OMITTED]

Information is available on salinity tolerance, growth, spawning, larval development and distribution of commercial individuals of C. japonica (Tanaka 1984a, 1984b, Yamamuro & Koike 1993, Nakamura 1997, Nakamura 2000, Takada et al. 2001). However, there is comparatively little information on larval recruitment (i.e., larval supply, larval settlement, recruitment) of the clam, particularly in the Kiso estuaries. Sekiguchi et al. (1991) worked on the spatio-temporal distribution of planktonic larvae and new settlers of the clam around the mouth of the Kiso River (commercial grounds have been established more upstream). Based on temporal changes in the vertical distribution of larval density of the clam over 24 h in July 1989, planktonic larvae occurred mainly in saline waters (the middle to bottom layers of the water column) throughout the lower part of the river, were flushed out into coastal, saline waters around the river mouth during the ebb tide and then returned to the river upstream during the flood tide. Deducing from their finding of new settlers at low density in the same month, which completely disappeared in September of the same year, they speculated that new settlers moved more upstream during the flood tide by drifting (byssus) threads, and they discussed the importance of examining larval recruitment throughout the Kiso Estuary (in the brackish water part of the Kiso River), including the spatio-temporal distribution of new settlers. Then, in surveys (100 meters between sampling stations) on tidal flats around the mouth of a separate, short river, Soutome et al. (2005) found high densities of bivalves (Ruditapes philippinarum and Musculista senhousia) that are common and abundant on the flats on the lower part of the river throughout the bivalve's ontogeny. On the other hand, they also found high densities of new settlers of C. japonica on the lower part of the flats but late juveniles and adults on the upper part.

Recently, studies dealing with larval recruitment of marine benthic populations indicate that larval recruitment may play an important role in determining the generation and maintenance of benthic populations (e.g., Underwood & Denley 1984, Connell 1985). The goal of our studies is to unveil larval recruitment processes of C. japonica in the Kiso estuaries to understand the mechanisms by which benthic populations and fishing grounds may be generated and maintained in these estuaries and then to establish reasonable resource management for the clam. In this study, as a first step toward our goal, we examined spatio-temporal variations in densities of different life stages (planktonic larvae, new settlers, small and large individuals and commercial individuals) of C. japonica.

MATERIALS AND METHODS

Study Area

The Kiso Rivers (Ibi-Nagara River and Kiso River), 3 of the biggest rivers in Japan, flow into the innermost part of Ise Bay along the Pacific coast of central Japan (Fig. 2). Their freshwater discharge accounts for ca. 80% of the total amount of freshwater delivered by rivers into the hay (Japan Society of Oceanography 1985): The Ibi River runs 121 km through a catchment of 1840 [km.sup.2] and has an annual freshwater discharge 2582 x [10.sup.6] [km.sup.3]. The Kiso River runs 227 km through a catchment of 5275 [km.sup.2] and has an annual freshwater discharge 8379 x [10.sup.6] [km.sup.3]. The Ibi and Nagara Rivers are separate except at their lowest part, where they unify into one called the Ibi-Nagara River, although the Nagara River has been closed 5 km upstream by the Nagara Dam since 1995 (Fig. 2).

[FIGURE 2 OMITTED]

The average depths along the river are shallower in the middle to upper parts of the Ibi-Nagara River than in the Kiso River (Fig. 3), although the maximum depths do not differ except in the lower parts, indicating that saline water might be detectable further upstream in the Kiso River than in the Ibi-Nagara River (Fig. 3).

[FIGURE 3 OMITTED]

The Kiso estuaries (the Ibi-Nagara Estuary and the Kiso Estuary), defined here as the areas where bottom water is detectable with salinity of at least 1 psu, are 2-10 m deep with a maximum tidal range of 3 m, reaching up to 30 km upstream in the Ibi River and 26 km upstream (to the dam site) in the Kiso River. With the recent progress of eutrophication in Ise Bay, oxygen-poor water develops widely in bottom waters of the central to innermost parts of the bay every summer to fall, so that the oxygen-poor water is often detectable even around the mouths of the Kiso Rivers (even at sites close to Nagara Dam, located 5 km upstream), as indicated in Figure 3, during the same seasons, occasionally causing mass mortality of benthic organisms, particularly bivalves, including C. japonica.

Sampling Procedures

Sampling was undertaken from May 2001 to October 2002. Planktonic larvae of C. japonica were obtained at stations (I-1a, I-1b, K-1a, K-1b) located 1 km seaward of each river mouth (Fig. 2), by using a vertical haul of plankton nets (22 cm in diameter, 133-[micro]m- mesh-openings) from 5 m depth to the surface every 2 wk (except for January to March 2002). Plankton samples were fixed immediately in 5% neutralized formalin seawater. Bivalve larvae were identified to specific level under an optical microscope according to Sakai & Sekiguchi (1992) and Kimura et al. (2004). For sampling benthic stages of the clam, 8 stations (2 or 3 sites with different depths) within each station area were located every 2 or 3 km, ending 12 km upstream from the river mouth in each estuary, and 2 or 3 sites were located 1 and 3 km seaward of the mouth (Fig. 2). All samples were collected from the flood tide to high water.

For sampling new settlers and small individuals of C. japonica, 1 sediment sample was collected in a core sampler (3.1 cm in diameter, 1.0 cm depth) from the surface layer of bottom sediments, which were obtained at each site within each station every 2 wk (except for January to March 2002) by using a Smith-McIntyre grab. Sediment samples were fixed in 10% neutralized formalin seawater mixed with dilute Rose-Bengal, and filtered through a 125-[micro]m-mesh sieve. Bivalve specimens were sorted out from the samples retained on the sieve under an optical stereomicroscope, and identified to specific level according to Sakai & Sekiguchi (1992) and Kimura et al. (2004). For sampling large and commercial individuals of the clam, 1 sediment sample was obtained at each site within each station every month by using a Smith-McIntyre grab. Sediment samples were filtered through a 1.0-mm-mesh sieve. Bivalves were sorted out from the samples, retained on the sieve and identified to specific level. Shell lengths of new settlers and small individuals of the clam were measured to the nearest 25 [micro]m with a micrometer under a optical stereomicroscope; those of large and commercial individuals were measured to the nearest 0.1 mm with a caliper.

Different life stages (planktonic larvae, new settlers, and small, large and commercial individuals) of C. japonica were distinguished according to Sekiguchi and his coworkers (e.g., Miyawaki & Sekiguchi 1999, 2000, Ishii et al. 2001a, b) except for "planktonic larvae" (i.e., "new settlers" were defined as having shell lengths <0.3 mm, "small individuals" as having [??] 0.3 mm and <1.0mm, "large individuals" as having shell lengths [greater than or equal to] 1.0 mm and <10.0 mm, and "commercial individuals" as having shell lengths [greater than or equal to] 10.0 mm. Unlike larvae of most marine or brackish water bivalves that have both D-shaped larvae (the earlier stage Veliger) and Umbo larvae (the late stage Veliger), C. japonica has D-shaped larvae only (Kimura et al. 2004). We defined here "planktonic larvae" as D-shaped larvae that were identified according to Kimura et al. (2004).

Data Processing 

Sediment samples were collected at 2 or 3 sites within each station as mentioned earlier. The density of each life stage of C. japonica

was not significantly different between sites within each station (t-test, P > 0.05). We used average densities of each life stage. Using log (n + 1)-transformed data of these averages, we statistically examined the density differences of each life stage between the 2 estuaries for the period from May 2001 to October 2002, using 2-way ANOVA (significant level [alpha] = 0.05).

Environmental Characteristics

For the last decade, the Kiso River Management Office of the Ministry of Transport and Infrastructure has continuously monitored and measured water temperature, salinity and dissolved oxygen content every hour in water 0.5 m above the bottom at a site (Zyonan, see Fig. 2 for location) 0.5 km upstream in the Ibi-Nagara Estuary. To examine environmental characteristics of the Kiso estuaries, we used averages over 24 h of these environmental data obtained from that office (see Fig. 5 later). Although we have no similar data available for the Kiso Estuary, environmental conditions are similar between the mouths of these estuaries (Mizuno et al. 2005). We drew a salinity profile during the flood tide on April 13, 2004 along the distance from the mouth of each estuary (Fig. 3): salinity was measured in water 0.5 m above the bottom with a conductivity, temperature-depth instrument (CTD).

[FIGURE 5 OMITTED]

We examined the sediment characteristics of the Kiso estuaries from the bottom sediments collected at all stations and determined the relative abundance (%) of the silt-clay fraction. This fraction (% by dry weight) was defined here as the fraction passing through a 63-[micro]m-mesh sieve. After sediments were dried in air for several days in the laboratory, a 100-g sample was filtered through the sieve, and the sediment retained on the sieve was weighed.

The silt-clay fraction of sediments in the Kiso estuaries was mostly less than 5.0% (Fig. 4), being much higher at the mouths and downstream parts of these estuaries, and lower in the upstream parts. Accordingly, bottom sediments are sandy 5 km or more upstream of these estuaries, although Mizuno et al. (2005) reported an extraordinarily high percentage (50% or more) of the silt-clay fraction in the area close to the Nagara Dam. Muddy sediment was common in troughs, so that the silt-clay fraction was very different between troughs and ridges even at the same distance from the mouths of these estuaries (Fig. 4).

[FIGURE 4 OMITTED]

RESULTS

Spatio-temporal Variation in Densities of Different Life Stages of Corbicula japonica

Planktonic Larvae

There were no significant differences in larval density between the 2 sites within each estuary (i.e., between st. I-1a and st. I-1b and between st. K-1a and st .K-1b) (Fig. 5). In 2002, larval density was significantly higher in the Kiso Estuary. However, in 2001 there were no significant differences between these estuaries. Temporal variation in larval density was notable, the density being higher from spring (March to May) to fall (September to November), reaching [greater than or equal to] 300 inds./[m.sup.3] in 2001 and [greater than or equal to] 100 inds./[m.sup.3] in 2002 in the Ibi-Nagara Estuary and [greater than or equal to] 200 inds./[m.sup.3] in 2001 and [greater than or equal to] 2000 inds./[m.sup.3] in 2002 in the Kiso Estuary. This study indicates a much longer spawning period for the clam than reported in previous studies (Nakamura 1997, Takada et al. 2001) that reported spawning exclusively from July to September.

Water temperature reached ca. 30[degrees]C in summer (June to August) and decreased below 10[degrees]C in winter (December 2001 to February 2002). On the other hand, although tending to become lower in summer and higher in winter, salinity did not indicate such a clear seasonal change but always showed irregular variations (ca. 15-32 psu) and occasionally marked lowering (down to <1 psu, corresponding to low larval density) owing to something else, high freshwater discharge through high rainfall in early summer. Larval densities were low or larvae were completely absent from the water column from December 2001 to April 2002, when the water temperature decreased to below 15[degrees]C. According to laboratory rearing experiments (Kimura et al. 2004), larvae of the clam reared at 15[degrees]C or lower failed to settle and recruit.

New Settlers

Significant differences in new settler density were detected between stations within each estuary in both years (Fig. 6). In 2002, the new settler density was significantly higher in the Kiso Estuary. However, in 2001, there were no significant differences in the density between estuaries. Temporal variation in density was notable, the density being higher in summer to fall. Higher densities, with 5,000 inds./[m.sup.2] and more, were found mainly in the upper part (7 km or more upstream) of the Ibi-Nagara Estuary, but in both the lower (from 2 km to 4 km upstream) and upper parts of the Kiso Estuary. The area of higher density of new settlers did not correspond to percentage of the silt-clay fraction in either estuary (see Fig. 4).

[FIGURE 6 OMITTED]

In the Ibi-Nagara Estuary, higher densities (>100 inds./[m.sup.3]) of new settlers were found in both years, being highest in 2002 (>5000 inds./[m.sup.2]) compared with 2001. In the Kiso Estuary, higher densities of planktonic larvae and new settlers were found in 2002. In 2001, much higher densities of new settlers were found in the Ibi-Nagara Estuary, whereas there were no significant differences in larval density between estuaries. On the other hand, in 2002 much higher densities of planktonic larvae were found in the Kiso Estuary than in the Ibi-Nagara Estuary, whereas the reverse was true for new settlers. For the Ibi-Nagara Estuary, higher densities of planktonic larvae in the fall of 2001 did not correspond to a higher density of new settlers in the same season. This was true also for the Kiso Estuary in the spring to early summer of 2002.

Small Individuals

In both years, significant differences in the density of small individuals were detected between stations within each estuary and between estuaries: the density was significantly higher in the Ibi-Nagara Estuary, as was seen in the new settlers (Fig. 7). There were no significant differences in density in each estuary between years. Temporal variation in density was notable, with densities decreasing during June and July. Spatial distribution patterns of small individuals were similar to those of new settlers.

[FIGURE 7 OMITTED]

The spatio-temporal distribution of higher densities (>5000 inds./[m.sup.2]) of small individuals did not always correspond to that of new settler density. In the Kiso Estuary, higher densities of new settlers were found in 2002, but the reverse was true for small individuals. No such difference was detected in the Ibi-Nagara Estuary. In 2001 and 2002, as was also seen for new settlers, much higher densities of small individuals were found in the Ibi-Nagara Estuary.

Large Individuals

In both years, significant differences in the density of large individuals were detected between stations within each estuary, but not between estuaries (Fig. 8). The density of large individuals decreased during fall to winter, but temporal variation in density was not as notable as in small individuals.

[FIGURE 8 OMITTED]

Spatial distribution patterns of large individual density were different from those of small individuals and between estuaries. In the Ibi-Nagara Estuary, higher densities (>500 inds./[m.sup.2]) of large individuals were found in the lower to middle parts (from 2 km to 7 km upstream), whereas small individuals occurred in the upper part. On the other hand, higher densities of large individuals were found in the upper part of the Kiso Estuary, where high densities of small individuals also occurred. Particularly in the Ibi-Nagara Estuary, spatio-temporal variation in higher densities (>500 inds./ [m.sup.2]) of large individuals did not correspond to that of small individuals. Spatio-temporal distributions of higher densities of large individuals were not much different between estuaries or within each estuary between years.

Commercial Individuals

In both years, significant differences in the density of commercial individuals were detected between stations within each estuary (Fig. 9). There were no significant differences in density within each estuary between years. In 2001, a significantly higher density of commercial individuals was found in the Ibi-Nagara Estuary. On the other hand, in 2001, there were no significant differences in density between estuaries. The density of commercial individuals decreased during fall to winter, although temporal variation in density was not as notable as in large individuals. Spatial distribution patterns of commercial individual density, particularly at higher densities (>500 inds./[m.sup.2]), were similar to those of large individuals.

[FIGURE 9 OMITTED]

DISCUSSION

Location of Larval Settlement of Corbicula japonica

In addition to C. japonica, 3 other bivalve species are common and abundant in the Kiso estuaries: Mactra veneriformis, Musculista senhousia and Ruditapes philippinarum (Sekiguchi et al. 1991, Mizuno et al. 2005). The clam Meretrix lusoria, which was previously important to local fisheries, has remained at an extremely low density, ruining the local fisheries in the last decade. Previous studies (Sekiguchi et al. 1991, Tsutsumi & Sekiguchi 1996, Soutome et al. 2005) and this study found new settlers of the earlier mentioned 3 bivalves mainly on the sediment around the river mouths, whereas those of C. japonica were found on the sediment of the upper part (7 km or more upstream) of the Kiso estuaries as clearly indicated in Figure 6.

The fact that higher-density areas of C. japonica new settlers were found in the upper parts of the Kiso estuaries may be explained by one of following alternative scenarios. (1) Planktonic larvae settled in the areas where the higher density of new settlers was found. (2) Planktonic larvae settled in a separate, downstream area, and thereafter moved to the higher-density area of new settlers through tidal migration by using drifting (or byssus) threads (e.g., Hamada & Ino 1954, Sigurdsson et al. 1976, Lane et al. 1985, Soutome et al. 2005). (3) Planktonic larvae settled over the whole area of each estuary, but died quickly after settlement, owing to unknown environmental factors, except those that settled in the higher-density area of new settlers. However, scenario 3 may not be accepted, because we could find no new settlers or dead shells of new settlers in all areas of estuaries (except in the higher-density area). On the other hand, scenario 2 may not be rejected, because all sampling was completed during the flood tide to high water. Scenario 2 may be accepted if larvae of the clam settled first in a separate, downstream area during the ebb tide to low water and then settled later in the higher-density area of new settlers during the flood tide through upstream migration, although our preliminary survey of new settlers failed to find evidence supporting the scenario. So we believe that scenario 1 is most likely.

It is not immediately apparent how the higher-density area of the new settlers is established. Sediment characteristics (e.g., the silt-clay fraction) of the higher-density area were not much different from the other areas (Fig. 4). We propose that the salinity level and its variation (in water just above the bottom) along the river distance may be critical for larval settlement, because tolerance to low salinity or freshwater increases from the planktonic larval stages to adulthood (Asahina 1941, Tanaka 1984a, b, Sekiguchi et al. 1991, and this study). According to Saito et al. (2002), who examined the change of salinity selectivity of the clam throughout their ontogeny in the laboratory, early and late D-shaped larvae (20-50 h and 50-100 h after fertilization, respectively) preferred water at 20 [+ or -] 5 psu and 15 [+ or -] 5 psu, respectively, whereas new settlers (100-180 h after fertilization) preferred water at 10 [+ or -] 5 psu and small individuals (180 h and more after fertilization) preferred even less saline water. In this study, D-shaped larvae were found in water at ca. 9-32 psu (Fig. 5). Using a computer simulation method, Kuwabara & Saito (2003) revealed complicated trajectories of C. japonica larvae along the river distance, assuming a real salinity profile along the river distance, change of salinity selectivity of larvae and new settlers and weak vertical swimming. It is possible for planktonic larvae of the clam to look for water at the preferred salinity by using vertical swimming, and for new settlers to quickly resuspend themselves into the water column to look for their preferred salinity on drifting threads. Drifting settlers could be transported to the higher-density area of the new settlers (i.e., higher larval settlement area) located in the upper parts of the Kiso estuaries as indicated in Figure 6. To understand the reasons why the higher-density area was located in the upper parts of the Kiso estuaries, we would need to examine environmental characteristics in those areas, particularly salinity data during the period when abundant new settlers of the clam were found (Figs. 5 and 6).

Differences in Corbicula japonica Density Between Estuaries

As summarized in Table 1, we compiled the data obtained during the period when planktonic larvae were collected, finding significant differences in densities of planktonic larvae, new settlers and small and commercial individuals in both 2001 and 2002 between estuaries: larval density was significantly higher in the Kiso Estuary, whereas the reverse was true for the densities of new settlers, and small and commercial individuals. Significant differences in density of large individuals between estuaries were not detected (Table 1). Because the local fishermen union (e.g., Akasuka) clearly gets higher annual catch yields of the clam in the Ibi-Nagara Estuary, putting higher fishing pressure (mortality) on commercial individuals in the estuary (Mizuno et al. 2005), commercial individuals in fact may have a much higher density in the Ibi-Nagara Estuary. In each estuary, the densities of larvae and new settlers were significantly higher in 2002, compared with 2001, but there was not a significant difference in the densities of the other life stages between years (Table 1).

In summary, larval supply was bigger in the Kiso Estuary, whereas densities of new settlers (i.e., strength of larval settlement) up to commercial individuals were higher in the Ibi-Nagara Estuary. This indicates that larval settlement processes may be critical for determining the densities of all stages between estuaries. However, it is not clear whether the strength of larval settlement controls population dynamics in each estuary. We need to examine variations in densities of different life stages by using cohort analyses. We will examine population dynamics as indicated by larval recruitment in these estuaries by cohort analysis in a coming study.

Ontogenetic Habitat Shift of Corbicula japonica

Habitat shift in relation to life stages (ontogenetic habitat shift thereafter) was detected in the Ibi-Nagara Estuary, but not in the Kiso Estuary (compare Figures 6-9): this was generated by the habitat shift of small to large individuals in the Ibi-Nagara Estuary. As indicated in Figure 3, depths along the river distance become deeper in the middle to upper parts of the Kiso Estuary than in corresponding locations in the Ibi-Nagara estuary, indicating that saline water may be detectable further upstream in the Kiso Estuary. Because juveniles and adults of the clam show strong resistance to low salinity or freshwater (Asahina 1941, Tanaka 1984a, 1984b, Sekiguchi et al. 1991), juveniles and adults (large and commercial individuals in this case) may locate comfortable habitats in the further downstream areas (i.e., in the lower to middle parts) in the Ibi-Nagara Estuary than in the Kiso Estuary. This could also be true for new settlers and small individuals (Asahina 1941, Tanaka 1984a, 1984b). We speculated that the main habitats of new settlers and small individuals would be located in more downstream (more saline) areas than favored by the large and commercial individuals. However, our data did not support this speculation; on the whole, the reverse was true for new settlers and small individuals: in the Ibi-Nagara Estuary, their habitats were located in the more upstream (less saline) areas than favored by the large and commercial individuals, as indicated in Figures 6-9. It is not immediately apparent why our speculation was not supported.

Previous studies dealing with larval recruitment of bivalves (Ma. veneriformis, Mu. senhousia and R. philippinarum) that are common and abundant in Japanese estuaries (Tsutsumi & Sekiguchi 1996, Miyawaki & Sekiguchi 1999, Miyawaki & Sekiguchi 2000, Ishii et al. 2001a, 2001b, Soutome et al. 2005), have not detected ontogenetic habitat shifts by these bivalves, unlike the habitat shift of C. japonica detected in the present study (see Figures 6-9; Soutome et al. 2005). As clearly seen in Figures 6-9, an ontogenetic habitat shift of C. japonica was detected in the Ibi-Nagara Estuary: higher densities of new settlers and small individuals were found mainly in the upper part (7 km or more upstream), whereas large and commercial individuals occurred in the lower to middle parts (2-7 km upstream). Such an ontogenetic habitat shift may be explained by one of the following alternative scenarios. (1) Ontogenetic habitat shift is generated by tidal migration on drifting (byssus) threads, as was reported for early juveniles of several bivalves and adults of Mer. lusoria and Corbicula species (Hamada & Ino 1954, Prezant & Chalermwat 1984, Lane et al. 1985, Sigurdsson et al. 1976). (2) There is no ontogenetic habitat shift within a cohort, but an apparent habitat shift is generated because each cohort occupies a separate area. (3) An apparent ontogenetic habitat shift within each cohort is generated by different site-specific mortality. (4) An apparent ontogenetic habitat shift is generated by different site (area)-specific mortality if each cohort occupies a separate area. Scenarios 1 and 2 can be examined or tested by cohort analysis, and then scenarios 3 and 4 can be tested after scenarios 1 and 2 are examined. However, it is not immediately apparent which scenario is supported. We will deal with this issue by cohort analysis in a coming study.

ACKNOWLEDGMENTS

The authors thank Dr Taeko Kimura of the Faculty of Bioresources of Mie University and staff of Mie Prefectural Science and Technology Promotion Center, for their moral and logical support during the course of this study; Akasuka Fishermen Union located in Kuwana; Mie Prefecture and staff of the authors' laboratory for helping in field sampling on board in the Kiso estuaries and the staff of the Kiso River Management Office of the Ministry of Transport and Infrastructure for using environmental data of the Kiso estuaries.

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RYOGEN NANBU, ETSUKO YOKOYAMA, TOMOMI MIZUNO AND HIDEO SEKIGUCHI *

Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu, Mie 514-8507, Japan

* Corresponding author: [e-mail]

TABLE 1.
Summaries of 2-way ANOVA for the density difference of each life
stage of Corbicula japonica for each year between the Ibi-Nagara
and the Kiso Estuaries and for each estuary between both years,
respectively.

May to December 2001      Ibi-Nagara   Kiso

Larvae                        --        --
New settlers                  --        --
Small individuals             O         X
Large individuals             --        --
Commercial individuals        O         X

January to October 2002   Ibi-Nagara   Kiso

Larvae                        X         O
New settlers                  O         X
Small individuals             O         X
Large individuals             --        --
Commercial individuals        --        --

Ibi-Nagara                   2001      2002

Larvae                        --        --
New settlers                  X         O
Small individuals             --        --
Large individuals             --        --
Commercial individuals        --        --

Kiso                         2001      2002

Larvae                        X         O
New settlers                  --        --
Small individuals             --        --
Large individuals             --        --
Commercial individuals        --        --

O, significant difference with higher density; X. significant
difference with lower density; --, no significant difference. Raw
data of density were log (n + 1)-transformed before statistical
examination using 2-way ANOVA (significant level [alpha] = 0.05).
Note that significant differences in density of each life stage
were examined between the Ibi-Nagara and the Kiso Estuaries in
each year and between both years in each estuary, using the data
obt for the same period, respectively.