Larval settlement and recruitment of a brackish water clam, Corbicula japonica, in the Kiso estuaries, central Japan

2007·6.9 MB·English

by  Ryogen Nanbu

Amer. Make. Bull. 22: 143-155
Larval settlement and recruitment of a brackish water clam, Corbicula japonica> in
the Kiso estuaries, central Japan
Ryogen Nanbu, Estuko Yokoyama, Tomomi Mizuno, and Hideo Sekiguchi
Faculty of Bioresources, Mie University, 1515 Kamihama-cho, Tsu,Mie 514-8507, Japan, [email protected]
Abstract: Population dynamicsofthebrackishwaterclam Corbiculajaponica wereexamined in the Kiso estuaries (the Ibi-Nagara Estuary
and the Kiso Estuary), central Japan, during the process oflarval recruitment. Based on temporal variation in densities (sampling every 2
weeks for planktonic larvae, newsettlers, and small individuals, sampling every month for large and commercially important individuals)
from May2001 toApril 2004, weconcludethatdensitiesoflargeand ofcommerciallyimportant individualsweredeterminednotbylarval
supplybutbybenthic processes. Density-dependent processesweredetectedbetween densitiesofnewsettlersandrecruits. These processes,
however, were detected for spring-summer cohorts, but not for autumn-winter cohorts. Spatial distributions ofeach cohort were almost
thesamewithin the Ibi-NagaraEstuaryandwithinthe Kiso Estuary, although cohortswerecollected mainlyin the middleto upperregions
ofthe Ibi-Nagara Estuarybut were collected in the upper region ofthe Kiso Estuary. A shift in an ontogenetic habitat within each cohort
was detected in the Ibi-Nagara Estuary but not in the Kiso Estuary: New settlers and small individuals were collected in the upper region
whilelargeand commerciallyimportant individualswerecollectedinthemiddle region. Theshift maybeexplainedbytidal migration using
byssalthreadsorbysite-specific differences in mortality, although it was not clearwhytheshiftwas detected in the Ibi-Nagara Estuarybut
not in the Kiso Estuary.
Key words: population dynamics, density-dependent processes, commercially important species
The brackish water clam Corbicula japonica (Prime, Recent studies on marine benthic invertebrates have
1864) is endemic to eastern Asia (Sakai et al. 1994, Harada emphasized the role oflarval recruitment in the population
and Nishino 1995). The clam is commonly found in estua- dynamics of intertidal and subtidal organisms that have
rine waters throughout Japan except for the Ryukyu Archi- complex life cycles (those that include planktonic and ben-
pelago, southern Japan, winch geographically belongs to the thic phases) {e.g., Roughgarden et al. 1988, Underwood and
Subtropical/Tropical Zone. The species is a target for clam Keough 2001), although few studies have been made in the
fisheries in Japan, especially in the Kiso estuaries (the Ibi- marine environment, probably due to difficulties in identi-
Nagara and the Kiso Rivers), central Japan (see Nakamura fying planktonic larvae {e.g., Sakai and Sekiguchi 1992), in
2000). Despite several regulations imposed to manage fish- examining the coupling of larval transport and dispersal
eries of Corbicula in Japan, the total annual catch yields of with oceanographic conditions (e.g., Roughgarden et al.
Corbicula species (of which approximately 99% is C. ja- 1988), and in discovering larval settlement processes (e.g.,
ponica) have decreased drastically over the last two or three Connell 1985, Gaines and Roughgarden 1985, Gaines et al.
decades (Mizuno etal. 2005), probablythrough the progress 1985). This situation has been true also for bivalves, includ-
ofeutrophication in the estuarine and coastal waters of Ja- ing the clams that are commercially important in Japan (see
pan. This is true forthe yields in the Kiso estuaries as well as Miyawaki and Sekiguchi 1999, 2000, Ishii et al. 2001a,
in other areas of Japan. Traditionally in Japan, the larger 2001b). Unfortunately, there is not sufficient data on larval
hard shell clam Meretrix lusoria (Roding, 1798) has been recruitment of Corbicula japonica, in contrast to the clam
commercially more important than C. japonica. A drastic Ruditapes pbilippinarum (Adams and Reeve, 1850), which
decrease in the yield ofM. lusoria in the Kiso estuaries oc- dominates Japanese tidal flats (Miyawaki and Sekiguchi
curred in the late 1970s, when the yield of C. japonica 1999, 2000, Ishii et al. 2001a, 2001b).
abruptly increased (Mizuno et al. 2005). This resulted in a Tolerance to varying salinity by planktonic larvae and
much greater fishing effort for C. japonica. However, the benthic stages of Corbicula japonica was examined in the
yields ot C. japonica in the Kiso estuaries has drastically laboratorybyTanaka (1984a, b) and Saito etal. (2002). Tidal
decreased since early 1980s despite several regulations im- transport of the larvae was investigated in relation to the
posed on the fishery. The causes or mechanisms by which salinity distribution in estuarine waters in the Kiso estuaries
the drastic decrease in the yield of C. japonica in the Kiso bySekiguchi etal. (1991) and in the laboratory by Kuwabara
estuaries, as well as in the other areas of Japan, may be and Saito (2003). In Lake Shinji where C. japonica is the
driven are not well understood (see Nakamura 2000). most important target species for clam fisheries, growth of
143
144 AMERICAN MALACOLOGICAL BULLETIN 22 • 1/2 • 2007
benthic stages of the clam was examined by Takada et al.
(2001) and Oshima et al. (2004). Takada et al. (2001) also
studied the seasonal abundance ofspat ofthe clam, estimat-
ing the season oflarval settlement. However, larval recruit-
ment ofthe clam has not been studied. Nanbu et al. (2005)
examined the spatio-temporal variations in densities ofdif-
ferent life stages (planktonic larvae, new settlers, and small,
large and commercially important individuals) of the clam
in the Kiso estuaries, central Japan, to understand larval
recruitment,bywhich benthicpopulations maybegenerated Pacific
Ocean
and maintained in the estuaries. In contrast to the other
common and abundant bivalves (Ruditapes philippinarum, 132° 143 E
Musculista senhousia [Benson, 1842], and Mactra venerifor-
mis Deshayes in Reeve, 1854), of which all life stages were KisoR. •NAGOYA
detected around the river mouths, C. japonica showed a
marked ontogenetic habitat shift in the Ibi-Nagara Estuary,
but not in the Kiso Estuary. The primary habitat for new
settlers and small individuals and oflarge and commercially Tsiy ISE BAY 2-
important individualsofC. japonica waslocated in the upper
and middle regions of the Ibi-Nagara Estuary, while the
benthic stages were primarily found in the upper region of Pacific
the Kiso Estuary. However, it is not clear whether the on- 1 13i7° E
togenetic habitat shift is generated by migration duringben-
thic stages or by differences in site-specific mortality ofnew Water Depth
recruits. 0 - 2 m
2 - 5 m
To understand the population dynamics of Corbicula 5 - 10 m
japonica in the Kiso estuaries, we examined which life stage > 10 m
may determine the strength of catch yields (or population
size) ofthe clam, and whether the ontogenetic habitat shift
of the clam may be generated by migration during benthic
Figure 1.Studyareaandlocationofsamplingstations.Solidcircles,
stages or by differences in site-specific mortality, using the sampling stations with 2 or 3 sites (e.g., I-la, I-lb) within each
cohort separation based on three years of data (from May samplingstation; bold solid lines seaward from the estuarymouth,
2001 untilApril 2004) collected in the Kiso estuaries. Nanbu protection bars against the tide.
et al. (2005) examined spatio-temporal distributions ofden-
sities ofeach life stage ofthe clam, using the first two years
ofdata from the present study. Environmental characteristics
For the last decade, the Kiso River Management Office
METHODS ofthe MinistryofTransport and Infrastructure has continu-
ously monitored water temperature and salinity every hour
Study area in water 0.5 m above the bottom at a site 0.5 km upstream
The Kiso Rivers (the Ibi, Nagara, and Kiso Rivers), three in the Ibi-Nagara Estuary (Zyonan, see Fig. 1 for location).
ofthe largest rivers in Japan, flow into Ise Bayon the Pacific To examine environmental characteristics of the Kiso estu-
coast ofcentral Japan (Fig. 1). The Ibi and the Nagara Rivers aries,we usedaveragesover24 h oftheseenvironmentaldata
join at their lower regions where both rivers are united into obtained from that office (Fig. 2A). Although we have no
the Ibi-Nagara River. The Nagara River, however, has re- similar data available for the Kiso Estuary, environmental
cently been closed at a point 5 km upstream by the Nagara conditions are similar at the mouths ofthese estuaries (Mi-
Dam (Fig. 1). The Kiso estuaries (the Ibi-Nagara and the zuno et al. 2005).
Kiso Estuaries), defined as the areaswhere bottom water has According to Mizuno et al. (2005), the silt-clay fraction
m
a detectable salinityof 1.0 psu, are 2-10 in depth andhave ofsediments in the Kiso estuariesis usuallylessthan 5.0% in
a maximal tidal range of3 m, reaching 30 km upstream for dry weight, being much higher at the mouths and down-
the Ibi River and 26 km upstream (where there is a dam) for stream areas of these estuaries, and lower in the upstream
the Kiso Estuary (Japan Society ofOceanography 1985). areas. Accordingly, bottom sediments are sandy 5 km or
LARVAL RECRUITMENT OF CORBICULA JAPONICA IN THE KISO ESTUARIES 145
Salinity Figure 2.A, Watertemperature and
salinity in the Kiso estuaries, central
Japan. B-F, Variation in average
densities of different life stages
(planktonic larvae, new settlers,
Temp.
small individuals, large individuals,
MJJASONDJFMAMJJASONDJFMAMJJASONDJFMA
and commercially important indi-
viduals) in the Kiso estuaries, cen-
2000/ tral Japan. Solid lines, the Ibi-
Nagara Estuary; dotted lines, the
Kiso Estuary.
JUL
MJJ A S ON DJFMAMJJ ASONDJFMAMJJ A SONDJ FMA
J A S ON DJFMAMJJ A SONDJFMAMJJ A SONDJ FMA
M J JA S ON DJFMAMJJ A SONDJFMAMJJ A SONDJ FMA
20, f£
15
ASON DJFMAMJJ SONDJFMAMJJ SONDJ FMA
M J J A A
5 F
5> —E
-c5 =o>
MJJASONDJFMAMJJASONDJFMAMJJASONDJFMA
2001 2002 2003 2004
Monthsofyear
more upstream ofthese estuaries, although an extraordinar- Sampling procedures
ily high percentage (50% or more) of the silt-clay fraction Sampling was undertaken in the Kiso estuaries from
occurs in the area closest to the Nagara Dam. Muddy sedi- May 2001 until April 2004. The study area, environmental
ment was common in troughs, so that the silt-clay fraction characteristics, sampling procedures, and data processing
was very different between troughs and ridges, even at the were described in detail in Nanbu et al. (2005).
same distance from the mouths ofthe estuaries. Planktonic larvae ofCorbiculajaponicawere obtained at
146 AMERICAN MALACOLOGICAL BULLETIN 22 • 1/2 • 2007
stations located 1 km seaward ofeach river mouth (I-la and (t-test, p >0.05). We used average densities ofeach life stage.
[-lb in the Ibi-Nagara Estuary, K-la and K-lb in the Kiso Based on these averages for the period from May 2001 to
Estuary) (Fig. 1) using a vertical haul of plankton nets (22 April 2004, we examined the differences in density of each
cm in diameter, 133 um mesh openings) from the bottomto life stage between the two estuaries and between sampling
the surface every 2 weeks (except for January to March year for each estuary, using Mann-Whitney's U-test (signifi-
2002). Larval density was indicated as individuals/m3. Bi- cance level, a = 0.05) and Kruskal-Wallis's H-test (signifi-
valve larvae were identified to species using a compound cance level, a = 0.05), respectively. We used Bonferroni's
microscope according to Sakai and Sekiguchi (1992) and method (a' = 0.05) when significant differences in density
Kimura et al. (2004). were detected between sampling years.
For sampling benthic stages of Corbicula japonica, 8 To separate each cohort, the shell lengths for the two
stations (1-3, I-1, II, 12, 15, 17, 19, and 112 for the Ibi-Nagara groups (new settlers/small individual group, large/
Estuary, K-3, K-l, Kl, K3, K5, K7, K9, and K12 for the Kiso commercial individual group) were compiled for all stations
Estuary) were locatedevery2 or 3 km up to 12 km upstream of each estuary. However, it was difficult to separate each
from the river mouth in each estuary and also 2 or 3 sites 1 cohort for commercial individuals due to their small num-
km and 3 km downstream/seaward from the mouth, respec- bers. Based on these data, cohorts within each group were
tively (Fig. 1). Samples were collected at 2 or 3 sites with identified by the method ofAkamine (1985), who separated
different depths within each sampling station area. To polymodal length distribution into two or more normal dis-
sample new settlers and small individuals, one sediment tributions. The growth curve of each cohort was estimated
sample was collected with a core sampler (3.1 cm in diam- based on temporal change ofthe mean shell length ofeach
eter, 1.0 cm depth) from the surface layer of bottom sedi- normal distribution.
ments which were obtained at each site within each station
using a Smith-Mclntyre grab. For sampling large and com- Estimation of densities of new settlers and recruits of
mercially important individuals, one sediment sample was Corbiculajaponica
obtained at each site within each station using a Smith- To examine the relationships between the densities of
Mclntyre grab. New settlers and small individuals of the new settlers and recruits (i.e., large individuals) ofthe clam,
clam were collected every 2 weeks while large and commer- using the data for cohorts that were successful in recruit-
cially important individuals were collected every month. ment, we estimated densities of new settlers and recruits,
Identification of new settlers and small individuals of C. respectively, as follows: Based on the growth curve of each
japonica was done following Sakai and Sekiguchi (1992) and successful cohort, we determined the daywhen average shell
Kimura et al. (2004). Density of the benthic stages of the lengths of new settlers reached 1.0 mm. Then, assuming
clam was indicated as individuals/0.0lm larval settlement day as day 0 when new settlers of each
.
The life stages of Corbicula japonica were defined as in successful cohort were first collected, we fitted a regression
Nanbu et al. (2005): "Planktonic larvae" were D-shaped lar- line to the temporal change of each cohort density using a
vae (i.e. early-stage veligers); "new settlers" were individuals graph in which the X-axis was days after larval settlement of
with shell lengths less than 300 pm; "small individuals" were each successful cohort and the Y-axis was the log-
mm
ones with shell lengths 0.3 or more but less than 1.0 transformed density of each successful cohort. Using the
mm; "large individuals" were ones with shell lengths of 1.0 density data estimated for successful cohorts with a signifi-
mm or more but less than 12.0 mm; "commercial individu- cant (p < 0.05) regression line, we examined the relation-
als" were ones with shell lengths of 12.0 mm or more. These ships between the densities ofnew settlers and recruits (i.e.,
definitions are the same as those used to describe bivalves based on the data for each estuaryand then for each estuary
that are common in Japanese tidal flats (e.g., Ruditapesphil- accordingto season), and then the relationshipsbetween the
ippinarum) by Sekiguchi and his co-workers (e.g., Miyawaki density of new settlers and the ratio of their densities (re-
and Sekiguchi 1999, 2000, Ishii et al. 2001a, b, Nanbu et al. cruits/new settlers) (i.e., based on the data for each estuary
2005) except for planktonic larvae. We defined "successful and then for each estuary according to season).
recruitment" as new settlers and small individuals that
mm
reached average shell lengths of 1.0 or more for each
cohort. RESULTS
Data analysis Variation in densities of different life stages of
Sediment samples were collected at 2 or 3 sites within Corbiculajaponica
each station. The density ofeach life stage ofC.japonica was The densities of planktonic larvae peaked primarily in
not significantly different between sites within each station May to December every year (Fig. 2B). There was no sig-
LARVAL RECRUITMENT OF CORBICULA JAPONICA IN THE KISO ESTUARIES 147
nificant difference in larval density between the Ibi-Nagara August everyyear (Fig. 2C). There was no significant differ-
and the Kiso Estuaries nor between sampling years for each ence in density between the two estuaries (except for 2003)
estuary (Table 1). As seen in Fig. 2B, however, larval density nor between sampling years for each estuary (Table 1), al-
in 2003 appeared to be lower than in the other years, prob- though the density in 2003 appeared to be lower than in the
ably due to more days with less than 10 psu in 2003 than in other years, possibly due to a lower larval density in 2003.
theotheryears, because salinitypreferredbyspawningofthe Higher densities of small individuals were found tor a
clam is in the range of9.35-21.82 psu (Asahina 1941). longer period than new settlers, occurring from August to
Water temperature reached about 30°C in summer and the following April (Fig. 2D), although the density appeared
decreased below 10°C in winter (Fig. 2A). On the other to be much lower in 2003 than in the other years. In each
hand, although tending to become lower in summer and year, density ofsmall individuals was significantly higher in
higher in winter, salinity did not indicate such a clear sea- the Ibi-Nagara Estuary than in the Kiso Estuary (Table 1 ).
sonal change but always showed irregular variations (about The Kiso Estuary had a higher density in 2002; other-
15-32 psu) and occasionally marked lowering (down to <1 wise there was no significant difference in the density of
psu, corresponding to low larval density) owing to freshwa- small individuals between sampling year for each estuary
ter discharge through high rainfall in early summer. Larval (Table 1).
densitieswereloworlarvaewere completelyabsent from the Variation in densities of large individuals was very
watercolumn usuallyfrom Decemberto the following April, similar between the two estuaries (Fig. 2E). There was
when the water temperature decreased to below 15°C. Ac- no significant difference in density between these estuaries
cording to laboratory rearing experiments (Kimura et al. and also between sampling year for each estuary (Table 1 ).
2004), larvae of the clam reared at 15°C or lower failed to As seen in Fig. 2E, however, the density of large indi-
settle and recruit. The larvae from earlier and later portions vidualsappearedtobehigher in 2003 than in theotheryears,
of a much longer period spawning of the clam every year in contrast to larvae and new settlers/small individuals
may not contribute to generating cohorts ofnew settlers. (Table 1).
The densities ofnew settlers peaked primarily in Julyto The density ofcommercially important individuals was
Table 1. Differences in densities ofdifferent life stages ofCorbiculajaponica between the Ibi-Nagara and the Kiso estuaries and between
sampling years for each estuary.
2001 Ibi-Nagara Kiso Ibi-Nagara Sampling year
Larvae Larvae 2002 =: 2001 = 2003
New
settlers
Small individuals O X New settlers 2002 = 2001 = 2003
Large individuals
Commercial individuals O Small individuals 2002 = 2001 = 2003
Large individuals 2003 = 2001 = 2002
2002 Ibi-Nagara Kiso
Larvae Commercial individuals 2001 > 2002 > 2003
New settlers i ,> _
Small individuals O X Kiso Sampling year
Large individuals Larvae 2002 = 2001 = 2003
Commercial individuals
New settlers 2002 = 2001 = 2003
2003 Ibi-Nagara Kiso
Larvae Small individuals 2002 = 2001 = 2003
L_ _J
New settlers O X
Small individuals O X Large individuals 2003 = 2002 = 2001
Large individuals Commercial individuals 2001 > 2002 > 2003
Commercial individuals
Mann-Whitney's U-test (significance level, a = 0.05). Kruskal-Wallis' H-test (significance level, a = 0.05).
O: significant difference with higher density. >: significant difference.
x: significant difference with lower density. =: no significant difference.
-: no significant difference. Multiple comparison (a' = 0.05; Bonferroni's method).
)
148 AMERICAN MALACOLOGICAL BULLETIN 22 • 1/2 • 2007
significantly lower in 2003 than in the other years (Table 1 and to succeed in recruitment through the three-year inves-
and their density appeared to decline from 2001 to 2004 tigation. Larval settlement for 2 cohorts (i, ii) occurred in
(Fig. 2F). August to September 2001. For 4 cohorts (iii-vi), the larval
settlement season in 2002 varied depending on cohort: Co-
Cohort separation of Corbiculajaponica horts iii, iv, and v settled in January, May to June, and
Inthe Ibi-NagaraEstuary, 21 cohorts ofnewsettlersand September, respectively. Larval settlement for the remaining
small individuals and 16 cohorts of large individuals were cohort occurred in April of 2003. Of these 7 cohorts, sig-
identified (Fig. 3B). Ofthese cohorts, 7 were found to settle nificant regression lines were fitted to the temporal change
Figure 3. Cohorts ofbenthic stages
of Corbicula japonica in the Ibi-
Nagara Estuary, central Japan. A,
Bold solid lines, larval density; thin
solidlines, salinity; dottedlines,wa-
tertemperature. B, i-vi, cohortssuc-
cessful in recruitment; solidoropen
circleswithverticallines,averagesof
shell lengths and standard error;
horizontalsolidline, shelllength 1.0
mm, which we defined as the size
for successful recruitment. C, i-vi,
cohorts same as in B.
MJJASONDJ FMAMJJASONDJ FMAMJJASONDJ FMA
MJ J ASONDJ FM A M J J A SONDJ FM A M J J AsONDJ FM A
2002
Monthsofyear
LARVAL RECRUITMENT OF CORBICULA JAPONICA IN THE KISO ESTUARIES 149
of density from larval settlement to recruitment for 6 co- August to September 2001. For 3 cohorts (3-5), larval
horts (cohort i-vi) (Fig. 3C). These cohorts (except cohort settlement occurred in Julyto September 2002. Larval settle-
iii) appeared to settle in August to November. ment lor the remaining 2 cohorts occurred in Decem-
In the Kiso Estuary, 12 cohortsofnewsettlers and small ber 2002 and in July 2003, respectively. Significant regres-
individuals and 13 cohorts oflarge individuals were identi- sion lines were fitted to the temporal change of density
fied (Fig. 4B). Ofthese cohorts, 7 were detected to settle and from larval settlement to recruitment for 5 cohorts (1-5)
to succeed in recruitment through the three-year investiga- (Fig. 4C). These cohorts appeared to settle in July to
tion. Larval settlement for cohorts 1 and 2 occurred in September.
Figure 4. Cohorts ofbenthic stages
ofCorbicula japonica in the KisoEs-
tuary, central Japan. A, Bold solid
solid lines, larval density; thin solid
lines, salinity; dotted lines, water
temperature. B, 1-5, cohorts suc-
cessful in recruitment; solidoropen
circleswithverticallines,averagesof
shell lengths and standard error;
horizontal solidline, shelllength 1.0
mm, which we defined as the size
for successful recruitment. C, 1-5,
cohorts same as in B.
MJJASONDJFMAMJJASONDJFMAMJJASONDJFMA
E 1000-
MJ ASONDJ FM AMJ ASONDJ FMAMJ ASONDJ FMA
J J J
2004
Monthsofyear
Kiso estuaries (combined) Kiso estuaries (combined)
n= 11 A n = 11
r= 0.76 r= 0.93
p = 0.005 p < 0.001
t5 A
I I
2 3 4 !
newsettlers(lo5;inds./m2) newsettlers(loginds./m )
IbhNagara Estuary Ibi-Nagara Estuary
n = 6 A n = 6
r = 0.74 r= 0.95
p > 0.05 p = 0.003
04
AA
A
A A
a\a
2 3I 4 e
newsettlers(loginds./m2) newsettlers(loginds./m )
Kiso Estuary Kiso Estuary
n = 5
r = 0.87
p > 0.05
o
o
o00
2 3
newsettlers(!oginds,/m ) newsettlers(loginds./m2)
Figure 5. Relationships between densities of new settlers and recruits of Corbicula japonica in the Kiso estuaries, central Japan. Upper
figures, data for the two estuaries combined; middle figures, data for the Ibi-Nagara Estuary; lower figures, data for the Kiso Estuary; left
figures,densityofnewsettlers vs. densityofrecruits;rightfigures,densityofnewsettlersvs. theratiooftheirdensities(recruits/newsettlers);
open circles, data for the Ibi-Nagara Estuary; triangles, data for the Kiso Estuary; n=number ofcohorts.
LARVAL RECRUILMENL OF CORBICULA JAPONICA IN THE KISO ESTUARIES 151
Based on the data for successful cohorts in the Kiso successful cohorts of benthic stages (compare Fig. 4B with
estuaries, it took nearly two years from larval settlement for Fig. 4A). However, as indicated by comparing Fig. 3B with
individuals to reach commercially viable shell lengths. Fig. 3A, peaks of densities of planktonic larvae did not al-
Takada etal. (2001) andOshima etal. (2004) estimatedshell ways contribute to the generation of successful cohorts of
growth ofclams using growth rings on the shell surface and benthic stages. The larvae from earlier and later periods of
found a similar growth rate as in the present study. spawning ofthe clam everyyear may not contribute to gen-
As indicated in Figs. 3B and 4B, the densities of new erating cohorts ofnew settlers.
settlers in the cohorts identified in 2003, which originated in
planktonic larvae and new settlers with much lower densi- Relationships between densities of new settlers and
ties, were considerably lower than in the other years (Figs. recruits of Corbiculajaponica
2B, 2C). Density peaks ofnew settlers appeared to generate Based on the data from 11 successful cohorts (6 from
Spring-Summer Spring-Summer
E
Autumn-Winter Autumn-Winter
n = 4 A n = 4
r= 0.98 r = 0.95
p = 0.016 p = 0.05
A
% 04
A
i
4C)
O
A
newsettlers(loginds./m ) newsettlers(loginds./m
Figure 6. Relationships between densities of new settlers and recruits of Corbicula japonica in the Kiso estuaries, central Japan. Upper
figures, data for spring-summer cohorts; lower figures, data for autumn-winter cohorts; open circles, data for the Ibi-Nagara Estuary,
triangles, data for the Kiso Estuary; n=the number ofcohorts.
152 AMERICAN MALACOLOGICAL BULLETIN 22 • 1/2 • 2007
the Ibi-Nagara Estuary, 5 from the Kiso Estuary), there was forautumn-winter cohorts, but therewas no significant cor-
a significantly positive correlation between densities ofnew relation between the density ofnew settlers and the ratio of
settlers and recruits in the combined estuaries, but not in their densities (recruits/new settlers).
each estuary (Fig. 5). There was a significant negative cor-
relation between the density ofnew settlers and the ratio of Ontogenetic habitat shift of Corbiculajaponica
theirdensities (recruits/newsettlers) forthese estuaries com- Of the cohorts identified in the Ibi-Nagara Estuary, 4
bined and also for each estuary individually (Fig. 5). cohorts (i-iv) were successful in reaching shell lengths (12.0
The 11 successful cohorts that were divided into two mm) ofcommercially important individuals (Fig. 3B). Each
groups, 7 spring-summer cohorts and 4 autumn-winter cohort had a similar spatio-temporal distribution (Fig. 7):
ones, according to the months with successful recruitment. new settlers and small individuals were collected mainly in
There was no significant correlation between densities of the upper region ofthe estuary, large and commercially im-
new settlers and recruits for spring-summer cohorts, portant individuals were found primarily in the central re-
whereas there was a significant negative correlation between gion. Nanbu et al. (2005) found an ontogenetic habitat shift
the density of new settlers and the ratio of their densities during the benthic stages (from newsettlersto commercially
(recruits/new settlers) (Fig. 6). The reverse was true for au- important individuals) in mixed cohorts of this clam. An
tumn-winter cohorts (Fig. 6): There was a significant posi- ontogenetic habitat shift was also detected within the same
tive correlation between densities ofnewsettlers and recruits cohort, as indicated in Fig. 7. However, this was not true for
Cohort Cohort
i ii
12 12
9 9
7 7
5 5
3 3-
I I
-1
I
-3
-3-I
MJJA SOND'j FMAMJJASOND'jFMAMJ JASOND'j FMA MJ JASONO'JFMAMJJASOND'jFMAMJJASOND\JFMA
Cohort ii Cohort iv
MJJASONDJFMAMJJASONDJ FMAMJJASONDJ FMA MJJASONDJ FMAMJJ AS ONDJFMAMJJA SONDJFMA
2001 2002 2003 2004 2001 2002 2003 2004
Months ofyear
Figure 7. Habitats ofdifferent cohorts ofCorbiculajaponica in the Ibi-Nagara Estuary, central Japan. Dots, sampling day/location; solid
lines,isoplethsofdensitiesofnewsettlers/smallindividualsorlarge/commercial individuals;numerals,densityininds./O.Olm2 graytoblack
;
areas, the highest density areas with 50 inds./O.Olm2 or more for new settlers/small individuals and 5 inds./O.Olm2 or more for large
individuals. Left and right isopleths in each figure indicate isopleths for new settlers/small individuals and large/commercial individuals,
respectively. Open space between isopleths ofnew settlers/small individuals and large/commercial individuals in each figure is due to the
transition between these two groups, as indicated in Figures 3 and 4.