ORIGINAL ARTICLE Biology

Small-scale variation in feeding environments for the Manila clamRuditapes philippinarum in a tidal flat in Tokyo Bay

Satoshi Watanabe Æ Satoshi Katayama ÆMasashi Kodama Æ Naritoshi Cho Æ Kaoru Nakata ÆMasaaki Fukuda

Received: 20 October 2008 / Accepted: 22 April 2009 / Published online: 9 June 2009

� The Japanese Society of Fisheries Science 2009

Abstract The relative contribution of particulate organic

matters (POMs) in water column and sediment as a food

source for the Manila clam, Ruditapes philippinarum, was

studied using carbon and nitrogen stable isotopic method

(d13C and d15N) in a tidal flat at Seaside Park, Yokohama,

Japan. Comparisons of d13C and d15N among R. philipp-

inarum and POMs in surface water, bottom water, and

sediment surface indicated that R. philippinarum larger

than 5 mm shell length (SL) mainly assimilated benthic

POM, and individuals smaller than 5 mm SL assimilated

benthic and pelagic POM. Continuous measurements of

chlorophyll concentrations in the bottom water revealed

tide-driven resuspension of the benthic phytopigments.

R. philippinarum showed differences in d13C and d15N

along an inshore–offshore transect, indicating small-scale

spatial differences in POM provision in the tidal flat. These

findings suggest that POM in the bottom water, supposedly

inhaled by R. philippinarum, is a mixture of a larger pro-

portion of resuspended benthic POM and a smaller pro-

portion of pelagic POM, and that the mixing ratio of the

POMs may be affected by the hydrodynamics of flooding

water associated with tidal flat topography.

Keywords Manila clam � Microphytobenthos �Resuspension � Ruditapes philippinarum �Stable isotope ratio

Introduction

The Manila clam Ruditapes philippinarum is an important

fishery resource in Japan; however, the commercial fishery

production of R. philippinarum has markedly declined in

many parts of Japan for the last two decades due to stock

depletion. The stock depletion has been considered to be

attributable in part to environmental changes, such as tidal

flat reclamation [1], hypoxia due to eutrophication of

coastal waters [2], outbreaks of flood events [3], strong

winter waves [4, 5], and the introduction of alien predators

associated with spat seeding. Due to the reduced produc-

tivity of fishing grounds, it has become increasingly

important to manage fishing and stocking of R. philippin-

arum properly for sustainable production.

Estimation of potential productivity of a fishing ground

is essential for proper fishery management; however, basic

biological knowledge is still insufficient for such estima-

tions. For instance, effects of food supply on the stock

conditions of R. philippinarum are not well understood

because of limited information on the feeding ecology.

Information on food supply is one of the essential param-

eters for ecological models [6–10] to estimate potential

productivity (i.e., growth, mortality, reproduction, and

stocking density) of R. philippinarum in a fishing ground.

These models, however, mostly use mean water column

chlorophyll-a concentration as the energy input, which was

S. Watanabe (&)

Japan International Research Center for Agricultural Sciences,

Tsukuba, Ibaraki 305-8686, Japan

e-mail: swat@affrc.go.jp

S. Katayama � N. Cho

National Research Institute of Fisheries Science, Yokosuka,

Kanagawa 238-0316, Japan

M. Kodama � K. Nakata

National Research Institute of Fisheries Science, Yokohama,

Kanagawa 236-8647, Japan

M. Fukuda

Hokkaido National Fisheries Research Institute, Kushiro,

Hokkaido 085-0802, Japan

123

Fish Sci (2009) 75:937–945

DOI 10.1007/s12562-009-0113-1

suggested to be a poor estimator of food resources by one

of the models [6] for the simulation of growth and repro-

duction of R. philippinarum.

Filter-feeding bivalves are generally held to feed on

particulate organic matter (POM) suspended in the water

column. R. philippinarum is also considered to feed on

marine POM [11], including phytoplankton [12], micro-

phytobenthos [13–16], and detritus [17, 18]. Relative

availability and importance of these pelagic and benthic

food particles have been the subject of debate, and much

importance has recently been focused especially on mi-

crophytobenthos [14–16]. However, since food particles in

near-bottom water may be a mixture of particles from both

pelagic and benthic origin, mechanisms of mixing and

provision of these particles to R. philippinarum need to be

further elucidated for the better understanding of R. phil-

ippinarum nutrition and estimation of food availability in a

fishing ground. Information on ontogenic diet change in

R. philippinarum should also be taken into consideration

for such estimations.

In the present study, stable isotopic investigations were

conducted in a tidal flat in Tokyo Bay in order to study

small-scale variations in feeding environments for

R. philippinarum of different shell length. Phytopigment

concentrations near the tidal flat bottom were also contin-

uously measured to ascertain the feeding environments for

R. philippinarum in association with tidal movements.

Materials and methods

Specimens and study site

This study was carried out in an artificial pocket beach at

Seaside Park of Yokohama (35�200N, 139�380E), which

was reclaimed in 1981 by the city of Yokohama. The tidal

flat at Seaside Park of Yokohama (Fig. 1, 1-km-long

shoreline, maximum subtidal zone 140 m, slope angle

1/100) was designed for benign wave conditions and little

sand erosion to enhance natural occurrence of clams for

recreational clam digging, and it has a high productivity of

R. philippinarum compared with other areas in Tokyo Bay

despite extremely high recreational fishing pressure.

Various-sized R. philippinarum (5.0–28.4 mm shell

length, SL) were collected from eight stations on an

inshore–offshore transect, 15 m (St. 1), 25 m (St. 2), 35 m

(St. 3), 45 m (St. 4), 60 m (St. 5), 75 m (St. 6), 95 m

(St. 7), and 105 m (St. 8) away from the shoreline for

analysis of carbon and nitrogen stable isotope ratios (d13C

and d15N) at around lower low tide on 8 April 2004. Rel-

ative ground height (converted from water depth) of the

stations was measured with a graduated staff, with the in-

shoremost St. 1 being 0 m.

Additional collections were made at St. 8 on 13 January

2005. R. philippinarum from smaller than 5 mm (not

individually measured) to 26.1 mm SL were collected at

around the higher low water at spring tide (approx 1 m

depth). Surface water, bottom water, and sediment surface

were collected as follows. Surface water was scooped with

a dipper (300 ml, n = 3). Bottom water was collected with

a vampire sampling suction pump (The Fluid Life Corpo-

ration) through an opening of a polyvinylchloride (PVC)

pipe fixed to the tidal flat with a metal plate (300 ml,

n = 3). The top 1 cm sediment of the tidal flat was col-

lected (n = 8) with a core sampler made from a 50-ml

injection syringe (inner diameter 29 mm, Terumo Corpo-

ration) by cutting off the needle end.

Upon being brought back to the laboratory, R. philipp-

inarum were allowed to defecate in filtered seawater

overnight and were frozen at -80�C until the stable iso-

topic analysis. Sediment samples were also stored at

-80�C. The water samples were filtered with a precombu-

sted glass fiber filter (GF/F filter, pore size: ca. 0.7 lm,

Whatman International Ltd.) immediately after being

brought back to the laboratory and kept in a desiccator.

Stable isotopic analysis

Whole-body soft tissue samples of R. philippinarum were

collected by gently scraping with forceps, lyophilized, and

homogenized to a fine powder. Powdered samples were

then defatted by the conventional Folch method [19], using

Seaside Parkof Yokohama

35°30’Tokyo

Bay

km200 10

Tokyo Bay

Study site140° 35°139°36’

N

500 m

Fig. 1 Map showing the study site. Dashed line indicates the

inshore–offshore transect line. Seaside park of Yokohama is located

at the closed-off section of Tokyo Bay. Four rivers flow into adjacent

Hirakata Bay (northernmost one not shown), which is connected to

Seaside Park of Yokohama by two channels alongside Nojima Island

938 Fish Sci (2009) 75:937–945

123

2:1 chloroform–methanol solution (v/v) and centrifugation.

Because the fat content of organisms often changes sea-

sonally, and lipid is known to have a lighter d13C than other

tissue fractions [20, 21], samples for stable isotopic anal-

yses are generally defatted to reduce this variation. The

defatted samples were oven-dried, and an aliquot of the

sample (ca. 0.8 mg) was put in a tin container for the

analyses. Samples of R. philippinarum larger than 5 mm

SL were measured individually; individuals smaller than

5 mm SL were pooled (n = 5) since they were lighter than

the instrument requirements (i.e., 0.8 mg dry weight) for

the measurements.

The sediment samples with added distilled water were

sonicated in a cold ultrasonic bath for 10 min; POM

accumulated at the sediment surface (SOM) was collected

with a Pasteur pipette into a test tube, and calcium car-

bonate was removed by adding 1 N HCl; they were then

rinsed with distilled water, centrifuged, and dried in a 50�C

oven overnight. POM samples in the surface water (SPOM)

and bottom water (BPOM) collected with GF/F filter were

decalcified with HCl fumes saturated in a glass jar over-

night, dried in a 50�C oven, scraped off with a spatula, and

put in a tin container.

The carbon and nitrogen isotope ratios (d13C and d15N)

were analyzed using an EA-1108 elemental analyzer (Carlo

Erba) coupled with an isotope ratio mass spectrometer

(Finnigan Mat ConFlo II, Mat 252). The isotope ratios

were expressed as per mil (%) deviation from international

standards (i.e., fossil calcium carbonate for C and air for

N): d13C, d15N = (Rsample/Rstandard - 1) 9 1,000, where R

is 13C/12C or 15N/14N. Instrumental precision was 0.2%.

Relationships between stable isotope ratios of POMs and

R. philippinarum were analyzed assuming that the enrich-

ment of d13C and d15N is ?0.6% and ?3.4% per trophic

level, respectively [20–23].

Atomic organic carbon:nitrogen (C/N) ratios of

R. philippinarum, SPOM, BPOM, and SOM were also

analyzed at the same time.

Bottom-water chlorophyll measurements

Phytopigment concentrations in the bottom water were

continuously measured at St. 8 from 7 to 8 December

2004. An optical chlorophyll/turbidity logger (Compact-

CLW, JFE ALEC Co., Ltd., calibrated against standard

chlorophyll, n = 9, r2 [ 0.99) was placed horizontally at

the station, with the sensor located about 3 cm above the

bottom. This device measures both chlorophyll-a and

phaeopigments, which are collectively referred to as

chlorophyll hereafter. The measurements were conducted

for 24 h at 1-min intervals, and obtained data were aver-

aged for 20 min. Tidal height information was obtained

from the tide table for adjacent Yokosuka provided by the

Hydrographic and Oceanographic Department, Japan Coast

Guard (i.e., phase shift from Seaside Park of Yoko-

hama = 0 cm, amplitude difference = 2 cm, therefore

considered almost equal). Correlation between tidal height

and chlorophyll concentration and between the absolute

value of change rate of tidal height in 20 min (DH) and

chlorophyll concentration were analyzed by linear

regression.

Statistical analyses

Differences of the mean d13C, d15N, and C/N ratio values

between samples collected at different stations and/or time

were statistically tested by either parametric or nonpara-

metric tests depending on the homogeneity of variance,

which was tested by Bartlett test. Data sets with homoge-

neous variance were tested by one-way analysis of variance

(ANOVA) and Tukey test for a posteriori comparison

among samples. Data sets with heterogeneous variance

were tested by Kruskal–Wallis test and Scheffe test for

a posteriori comparisons. A P value less than 0.05 was

considered statistically significant.

Results

Stable isotope ratios of R. philippinarum and POMs

in water column and sediment

Stable isotopic ratios of POMs in the water column and

sediment surface were significantly different (Fig. 1).

SPOM (-26.3 ± 1.1% standard error, SE), BPOM

(-21.8 ± 0.5%), and SOM (-17.4 ± 0.2%) had signifi-

cantly different mean d13C values (ANOVA P \ 0.0001;

Tukey test P \ 0.0001). The mean d15N in SPOM

(4.8 ± 1.4%), BPOM (9.5 ± 0.01%), and SOM

(9.2 ± 0.2%) was also significantly different (Kruskal–

Wallis P = 0.03); however, the posterior multiple com-

parison did not detect significant differences due to large

variance in SPOM (Scheffe P [ 0.06).

Stable isotope ratios of R. philippinarum differed among

different size classes, and individuals smaller than 5 mm

SL had widely different values from larger individuals

(Fig. 2). The mean d13C of R. philippinarum \ 5 mm SL

(class 1, -18.8 ± 0.4%) was significantly smaller than

that of 5–10 mm SL class (class 2, -16.7 ± 1 9 10-5%,

Tukey test P \ 0.0001), 10–20 mm SL class (class 3,

-16.3 ± 0.1%, P \ 0.0001), and [20 mm SL class

(class 4, -15.7 ± 0.2%, P \ 0.0001). The differences

between class 2 and class 4 and between class 3 and

class 4 were also significant (P \ 0.0001), and there was a

tendency to increase from class 2 to 4. The mean d15N of

R. philippinarum had a small range and was significantly

Fish Sci (2009) 75:937–945 939

123

different only between size class 1 (11.5 ± 0.5%) and

class 4 (12.5 ± 0.1%, Tukey test P \ 0.001). The mean

d15N of class 2 and class 3 was 11.4 ± 1 9 10-5% and

11.9 ± 0.1%, respectively.

Based upon previously reported fractionation of d13C

and d15N per trophic level (?0.6% and ?3.4%, respec-

tively) [20–23], classes 2–4 were considered to feed on

SOM, class 1 to feed on a mixture of SOM and BPOM, and

the contribution of SPOM seemed to be small (Fig. 2).

Contribution of SOM and BPOM to class 1 was estimated

to be 45.5% and 54.5%, respectively, by ratio calculation

of d13C, i.e., d13Cclass 1 = d13CBPOM 9 R ? d13CSOM 9

(1 - R), where R is the contribution ratio of BPOM.

The mean C/N ratio of SPOM (9.5 ± 1.03) and SOM

(6.07 ± 0.21) was significantly different (Fig. 3, Tukey

test P \ 0.001). The mean C/N ratio of BPOM (7.7 ±

0.56) lay between that of SPOM (9.5 ± 1.03) and SOM

(6.1 ± 0.21) and was not significantly different from these

two (Tukey test P = 0.11 and 0.07, respectively).

Stable isotope ratios and C/N ratio of R. philippinarum

along inshore–offshore transect

The mean d15N and d13C of R. philippinarum had small

ranges (11.7–12.9% and -16.3% to -15.2%, respec-

tively) along the inshore–offshore transect, but the differ-

ences were significant among the stations (Fig. 4, ANOVA

P \ 0.0001 for d15N and d13C). The stable isotope ratios of

R. philippinarum coincided with the topography of the

tidal flat. For instance, both d15N and d13C had lower

values at St. 4 (11.8 ± 0.1% and -16.3 ± 0.1%,

respectively) where ground height was elevated than at

other stations, such as St. 7 (d15N: 12.2 ± 0.1% and d13C:

-15.2 ± 0.1%); however, neither d15N nor d13C was

significantly correlated with relative ground height

(r2 = 0.045 and 0.16; P = 0.76 and 0.61, respectively).

Shell length (i.e., 5.0–28.4 mm) and d15N of R. philipp-

inarum had a weak significant positive correlation: d15N =

0.026 9 SL ? 11.8 (Fig. 5, r2 = 0.089; P \ 0.01),

whereas shell length and d13C did not have a significant

correlation (r2 = 0.027; P [ 0.1).

The mean C/N ratio of R. philippinarum ranged from

3.48 to 3.93 and significantly differed among stations

(Fig. 6, Kruskal–Wallis, P \ 0.0001). R. philippinarum

tended to have lower C/N ratios in near-shore stations (e.g.,

St. 1: 3.48 ± 0.033) and offshore stations (St. 8:

3.79 ± 0.065) than in middle stations (St. 4: 3.93 ± 0.10).

The C/N ratio did not have a significant correlation with

shell length of R. philippinarum (r2 \ 0.01; P = 0.39), but

had a significant negative correlation with d13C and d15N

(Fig. 7).

Chlorophyll concentrations in bottom water

Chlorophyll concentration in bottom water changed sinu-

soidally from 3.5 to 21.9 lg/l over the 24-h continuous

measurements. The bottom-water chlorophyll concentra-

tion seemed to change over a cycle that was half that of the

semidiurnal tidal rhythm (Fig. 8a). The chlorophyll con-

centration gradually increased from 9.8 to 21.5 lg/l as the

tidal level decreased from high tide at 13:00 hours to mid-

tide at 15:40 hours, and decreased to 10.8 lg/l towards low

tide at 20:00 hours. As the tidal height rose after

20:00 hours to mid-tide at 23:00 hours, the chlorophyll

concentration increased again to 18.0 lg/l, and then

14

4

6

8

10

12

14δ15

N (

‰)

SPOM < 5 mmBPOM 5 - 10 mm

0

2

-28 -26 -24 -22 -20 -18 -16 -14

δ13C (‰)

SOM 10 - 20 mm> 20 mm

Fig. 2 Plot of d15N and d13C of whole soft tissue of Ruditapesphilippinarum, particulate organic matter in surface water (SPOM),

bottom water (BPOM), and sediment surface (SOM) at Seaside Park

of Yokohama in January 2005. Points and bars indicate mean ± stan-

dard error. R. philippinarum were divided into four shell-length

classes (open points). Dotted arrows indicate potential consumers of

particulate organic matters predicted from isotopic trophic enrichment

(?3.4% and ?0.6% per trophic level for d15N and d13C,

respectively)

11 a

7

8

9

10

11

ab

5

6

SPOM BPOM SOM

PO

M C

/N r

atio

b

Fig. 3 Mean C/N ratio of particulate organic matter in surface water

SPOM, bottom water (BPOM), and sediment surface (SOM) at Seaside

Park of Yokohama in January, 2005. Bars indicate ± standard error

940 Fish Sci (2009) 75:937–945

123

decreased to 10.4 lg/l towards high tide at 01:40 hours.

The pattern thereafter towards the low tide at 07:20 was not

clear. The bottom-water chlorophyll concentration did not

have a significant correlation with tidal height (r2 = 0.003,

P = 0.66), but it had a significant positive correlation with

rate of change of tidal height (DH) (Fig. 8b, r2 = 0.21,

P \ 0.0001).

Discussion

Stable isotope ratios of carbon and nitrogen reflect assimi-

lated diet among ingested food items [20, 22, 24], and food-

chain structures of various biota have been ascertained

using this method. The stable isotopic method is especially

useful for studying the diet of filter-feeding bivalves whose

gut contents are often detrital and therefore hard to identify

visually. In this study, comparison of d13C and d15N in

R. philippinarum and POMs in water column and sediment

surface indicated that R. philippinarum, especially indi-

viduals larger than 5 mm SL, mainly assimilated particles

in the sediment surface (i.e., SOM) rather than in the water

column (BPOM) at Seaside Park of Yokohama. Individuals

smaller than 5 mm SL seemed to assimilate relatively more

particles from water column (i.e., estimated contribution of

BPOM was 54.5%) as compared with larger individuals.

Stable isotope ratios of POM in the surface water (SPOM)

had widely different values from those of R. philippinarum,

indicating its less significant contribution as a food source

for R. philippinarum. The mean d13C of SPOM (-26.3%)

was close to values reported for terrestrial plant detritus [11,

25, 26], and the higher mean C/N ratio of SPOM (9.5) also

0-14014 baδ15NGround height

δ13CGround height

-30

-20

-10

-16

-15

δ13C

(‰

)

-30

-20

-10

12

13

δ15N

(‰

)

a a

b bc

c ca a a

ad

b bc

cd

-40-17

Rel

ativ

e gr

ound

hei

ght (

cm)

-4011 0 20 40 60 80 100 120

Rel

ativ

e gr

ound

hei

ght (

cm)

Distance from shoreline (m)

0 20 40 60 80 100 120

Distance from shoreline (m)

bb b

bb

bc

Fig. 4 d15N (a) and d13C (b) of

whole soft tissue of Ruditapesphilippinarum at eight stations

(St. 1: inshoremost—St. 8:

offshoremost) on an inshore–

offshore transect at Seaside Park

of Yokohama in April 2004.

Points and bars indicate

mean ± standard error. Relative

ground height indicates

difference from St. 1. Differentletters indicate significant

difference (Tukey test

P \ 0.01)

ba 14 -14

11

12

13

δ15N

(‰

)

-17

-16

-15

δ13C

(‰

)

10

SL (mm)

-180 10 20 30 0 10 20 30

SL (mm)

Fig. 5 Relationship between

shell length and whole soft

tissue d15N (a) and d13C (b) of

Ruditapes philippinarum at

Seaside Park of Yokohama in

April 2004. Shell length (SL)

and d15N have a weak

significant positive correlation:

d15N = 0.026 9 SL ? 11.8

(r2 = 0.089; P \ 0.01),

whereas SL and d13C are not

significantly correlated

(r2 = 0.027; P [ 0.1)

04.2 C/N ratio

-30

-20

-10

3.6

3.8

4

C/N

rat

io

ababc

bc c

bc abc

abc

Ground height

-403.2

3.4

0 20 40 60 80 100 120

Rel

ativ

e gr

ound

hei

ght (

cm)

Distance from shoreline (m)

a

Fig. 6 Atomic carbon:nitrogen (C/N) ratio of whole soft tissue of

Ruditapes philippinarum at eight stations (St. 1: inshoremost–St. 8:

offshoremost) on an inshore–offshore transect line at Seaside Park of

Yokohama in April 2004. Points and bars indicate mean ± standard

error. Relative ground height indicates difference from St. 1. Lettersindicate significant difference (Scheffe test P \ 0.05)

Fish Sci (2009) 75:937–945 941

123

indicates influence of terrestrial detritus [11, 27, 28]. It is,

therefore, possible that SPOM contained terrestrial particles

originating from adjacent river water that was stratified at

the water surface. Kasai et al. [11] reported that terrestrial

particles are a minor component of R. philippinarum diet.

Although microalgal compositions in the POMs and gut

contents of R. philippinarum were not examined in this

study, estimation of the relative contribution of phyto-

plankton and microphytobenthos can be made based upon

d13C values. Given that phytoplankton generally has lower

d13C (from -26.3% to -16.6%) as compared with mi-

crophytobenthos (from -19% to -11%) [29–32], isotopic

changes in plant tissues by decomposition are negligible

[33], and enrichment of d13C per trophic level is about

?1% [20, 24] (reported to be ?0.6% for R. philippinarum

specifically) [23], strong influence of microphytobenthos

and/or its detritus is indicated for the diet of R. philippin-

arum larger than 5 mm SL (i.e., mean d13C: from -15.7%to -16.7%). Similarly, the lower mean d13C value of

R. philippinarum smaller than 5 mm SL (-18.8%) indi-

cates that smaller individuals assimilate relatively more

phytoplankton as compared with larger individuals. This

estimation is only approximate since the reported d13C

values for phytoplankton and microphytobenthos partially

overlap due to the fluctuations associated with the growth

rate of the microalgae and availability of aqueous CO2

[34]; however, it is consistent with the results of stable

isotopic comparisons between POMs and R. philippinarum

(i.e., sediment organic matter contains more microphyto-

benthos than phytoplankton). Therefore, it is reasonable to

ascertain that microphytobenthos and/or its detritus are the

major food source for R. philippinarum larger than 5 mm

SL at Seaside Park of Yokohama.

Microphytobenthos is generally in higher abundance as

compared with phytoplankton [35] due to high nutrient

provision from the sediment and the fact that benthic dia-

toms, the predominant microphytobenthos, have wider

optimum ranges for environmental factors than do plank-

tonic diatoms [36, 37]. It is considered that microphyto-

benthos on the sediment surface is more readily available

than phytoplankton to R. philippinarum, which inhales

food particles through the siphon opening at the near sur-

face of the tidal flat bottom. Since small juveniles of

R. philippinarum up to about 2 mm SL are often seen on

the tidal flat surface (Watanabe, personal observation), they

may encounter and ingest relatively more pelagic particles

than do larger individuals burrowing themselves deeper in

the sediment. Thus, although the causes of the observed

size-dependent d13C differences are not understood, it is

suggested that food availability for smaller juveniles be

estimated separately from that for larger individuals.

In order for R. philippinarum to inhale sediment parti-

cles, the particles must be resuspended in the water

column. Sediment particles are known to be resuspended

ba 5 5

3.5

4

4.5

C/N

rat

io

3.5

4

4.5

C/N

rat

io

310 11 12 13 14

δ15N (‰)

3-18 -17 -16 -15 -14

δ13C (‰)

Fig. 7 Relationship between

atomic carbon:nitrogen (C/N)

ratio and d15N (a) and d13C (b)

of whole soft tissue of

Ruditapes philippinarum at

Seaside Park of Yokohama in

April 2004.

R = -0.22 9 d13C ? 0.33

(r2 = 0.17, P \ 0.001);

R = -0.15 9 d15N ? 5.55

(r2 = 0.10, P \ 0.01), where

R denotes C/N ratio

25200

25

30ChlorophyllTidal height

ba

5

10

15

20

40

80

120

160

10

15

20

Chl

orop

hyll

(µg/

l)

00 1 2 3 4 5 6

Chl

orop

hyll

(µg/

l)

DH (cm/20 min)

00

5

8 10 12 14 16 18 20 22 0 2 4 6 8

Tid

al h

eigh

t (cm

)Time

Fig. 8 Relationship between

bottom-water chlorophyll

concentration (averaged over

20 min) and tidal height (a) and

rate of change of tidal height (b)

during 24-h continuous

measurement at Seaside Park of

Yokohama in December 2004.

C = 1.2 9 DH ? 8.2

(r2 = 0.21; P \ 0.001), where

C denotes chlorophyll

concentration, and DH denotes

absolute value of tidal height

difference in 20 min

942 Fish Sci (2009) 75:937–945

123

by tidal movements [38–40], and the results of this study also

supported this phenomenon. Chlorophyll concentration in

the bottom water fluctuated sixfold and had a significant

positive correlation with the rate of change of tidal height

(i.e., chlorophyll concentration was higher when the tidal

level was rapidly changing), indicating that tide-driven

bottom-water currents and/or waves resuspend phytopig-

ments on the tidal flat. Thus, it is suggested that R. philipp-

inarum inhales bottom water with resuspended benthic

particles and perhaps smaller amounts of pelagic particles.

The BPOM samples were taken at low tide where resus-

pension of benthic particles is considered low, and this could

be the reason for BPOM having different d15N and d13C

values from SOM. Although R. philippinarum is known to

have a semidiurnal metabolic rhythm [41], details of feeding

rhythm are not known. This study validates the importance of

elucidating the feeding rhythm of R. philippinarum in

association with tidal movements in order to evaluate prop-

erly the amount of food supply to R. philippinarum.

The mixing ratio of benthic and pelagic particles in the

bottom water is considered to cause the differences in the

stable isotope ratios of R. philippinarum at the eight sta-

tions along the inshore-offshore transect. The stations were

only 10–15 m apart, and it is improbable that d13C and

d15N of phytoplankton provided from the offshore water

are segregated in such a special way that results in sig-

nificant differences in the d13C and d15N of R. philippin-

arum at each station. Resuspended benthic particles are

also transported and dispersed by tidal currents, and they

may not have a clear small-scale distributional pattern,

either. Therefore, it seems more reasonable to hypothesize

that the differences in the stable isotope ratios of R. phil-

ippinarum are attributable to the mixing ratio of phyto-

plankton and microphytobenthos at each station.

R. philippinarum with lower d13C in stations around the

crest of a sand wave (i.e., stations 3–5) are considered to

assimilate relatively larger proportion of phytoplankton

with lower d13C values [29–32] as compared with at other

stations (note that microphytobenthos is still the main food

source). This may be related to the differences in resus-

pension of microphytobenthos and provision amount of

phytoplankton associated with hydrodynamics and topog-

raphy of the tidal flat. Thus, although mechanisms are not

known, this study implies that small-scale spatiotemporal

variations in food provision must be taken into consider-

ation to evaluate the amount of food supply for R. phil-

ippinarum in a tidal flat.

The mean C/N ratio of R. philippinarum also showed

differences among the stations along the inshore–offshore

transect. Soft tissue C/N ratio is used as an index of

nutritional conditions [42] (i.e., higher C/N ratio indicates

more carbohydrate and/or lipid reserve); since the samples

in this study were defatted, the C/N ratio is considered to

reflect the amount of carbohydrate, especially glycogen

content [43–45]. The mean C/N ratio was higher at the

crest of a sand wave (stations 4 and 5) where d13C was low,

and there was a negative correlation between d13C and C/N

ratio, indicating that R. philippinarum feeding on relatively

more phytoplankton are in better nutritional condition. Two

explanations are possible for the C/N differences: (1)

phytoplankton is more nutritional than microphytobenthos,

and (2) phytoplankton is a surplus food source increasing

the overall amount of food supply. A laboratory experiment

has shown that R. philippinarum readily ingests and digests

various kinds of algae, bacteria, and rotifers, and assimi-

lation and growth efficiency of Tetraselmis sp. and Nitzs-

chia sp. were about the same [46]. Thus, although further

qualitative and quantitative information is needed, phyto-

plankton also seems to be a good food source for

R. philippinarum.

In summary, this study implies that resuspended benthic

POM, containing mainly microphytobenthos and its detri-

tus, sustains the high productivity of R. philippinarum in

the tidal flat of Seaside Park of Yokohama. This coincides

with previous studies that suggested importance of micro-

phytobenthos in many other areas in Japan [13–18].

However, this study also showed that phytoplankton can

also be a good food source if available in a large quantity,

endorsing the case of Shirakawa River tidal flat in Ariake

Sound, Japan [12]. The relative contribution of phyto-

plankton and microphytobenthos may simply be dependent

upon their relative availability to R. philippinarum rather

than on their food quality. In order to estimate the amount

of food supply in a fishing ground for making an ecological

fishery model for planning fishery management, one should

ideally use the chlorophyll-a concentration of bottom water

at the time of feeding regardless of the microalgal contents.

Elucidation of feeding rhythm of R. philippinarum asso-

ciated with tidal movement is indispensable.

Acknowledgments We are grateful to Dr. M. Toyokawa and Ms.

T. Kawashima at the National Research Institute of Fisheries Science

for technical assistance. We thank Dr. T. Kawamura at Ocean

Research Institute, The University of Tokyo for helpful discussion

and Mr. J. O’Connell for English proof-reading.

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