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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 218: 239–248, 2001 Published August 20
INTRODUCTION
Barnacles accumulate extremely high concentra-
tions of trace metals, exceeding those in most other
invertebrates (Rainbow 1987, 1998), a feature underly-
ing their employment as biomonitors of trace metal
availabilities in coastal waters (Phillips & Rainbow
1988, 1994, Rainbow & Phillips 1993, Rainbow 1995).
Barnacles have comparatively very high uptake rates
of trace metals from solution (Rainbow & White 1989,
1990, Rainbow 1998), but also assimilate trace metals
from food with high efficiencies (Wang et al. 1999a,b,
Wang & Rainbow 2000). They feed by filtering sus-
pended matter in large quantities, and trophic transfer
appears to be the predominant source of the large
quantities of trace metals taken up and subsequently
accumulated (Wang et al. 1999b). Previous studies
have clearly demonstrated that the high accumulated
concentrations of metals such as Zn are due mainly to
© Inter-Research 2001
*E-mail: psr@nhm.ac.uk
Comparative assimilation of Cd, Cr, Se, and Zn
by the barnacle Elminius modestus from
phytoplankton and zooplankton diets
Philip S. Rainbow
1,
*
, Wen-Xiong Wang
2
1
Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
2
Department of Biology, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay,
Kowloon, Hong Kong, PR China
ABSTRACT: Assimilation from the diet is established as a key factor in the accumulation of very high
trace metal concentrations by the barnacle Elminius modestus. Assimilation efficiencies (AE) of Cd,
Cr, Se and Zn were measured from a diet of different phytoplankton (2 diatoms, a dinoflagellate, a
prasinophyte and a chlorophyte) and of zooplankton (the copepod Acartia spinicauda with metal
accumulated from solution or diet). AEs of Cd, Se and Zn (but not Cr) varied greatly with phyto-
plankton type, and for Cd and Se the AE was correlated with the percentage of metal burden held in
the cytoplasm of the phytoplankton diet. AE was generally higher from the zooplankton diet than
from a phytoplankton diet. Variation in AEs of Cd, Se and Zn was not explained by any correlation
with the percentage of copepod metal burden held in the soft tissues, nor was there variation for
either Cd or Zn according to whether the copepod prey had accumulated metal from dissolved or
food sources. Comparisons of the assimilation and efflux of accumulated metals by the archaeo-
balanid barnacle E. modestus and literature data for (phylogenetically younger) balanid species of
the genus Balanus indicate some differences in digestive physiology of barnacles from the 2 families,
tending towards higher AEs in the balanids. Modeling of the accumulation of Cd and Zn by E. mod-
estus predicts that for each metal >97% of accumulated metal has been derived from dietary inges-
tion. The dominance of dietary ingestion in trace metal accumulation is a function of the conspicu-
ously high assimilation efficiencies and high ingestion activity of barnacles. This study adds to the
small but growing list of examples highlighting the significance of trophic transfer in metal accumu-
lation by aquatic invertebrates.
KEY WORDS: Barnacles · Elminius modestus · Dietary uptake · Assimilation · Cadmium · Chromium ·
Selenium · Zinc
Resale or republication not permitted without written consent of the publisher
Mar Ecol Prog Ser 218: 239–248, 2001
their binding in detoxified form as phosphates in the
tissues beneath the midgut epithelium (Walker et al.
1975a,b, Rainbow 1987, Pullen & Rainbow 1991).
Recently interest has been turning increasingly to
the significance of assimilation of trace metals from
food, and in the trophic transfer of metals in marine
food chains (Fisher & Reinfelder 1995, Reinfelder et al.
1998, Wang & Fisher 1999). In spite of the considerable
attention paid to the trace metal biology of barnacles
(Rainbow 1987, 1998), there are still few studies on the
uptake of metals from the diet by these ecologically
important inhabitants of hard surfaces, littoral and sub-
littoral, in coastal waters. Wang et al. (1999a,b) mea-
sured the assimilation efficiencies in the barnacle Bal-
anus amphitrite, and Wang & Rainbow (2000) have
investigated another species of the same genus, B.
trigonus. These studies have demonstrated that the
high concentrations of metals in barnacles can be
accounted for physiologically by the efficient assimila-
tion and slow efflux of trace metals. Both B. amphitrite
and B. trigonus are important ecologically (typically as
fouling species) in tropical and subtropical coastal
waters, but information is lacking for ecologically sig-
nificant temperate species of barnacle.
Two such ecologically important species in the north
Atlantic and adjacent European coastal waters are the
littoral barnacles Semibalanus balanoides and Elmin-
ius modestus, an antipodean immigrant now common
in NW Europe. Moreover, both these barnacles belong
to the family Archaeobalanidae, as opposed to the Bal-
anidae to which the genus Balanus belongs (Newman
& Ross 1976). The Archaeobalanidae is considered to
be a phylogenetically older family than the Balanidae,
and archaeobalanids may therefore differ physiologi-
cally from species of the genus Balanus, particularly
since changes in feeding methods and consequently
food type are a strong feature of balanomorph barna-
cle evolution (Anderson 1994). We decided, therefore,
to undertake a comparative investigation of the assim-
ilation of trace metals in one of these ecologically
important barnacles. Our choice fell on E. modestus
because, for a valid comparison, we intended to carry
out all experiments on this barnacle under the condi-
tions used for the studies of B. amphitrite and B.
trigonus (Wang et al. 1999a,b, Wang & Rainbow 2000).
S. balanoides is a northern barnacle species, less likely
to withstand raised temperatures and lower salinities
than E. modestus (Rainbow 1984). It is for the same
reason that barnacles, after collection in England,
were immediately shipped out to the laboratories in
Hong Kong for experiments.
The specific objectives of this study were therefore
to: (1) determine the effects of different phytoplankton
diets on the assimilation efficiencies of Cd, Cr, Se and
Zn by the barnacle Elminius modestus; (2) determine
the assimilation efficiencies of trace metals from cope-
pod prey radiolabeled from solution or from a diet of
diatoms; (3) compare the assimilation of trace metals
by the archaeobalanid barnacle E. modestus with the
assimilation of trace metals by the 2 balanid barnacles
Balanus amphitrite and B. trigonus; (4) determine the
efflux rates of trace metals assimilated from the diet by
E. modestus; and (5) model the accumulation of Cd and
Zn by the barnacle E. modestus.
MATERIALS AND METHODS
Barnacles and metals. Barnacles Elminius modestus
Darwin were collected intertidally on mussels (Mytilus
edulis) from Southend-on-Sea, England on 16 October
2000, before air transport to Hong Kong. Individual
barnacles were isolated on small pieces of mussel shell
for experiments. Radioisotopes of Cd, Cr, Se and
Zn were obtained from NEN Research Product and
Lawrence Livermore National Laboratory, California,
USA (
109
Cd, in 0.1 N HCl;
51
Cr(III), in 0.1 N HCl;
75
Se,
as Na
2
SeO
3
in distilled water; and
65
Zn, in 0.1 N HCl).
The radioactivity was measured by a Wallac gamma
counter. Spillover of radioisotopes was corrected and
all counts were related to standards for each isotope
and corrected for radioactive decay. The gamma
emissions of
109
Cd were determined at 88 keV,
51
Cr at
320 keV,
75
Se at 264 keV, and
65
Zn at 1115 keV. Count-
ing times were adjusted so that the propagated count-
ing errors were typically <5%. All experiments de-
scribed below were carried out at a temperature of
18°C and a salinity of 30 ppt.
Assimilation efficiency of metals from ingested food.
The assimilation efficiencies (AE) of Cd, Cr, Se, and Zn
in barnacles feeding on different prey including both
phytoplankton and zooplankton were determined.
Phytoplankton diets considered were: the diatoms
Thalassiosira weissflogii (CCMP 1048) and Phaeo-
dactylum tricornutum (CCMP 630), the dinoflagellate
Prorocentrum minimum (CCMP 696), the prasinophyte
Tetraselmis levis (CCMP 896), and the chlorophyte
Chlorella autotrophica (CCMP 243). The phytoplank-
ton were obtained from the Provasoli-Guillard Phyto-
plankton Collection Center, West Boothbay Harbor,
Maine, USA, and maintained in f/2 medium (Guillard
& Ryther 1962) at 18°C and a light illumination of
100 µmol m
–2
s
–1
with a 14:10 h light:dark cycle. The
copepods (Acartia spinicauda) were collected by net
tows from Clear Water Bay, Hong Kong, and resus-
pended in filtered seawater.
The phytoplankton were radiolabeled as described
in Wang & Rainbow (2000). Briefly, the cells were
removed from their culture, filtered and resuspended
in 50 ml 0.2 µm filtered seawater enriched with f/2 lev-
240
Rainbow & Wang: Comparative assimilation of Cd, Cr, Se, and Zn by barnacles
els of N, P, Si, vitamins, and f/20 levels of trace metals
minus EDTA, Cu, and Zn (Guillard & Ryther 1962).
Each radioisotope was added at 555 kBq l
–1
(corre-
sponding to 66 nM for
109
Cd, 1.4 nM for
51
Cr, 10 nM for
75
Se, and 0.8 nM for
65
Zn). The phytoplankton were
grown for 4 d, to allow the cells to be uniformly radio-
labeled, after which the cells were filtered from the
radioactive medium and rinsed with filtered seawater
before being fed to the barnacles.
The copepods were radiolabeled with radiotracers
both from solution and in the diet in 500 ml of 0.2 µm
filtered seawater at a density of 1 ind. ml
–1
. In the
dietary phase treatment, the diatom Thalassiosira
weissflogii was radiolabeled as described above and
fed 4 times a day to the copepods for a total of 2 d. In
the solution radiolabeling treatment, the copepods
were exposed to radiotracers in the dissolved phase for
1 d. Radioisotope additions were 148 kBq l
–1
for each
isotope. However, we found that the uptake of
51
Cr(III)
and
75
Se(IV) by copepods from the dissolved phase
was slow, and we were not able to measure the AEs of
Cr and Se by barnacles from ingested copepods that
had accumulated either of these radiolabeled metals
from solution. In addition, the uptake of Cr by cope-
pods from ingested food was also slow (presumably
due to low assimilation of Cr by the copepods), and the
AE of Cr by barnacles feeding on copepods radiola-
belled by dietary exposure was also not determined.
The distributions of metals in the cytoplasm of phyto-
plankton were determined as described in Fisher et al.
(1983), and the distribution of metals in the soft tissues
of copepods was determined as in Wang & Fisher
(1998).
The AEs of metals were determined with a pulse-
chase feeding technique as described in Wang & Fisher
(1999) and Wang & Rainbow (2000). Barnacles were
placed in 100 ml filtered seawater and fed on radio-
labeled food particles at a cell density of about 2.2 mg
l
–1
for all phytoplankton diets, and at a density of 1 to
2 individuals copepods ml
–1
for Acartia spinicauda.
After 30 to 45 min feeding, before the egestion of ra-
dioactive faeces, individual barnacles were rinsed with
non-radiolabeled water to remove any remaining phy-
toplankton cells and their radioactivity was counted.
(Adsorption of radioisotopes onto the mussel shell
pieces was confirmed on later dissection to be negligi-
ble.) Five to 6 replicate individuals were then placed in-
dividually in beakers containing 120 ml filtered sea-
water with unlabeled diatom Thalassiosira weissflogii
to promote depuration of ingested radiolabeled food.
Faeces produced by the barnacles were removed at
frequent time intervals and their radioactivity analyzed.
The radioactivity remaining in the barnacles was mea-
sured at frequent time intervals over a period of 48 h.
Water and food were renewed in the individual ex-
perimental beakers on each occasion when the radio-
activity in the barnacle was being counted. Because our
results indicated that there was negligible egestion of
unassimilated metals following 30 h of depuration (see
Results), the AE was therefore defined as the percent-
age of ingested radioisotope retained in the barnacles
following 30 h of depuration.
Efflux rate measurements. The diatom Thalassiosira
weissflogii was radiolabeled in 200 ml of 0.2 µm filter-
ed seawater as described above. Radiolabeled diatoms
were removed from the radioactive medium each day,
filtered and fed to individual barnacles for 1 h. The
barnacles were subsequently removed from the radio-
labeled feeding beakers and placed in filtered seawa-
ter in the presence of unlabelled diatom food (T. weiss-
flogii). The barnacles were fed under these conditions
for 7 d. After 7 d, the radioactivity retained in the bar-
nacles was measured. Two individuals were dissected
to determine the distribution of metals in the shell and
tissues. The remaining 11 individuals were then depu-
rated in 240 ml unlabeled seawater for a period of 45 d,
during which they were fed with the diatom T. weiss-
flogii (unlabeled). Seawater and food were renewed on
a daily basis. The radioactivity retained in the barna-
cles was monitored at time intervals. The efflux rate
constant was defined as the rate constant of the physi-
ological turnover, which was calculated from the slope
of the slower exchanging compartment.
RESULTS
The barnacles assimilated all 4 metals from the
ingested phytoplankton and Cd, Se and Zn from the
copepods. Radioactive counting of the faecal pellets
egested by barnacles fed on radiolabeled Thalassiosira
weissflogii showed that unassimilated metal passed
through the gut within 5 h for Zn and Cr, within 10 h
for Cd and within 24 h for Se (Fig. 1). After the initial
egestion of unassimilated metal in the faeces, the per-
centage of assimilated metal retained in the barnacles
remained essentially constant or declined very slowly
with considerable variation between diet types (Figs 2
& 3). Table 1 presents the calculated AEs of the metals,
defined as the % retained in the barnacles after 30 h of
depuration, for different food types. The AEs for Cd, Se
and Zn showed great variation between different
phytoplankton diets, falling in the range 21 to 48% for
Cd, 34 to 66% for Se, and 37 to 92% for Zn. The varia-
tion of Cr between phytoplankton diets was much less,
and the AEs themselves were much lower (8 to 13%)
than those of the other trace metals (Table 1, Fig. 2).
For each phytoplankton diet type except Tetraselmis
levis, the AEs were highest for Zn. The AE for Se was
usually next highest, followed by the AE for Cd, with
241
Mar Ecol Prog Ser 218: 239–248, 2001
the AE for Cr always the lowest. In the case of the
phytoplankton diets, Cd, Se and Zn incorporated into
the diatom T. weissflogii always had the highest AE.
There was a significant positive relationship between
the AEs of Cd and Se, but not of Cr or Zn, from phyto-
plankton and the percentage of accumulated radio-
labeled metal present in the phytoplankton cytoplasm
(Fig. 4).
The AEs for Cd, Se and Zn from the copepod diet
were generally higher than from the phytoplankton
diets, whether the metal had been incorporated into
the copepod from solution or via a radiolabelled diet of
Thalassiosira weissflogii (Table 1, Fig. 3). In the cases
of the assimilation of Cd and Zn, for which data are
available to make the comparison, there was no differ-
ence between the AE of each metal by Elminius mod-
estus from copepod prey that had accumulated the
metal from either solution or food (Table 1). Between
54 and 99% of radiolabeled metal was present in the
soft tissue (as opposed to the exoskeleton) of the cope-
pods fed to the barnacles, and there was no significant
correlation between this percentage and the assimila-
tion efficiency of the barnacles.
Barnacles fed daily on pulses of radiolabeled Thalas-
siosira weissflogii for 7 d were subsequently counted
regularly in order to assess efflux rate constants and
biological retention half-lives of assimilated metals.
Results are presented in Fig. 5 and Table 2. The efflux
of all assimilated metals is very slow with efflux rate
constants varying from 0.0022 d
–1
for Zn to 0.0181 d
–1
for Cd, with corresponding retention half-lives varying
from 1346 d (Zn) to 43.5 d (Cd) (Table 2). Cd and Zn
efflux rates are significantly correlated, but there is no
significant relationship between the efflux rate con-
stant of Cd and that of either Cr or Se. The percentage
distributions of the assimilated metals were also mea-
sured after feeding on radiolabeled diatoms for 7 d (n =
2) and after a further 45 d depuration (n = 11). Distrib-
utions in the total soft tissues after 7 d feeding on radi-
olabeled diatoms were 99% for Cd, 77% for Cr, 74%
242
Fig. 1. Elminius modestus. Egestion rate of metals (Cd, Cr, Se,
and Zn) in the barnacles following pulse feeding on radiola-
beled diatoms Thalassiosira weissflogii. #1, #2, #3, #4, #5, and
#6 represent different experimental individuals
Fig. 2. Elminius modestus. Retention of Cd, Cr, Se, and Zn in
the barnacles following pulse feeding on different phyto-
plankton diets. (
d
) Thalassiosira weissflogii, (
s
) Prorocentrum
minimum, (
Z
) Tetraselmis levis, (
y
) Chlorella autotrophica,
(
J
) Phaeodactylum tricornutum. Data shown are means + SD
(n = 5 to 6)
Fig. 3. Elminius modestus. Retention of Cd, Se, and Zn in the
barnacles following pulse feeding on copepods Acartia spini-
cauda. (
d
) Copepods radiolabeled with metals from dissolved
phase, (
s
) copepods radiolabeled with metals from food
source. Data shown are means ± SD (n = 4 to 6)
Rainbow & Wang: Comparative assimilation of Cd, Cr, Se, and Zn by barnacles
for Se, and 88% for Zn. After 47 d depuration, 87% of
Cd, 61% of Cr, 54% of Se, and 88% of Zn were found
in the total soft tissues. The distributions of metals into
different body parts were not determined because of
the small sizes of barnacles used.
DISCUSSION
The assimilation efficiencies (AE) of the trace metals
Cd, Se and Zn (but not Cr) by Elminius modestus did
vary with the type of ingested phytoplankton provid-
ing the dietary source of the metal. The AEs varied by
2.2×, 1.9×, and 2.5× for Cd, Se, and Zn, in barnacles
feeding on different phytoplankton diets. For Cd and
Se, the AEs were correlated with the percentage of
metal in the cytoplasm of the phytoplankton diet, as
Wang & Rainbow (2000) found for Cd AE in the case of
Balanus trigonus. There was no such correlation for the
AE of Cr (Fig. 4), which is hardly surprising given the
lack of variation of Cr AE between phytoplankton
diets, again in agreement with results for B. trigonus
(Wang & Rainbow 2000). The AE of Zn by E. modestus
did vary between phytoplankton diets but this varia-
tion could not be ascribed to any correlation with the
amount of phytoplankton metal accumulated in the
cytoplasm (Fig. 4), contrary to the situation for B.
trigonus (Wang & Rainbow 2000). Differences in metal
243
Food type Cd Cr Se Zn
Phytoplankton
Thalassiosira weissflogii 47.7 ± 7.2 9.5 ± 2.2 65.5 ± 4.5 92.2 ± 3.7
Phaeodactylum tricornutum 25.5 ± 6.1 8.6 ± 5.1 57.6 ± 4.4 79.6 ± 11.3
Prorocentrum minimum 41.3 ± 4.6 9.0 ± 4.7 34.1 ± 2.2 82.8 ± 7.3
Tetraselmis levis 28.2 ± 3.6 13.0 ± 4.6 50.3 ± 7.8 37.1 ± 6.0
Chlorella autotrophica 21.4 ± 2.5 8.7 ± 2.9 ND 67.7 ± 4.2
Copepod
Acartia spinicauda (water radiolabeled) 63.7 ± 8.2 ND ND 89.8 ± 9.2
Acartia spinicauda (food radiolabeled) 76.9 ± 11.4 ND 73.6 ± 13.3 93.7 ± 9.2
Table 1. Elminius modestus. Assimilation efficiencies (%) of Cd, Cr, Se, and Zn in barnacles feeding on different planktonic prey
after 30 h of depuration. Mean ± SD (n = 4 to 6). ND: not determined
Fig. 4. Elminius modestus. Relationship between the assimi-
lation efficiency of Cd, Cr, Se, and Zn in the barnacles after
30 h of depuration and the metal distributions in the cyto-
plasm of phytoplankton cells. Mean ± SD (n = 4 to 6)
Fig. 5. Elminius modestus. The depuration of Cd, Cr, Se, and
Zn in the barnacles following 7 d feeding on radiolabeled
diatom Thalassiosira weissflogii. Mean ± SD (n = 10)
k
e
(d
–1
) t
1/2
(d) % in compartment
Cd 0.0181 ± 0.0062 43.5 ± 15.9 89.0 ± 14.7
Cr 0.0066 ± 0.0046 126 ± 56.3 88.6 ± 12.1
Se 0.0137 ± 0.0035 54.0 ± 13.0 77.8 ± 7.6
Zn 0.0022 ± 0.0019 1346 ± 2659 96.6 ± 2.8
Table 2. Elminius modestus. The calculated efflux rate con-
stants and biological retention half-lives of Cd, Cr, Se, and Zn
in the barnacles. The % of metals in the slower exchanging
compartment is also included. Mean ± SD (n = 10)
Mar Ecol Prog Ser 218: 239–248, 2001
distributions in phytoplankton cytoplasm have been
demonstrated to be partially responsible for the varia-
tion of metal AEs observed in several filter feeding
invertebrates such as copepods and bivalves (Rein-
felder & Fisher 1991, Wang & Fisher 1996, Xu et al.
2001).
AEs of trace metals from a zooplankton (copepod)
diet by Elminius modestus also varied (63 to 94%)
between metals, without variation for either Cd or Zn
according to whether the copepod prey had accumu-
lated radiolabeled metal from a dissolved or a food
source. The variation between AEs of Cd, Se and Zn in
E. modestus could not, however, be explained by any
correlation with the percentage of copepod metal bur-
den held in the soft tissues, as for both Balanus
amphitrite (Wang et al. 1999a,b) and B. trigonus (Wang
& Rainbow 2000). Consistent with our previous studies
in barnacles (Wang et al. 1999a,b, Wang & Rainbow
2000), the AEs of metals were generally higher for zoo-
plankton diets than for phytoplankton diets, especially
for Cd and Se. Similarly, recent studies have shown
that metal assimilation is much higher in carnivorous
invertebrates feeding on animal tissues than in herbi-
vores feeding on plant tissues (e.g., Fowler & Tessie
1997, Wang & Ke in press).
Table 3 synthesizes the comparative data available
for the assimilation efficiencies of the archaeobalanid
barnacle Elminius modestus and the balanids Balanus
amphitrite and B. trigonus. Barnacles in fact have very
high trace metal AEs compared to other aquatic inver-
tebrates (Wang & Fisher 1999, Wang et al. 1999a,b,
Wang & Rainbow 2000). When taking up metals from
ingested diatoms, E. modestus has similar AEs for Se
and Zn to those of the 2 Balanus species. The AE of E.
modestus for Cr is lower than those of B. amphitrite but
not of B. trigonus. AEs of trace metals from a dino-
flagellate diet are similar in archaeobalanids and bal-
anids, whilst the AEs of E. modestus for Cd and Zn
from the prasinophyte Tetraselmis levis are lower than
those of B. trigonus. There is indication, therefore, that
there are differences in the digestive physiology of ar-
chaeobalanid and balanid barnacles tending towards
higher trace metal AEs in the balanids, but the com-
parative data set does need expansion.
The efflux rates of Cd, Cr, Se and Zn accumulated
from a diet of diatoms Thalassiosira weissflogii are
very low (Fig. 5, Table 2), as found in Balanus
amphitrite (Wang et al. 1999a,b). Indeed, the efflux
rate constants of Zn in barnacles are 1 or 2 orders of
magnitude lower than typical efflux rate constants
measured in marine bivalves, whereas the efflux rate
constants of Cd and Se are somewhat comparable to
those measured in marine bivalves (Wang et al. 1996,
Wang & Fisher 1999, Wang et al. 1999b). Biological
retention half-lives represent a straightforward crite-
rion for comparing efflux between the 2 barnacle spe-
cies under identical experimental conditions. Elminius
modestus half-lives are longer than those of B.
amphitrite (Wang et al. 1999a,b) for Cr (126 vs 36 d)
and Zn (1346 vs 230 d), the same for Se (54 vs 52 d),
and shorter for Cd (44 vs 126 d). As in the case of
assimilation efficiencies more data are needed, but
again there may be differences between the trace
metal physiologies of the archaeobalanid and balanid
barnacles.
The correlation between Cd and Zn efflux rates of
Elminius modestus is also present in Balanus amphi-
trite (Wang et al. 1999a), and suggests that these 2
metals may be in part sharing similar physiological
routes of detoxification in barnacles (see Rainbow
1987, 1998). The very different absolute efflux rates of
Cd and Zn in E. modestus (Table 2) do, however, indi-
cate that the sharing is not total (see also Rainbow
1987, 1998, Pullen & Rainbow 1991). The lack of corre-
lations between efflux rates of the other trace metals,
either in E. modestus or in B. amphitrite (Wang et al.
1999a), and the different efflux rates indicated that the
accumulated trace metals are following different meta-
bolic pathways.
It is possible to model the accumulation of metals in
the barnacles. According to a biokinetic model, metal
concentrations can be predicted from the following
equation, assuming that metals in barnacles were
taken up from both the aqueous and dietary phases,
and that the influx from each pathways is a first order
process (Thomann 1981, Landrum et al. 1992, Wang et
al. 1996):
C = [(k
u
× C
w
) + (AE × IR × C
f
)]/(k
e
+ g) (1)
where C is the metal concentration in the barnacles
(µg g
–1
dry wt), k
u
is the metal uptake rate constant
from the dissolved phase (mol g
–1
d
–1
per mol l
–1
, i.e.
lg
–1
d
–1
), C
w
is the metal concentration in the dissolved
phase (µg l
–1
), AE is the metal assimilation efficiency
from ingested particles, IR is the ingestion rate of bar-
nacles (g g
–1
d
–1
dry wt), C
f
is the metal concentration
in ingested particles (µg g
–1
dry wt), k
e
is the efflux rate
constant (d
–1
), and g is the growth rate constant (d
–1
).
In this study, we only modeled Cd and Zn concentra-
tions in barnacles because the metal geochemical
factors of these metals are relatively well measured.
Concentrations of Cr and Se were ignored due to their
complex redox speciation in seawater.
Rainbow (1998) and Rainbow & White (1990) calcu-
lated uptake constants for Cd and Zn from the data of
Rainbow & White (1989) to be 0.1 l g
–1
d
–1
and 0.3 l g
–1
d
–1
for Elminius modestus at 10°C and 33 ppt. Tappin
et al. (1995) quote relevant data for dissolved and par-
ticulate suspended matter concentrations of Cd and Zn
near Southend, UK. At a station opposite the mouth of
244
Rainbow & Wang: Comparative assimilation of Cd, Cr, Se, and Zn by barnacles
the Thames estuary in January 1989, dissolved con-
centrations were 0.028 µg Cd l
–1
and 0.65 µg Zn l
–1
, and
suspended particulate matter (18 mg l
–1
) contained
1.5 µg Cd g
–1
and 70 µg Zn g
–1
(Tappin et al. 1995).
These concentrations can be considered as acceptable,
for Statham et al. (1993) measured dissolved metal
concentrations of 0.022 ± 0.010 µg Cd l
–1
and 0.61 ±
0.33 µg Zn l
–1
in the nearby Strait of Dover, and quoted
dissolved concentrations of 0.019 ± 0.008 µg Cd l
–1
and
0.24 ± 0.13 µg Zn l
–1
in the central southern North Sea
from Burton et al. (1993). Suspended particulate matter
in the upper waters of the Strait of Dover (1990 to 1991)
varied from 6 to 37 mg l
–1
, with Cd concentrations of
0.1 to 0.36 µg Cd g
–1
(James et al. 1993). We therefore
used the data of Tappin et al. (1995) for metal concen-
trations in the dissolved phase and suspended particles
in the modeling analysis.
The literature offers 2 estimates of the ingestion rate
of Elminius modestus. Crisp (1964) reported the inges-
tion rate at 13 to 17°C for a barnacle of this species of
10 mm diameter to be 0.6 mg tissue dry wt d
–1
, equiva-
lent to 0.44 g g
–1
d
–1
, given a dry weight of 1.37 mg
for a 10 mm barnacle (P.S.R. unpubl., for Southend
E. modestus). In addition, Crisp & Southward (1961)
quoted a 11.4 ml h
–1
filtration rate per individual E.
modestus. Assuming a typical seston concentration of
2 mg l
–1
in the sea, this ingestion rate is 0.55 mg d
–1
, or
0.40 g g
–1
d
–1
for a barnacle of 1.37 mg tissue dry
weight. These values are comparable to each other
and similar to the ingestion rate used in modeling
245
Barnacle Cd Cr Se Zn
Food type
Phytoplankton
Diatoms
Elminius modestus
Thalassiosira weissflogii 47.7 ± 7.2 9.5 ± 2.2 65.5 ± 4.5 92.2 ± 3.7
Phaeodactylum tricornutum 25.5 ± 6.1 8.6 ± 5.1 57.6 ± 4.4 79.6 ± 11.3
Balanus amphitrite
Chaetoceros muelleri 34.8 ± 5.1 21.6 ± 4.20 78.6 ± 11.5 76.1 ± 6.0
Skeletonema costatum 86.2 ± 8.2 25.7 ± 8.30 ND 87.2 ± 2.4
Balanus trigonus
Thalassiosira weissflogii 62.0 ± 3.3 6.1 ± 2.4 ND 84.7 ± 7.0
Skeletonema costatum 71.4 ± 7.3 10.5 ± 6.20 ND 76.3 ± 7.5
Dinoflagellates
Elminius modestus
Prorocentrum minimum 41.3 ± 4.6 9.0 ± 4.7 34.1 ± 2.2 82.8 ± 7.3
Balanus trigonus
Prorocentrum minimum 40.8 ± 10.3 3.2 ± 1.9 ND 69.8 ± 2.9
Prasinophytes
Elminius modestus
Tetraselmis levis 28.2 ± 3.6 13.0 ± 4.60 50.3 ± 7.8 37.1 ± 6.0
Balanus trigonus
Tetraselmis levis 49.0 ± 3.0 5.7 ± 5.0 ND 54.1 ± 5.4
Chlorophytes
Elminius modestus
Chlorella autotrophica 21.4 ± 2.5 8.7 ± 2.9 ND 67.7 ± 4.2
Copepods
Water radiolabeled
Elminius modestus
Acartia spinicauda 63.7 ± 8.2 ND ND 89.8 ± 9.2
Balanus amphitrite
Canthocalanus pauper 71.6 ± 17.4 31.6 ± 11.6 62.9 ± 6.7 93.2 ± 4.5
Temora turbinata 87.9 ± 8.9 36.2 ± 9.80 66.4 ± 8.2 93.2 ± 5.8
Balanus trigonus
Paracalanus aculeatus 78.4 ± 6.6 ND ND 85.5 ± 7.9
Food radiolabeled
Elminius modestus
Acartia spinicauda 76.9 ± 11.4 ND 73.6 ± 13.3 93.7 ± 9.2
Balanus trigonus
Paracalanus aculeatus 76.8 ± 15.5 ND ND 88.4 ± 9.3
Table 3. Comparative assimilation efficiencies (%, mean ± SD) of Cd, Cr, Se, and Zn in barnacles Balanus amphitrite (from Wang
et al. 1999a,b), B. trigonus (from Wang & Rainbow 2000) and Elminius modestus (this study) feeding on different planktonic prey.
ND: not determined
Mar Ecol Prog Ser 218: 239–248, 2001
metal accumulation in the barnacle Balanus amphitrite
(Wang et al. 1999a). We therefore employed a daily
ingestion rate of 0.40 g g
–1
d
–1
tissue dry weight in the
modeling. The growth rate constant of E. modestus is
unknown. Although Crisp (1964) quoted a dry tissue
weight increment (13 to 17°C) in body excluding shell
as 0.16 to 0.25 mg dry wt body
–1
d
–1
(0.12 to 0.18 d
–1
) for
this species, such a growth rate is much higher than
measurements in other barnacles (Wang et al. 1999a).
In this study, we have therefore employed a range
of growth rate constants (0.002 to 0.01 d
–1
) as deter-
mined in other barnacles in the modeling analysis.
Metal AEs and efflux rate constants have been taken
from present measurements. The AEs in our study
were modeled as a range of AE (e.g., 20 to 50% for Cd
and 40 to 90% for Zn).
We did not consider zooplankton as the main dietary
source for metal accumulation in barnacles because
our previous modeling study in barnacle Balanus am-
phitrite indicated that zooplankton may not be the
dominant food. However, because of the contrasting
metal assimilation from phytoplankton and zooplank-
ton diets, metal assimilation from copepods by barna-
cles was also examined in the first part of this study.
The metal concentrations in barnacles were therefore
modeled as a function of metal AEs determined for dif-
ferent phytoplankton foods and barnacle growth rate
constants, using the mean values of metal concentra-
tions in the dissolved phase and particulate phase,
metal efflux rate constant and ingestion rate as sum-
marized in Table 4. Our model predicts that the likely
Cd and Zn concentrations in Elminius modestus would
be 4.4 to 15.1 µg g
–1
for Cd and 1000 to 6050 µg g
–1
for
Zn (Fig. 6). Using the median metal AE (35% for Cd,
and 65% for Zn), the predicted Cd and Zn concentra-
tions in E. modestus would be 7.6 to 10.6 µg g
–1
for Cd
and 1500 to 4400 µg g
–1
for Zn, within the range of
growth rate constants (0.002 to 0.01 d
–1
). The predicted
Cd concentrations were about 2 to 4× lower than the
field measurements of its concentration in E. modestus
from Southend (15.7 to 27.3, mean concentration of
23.3 ± 6.6 µg g
–1
, measured during the summer of
2000). For Zn, the field concentrations (2470 to 4730,
mean concentration of 3463 ± 1155 µg g
–1
, measured
during the summer of 2000) fall within the range of the
predicted Zn concentrations in the barnacles (1000 to
6050 µg g
–1
). Furthermore, the model predicted that
>97% of both Cd and Zn in E. modestus is accumu-
lated from dietary ingestion, and uptake from the dis-
solved phase only contributes to <3% of total metal
accumulation in the barnacles.
The somewhat lower predicted Cd concentrations in
the barnacles, compared to Cd concentrations mea-
sured in field collected samples, may be partially due
to the relatively high efflux rate con-
stant (0.0181 d
–1
) measured in this spe-
cies in this study. Among the 4 metals
considered in the study, the efflux rate
constant of Cd was the highest. In the
barnacle Balanus amphitrite, Wang et
al. (1999a) recorded an efflux rate con-
stant of 0.007 d
–1
for Cd. Our modeling
for Zn, however, does indicate that the
measurements of several metal physi-
ological and geochemical parameters
and the barnacle’s physiological para-
meters were accurate in predicting the
246
Parameters Cd Zn
Dissolved metal concentration (µg l
–1
) 0.028 0.65
Metal concentration in seston (µg g
–1
) 1.5 70
Uptake rate constant (l g
–1
d
–1
) 0.1 0.3
Assimilation efficiency (%) 20–50 40–90
Ingestion rate (g g
–1
d
–1
) 0.40 0.40
Efflux rate constant (d
–1
) 0.0181 0.0022
Growth rate constant (d
–1
) 0.002–0.01 0.002–0.01
Table 4. Elminius modestus. Numeric values of parameters used in modeling Cd
and Zn bioaccumulation in barnacles
Fig. 6. Elminius modestus. Model-predicted Cd and Zn con-
centrations in barnacles as a function of metal assimilation
efficiency (AE) from ingested phytoplankton diets and barna-
cle growth rate constant. In modeling the metal concentra-
tions in the barnacles, the mean values of other physiological
and geochemical parameters were used
Rainbow & Wang: Comparative assimilation of Cd, Cr, Se, and Zn by barnacles
Zn concentrations in the barnacles. In the modeling
analysis, we only considered the variability of metal
AE and the barnacle’s growth rate constant. It should
be noted that other parameters may also vary consider-
ably under different environmental and biological con-
ditions, including occasional peaks of bioavailable
metal concentrations in seston or solution. The vari-
ability of these parameters may also need to be consid-
ered in analyzing the variation of metal concentrations
in barnacles.
The modeling results further indicated that the
majority of Cd and Zn are indeed accumulated in bar-
nacles from ingestion of food particles. Uptake from
the dissolved phase, despite the relative high uptake
rate, only contributed to <3% of total metal accumula-
tion in these barnacles. The dominance of dietary
ingestion appears to be caused by the efficient assimi-
lation of metals and the high ingestion activity of the
barnacles. Consistently, previous modeling in Balanus
amphitrite suggested that Cd and Zn were overwhelm-
ingly obtained from ingestion of diets rather than from
uptake from the dissolved phase (Wang et al. 1999a,b).
This study has therefore added a further example of
the significance of trophic transfer in metal accumula-
tion in aquatic invertebrates, especially in those ani-
mals that can efficiently assimilate metals from the
diet.
Acknowledgements. The research was supported by the joint
research scheme of the Hong Kong Research Grant Council
and the UK British Council (JRS99/42, to W.-X.W. and P.S.R.),
and a RGC/CERG grant (HKUST6113/00M, to W.-X.W.). We
are grateful for the technical assistance of Brian Smith and
Robert Dei.
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Editorial responsibility: Otto Kinne (Editor),
Oldendorf/Luhe, Germany
Submitted: February 2, 2001; Accepted: May 25, 2001
Proofs received from author(s): July 25, 2001
