Journal of Plankton Research Vol.21 no.2 pp.299–308, 1999
Production, metabolism and production/biomass (P/B) ratio of
Themisto japonica (Crustacea: Amphipoda) in Toyama Bay,
southern Japan Sea
Tsutomu Ikeda and Naonobu Shiga
Biological Oceanography Laboratory, Faculty of Fisheries, Hokkaido University,
3-1-1 Minatomachi, Hakodate 041, Japan
Abstract. The production and metabolism of the amphipod Themisto japonica in Toyama Bay,
southern Japan Sea, were estimated based on their biomass and population structure data collected
from every 2-week samplings from 1 February 1990 through 30 January 1991 (363 days). Over the
sampling period, the mean biomass (B) was 370 mg C m–2. Production (P) was calculated as the sum
of somatic (Pg) and molt (Pe) production (P = Pg + Pe), and metabolism (M) as the sum of routine
metabolism (Mrtn) and diel vertical migration (Mdvm). Integrating over the entire sampling period, Pg
and Pe were 1934 and 176 mg C m–2, respectively, and Mrtn and Mdvm were 4100 and 1778 mg C m–2,
respectively. Mean daily P/B and Pg/B ratios were 0.016 and 0.014, respectively, and mean Pg/M and
P/M ratios were 0.33 and 0.36, respectively. Assuming assimilation efficiency of 0.904, ingestion was
computed as 8837 mg C m–2 per 363 days. For the daily maintenance of growth and metabolism, the
T.japonica population needs to ingest an amount of prey which equates to 6.6% of their biomass, or
30% of possible total production of their prey animals (copepods and small euphausiids) in Toyama
Bay.
Introduction
The hyperiid amphipod Themisto japonica is distributed in the Okhotsk Sea,
Japan Sea, western subarctic Pacific and southern Kuriles (Bowman, 1960). In the
southern Japan Sea, T.japonica is the most abundant pelagic amphipod and ranks
second to fourth in the total net zooplankton biomass (Ikeda et al., 1992). As a
typical carnivore, T.japonica preys on various other zooplankton species (largely
copepods and small euphausiids) and is preyed upon by masu salmon
(Oncorhynchus masou), pink salmon (Oncorhynchus gorbuscha), the common
squid (Todarodes pacificus), Alaska pollack (Theragra chalcogramma) and Atka
mackerel (Pleurogrammus azonus) (cf. Ikeda et al., 1992). Thus, T.japonica is a
vital link between secondary production and production of animals at higher
trophic levels in the southern Japan Sea. Despite its importance in trophodynamics, no information is presently available about the quantitative production
processes of the T.japonica population in the field.
The early life cycle of Themisto amphipods is characterized by the hatching of
juveniles and their first two moltings within the female’s marsupium (Sheader,
1977; Ikeda, 1990). Then the juveniles are released into ambient water. Ikeda et
al. (1992) investigated the population structure of T.japonica in Toyama Bay over
a 1 year period and found that its reproduction continued throughout the year.
According to their results, the minimum and maximum maturity sizes of T.japonica are 6 and 12 mm for males, and 9 and 17 mm for females. However, the growth
pattern was difficult to analyze from field population structure data because of
overlapping cohorts due to the continuous reproduction mode of this species. As
an alternative approach, Ikeda (1990) determined the growth rate of T.japonica
© Oxford University Press
299
T.Ikeda and N.Shiga
based on the data of intermolt period and molt increment as a function of temperature of laboratory-reared specimens. Further, Ikeda (1991) combined data of
growth, metabolism (= oxygen consumption), molting and fecundity of this
species, and established lifetime budgets of assimilated carbon for T.japonica.
In this study, information gained from field survey and laboratory experiments
on T.japonica is integrated to calculate production (somatic and molts), metabolism (routine and diel vertical migration) and the production:biomass (P:B) ratio
of the population of this amphipod in Toyama Bay.
Method
Population data
Numerical abundance and body length distribution of T.japonica were estimated
from a series of vertical hauls (500 m to the surface) at an interval of 2 weeks over
one full year (24 dates, February 1990 through January 1991) at an offshore
station (37°009N, 137°149E) in Toyama Bay. In these data, the body length (BL;
the maximum distance between the tip of the head and the distal end of the
uropods of the straightened body) was divided into 1 mm increments (16 size
classes over the entire range of 1–17 mm BL, covering juveniles just released from
the female’s marsupium to mature adults). Details of these results and environmental data (temperature, salinity, total net zooplankton biomass) collected
concurrently may be found in Ikeda et al. (1992).
Body allometry/carbon content
The relationships between BL (mm) and dry weight (DW; mg) and wet weight
(WW; mg) for T.japonica have been established as DW = 0.0049 BL2.957 and WW
= 0.0304 BL2.832 (Ikeda, 1990). DW carbon contents of T.japonica are 36.15% for
<1 mg DW, 36.50% for 1–5 mg DW, 37.20% for 5–10 mg DW, and 38.31% for >10
mg DW specimens, with no appreciable differences between males and females.
The carbon content of dried molts is 23.70% (Ikeda, 1991).
Diel vertical migration/habitat temperature
Except for early juveniles (1–3 mm), which stay at or near the surface both day
and night, older juveniles and adults of T.japonica migrate over an extensive
vertical distance each day. The vertical range of the migration varies seasonally
from 150 (September) to 400 m (June) for the Toyama Bay population (Ikeda et
al., 1992; T.Ikeda, unpublished data) with an annual mean of 250 m. The range
of habitat temperature encountered by vertically migrating T.japonica varies
seasonally, from 1 to 2°C during the daytime and from 5 to 16°C during nighttime (Ikeda et al., 1992), and estimated daily mean temperature is 6.4°C over the
year. Ikeda (1992) reared T.japonica in the laboratory under a fluctuating
temperature regime (1–15°C, with an integrated daily mean of 8°C) and at
constant temperature (8°C) as a control. Comparison of T.japonica reared in
these two thermal regimes revealed no significant effect of thermal modes on the
300
Production of Themisto japonica
daily growth and metabolism (= oxygen consumption) of this animal. Thus, daily
integrated temperature is a good estimator of habitat temperature for vertically
migrating T.japonica. In the present calculation, the integrated habitat temperature of T.japonica was chosen as 15°C for 1–3 mm specimens and 6.4°C for >3
mm specimens.
Results
Biomass
Biomass of T.japonica expressed in carbon units (B; mg C m–3) increased gradually from the beginning of the year, forming several peaks in spring through
summer (1.30 mg C m–3 in May, 2.18 mg C m–3 in August and 2.24 mg C m–3 in
September), then decreased toward the end of the year (Figure 1), with an integrated mean B over the entire study period (363 days) of 0.739 mg C m–3 or 370
mg C m–2 (Table I).
Somatic production
Somatic production of T.japonica at a given sampling date was computed as the
sum of growth increments
of 16 size classes multiplied by the abundance of each
s
N
(CW
size class: Pg =i ∑
i
i + 1 – CWi)/Di, where Pg is the daily somatic production
=1
(mg C m–3 day–1), CWi and CWi + 1 are the weights (mg C) at the beginning and
end of the size interval, Di is the developmental time (days) from CWi to CWi + 1,
Ni is the abundance (number m–3) of each size class, and s is 16. The BL data were
Fig. 1. Changes with season in daily somatic production (Pg), molt production (Pe), routine metabolism (Mrtn) and diel vertical migration metabolism (Mdvm) (all mg C m–3 day–1) and biomass (B, mg
C m–3) of the T.japonica population in Toyama Bay, southern Japan Sea.
301
T.Ikeda and N.Shiga
Table I. Summary of a 363 day carbon budget for the T.japonica population in Toyama Bay (1
February 1990 through 30 January 1991). Data are expressed m–3 and m–2; the former was multiplied
by 500 for the latter
mg C m–3
Mean biomass (B)
mg C m–2
0.739
Production (P = Pg + Pe)
4.221
Somatic (Pg)
3.868
Molt (Pe)
0.353
Metabolism (M = Mrtn + Mdvm)
11.755
Routine (Mrtn)
8.199
Diel vertical migration (Mdvm)
3.556
Assimilation (A = P + M)
15.976
Ingestion (I = A/0.904)
17.673
Ratios
P/B
Pg/B
P/M
Pg/M
370
(% of A)
(26.4)
(24.2)
(2.2)
(73.6)
(51.3)
(22.3)
2110
1934
176
5878
4100
1778
7988
8837
=
=
=
=
5.71
5.23
0.359
0.329
converted to dry weight (DW) using the allometric equation, then to C units using
C content percentages for each BL class mentioned above. Di was estimated from
cumulative developmental time (t; days) as a function of BL and temperature (T,
°C) using the modified growth equation of Ikeda (1990) for T.japonica: t = (1.033
– e–0.0426BL)/(0.0246eb), where b = 2.3023(–0.4503 – 10–0.0267T – 0.0366).
Pg thus calculated varied greatly with season, from 0.003 (January) to 0.029 mg
C m–3 day–1 (August, cf. Figure 1), with an integrated Pg over the entire study
period (363 days) of 3.868 mg C m–3 or 1934 mg C m–2 (Table I).
Molt production
s
The production of molts was given by the equation: Pe = i ∑
(MDWi 3 Ni 3 a/Di),
=1
where Pe is the molt production (mg C m–3 day–1), MDWi is the geometric mean
DW [= (DWi + 1 3 DWi)0.5] of each size-class, Ni is the abundance of each size
class, a is the percent loss in body DW per molting (6.8%; Ikeda, 1991), multiplied
by the carbon content of molts (23.7% of DW; Ikeda, 1991) and Di is the intermolt
period (IP; days) estimated from the modified equation of Ikeda (1990) as a function of BL and temperature (T; °C) (IP = 10b + 0.0709BL, where b = 100.0366 – 0.0267T)
and BL versus DW allometry mentioned above.
Pe ranged from 0.0003 (January) to 0.0023 (August) mg C m–3 day–1, with an
integrated Pe over the entire study period of 0.353 mg C m–3 or 176 mg C m–2
(Table I).
Metabolism
The metabolism M (mg C m–3 day–1) was partitioned into two components: routine
metabolism (Mrtn) and diel vertical migration metabolism (Mdvm). Mrtn was calculated from oxygen consumption rates (R; µl O2 individual–1 h–1) determined by
302
Production of Themisto japonica
placing specimens in the standard cell (7.9 ml capacity) fitted with a YSI oxygen
electrode to monitor the change in oxygen content in the cell (Ikeda, 1991). The
specimens used for Ikeda’s (1991) study were those collected 4–20 h prior to the
experiments; therefore, extra energy associated with feeding (‘specific dynamic
action’, cf. Kiørboe et al., 1985) is assumed to be included in his oxygen consumption data. R is a function of the size (DW; mg) of specimens and temperature (T;
°C): R = DW0.788 3 100.05355T – 0.2316 (modified from Ikeda, 1991). Mdvm is the
amount of oxygen consumed for diel vertical migration (R9; µl O2 individual–1
km–1), which was calculated from the equation of net cost of transport as a function of weight (mg WW) of animals established for pelagic crustaceans (‘multiplepaddle’ propulsive system as compared with the ‘undulatory’ propulsion system
of fishes): R9 = 9.02WW0.72 (Torres, 1984; the original equation based on energy
unit was modified using oxycalorific equivalent 1 cal = 208.33 µl O2). R9 is independent of temperature in theory (cf. Morris et al., 1990) and is 0.5R9 for T.japonica (>3 mm) migrating 250 m daily (i.e. 0.5 km for round trip). In calculating R or
R9, DW or WW of specimens was representeds by the geometric mean DW or WW
(Ri 3 Ni) and Mdvm = 0.5 3 10–3 g
ofs each size class. Thus, Mrtn = 24 3 10–3 g i ∑
=1
∑ (R9i 3 Ni), where 24 is to convert hourly rate to daily rate, 10–3 is to convert
i=1
micrograms to milligrams, and g (= 0.97 3 12/22.4) is to convert oxygen units to
carbon units assuming protein metabolism (RQ = 0.97; Gnaiger, 1983).
Both Mrtn and Mdvm thus calculated were the highest in the summer season, and
the seasonal range was from 0.005 to 0.063 mg C m–3 day–1 for the former and
from 0.003 to 0.025 mg C m–3 day–1 for the latter (Figure 1). Integrated values
over the entire study period (363 days) were 8.119 mg C m–3 or 4100 mg C m–2
for Mrtn, and 3.556 mg C m–3 or 1778 mg C m–2 for Mdvm (Table I).
Assimilation and ingestion
Carbon assimilated by T.japonica (A; mg C m–3 day–1) is defined as A = P + M =
Pg + Pe + Mrtn + Mdvm, assuming no leakage of soluble organic matter. The
amount of ingested carbon (I; mg C m–3 day–1) was computed adopting an assimilation efficiency value of 90.4% determined on a bentho-pelagic amphipod
Calliopius laeviusculus by Dagg (1976), i.e. I = A/0.904.
A ranged from 0.008 to 0.087 mg C m–3 day–1, and I from 0.012 to 0.117 mg C
–3
m day–1. Integrated A and I values over the entire study period (363 days) were
15.98 and 17.67 mg C m–3, or 7988 and 8837 mg C m–2, respectively (Table I).
Ratios between parameters (Pg /B, P/B, Pg /M, P/M, B/N)
The ranges of seasonal variations were 0.011–0.026 for somatic production to
biomass (Pg/B) ratios, 0.011–0.028 for total production to biomass (P/B) ratios,
0.28–0.44 for somatic production to metabolism (Pg/M) ratios and 0.31–0.48 for
total production to metabolism (P/M) ratios. Since the seasonal trends of P/B and
P/M ratios were similar to those of Pg/B and Pg/M ratios, respectively, only the
latter two ratios are shown in Figure 2. A population (= size) structure index B/N
303
T.Ikeda and N.Shiga
(mg C individual–1) fluctuated irregularly from 0.174 to 1.867 (Figure 2). From
Figure 2, it is seen that the seasonal patterns of Pg/B and Pg/M ratios paralleled
each other. The seasonal pattern of the B/N ratios was entirely different from
those of Pg/B or Pg/M ratios, and it changed in an opposite fashion to the latter
two ratios.
Discussion
Despite the widespread distribution of hyperiid amphipods over the world ocean
(Shih, 1982), there is no information about the production of this group of
zooplankton. The only related information presently available is a production
estimate for total planktonic gammarid amphipods (not species, but as a group)
on Georges Bank by Avery et al. (1996). However, Avery et al.’s (1996) data are
not directly comparable to the present results since the P/B ratio they used is of
benthic gammarids in the same region.
On the bases of similar habitat (marine), ecology (planktonic) and trophic type
(carnivore), but disregarding phylogenetic differences, the present estimates of
production and Pg/B ratio of T.japonica are compared with the chaetognaths
Sagitta elegans and Sagitta hispida (Sameoto, 1973; Reeve and Baker, 1975), and
the ctenophores Pleurobrachia bachai and Mnemiopsis maccradyii (Hirota, 1974;
Reeve and Baker, 1975) in Table II. Somatic production (Pg) and Pg/B were used
instead of total production (P) and P/B for the basis of comparison since daily
production of non-crustacean carnivorous zooplankters in Table II represents
somatic production (Pg) only. Whereas daily production of T.japonica (5.33 mg
C m–2 day–1) is the greatest among these carnivorous zooplankers, its daily Pg/B
Fig. 2. Changes with season in somatic production to biomass (Pg/B; day–1) ratios, somatic production to metabolism (Pg/M) ratios and a population size structure index (B/N; mg C individual–1) of
the T.japonica population in Toyama Bay.
304
aCalculated
12.5–20
2.0
1.37
Toyama Bay
6–15
26a
0.5–14
26a
Temperature
(ºC)
1.06
0.30
0.157
Maximum
size (mg C)
South Florida
inshore water
La Jolla Bight
South Florida
inshore water
Nova Scotia water
Habitat
from annual or near annual data.
Sagitta elegans
Ctenophores
Mnemiopsis
mccradyii
Pleurobrachia
bachiai
Amphipods
Themisto japonica
Chaetognaths
Sagitta hispida
Animal group/species
0.21 (mean)
0.20a
0.014a
0.74a
5.33a
0.006a
0.55a
0.50–1.01
0.31 (mean)
Daily Pg:B
2.00–4.80
Daily Pg
(mg C m–2)
Table II. Daily somatic production (Pg) and daily production/biomass ratios (Pg/B) of carnivorous zooplankton
This study
Hirota (1974)
Reeve and Baker (1975)
Sameoto (1973)
Reeve and Baker (1975)
Source
Production of Themisto japonica
305
T.Ikeda and N.Shiga
ratio (0.016) is modest, falling within the wide range of the Pg/B values of other
carnivorous zooplankters (0.006–0.31).
The P/B ratio (often equivalent to the Pg/B ratio, as noted above) is an appropriate basis for comparing the production potential of various invertebrates, and
is largely a function of their sizes (cf. Banse and Mosher, 1980), i.e. the greater
the sizes, the lower the P/B ratios. The population size structure index (B/N) used
in the present study is not an accurate measure of mean body size of T.japonica
unless the size distribution is proved to be the normal. Nevertheless, lower Pg/B
ratios associated with larger B/N are seen in this study (Figure 2). Because of the
lack of appropriate information, the effect of habitat temperature on the P/B
ratio has not been evaluated, but recent results of extremely higher P/B ratios on
tropical marine copepods (cf. Webber and Roff, 1995) suggest that habitat
temperature is an additional factor affecting the P/B ratios of marine zooplankton. From this view, higher Pg/B ratios seen in S.hispida and Mnemiopsis
maccradyii than that of T.japonica may be explained partly by higher habitat
temperature and/or their smaller body sizes. Despite a smaller size than that of
T.japonica, the lower Pg/B ratio of S.elegans may be a result of the pronounced
effect of their lower habitat temperature. Banse and Mosher (1980) noted that
the phylogeny of aquatic and terrestrial invertebrates had little effect on the
broad relationship between P/B ratio and body size.
McLaren et al. (1989) calculated Pg/B ratios of 10 copepod species from
Emerald Bank, Scotian Shelf, where the thermal regime is roughly similar to that
of Toyama Bay. Among the 10 copepods they studied, Calanus hyperboreus is of
special interest because this copepod has an adult body size (2.7 mg DW) similar
to that of T.japonica studied here (mid adult size: 3.7 mg DW; cf. Ikeda, 1991).
The daily Pg/B ratio of C.hyperboreus is 0.020 (calculated from the annual Pg/B
= 7), which is near our estimate of 0.014 for T.japonica. Because C.hyperboreus
and T.japonica are a typical herbivore and a typical carnivore, respectively,
trophic type appears to have little or no effect on the Pg/B ratios in zooplankton.
All these results together suggest that the production potential of T.japonica, as
judged by Pg/B ratios, does not differ appreciably from those of other zooplankters if differences in body size and/or habitat temperature are taken into account.
Metabolism accounted for the greatest portion (73.6%; Table I) of carbon
assimilated by T.japonica. In contrast to terrestrial and aquatic benthic invertebrates, field comparisons between population metabolism and population
production are extremely scarce for marine zooplankton (Humphreys, 1979;
Banse and Mosher, 1980). As the only data available, Sameoto (1973) estimated
field population metabolism and production of the chaetognath S.elegans in
Bedford Basin, Nova Scotia, assuming that the extra metabolism for undefined
activity of wild specimens equals twice the metabolism determined on captive
specimens in the laboratory (wild metabolism = laboratory-determined metabolism 3 2). This correction factor of 32 for estimating metabolism of wild specimens is empirical, derived from studies of fish energetics (cf. Winberg, 1956). In
this light, application of the relationship between energy cost for locomotion and
body mass established by Torres (1984) is an alternative method providing a
logical basis for estimating extra metabolism of wild zooplankton. The present
306
Production of Themisto japonica
results suggest that the extra metabolism (Mdvm) needed for the diel vertical
migration of T.japonica is 30.30 that of captive specimens (Mrtn), which is much
less than 32. Despite these methodological differences, field population metabolism/production ratios (Pg/M) yielded for S.elegans (0.25) by Sameoto (1973)
and T.japonica (0.33) in this study are quite close to each other.
Lasker (1966) noted that molt production by the euphausiid Euphausia pacifica
amounted to 300 mg C m–2 year–1 in the northern North Pacific, and is an important source of oceanic detritus. Molt production by T.japonica in Toyama Bay was
calculated as 177 mg C m–2 year–1 (176 3 365/363), which is the same order of
magnitude as that of E.pacifica in the northern North Pacific. Comparison of carbon
budgets of assimilated carbon in E.pacifica (Lasker, 1966) and T.japonica (this
study) revealed that the fraction invested to molt production was greater in the
former (15.3%) than the latter (2.2%). Production of larger molts may be one possible cause for greater partition of assimilated carbon to molt production in E.pacifica. However, the size of a single molt, as assessed by the loss in body carbon at
each molting, is similar between E.pacifica (4.1%, calculated from Lasker’s data)
and T.japonica [4.4%, calculated from Ikeda’s (1991) data]. Alternatively, higher
molting frequency of E.pacifica than T.japonica may be the case. From intermolt
periods expressed as a function of body size (BL) and temperature for E.pacifica
(Iguchi and Ikeda, 1995) and T.japonica (this study), intermolt periods of specimens
with similar body size (BL = 10 mm) and at the same temperature (6.4°C) are
predicted as 7.9 days for the former and 27.7 days for the latter. Thus, E. pacifica
can produce 3.5 molts, while T.japonica casts one molt (although molt increment
in terms of body length of the former is 0.6 times the latter), indicating that this
dissimilar molting frequency is the major cause which led to different partition of
assimilated carbon to molt production in these two crustaceans.
To maintain somatic growth, molting and metabolism, the population of T.japonica in Toyama Bay needs to ingest prey animals of 8886 mg C m–2 per year (8837
3 365/363). Comparing this to the annual mean population biomass of 371 mg C
m–2 (370 3 365/363), the mean daily food requirement of the population is estimated as 6.5% of their biomass. In Toyama Bay, the major prey animals of T.japonica are copepods and young euphausiids (E.pacifica, body length <10 mm) (Ikeda
et al., 1992). Assuming the annual Pg/B ratio to be 49 for copepods [a mean of six
copepod species of which adult DW ranges from 2 to 107 µg in McLaren et al.
(1989)], annual copepod production is calculated from their seasonal biomass data
(Hirakawa et al., 1992) as 28 992 mg C m–2 in Toyama Bay. From production data
of E.pacifica in Toyama Bay (N.Iguchi and T.Ikeda, unpublished), annual production of <10 mm E.pacifica is computed as 350 mg C m–2. Thus, comparison of annual
ingestion by the T.japonica population (8886 mg C m–2) with annual production of
their prey animals (28 992 + 350 = 29 342 mg C m–2) led us to conclude that T.japonica consume about one-third (30.4%) of the annual production of their prey.
Acknowledgement
We are grateful to D.L.Mackas for his critical reading of the manuscript and
valued comments.
307
T.Ikeda and N.Shiga
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Received on June 4, 1998; accepted on October 2, 1998
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