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Species composition and depth
distribution of chaetognaths in a
Kuroshio warm-core ring and
Oyashio water
TRAVIS B. JOHNSON* AND MAKOTO TERAZAKI
OCEAN RESEARCH INSTITUTE, THE UNIVERSITY OF TOKYO, 1-15-1 MINAMIDAI, NAKANO-KU, TOKYO 164-8639, JAPAN
*CORRESPONDING AUTHOR:
[email protected]
Chaetognath species composition, diversity and depth distribution were investigated inside and outside
a warm-core ring oV the north-east coast of Japan. Time series samples were collected at a station
inside a Kuroshio warm-core ring (KWCR) and at a station in the surrounding Oyashio water.
Greater mean abundance of 2.51 ± 0.116 (mean ± SE) chaetognaths m–3 was found outside the
ring compared with 1.75 ± 0.321 chaetognaths m–3 collected within the ring. However, species
diversity values were higher within the KWCR (Shannon–Weaver index). Eukrohnia hamata was
the dominant species outside the ring, comprising 79.8–87.9% of the total chaetognaths per haul.
Inside, Sagitta minima was dominant, comprising 35.1–44.3%. Most E. hamata were collected in
deeper layers within the KWCR and their abundance was on average only 9% of that found in the
Oyashio region. Only E. hamata and Sagitta scrippsae had diVerent depth distributions in the
KWCR. The vertical distribution of E. hamata by body size appeared altered by the KWCR.
Although the mean length of E. hamata was not signiWcantly diVerent between regions, sexual
development appeared inhibited in the ring. Sagitta elegans collected in the KWCR were mostly small
in size (<10 mm), signiWcantly smaller than in the Oyashio water.
INTRODUCTION
Warm-core rings are mesoscale events that are formed as
eddies of warm water that detach from large meandering
current systems such as the Gulf Stream or Kuroshio
Current (Lalli and Parsons, 1997). These rings spin oV
as independent bodies of circulating water into the colder
water on each side of the current system. As is commonly
known, these rings can transport water and organisms
from one water mass into another and thus can have a
major inXuence on marine ecosystem processes. They
continue to be the focus of much research, including
studies on the eVect they have on primary productivity,
zooplankton biomass and distribution of Wsh (Roman
et al., 1985; Saitoh et al., 1986; Craddock et al., 1992;
Hama, 1992). Warm-core rings can be several hundred
kilometers in size with radically diVerent hydrographic
proWles than neighboring waters and can exist for months
to years. Since they contain species from their source
water, and can entrain species from surrounding waters
as they move, it is possible to study the eVect the ring has
on species distribution, biomass variation and life success.
Chaetognaths are recognized as important components in most marine planktonic communities, as they
are found in every marine habitat and are often second
in abundance only to copepods (their chief prey) among
all marine zooplankton groups (Feigenbaum and Maris,
1984). The biomass of chaetognaths is estimated to be
10–30% of that of copepods in the world oceans; thus,
they play a signiWcant role in the transfer of energy from
copepods to higher trophic levels (Bone et al., 1991). As
abundant predators, they can place heavy pressure on
copepod communities (Øresland, 1990) and are an important component in ocean carbon Xux (Terazaki, 1995).
Chaetognaths have also been the subject of research
by Wsheries scientists. For example, their role as predators
and competitors of larval Wsh has been evaluated in
recent years (Baier and Purcell, 1997; Brodeur and
Terazaki, 1999) and their distribution has been linked
doi: 10.1093/plankt/fbg085, available online at www.plankt.oupjournals.org
Journal of Plankton Research 25(10), # Oxford University Press; all rights reserved
JOURNAL OF PLANKTON RESEARCH
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to salmon catches in the Gulf of Alaska (LeBrasseur, 1959).
Furthermore, chaetognaths are prey for adult PaciWc saury
oV the coast of north-eastern Japan (H. Sugisaki, personal
communication) and they are one of the main food items
for Walleye pollock in the Japan Sea during spring (Kooka
et al., 1998).
Many chaetognath species are uniquely aVected by the
hydrographic conditions of their environment (e.g. salinity,
temperature and dissolved oxygen), and they display
species-speciWc relationships to water masses in addition
to distinct vertical distribution proWles in the water column
(Bieri, 1959; Sullivan, 1980; Terazaki and Miller, 1986;
Terazaki, 1992; Ulloa et al., 2000). As a result, chaetognaths are often categorized according to the type of water
mass to which they are best adapted (e.g. warm water, cold
water, mixed water). The surrounding hydrographic conditions can also inXuence aspects of chaetognath ecology,
such as growth, sexual development and feeding rate
(McLaren, 1963; Feigenbaum, 1982).
The warm Kuroshio Current Xows north along Japan’s
southern coast and gradually turns east near 36 N, leaving
Honshu Island to form the Kuroshio Extension. Where
the Kuroshio Extension meets the cold Oyashio Current
Xowing south out of the Okhotsk and Bering Seas, an
intermediate zone develops between the Kuroshio and
Oyashio Front. This zone, termed the Perturbed Area
by Kawai (Kawai, 1972), is where Kuroshio warm-core
rings (KWCRs) form—a process that has been thoroughly
studied (Kawai, 1972; Hata, 1974; Saitoh et al., 1986;
Tomosada, 1986).
The waters oV the coast of north-eastern Japan are the
site of much Wshing eVort and thus have commercial
importance. Given the proliferation of warm-core rings
in this region and the recognized role that chaetognaths
play in marine ecosystems, learning about the interactions
between KWCRs and chaetognaths is important for gaining a more complete understanding of the ecology of the
region. The objectives of this study were to determine the
eVects of a KWCR on chaetognaths in terms of their
(i) species composition, (ii) abundance and vertical distribution, and (iii) body size and development.
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at stations C11 (38 559N, 143 309E) and C31 (38 009N,
144 009E), respectively (Figure 1). Station C31 was inside
a KWCR, and station C11 was outside the ring, ~120 km
to the north. This ring was formed in August 1997 and at
the time of sampling the position of the ring center was
~37 459N, 1449E. The ring was elliptical in shape,
~200 km between the widest points and 110 km between
the narrowest parts.
A surface-drifting buoy was launched at the start of the
sampling period and hauls were taken as close to the
buoy as possible in an eVort to track the same parcel of
water. The net sampled seven discrete layers in the water
column at station C11, which were a 200-m layer from
600 to 800 m, and then 100-m increments to the surface.
At station C31, eight 100-m layers were sampled from
800 m to the surface. Hauls were repeated four times at
station C11 and three times at station C31. The times of
all hauls were recorded in local solar time. The net was
retrieved at a rate of 0.8–1 m s–1 and the average volume
of water Wltered was 23.75 m3 per 100 m layer. The
water depth was 2400 m at station C11 and 6500 m at
station C31. Nets were rinsed down into the cod end and
the samples were Wxed with a 10% buVered formalin and
seawater solution. Water temperature and salinity data
were collected by regular CDT casts throughout the
duration of the cruise.
METHOD
Sampling
Samples were collected during the KH-98-4 cruise
conducted by the R/V ‘Hakuho Maru’ of the Ocean
Research Institute, University of Tokyo, using an ORIVMPS net [Vertical Multiple Plankton Sampler, with a
mouth area of 0.5 3 0.5 m and 0.33 mm mesh; (Terazaki
and Tomatsu, 1997)]. Successive tows were taken from
September 13 to 14 and from September 19 to 20, 1998
Fig. 1. Sampling area and stations C11 and C31 in the waters of the
western North Pacific adjacent to the Japanese islands of Honshu and
Hokkaido.
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Laboratory analysis
Chaetognaths were separated out from the rest of the
samples and sorted into species. The Shannon–Weaver
diversity index (H9) was used to compare species diversity
inside and outside the warm-core ring (Fowler et al.,
1998).
Eukrohnia hamata and Sagitta elegans were selected as
the focus for much of the analysis in this study as
they were the dominant species in the Oyashio water
and were numerous enough inside the ring to warrant
comparison.
To test for diVerences in vertical distribution inside
and outside the ring, the weighted mean depth (WMD)
of E. hamata and S. elegans was calculated for each haul:
WMD = ðSni di Þ=ðSni Þ
where ni is the number of individuals in depth stratum
i and di is the midpoint of the stratum. Mann–Whitney
U-tests were used to compare WMDs between stations.
The standard length (front of the head to the end of
the tail, excluding the Wn) of all E. hamata from station
C31 (109 individuals) and 112 randomly selected individuals from station C11 was recorded. Standard length
was also recorded for the entire S. elegans collection
(40 individuals from station C31; 113 individuals from
station C11). A t-test was used to compare the mean
standard length of E. hamata between stations.
To compare the vertical distribution of E. hamata
according to body size, specimens were sorted and
divided into size classes as follows: ˘ 10 mm, >10 mm to
˘ 15 mm, >15 mm to ˘ 20 mm and >20 mm. These classes
will subsequently be abbreviated as <10 mm, 10+ mm,
15+ mm and 20+ mm, respectively. The WMDs of
diVerent size classes within each station were compared
using Kruskal–Wallis tests. Mann–Whitney U-tests were
used to compare WMDs of like size classes between
stations. Sagitta elegans was excluded from this analysis
because only Wve specimens >10 mm were collected in
the ring, thus meaningful comparison of the two regions
was not possible.
All E. hamata and S. elegans >15 mm in length were
checked for degree of sexual maturity. Individuals
<15 mm in this collection did not yet show signs of sexual
development. Stage of maturity for E. hamata and
S. elegans was based on development of the gonads according to Alvariño (Alvariño, 1967) and King (King, 1979),
respectively. The stages of E. hamata can be summarized
as: stage I, ovaries and testes as Wne tubes; stage II, tail
segment full and ovaries longer; stage III, tail segment
partially discharged and ova developing; stage IV, tail
segment discharged and ovaries stretching two-thirds of
the distance to ventral ganglion. Likewise, S. elegans can
be summarized as: stage I, no visible testes or ovaries;
stage II, ovaries and testes visible but immature; stage III,
seminal vesicles present and a few large ova visible; stage
IV, ova large and seminal vesicles Wlled.
RESULTS
Species composition
The KWCR contained more species from more genera
and had a higher level of diversity than the Oyashio
water. The chaetognath population at station C11 outside the ring consisted of 13 species from three genera
(Table I). Eukrohnia hamata was the dominant species and
accounted for 79.8–87.9% of the total chaetognaths per
haul (Figure 2). The remainder consisted mainly of
Eukrohnia bathypelagica and S. elegans (3.6–7.7 and 4.3–
8.7%, respectively).
At station C31 inside the KWCR, 18 species from four
genera were identiWed (Table I). Here the dominant
species was Sagitta minima, which comprised 35.1–44.3%
of the total chaetognaths per haul (Figure 2). Eukrohnia
hamata was less abundant in the KWCR, accounting for
only 10.5–13.7% of the chaetognaths collected. The
other major chaetognath species inside the ring were
Sagitta enXata (11.4–17.7%), Sagitta scrippsae (6.5–11.4%),
Sagitta regularis (5.7–12.8%), Sagitta nagae (5.9–7.7%) and
S. elegans (1.4–7.4%).
H9 calculated for the chaetognaths collected in each
haul ranged from 1.78 to 2.05 inside the ring, and from
0.56 to 0.80 outside, which indicated that there was a
higher level of species diversity within the KWCR.
Furthermore, seven species comprised 93% of the
chaetognaths that were collected from each tow inside
the ring on average, but in the less diverse Oyashio
water, just three species accounted for 95% on average
(Figure 2).
There were seven species found exclusively at station
C31, two species exclusively at station C11 and 11 species present at both locations. Mesopelagic and coldwater species were dominant in the Oyashio water and,
in the KWCR, mixed-water and warm-water species
were dominant. No regular Xuctuation was apparent in
the overall composition of the chaetognaths over time in
the tows at each station.
Hydrographic proWle
The Oyashio water at station C11 had a single thermocline between the surface and ~100 m (Figure 3). The
halocline was somewhat shallower and not as clearly
deWned. The greatest change in salinity occurred from
the surface to ~50 m and continued to gradually increase
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Table I: The total number of individuals and their per cent of the total number of chaetognaths for each
species collected at stations C11 in the Oyashio water and C31 in a KWCR to the east of northern Japan
Station
Species
No. of individuals
Per cent of total
C11
Eukrohnia hamata
1569
83.72
Meso
Sagitta elegans
113
6.03
Cold
Eukrohnia bathypelagica
107
5.71
Meso
Sagitta nagae
32
1.71
Mix
Sagitta scrippsae
18
0.96
Mix
Sagitta minima
14
0.75
Mix
Sagitta enflata
13
0.69
Warm
Sagitta macrocephala
2
0.11
Meso
Sagitta decipiens
2
0.11
Meso
Sagitta zetesios
1
0.05
Meso
Krohnitta subtilis
1
0.05
Warm
Sagitta hexaptera
1
0.05
Warm
Sagitta lyra
1
0.05
Warm
Total
C31
Water mass
1874
Sagitta minima
364
38.00
Mix
Sagitta enflata
137
14.30
Warm
Eukrohnia hamata
109
11.38
Meso
Sagitta scrippsae
77
8.04
Mix
Sagitta regularis
69
7.20
Warm
Sagitta nagae
62
6.47
Mix
Sagitta elegans
40
4.18
Cold
Sagitta spp. juvenilesa
36
3.76
N/A
Sagitta decipiens
15
1.57
Meso
Pterosagitta draco
11
1.15
Warm
Krohnitta subtilis
7
0.73
Warm
Sagitta pacifica
7
0.73
Warm
Sagitta bipunctata
6
0.63
Warm
Eukrohnia bathypelagica
4
0.42
Meso
Sagitta ferox
4
0.42
Warm
Sagitta neglecta
4
0.42
Warm
Sagitta lyra
3
0.31
Warm
Sagitta zetesios
2
0.21
Meso
Krohnitta pacifica
1
0.10
Warm
Total
958
The water masses that each species is typically associated with are shown.
Meso, mesopelagic water; Mix, mixed water; Cold, cold water; Warm, warm water.
Unidentified.
a
with depth. There were two oxyclines, indicating increasing oxygen concentrations between the surface and
~50 m, and decreasing oxygen concentrations between
~50 and 300 m.
At station C31 within the ring, there were two rapid
decreases in temperature that were evident in the proWle.
The Wrst thermocline was between the surface and ~150 m.
The second thermocline was found between ~350 and
450 m. Salinity also Xuctuated in a step-like manner:
it increased to a maximum at ~100 m, decreased to
a low near 450 m, and then rose through the rest of the
sampling range. The dissolved oxygen proWle was roughly
the opposite of the salinity proWle, with a minimum at
~100 m, a maximum at ~450 m and a steady decrease
to 1000 m.
Abundance and vertical distribution
Chaetognath abundance was lower inside the ring and
the vertical distribution varied among species. At station
C11 in the Oyashio water, the overall abundance of
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widely distributed across the thermocline outside the ring, it
was not widely distributed across the lower thermocline
inside the ring.
The vertical distribution of S. elegans did not diVer
signiWcantly between locations (Mann–Whitney U-test
on WMD, P = 0.48), but abundance inside the ring was
on average ~50% of that outside. Peaks in abundance
occurred in the upper 400 m at both stations, and
S. elegans was distributed across all thermoclines. The
vertical distribution and abundance for S. enXata, S. minima,
S. nagae and S. scrippsae are summarized in Table II.
Length, size class and maturity
Fig. 2. Relative abundance of the most abundant species of chaetognaths at stations C11 in the Oyashio water and C31 in a KWCR to the
east of northern Japan. The times shown are for the middle of each haul.
chaetognaths integrated over each haul ranged from 2.26
to 2.76 individuals (ind.) m–3, with a mean of 2.51 ± 0.116
(mean ± SE). Within the KWCR at station C31, abundance ranged from 1.27 to 2.36 ind. m–3, with a mean of
1.75 ± 0.321. However, of the 11 chaetognath species
present at both stations, all but three (E. hamata, S. elegans
and E. bathypelagica) were more abundant at station C31.
There were six species present both inside and outside
the ring in numbers suitable for comparison: E. hamata,
S. elegans, S. enXata, S. minima, S. nagae and S. scrippsae. In
the Oyashio water, E. hamata consistently had peak
abundances just below the thermocline at depths of
100–300 m (Figure 4). However, E. hamata was found
at deeper depths inside the KWCR and their WMD
values were signiWcantly diVerent to those outside the
ring (Mann–Whitney U-test, P = 0.034). Most individuals
were found in deep cold waters below the core of the ring
where temperature had stabilized. Additionally, the
abundance of E. hamata at station C31 was on average
~10% of that at station C11. Although E. hamata was not
The standard length of the individuals within E. hamata
populations did not diVer signiWcantly between stations
(t-test, P = 0.58) (Figure 5). Outside the ring, standard
length ranged from 4.0 to 21.5 mm with a mean of
12.3 ± 0.42 mm (mean ± SE). Inside the ring, standard
length ranged from 5.0 to 17.5 mm with a mean of
12.6 ± 0.30 mm.
In contrast, the standard length of individuals within the
S. elegans population was obviously very diVerent between
stations (Figure 6). Standard length inside the ring had a
range of 4.0–26.0 mm with a mean of 7.2 ± 0.83 mm.
Standard length outside the ring had a range of 5.0–31.0 mm
and mean of 19.2 ± 0.57 mm. Furthermore, in the KWCR,
87.5% of the individuals were <10 mm in length and 93.8%
of the individuals from the Oyashio water were >10 mm.
The vertical distribution of size classes among E. hamata
was diVerent at the two locations. Outside the ring, size
classes segregated vertically into diVerent depths and a
statistically signiWcant diVerence was found among
the WMD of the size classes (Kruskal–Wallis test, P =
0.0097). Small individuals were concentrated in shallow
waters and larger individuals were concentrated in increasingly deeper waters (Figure 7). SpeciWcally, ‘<10 mm’
Fig. 3. Temperature, salinity and dissolved oxygen profiles for stations C11 in the Oyashio water and C31 in a KWCR to the east of northern Japan.
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Fig. 4. Abundance (ind. m–3) and vertical distribution of E. hamata and S. elegans at stations C11 in the Oyashio water and C31 in a KWCR to
the east of northern Japan. The times shown are for the middle of each haul. Night-time hauls are in black.
individuals peaked between 100 and 200 m depths,
‘10+ mm’ individuals peaked between 200 and 300 m,
and ‘15+ mm’ individuals mostly had peaks between 300
and 500 m.
Within the warm-core ring, size classes were also
segregated and WMDs were signiWcantly diVerent
(Kruskal–Wallis test, P = 0.027), but the speciWc segregation pattern diVered, with the smallest individuals being
concentrated in deep waters (Figure 7). Individuals
<10 mm in length had peaks between 600 and 800 m
depth, ‘10+ mm’ individuals mostly peaked between 300
and 500 m, and ‘15+ mm’ individuals peaked between
500 and 700 m. The data for E. hamata >20 mm in length
were not analyzed in this study as they were absent from
station C31 samples and made up an insigniWcant
proportion of the samples from station C11 (0.3–0.5%).
In the Oyashio water at station C11, 12.5% of the
E. hamata were >15 mm in length and they fairly evenly
represented the Wrst three maturity stages: stage I =
25.6%, stage II = 36.9% and stage III = 37.5%. In the
warm-core ring at station C31, 21.1% of the E. hamata
were >15 mm; however, 95.7% of these were at stage I
maturity. Furthermore, conWning the analysis of Oyashio
individuals to between 15.0 and 17.5 mm for the purpose
of matching the maximum length (17.5 mm) of KWCR
individuals, a similar composition of maturity stages was
found outside the ring: stage I = 33%, stage II = 43% and
stage III = 23%.
Table II: Summary of the vertical distributions and peak abundance values of the chaetognath species
S. enflata, S. minima, S. nagae and S. scrippsae at stations C11 in the Oyashio water and C31 in a
KWCR to the east of northern Japan
Species
Station C11
Vertical distribution (m)
Station C31
Peak abundance per
Vertical distribution (m)
haul (ind. m–3)
Peak abundance per
haul (ind. m–3)
Sagitta enflata
Mostly 0–100
0.04–0.21
Mostly 0–100
0.93–3.07
Sagitta minima
Mostly 0–100
0.04–0.21
Mostly 0–100
3.12–7.24
Sagitta nagae
Mostly 0–100
0.04–0.63
Mostly 0–100
0.34–1.09
Sagitta scrippsae
Mostly 0–200
0.08–0.21
0–800 with random peaks
0.29–0.42
through mesopelagic region
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Fig. 5. Length–frequency distributions of E. hamata at stations C11 in
the Oyashio water and C31 in a KWCR to the east of northern Japan.
Fig. 6. Length–frequency distributions of S. elegans at stations C11 in
the Oyashio water and C31 in a KWCR to the east of northern Japan.
Fig. 7. Abundance (ind. m–3) and vertical distribution of E. hamata size classes ‘<10’, ‘10+’ and ‘15+’ at stations C11 in the Oyashio water and C31
in a KWCR to the east of northern Japan. The times shown are for the middle of each haul. Night-time hauls are in black.
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Compared with E. hamata, S. elegans had a greater
proportion of large individuals in the Oyashio water:
individuals >15 mm accounted for 71.7% of the S. elegans
collected. Like E. hamata, multiple maturity stages were
represented: stages I, II, III and IV comprised 6.2, 59.3,
23.5 and 11.1% of the sample, respectively. Within the
KWCR, 90% of the S. elegans collected were juveniles
<15 mm in length.
DISCUSSION
This study found that the chaetognaths in the KWCR
had low abundances (mean = 1.75 ± 0.321 ind. m–3) and
a high species diversity (H9 = 1.78–2.05) compared with
Oyashio water (mean abundance = 2.15 ± 0.116 ind.
m–3; H9 = 0.56–0.80). This is probably linked to the
diVerences in productivity of the two areas. High species
richness is often associated with regions of low productivity (Angel, 1993) and the primary productivity in
KWCRs tends to be lower than that in the Oyashio
water (Hama, 1992).
We found that the assemblage of chaetognaths in the
KWCR was quite similar to the assemblages observed in
Kuroshio Current water during previous studies. Kuroda
conducted extensive sampling surveys through the
Kuroshio Current south of Japan which collected 23
species of chaetognaths representing Wve genera, and
found that S. enXata, S. regularis, Sagitta paciWca (winter
and spring) and Krohnitta paciWca (summer and autumn)
were Kuroshio water indicator species (Kuroda, 1976,
1977). In addition, Pterosagitta draco, Sagitta lyra (especially
winter), Sagitta hexaptera, Krohnitta subtilis and Sagitta
pseudoserratodentata were recognized as oVshore water
indicator species. The mixed-water species S. minima
was regarded as an important species in terms of abundance through the Kuroshio Current (Kuroda, 1976);
however, due to its adaptation and presence in a wide
range of environments, it was not regarded as an
indicator species of Kuroshio water (Furuhashi, 1961).
All of these indicator species except S. hexaptera and
S. pseudoserratodentata were recovered from our hauls in
the KWCR.
Furthermore, only three species present in our KWCR
collection did not appear in Kuroda’s collection from the
Kuroshio Current: S. elegans, E. bathypelagica and S. scrippsae.
This is most likely due to the shallow sampling depth
(150 m) of Kuroda’s surveys, because all three of these
species were caught in deep hauls (1000 m) in Sagami Bay,
and E. bathypelagica and S. scrippsae were also collected in
Suruga Bay (Marumo and Nagasawa, 1973). Both bays
are on the southern edge of Japan, receive strong input
from Kuroshio water and have temperature–salinity
proWles similar to a KWCR (Marumo and Nagasawa,
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1973). Sagilta elegans is present in deep water in Sagami
Bay, entering with the Oyashio Current, which forms the
intermediate water of the bay, but does not appear to be
present in the warm Kuroshio water found at shallower
depths inside or outside the bay (Marumo and
Nagasawa, 1973; Kuroda, 1976). Since S. elegans is
known to migrate vertically (Alvariño, 1965; Kotori,
1976; Terazaki and Miller, 1986), it has been suggested
that it enters the core water of KWCRs by migrating up
from intermediate waters (Terazaki, 1992). The presence
of S. elegans in the surface waters in the KWCR of this
study suggests that, in addition to vertical migration, it
may also be entrained from surrounding Oyashio waters
and carried within the surface layers into the ring center
by the mixing action of the ring water. Olson and Backus
reported similar transport of a cold-water Wsh to the
center of a warm-core Gulf Stream ring and suggested
that this mechanism was widely applicable to planktonic
plants and animals (Olson and Backus, 1985). A similar
entrainment mechanism is missing in the Kuroshio
Current and Sagami Bay, plus there are no neighboring
cold surface waters from which to entrain S. elegans. The
S. elegans of Sagami Bay can probably only enter the
Kuroshio water by vertical migration from the intermediate water and most likely cannot adapt to the
warm environment well enough to rise all the way to
the surface. The combination of these factors is probably
why S. elegans is not found in the surface waters of
Kuroshio Current water south of Japan but did appear
in the KWCR of this study.
The Kuroshio Current waters and the KWCR also
showed similarities in species diversity and abundance.
We calculated species diversity (H9) from Kuroda’s mean
summer data for Kuroshio water to be 2.07, comparable
with our values of 1.78–2.05 in the warm-core ring. We
considered it legitimate to compare our data with
Kuroda’s summer data because the ring from our study
was formed in August. The species S. enXata, S. minima
and S. regularis dominated Kuroda’s summertime data set
and were also some of the most abundant species in our
data set (Table I). Furthermore, the seasonal nature of
the Kuroshio indicator species and time of ring formation matched well with their respective numbers in our
collection.
Because of the higher temperature and salinity, the
KWCR is probably a better environment for mixedwater and warm-water chaetognath species than for
cold-water species. As would be expected, increased
abundance of warm- and mixed-water species in the
ring coupled with decreased abundance of cold-water
species was found during this study. These Wndings are
similar to those of Terazaki, who suggested that S. elegans
and E. hamata could not survive in the central core water
1286
T. B. JOHNSON AND M. TERAZAKI
j
CHAETOGNATHS IN A WARM-CORE RING AND OYASHIO WATER
of a KWCR because of the temperature and salinity
Xuctuations, while S. nagae, S. minima and S. scrippsae
were able to adapt and live there (Terazaki, 1992).
Although in this study the abundance of mixed- and
warm-water species was found to be higher in the ring,
overall chaetognath abundance was higher outside the
ring compared with inside due to large numbers of
E. hamata.
Although S. scrippsae have been previously reported in
Oyashio water (Kotori, 1976), their increased abundance
in the ring during this study suggests that a KWCR
might be a better environment for them. Sagitta scrippsae
are often found in cold or cool waters throughout the
PaciWc Ocean, such as in the Alaska Gyre, California
Current and Bering Sea (Alvariño, 1965; Kotori, 1972).
However, in the eastern PaciWc, they appear to show the
characteristics of a mixed-water species, occurring in
abundance where the Kuroshio and Oyashio currents
meet (M. Terazaki, unpublished data) as well as in
KWCRs (Terazaki, 1992).
In the Oyashio region, the vertical distributions of the
six chaetognaths species that were analyzed in this study
(Figure 4; Table II) were consistent with previous data on
those species from other regions (LeBrasseur, 1959;
Terazaki and Miller, 1986; Sameoto, 1987; Duró et al.,
1994; Nishihama, 1998). Within the KWCR, this study
agreed with vertical distributions outlined by Terazaki
(Terazaki, 1992). Only the vertical distribution of
E. hamata and S. scrippsae was noticeably diVerent within
the ring compared with outside. Eukrohnia hamata were
present in small numbers below the core water of the ring
and seemed to persist in the KWCR area by staying
mostly below the warm-water region where temperatures
were similar to the surrounding Oyashio area. The same
deep distribution pattern of this species was seen in the
waters of Sagami and Suruga bays, which are inXuenced
by the Kuroshio Current (Marumo and Nagasawa,
1973). The reported temperature range for E. hamata is
between –2 and 16.5 C, and for S. elegans it is between
–0.5 and 21 C (Alvariño, 1965), and this may explain
why S. elegans was present in the warm surface waters of
the ring while E. hamata was not. However, it does not
explain why over twice as many E. hamata as S. elegans
were collected from within the ring.
The distribution of small E. hamata below large
E. hamata in the KWCR is one of the more interesting
Wndings of this study. The concentration of small
E. hamata near the surface and larger E. hamata in deeper
layers, as seen in the Oyashio water, is a phenomenon
that has been documented previously in the Antarctic
(Øresland, 1995), Arctic (Sameoto, 1987; Timofeev,
1998) and the North PaciWc (Sullivan, 1980). This segregation among depths is believed to be part of the life cycle
of E. hamata, with sexually mature adults spawning in
deep mesopelagic waters, followed by upward migration
of hatched juveniles to the surface, and then a return to
mesopelagic layers as they mature (Sullivan, 1980;
Hagen, 1985; Sameoto, 1987; Øresland, 1995;
Timofeev, 1998; Duró and Gili, 2001). Duró reported
juvenile E. hamata in the Weddell Sea to be seasonally
distributed in mesopelagic layers immediately following
hatching before they made their way back to the surface
layer (Duró and Gili, 2001). Outside this period following
hatching, our study provides the Wrst report of immature
E. hamata being distributed below larger individuals of
which we are aware. The warm-core ring environment
may have caused this phenomenon, but the sample sizes
of this study are too small to reach a statistically significant conclusion and further study in both KWCRs and
the Kuroshio Current will be required to adequately
investigate this anomaly.
Such alteration of their vertical distribution would
probably inhibit the growth and development of E. hamata,
because vertical distribution is an important part of their
life cycle, as described above. Yet the highly similar standard lengths observed at both stations suggest that
growth in body size is unaVected. Sexual development,
however, may be aVected by this alteration of vertical
distribution. Although there was a higher percentage of
large E. hamata inside the ring compared with outside,
virtually none of the individuals inside the ring had
matured past stage I, but outside there was a fairly even
mix of stages I, II and III. This is probably not a response
to temperature because higher temperatures tend to
encourage faster maturation at a smaller body size
(McLaren, 1963). Furthermore, they did not appear to
be undernourished, because inside the ring 7% of the
E. hamata population had food in their gut, while outside
5% had food in their gut. One possible explanation for
this is that the E. hamata inside the ring may be shifting their
energy resources towards growth rather than reproduction as they try to cope with the warmer environment
within the ring.
With respect to S. elegans, the uniformly small body
length in the KWCR could be caused by temperaturerestricted growth and development or an inability to
survive among large, mature individuals inside the ring.
The Belehrádek function used by McLaren (McLaren,
1963) showed the mean length of adult S. elegans to be
inversely proportional to temperature, i.e. small-sized
adults being found in warm water. However, no mature
adult S. elegans were collected within the ring and most of
them lacked any signs of sexual development. Therefore,
it appears that large, mature S. elegans cannot survive in
the KWCR. Juvenile S. elegans in the subarctic PaciWc
Ocean have been shown to inhabit a wider range of
1287
JOURNAL OF PLANKTON RESEARCH
j
VOLUME
temperatures compared with those at more advanced
stages of sexual development (Terazaki et al., 1995), and
this is a likely explanation for the results of this study.
While it will take additional research to validate this
assumption, it appears that it is possible that the
warm-core ring is aVecting S. elegans in a deleterious manner.
A limitation of the data set compiled for this study was
the lack of samples from within the Kuroshio Current
oV the south-eastern coast of Japan. Future studies
comparing the chaetognath assemblages and their depth
distributions within the Oyashio water and a KWCR
should incorporate sampling within the Kuroshio Current
in order to verify whether the results recorded within the
ring are unique to the ring environment or simply reXect
the relationship of chaetognaths to warm Kuroshio water
in general.
25
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PAGES
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2003
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ACKNOWLEDGEMENTS
We thank the captain and crew of the R/V ‘Hakuho
Maru’ for their assistance during the KH-98-4 cruise.
Special thanks to Dr M. J. Miller for several critical
reviews of the manuscript. In addition, we thank
Drs C. B. Clarke, M. J. Gibbons and S. Toczko for
their constructive comments on the manuscript.
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