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ORIGINAL ARTICLE
Vertical distribution, population structureand developmental characteristics of theless studied but globally distributed
mesopelagic copepod Scaphocalanus magnus
in the western Arctic Ocean
ATSUSHI YAMAGUCHI

1,2, *, CARIN J. ASHJIAN3 , ROBERT G. CAMPBELL4 AND YOSHIYUKI ABE5

1

graduate school of fisheries sciences, hokkaido university, 3-1-1 minato-cho, hakodate 041-8611, japan, 2 arctic research centre,
hokkaido university, kita-21 nishi-11 kita-ku, sapporo 001-0021, japan, 3 biology department, woods hole oceanographic institution, 266
woods hole rd, woods hole, ma 02543, usa, 4 graduate school of oceanography, university of rhode island, narragansett, ri 02882, usa,
and 5 ura station, hokkaido university, kita-21 nishi-10 kita-ku, sapporo 001-0021, japan*corresponding author: a-yama@ﬁsh.hokudai.ac.jp
Received August 2, 2019; editorial decision March 27, 2020; accepted April 3, 2020
Corresponding editor: Marja Koski

Scaphocalanus magnus is a large copepod species with a worldwide distribution. Information on the ecology of this species
is scarce. In this study, S. magnus was collected using vertically stratiﬁed net sampling from an ice station in the western
Arctic Ocean over a year. The vertical distribution differed between periods of polar night and midnight sun, with core
depth distributions of 264–381 m for the polar night and 518–745 m for the midnight sun. The shallower distribution
during the polar night may have resulted from the animals moving upwards to ﬁnd sufficient food during the lessproductive season. The abundance of early copepodite stages showed clear seasonality. The C2 and C3 stages wereabundant June–August and August–September, respectively. After C4, seasonality in abundance was not clear. For C4
and C5, body sizes and masses were greater for males than for females while the opposite was the case for C6. Adult
males do not feed and thus have shorter life spans than females, skewing the adult sex ratio towards females. For thesemesopelagic particle feeding copepods, a long residence time at C6F suggests that in a varying food environment, it ispossible to wait to initiate reproduction until favourable food conditions occur.
KEYWORDS: copepods; mesopelagic; life cycle; vertical distribution

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J. Plankton Res. (2020) 42(3): 368–377. First published online May 19, 2020 doi:10.1093/plankt/fbaa021

A. YAMAGUCHI ET AL. |MESOPELAGIC COPEPOD SCAPHOCALANUS MAGNUS IN THE WESTERN ARCTIC OCEANINTRODUCTIONMETHOD
Zooplankton samples were collected during the year-long
Surface Heat Budget of the Arctic Ocean (SHEBA) expedition (e.g. Ashjian et al., 2003) from along the ice station
drift track through the Canada Basin to the Mendeleev
Plain (Electronic supplement 1). Stratiﬁed vertical hauls(nominally 0–50, 50–100, 100–200, 200–1500 and 1500–
2800 m, depending on bottom depth) were made tojust above the seaﬂoor at 10–14 days intervals (total 30
sampling dates) using a 1-m mouth diameter opening–closing net equipped with 53 or 150 μm mesh from27 October 1997 to 29 September 1998 (Ashjian et al.,2003) (Electronic supplement 2). The samples were preserved with 4% buffered formalin seawater. Vertical proﬁles of temperature were collected using a conductivity–
temperature–depth package. Previous analyses of thesesamples had enumerated only the dominant zooplankton
taxa (Ashjian et al., 2003). In the present analysis, lessabundant or rare species were quantiﬁed by examining entire samples rather than only quantitative portions(splits).
S. magnus were sorted by copepodite stages. Morphologically, females and males were distinguishable at theC4–C6 stages. At stages C4 and C5, females have a thin,short swimming leg 5 (P5) that extends only to the ﬁrst 1–2
segments of the urosome while males have thicker, longer
P5s that extend to segments 2–3 of the urosome. For
details on the morphology of each stage, see Electronic
supplement 3 (photographs). A small number of C1 stagesof S. magnus was present, likely because the diagonal sizeof the mesh (212 μm) dictated that only stages >C2[prosome width: 297 ± 19 μm (mean ± standard deviation, n = 9)] were collected quantitatively. While we used
53 μm mesh nets for several occasions, because of theusage period (2 months) and depth (0–325 m) were limited(see Electronic supplement 2), the effect of the differencesin collection mesh size on the results of this study may
be ignored for the deep-dwelling S. magnus. Adult female(C6F) specimens with spermatophores attached also wereidentiﬁed (Electronic supplement 3).

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Scaphocalanus magnus is a calanoid copepod of 4.5–5.2 mm
total adult length belonging to the Scolecitrichidae
and having a worldwide distribution from the Arctic
to the Antarctic, including tropical regions (Brodsky,
1950; Tanaka, 1969). To date, there is no evidenceof cryptic speciation based on worldwide molecular
genetic/phylogeographic analyses for S. magnus (Ann
Bucklin personal communication). S. magnus is a deepwater species, synonymous with Scaphocalanus acrocephalus
and Scaphocalanus cristatus (Vervoort, 1957; Park, 1982). In
the Arctic Ocean, although a minor component of total
zooplankton biomass, S. magnus occurs consistently (Johnson, 1963). During summer, S. magnus (=S. acrocephalus)
is found at 100–300 m and accounts for 0.22–0.28%
of the abundance and 1.33–3.58% of the biomass ofthe total zooplankton community in the Arctic basins(Amundsen, Makarov and Canada Basin) (Kosobokova
and Hirche, 2000). For the Paciﬁc-side western Arctic
Ocean, S. magnus is reported to be an oceanic deep-sea
species and characteristic of the basin (Kosobokova andHopcroft, 2010; Matsuno, 2014; Smoot and Hopcroft,2017a).
Available ecological information on S. magnus mostly
focuses on feeding modes, reproduction and others. S.
magnus have specialized vermiform and brushform sensory ﬁlaments on the maxilla and two long vermiform
sensory ﬁlaments and one short brushform sensory ﬁlament on the maxilliped (Park, 1982). Consequently,
S. magnus are thought to be omnivorous, consuming awide range of food items ranging from phytoplankton to
micro- and mesozooplankton and detritus (Arashkevich,
1969; Blachowiak-Samolyk et al., 2007; Pomerleau et al.,2015). S. magnus is a broadcast spawner, producing eggs
that are large relative to their body size and with thesmallest clutch size (1.6 eggs/clutch) reported for calanoid
copepods (Kosobokova et al., 2007; Brun et al., 2017). The
carbon and nitrogen mass, protein content and electrontransport system activity of S. magnus are reported only forthe subarctic Paciﬁc (Ikeda et al., 2006). While bioluminescence is common in the mesopelagic copepods, S. magnus
do not exhibit bioluminescence (Takenaka et al., 2012).
S. magnus is known to be a host of epibiont suctorianciliates (Yamaguchi, 2006). While such ecological information is valuable, little information is available for population structure, life cycle and chemical composition,
despite the possible worldwide distribution of this species.
In the present study, vertical distributions, seasonal
population structure changes, life stage-speciﬁc lengthand mass changes, and moult intervals over an annual
cycle were investigated for S. magnus using vertically stratiﬁed samples collected every 10–14 days over a single year

(1997–1998) at an ice station in the western Arctic Ocean.
Since ambient environments (e.g. light or food availability)
in the Arctic Ocean show greater seasonality, seasonal
phenology is expected to be present even for the deepsea copepods. Adding vertical distribution and population
structure observations, measurements of length and mass
with each copepodite stage (female and male separation
were also made for juveniles based on their morphology)
provides deep insights into their development and growth.

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Seaﬂoor depth varied between 352 and 3850 m over
the year-long drift track (Ashjian et al., 2003). Becauseof this, sampling depths varied from two to eight (number of samples on each date) over 0–100 to 0–3500 m,
respectively (Electronic supplement 2). Since S. magnus
was found primarily below 250 m (see below, Fig. 1),
samples with maximum sampling depths shallower than
400 m were omitted from the analysis, reducing the number of available sampling dates to 20 (from an original30). However, the time interval between the 20 successive dates was at most 1 month, permitting analysis ofseasonal abundance and population structure (Electronic
supplement 2).
The core depth distribution (D50% ) was calculated
based on abundances (ind. m−2 ) in each sampling layer(Pennak, 1943):
D50% = d1 + (d2 − d1 ) ×

50 − p1p2

where d1 is the depth (m) of the upper depth of the 50%
individual occurrence layer, d2 is the maximum depth (m)of the 50% individual occurrence layer, p1 is the cumulative individual percentage (%) that occurred at depths
shallower than the 50% individual occurrence layer, andp2 is the individual percentage (%) at the 50% individual
occurrence layer. The means and standard deviations of
the D50% of each copepodite stage were calculated during
the period of the midnight sun (late April–August) and

PAGES 368–377

2020

of the polar night (mid-November–early February), andthe differences between them were tested with Mann–
Whitney U test. For the evaluation of vertical distributions, data with maximum sampling depths shallower
than 900 m and data collected during transitions outsideof the periods of the midnight sun or polar night wereomitted. According to these criteria, three dates during
the midnight sun and six dates during the polar night wereretained. For details of sampling depths and dates, seeElectronic supplement 2. Ontogenetic changes in vertical
distribution were tested by one-way analysis of variance(ANOVA) on the D50% of each copepodite stage during
the midnight sun and polar night.
Body length (prosome length: PL) and masses (wetmass; WM, dry mass; DM, ash-free dry mass; AFDM)
were measured for each stage using formalin-preserved
samples. Specimens of each copepodite stage wereselected and photographed using a stereomicroscope.
The PLs were measured from the images with 10 μm
precision using ImageJ (https://imagej.nih.gov/ij/). The
WM was determined for batches of pooled specimens(3–19 individuals of each stage) that had been brieﬂyrinsed in distilled water, blotted dry on a ﬁlter paper andweighted in a pre-weighed combusted aluminium pan.
Those samples then were placed in an oven at 60◦ C for 5 h
and weighed after drying to determine DM. Ash content
was determined by weighing the same specimens before
and after combustion at 480◦ C for 5 h; AFDM was thendetermined by difference. For all determinations, a mass
was determined to a precision of 1 μg using a CAHN (C33) microbalance. Moult increments (MI, %) at Cn-1 /Cnstage in PL, WM, DM and AFDM were calculated
according to the equation of Mauchline (1998):
MI = 100 × (value of Cn —value of Cn-1 )/value ofCn-1 .
To identify at which stage the growth of PL andmasses are achieved, the proportion of growth (PG, %) at
Cn-1 /Cn for PL, WM, DM and AFDM was determined
for females and males, treating the values of adults as
100%:
PG = 100 × (value of Cn —value of Cn-1 )/value ofadult (C6).

RESULTS
Vertical distribution
The mean vertical distribution core (D50% ) of eachcopepodite stage varied from 264 m (C6F, polar night)
to 745 m (C2, midnight sun) (Fig. 1). Depth differences
between the midnight sun and the polar night wereobserved for stages C3–C6F (note that stage C2 did not

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Fig. 1. Ontogenetic changes in the core depth distributions for S.
magnus. Mean D50% , (symbols) and standard deviations (bars) are shown.
Open and solid symbols denote data collected during the periods of themidnight sun (late April to August, n = 3) and of the polar night (midNovember to early February, n = 6), respectively. The mean temperatures during the midnight sun (solid line) and polar night (dotted line) are
shown in the right panel. Differences in D50% between midnight sun andpolar night were tested using a U test. ∗ : P < 0.05, ∗∗ : P < 0.01, ns: notsigniﬁcant. Ontogenetic changes within each season were not signiﬁcantfor both midnight sun and polar night (P > 0.05, one-way ANOVA).

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A. YAMAGUCHI ET AL. |MESOPELAGIC COPEPOD SCAPHOCALANUS MAGNUS IN THE WESTERN ARCTIC OCEAN

Population structure
During the observation period, there were three transitions in water mass type in the upper water column(Ashjian et al., 2003) that had little effect on theabundances of each copepodite stage of S. magnus(Fig. 2). Clear seasonal changes were detected forabundances of C2 and C3, with C2 abundant from
June to August, and C3 dominant from August to
September. C4 was abundant in August–September
and January–February, and C5 and C6 were numerous
during May–June. The sex ratio was ∼1:1 in C4 and C5[females were 51.4 ± 15.6% of C4s and 47.1 ± 11.4%(mean ± standard deviation)] of C5s]. However, the sex
ratio was skewed towards females at C6 with 84.2 ± 7.8%of that stage being females. Spermatophores attached to
C6F were very scarce, observed seasonally from Juneto September for only 0–7.7% of the C6F (Fig. 2).
On average, the proportion of females were highest inC6 (44.3 ± 14.5%) while the proportion of males werehighest in C5 (54.6 ± 14.1%) (Fig. 2). Seasonal changes
in the relative proportions of C4–C6 were observed
for males, with high proportions of C4, C5 and C6
during October–January, February–July and April–July,
respectively. However, the relative proportions of thosestages remained fairly constant seasonally for females.

Fig. 2. Seasonal abundance of each copepodite stage of S. magnus.
Females and males were morphologically distinguishable for stages C4–
C6. Stage compositions within the female and male populations areshown in the bottom panels. Arrows in the upper panel indicate timings
of water mass changes (cf. Ashjian et al., 2003). Solid triangles in
the C6 panel represent occurrences of spermatophore-attached C6F.
Horizontal dashed lines and values in the parentheses show annual
mean abundances for each stage (ind. m−2 ).

Length and mass
PL ranged from 1.03 mm (C2) to 3.80 mm (C6F) andincreased with development (Table I). Masses (Y : WM,
DM and AFDM, μg) and PLs (X : mm) had highly signiﬁcant relationships, expressed as power functions (Y = a
Xb ) (r2 = 0.991–0.993, P < 0.0001), with b between 3.024
and 3.349. Males tended to be larger and heavier thanfemales at stages C4 and C5 (Table II). In particular, themasses of C5M were 1.39–1.67 times heavier than thoseof C5F (P < 0.0001; U test, Tables I and II). Female C6s
tended to be longer (PL) and heavier in WM (P < 0.0001)

than male C6, but no differences were observed for DM
and AFDM (Table II).

Chemical composition
The water content ranged between 75.6 ± 1.6% WM(mean ± standard deviation) of C5M and 85.9 ± 2.6%
WM of C4F, and was signiﬁcantly different between
stages (P < 0.0001, one-way ANOVA, Table I). The

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occur in polar night samples). The D50% of C3–C6F
ranged from 518 to 745 m (653 ± 86 m, mean ± standard
deviation) during the midnight sun but from 264 to
381 m (322 ± 45 m) during the polar night. Thus, thedistribution depths of C3–C6F were shallower during thepolar night than during the midnight sun with differences
ranging from 25 to 395 m (331 ± 51 m) (P < 0.05, Utest). In contrast, the D50% of C6M varied between
628 and 735 m throughout the year and no differenceswere detected between midnight sun and polar night(P = 0.593). Ontogenetic changes in vertical distribution
were not detected for C2–C6F/M either during midnight
sun or polar night (P = 0.052–0.205, one-way ANOVA).

1.03 ± 0.03
(9)
1.54 ± 0.06
(9)
2.02 ± 0.13
(14)
2.29 ± 0.09
(12)
2.97 ± 0.06
(9)
3.20 ± 0.07
(10)
3.80 ± 0.08
(10)
3.50 ± 0.08
(10)

C2

9.795
3.349
0.991
<0.0001

AFDM-PL(P < 0.05)

df
7
F
24.3
P
<0.0001
Tukey-Kramer C5M

WATER

one-way ANOVA

11.11 ± 1.00
(4)40.00 ± 3.49(6)
100.6 ± 14.4
(6)
141.1 ± 11.1
(6)
387.8 ± 18.3
(5)
647.6 ± 75.0(5)706.6 ± 79.9
(5)640.2 ± 28.1
(5)

AFDM (μg)

C6M C5F C6F C4M C3

84.4 ± 0.6(4)83.1 ± 1.3(6)85.9 ± 2.6(6)82.4 ± 1.2(6)79.2 ± 0.8(5)75.6 ± 1.6(5)79.4 ± 2.2(5)
78.5 ± 0.6
(5)

WATER
(% of
WM)

C2

C4F

7
5.43
<0.0001
C4F C4M

AFDM

91.7 ± 1.4(4)92.5 ± 0.9(6)90.8 ± 0.5(6)91.4 ± 0.5(6)92.1 ± 0.7(5)94.1 ± 0.8(5)91.5 ± 1.2(5)92.7 ± 0.3(5)

AFDW (%of DM)C6F C2C5FC3

C6M C5M

PAGES 368–377

The number of replicates is shown in the parentheses. For analysing body allometry, the power regression model Y = aXb was used, where Y is WM, DM or AFDM (μg) and X is PL (mm).
Differences between stages in WATER and AFDM were tested by one-way ANOVA and post hoc test (Tukey-Kramer). Any two stages not underscored by the same line are signiﬁcantly different.

75.34
10.69
3.024
3.341
0.993
0.992
<0.0001 <0.0001

DM-PL

12.13 ± 1.28
(4)43.27 ± 3.81
(6)
110.7 ± 15.5
(6)
154.4 ± 11.9
(6)421.2 ± 19.1
(5)687.9 ± 74.7
(5)
771.5 ± 78.2
(5)
690.5 ± 30.2
(5)DM (μg)NUMBER 3

Constant (a)Power (b)
r2
P

WM-PL

Regression statistics for body allometry

77.71 ± 6.02
(4)
257.2 ± 20.4
(6)800.2 ± 138.6(6)
881.9 ± 72.6
(6)2024 ± 87(5)2810 ± 156(5)3747 ± 119(5)3214 ± 106(5)WM (μg)VOLUME 42

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372

C6M

C6F

C5M

C5F

C4M

C4F

C3

PL (mm)Stage

Table I: Summary data as the mean ± standard deviation of prosome length (PL), wet mass (WM), dry mass (DM), ash-free dry mass (AFDM),
water content (WATER, % of WM) and organic content (AFDM, % of DM) for S. magnus

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A. YAMAGUCHI ET AL. |MESOPELAGIC COPEPOD SCAPHOCALANUS MAGNUS IN THE WESTERN ARCTIC OCEAN

Table II: Comparisons between S. magnus female and male prosome length (PL), wet mass (WM), dry
mass (DM) and ash-free dry mass (AFDM) for stages C4–C6
Stage

ns

F=M
F < M∗∗∗∗F > M∗∗∗∗

WM

DM

AFDM

F < M∗∗∗∗F < M∗∗∗∗F < M∗∗∗F < M∗∗∗∗F < M∗∗∗F < M∗∗∗∗
F = Mns

F > M∗∗∗∗

F = Mns

For raw data of each parameter, see Table I. Differences between female (F) and male (M) were tested using the Mann–Whitney U test.
∗∗∗ : P < 0.001, ∗∗∗∗ : P < 0.0001, ns: not signiﬁcant.

Fig. 3. Moult increment (MI) in prosome length (PL) and masses for S.
magnus (WM: wet mass, DM: dry mass, AFDM: ash-free dry mass). Raw
data are shown in Table I.

organic content was between 90.8 ± 0.5% DM at C4F
and 94.1 ± 0.8% DM at C5M and also was signiﬁcantly
different between stages (P < 0.0001, one-way ANOVA,
Table I). It is notable that males always had lower watercontent and higher organic content than females within
the same copepodite stage (Table I).

Moult increment
The MI of PL of adjacent stages were 9.4–49.3%, while
those of masses (WM, DM and AFDM) ranged from
−1.1 to 358.8% and showed greater differences (Fig. 3).
The MI also differed substantially between females andmales. For both C3/C4 and C4/C5, MIs of males werelarger and greater than females, especially for the massmeasurements. For the moult from C5 to C6, all female
growth was positive while male growth was extremely
small, including negative growth in AFDM.
The proportion of PL growth (as a percentage of themean adult female and male values) did not vary with astage between males and females, with the largest proportions for both seen for the <C2 (27.1–29.4%) (Fig 4). By
contrast, proportions for changes in mass (WM, DM and
AFDM) in the older stages differed between males andFig. 4. The proportional increase in length and mass as a percentage of
the C6 values by the stage for S. magnus. Raw data are shown in Table I.
(PL: prosome length, WM: wet mass, DM: dry mass, AFDM: ash-freedry mass).

females. Females had the greatest increase in mass (45.1–
46.0%) as C6 with substantial increases also during C5
while males had the largest increase (60.0–79.1%) as C5.

DISCUSSION
Vertical distribution
In the present study, C3–C6F of S. magnus were distributed at 518–745 m during the period of the midnight
sun and at shallower depths, 264–381 m, during the polarnight (Fig. 1). By contrast, C6M was found between 628
and 735 m with no day–night difference. Since water temperature did not vary between polar night and midnightsun, the seasonal changes in vertical distributions do notappear to be related to water temperature.

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C4
C5
C6

PL

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2020

In the Arctic Ocean, because of the constant solar radiation during summer, primary production and exportsinking ﬂux have seasonal peaks in summer (Lalande et al.,2019). Conversely, during the polar night, without radiation, primary production is much reduced or absent andparticle ﬂux to the deep sea is lower (Lalande et al., 2019).
Sinking particle ﬂux from the surface layer in the ocean
is known to decrease exponentially with increasing depth(Suess, 1980; Pace et al., 1987). Therefore, during winter,S. magnus may ascend to the shallower depths to exploit thepresumably greater food availability. During C6, feeding
appendages of males degenerate, digestive tracts becomethread-like, and animals cease feeding (Harding, 1974),
so there is no need for them to ascend to ﬁnd foodduring the polar night (Fig. 1) and they can remain in the
deep layer during both midnight sun and polar night toavoid predation. This behaviour can be thought of as ananalogue to DVM, although on a longer temporal scale,since upwards migration is similarly motivated by foodavailability and downwards migration by avoidance of
predation.

Abundance and biomass
Abundances of S. magnus in the Arctic Ocean from previous studies are comparable to those seen in this study(ca. 100–>200 ind. m−2 , Table III). However, the biomassof S. magnus from around Svalbard (4 ± 15 mg DM m−2 )is much lower than in this study (65.2 ± 31.8 mg DM
m−2 ). Most of the sampling locations from that study
were in water depths of shallower than ∼500 m. It
may be that S. magnus is less abundant there because
they prefer greater water depths (518–745 m) during themidnight sun (Fig. 1). The abundance and biomass of S.
magnus in this study corresponded with the reported values
in the Canada Basin (Kosobokova and Hopcroft, 2010)(Table III).

Difference in development time between
females and males
Although cohort development to C2–C3 could be traced
for S. magnus, it was unclear after C4 stages (Fig. 2). This
uncertainty likely resulted because development times
differ between females and males. Comparisons of theratio of C4–C6 for each sex showed distinct differencesin cohort growth, especially for males (Fig. 2). The difference in development time between females and males
is most prominent in C6. Since C6M are not feeding,their longevity should be short. This short duration has amarked effect on the sex ratio of S. magnus in the C6 stage.
The sex ratio of C4 and C5 was ∼1:1, whereas in C6 it

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The vertical distribution of S. magnus has been reported
previously. In the Arctic Ocean central basin, S. magnus had a distribution centre at 50–500 m during thesummer (midnight sun) (Mumm et al., 1998; Kosobokova
and Hirche, 2000; Auel and Hagen, 2002). Elsewhere, S.
magnus occurred below 150 m in the Bering Sea (Minoda,1971), 495–775 m in the Oyashio region (Furuhashi,1966), 500–1000 m in the Sargasso Sea (Deevey and
Brooks, 1977), 570–950 m in the Canary Islands (Roe,1972) and 1000–3000 m in the equatorial Indian Ocean(Grice and Hulsemann, 1967). Thus, the distribution
depth of S. magnus becomes deeper at lower latitudes,
presumably as this cold-water species avoids warm surface
waters.
Diel vertical migration (DVM) is a common pattern
in calanoid copepod vertical distribution (e.g. Mauchline, 1998). However, the temporal sampling of the net
tows in this study did not permit identiﬁcation of true
DVM. The seasonal day–night differences are somewhat
analogous, although on a completely different temporalscale (months vs. a day). DVM of S. magnus, has beenreported in the North Atlantic Rockall Trough with the
day distribution peak at 500 m and the night distribution
peak at 100 m (Mauchline, 1998). In the Canary Islands,
peak abundance was reported at 800 m during the day
and at 660 m during the night, shoaling slightly (Roe,1972). On the other hand, no DVM was reported in the
Bering Sea (Minoda, 1971) or in the Arctic Ocean, likely
because the sampling was conducted in summer during
the midnight sun (Mumm et al., 1998; Kosobokova andHirche, 2000; Auel and Hagen, 2002).
Why are S. magnus found at shallower depths during
the polar night in this study? It has been reported thatdeep-sea copepods in polar regions have different vertical
distributions and preferred food items than other speciesof the same family and genus from outside polar regions(Laakmann et al., 2009). For the Scolecitrichidae, to which
S. magnus belongs, altering the vertical distribution andbody size in the mesopelagic layer is known to maintainsympatric species diversity (Kuriyama and Nishida, 2006).
In the Beaufort Sea Scolecitrichidae’ depth distributions
are known to differ within the family and genus, with
Scolecithricella minor at the shallowest depths (0–500 m), S.
magnus at intermediate depths (200–500 m) and Scaphocalanus brevicornis at greatest depths (300–1000 m) (Smootand Hopcroft, 2017b). Additionally, vertical partitioning within the Scaphocalanus genus has been reported in
the central Arctic Basin, with the shallowest for S. magnus, intermediate for S. brevicornis and the deepest for
S. polaris (Kosobokova et al., 2011). These observations
suggest that competition for food is strong in the Arctic
Ocean.

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A. YAMAGUCHI ET AL. |MESOPELAGIC COPEPOD SCAPHOCALANUS MAGNUS IN THE WESTERN ARCTIC OCEAN

Table III: Comparison of abundance and biomass of S. magnus in the Arctic Ocean
Location

Depth

Mesh size

Abundance

Biomass

References

(m)

(μm)

(ind. m−2 )(mg DM m−2 )Jun.–Oct.

0–500

150–300

100– > 200-

Mumm et al. (1998)

Aug.–Sep.
May–Aug.

0–1500
0–600

200
180

174.7
-

4 ± 15

Auel and Hagen (2002)
Blachowiak-Samolyk et al. (2008)

June–July
Year round

0–3000
0–2800150
53, 150200 ± 22∗176.0 ± 101.1

48.5 ± 3.5∗
65.2 ± 31.8

Kosobokova and Hopcroft (2010)
This study

Where available, means ± standard deviations are shown. ∗ : standard errors.

was largely skewed towards females (Fig. 2). In a study ofC6 S. magnus sex ratios in the Canary Islands, males were
only 2.7–12.1% of the total C6 population and females
predominated (Roe, 1972), similar to the annual average
percentage of adult males (15.8 ± 7.8%) from the present
study.
The proportions of C4–C6 in the female and malepopulations reﬂect the residence time in each stage ofdevelopment (cf. Mauchline, 1998). High proportions
were seen in the female C6 and male C5 stages, withfemales quickly developing to C6F and males spending
longer as C5M. It is well known that the reproductionof most copepods is dependent on food availability(Mauchline, 1998). Thus, spending a long time as C6F
permits females to reproduce in response to periodsof greater food availability within that stage duration(Yamaguchi and Ikeda, 2000b). Males are potential
competitors for available food. The cessation of malefeeding thus reduces competition for food with other life
stages. This may explain why the feeding appendagesof the C6M degenerate in several mesopelagic families(Euchaetidae, Aetideidae, Scolecitrichidae; Yamaguchi
and Ikeda, 2000b).

Developmental characteristics
In this study, weights were determined for formalinpreserved individuals, potentially resulting in underestimates because of the dissolution of organic matter duringstorage (Fudge, 1968; Hopkins, 1968). Nonetheless, themagnitude of the error caused by formalin preservation isrelatively small and unimportant for broad comparisons
between stages of dissimilar species (Gruzov and Alekseyeva, 1970). The DM of S. magnus has been reported
in the Greenland Sea to be 722 ± 23 μg for C6F and
635 ± 62 μg for C6M (Richter, 1994) corresponding
closely to the DMs measured in this study (772 ± 78 μg(C6F) and 691 ± 30 μg (C6M), respectively, Table I).
Here, allometric formulas were obtained between PL
and body masses (WM, DM and AFDM), with power (b)between 3.024–3.349. According to Mauchline (1998),
the power (b) of allometric equations between PL andbody mass of calanoid copepods usually lies between
2.5 and 3.3, corresponding to the values observed in thepresent study. The difference in power is considered tobe due to the body shape of each species (Gruzov andAlekseyeva, 1970).
The water content (75.6–85.9% WM) of S. magnus
measured in this study was in the range of most copepods (67–92% WM, Båmstedt, 1986). The organic matter (=AFDM) content (90.8–94.1% DM) of S. magnus
also corresponded well to the range (70–98% DM) andmode (92–94% DM) seen for copepods (Båmstedt, 1986).
Noteworthy here is that males had a lower water content
and a higher AFDM content than females when compared within the same developmental stages of C4–C6(P < 0.0001, one-way ANOVA, Table I).
MI of the copepodite stages of calanoid copepods are
known primarily for epipelagic species, with observed MI
in PL between 14.5 and 29.3% and in DM of 61.7–94.0%(Mauchline, 1998). Here, the MI of S. magnus was between
9.4 and 49.3% in PL and 0.4–345.5% in DM (Fig. 3). The
maximum reported values of MI for DM of mesopelagic
copepods are 498% (Heterorhabdus tanneri, Yamaguchi and
Ikeda, 2000a) and 550% (Paraeuchaeta elongata, Ikeda and
Hirakawa, 1996). Thus, the MIs, expressed as DM, of
mesopelagic copepods are much greater than those of
epipelagic copepods. For S. magnus, it is notable that theMIs of both C3/C4 and C4/C5 were larger in males thanin females, and this pattern was pronounced in DM and
AFDM (Fig. 3). On the other hand, in C5/C6, all female
growth was positive, but male growth was extremely
small, and negative growth was seen in AFDM.
S. magnus in the Arctic Ocean and the aetideid copepod Gaetanus (= Gaidius) variabilis, a medium-sized (PL of
3.02 mm in C6F) mesopelagic copepod found at 600–
1000 m in the Oyashio region, are similar in several
aspects of their ecology. Both are suspension feeders
and omnivores and both have feeding appendages thatdegenerate at C6M (Yamaguchi et al., 2005, 2007). At

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Nansen, Amundsen and
Makarov BasinCentral Arctic Ocean
Northern Svalbard
watersCanada Basin
Western Arctic Ocean

SeasonJOURNAL OF PLANKTON RESEARCH

VOLUME 42

CONCLUSION
Through this study, ecology (vertical distribution, population structure and developmental characteristics) of
mesopelagic copepod S. magnus was evaluated. Since
seasonality in environmental parameters (light andfood availability) is large, their vertical distribution andpopulation structure, especially early copepodite stages,
showed seasonality which related to their adaptation
for these conditions. In addition to these ordinary
observations (counting with each stage), measurements oflength and masses (WM, DM and AFDM) with each stage
throughout the life stages provide deep insights on thegrowth and development of the mesopelagic copepods.
This study emphasizes the importance and usefulness ofthe measurements of weights with each copepodite stage
especially for studies on the life cycle of the longer lived
mesopelagic copepods.

PAGES 368–377

2020

ACKNOWLEDGEMENTS
Many thanks to the crew members of the CCGS Des Groseilliers and to our collaborators at the SHEBA ice camp, especially Harold ‘Buster’ Welch, for assistance in collecting thesamples and CTD data. The collection of the samples was
supported in part by grants #OCE9707184 to C.J. Ashjian
and #OCE9707182 to R.G. Campbell from the US National
Science Foundation. Part of this study was supported by a Grantin-Aid for Scientiﬁc Research 19H03037 (B), 17H01483 (A)and 15KK0268 (Joint International Research) from the Japan
Society for the Promotion of Science (JSPS). This work waspartially conducted for the Japanese ‘Arctic Challenge for Sustainability’ (ArCS II) project. The comments and suggestions of
two anonymous reviewers are gratefully acknowledged.

FUNDINGUS National Science Foundation (#OCE9707184 to C.J.A.
and #OCE9707182 to R.G.C.). Grant-in-Aid for ScientiﬁcResearch (19H03037 (B), 17H01483 (A)). Joint International
Research from the Japan Society for the Promotion of Science(15KK0268).

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