Zoosymposia 19: 041–050 (2020)
https://www.mapress.com/j/zs
Copyright © 2020
· Magnolia Press

ISSN 1178-9905 (print edition)ZOOSYMPOSIAISSN 1178-9913 (online edition)
https://doi.org/10.11646/zoosymposia.19.1.9
http://zoobank.org/urn:lsid:zoobank.org:pub:CD4EAFC3-2ED4-4ABD-8F8E-650837159770

Community structure and seasonal changes in population structure of pelagic
polychaetes collected by sediment traps moored in the subarctic and subtropicalwestern North Pacific Ocean
KANAKO AMEI¹,*, NAOTO JIMI², MINORU KITAMURA³, NAOYA YOKOI4 & ATSUSHI YAMAGUCHI4,5
Faculty of Fisheries, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido 041–8611, Japan� kanakoamei@eis.hokudai.ac.jp, https://orcid.org/0000-0003-0418-0988
2
Bioscience Group, National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190–8518, Japan� beniimo7010@gmail.com, https://orcid.org/0000-0001-8586-3320
3
Japan Agency for Marine-Earth Science and Technology, 2–15 Natsushima-cho, Yokosuka, Kanagawa 237–0061, Japan� kitamura@jamstec.go.jp, https://orcid.org/0000-0002-8062-8177
4
Graduate School of Fisheries Science, Hokkaido University, 3-1-1 Minato-cho, Hakodate, Hokkaido 041–8611, Japan� naoya.yokoi@gmail.com
5
Arctic Research Center, Hokkaido University, Kita-21 Nishi-11 Kita-ku, Sapporo, Hokkaido 001–0021, Japan� a-yama@fish.hokudai.ac.jp, https://orcid.org/0000-0002-5646-3608*Correspondence: � kanakoamei@eis.hokudai.ac.jp
1

Abstract
Community structure and seasonal changes in the population structure of pelagic polychaetes were studied based on
zooplankton samples collected by sediment traps moored at 200 m depth in the subarctic and subtropical western North
Pacific throughout the year. Eight species belonging to seven genera and seven families occurred at the subarctic station,
while twelve species belonging to ten genera and seven families were identified at the subtropical station. Polychaete
abundance was 5.37 ± 0.44 ind. m-2 day-1 (annual mean ± standard error) at the subarctic station, and 1.36 ± 0.15 ind.
m-2 day-1 at the subtropical station. Polychaete abundance at the subarctic station was high from May to August, but
no seasonal patterns were observed at the subtropical station. The dominant species in the subarctic was Tomopteris
septentrionalis, which accounted for 62.9% of annual mean abundance; at the subtropical station, the dominant species
was Pelagobia sp. (22.8%). In the subarctic, small specimens of T. septentrionalis (<3 mm in body length) occurred only
in winter (December-March). No clear seasonal changes in population structure of the subtropical Pelagobia sp. weredetected. The latitudinal patterns we observed in the polychaete communities of the western North Pacific were similar to
those previously observed in the eastern North Pacific. Changes in the population structure of T. septentrionalis suggestthat the life cycle of this species is seasonal in the subarctic region.

Introduction
Pelagic polychaetes play important roles in food webs and in material fluxes in oceanic marine ecosystems(Jumars et al. 2015; Halanych et al. 2017). The pelagic polychaete Pelagobia longicirrata Greeff, 1879, forexample, is reported to be a herbivore (Day 1967; Hopkins 1985, 1987; Hopkins & Torres 1989), and is known
to be the prey of various taxa such as copepods and amphipods (Hopkins 1985; 1987). Polychaetes (mainly
P. longicirrata) account for 34% of the prey items consumed by the carnivorous chaetognath Pseudosagitta
gazellae (Ritter-Záhony, 1909) (Øresland 1990). Some carnivorous polychaetes such as Tomopteris (Lebour
1923; Rakusa-Suszczewski 1968; Fauchald & Jumars 1979) and Typhloscolex (Feigenbaum 1979) are known
to feed on chaetognaths. Poeobius meseres Heath, 1930 creates a mucus web in the water column, passively
collecting detrital particles on the web and then feeding on it with its ciliated tentacles (Uttal & Buck 1996).
Thus, the feeding modes of pelagic polychaetes are varied, and they may have an important role in the food
web and organic matter fluxes in the pelagic ecosystem.
Despite their importance, information about the ecology and life histories of pelagic polychaetes is scarce.
Submitted: 20 Sept. 2019; Accepted by Bruno Pernet: 24 Apr. 2020; published: 28 Dec. 2020

41

Licensed under Creative Commons Attribution-N.C. 4.0 International https://creativecommons.org/licenses/by-nc/4.0/

Most information about polychaete reproduction, development and life cycles concerns benthic species living in the neritic region (cf. Giangrande 1997 and references therein). For the pelagic realm, most ecologicalinformation on polychaetes concern planktonic larval stages in the neritic zones (cf. Blake 2017 and references
therein). Thus, most ecological information on polychaetes is derived from the neritic region, and limited ecological information is available for pelagic polychaetes (Halanych et al. 2007).
The paucity of ecological information (especially on seasonal patterns in abundance and on life cycles) for
oceanic pelagic polychaetes may due to the lack of sufficient sampling to collect seasonal time-series samples.
Few attempts have been made to collect seasonal time-series samples of polychaetes in the oceanic region(cf. Accornero et al. 2003; Guglielmo et al. 2014). One method to collect such samples is by using automatedsediment traps, which collect passively settling particles in an exposed sample cup for a certain period of time,then rotate to close that cup and expose a new one. Such traps can be moored in the field where they collectmultiple samples over the course of a year. Such samples have previously been used to estimate growth rates
and describe the life cycles of oceanic copepods and chaetognaths (Ohashi et al. 2011; Matsuno et al. 2014).
Though they may be very useful for studying pelagic polychaetes, as well, few attempts have been made to use
sediment traps for this purpose (Accornero et al. 2003).
In the present study, we evaluate community structure and seasonal changes in the population structureof pelagic polychaetes based on zooplankton swimmer samples collected by sediment traps moored at 200
m depths of the subarctic and subtropical western North Pacific throughout the year. For the most dominant
species in each region, body length and segment numbers were quantified, and their seasonal changes were
evaluated. We compare data we obtained on polychaete community structure in the western North Pacific withdata from the eastern North Pacific (California Current System) (Fernández-Alamo et al. 2003). Based on
seasonality in population structure, we identify some features of the life cycles of the dominant species.
Methods
Sampling location
With the goal of quantitatively evaluating carbon and material cycles, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) established two time-series sites in the western Pacific, one in the subarctic
gyre (St. K2: 47˚00’N, 160˚00’E, bottom depth: 5200 m), and one in the subtropical gyre (St. S1: 30˚00’N,
145˚00′E, bottom depth: 5700 m). As part of the “K2S1” project, JAMSTEC scientists have conducted timeseries sampling at these sites since 2010 (Honda 2016). Preliminary cruise reports and metadata from the K2S1
project are available from JAMSTEC “DARWIN” database (http://www.godac.jamstec.go.jp/darwin/e), andenvironmental data such as dissolved oxygen, nutrients, chlorophyll and fluxes of settling particles are available from the “K2S1 Database” (https://ebcrpa.jamstec.go.jp/k2s1/en/).
Sample collection
Samples were collected using time-series sediment traps (SMD26S-26 with 26 collecting cups, conical-shaped,
and an open mouth area of 0.5 m2; Nichiyu Giken Kogyo Co., Ltd., Kawagoe, Japan). These were moored at
a depth of 200 m at both subarctic (St. K2) and subtropical (St. S1) sites from 25 July 2013 to 15 May 2014
(St. K2) and 18 July 2013 to 4 July 2014 (St. S1) (Fig. 1). The traps were anchored with rope to the sea floor
at each station. A solution of 10% buffered formalin seawater was added to each trap cup before deployment.
Each sample cup was exposed for 7 or 14 days at St. K2, and 9 or 18 days at St. S1. After recovery, collected
samples were gently sieved on to a 1-mm mesh, and the fraction captured on the mesh (≥1 mm) was examined.
Note that samples were unavailable at St. K2 from 3 April to 1 May 2014. For details on the abundance of therest of the zooplankton community and on the abundance of particles <1 mm in size, see Yokoi et al. (2018)
and Honda et al. (2016), respectively.
Polychaetes
Pelagic polychaetes were sorted from whole zooplankton swimmer samples, identified to the lowest levelpossible under a stereomicroscope, and counted. Identifications were made using Dales (1957). Polychaete
abundance (ind. m-2 day-1) was calculated using the following equation:
Abundance = N / 0.5 / D
where N is the number per sample (ind. sample-1), 0.5 is the mouth area (m2) of the sediment trap, and D is thelength of time over which the trap sample was captured (day).

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To evaluate seasonal changes in polychaete abundance, abundance data were log-transformed (log [X+1])
to reduce bias due to quantitative redundancy. Similarities between samples were identified using the BrayCurtis method. To group the samples, similarity indices were coupled with hierarchical agglomerative clustering using an unweighted pair group method using the arithmetic mean. For detailed methods, see Yokoi et al.
(2018) and references therein.

FIGURE 1. Location of sampling stations. K2 is in the subarctic and S1 is in the subtropical western North Pacific. The
approximate directions of current flows are shown with arrows (Yasuda 2003).

For the dominant species at each station (Tomopteris septentrionalis Steenstrup, 1849 at St. K2 and Pelagobia sp. at St. S1), body length was measured from the tip of prostomium to end of the anal cirrus by theaid of an eye-piece micrometer or calipers, with a precision of 0.1 mm. Body length data for each month werepooled within season: spring (approximately April to June, slightly varied with the station), summer (JulySeptember), autumn (October-December), and winter (January-March), and histograms of size structure were
made. Since body length of T. septentrionalis had a normal distribution (p<0.05, Kolmogorov-Smirnov test),
seasonal differences in body length were tested by one-way ANOVA and post-hoc test (Tukey-Kramer). For
Pelagobia sp., body length was not normally distributed (p>0.05, Kolmogorov-Smirnov test), so seasonality
in body length was evaluated by the Welch test.

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Results
Polychaete species composition
A total of 572 individual pelagic polychaetes were collected at the two stations across the sampling period. At
the subarctic station (K2) a total of 10 species were collected, in seven families. Eight of these were identified
as members of seven genera; two were identified only to the family level. At the subtropical station (S1), a totalof 16 species were collected, in seven families. Twelve of these were identified as members of ten genera, andfour were identified only to the family level (Table 1). Members of five families – Alciopidae Ehlers, 1864;
Iospilidae Bergström, 1914; Lopadorrynchidae Claparède, 1870; Tomopteridae Grube, 1850; and Typhloscolecidae Uljanin, 1878 – occurred at both stations. The number of species belonging to Alciopidae, Lopadorhynchidae and Typhloscolecidae was greater at the subtropical station than at the subarctic station.
TABLE 1. List of species and families of pelagic polychaetes collected by sediment traps moored at 200 m of St. K2(25 July 2013–15 May 2014) in the subarctic and St. S1 (18 July 2013–4 July 2014) in the subtropical western North
Pacific.
K2
Alciopidae Ehlers, 1864
Rhynchonereella angelini (Kinberg, 1866)
Iospilidae Bergström, 1914
Iospilidae sp.
Phalacrophorus sp.
Lopadorrhynchidae Claparède, 1870
Pelagobia sp.
Polynoidae Kinberg, 1856
Polynoidae sp.
Spionidae Grube, 1850
Spionidae sp.
Tomopteridae Grube, 1850
Tomopteris (Johnstonella) pacifica (Izuka, 1914)
Tomopteris septentrionalis Steenstrup, 1849
Typhloscolecidae Uljanin, 1878
Travisiopsis sp.
Typhloscolex muelleri Busch, 1851

S1
Alciopidae Ehlers, 1864
Alciopidae sp.1
Alciopidae sp.2
Plotohelmis tenuis (Apstein, 1900)
Plotohelmis capitata (Greeff, 1876)
Rhynchonereella angelini (Kinberg, 1866)
Vanadis crystallina Greeff, 1876
Amphinomidae Lamarck, 1818
Amphinomidae sp.
Glyceridae Grube, 1850
Glyceridae sp.
Iospilidae Bergström, 1914
Phalacrophorus sp.
Lopadorrhynchidae Claparède, 1870
Lopadorrhynchus uncinatus Fauvel, 1915
Maupasia coeca Viguier, 1886
Pelagobia sp.
Tomopteridae Grube, 1850
Tomopteris sp.1 (no tail)Tomopteris sp.2 (with tail)Typhloscolecidae Uljanin, 1878
Travisiopsis sp.
Typhloscolex muelleri Busch, 1851

Community structure
A cluster analysis divided the samples into two large groups at 26% similarity, which corresponded with thetwo geographic stations (K2 vs S1) (Fig. 2). For polychaete species composition, three species groups wereidentified at 9% similarity. Differences in species groups were a reflection of abundance patterns at each station. One species group containing Maupasia coeca Viguier, 1886 and Plotohelmis spp. occurred only at St.
S1. The largest species group was divided into two species sub-groups: i.e. a species group characterized byhigh abundance at St. S1 (Pelagobia sp. and Travisiopsis sp.), and a species group with high abundance at St.
K2 (Tomopteris spp. and Typhloscolex spp.).
Annual mean polychaete abundance at St. K2 was 5.37 ± 0.44 ind. m-2 day-1 (mean ± standard error) andwas high from May to August (Fig. 3A). Throughout the year, T. septentrionalis was dominant and accounted
for a mean of 62.9% of total polychaete abundance at this site.
Annual mean polychaete abundance at St. S1 was 1.36 ± 0.15 ind. m-2 day-1 (mean ± standard error), andshowed no clear seasonal pattern (Fig. 3B). The most abundant species at St. S1 was Pelagobia sp., and thisspecies accounted for a mean of 22.8% of total polychaete abundance at this site.

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FIGURE 2. Results of cluster analyses on pelagic polychaete abundance at St. K2 (25 July 2013–15 May 2014) in thesubarctic and St. S1 (18 July 2013–4 July 2014) in the subtropical western North Pacific. Horizontal and vertical axes
indicate groups with samples and species, respectively. Differences in color represent abundance values (ind. m-2 day-1).
Dashed lines demarcate groups.

Population structure
For the dominant species at St. K2, T. septentrionalis, body length and segment number ranged from 1.9-28.5mm and 8-25 segments, respectively (Fig. 4A). Body length of T. septentrionalis varied significantly acrossseasons (one-way ANOVA, p<0.05). Shorter body length (<3 mm) individuals and those with few segments(<10) were observed only in winter (December-March).
For the dominant species at St. S1, Pelagobia sp., body length and segment numbers ranged from 0.5-7.1
mm and 8-20 segments, respectively (Fig. 4B). While seasonal changes in body length of Pelagobia sp. were
present (Welch test, p<0.05), these do not show a clear seasonal pattern, especially with respect to the occurrence of the smaller specimens.
Discussion
Community structure
The diversity (as species number) of pelagic polychaetes was higher at the subtropical station (16 species)
compared to the subarctic station (10; Table 1). This is similar to the pattern observed in copepods, a predominant component of zooplankton; in this group, species diversity is known to have a clear latitudinal pattern,high in subtropical regions and low in subarctic regions (Rombouts et al. 2009). In benthic polychaetes, Gobin& Warwick (2006) mentioned that latitudinal patterns of species diversity were unclear, but that species diversity varied with environmental conditions, habitat, and resources.
Polychaete community structure differed between the stations, and three species groups were identified: 1)
species occurring only at the subtropical station, 2) species occurring in both regions with high abundance atthe subarctic station, and 3) species occurring in both regions but with high abundance at the subtropical station(Fig. 2). We can compare these data with data on the pelagic polychaete community from the eastern North
Pacific from the CalCOFI program, which collected from a grid of stations covering 41˚-21˚N in the California
Current System (Fernández-Alamo et al. 2003). Within these regions, the pelagic polychaete community isreported to be divided into three groups: i.e. northern, southern, and transition communities (Fernández-Alamoet al. 2003). We found three species of Lopadorrhynchidae, at the subtropical station; members of this family
are known to be common in warm low latitude oceans (Rouse & Pleijel 2001).
changes of pelagic polychaetes

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FIGURE 3. Seasonal changes in abundance and taxonomic composition of pelagic polychaetes at St. K2 in the subarctic(25 July 2013–15 May 2014) (A) and St. S1 in the subtropical (18 July 2013–4 July 2014) (B) western North Pacific.

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The annual mean abundance of pelagic polychaetes was greater at the subarctic station than at the subtropical station, by a factor of 3.9 times (Fig. 3). Concerning seasonality, polychaetes were more abundant in May toAugust at the subarctic statin, where they were dominated in abundance by T. septentrionalis (composed 63%
in annual mean abundance). In contrast, they showed less seasonality at the subtropical station, with lower
abundance of the dominant species (Pelagobia sp.; 23%). Similar patterns are reported for the California Current System: the abundance of pelagic polychaetes at depths of 0-200 m is higher for the northern community(mean: 242.0 ind. 1000 m-3), and lower for the southern (75.6 ind. 1000 m-3) and transition (4.6 ind. 1000 m3) zones (Fernández-Alamo et al. 2003). Dominance of T. septentrionalis in the northern community is also
reported (Fernández-Alamo et al. 2003). This indicates that similar latitudinal patterns in pelagic polychaete
abundance and common dominant species are present for the eastern and western North Pacific.
While polychaete abundance differed between the two stations (p<0.05, Mann-Whitney U-test), the abundance and biomass of polychaetes as a fraction of the total zooplankton community did not differ between
subarctic and subtropical station (Yokoi et al. 2018). Zooplankton abundance and biomass are well known tobe greater in the subarctic than in the subtropics (Longhurst 1998). This was true at our stations: zooplankton
was higher in both abundance (2.3 times) and biomass (6.8 times) at the subarctic station compared to the subtropical station (Yokoi et al. 2018). These facts suggest that the regional differences in polychaete abundance
observed by Fernández-Alamo et al. (2003) and this study may be related to regional differences in zooplankton standing stock.
The dominant species at the subarctic station, T. septentrionalis, is known to occur broadly in the northern
North Pacific (Dales, 1957). Seasonal patterns of polychaete abundance at this station (Fig. 3A) are likely related to clear seasonal patterns in both primary production and zooplankton abundance in the subarctic Pacific(Longhurst 1998). Planktonic chaetognaths have been reported as prey items for Tomopteris spp. from various
locations (Lebour 1923; Rakusa-Suszczewski 1968; Fauchald & Jumars 1979). In the present study, there wasno correlation between the abundance of polychaetes and chaetognaths in the same zooplankton swimmer
samples (r=0.023-0.054). This fact suggests that the seasonality of polychaete abundance in the subarctic region has no direct relationship with the seasonality of chaetognaths in the same region.
Population structure
Segment number of Tomopteris spp. is known to increase with increasing body length (Åkesson 1962). In thisstudy, specimens of T. septentrionalis with 8-25 segments were observed, and segment number increased withincreasing body length (Fig. 4A). For population structure, significant seasonal changes in body length weredetected, and smaller individuals (<3 mm body length and <10 segments) occurred only in winter (DecemberMarch) (Fig. 4A). This suggests that the main reproductive period of T. septentrionalis at this location might
be in winter.

FIGURE 4. Seasonal changes in body length of (A) Tomopteris septentrionalis at St. K2 in the subarctic and (B) Pelagobia sp. at St. S1 in the subtropical western North Pacific. Column patterns indicate differences in segment number.

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Large individuals of Tomopteris spp. are primary carnivores (Jumars et al. 2015). Copepods composed
50% of abundance and 60% of the biomass in the zooplankton swimmer samples here examined (Yokoi et al.
2018). Copepods may be a food source of T. septentrionalis. However, we detected no correlation was detected
between the abundances of polychaetes and copepods (r=0.196, p=0.394). Small Tomopteris spp., are reported
to ingest diatoms as an important dietary component (Hopkins 1985, Hopkins & Torres 1989, Jumars et al.
2015). Since phytoplankton in the subarctic Pacific bloom saround April-May (Longhurst 1998), reproductionof T. septentrionalis in winter (December-March) might match early juvenile development with the spring
phytoplankton bloom. Analyses of gut contents and stable isotope composition may valuable to test this hypothesis (Jumars et al. 2015).
The dominant polychaete at the subtropical station, Pelagobia sp., is reported to be a primary herbivore,
feeding on phytoplankton (Day 1967; Hopkins 1985, 1987; Hopkins & Torres 1989). Since the sediment trapof this study was moored at 200 m depth, it might be difficult to feed on fresh phytoplankton at that depth.
In the subtropical North Pacific, the nutrients are depleted at shallower depths, and pico-sized phytoplankton(Prochlorococcus) form a sharp peak around 100 m depth which corresponds with the nutrient-cline at that
depth (Furuya 1990; Campbell et al. 1994). Considering this, the food for Pelagobia sp. might be provided
through the microbial loop (picophytoplankton - heterotrophic nanoflagellates [HNF] - ciliates) in the subtropical North Pacific. In the present study, no clear seasonal pattern was not detected for the population structure of Pelagobia sp. at the subtropical site (Fig. 4B). In the subtropical North Pacific, primary productivity is
known to be aseasonal (Longhurst 1998), and this may partially explain the lack of seasonality in the population structure of Pelagobia sp.
Acknowledgments
The sediment trap samples used in this study were moored and recovered by R/V Mirai of JAMSTEC. Partof this study was supported by a Grant-in-Aid for Scientific Research 20H03054 (B), 19H03037 (B) and
17H01483 (A), and Challenging Research (Pioneering) 20K20573 from the Japan Society for the Promotionof Science (JSPS). This work was partially conducted for the Arctic Challenge for Sustainability II (ArCS II)
project.

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