Laboratory of Biological Oceanography, Graduate School of Agricultural Science, Tohoku University, 981-8555, JapanLaboratory of Taxonomy, Department of Biology, Faculty of Science, Toho University, 2-2-1, Miyama, Funabashi, Chiba 274-8510, JapanLaboratory of Marine Biology, Department of Environmental Science, Faculty of Science, Toho University, 2-2-1, Miyama, Funabashi, Chiba 274-8510,
apan
r t i c l e i n f o
rticle history:eceived 19 September 2013eceived in revised form1 November 2013ccepted 16 December 2013vailable online xxx
eywords:
a b s t r a c t
Large numbers of swimming and stranding Urechis unicinctus were observed at night during low tide inSasuhama, Miyagi Prefecture, northeastern Japan, during the periods from January to February in 2012and 2013. Worms did not drift passively but swam actively, therefore hinting at a certain purpose for suchbehavior. As trochophore larvae of U. unicinctus were observed to occur simultaneously in the plankton,we infer the possibility that this is an event of reproductive swarming. Anatomical observations of bothswimming and stranding U. unicinctus showed that none of the specimens had gametes, which maysuggest that these were completely spent after spawning. Urechis unicinctus seemed to begin swimming
after dusk and the observed swimming behavior occurred during the evening ebb tide throughout thenight low tide during winter time. Stranding U. unicinctus have long been known in Japan and have beenattributed to sea storms. The present study shows for the first time the possibility that U. unicinctus swimsin order to reproduce at night and that this swimming behavior is closely linked to the stranding of U.unicinctus individuals.
Echiurans (Annelida), popularly known as spoon worms ornnkeeper worms, occur in shallow- to deep-water habitatshroughout the world’s oceans, from tropical seas to subpolaregions (Biseswar, 2009, 2010, 2012). In recent years, much atten-ion has been paid to the systematic position of echiuran wormsMcHugh, 1997; Struck et al., 2007, 2011). Traditionally, echiuransere excluded from annelids and recognized as a separate phy-
um because of their unsegmented body (Newby, 1940; Stephennd Edmonds, 1972), and considered to be close relatives ofnnelids based on their developmental and morphological char-cteristics (Stephen and Edmonds, 1972; Edmonds, 2000; Ruppert
Please cite this article in press as: Abe, H., et al., Swimming behavior o(2014), http://dx.doi.org/10.1016/j.zool.2013.12.001
t al., 2004). However, morphological observations (Hessling andestheide, 1999, 2002; Purschke et al., 2000; Hessling, 2002) andolecular phylogenetic analyses (McHugh, 1997, 1999; Brown
∗ Corresponding author. Current address: Tohoku National Fisheries Researchnstitute, Fisheries Research Agency, 3-27-5 Shinhama-cho, Shiogama, Miyagi 985-001, Japan. Tel.: +81 223651191; fax: +81 223671250.
et al., 1999; Bleidorn et al., 2003a,b, 2006; Struck et al., 2007,2011; Bourlat et al., 2008; Dunn et al., 2008; Yokobori et al., 2008;Wu et al., 2009) gave consistently support for the inclusion ofEchiura within Annelida, and they are currently treated as a derivedpolychaete group (Struck et al., 2007, 2011). In contrast to their sys-tematic position within the annelids, the higher-level relationshipswithin echiurans remain poorly understood (Ruppert et al., 2004)and it has been suggested that the currently accepted classificationshould be revised (Nishikawa, 2002). The results of echiuran rela-tionships given by molecular phylogenetic analyses also contradictthe currently accepted higher-level classification (Goto et al., 2013).
Urechis unicinctus is an echiuran that lives in marine sedi-ments of the lower intertidal and subtidal zones in coastal mud orsandy areas, being mainly distributed around the coasts of China,Korea, Russia and Japan (Satô, 1939; Zhou et al., 2007; Murina andChernyshev, 2008). Worms construct a U-shaped burrow and filtersuspended materials from seawater pumped through the burrowusing a mucus net (Li et al., 1997). These worms play an impor-
f the spoon worm Urechis unicinctus (Annelida, Echiura). Zoology
tant role in improving the quality of polluted sediment and asprey to several species of demersal fishes. Several commensal orga-nisms dwell within the tubes of U. unicinctus, including the varunidcrabs Acmaeopleura balssi and A. toriumii, and the pinnotherid crab
seudopinnixa carinata (Anker et al., 2005; Itani et al., 2005). U.nicinctus is commonly used as food in Japan, China and especiallyorea (Nishikawa, 1995; Murina and Chernyshev, 2008). They arelso used as fishing bait, so they can be considered an economi-ally important species (e.g., Ishikawa, 1938; Saito et al., 2011). U.nicinctus was previously reported as a common species occurring
n high densities in tidal flats and neritic waters in Japan. However,t has been pointed out that the habitats for this species in Japaneseidal flats have been reduced and deteriorated leading to a dramaticecrease of its populations (Wada et al., 1996; Nishikawa, 2007,012). Although taxonomic studies of echiurans are well underay, their ecology is still little understood.
The spawning season of this species in Japan is reported to ben winter, from December to January (Sato, 1935) or October to
arch (Hiraiwa and Kawamura, 1936; Ohkawa, 1958; Sakiyama,958; Sugiura, 1962), whereas in China it is reported to be in spring,rom April to May (Li et al., 1997). Gametes are liberated in theoelom where they accumulate in two pairs of gonoducts fromhich mature eggs or sperm are released afterwards (Sato, 1935).ature eggs are 130 �m in diameter, and trochophore larvae were
ormed 18–24 h after fertilization at 7–13 ◦C (Satô and Itô, 1961;an et al., 1983). Because artificial insemination is relatively easy, U.nicinctus has often been used as experimental material in embry-logy, physiology, and biochemistry studies (e.g., Sakiyama, 1958;awada and Noda, 1963; Ochi, 1976). However, little is knownbout its reproduction in the natural habitat. Although reproduc-ive swarming of echiuran worms is not known, swimming andtranding events of U. unicinctus have long been reported in JapanIkeda, 1924; Ishikawa, 1938; Nishikawa, 1995, 2007).
During the periods from January to February in 2012 and 2013,arge numbers of swimming and stranding U. unicinctus werebserved at night during low tide in Sasuhama, Miyagi Prefecture,ortheastern Japan. In the present study, we report on the swim-ing behavior of U. unicinctus and discuss the possibility that thisas an event of reproductive swarming.
. Materials and methods
Observation and sampling was carried out in Sasuhama38◦24.35′N, 141◦22.13′E), Ishinomaki city, Miyagi Prefecture,ortheastern Japan (Fig. 1), from January to March in 2012 and013. Table 1 summarizes the year, date, phase of the moon (theumber of days past New Moon), high tide time, low tide time,nd observation period. Each day three people searched the coastor 20–30 min for swimming and stranded worms using flashlightsnd headlights. Swimming and stranding worms were collected bysing a spoon net or by hand on January 11 and 24, February 11 and1, 2012 and January 3 and 11, 2013. Body length, maximum width,nd wet weight of collected U. unicinctus were measured after anes-hesia with magnesium chloride (Stephen and Edmonds, 1972).
et weight was measured after removing as much seawater asossible from the intestine by gently squeezing the worms. Swim-ing behavior was recorded with a digital camera. On February 11,
012 and January 3, 2013 five individuals of the stranded wormsollected were artificially submerged in seawater to test whetherhey could swim or not. All specimens were then fixed in either 99%thanol or 10% neutral formalin solution.
In order to determine the sex and degree of maturity of theollected material, 15 formalin-fixed specimens (4 stranded and1 swimming specimens), which had been collected on January1 and February 21, 2012 and January 3 and 11, 2013, were
Please cite this article in press as: Abe, H., et al., Swimming behavior o(2014), http://dx.doi.org/10.1016/j.zool.2013.12.001
issected and their gonoducts and coelomic fluid observed under stereo microscope. All dissected specimens are deposited in theohoku University Museum (TUM), Sendai, Japan and the Nationaluseum of Nature and Science (NSMT), Tsukuba, Japan, under
PRESSx (2014) xxx–xxx
the registration numbers TUMC-111414–111416 and NSMT-Ec108–110, respectively.
Zooplankton was also collected at the same sampling stationusing a plankton net of 30 cm diameter and a mesh size of 100 �mfor a horizontal haul of approximately 20 m. Plankton sampleswere fixed in 5% neutral formalin solution. Planktonic larvae of U.unicinctus were identified and counted under a stereo microscope.Water volume filtered by the plankton net was calculated from thedistance towed and the opening area of the net, assuming no over-flowing (i.e. 100% filtration efficiency), and the larval density wascalculated from the filtered water volume.
To ensure the accurate identification of planktonic larvae ofU. unicinctus, nuclear 18S rRNA gene sequences of both adultsand larvae were analyzed and compared. Genomic DNA wasextracted from living specimens (three planktonic larvae andthree adults) for use in polymerase chain reactions (PCR). PCRtubes (0.2 ml) containing 50 �l of 10% Chelex 100 suspension(Bio-Rad Laboratories Inc., Richmond, CA, USA) and animal tis-sues were heated to 95 ◦C for 20 min to extract DNA accordingto the method of Richlen and Barber (2005). All PCRs wereperformed in a thermal cycler in a reaction mixture (25.0 �l)containing 1.0 �l of template DNA, 0.2 mM of each deoxynucleo-side triphosphate (dNTP), 1 × PCR buffer, 2.0 mM MgSO4, 0.4 U ofKOD-Plus-Ver. 2 DNA polymerase (with intensive 3′ → 5′ exonu-clease activity; Toyobo, Osaka, Japan), and 0.2 �M of each primer.Three primer pairs (18S-1F1/18S-1R632, 18S-2F576/18S-2R1209,and 18S-3F1129/18S-R1772; Teramoto et al., 2013) were used toamplify the nuclear 18S rRNA gene. The PCR cycling conditions wereas follows: initial denaturation at 94 ◦C for 2 min; 38 cycles at 94 ◦Cfor 15 s, 54 ◦C for 30 s, and 68 ◦C for 45 s. PCR amplification productswere checked on 1.5% agarose gels, with ethidium bromide staining.To remove unincorporated PCR primers and dNTPs, 1.5 �l of the PCRproducts was treated with 0.6 �l exonuclease I and shrimp alkalinephosphatase (Exo SAP-IT) (USB Corp., Cleveland, OH, USA) at 37 ◦Cfor 15 min, followed by incubation at 80 ◦C for 15 min to inacti-vate the enzymes. The PCR products were sequenced directly usingan automated DNA sequencer (ABI PRISM 3100; Applied Biosys-tems/Life Technologies Corp., Carlsbad, CA, USA). The forward andreverse complementary sequences were combined and the threenucleotide sequences obtained with the three primer pairs werealigned using GENETYX software (Genetyx Corp., Tokyo, Japan).Partial sequences of the nuclear 18S rRNA gene of three planktoniclarvae and three adults obtained in this study were compared witheach other and against the sequence of U. unicinctus registered inGenBank (AB771464).
Water temperature, salinity, and dissolved oxygen (DO) concen-tration were measured using a mercurial thermometer, a portableconductivity meter (CM-21P; DKK-TOA Corp. Tokyo, Japan), and aportable dissolved oxygen meter (HQ40d; HACH Co., Loveland, CO,USA), respectively, on the collecting days and on some additionaldays from September 2011 to April 2012 and November 2012 toFebruary 2013.
3. Results
Water temperature, salinity, and DO concentration rangedfrom 3.0 to 21.3 ◦C, from 27.8 to 34.5 PSU, and from 7.4 to12.6 mg l−1, respectively, from September 2011 to April 2012 andfrom November 2012 to February 2013 (Fig. 2).
Swimming individuals of U. unicinctus were observed at nighton January 11 and 24, February 11 and 21, 2012, and on January3, 5, and 11, 2013 (Table 1). Additionally, stranding worms were
f the spoon worm Urechis unicinctus (Annelida, Echiura). Zoology
observed on January 11 and 24, February 11, 2012, and January3, 2013. On January 11, although the number of swimming andstranding worms was not accurately counted, more than 50swimming and 20 stranding worms were observed. Additionally,
Fig. 1. (a) Locations of the present study site and of previous reports of s
uring nighttime on January 24 and December 27, 2012, one andwo individuals, respectively, were observed lying on the shallowottom in Mangoku-ura Inlet (Fig. 1). Swimming behavior of the
Please cite this article in press as: Abe, H., et al., Swimming behavior o(2014), http://dx.doi.org/10.1016/j.zool.2013.12.001
ereidid polychaete Nectoneanthes oxypoda, the glycerid poly-haete Glycera americana, and of nemerteans was also observed inasuhama at the same time (Table 1).
able 1ccurrence of swimming and stranding individuals of Urechis unicinctus and other worm
Year Date Moon phase High tidetime
Low tidetime
Observation ti
2012 January 11 17.4 16:38 23:32 23:00–23:30
January 24 0.8 15:36 22:33 15:00–15:20
22:00–22:30
February 11 18.8 17:59 24:03 23:45–24:15
February 17 24.8 9:35 18:34 22:30–23:00
February 19 26.8 13:02 20:23 18:00–18:30
21:00–21:30
February 21 28.8 14:54 21:38 10:30–10:50
13:00–13:20
15:00–15:20
17:30–17:50
19:00–19:30
19:30–20:00
20:00–20:30
22:00–22:30
24:30–24:50
February 24 2.2 16:57 23:08 15:00–15:20
March 13 20.2 19:24 24:13 13:00–13:20
November 16 2.2 16:10 23:21 21:00–22:00
2013 January 03 20.8 19:14 25:37 22:30–23:00
24:00–24:30
January 05 22.8 22:02 27:37 23:00–24:00
January 08 25.8 11:39 19:36 17:20–17:50
19:20–19:50
January 11 28.8 14:28 21:43 20:30–21:00
January 13 1.3 15:55 22:54 22:30–23:00
January 24 12.3 13:05 20:37 18:50–19:20
20:30–21:00
February 08 27.3 13:34 20:48 20:30–21:00
–: no individuals observed. Blank: no data.a Larvae = density (ind m−3) of planktonic larvae of Urechis unicinctus.
ing and stranding Urechis unicinctus; (b) detailed map of the study site.
Body length, maximum width, and wet weight of U. unicinctuscollected on January 24, 2012 (N = 15) ranged from 8.4 to 24.7 cm(average ± SD: 12.9 ± 4.1 cm), 1.1 to 3.6 cm (2.1 ± 0.6 cm), and 3.8 to
f the spoon worm Urechis unicinctus (Annelida, Echiura). Zoology
46.9 g (12.8 ± 10.7 g), respectively. For those collected on January 3,2013 (N = 44), these parameters were 4.0 to 17.0 cm (10.2 ± 3.3 cm),0.7 to 2.8 cm (1.8 ± 0.6 cm), and 1.2 to 34.9 g (10.3 ± 7.0 g),
Fig. 2. Water temperature, salinity, and dissolved oxygen (DO) concentration atthe sampling station in Sasuhama (a) during the period from September 2011 toApril 2012 and (b) from November 2012 to February 2013. The symbols representts
rnpt
oo
sstbv
a tonic (Nishikawa, 1995, 2007). Ikeda (1924) also reported that
F(
he measurement dates; gray shaded regions correspond to the periods when thewimming and stranding of Urechis unicinctus were observed.
espectively. Body length and maximum width values were almostormally distributed over the two years (Fig. 3). Wet weight wasrobably slightly affected by the amount of remaining seawater inhe intestine of the worms.
All specimens dissected in this study lacked gametes in the gon-ducts and coelomic fluid. Thus, sexuality and degree of maturityf these specimens could not be determined.
Swimming U. unicinctus twisted the body spirally from side toide (see Movie 1 in the supplementary online Appendix). Duringwimming, the anterior end of the body was positioned consis-
Please cite this article in press as: Abe, H., et al., Swimming behavior o(2014), http://dx.doi.org/10.1016/j.zool.2013.12.001
ently in an upward direction, whereas the central part of the bodyecame a helical axis and, relatively speaking, did not move. Theentral body surface was directed consistently towards the inside
Num
ber o
f ind
ivid
uals
0
4
8
12
16
20 a
121086420
0
5
10
15
20
25 b
21.51.00.50
02468
1012 c
1614121086420
Body le
Maximum
Wet w
ig. 3. Size-frequency distribution of Urechis unicinctus collected on January 24, 2012 (soa) body length (cm), (b) maximum width (cm), and (c) wet weight (g).
PRESSx (2014) xxx–xxx
of the spiral. This movement took place at a rate of 30–40 times perminute. Four out of five U. unicinctus found stranded swam whensubmerged in seawater.
Larvae of U. unicinctus occurred in the plankton from January 11to February 21 except for February 17, 2012, and the density was0–113 individuals m−3 (Table 1). Larvae found on January 11, 2012were in the early trochophore stage (ca. 300 �m long) while thosefound on February 21, 2012 were in the segmented trochophorestage (ca. 700 �m) (Fig. 4). Unfortunately, we did not observe anylarvae in 2013.
All nuclear 18S rRNA gene sequences (1758 bp) of both adultsand larvae obtained in this study (GenBank accession number:AB830710) were identical and matched the sequences for U.unicinctus registered in GenBank (AB771464).
4. Discussion
According to Ishikawa (1938) “a copious amount of U. unicinc-tus floated up from the sea bed and got tangled in the brushwoodset in the sea to gather lavers seaweed in Hinaga, Aichi Prefecture(Fig. 1), central Japan in December 1936”. He also reported that“in Mihonoseki, Tottori Prefecture, Sea of Japan coast (Fig. 1), itis usual to catch U. unicinctus which floats up to the surface byusing a scoop net every year during the period spanning Novemberthrough March”. Although the precise cause of this floating phe-nomenon was not known, it was believed that U. unicinctus woulddrift because of a bottom sediment disturbance caused by turbulentwaves during stormy weather (Ishikawa, 1938). At the end of theEdo period (about 150 years ago) in Shiretoko Peninsula, Hokkaido(Fig. 1), people ate stranded U. unicinctus after winter storms as
f the spoon worm Urechis unicinctus (Annelida, Echiura). Zoology
numerous specimens of U. unicinctus (as Echiurus unicinctus) werediscovered stranding at Zenibako, Hokkaido (Fig. 1). Even now, itseems that U. unicinctus is occasionally found stranded on Japanese
26242220181614
2.5.0 3.5 4.03.0
4624222018 48363426
ngth (cm)
width (cm)
eight (g)
lid bar, N = 15) and January 3, 2013 (open bar, N = 44). The horizontal axis indicates
ig. 4. Light micrographs showing live planktonic larvae of Urechis unicinctus collec1, 2012.
hores and is used by the local people for food or as fishing bait.uring the period of the present study, stranding of U. unicinctus inamamasu, Hokkaido (Fig. 1) on December 8, 2012 was reported
n a local newspaper (Tajima, 2013). People in these areas seem toelieve that the stranding of these worms is due to bottom sedimentisturbance by turbulent waves during winter storms (Tajima,013). On the other hand, Sato (1943) reported the observation thatU. unicinctus often swims at night in a rearing position, especiallyuring their reproductive period (from October to February)” ande pointed out the possibility that this behavior could be related tohe reproductive swarming of this species.
In the present study, the swimming behavior of U. unicinc-us was observed in January and February, the period of theowest water temperature during the year (Table 1 and Fig. 2).lthough monthly field observations for other purposes have beenonducted at the same place in Sasuhama for several years, swim-ing and stranding of this species have never been observed in
ther seasons. In Ishinomaki-city, daily maximum wind velocity inanuary 2012 and 2013 was 4.7–13.3 and 4.8–13.7 ms−1, respec-ively (Japan Meteorological Agency, 2013), and significant waveeight in January 2013 was 0.16–1.45 m (NOWPHAS, 2013). It ispparent from these data that no storms had occurred when thewimming behavior was observed. Therefore, it is not likely thathe swimming behavior of U. unicinctus was due to the disturb-nce of bottom sediments by turbulent waves. Even if the sea hadeen rough, it is highly unlikely that U. unicinctus would be sweptut from the sediment by turbulent waves. Indeed, they burrownto the sediment reaching a depth of 15–30 cm in winter and
ore than 100 cm in summer (Ishikawa, 1938). As shown in Movie (see the supplementary online Appendix), U. unicinctus did notrift passively but swam actively, therefore hinting at a purposefulehavior. As such we consider that this phenomenon was more
ikely related to the reproductive behavior pointed out by Sato1943). There is almost no doubt that the reproduction of U. unicinc-us occurred in this period since the occurrence of planktonic larvaeas confirmed for this period as well (Table 1).
The planktonic larvae of U. unicinctus observed on January 11,012 were in the trochophore stage, about 300 �m long (Fig. 4). Fer-ilized eggs of U. unicinctus have been reported to reach the stagef early trochophore (about 130 �m long) 23 h after fertilization;hey reach a length of ca. 182 �m in 48 h, 560 �m in 16 days, and09 �m in 19 days at 16.5–23 ◦C (Li et al., 1997). Considering theseesults, the peak period of reproduction may be presumed to haveccurred prior to January 11, 2012 precisely when a lot of swim-
Please cite this article in press as: Abe, H., et al., Swimming behavior o(2014), http://dx.doi.org/10.1016/j.zool.2013.12.001
ing individuals were observed (Table 1). Although our presentata is insufficient to confirm that the planktonic larvae found on
anuary 11 and February 21, 2012 derived from the same cohort,ur data show that the trochophore larvae reached a size of 700 �m
om the sampling station in Sasuhama (a) on January 11, 2012, and (b) on February
(from 300 �m) in 41 days, which is considerably slower than thedata of Li et al. (1997) suggest. However, this slow growth rate couldhave been caused by the low water temperature (Fig. 2).
Anatomical observations of both swimming and stranding U.unicinctus specimens showed that none of them had gametes. Inechiurans, gametes are produced in special gonadal regions whichtypically cover the posterior-most portion of the ventral bloodvessel and are released into the coelom to mature, and then accu-mulate in the gonoducts until spawning occurs (Barnes, 1980;Pilger, 1993; Brusca and Brusca, 2003). Urechis unicinctus is a gono-choristic species, and when mature, sexes can be easily recognizedexternally as the color of their gametes can be seen through theirsemi-transparent body (Satô and Itô, 1961). Since all specimensdissected in the present study lacked gametes, sexuality couldnot be determined. In its congeneric species, Urechis caupo, theduration of the coelomic phase of oogenesis was estimated to be4.5 months (Das, 1976), whereas other studies report that theproduction of oocytes takes place continuously throughout theyear (Gould-Somero, 1975; Suer, 1984). Fisher (1946) also statedthat stored sex products of U. caupo were found in the nephridia(=gonoducts) throughout the year. MacGinitie (1938) noted thatU. caupo spawning in the laboratory emptied their storage organsalmost completely. This information allows us to conclude that thegametes of U. unicinctus examined in the present study might havebeen completely spent after spawning.
Although the possibility of reproductive swarming of U. unicinc-tus has been pointed out in the present study, there are some factsthat stand in conflict with this hypothesis. Sexual maturation inU. unicinctus has been reported to occur at 7 cm body length (Liet al., 1997). However, since the smallest individual collected inthe present study was 4.0 cm long, we may assume that not all col-lected specimens were mature. Additionally, since the location ofthe adult habitat could not be identified, the present study cannotcompletely refute the possibility that the worms spawn in theirburrows in the bottom sediment before they start their swimmingbehavior. However, given the fact that swimming incurs a high riskof predation, it is unlikely that they start swimming after spawningin their burrows. It would be more reasonable to assume that theyswim for the purpose of reproduction.
In addition to the swimming behavior, a large number ofstranded U. unicinctus was observed at the same time. As the seawas not stormy at that time, it is assumed that the stranded spec-imens were just washed ashore during ebb tide. This would alsoexplain why four out of the five stranded individuals tested swam
f the spoon worm Urechis unicinctus (Annelida, Echiura). Zoology
when they were submerged in seawater. There is an artificial slopeat the sampling site, and this might easily have induced stranding(Fig. 5). Since stranded individuals were observed especially nearthe high tide line, it may be assumed that they had been swimming
Fig. 5. Pictures of (a) the sampling station at flood tide, (b) a stranded individual of Urechis unicinctus. Dotted and dashed lines indicate the high and low tide line, respectively.T his un
auiaosliimrlssas
itsw“fH(abgohkmoicwahDsiArt
279–288.Bleidorn, C., Vogt, L., Bartolomaeus, T., 2003b. A contribution to sedentary polychaete
phylogeny using 18S rRNA sequence data. J. Zool. Syst. Evol. Res. 41, 186–195.
he area enclosed by these lines and the solid lines is the area where stranded Urec
t the time of high tide. However, although a lot of stranding U.nicinctus were observed at 22:00 on 24 January, no swimming
ndividuals were observed at 15:00, during high tide. Taking intoccount the fact that the swimming behavior of U. unicinctus wasbserved only at night (Table 1), U. unicinctus might have begunwimming soon after dusk and continued during ebb tide untilow tide at night. The fact that both swimming and strandingndividuals were observed around spring tides and half tidesndicates that this phenomenon may be related to the phase of the
oon. However, further observations are needed to confirm thiselationship. As mentioned above, stranding U. unicinctus haveong been known in Japan, but this phenomenon has never beentudied in detail. The present study shows for the first time thetrong possibility that U. unicinctus swims in order to reproducet night and that this swimming behavior is closely linked to thetranding of U. unicinctus individuals.
Temporal and spatial determination of the release of gametess a critical step in the life history of free spawning marine inver-ebrates (Thorson, 1950; Pennington, 1985). Mass swimming ofexually mature adults with the aim of spawning in the surfaceater is a well-known conspicuous reproductive behavior. Indeed,
swarming” is well documented in many polychaetes of theamilies Nereididae (Clark, 1961; Imajima, 1972; Schroeder andermans, 1975; Wu et al., 1985; Hanafiah et al., 2006), Eunicidae
Stair, 1897; Itano and Buckley, 1988), Glyceridae (Simpson, 1962),nd Syllidae (Franke, 1999). In the present study, swimmingehavior of the nereidid polychaete Nectoneanthes oxypoda, of thelycerid polychaete Glycera americana, and of nemerteans was alsobserved. Reproductive swarming of N. oxypoda and G. americanaas been reported previously (Simpson, 1962; Sato, 2013). It isnown that the morphology of these species is characteristicallyodified prior to spawning. However, the fact that the individuals
f these two species collected for the present study were allmmature stages (atokes) without epitokous metamorphosis indi-ates that the swimming behavior observed in the present studyas not for the purpose of reproduction. Nocturnal swimming
ctivities of benthic worms not for the purpose of reproductionave also been previously reported (Thomas and Jelley, 1972;ean, 1978a,b), although the exact reason for non-reproductive
wimming behavior is not known. Dean (1978b) suggested that its a much more common phenomenon than recognized heretofore.
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s he pointed out, most field studies in near-shore marine envi-onments are conducted during daylight hours and mainly at lowide. We may thus conclude that a more detailed understanding of
icinctus were found.
the swimming behavior of marine worms including U. unicinctusrequires further observations during nighttime.
Acknowledgements
We thank Dr. Masanori Sato for reading and improving an ear-lier version of the manuscript as well as species identification andmorphological observations of nereidid and glycerid polychaetes.We also thank Mr. Yusuf Shuaib Ibrahim for species identifica-tion and morphological observations of glycerid polychaetes. Weare grateful to Drs. Teruaki Nishikawa and Helena Fortunato forcritical reading and valuable comments on an earlier version ofthe manuscript. We are also grateful to two anonymous review-ers for their constructive comments that improved the manuscript.Thanks are also due to Messrs. Noritaka Ayakoji, Jiro Endo, DaikiFujii, Hiromasa Ohno, and Yuma Sato for their assistance duringfield observations on some cold nights. This study was partiallysupported by a research grant from the Research Institute of MarineInvertebrates Foundation to H.A.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.zool.2013.12.001.
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