著者

Ron M. L. Ates

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.12, no.1, pp.71-77, 2017-02-28 (Released:2017-02-28)

参考文献数

86

被引用文献数

1 25

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Instances of predation and scavenging of jellyfish (pelagic Scyphozoa, Hydrozoa and Ctenophora) by benthic invertebrates are reviewed and presented. Sea anemones and decapod crabs are widespread as well as common predators or scavengers of jellyfish. Dead or dying jellyfish are also preyed upon by species of Echinodermata.

著者

Ichiro Takeuchi Momoko Shiraishi Dhugal Lindsay

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.18, no.4, pp.190-197, 2023-11-27 (Released:2023-11-30)

参考文献数

43

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During an ROV Hyper-Dolphin dive to bathyal depths in the Kamaishi Marine Canyon, off Tohoku District, the north Pacific, the association of a large specimen of Caprella (Crustacea: Malacostraca: Amphipoda) was observed, surrounded by juveniles, clinging to the dorsal tube feet of the deep-sea holothurian Scotoplanes globosa (Théel, 1879) at 1142 m depth. Scotoplanes globosa “walks” on the seafloor using long tube-like limbs extending from its body. The present observation indicated that “walking” deep-sea holothurians are an underrated substrate for Caprella spp. in the bathyal depths. Approximately 25 juveniles were collected with the mature female specimen from the same holothurian. These individuals were identified as Caprella fimbrillata Vassilenko, 1993 which has previously been reported from the eastern seaboard of Etorofu/Iturup Island, Chishima/Kuril Islands, in 605 to 1200 m water depth; these sites are located approximately 600 km northeast from the present site off Tohoku in the northwest Pacific. The mature female was closely surrounded by juveniles on the same deep-sea holothurian, suggesting maternal care by the mother caprellid in the bathyal depth environment.

著者

Masumi Kamata Naohisa Wada Nina Yasuda Daniel Varela Jorge I. Mardones Javier Paredes-Mella Satoshi Nagai

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.18, no.1, pp.55-59, 2023-02-28 (Released:2023-03-02)

参考文献数

35

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Outbreaks of paralytic shellfish poisoning caused by the toxic dinoflagellate Alexandrium catenella (Dinophyceae) are currently a serious global problem both from economic and food hygiene perspectives. In Chile, A. catenella was first recorded in 1972 and is currently the main harmful algae species in the country, in terms of seafood security. As no relevant microsatellite markers for Chilean A. catenella populations were available, we isolated 23 new polymorphic microsatellite loci of A. catenella from Chile, of which 15 are applicable to both Chilean and Japanese populations. We found a strong genetic break between Chilean and Japanese A. catenella populations (FST=0.298, P<0.001), consistent with the morphological differences between them. In contrast to the Japanese population, for which genetic diversity ranged from 0.268 to 0.937, the Chilean population had a lower genetic diversity, ranging between 0.065 and 0.512. This study successfully developed A. catenella microsatellite markers that can be used to investigate its genetic spatial and temporal bloom diversity and assess further genetic connectivity between the population from southern and northern Chilean areas.

著者

Takeshi Yuhara Hajime Ohtsuki Jotaro Urabe

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.18, no.2, pp.106-109, 2023-05-31 (Released:2023-05-30)

参考文献数

29

被引用文献数

1

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Ghost crabs (genus Ocypode spp.) found on sandy beaches are susceptible with the loss of an ecotone, making them useful as indicators of anthropogenic alteration, such as seawall construction. On the sandy beaches of Sendai Bay on the Pacific coast of the Tohoku region of northern Japan, three Ocypode species have been sympatrically observed in recent years: O. stimpsoni, O. ceratophthalmus, and O. sinensis. However, these three Ocypode species only occur sympatrically from late summer to autumn, immediately after new recruitment. Since they are all juveniles at that period, it is difficult to identify them by morphological characteristics. Thus, we have developed a method for identifying these three Ocypode species. By using two restriction enzymes of AciI and BfaI, we can perform polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis on the 12S-rRNA region of the mitochondrial DNA. As a result, our PCR-RFLP analysis is able to identify all three Ocypode species. Therefore, this method will be a helpful tool for understanding population dynamics and evaluating the sandy beach ecosystem in cases where the three Ocypode species occur sympatrically but identification by morphological features is difficult.

著者

Kenji Toyota Takashi Ichikawa Nobuo Suzuki Tsuyoshi Ohira

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.18, no.2, pp.84-92, 2023-05-31 (Released:2023-05-30)

参考文献数

32

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Semi-terrestrial brachyuran sesarmid crabs are the most diverse and important faunal components of mangrove forest and temperate coastal communities worldwide. To establish a stable larval rearing method for understanding their larval developmental processes including molecular mechanisms of molting and metamorphosis, we examined the effects of diet on the survival and growth of three sesarmid crabs, Chiromantes haematocheir, Orisarma intermedium, and O. dehaani under larval development. Zoea of each species, immediately after hatching, were reared under different feeding conditions as follows: non-feeding; fed only nutrition-enriched rotifers; fed only Artemia (brine shrimp) nauplii; and fed a combination of both nutrition-enriched rotifers and brine shrimp. Larvae of each species survived almost to megalopa when they were fed both rotifer and brine shrimp, but all C. haematocheir larvae had died by zoeal stage 2 when they were only fed brine shrimp. Chiromantes haematocheir larvae could survive into the megalopa stage when fed only nutrition-enriched rotifer, whereas both O. dehaani and O. intermedium showed 50% or less survival and metamorphosis rates into megalopa. The duration of larval development and survival rates of the three crabs were shorter and higher, respectively, in the mixed feeding group (both rotifer and brine shrimp) than when only fed rotifer or brine shrimp. This study provides baseline knowledge of the interspecific variability in larval survival and duration under different diets for sympatric sesarmid crab species.

著者

Ma'ruf Kasim Hiroshi Mukai

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.4, no.3, pp.104-114, 2009-08-25 (Released:2009-09-25)

参考文献数

29

被引用文献数

14 18

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The gut content was analyzed in order to clarify the potential food sources of the oyster (Crassostrea gigas) and the clam (Ruditapes philippinarum) in the Akkeshi-ko estuary. Benthic diatoms in the diets of oysters were most abundant and accounted for 70% in 2003 and 67% in 2004, followed by pelagic diatoms and dinoflagellates. In gut of clams, benthic diatoms represented 78% in 2003 and 87% in 2004, also followed by pelagic diatoms and dinoflagellates. There are no differences in dietary preference between oyster and clam. There are no significant correlations between phytoplankton compositions in gut of oyster/clam and water column. The Index of electivity was high particularly in dinoflagellates and several species of benthic diatoms such as Amphora sp., Cocconeis scutellum, Nitzschia sp., Paralia sulcata, Rhoicophenia curvata and Thallasiosira sp., Gramatophora marina, and so on.

著者

Ilka Straehler-Pohl Gisèle Flodore Youbouni Ghepdeu Durane Tchatchouang Chougong François Tchoumbougnang André Carrara Morandini

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.17, no.4, pp.406-429, 2022-11-30 (Released:2022-11-30)

参考文献数

73

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Ernst Haeckel described four new chirodropid species in 1880. Chirodropus gorilla was seen only on a few occasions along the Western coasts of Africa, while Chirodropus palmatus (from St. Helena Island) was never recorded again. Type specimens of both species are lost, leading some scientists to doubt the validity of C. palmatus. New specimens assignable to C. gorilla from European and South African Museum collections shed light on the identification of both species. Among the C. gorilla samples, small mature individuals with more pedalial branches than in the larger specimens were discovered. Further observations on living specimens of the smaller chirodropid from Cameroon suggested that they must be C. palmatus because there were only two chirodropid species described from West African waters; comparison with Haeckel’s descriptions and drawings confirmed the identification. Additionally, our data showed that Chirodropus palmatus must be classified into the family Chiropsalmidae and accommodated in its own genus, Chimaerus gen. nov. We also revised definitions of the families Chirodropidae and Chiropsalmidae and re-described both species.

著者

Haruo Yamaguchi Yuji Tomaru

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.17, no.1, pp.100-103, 2022-02-22 (Released:2022-02-23)

参考文献数

14

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This study investigated the temporal and spatial variations of Takayama blooms in Uranouchi Inlet, Japan. From July to September 2017, Takayama cells appeared (≥1 cell mL−1) at depths of 0–10 m, temperatures of 25.4–31.7°C, and salinities of 23.1–34.2. The maximum cell density was 1.37×103 cells mL−1. By analyzing partial rDNA sequences of Takayama strains collected at the bloom initiation, we found Takayama acrotrocha/Takayama xiamenensis and Takayama tasmanica in the Uranouchi Inlet. This study contributes to understanding of the dynamics of and risk to aquaculture of Takayama blooms in the coastal waters of Japan.

著者

Takehiro Kazama Seiji Ishida Satoshi Shimano Jotaro Urabe

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.7, no.3, pp.111-125, 2012-08-25 (Released:2013-06-18)

参考文献数

57

被引用文献数

7 6

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Tintinnid ciliates have been traditionally classified according to morphological and morphometric features of the lorica. To examine if the morphological characteristics of loricae reflect the phylogenetic relationships, we collected 23 tintinnid morphospecies (11 genera) and reconstructed phylogenetic trees based on 84 partial sequences of nuclear small subunit rRNA (SSrRNA) gene fragments of 44 morphospecies from a total of eight families (55 sequences from this study, and 29 sequences from the literature). We found that tintinnid ciliates could be classified into six clades consisting of five major clades. While one clade consisted of one family, each of the other five clades consisted of more than two families. Although two families were found only in one clade, the remaining six families were found in more than two clades. In addition, each of six morphospecies was found in more than two major clades, indicating that they are polyphyletic. Principal coordinate analysis showed that morphology of the lorica overlapped substantially between the clades. These results imply that most of the morphological and morphometric traits of marine tintinnid species do not reflect SSrRNA genetic distances even at family levels. It is necessary to identify new characteristics that reflect phylogenetic relationships robustly.

著者

Xiaodong Wang Yuta Yagi Shotaro Tojima Izumi Kinoshita Yuichi Hirota Shinji Fujita

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.16, no.3, pp.210-220, 2021-08-06 (Released:2021-07-31)

参考文献数

46

被引用文献数

1 2

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The ontogeny of Ilisha elongata was reviewed, and their early distribution was examined because information on the early life history of this species is limited. Revision in ontogeny revealed several unique characteristics to this species, such as two chorions in the egg and a more anterior pelvic bud (fin) position. These characteristics contrast strikingly with the clupeid fishes, suggesting that this species belongs to the Pristigasteridae and not to the Clupeidae. Data from ichthyoplankton collected in March, May, July and November suggest that I. elongata spawned just off river mouths in summer and attained peak spawning in June. The larvae with elliptical eyes migrated inside the rivers, where the waters were more turbid. Thus, the spawning and nursery grounds of I. elongata almost entirely overlapped with those of the clupeid Sardinella zunasi both temporally and spatially, which may result in larval competition between the two species.

著者

Hiroshi Ueda Sakiko Orui Sakaguchi

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.14, no.1, pp.29-38, 2019-02-27 (Released:2019-03-12)

参考文献数

26

被引用文献数

3

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The brackish-water calanoid copepod known as Pseudodiaptomus inopinus in the mainland of Japan consists of two genetically separate species. One is P. japonicus, which was once synonymized with P. inopinus but was recently revived. This paper describes the other species as Pseudodiaptomus yamato n. sp., which is confirmed to have morphological differences from P. inopinus s.s. based on specimens from the type locality (Lake Taihu, China) of the latter. We also redescribe P. japonicus and P. inopinus s.s. for comparative purposes. The three species are distinguishable by the combination of the following morphologies: 1) weak or prominent posterior round projections of the female last pediger; 2) relative length of posterior processes of the female genital operculum; 3) presence or absence of medial spinules on the first exopodal segment of the female leg 5; and 4) the size of spinules at the center of the ventral surface of the male second urosomite. Significant inter-population variation is observed in some spinules of P. japonicus. The past and present records indicate that Pseudodiaptomus yamato n. sp. is endemic to Japan and confined to the coasts affected by the warm Kuroshio Current from western Kyushu to the middle of Honshu, while P. japonicus is widespread in northern East Asia without overlapping the range of P. yamato n. sp. The range of Pseudodiaptomus inopinus s.s. most certainly does not extend to those of P. yamato n. sp. and P. japonicus.

著者

Keiji Matsuoka Takahiro Hirano

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.15, no.3, pp.232-237, 2020-08-14 (Released:2020-08-04)

参考文献数

25

被引用文献数

1

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A new species of viviparid gastropod belonging to the genus Heterogen Annandale, 1921, is described from the Pleistocene Katata Formation of the Kobiwako Group in the southern part of Lake Biwa, central Japan. This new species is continuously recorded from the Nijigaoka Clays (1.2 Ma) to the Hiraen Clays (0.8 Ma) of the Katata Formation. This new species, Heterogen praelongispira sp. nov. is similar to the extant H. longispira in Lake Biwa, but the details of shell morphology are different from those of H. longispira, including fine-to-deep suture, flattened whorl side, and oval shape of embryonic shell.

著者

Hiroshi Kajihara Atsushi Yamaguchi

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.15, no.4, pp.337-341, 2020-11-18 (Released:2020-11-12)

参考文献数

31

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The three currently recognized species comprising the pelagic polystiliferous hoplonemertean genus Protopelagonemertes Brinkmann, 1917 are supposedly distinguishable chiefly with respect to the number of proboscis nerves: 29 (varying from 22 to 30) in P. hubrechti (Brinkmann, 1917); 19–21 (varying from 19 to 22) in P. beebei Coe, 1936; and 36 in P. joculatori Van der Spoel, 1988. A single specimen collected off the Pacific coast of Hokkaido, Japan, herein identified as P. beebei, was found to possess 19–23 primary proboscis nerves. In addition to the primary proboscis nerves, the material also possessed 12–16 secondary proboscis nerves, which are discernible from the primary nerves in that they coexist with putative glial cells. Although secondary proboscis nerves have been identified in some other species in the Pelagica, they have not previously been described for any members of the Protopelagonemertes. Our findings thus prompted us to question the validity of morphological species delimitation within this genus, given that the distinction between primary and secondary proboscis nerves has previously not been taken into consideration. Close examination of the specimen in the living state revealed that it is characterized by a pair of cephalic furrows, a structure commonly found in benthic representatives of the phylum, but herein confirmed for the first time among the Pelagica. A 658-bp partial sequence of the mitochondrial cytochrome c oxidase subunit I gene from the newly examined specimen was found to be identical to sequences derived from two other specimens previously collected in Sagami Bay, thereby indicating that P. beebei may occur commonly in Japanese meso- to bathypelagic waters.

著者

Jamie E. Seymour Emily P. O’Hara

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.15, no.2, pp.73-77, 2020-05-27 (Released:2020-05-30)

参考文献数

36

被引用文献数

3

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Pupillary response under varying conditions of bright light and darkness was compared in three species of Cubozoa with differing ecologies. Maximal and minimal pupil area in relation to total eye area was measured and the rate of change recorded. In Carukia barnesi, the rate of pupil constriction was faster and final constriction greater than in Chironex fleckeri, which itself showed faster and greater constriction than in Chiropsella bronzie. We suggest this allows for differing degrees of visual acuity between the species. We propose that these differences are correlated with variations in the environment which each of these species inhabit, with Ca. barnesi found fishing for larval fish in and around waters of structurally complex coral reefs, Ch. fleckeri regularly found acquiring fish in similarly complex mangrove habitats, while Ch. bronzie spends the majority of its time in the comparably less complex but more turbid environments of shallow sandy beaches where their food source of small shrimps is highly aggregated and less mobile.

著者

Ilka Straehler-Pohl George I. Matsumoto Melisa J. Acevedo

出版者

日本プランクトン学会、日本ベントス学会

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.12, no.2, pp.129-138, 2017-05-30 (Released:2017-06-06)

参考文献数

53

被引用文献数

9

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A lot of confusion (over the last 90 years) surrounds the naming of the Californian carybdeid population, sighted near La Jolla and Santa Barbara, since its first description by Stiasny in 1922. The specimens were first identified as Carybdea rastonii and later as Carybdea marsupialis but the identification was doubted by several scientists. To clear up the confusion, specimens of the Californian population were compared to specimens of all known carybdeid species. This comparison revealed that the Californian population represents an undescribed carybdeid species, named Carybdea confusa n. sp., being identified by the combination of the following characters: Gastric phacellae (single rooted, single stemmed), velarial canals (2 velarial canal roots/octant; canals multiple-branched with rounded tips) and pedalial canal (knee bend with thorn-like appendage).

著者

Ilka Straehler-Pohl

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.14, no.4, pp.261-270, 2019-11-27 (Released:2019-11-25)

参考文献数

25

被引用文献数

2

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A new carybdeid species, Carybdea irregularis sp. nov., is described from the Dana Expedition 1928–30 collections of the Natural History Museum of Denmark, Copenhagen, originally sampled from French Polynesia. It can be distinguished from its congeners by the combination of the following morphological features: pedalial canal knee bend rounded, 2 very narrow velarial canal roots/octant with slim, irregularly shaped canals (no canal resembles another one) and bottle tree-like gastric filaments.Description, photo and and drawings by Bigelow (1909) on three French Polynesian carybdeid medusae, sampled in Rikitea Harbor, Mangareva, Gambier Islands (French Polynesia) during the Albatross Expedition in 1905 and identified by him as “Carybdea rastonii”, fit exactly the anatomical structures of this new species and are therefore designated to it.Up to now, Carybdea irregularis sp. nov. is the smallest species of the genus Carybdea, maturing with at a bell height of 15 mm.

著者

Hiroki Kise Naoko Dewa James Davis Reimer

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.13, no.3, pp.136-141, 2018-08-27 (Released:2018-08-29)

参考文献数

32

被引用文献数

4

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The species Epizoanthus planus is the only known described zoantharian with an association with echinoderms, and it is known to live on the spines of living sea urchins from the family Cidaridae. This species has been reported from 741 to 1019 m in the Indian Ocean and the East China Sea. However, diagnostic characters are lacking in both the original description and subsequent studies. The present study reports the first record of E. planus from Japanese waters (300–400 m) based on a newly collected specimen, and this record slightly extends the species distribution range in the northwest Pacific Ocean. This new record increases the number of Epizoanthus species in Japanese waters to at least five species. Although many taxonomy and diversity studies have focused on specimens from SCUBA-based collections in Japanese waters, few studies have focused on zoantharians in deeper waters below SCUBA diving limits. The results of the present study highlight the importance of examining specimens from deeper waters to better understand the overall diversity of the order Zoantharia.

著者

Hiroshi Ueda

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.13, no.1, pp.28-31, 2018-02-28 (Released:2018-03-14)

参考文献数

14

被引用文献数

1

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High-concentration pure glycerol has been used as a medium for permanent preservation of micro-crustacean specimens. However, glycerol-mounted permanent slide preparations are rare because of the difficulty in sealing the coverslip over the long term. I devised a glycerol-mounted permanent preparation method, especially for a small dissected piece of a micro-crustacean using a glass depression slide and a coverslip with secure sealing, in order to overcome some of the disadvantages of the popular permanent mountants that solidify, such as discoloration of specimens and difficulty in remounting. Unique characteristics of the preparation method are: (1) the central glycerol being of a few millimeters in diameter, which embeds the specimen and is retained in the depression center by a paraffin wax circle painted on the depression bottom; (2) the marginal glycerol, which fills up the clearance between the glass slide and the coverslip and prevents intrusion of the sealant into the clearance; (3) sealing with nail polish first and Canada balsam second, where the latter can seal securely over the long term and the former, being a fast-drying sealant, prevents intrusion of the latter into the marginal glycerol. By following this method, even a small dissected piece can be easily found under a microscope and taken out from the preparation for detailed examination with a low risk of going missing. The procedure to make the preparation is presented in detail.

著者

Minoru Ijichi Gyo Itani Hiroshi Ueda

出版者

日本プランクトン学会、日本ベントス学会

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.12, no.3, pp.145-150, 2017-08-22 (Released:2017-08-24)

参考文献数

26

被引用文献数

4

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The life cycle and precopulatory mate guarding behavior of the poecilostomatoid copepod Goidelia japonica associated with the spoon worm Urechis unicinctus (Echiura) are described based on specimens from 19 worms collected from western Japan. A total of 676 copepods were collected from the host’s rectum (494 copepods) and body surface (182 copepods). Copepod numbers were significantly correlated with host weight. Copepods in the rectum consisted almost completely of copepodids (35%) and adult males (65%), with only one non-ovigerous adult female recorded. In contrast, those on the body surface were all adult males (24%) or females (76%), of which 58% were ovigerous. No nauplii or CI occurred. In the rectum, 36% of the adult males guarded 67% of the copepodids, consisting of CII–CV. From these results, their life cycle is considered as follows: nauplii and CI live as plankton; just before or after molting to CII, they enter the host’s rectum and females are probably soon guarded by males; they grow up to CV there; CV females guarded by males move to the body surface just before the final molt; on the body surface females molt to adults and spawn eggs. Guarding the first symbiotic stage (CII) is a unique precopulatory behavior for symbiotic poecilostomatoid copepods. This can be explained from the viewpoint of evolutionary resolution of the intersexual conflict and advantage for early copepodids in precopula.

著者

Hiroaki Tosuji Kiho Nishinosono Hwey-Lian Hsieh Christopher J. Glasby Takeru Sakaguchi Masanori Sato

出版者

The Plankton Society of Japan, The Japanese Association of Benthology

雑誌

Plankton and Benthos Research (ISSN:18808247)

巻号頁・発行日

vol.14, no.4, pp.287-302, 2019-11-27 (Released:2019-11-25)

参考文献数

39

被引用文献数

10 12

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Taxonomic reexamination of Japanese populations of the Perinereis nuntia species group, which constitutes a major polychaete component in intertidal benthic communities, was carried out by analyzing the mitochondrial 16S rDNA and the nuclear ribosomal ITS sequences of 80 specimens, newly collected from 27 sites in Japan, together with 18 museum-preserved specimens collected from southern neighboring countries outside Japan. The Japanese populations of the Perinereis nuntia species group could be divided into four genetically different groups, which corresponded morphologically to four nominal species (P. mictodonta, P. wilsoni, P. shikueii, and P. nuntia), with some exceptions. Perinereis nuntia and P. shikueii were recorded as new to the Japanese fauna, with their distributions restricted to southern Japan. The clade containing P. shikueii was subdivided into two sister clades (forms A and B), indicating that cryptic speciation has occurred within this clade. Our results revealed a remarkable variability in the number of bars in area VI of the proboscis in form B of P. shikueii, which appeared to be caused by breakage of the long bar into short bars during growth from a juvenile to an adult.