著者

Noriko Yasuoka Yoichi Yusa

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.1, pp.78-81, 2017-02-28 (Released:2017-02-28)

参考文献数

27

被引用文献数

2

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Two Saccostrea oysters (S. glomerata and S. cucullata) are considered to be protandric hermaphrodites based on indirect observations. However, no other information is available on the sexual systems of Saccostrea oysters and the potential for bi-directional sex change has not previously been reported in them. Therefore, the present study was undertaken to investigate the sexual systems of S. kegaki and S. mordax in western Japan, utilizing field sampling and in situ experiments. In the field sampling, shell length (SL) and sex were determined during the reproductive season of these oysters (June–October) in 2012. Then, sex change was directly observed between 2013 and 2016 by determining the sex of individuals through gonadal biopsy, re-attaching the individuals in the field, collecting the survivors in the following year, and determining their sex by dissection. In both species, the proportion of males decreased with increasing SL, although the SL of males and females greatly overlapped. Direct observations showed that both male-to-female and female-to-male sex change occurred. Thus, our results indicated that both Saccostrea species tend to be protandric, but have the ability to change sex in both directions under natural conditions.

著者

Ken-Ichiro Ishii Ryoma Kamikawa

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.164-170, 2017-08-22 (Released:2017-08-24)

参考文献数

42

被引用文献数

7

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The non-photosynthetic diatoms Nitzschia spp. are known to have evolved from photosynthetic species to heterotrophic species by the loss of photosynthesis. We investigated their ability to tolerate wide ranges of temperatures and salinities. Nitzschia spp. were capable of surviving or thriving even at 5°C and 35°C. In addition, these diatoms were also capable of surviving at salinities of 0.5 and 12.0, while thriving at those from 1.0 to 9.0. Such tolerance to a wide range of temperatures and salinities would allow these non-photosynthetic diatoms to thrive in mangrove estuaries, where environmental conditions often drastically fluctuate. Our experiments revealed that the growth rates of the non-photosynthetic diatoms were larger than those estimated based on cell volumes and temperature, suggesting that these non-photosynthetic diatoms may be an important group of organisms contributing to material circulation by growing heterotrophically in mangrove estuaries.

著者

Kazuki Yoshida Tatsuma Sato Kaoru Narita Takeshi Tomiyama

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.1, pp.53-60, 2017-02-28 (Released:2017-02-28)

参考文献数

25

被引用文献数

2

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The moonsnail Laguncula pulchella (Naticidae, formerly known as Euspira fortunei) is an invasive species that has had a negative impact on the Manila clam (Ruditapes philippinarum) population in northern Japan. In southern Japan, few records of L. pulchella exist; furthermore, the knowledge concerning the ecology of this organism is currently limited. Unexpectedly, we found many L. pulchella individuals in the Misuji River estuary, located in Hiroshima in southern Japan, during May 2015. We conducted a field survey on the density and body size of L. pulchella at a tidal flat in the Misuji River estuary (Site H) and compared those to L. pulchella in Matsukawaura Lagoon, located in Fukushima in northern Japan (Site F). The adult population of L. pulchella in a clam fishing ground (40,500 m2) at Site H was estimated to be 2,048 individuals in 2015. Densities of both surfaced individuals and egg collars of L. pulchella were lower at Site H than at Site F. Sizes of L. pulchella individuals and their egg collars as well as the somatic weight of the individuals at Site H were significantly smaller than those at Site F. These results suggest that the biological characteristics of L. pulchella largely differ between the two sites.

著者

Aiko Mitsuzawa Hiroyuki Miyamoto 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.2, pp.115-122, 2017-05-30 (Released:2017-06-06)

参考文献数

29

被引用文献数

13

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Copepod nauplii are the primary food item for early-stage marine fish larvae, but their feeding selectivity on the nauplii of different copepod species has not been well studied. This study revealed feeding selectivity in the dominant preflexion larvae of three fishes Sebastiscus marmoratus, Sebastes sp. and Ammodytes japonicus in the Seto Inland Sea, Japan, in spring and winter. Copepod nauplii and eggs numerically constituted 80–91% of the prey items in their guts. Chesson’s preference index for the nauplii of various copepod species was positive for Calanus sinicus and Paracalanus parvus sensu lato, negative for Acartia omorii, and negative or neutral for Oithona similis. This result is probably attributable to the size and swimming behavior of the prey species. The preference index in A. japonicus larvae was not significantly different between the nauplii of different species, suggesting that the larvae are less selective than the other two fishes. The preference index for Calanus eggs was very high, especially in Sebastiscus marmoratus and Sebastes sp., and most of the eggs in the guts were shrunken, probably due to digestion. In terms of volume, C. sinicus nauplii and eggs in gut contents represented 36–72% of total copepod nauplii and >80% of total copepod eggs, respectively, because of high selectivity by fish larvae and their large individual volumes compared with the dominant copepods. This indicates that C. sinicus is the most important prey species for early fish larvae despite their low abundance in the environment.

著者

A'an J. Wahyudi Shigeki Wada Masakazu Aoki Takeo Hama

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.8, no.2, pp.55-67, 2013-05-25 (Released:2014-08-30)

参考文献数

50

被引用文献数

1 6

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Gaetice depressus, a varunid crab common on intertidal boulder shores, is a potential key organism for monitoring organic matter flow through the food web. In order to elucidate its biogeochemical role, the diet source and trophic position of this crab on the boulder shore of an island off the Izu Peninsula, Japan, were estimated using three approaches: foregut content examination, stable isotope signature (δ13C and δ15N) and pigment biomarkers. The results suggest that G. depressus utilizes green macroalgae (Ulva sp., Ulvaria sp.) as its main diet source together with red macroalgae (Gloiopeltis complanata, Gl. furcata). This crab also utilizes periphytic and planktonic microautotrophs (occasionally tissues of heterotrophs) when macroalgae prove insufficient due to seasonal change. Therefore, G. depressus can be considered to be an omnivore since it consumes both autotrophs and heterotrophs, although it obtains organic matter mostly from autotrophs.

著者

Taketoshi Kodama Kazutaka Takahashi Ken-ichi Nakamura Shinji Shimode Takafumi Yamaguchi Tadafumi Ichikawa

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.10, no.1, pp.75-79, 2015-03-25 (Released:2015-03-20)

参考文献数

25

被引用文献数

1 4

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Short-term variation in the ammonium excretion rate by Calanus sinicus during the post-capture period was evaluated in 1-h incubations. Incubation water volume (13 mL vs. 100 mL) did not significantly affect the excretion rate, and the overall mean excretion rate of C. sinicus was similar to reported values from congeneric species. However, the 24-h variations in the excretion rate denoted that acclimation stress decreased the excretion rates 1–2 h after introduction to a no-food condition. The effect of presence/absence of food was not significant up to 24 h after capture.

著者

Shozo Sawamoto Rui Matsumoto

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.7, no.4, pp.203-206, 2012-11-25 (Released:2013-06-18)

参考文献数

15

被引用文献数

1 16

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More than 50 specimens of the megamouth shark Megachasma pelagios have been reported so far, but biological observations on the species are still limited. We examined a female megamouth specimen that was captured by the bonito purse seine fishery in the Kuroshio Extension in July 2007 and donated to the Okinawa Churaumi Aquarium. The specimen was an immature female, 3,667 mm in total length and 361 kg in weight. The stomach contents measured 2,200 mL excluding the portion that was lost from the specimen before measurement. Detailed examination of a small part (14.6 mL) of the stomach contents revealed undamaged worm-like organisms, 10 partially damaged euphausiids and abundant fragmented pieces of euphausiids. The worm-like organisms were probably parasites such as tapeworms. The partially damaged euphausiids were identified as Euphausia pacifica except for one individual of Nematoscelis difficilis. Based on the number of fragmented right mandibles, which were less damaged than the other fragmented appendages, the total number of euphausiids in the stomach contents is estimated to be at least 18,000 individuals. The high abundance of euphausiids in the stomach contents suggests that the present specimen has fed on a swarm of E. pacifica in an oceanic area.