Seasonal abundance, population structure, sex ratio and gonad maturation of Metridia okhotensis Brodsky, 1950 in the Okhotsk Sea: analysis of samples collected by pumping up from deep water

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In the Okhotsk Sea, the calanoid copepod Metridia okhotensis Brodsky, 1950 is the dominant component of zooplankton, accounting for 61% of the annual mean total pelagic copepods. Although this organism is important, little ecological information is available for M. okhotensis in the Okhotsk Sea because of the ice cover during winter, which prevents the collection of seasonal samples in this region. Here, we report the seasonal changes in the population structure, sex ratio and female gonad maturation of M. okhotensis. The data are from samples collected using water pumped from a depth of 350 m off Rausu Harbour in the Okhotsk Sea at 2-week intervals over a 2.5-year period. Due to the mesh size of the strainer (420 μm), M. okhotensis was collected from C3 to adults. The sex ratio of C5 (female : male) was approximately 1 : 1 throughout the year. In contrast, the sex ratio of C6 (adult) showed a clear seasonality, with males (C6M) occurring only from December to May and females (C6F) dominating during the other seasons. The gonad maturation of C6F was scored using five categories, and their composition also showed clear seasonality. From January to April, gonads developed rapidly from stage I (immature) to V (spawning). During the other seasons, the majority of C6F had immature gonads. Based on these data, we conclude that this species likely has a diapause phase for C6F, with immature gonads, and C5M from June to November. Moulting from C5M to C6M began in December. Accompanying the occurrence of C6M, C6F were fertilized from December to January. C6F underwent gonad maturation from January to April and performed primary reproduction from April to May. Thereafter, M. okhotensis entered diapause from June to November.

Crustaceana

International Journal of Crustacean Research

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References

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Figures

  • A, Location of Rausu, eastern Hokkaido; and B, schema of Rausu Deep-Ocean Water sampling site. This figure is published in colour in the online edition of this journal, which can be accessed via http://booksandjournals.brillonline.com/content/journals/15685403.

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  • A, Seasonal changes in temperature and zooplankton abundance (copepods and others) at 350 m and B, copepod abundance and species composition (Metridia okhotensis Brodsky, 1950, Metridia pacifica Brodsky, 1950 and other copepods) observed in the Rausu deep-ocean water from June 2007 to November 2009.

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  • A, Seasonal changes in abundance; B, stage composition; C, sex ratio of C5 and C6; and D, gonad maturation of C6F of Metridia okhotensis Brodsky, 1950 in the Rausu deep-ocean water from June 2007 to November 2009.

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  • Seasonal changes in sea surface temperature, chlorophyll a and ice coverage (shown as white bars) around the Rausu Harbour from 1 January 2007 to 26 December 2009 determined using satellite images (MODIS-Aqua).

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