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Comparative Life-History Study on Sympatric Hyperiid Amphipods (Themisto Pacifica and T

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Comparative Life-History Study on Sympatric Hyperiid Amphipods (Themisto Pacifica and T Marine Biology (2004) 145: 515–527 DOI 10.1007/s00227-004-1329-3 RESEARCH ARTICLE Y. Yamada Æ T. Ikeda Æ A. Tsuda Comparative life-history study on sympatric hyperiid amphipods (Themisto pacifica and T. japonica) in the Oyashio region, western North Pacific Received: 17 October 2003 / Accepted: 4 February 2004 / Published online: 26 March 2004 Ó Springer-Verlag 2004 Abstract Life-history features of the sympatric amphi- specimens, the minimum and maximum generation pods Themisto pacifica and T. japonica in the western times of females at a temperature range of 2–12°C were North Pacific were analyzed based on seasonal field computed as 32 days (12°C) and 224 days (2°C), samples collected from July 1996 through July 1998, and respectively, for T. pacifica, and 66 days (12°C) and data from laboratory rearing experiments. T. pacifica 358 days (2°C), respectively, for T. japonica. The num- occurred throughout the year, with populations peaking bers of eggs or juveniles in females’ marsupia increased from spring to summer. In contrast, T. japonica were with female body length and ranged from 23 to 64 for rare from autumn to early winter, but became abundant T. pacifica and from 152 to 601 for T. japonica. Taking in late winter to spring. Mature T. pacifica females and into account the number of mature female instars, life- juveniles occurred together throughout the year, indi- time fecundities were estimated as 342 eggs for T. cating year-round reproduction. Mature T. japonica pacifica and 1195 eggs for T. japonica. Possible mecha- females were observed only in spring, and juveniles nisms for the coexistence of these two amphipods in the occurred irregularly in small numbers, suggesting lim- Oyashio region are also discussed. ited, early-spring reproduction in this study area. Size composition analysis of T. pacifica identified a total of eight cohorts over the 2 years of the study. Due to the smaller sample size and rarity of mature females Introduction (>9.6 mm) and males (>7.1 mm), cohort analyses of T. japonica were not comparable. Laboratory rearing of Hyperiid amphipods are common components of marine specimens at 2°C, 5°C, 8°C and 12°C revealed that a zooplankton communities throughout the world. Among linear equation best expressed body length growth by hyperiid amphipods, those belonging to the genus T. pacifica, while a logistic equation best expressed body Themisto are numerous, both in abundance and biomass, length growth by T. japoncia. Combining these labora- in high-latitude seas worldwide. They are generally car- tory-derived growth patterns with maturity sizes of wild nivores, preying on other zooplankton. In turn, they form important prey components of fishes (Lambert 1960; LeBrasseur 1966; Lønne and Gulliksen 1989; Communicated by O. Kinne, Oldendorf/Luhe Dalpadado et al. 2000), sea birds (Ogi and Hamanaka Y. Yamada (&) Æ T. Ikeda 1982; Ogi et al. 1985; Pedersen and Falk 2000; Bocher Marine Biodiversity Laboratory, Graduate School of Fisheries et al. 2000), and marine mammals (Kawamura 1970; Sciences, Hokkaido University, 3-1-1 Minato-machi, Nemoto and Yoo 1970; Wathne et al. 2000). Themisto 041-8611 Hakodate, Hokkaido, Japan E-mail: [email protected] amphipods play an important intermediate role between Fax: +81-3-53516481 primary production and the production of higher trophic level animals in polar sea ecosystems (Pakhomov and A. Tsuda Hokkaido National Fisheries Research Institute, Perissinotto 1996; Froneman et al. 2000; Bocher et al. 116 Katsurakoi, 085-0802 Kushiro, Hokkaido, Japan 2001; Auel et al. 2002). In the western subarctic Pacific and its marginal seas, Present address: Y. Yamada Ocean Research Institute, University of Tokyo, Themisto pacifica and T. japonica are the most abundant 1-15-1 Minamidai, 164-8639 Nakano, Tokyo, Japan hyperiid amphipods (Bowman 1960). T. pacifica is dis- Present address: A. Tsuda tributed widely in the western through eastern North Ocean Research Institute, University of Tokyo, Pacific and Bering Sea, but not in the Japan Sea and 1-15-1 Minamidai, 164-8639 Nakano, Tokyo, Japan Okhotsk Sea. Compared with the wide geographical 516 distribution of T. pacifica, the distribution of T. japonica reason, only incidental information is currently available is limited to the Japan Sea and the Okhotsk Sea, off the on Themisto amphipods in the western subarctic Pacific. east coast of northern Japan and off the southern Kur- Studies on these amphipods have examined diel vertical iles (Bowman 1960). The trophic importance of these distribution for Themisto spp. (Yoo 1970; Sugisaki two amphipods is suggested by their frequent occurrence 1991), population growth for T. japonica (Sugisaki et al. in the stomach contents of the salmon Oncorhynchus 1990), and horizontal distribution patterns for T. pacif- gorbuscha, O. masou (Fukataki 1967, 1969) and O. keta ica (Bowman 1960; Vinogradov 1992). (Tadokoro et al. 1996), the walleye pollock Theragra As part of a research program to evaluate trophody- chalcogramma (Fujita et al. 1995; Kooka et al. 1997; namics in the pelagic ecosystem in the Oyashio region, Yamamura et al. 2002), the common squid Todarodes western North Pacific, we investigated seasonal changes pacificus (Okiyama 1965) and of mesopelagic fishes, such in abundance, biomass and population structure (size as Diaphus theta, Stenobrachisu leucopsarus and Gonos- and maturity) and brood size of T. pacifica and toma gracile (Beamish et al. 1999; Moku et al. 2000; T. japonica as a basis for evaluating their life-cycle pat- Uchikawa et al. 2001). terns. In conjunction with field population analyses, Numerous studies have examined various aspects of laboratory rearing experiments facilitated analyses of T. japonica living in the Japan Sea, including vertical growth and maturation patterns. From these combined distribution patterns (Semenova 1974; Ikeda et al. 1992), results, possible mechanisms that enable T. pacifica and life cycles (Ikeda et al. 1992), growth models (Ikeda T. japonica to coexist in the Oyashio region are discussed. 1990), lifetime carbon budgets (Ikeda 1991), and popu- lation production calculations (Ikeda and Shiga 1999). In contrast to the Japan Sea, the western North Pacific Materials and methods has two co-occurring Themisto amphipods (T. pacifica and T. japonica). T. pacifica and T. japonica are mor- Field sampling phologically similar and difficult to distinguish. For this Seasonal samples were collected at night using oblique tows of bongo nets (mouth opening: 70·2 cm, mesh size: 333 lm) from a Table 1 Sampling data of oblique tows of bongo nets in the depth of about 500 m (range: 386–900 m) to the surface (Tsuda Oyashio region from July 1996 to July 1998 (n.a. failed to record) et al. 1999). Samples were taken from July 1996 to July 1998 (Table 1) in the Oyashio region within the rectangle defined by Year Date Time of day Max. net depth (m) 41°30¢–42°30¢N and 145°00¢–146°00¢E (hereafter referred to as ‘‘site H’’; Fig. 1). The vessels used for sampling were the F.R.V. ‘‘Hokko Maru’’ and ‘‘Tankai Maru’’ of the Hokkaido National 1996 8 Jul 23:40–00:31 508 Fisheries Research Institute and the F.R.V. ‘‘Hokushin Maru’’ of 2 Aug 18:58–19:38 479 the Hokkaido Fisheries Experimental Station at Kushiro. Net 3 Oct 20:12–21:03 n.a. depth was measured with a depth-meter (Rigosha), and the volume 5 Dec 05:15–06:01 488 of water filtered through the nets was measured by a flow-meter 1997 13 Jan 23:05–23:58 851 (Rigosha). As mentioned below (see ‘‘Vertical distribution’’ sub- 13 Mar 22:13–00:07 574 section in ‘‘Results’’), the depth reached by the net is sufficient to 15 Apr 20:25–21:30 521 collect almost the entire populations of the two amphipods. Tem- 13 May 00:13–01:00 734 perature and salinity profiles were determined concurrently with a 4 Jul 01:35–02:26 435 CTD system during each sampling. 22 Aug 02:00–02:51 748 To evaluate day/night vertical distribution patterns, zooplank- 3 Oct 22:49–22:56 685 ton samples were collected from 15 depth horizons between the 1998 17 Jan 18:50–20:07 883 surface and a depth of 1100 m, using a set of MTD nets (mouth 14 Mar 23:06–00:01 386 opening: 56 cm; mesh opening: 0.33 mm; Motoda 1971) on 1 19 Apr 20:07–21:07 408 September 1992 aboard the T.S. ‘‘Oshoro Maru’’. The nets were 15 May 18:42–19:41 900 towed at a speed of 1.0–1.5 knots for 20 min, and the volume of 7 Jul 02:14–03:00 448 water that passed through each net was registered with flow-meters Fig. 1 Location of sampling area (site H, shaded box) and station E-16 (dot) in the western North Pacific 517 (Rigosha) mounted in the center of the net mouth rings. All zoo- maintained at ca. 5°C and transported to the laboratory on land. plankton samples collected with bongo nets and MTD nets were At each sampling, seawater was collected with Van Dorn or Niskin preserved immediately in 5% formalin-seawater buffered with water samplers from 100 m depth and kept in 20-l containers for borax. use in the experiments. Live specimens were placed individually in Zooplankton samples were weighed (wet biomass) at the labo- 50- to 1000-ml glass containers filled with seawater. The containers ratory on land. Themisto spp. were sorted from the zooplankton were placed in refrigerated incubators, where temperatures were samples for the following analyses. adjusted to 2°C, 5°C, 8°C, or 12°C. Specimens were reared in the dark. Cut whelk (Neptunes lyrata) meat was provided in excess as food.
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