Mar Biol (2007) 150:609–625 DOI 10.1007/s00227-006-0382-5 RESEARCH ARTICLE Vertical distribution, population structure and life cycles of four oncaeid copepods in the Oyashio region, western subarctic PaciWc Yuichiro Nishibe · Tsutomu Ikeda Received: 5 April 2006 / Accepted: 7 June 2006 / Published online: 28 June 2006 © Springer-Verlag 2006 Abstract Vertical distribution and population struc- T. borealis and O. parila copepodids, no clear seasonal ture of four dominant oncaeid copepods (Triconia succession was observed thus estimation of their gener- borealis, Triconia canadensis, Oncaea grossa and ation time was uncertain. The present comprehensive Oncaea parila) were investigated in the Oyashio results of vertical distribution and life cycle features for region, western subarctic PaciWc. Seasonal samples T. borealis, T. canadensis, O. grossa and O. parila are were collected with 0.06 mm mesh nets from Wve dis- compared with the few published data on oncaeid spe- crete layers between the surface and 2,000 m depth at cies distributing in high latitude seas. seven occasions (March, May, June, August and Octo- ber 2002, December 2003 and February 2004). The depth of occurrence of major populations of each spe- Introduction cies diVered by species; the surface–250 m for T. bore- alis, 250–1,000 m for T. canadensis, 250–500 m for The copepod family Oncaeidae is a diverse group of O. grossa and 500–1,000 m for O. parila. The ontogenetic marine pelagic cyclopoids (Böttger-Schnack and Huys vertical migration characterized by deeper occurrence 1998; Boxshall and Halsey 2004). They inhabit all parts of early and late copepodid stages, and shallower of the world oceans, ranging from coastal to oceanic occurrence of middle copepodid stages was observed in waters, from tropical to polar regions (Malt 1983; T. canadensis and O. parila. Of the four oncaeid cope- PaVenhöfer 1993) and from epi- to bathypelagic zones pods, almost all copepodid stages occurred throughout (e.g. Boxshall 1977; Deevey and Brooks 1977; Böttger- the study period, suggesting that their reproduction Schnack 1994; Richter 1994; Nishibe and Ikeda 2004). continues throughout the year in the region. Neverthe- While oncaeid copepods are abundant also in coastal less, a clear developmental sequence of stage-to-stage waters (PaVenhöfer 1983; Uye et al. 1992; Noda et al. was traced for T. canadensis and O. grossa copepodids, 1998), their relative importance in the copepod com- implying their generation time to be 1 year. For munities becomes more evident in oceanic waters, especially in the meso- and bathypelagic zones (Bött- ger-Schnack 1995, 1996, 1997; Webber and RoV 1995; Satapoomin et al. 2004; Hopcroft et al. 2005). In these Communicated by S. Nishida, Tokyo depth layers, oncaeid copepods usually account for more than 50% and up to 90% of total copepod num- Y. Nishibe · T. Ikeda bers based on the sampling with Wne mesh nets (e.g. Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-cho, Hakodate 041-8611, Japan Böttger-Schnack 1994; KrniniT 1998; KrniniT and Grbec 2002; Yamaguchi et al. 2002). Present Address: Despite their ubiquitous distribution and high Y. Nishibe (&) abundances, our knowledge on the ecology of oncaeid Center for Marine Environmental Studies, Ehime University, W V 3 Bunkyo-cho, Matsuyama 790-8577, Japan copepods is still de cient (Pa enhöfer 1993; Böttger- e-mail: [email protected] Schnack et al. 2004; Turner 2004). In particular, there 123 610 Mar Biol (2007) 150:609–625 is little information available about the population referred to as Site H; Fig. 1), in 10 March, 30 May, 18 dynamics and life cycle strategies of oncaeid copepod June, 9 August and 9 October 2002 (Table 1). Addi- species. Metz (1996) examined the vertical distribution tional sampling was made aboard the T. S. ‘Oshoro and population structure of three dominant oncaeid Maru’ in 17 December 2003 and 9 February 2004 at species, Oncaea curvata, Oncaea parila and Triconia Site H, to complete the seasonal cycle. During the sur- antarctica, in the Bellingshausen Sea during two sea- vey in 2002, zooplankton samples were collected with a sons and extrapolated their life cycle patterns. Some closing type net (60 cm mouth diameter, 0.06 mm mesh fragmentary information on the life cycles of Triconia size, Kawamura 1989) equipped with a Xowmeter (Rhi- borealis was provided by Pavshtiks (1975) and Richter gosha) inside the mouth of the net and a RMD depth (1994) from the Davis Strait and the Greenland Sea, meter (Rhigosha) on its suspension cable to read the respectively. Recently, Böttger-Schnack and Schnack depths the net reached. The net was hauled vertically (2005) studied the population structure and fecundity at speeds of 0.5–1.0 m s¡1 from Wve discrete layers: sur- of the warm-water species Oncaea bispinosa in the Red face to the bottom of the thermocline (Th), Th–250, Sea and discussed their reproduction traits. 250–500, 500–1,000 and 1,000–2,000 m (Table 1). For While a total of 40 oncaeid copepod species have the sampling on 17 December 2003 and 9 February been recorded in the Oyashio region, western subarctic 2004, a vertical multiple plankton sampler (VMPS; PaciWc, the four species T. borealis (Sars), Triconia 50 cm £ 50 cm mouth-opening, 0.06 mm mesh size; canadensis (Heron and Frost), Oncaea grossa Heron Terazaki and Tomatsu 1997) was employed. The and Frost, and O. parila Heron are most abundant in VMPS was hauled at a speed of 1.0 m s¡1 from the terms of both numbers and biomass (Nishibe and same depth stratum as described above. Average Wltra- Ikeda 2004; Nishibe 2005). T. borealis has been tion eYciencies for the closing type net and VMPS recorded from high latitude seas in the northern hemi- were 71 and 79%, respectively. Because the thermo- sphere such as the subarctic Atlantic (Malt 1983; cline was not recognized in 10 March 2002, the Th was Heron et al. 1984), the subarctic PaciWc (Heron and assumed arbitrarily as 100 m depth (Table 1). To inves- Frost 2000), and the Arctic Ocean (Heron et al. 1984), tigate the diel vertical migration pattern, day and night and thus is considered to be a genuine arctic/subarctic sampling was conducted on 9 October 2002 (Table 1). species. Conversely, O. parila occurs in high latitude After collection, samples were preserved immediately seas of both hemispheres such as the subarctic PaciWc on board ship in a 2% formaldehyde-seawater solution (Heron and Frost 2000), the Arctic Ocean (Heron buVered with borax. At each zooplankton sampling et al. 1984) and the Southern Ocean (Heron 1977). For date, vertical proWles of temperature and salinity were T. canadensis and O. grossa, previous records outside determined by using a CTD rosette system (SBE-9 the Oyashio region has been restricted to the eastern plus, Sea Bird Electronics). subarctic PaciWc (Heron and Frost 2000). In the present study, we investigated the vertical distribution, abundance and population structure of T. borealis, T. canadensis, O. grossa and O. parila in W the Oyashio region, western subarctic Paci c by analy- Okhotsk Sea sing seasonal samples collected from the surface to 2,000 m depth. We compare the present results with those from the other high latitude seas, and discuss life cycle features, such as ontogenetic vertical migration, generation time and reproduction of the four oncaeid 44°N copepod species. Hokkaido 1000 m 3000 m 5000 m 7000 m Materials and methods Site H Field samplings Pacific Ocean 40°N Seasonal zooplankton samples were collected on board 140°E 144°E 148°E the T. S. ‘Oshoro Maru’ and R.V. ‘Ushio Maru’ at a Ј Ј Fig. 1 Location of sampling site (Site H; circled star) in the Oy- station (41°30 N; 145°47 E, 6,670 m deep) in the ashio region, western subarctic PaciWc. Bathymetric counters Oyashio region oV southeastern Hokkaido (here after (1,000, 3,000, 5,000 and 7,000 m) are also shown 123 Mar Biol (2007) 150:609–625 611 Table 1 Summary of zoo- Sampling date Time (local time) Ship Sampling depth (m) plankton sampling data at Site Ј Ј H (41°30 N; 145°47 E) 10 March 2002 09:16–12:18 Os 0–100, 100–250, 250–500, 500–1,000, 1,000–2,000 30 May 2002 13:26–15:38 Os 0–50, 50–250, 250–500, 500–1,000, 1,000–2,000 18 June 2002 02:24–06:17 Os 0–50, 50–250, 250–500, 500–1,000, 1,000–2,000 9 August 2002 19:30–22:31 Us 0–70, 70–250, 250–500, 500–1,000, 1,000–2,000 9 October 2002 09:10–12:19 (D), Us 0–70, 70–250, 250–500, 500–1,000, 1,000–2,000 19:00–22:09 (N) D Day sampling, N night sam- 17 December 2003 04:15–06:33 Os 0–100, 100–250, 250–500, 500–1,000, 1,000–2,000 pling, Os TS ‘Oshoro Maru, 9 February 2004 17:54–20:29 Os 0–180, 180–250, 250–500, 500–1,000, 1,000–2,000 Us RV ‘Ushio Maru’ IdentiWcation and enumeration all developmental stages were quantitatively retained for T. canadensis, O. grossa and O. parila. Copepodid stages of T. borealis, T. canadensis, O. grossa As an index of breeding activity, C6 females carrying and O. parila were sorted from the entire sample or egg sacs or having spermatophores attached to the geni- aliquots taken by using a box type splitter (Motoda tal double-somite were counted separately for the four 1959) and enumerated under a dissecting microscope.