Fig. 4 in Cymonomus curvirostris Sakai 1965

Length-weight Relationships and Chemical Composition of the Dominant Mesozooplankton Taxa/species in the Subarctic Pacific, with Special Reference to the Effect of Lipid Accumulation in Copepoda Asami Nakamura1,4, Kohei Matsuno2,5, Yoshiyuki Abe1, Hiroshi Shimada3, and Atsushi Yamaguchi1,* 1Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1, Minato-cho, Hakodate, Hokkaido, 041-8611, Japan. E-mail: [email protected](Nakamura); y[email protected](Abe) 2National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo, 190-8518, Japan. E-mail: [email protected] 3Hokkaido Research Organization, Central Fisheries Research Institute, 238 Hamanaka-cho, Yoichi, Hokkaido 046-8555, Japan. E-mail: [email protected].jp 4Present address: You Story Co. Ltd., 355-14, Oiwake, Karuizawa, Kita-Saku-Gun, Nagano, 389-0115, Japan 5Present address: Australian Antarctic Division, Kingston, Tasmania 7050, Australia (Received 6 July 2016; Accepted 28 April 2017; Published 23 May 2017; Communicated by Ryuji Machida) Asami Nakamura, Kohei Matsuno, Yoshiyuki Abe, Hiroshi Shimada, and Atsushi Yamaguchi (2017) While length-weight (L-W) regressions for warm-water zooplankton taxa from the waters neighbouring Japan already exist, they are still missing for comparable cold-water species. In this study, the L-W regressions of 41 species belonging to 12 taxa that are dominant in the Oyashio region were reported. The body length and volume of zooplankton were measured with an image-analysis system, and the effects of lipid accumulation in Copepoda on their mass and chemical composition were quantified. The L-W regressions had a high coefficient of determination (mean r2 = 0.886). For the chemical composition, the water composition ranged from 69.8 to 95.2% wet mass (WM), carbon (C) composition from 3.8 to 60.8% dry mass (DM) and nitrogen (N) composition from 1.0 to 10.1% DM. Taxon-specific differences in the chemical composition were marked for the gelatinous taxa (Appendicularia, Cnidaria, Salpida), which also had high water and low C composition. Because C is an index of lipids, high water compositions together with low lipid compositions are considered to be characteristics of the gelatinous taxa. The most significant effects of lipid accumulation in the Copepoda are changes in DM and C. Within the same developmental stage, the DM and C compositions of the full lipid-containing specimens showed 495% and 741% increases, respectively, over those of the low lipid-containing specimens. These differences exceeded the changes after moulting (78.1%) for general copepod species. Thus, lipid accumulation should be evaluated for the accurate mass estimation of boreal Copepoda by image analysis. Key words: Mass, L-W equation, Zooplankton, C/N, Lipids, Oil sac volume, Image analysis. *Correspondence: E-mail: [email protected] BACKGROUND In the marine ecosystem, mesozooplankton play an important role as a vital link connecting primary producers and higher trophic levels (Hunt et al. 1998; Beamish et al. 1999; Ikeda et al. 2008). The feeding preference of fishes is affected by the size of mesozooplankton (Sheldon et al. 1977), and the energy cost, growth and mortality of fishes also vary with the size of mesozooplankton (van der Meeren and Næss 1993). These facts indicate that information about mesozooplankton’s size and biomass are of prime importance when evaluating energy transfer in marine ecosystems. On the other hand, mesozooplankton play an important role in the vertical material flux down to the deep layer. Mesozooplankton feed on phytoplankton, egest fast-sinking faecal pellets, and actively Zoological Studies 56: 13 (2017) doi:10.6620/ZS.2017.56-13 1 transport materials by the diel vertical migration (DVM), thus they have central role in the “biological pump” (Longhurst and Harrison 1989). Because the flux of the faecal pellets egested during DVM is correlated with mesozooplankton’s size and biomass (Paffenhöfer and Knowles 1979; Uye and Kaname 1994), an accurate estimation of the biomass and size of mesozooplankton is also of primary importance from the perspective of the biological pump. The chemical composition of mesozooplankton varies by taxon. For instance, the Cnidaria, Appendicularia, and Salpida are known to have high water and lower organic compositions and are called “gelatinous taxa” (Larson 1986; Gorsky et al. 1988; Molina-Ramírez et al. 2015). The chemical composition is also known to vary with region and depth (e.g., geographically and vertically). For instance, the organic composition and lipid levels are higher for high-latitude species (Lee et al. 1971; Båmstedt 1986), while higher carbon and low nitrogen compositions are reported for deep-sea species (Ikeda et al. 2006). Chemical composition differences may also affect lengthweight (L-W) relationships, and these relationships also vary widely by taxon and region (Uye 1982; Mizdalski 1988; Hirst 2012). Concerning the waters around Japan, information on the L-W relationships and chemical compositions of mesozooplankton is available for the warm-water regions (Uye 1982); however, little information is available for the coldwater (Oyashio) region. In the Oyashio region, large amounts of nutrients are provided at the surface layer by strong wintertime mixing; phytoplankton form massive diatom blooms during the spring (Kasai et al. 1997). The mesozooplankton in this region is dominated by Copepoda, which utilize the spring phytoplankton bloom as energy for their growth and reproduction (Miller et al. 1984). For instance, the dominant copepods in this region, Neocalanus spp., achieve growth at the surface layer during the spring bloom, store lipids, and then migrate down to the deep layers for diapause and reproduction (Kobari and Ikeda 1999; Tsuda et al. 1999), during which the stored lipids are used for energy (Lee et al. 1970; Miller et al. 1998; Jónasdóttir 1999). Thus, the C/N ratio of copepods is known to vary seasonally and to be correlated with the amount of the lipid store (Omori 1969). While the importance of their lipid store is known, information on the effects of the lipid store of Copepoda on their L-W relationship and chemical compositions is scarce. In this study we report the L-W relationships of various taxa (41 species belonging to 12 taxa) that are dominant in the mesozooplankton community in the Oyashio region. For the chemical composition, the water, carbon and nitrogen compositions were quantified and the differences were evaluated by separating gelatinous and semigelatinous taxa (Larson, 1986). For the Copepoda, the amount of stored lipid was quantified using image-analysis methods (Shimada and Oku 2014). Based on the amounts of stored lipids, the body volume, mass and chemical compositions of the Copepoda were compared and the effects of the lipid stores were evaluated for each parameter. MATERIALS AND METHODS Present study is a compilation of published and unpublished studies, some of which has been submitted as theses project of Hokkaido University. Field sampling Specimens of 41 species belonging to 12 taxa used for the L-W relationship estimation were mainly collected between the sea surface and a depth of 3000 m in the Oyashio region in the western subarctic Pacific Ocean and the adjacent northern Japan Sea (Table 1). Zooplankton samples were collected at four stations (41°N, 145°E; 43°N, 155°E; 40°N, 155°E; 37°N, and 148°E) by vertical tows of a NORPAC net (45 cm mouth diameter, 335 µm mesh) at depths of 150 m or 500 m to the sea surface between 8 and 19 May 2015. The freshly collected samples were taken according to the procedures discussed below. Additional samples were collected by a 0-300 m vertical tow of a NORPAC net at St. O26 (45°N, 143°E) in the southern Okhotsk Sea on 5 June 2015. Samples for analysing the effect of the lipid stores of copepods were collected for five species: Eucalanus bungii copepodid stage six female (C6F), Metridia okhotensis C5M, C6F, Neocalanus cristatus C5, N. flemingeri C5 and C6F, and N. plumchrus C5. The specimens were sorted into three categories of lipid storage amounts (low, medium, and full) as defined by the following references: Shoden et al. (2005) for E. bungii, Padmavati et al. (2004) for M. okhotensis and Ikeda et al. (1990) for Neocalanus spp. page 2 of 15Zoological Studies 56: 13 (2017) Table 1. Summary on mass-length regressions for various zooplankton taxa/species which dominated in the western subarctic Pacific and their adjacent seas. Note that length units are μm for meso-size taxa (Ostracoda, Copepoda and Appendicularia) and mm for the remaining macro-size taxa. For masses, units are in μg for all taxa. WM: wet mass, DM: dry mass, C: carbon, N: nitrogen, BH: bell height, TL: total length, SL: standard length, PL: prosome length, BL: body length, L: length, TrL: trunck length. Detailed diagrams of measured parts are shown in figures 1 and 2 Taxa Species (taxonomic category) Regression Unit r2n p References Mass Length Cnidaria Aglantha digitale Log10DM = 1.125(Log10BH)2+ 1.268Log10BH+0.667 DM (μg) BH (mm) 0.976 72 Ikeda and Imamura (1996) Other Cnidaria spp. Log10DM = 2.94Log10TL+0.82 DM (μg) TL (mm) 0.620 35 Imao (2005) Annelida Annelida spp. Log10DM = 1.53Log10TL+1.49 DM (μg) TL (mm) 0.810 43 Imao (2005) Ostracoda Discoconchoecia pseudodiscophora Log10DM = 2.61Log10SL-7.751 DM (μg) SL (μm) 0.992 8 Kaeriyama and Ikeda (2002) Metaconchoecia skogsbergi Log10DM = 2.42Log10SL-7.143 DM (μg) SL (μm) 0.996 7 Kaeriyama and Ikeda (2002) Orthoconchoecia haddoni Log10DM = 2.53Log10SL-7.511 DM (μg) SL (μm) 0.996 8 Kaeriyama and Ikeda (2002) Copepoda Calanus pacificus (C1-C5) Log10DM = 1.871Log10PL-4.309 DM (μg) PL (μm) 0.783 21 < 0.001 Ueda et al. (2008) Log10C = 3.573Log10PL-11.008 C (μg) PL (μm) 0.857 20 < 0.001 Ueda et al. (2008) Log10N = 2.382Log10PL-7.788 N (μg) PL (μm) 0.625 13 < 0.01 Ueda et al. (2008) Eucalanus bungii (C1-C5) Log10DM = 2.052Log10PL-5.408 DM (μg) PL (μm) 0.771 93 < 0.001 Ueda et al. (2008) Log10C = 2.828Log10PL-8.97 C (μg) PL (μm) 0.879 92 < 0.001 Ueda et al. (2008) Log10N = 2.727Log10PL-9.63 N (μg) PL (μm) 0.788 53 < 0.001 Ueda et al. (2008) Gaetanus variabilis (C1-C6) Log10DM = 3.169Log10PL-8.317 DM (μg) PL (μm) 0.982 34 Yamaguchi and Ikeda (2000a) Heterorhabdus tanneri (C3-C6) Log10DM = 3.530Log10PL-9.579 DM (μg) PL (μm) 0.998 20 Yamaguchi and Ikeda (2000b) Metridia pacifica (C1-C5) Log10DM = 1.405Log10PL-2.865 DM (μg) PL (μm) 0.689 45 < 0.001 Ueda et al. (2008) Log10C = 2.967Log10PL-9.113 C (μg) PL (μm) 0.924 44 < 0.001 Ueda et al. (2008) Log10N = 2.902Log10PL-9.209 N (μg) PL (μm) 0.940 45 < 0.001 Ueda et al. (2008) Neocalanus cristatus (C1-C5) Log10DM = 2.418Log10PL-6.242 DM (μg) PL (μm) 0.890 67 < 0.001 Ueda et al. (2008) Log10C = 2.964Log10PL-8.931 C (μg) PL (μm) 0.900 67 < 0.001 Ueda et al. (2008) Log10N = 3.115Log10PL-10.288 N (μg) PL (μm) 0.893 62 < 0.001 Ueda et al. (2008) Neocalanus flemingeri (C4-C5) Log10DM = 4.954Log10PL-15.005 DM (μg) PL (μm) 0.669 15 < 0.001 Ueda et al. (2008) Log10C = 6.395Log10PL-20.449 C (μg) PL (μm) 0.681 14 < 0.001 Ueda et al. (2008) Log10N = 4.905Log10PL-16.345 N (μg) PL (μm) 0.306 14 Ueda et al. (2008) Neocalanus plumchrus (C1-C5) Log10DM = 2.044Log10PL-4.881 DM (μg) PL (μm) 0.933 49 < 0.001 Ueda et al. (2008) Log10C = 3.237Log10PL-9.794 C (μg) PL (μm) 0.957 49 < 0.001 Ueda et al. (2008) Log10N = 2.235Log10PL-7.043 N (μg) PL (μm) 0.874 49 < 0.001 Ueda et al. (2008) Paraeuchaeta birostrata (Egg-C6) Log10DM = 2.882Log10PL-7.252 DM (μg) PL (μm) 0.991 358 Yamaguchi and Ikeda (2002) Paraeuchaeta elongata (Egg-C6) Log10DM = 3.167Log10PL-8.358 DM (μg) PL (μm) 0.975 359 Yamaguchi and Ikeda (2002) Paraeuchaeta rubra (Egg-C6) Log10DM = 2.854Log10PL-7.1102 DM (μg) PL (μm) 0.984 172 Yamaguchi and Ikeda (2002) Pleuromamma scutullata (C1-C6) Log10DM = 2.723Log10PL-6.892 DM (μg) PL (μm) 0.995 28 Yamaguchi and Ikeda (2000b) Pseudocalanus newmani (N2-N6) Log10DM = 2.515Log10TL-6.57 DM (μg) TL (μm) 0.951 5 Lee et al. (2003) Pseudocalanus newmani (C1-C5) Log10DM = 2.08Log10TL-5.456 DM (μg) TL (μm) 0.988 9 Lee et al. (2003) Scolecithricella minor (C2-C6) Log10DM = 3.669Log10PL-9.739 DM (μg) PL (μm) 0.989 22 Yamaguchi (1999) Cyclopoid Copepoda (3 species, C1-C6) Log10DM = 1.997Log10PL-5.3245 DM (μg) PL (μm) 0.755 56 Kaneko (2005) Poecilostomatoid Copepoda (12 species, C5-C6) Log10DM = 2.875Log10PL-7.458 DM (μg) PL (μm) 0.976 24 < 0.0001 Nishibe (2005) Other Copepoda spp. Log10DM = 2.62Log10TL-6.40 DM (μg) TL (μm) 0.670 242 Imao (2005) Mysidacea Meterythrops microphtalma Log10DM = 3.10Log10BL+0.26 DM (μg) BL (mm) 0.987 46 Ikeda (1992) Amphipoda Cyphocaris challengeri Log10DM = 2.83Log10BL+0.69 DM (μg) BL (mm) 0.992 17 < 0.0001 Yamada (2002) Primno abyssalis Log10DM = 2.71Log10BL+0.76 DM (μg) BL (mm) 0.992 22 < 0.0001 Yamada et al. (2002) Themisto japonica Log10DM = 2.12Log10BL+1.11 DM (μg) BL (mm) 0.912 30 < 0.0001 Yamada (2002) Themisto pacifica Log10DM = 2.72Log10BL+0.690 DM (μg) BL (mm) 0.992 21 < 0.0001 Yamada (2002) Euphausiacea Euphausia pacifica (Furcilia-Adult) Log10WM = 3.130Log10BL+0.914 WM (μg) BL (mm) 0.990 67 < 0.01 Kim (2009) Thysanoessa inspinata (Furcilia-Adult) Log10WM = 3.190Log10BL+1.041 WM (μg) BL (mm) 0.988 53 < 0.01 Kim (2009) Thysanoessa longipes (Furcilia-Adult) Log10WM = 3.263Log10BL+0.929 WM (μg) BL (mm) 0.992 55 < 0.01 Kim (2009) Chaetognatha Eukrohnia fowleri Log10DM = 3.32Log10TL-1.14 DM (μg) TL (mm) 0.950 85 Imao (2005) Sagitta elegans Log10DM = 2.91Log10TL-0.79 DM (μg) TL (mm) 0.970 54 Imao (2005) Other Chaetognatha spp. Log10DM = 2.80Log10TL-0.6 DM (μg) TL (mm) 0.900 96 Imao (2005) Doliolida Dolioletta toritonis (nurse) Log10WM = 2.16Log10L+1.56 WM (μg) L (mm) 0.915 20 Aono (1999) Dolioletta toritonis (phorozooid) Log10WM = 2.24Log10L+1.77 WM (μg) L (mm) 0.895 7 Aono (1999) Dolioletta toritonis (gonozooid) Log10WM = 2.39Log10L+1.66 WM (μg) L (mm) 0.934 23 Aono (1999) Salpida Cyclosalpa bakeri Log10DM = 3.03Log10L+0.2 DM (μg) L (mm) 0.968 14 Aono (1999) Salpa aspera Log10DM = 3.66Log10L-0.74 DM (μg) L (mm) 0.963 25 Aono (1999) Salpa fusiformis Log10DM = 2.73Log10L+0.36 DM (μg) L (mm) 0.947 24 Aono (1999) Thalia democratica (solitary zooid) Log10DM = 2.26Log10L+0.86 DM (μg) L (mm) 0.839 19 Aono (1999) Thalia democratica (aggregate zooid) Log10DM = 2.86Log10L+0.53 DM (μg) L (mm) 0.939 25 Aono (1999) Appendicularia Oikopleura longicauda Log10DM = 1.988Log10TL-4.264 DM (μg) TrL (μm) 0.982 29 < 0.001 Shichinohe (2000) Mollusca Mollusca spp. Log10DM = 1.13Log10TL+2.29 DM (μg) TL (mm) 0.440 9 Imao (2005) page 3 of 15Zoological Studies 56: 13 (2017) Mass and chemical composition measurements Fresh specimens/samples were removed from seawater, placed on a 100-µm mesh and then rinsed briefly with distilled water to remove salt. Samples on the mesh were then placed on clean dry tissues to remove water. The waterfree samples were then placed in a pre-weighed aluminium pan and frozen at -20°C. In a laboratory on land, the wet mass (WM) was measured using a microbalance (Mettler Toledo MT5) with a precision of 1 µg, and the sample was freeze-dried for five hours and stored in a drying oven at 60°C for twelve hours. After cooling in a desiccator for several hours, the dry mass (DM) was measured with a microbalance. The water compositions (% of WM) were calculated according to the differences in masses (water = 100 (WM - DM)/WM). The dried samples were ground, weighed, and placed in a tin cup, after which carbon (C) and nitrogen (N) were measured by a CHN coder (Vrio EL III). The C and N compositions were expressed as a percentage of the DM. Body length and volume measurements The measurements of the lengths of the body parts of the various taxa treated in this study are shown in figure 1. The bell height (BH) of the Cnidaria, total length (TL) of the Annelida, standard length (SL) of the Ostracoda, body length (BL) of the Mysidacea, body length (BL) of the Amphipoda, body length (BL) of the Euphausiacea, total length (TL) of the Chaetognatha, length (L) of the Doliolida, length (L) of the Salpida, trunk length (TrL) of the Appendicularia, and total length (TL) of the Mollusca were measured. For the Copepoda, only the prosome length (PL) was measured for all of the species (Fig. 2A). The lipid stores were quantified for five species (see later), and image analyses from the dorsal and lateral views were performed using the equipment of Shimada and Oku (2014). The length (L) and width (W) of the prosome (PL and PW), urosome (UL and UW) and oil sac (OSL and OSW) were measured to a precision of 1 µm with the aid of Image J software (Figs. 2A, B). The volumes (V: mm3 ind.-1) of the prosome (PV), urosome (UV) and oil sac (OSV) were calculated with the following equation (Escribano and McLaren 1992): V = 1/6 × L × π × (W/2)2. The total volume (TV) was calculated as the sum of the PV and UV (TV = PV + UV). To evaluate the effect of the viewing direction (dorsal Fig. 1. Illustration showing the length measurements of various zooplankton taxa. A: bell height (BH) of Cnidaria, B: total length (TL) of Annelida, C: standard length (SL) of Ostracoda, D: body length (BL) of Mysidacea, E: body length (BL) of Amphipoda, F: body length (BL) of Euphausiacea, G: total length (TL) of Chaetognatha, H: length (L) of Doliolida, I: length (L) of Salpida, J: trunk length (TrL) of Appendicularia, K: total length (TL) of Mollusca. Note that the length measurements for Copepoda are summarized in figure 2. (A) (B) (C) (H) (I) (G) (J) (F) (E) (D) (K) page 4 of 15Zoological Studies 56: 13 (2017) Fig. 2. Diagrams of the length and volume measurements of Copepoda (Eucalanus bungii C6F with full lipid) in dorsal (A) and lateral (B) views. C: E. bungii C6F with low lipid, D: Neocalanus cristatus C5 with full lipid (left) and low lipid (right), E: Metridia okhotensis C5F with full lipid (left) and low lipid (right). PL: prosome length, PW: prosome width, OSL: oil sac length, OSW: oil sac width, UL: urosome length, UW: urosome width. and lateral views) on the volume value, we compared the volumes from both the dorsal (VD) and lateral (VL) views. Length-weight equation and analysis To express the L-W relationship, we applied the power-law equation, which was also used by Uye (1982) for the warm-water region of Japan: Log10M = a × Log10L + b, where a and b are fitted constants, M is the mass in µg and L is the length in µm for mesozooplankton (Ostracoda, Copepoda and Appendicularia) and in mm for macrozooplankton taxa (Uye 1982). For the cnidarian Aglantha digitale, we applied an expression in a quadratic equation based on the literature (Ikeda and Imamura 1996). Most of the L-W relationships were unpublished data in theses submitted to Hokkaido University. To evaluate the changes in the volume and mass together with the lipid store (from low to medium and full lipid), we calculated the “percentage change” index according to Hopkins et al. (1984) for E. bungii C6F, M. okhotensis C6F and N. cristatus C5. This value means that the percentage changes in the volume and mass from the values at low lipid store are expressed as 100%: Percent change = 100 × ([Full or Medium] - [Low]) / Low, where Full, Medium, and Low indicate the values at full, medium and low lipid levels, respectively (Hopkins et al. 1984). The percentage changes were calculated for volumes (PV, OSV and TV), masses (µg WM and µg DM), and chemical compositions (µg Water, µg C and (A) (B) (C) (D) (E) page 5 of 15Zoological Studies 56: 13 (2017) µg N); the effects of lipid storage on the volume, mass, and chemical compositions were then evaluated. RESULTS Length-weight equations and chemical compositions The L-W equations of 41 species belonging to 12 taxa are summarized in table 1. For the Euphausiacea and Doliolida, the mass units were in WM and were in DM for the other taxa. For five species of Copepoda (Calanus pacificus, E. bungii, Metridia pacifica, N. cristatus, N. flemingeri and N. plumchrus), the units of C and N are also shown as presented in Ueda et al. (2008). The coefficient of determination (r2) ranged between 0.306 and 0.998, and the mean value was 0.886 ± 0.145 (mean ± 1 SD). The water, C, and N compositions of each taxon are summarized in table 2. The water compositions ranged between 69.8% WM (Thysanoessa longipes) and 95.2 ± 0.5%WM (Aglantha digitale). The C compositions ranged Table 2. Summary on water composition (% of wet mass, %WM), carbon (C) and nitrogen (N) composition (% of dry mass, %DM) for various zooplankton taxa/species which dominated in the western subarctic Pacific and their adjacent seas. Values are mean ± 1 SD Taxa Species (taxinomic category) Water (%WM) C (%DM) N (%DM) References Cnidaria Aglantha digitale 95.2 ± 0.5 15.4 ± 1.6 4.3 ± 0.4 Ikeda (2014a) Other Cnidaria spp. 3.8 ± 4.3 1.0 ± 1.4 This study Annelida Annelida spp. 85.2 ± 1.15 32.8 ± 4.28 7.9 ± 0.71 This study Ostracoda Discoconchoecia pseudodiscophora 75.9 50.8 ± 4.7 7.8 ± 0.8 Ikeda (1990), Kaeriyama and Ikeda (2004) Metaconchoecia skogsbergi 39.8 ± 1.6 9.4 ± 0.5 Kaeriyama and Ikeda (2004) Orthoconchoecia haddoni 44 8.95 Kaeriyama and Ikeda (2004) Copepoda Calanus pacificus (C1-C5) 18.9 ± 13.4 3.0 ± 2.3 Ueda et al. (2008) Eucalanus bungii (C1-C5) 92.8 ± 1.5 15.1 ± 5.8 1.5 ± 0.7 Shoden (2000), Ueda et al. (2008) Gaetanus variabilis (C5-C6) 79.7 ± 3.5 47.1 ± 4.9 10.1 ± 1.1 Ikeda et al. (2006) Heterorhabdus tanneri (C6) 88.1 43.2 9.4 Ikeda et al. (2006) Metridia pacifica (C1-C5) 8.0 ± 5.3 1.5 ± 0.8 Ueda et al. (2008) Neocalanus cristatus (C1-C5) 83.9 ± 8.5 30.9 ± 20.5 3.8 ± 2.3 Kobari et al. (2003), Ueda et al. (2008) Neocalanus flemingeri (C4-C5) 81.6 ± 6.3 47.8 ± 11.6 4.5 ± 1.8 Kobari et al. (2003), Ueda et al. (2008) Neocalanus plumchrus (C1-C5) 86.9 ± 4.5 22.2 ± 15.0 4.5 ± 2.1 Kobari et al. (2003), Ueda et al. (2008) Paraeuchaeta birostrata (C5-C6) 70.7 ± 4.1 58.8 ± 2.2 7.5 ± 0.5 Ikeda et al. (2006) Paraeuchaeta elongata (C4-C6) 75.0 ± 5.0 56.5 ± 2.8 7.9 ± 0.9 Ikeda et al. (2006) Paraeuchaeta rubra (C5-C6) 69.9 ± 2.6 60.8 ± 3.1 7.5 ± 0.6 Ikeda et al. (2006) Pleuromamma scutullata (C6) 80.3 47.7 9.6 Ikeda et al. (2006) Pseudocalanus newmani (C6) 47.9 ± 5.3 9.7 ± 1.3 Lee et al. (2001) Scolecithricella minor (C4-C6) 80.9 ± 2.3 Yamaguchi (1999) Cyclopoid Copepoda (3 species) 81.4 ± 5.1 44.8 ± 5.9 9.8 ± 1.8 Ikeda (2014b) Poecilostomatoid Copepoda (12 species, C6) 52.9 ± 5.3 8.5 ± 1.3 Nishibe and Ikeda (2008) Other Copepoda spp. 81.4 ± 5.1 51.6 ± 7.5 9.8 ± 1.8 Imao (2005), Ikeda (2014b) Mysidacea Meterythrops microphtalma 84.8 ± 1.7 46.1 ± 8.0 8.6 ± 1.9 Ikeda (1992), Ikeda (2013b) Amphipoda Cyphocaris challengeri 80.1 ± 4.7 36.8 ± 4.8 6.8 ± 0.9 Yamada and Ikeda (2003) Primno abyssalis 77.4 ± 4.2 54.3 ± 5.4 7.6 ± 0.3 Yamada and Ikeda (2003) Themisto japonica 77.7 ± 2.6 46.3 ± 2.3 8.8 ± 0.5 Yamada and Ikeda (2003) Themisto pacifica 77.2 ± 2.8 47.9 ± 7.2 8.3 ± 1.1 Yamada and Ikeda (2003) Euphausiacea Euphausia pacifica (Adult) 76.8 ± 0.9 34.5 ± 1.7 9.3 ± 0.2 Kim (2009) Thysanoessa inspinata (Adult) 77.2 ± 1.6 36.3 ± 1.0 9.8 ± 0.3 Kim (2009) Thysanoessa longipes (Juvenile-Adult) 69.8 41.0 ± 9.8 8.6 Iguchi and Ikeda (2005) Chaetognatha Eukrohnia fowleri 90.3 ± 1.5 41.1 ± 10.3 8.5 Imao (2005), Ikeda and Takahashi (2012) Sagitta elegans 91.0 ± 0.2 44.2 ± 4.5 12.1 Imao (2005), Ikeda and Takahashi (2012) Other Chaetognatha spp. 89.6 ± 2.5 39.5 ± 5.3 9.9 ± 2.0 Imao (2005), Ikeda and Takahashi (2012) Doliolida Dolioletta toritonis Salpida Salpa fusiformis 4.6 ± 0.2 1.0 ± 0.03 This study Appendicularia Oikopleura longicauda 29.5 ± 2.9 7.3 ± 2.0 Shichinohe (2000) Mollusca Mollusca spp. 80.2 ± 7.7 28.0 ± 0.9 5.2 ± 2.6 Imao (2005), Ikeda (2014b) page 6 of 15Zoological Studies 56: 13 (2017) Fig. 3. Scatter plots of the water (water, % wet mass [WM]), carbon (C, % dry mass [DM]) and nitrogen (N, % DM) compositions for various zooplankton taxa (cf. Table 2). Plotted areas for the gelatinous taxa (Annelida, Appendicularia, Chaetognatha, Cnidaria, Mollusca and Salpida) are indicated by dashed circles. For taxa other than the gelatinous taxa, regression lines are calculated for each panel. The separation of gelatinous and semi-gelatinous taxa was derived from Larson (1986). (A) (B) (C) from 3.8 ± 4.3% DM (various cnidarian species) to 60.8 ± 3.1% DM (Paraeuchaeta rubra). The N compositions were observed between 1.0 ± 0.03% DM (Salpa fusiformis) and 10.1 ± 1.1% DM (Gaetanus variabilis). The relationships among water, C, and N compositions are shown with scatter plots (Fig. 3). When comparing the water and C compositions, a significant negative correlation (high C composition implying low water composition) was detected for the non-gelatinous taxa (p < 0.01, Fig. 3A). The gelatinous taxa plotted at positions of high water and low C compositions. The plots of the semigelatinous taxa appear between the other two types of taxa. The comparison between the N and water compositions showed no correlation among them (Fig. 3B). The comparison between the C and N compositions were positively correlated in both gelatinous and non-gelatinous taxa (p < 0.0001, Fig. 3C). Effect of lipid storage in Copepoda The mean values of the body volumes (PV, OSV, and TV), masses (WM and DM) and chemical compositions (water, C, and N) of the Copepoda were summarized for the three lipidaccumulation categories (low, medium, and full) (Table 3). Lateral (VL) view body volumes were smaller than the dorsal view (VD) values (Table 3). From a regression analysis between VL and VD in the form of VL = a × VD, where a is a fitted constant, highly significant correlations were observed for all volume units (p < 0.0001) (Fig. 4). The slopes (a) of the regressions ranged between 0.538 and 0.896. Based on the mean slope values, the volume values from the lateral view (VL) were 82.1 ± 5.4% of the dorsal view (VD) for PV, 82.4 ± 5.5% for TV and 70.0 ± 12.6% for OSV (Fig. 4). By applying the “percent change” of Hopkins et al. (1984), we analysed the volume and mass changes along with the lipid accumulation (Fig. 5). Within the treated units, the value increases were prominent, especially for OSV, DM, and C (Fig. 5). For OSV, the percent changes of the values for the medium and full lipids were 550% and 3212% of the values for the low lipids, respectively (Fig. 5). These observed values indicate a 5-fold and 32-fold OSV increase from the low-lipid accumulation to specimens with medium and full lipid accumulations, respectively. The maximum percentage change was 495% and 741% for DM and C, respectively. Species-specific differences were also detected; thus, small increases of volume and mass along with a change in lipids were observed for E. bungii C6F, while the greatest increases were observed for N. cristatus C5; the M. okhotensis C6F values were between those of these two species (Fig. 5). The proportion of OSV to TV was standardized by calculating OSV/TV, and page 7 of 15Zoological Studies 56: 13 (2017) Table 3. Comparison on body volume, mass and chemical composition of Copepoda (Eucalanus bungii, Metridia okhotensis, Neocalanus cristatus, Neocalanus flemingeri and Neocalanus plumchrus) between full, medium and low lipid contents Volume (mm3 ind.-1) Species Lipid Stage (n) PVLPVDOSVLOSVDTVLTVD E. bungii Full C6F 7 10.504 12.760 0.210 0.330 10.559 12.809 Medium C6F 9 9.413 11.199 0.034 0.036 9.463 11.247 Low C6F 1 11.028 10.145 0.000 0.000 11.087 10.202 M. okhotensis Full C5M 5 1.110 1.274 0.247 0.288 1.140 1.308 Full C6F 2 2.195 2.069 0.407 0.388 2.271 2.153 Medium C6F 6 2.259 2.592 0.052 0.104 2.354 2.685 Low C6F 2 2.011 2.214 0.012 0.012 2.095 2.290 N. cristatus Full C5 6 18.553 22.383 2.944 2.488 18.795 22.623 Medium C5 16 12.929 17.381 0.398 0.497 13.135 17.579 Low C5 6 7.114 11.337 0.226 0.292 7.243 11.459 N. flemingeri Full C5 9 4.110 4.678 1.333 1.826 4.157 4.719 Full C6F 2 7.521 9.219 2.141 2.013 7.632 9.345 Medium C5 10 3.168 3.676 0.478 1.012 3.213 3.719 Low C6F 1 4.791 7.521 0.000 0.000 4.907 7.657 N. plumchrus Medium C5 12 2.103 2.672 0.138 0.134 2.134 2.705 Mass (mg ind.-1) Chemical Composition Species Lipid Stage (n) WM DM (n) Water (%WM) (n) C (%DM) N (%DM) E. bungii Full C6F 5 12.399 1.158 5 90.48 3 34.27 7.057 Medium C6F 5 11.152 0.768 5 93.09 2 28.60 7.730 Low C6F 2 11.501 0.861 2 92.49 1 28.00 6.470 M. okhotensis Full C5M 6 1.167 0.370 6 68.22 2 54.20 6.725 Full C6F 2 2.738 0.665 2 75.72 2 46.35 6.080 Medium C6F 15 2.760 0.548 15 80.14 6 43.83 8.685 Low C6F 2 2.720 0.393 2 85.47 1 37.40 10.220 N. cristatus Full C5 25 22.989 6.047 25 73.74 15 54.65 6.942 Medium C5 35 17.394 1.927 35 89.24 14 39.91 8.061 Low C5 11 11.059 1.016 11 90.91 6 38.63 9.740 N. flemingeri Full C5 9 5.418 1.959 9 66.69 11 51.86 7.445 Full C6F 1 11.093 3.813 1 65.63 1 60.40 5.900 Medium C5 26 3.963 1.171 26 71.11 18 53.04 7.472 Low C6F 1 5.963 0.334 1 94.39 1 18.50 2.610 N. plumchrus Medium C5 6 2.663 0.420 6 84.95 2 43.90 7.365 PV: prosome volume, OSV: oil sac volume, TV: total volume (= PV + urosome volume [UV]), WM: wet mass, DM: dry mass, Water: water content, C: carbon content, N: nitrogen content. Lower letters in volume indicate observed direction: i.e., L: lateral and D: dorsal views. (n): observed number. correlation analyses were then performed on the chemical compositions (water, C and N) (Fig. 6). With increasing OSV/TV, the water compositions decreased (r2 = 0.85, p < 0.0001, Fig. 6A), while the C compositions increased (r2 = 0.64, p < 0.001, Fig. 6B). For the N compositions, no correlations were observed for OSV/TV (Fig. 6C). DISCUSSION Length-weight Equation Throughout this study, species-specific differences in L-W equations were noted, even within the same taxa. For the Copepoda, the body sizes of Paraeuchaeta spp., Neocalanus spp. and E. bungii were similar, but marked differences in page 8 of 15Zoological Studies 56: 13 (2017) mass were observed. Thus, for the same body size (PL = 3 mm), the DM values of Neocalanus spp. and Paraeuchaeta spp. were 3 or 12 times greater than that of E. bungii (Fig. 7). These speciesspecific differences in mass may be caused by their species-specific differences in chemical composition. Flint et al. (1991) revealed that the lipid and protein compositions of Eucalanus spp. were extremely low, 1/7-1/10 (lipid) and 1/5-1/20 (protein), respectively, of those in Calanus spp., which are called “jelly-bodied copepods” because of their low organic and high water compositions. The water composition of E. bungii evaluated in this study (92.8 ± 1.5%) (Table 2) was similar to those of the gelatinous zooplankton (i.e., > 95%, Alldredge and Madin 1982). According to Ohman (1997), the water compositions of three sympatric copepods (Rhincalanus nasutus, C. pacificus and M. pacifica) (82.3-84.3%) differed significantly from those of Eucalanus californicus (92.9%). These facts suggest that the chemical compositions of Eucalanus spp. may be similar to those of previously reported gelatinous zooplankton taxa. The C and N compositions of E. bungii were 15.1% and 1.5% DM, respectively; those of Neocalanus spp. were 22.2-47.8% and 3.84.5% DM respectively; and those of Paraeuchaeta spp. were 56.5-60.8% and 7.7-7.9% DM (Table 2), respectively. Paraeuchaeta spp., the heaviest DM species with the same body size, had high C and N compositions, while both the C and N compositions were the lowest for the lightest, E. bungii, and all of the values of Neocalanus spp. were intermediate between these two species. For zooplankton, C and N are the lipid and protein indices, respectively (Postel et al. 2000). The low C and N compositions of E. bungii suggest that individuals of this species have low lipid and protein compositions in their bodies. Because of the low organic compositions (lipid and protein), E. bungii show high water compositions as well as a transparent body colour and structure, which may function to reduce predation pressure by visual predator fishes. From the viewpoint of feeding modes, Paraeuchaeta spp. are categorized as carnivores (Yen 1983), while Eucalanus spp. mainly feed on phytoplankton (Ohtsuka et al. 1993), and Neocalanus spp. are suspension feeders (Dagg 1993; Gifford 1993). For carnivorous Paraeuchaeta spp., high protein compositions may provide a high swimming ability, which allows them to capture prey. For herbivorous Eucalanus spp., a high swimming ability may not be required; Fig. 4. Comparison of volumes (prosome volume: PV, total volume: TV, oil sac volume: OSV) of five copepod species between those in lateral views (Y-axis) and dorsal views (X-axis). Positions of 1:1 are shown with dashed lines. All regressions were significant (p < 0.0001). page 9 of 15Zoological Studies 56: 13 (2017)