Biomass and estimated production, and feeding pressure on zooplankton of chaetognaths in the Yellow Sea, China

The biomass and estimated production, feeding rate of chaetognaths were studied in the Yellow Sea. Sagitta crassa, Sagitta enflata, Sagitta bedoti, and Sagitta nagae were found during the study period, and S. crassa and S. nagae were the dominant species. The mean chaetognath biomass was in the range of 2.56 5.57 mg DM m-3, and S. crassa and S. nagae comprised 49 90 and 10 33%, respectively. The geographical distribution pattern was differed between different species. The annual average estimated feeding rate and production of chaetognaths was evaluated as 55.24 mg C m-3 year-1 and 15.90 mg C m-3 year-1, respectively. The body length of Sagitta species less than 9 11 mm had relatively high feeding rate in August, September, and December because of their relatively high total biomass, while the body length with 8 16, 11 15, and 6 16 mm groups had relatively high feeding rate in March, May, and June, respectively. The average feeding pressure on zooplankton biomass and production by chaetognaths could be evaluated as 1.07 and 18.95%, respectively, and the biomass and production of small copepods, and nauplius and copepodite of large copepods would be seriously impacted during whole year. Article history: Received 5 December 2018 Revised 23 June 2019 Accepted 26 June 2019


INTRODUCTION
The chaetognaths are major pelagic invertebrates which are extremely abundant in marine systems, often second only to copepods in number (Feigenbaum and Maris 1984). Chaetognaths are well known as important carnivores of secondary producers in marine ecosystems (Terazaki 2001), and their predation may have a significantly impact on copepod population dynamics (Reeve 1970;Sameoto 1972), especially under conditions of low environmental productivity (Kimmerer 1984;Øresland 1990).
Biomass, production and consumption of chaetognaths and dominant chaetognath species have been reported in some sea areas. Chaetognaths had a mean biomass of 0.109 -0.146 mg C m -3 in Southern Ocean (Kruse et al. 2010). The yearly production and required a prey production of S. crassa was estimated to be 3.8 g C m -2 and 13.1 g C m -2 year -1 in Tokyo Bay, respectively. Chaetognaths were identified as the second most important group composing at times up to 30% (mean 14.7%) of total zooplankton abundance in the pelagic system of the Prince Edward Islands . Trophic ecology of chaetognaths or dominant chaetognath species was studied by many researchers, which could be indicated by the feeding pressure on zooplankton biomass and production. Feeding pressure of chaetognaths on zooplankton varied significantly under different environmental conditions and between different sea areas. Kehayias et al. (2005) reported that the feeding pressure of chaetognaths on zooplankton standing stocks was 0.02 -2.76% in Aegean Sea, while Baier and Purcell (1997b) Terr. Atmos. Ocean. Sci., Vol. 31, No. 1, 61-75, February 2020 reported that chaetognaths could consume up to 44% d -1 of standing stocks of large copepods in South Atlantic Bight. Feeding pressure of Sagitta elegans on zooplankton production was only 0.7 -1.0% in St. Margaret's Bay (Sameoto 1972), while the dominant species Sagitta tenuis consumed up to 5 -15% of zooplankton production in Chesapeake Bay (Canino and Grant 1985).
The Yellow Sea is a marginal sea of the northwest Pacific, surrounded by the eastern coast of China and the Korean peninsula, with a mean depth of 44 m and maximum depth of 103 m, and as a shallow shelf water, the Yellow Sea contains complex physical environments and is affected greatly by climatic conditions (Huo et al. 2012). The Yellow Sea is a major fishing ground, and the bordering countries derive a substantial part of their food from fishing in coastal waters. It is our responsibility to preserve the health of this productive ecosystem in order to sustain fishing in the future. The relationships between zooplankton and higher trophic levels in the Yellow Sea was a key scientific question, and how to describe variations of zooplankton biomass and production and make it easier to be modeled in the Yellow Sea ecosystem is the main purpose . The zooplankton functional groups approach, which is considered a good method of linking the structure of food webs and the energy flow in the ecosystems, is used to describe this process in the Yellow Sea ecosystem .
Chaetognaths is one gelatinous carnivorous zooplankton groups in the Yellow Sea , which seem to be opportunistically positioned to utilize secondary production that is ordinarily consumed by fish (Mills 1995). In addition, the chaetognaths group is also a food resource for fish . However, to our knowledge, there has few studies on chaetognaths in the Yellow Sea. Zuo et al. (2004) and Lee et al. (2016) described the relationship between distribution patterns of chaetognaths and environmental factors; Huo et al. (2010) reported that the life cycle of S. crassa in the Southern Yellow Sea; Kang et al. (2012) summarized that chaetognaths also showed a downward trend after the mid-1990s, but high positive anomalies were observed in 2002 and 2006 in the Eastern Yellow Sea; Choi and Park (2013) found that the abundance of chaetognaths fluctuated around the long-term mean value during the past four decades in the western waters of the Korean Peninsula in the Yellow Sea.
However, the trophic ecology of chaetognaths is unknown in the Yellow Sea. Our hypothesis is that chaetognaths would require a significant prey production in the Yellow Sea. In this present study, the biomass, and estimated production and feeding of chaetognaths as well as the feeding pressure of chaetognaths on zooplankton biomass and secondary production were evaluated based on data obtained from the six cruises during period from September 2006 to August 2007 in the Yellow Sea, which is very important for "parameterization" of food web structure models of the Yellow Sea.

Study Area and Sampling Methods
The chaetognaths was sampled during six cruises on the R/V "Beidou" in the Yellow Sea during period from September 2006 to August 2007. Figure 1 shows the location of transects and sampling stations during these six cruises. The maximum water depth of these stations was less than 100 m. Vertical profiles of temperature and salinity were recorded at each station with a Sea-Bird (SBE 25) CTD instrument lowered and retrieved from the sea surface to near the sea bed. Seawater samples for the measurement of chlorophyll a (Chl a) were collected at different depths from the sea surface to near the sea bed, filtered onto GF/F glass-fiber filters, extracted using 90% aqueous acetone and the concentration of Chl a determined fluorometrically using a Turner Designs fluorometer. Bad weather prevented experiments being done at some stations in certain cruises.
The chaetognaths were sampled using the conical plankton net with 500 μm mesh size and 0.8 m mouth diameter Sun et al. 2010), which were towed vertically at ~1 m s -1 from near the sea bottom of the water column to the sea surface. A TSK flow-meter and depth sensors were mounted at the mouth of the each net to measure the volume of water filtered and the sampling depth, respectively. After the nets were retrieved, the zooplankton samples were immediately preserved in 5% neutralized formalin seawater solution. In the laboratory, the abundance of chaetognaths was all counted, and were enumerated under a dissecting microscope.

Biomass Determination
Only four chaetognath species, Sagitta crassa, Sagitta enflata, Sagitta bedoti, and Sagitta nagae, was found during the study period in the Yellow Sea. The biomass of each chaetognath species was determined based on the relationship between the dry weight and the body length. The biomass of S. crassa and S. enflata could be obtained based on the following equations: W = 0.197L 3.01 for S. crassa (Nagasawa 1984) and W = 0.0001352L 3.1545 for S. enflata (Feigenbaum 1979), respectively. Because the abundance of S. bedoti was rather low compared with other Sagitta species, and the body length of S. bedoti was similar to that of S. crassa, so the biomass of S. bedoti was evaluated according to the relationship between the dry weight and the body length of S. crassa. Total lengths of 170 undamaged live S. nagae were measured to the nearest 0.1 mm. These samples were rinsed with distilled water to remove external salts, dried at 60°C for 24 -36 h, weighed to the nearest 0.1 μg. The range of lengths was 8.50 -25.75 mm. The relationship between the dry weight and the body length of S. nagae could be described as following equation: W = aL b (Uye 1982), where a and b were the constant parameters.

Production and Feeding Rate Evaluation
According to Huo et al. (2012), the respiration rate of each Sagitta species for the feeding and production estimates was calculated based on the Ikeda and Motoda's physiological model (Ikeda andMotoda 1975, 1978). Respiration was expressed as carbon units (R C , mg C animal -1 h -1 ) as the following equation, assuming an ammonia RQ of 0.97 (Gnaiger 1983): R C = R O2 × 0.97 × 12/1000 × 22.4, where 12 is the molar mass of carbon (g mol -1 ), 22.4 is the molar volume of oxygen (L mol -1 ) and R O2 is respiration rate (μl O 2 animal -1 h -1 ) which was calculated as a function of body weight (W) and temperature (T, °C): ln R O2 = -0.2512 + 0.7886 ln W + 0.0490 T, where R O2 is respiration rate (μl O 2 animal -1 h -1 ) and W is body dry weight (mg animal -1 ) (Ikeda 1985). Equations of feeding and production of each Sagitta species were suggested as follows (Nagasawa 1991): F = 1.6R C , P = 0.46R C , where F is feeding and P is production. The feeding and production per day of each Sagitta species were obtained as F and P multiplying the abundance of each Sagitta species (ind. m -3 ) and 24 h.
In the Yellow Sea, the strong stratification appeared in the summer time from June to September resulted by the Yellow Sea Cold Bottom Water (YSCBW). During the summer time at the YSCBW sea area, S. enflata, S. nagae and S. bedoti were mostly distributed in the upper layer and outside the cold water mass sea area, but S. crassa inhabited mostly inside the thermocline . So in the June, August, and September, when calculating the respiration rate of S. crassa at the YSCBW sea area, the parameter T was the average temperature of the thermocline, while calculating the respiration rate of the other three Sagitta species, the parameter T was the average temperature of water column upper the thermocline. And in other months when the strong stratification disappeared, the parameter T was the average temperature of water column from sea surface to the bottom. Based on the results reported by Huo et al. (2012), we could confirm that S1-3, S1-4, S1-5, S1-6, S1-7, S3-2, S3-3, S3-4, S3-5, S3-6, S3-7 located inside the YSCBW sea area in June 2007, and S1-2, S1-3, S1-4, S1-5, S1-6, S1-7, S3-3, S3-4, S3-5, S3-6, S3-7 in August 2007, and S1-2, S1-3, S1-4, S1-5, S1-6, S1-7, S3-7 in September 2006, respectively. Huo et al. (2012) reported the biomass (mg C m -3 ) and estimated production (mg C m -3 d -1 ) of zooplankton at the same sapling sites in the Yellow Sea. So the feeding pressure could be evaluated based on the feeding rate of each Sagitta species and total four Sagitta species divided by the biomass and production of zooplankton as follows: F Biomass = F/B Zooplankton , F Production = F/P Zooplankton , where F is feeding rate of chaetognaths, F Biomass is feeding pressure of chaetognaths on zooplankton biomass, F Production is feeding pressure of chaetognaths on zooplankton production, B Zooplankton is zooplankton biomass and P Zooplankton is zooplankton production, respectively.

Data Analysis
The sampling map and geographical distribution of chaetognath biomass were constructed using Surfer 8.0 software. Biomass, feeding rate and estimated production of chaetognaths were reported as mean values ± standard deviation (mean ± SD). Rang and average of biomass (mg DM m -3 ), feeding rate (μg C m -3 d -1 ) and production (μg C m -3 d -1 ) of four Sagitta species and total chaetognaths were calculated as the range between the lowest and highest value, and mean value from all sampling sites, respectively. Statistical analyses were conducted using SPSS 19.0.
Multivariate ordination techniques were used to analyze the relationship of environmental variables with the chaetognaths by CANOCO for Windows 4.5. The environmental parameters were adopted as the explanatory variables. All of these environmental parameters were log10transformed before analysis. The chaetognath species data were log10(x + 1) transformed before analysis to obtain consecutive distributions. Detrended correspondence analysis (DCA) for the chaetognath species data was employed to decide whether linear or unimodal ordination methods should be applied in this study. DCA revealed that the maximum gradient length of the four axes was lower than 3, therefore, redundancy analysis (RDA) was used. Monte Carlo simulation was used to test the significance of the environmental parameters to explain the chaetognaths data in the RDA.

Biomass Distribution of Four Sagitta Species
The total length-dry weight relationship of S. nagae is described by the following equation: W = 0.0565L 3.3426 , where W is dry weight (μg) and L is total length (mm). The mean total biomass of chaetognaths in the Yellow Sea peaked in May 2007 with 5.57 mg DM m -3 , and was lowest in August 2007 with 2.56 mg DM m -3 , and in other months, the biomass was in the range of 2.85 -4.12 mg DM m -3 (Table 1). S. crassa and S. nagae comprised 49 -90 and 10 -33% of the total biomass throughout the year, respectively. High biomass of S. crassa was mainly distributed at the northern part of the investigated area throughout the year (Fig. 2). S. enflata, the oceanic species, was mainly distributed at the southern part of the study area. In March 2007, relative high biomass of S. nagae located at the northern part of study area, while in other investigated months, it was mainly distributed at the central and southern part of the study area. S. bedoti had lowest biomass compared with other three Sagitta species during the study period in the Yellow Sea.
During the study period, the spatio-temporal distribution of the environmental variables varied between the investigated months. The results observed for these environmental variables were reported by Huo et al. (2012). The relationship between environmental factors and chaetognaths biomass in each month investigated was analyzed by RDA (Table 2 and

Estimated Feeding Rate and Production of Four Sagitta Species
The mean total feeding rate and production of four Sagitta species was also highest in May 2007 with 222.84 and 64.39 μg C m -3 d -1 , respectively, but was lowest in December 2006 with 115.74 and 33.28 μg C m -3 d -1 , respectively (Tables 1 and 3). In September 2006 and June, August 2007, chaetognaths also had high feeding rate and production (Huo et al. 2012), even though their body weights were relatively low in August and September (Tables 1 and 3). S. crassa and S. nagae had high feeding rate and production during the whole study period because of relatively their high biomass    (Fig. 4).

Feeding Rate of Different Body Length of Chaetognaths
Feeding rate of different body length of chaetognaths was analyzed at the interval of 1 mm which was showed in Fig. 5. The feeding rate of 5 -9 mm body length of chaetognaths was relatively higher than those of other chaetognath body length groups in September 2006 and August 2007. In December 2006, the 7 -11 mm body length of chaetognaths had relatively high feeding rate than other chaetognath body length groups. And in the March, May, and June of 2007, the relatively high feeding rate of chaetognaths was in the body length of 8 -16, 11 -15, and 6 -16 mm groups, respectively.

Feeding Pressure on Zooplankton Biomass and Production
Based on the results of zooplankton biomass and production obtained from the same sampling sites of the Yellow Sea reported by Huo et al. (2012), the feeding pressure on zooplankton biomass and production by four Sagitta species in the Yellow Sea could be evaluated as Table 4 showed. Feeding pressure on zooplankton biomass and production was lowest in May 2007 with 0.80 and 13.54%, respectively. Feeding pressure on zooplankton production was as high as 26.90%, but was only 0.99% on zooplankton biomass in March 2007. In other investigated months, the feeding pressure was in the range of 1.05 -1.22 and 14.50 -21.21% on zooplankton biomass and production, respectively. Based on these data, the annual average feeding pressure on zooplankton biomass and production was evaluated as 1.07 and 18.95%, respectively, in the Yellow Sea. S. crassa and S. nagae contributed most to the feeding pressure on zooplankton biomass and production during the whole study period because of their relatively high biomass compared with other two species (Table 4).

Methods of Evaluating the Feeding Rate of Chaetognaths
Feeding rate of chaetognaths can be evaluated as the following equation proposed by Bajkov (1935):  Table 3. Rang and average of feeding rate (μg C m -3 d -1 ) and production (μg C m -3 d -1 ) of four Sagitta species and total chaetognaths in different investigated months in the Yellow Sea. of prey taken per 24 h), and DT is digestion time (h), and NPC represents mean number of prey ingested per chaetognath. NPC can be obtained based on the abundance of copepod mandible or chaetognath hooks or other zooplankton detected from the gut content analysis. DT can be evaluated using two methods. The first method is that chaetognaths which contain food in their guts are cultured in situ environmental conditions, and then the digestion time is observed (Feigenbaum and Maris 1984). The other method is that the digestion time is calculated using the empirical equation: DT = 10.96e -0.086T derived for the Sagitta species by Stuart and Verheye (1991). The accuracy of the quantitative analysis of gut contents in samples collected by planktonic nets can be affected by various factors. Among the main factors that can cause uncertainties in the estimates is the cod-end feeding and also the regurgitation and evacuation of prey during the handling and preservation processes (Kehayias 2003). Baier and Purcell (1997a) indicated that tow duration should be minimized and prey in the foregut from gut content analysis should be excluded in order to reduce errors due to codend feeding, and they also reported that prey loss due to evacuation can be as much as 50% of the total prey in tows of greater than 2 min duration, so some researcher suggest that the NPC values in the above feeding rate equation were multiplied by two (Baier and Purcell 1997b;Kehayias et al. 2005). In addition, the possibility of chaetognaths regurgitating their prey during sampling can't be discounted (Feigenbaum and Maris 1984). Feigenbaum and Maris (1984) reviewed digestion times of some chaetognath species evaluated in laboratory or in situ environmental conditions. However, it is evident that digestion time at any temperature can be extremely variable because of the size, type, abundance, chemical composition of prey, as well as the size, development stage of chaetognath species, and that variation between species may not be as great as within species (Stuart and Verheye 1991;Øresland 2000).
The Ikeda-Motoda's physiological methods is convenient to be used to calculate feeding and production of chaetognaths if body weight and water temperature were obtained (Ikeda and Motoda 1978;Ikeda 1985). However, the gross growth efficiency and assimilation efficiency of 0.3 and 0.7 (Hirst and Sheader 1997;Ikeda 1985;Runge and Roff 2000) may vary greatly due to different species (Alldredge 1984;Omori and Ikeda 1984). And the coefficient between the feeding rate, and production and respiration rate is also affected by various factors, such as chaetognath species, water temperature, size and type of prey.
In the present study, we checked the gut contents of four Sagitta species on board during the investigated period, but few Sagitta had food in their gut, and these few Sagitta had hardly excreted over long time mainly because of environmental change. The choice of which of the methods mentioned above depends on the research questions and in  Table 4. Rang and average of estimated feeding pressure on zooplankton biomass (%) and production (%) per day of four Sagitta species and total chaetognaths in different investigated months in the Yellow Sea. some cases, a global model of growth may be a sufficient approximation of growth and reproductive rates of zooplankton (Runge and Roff 2000). Therefore, Ikeda and Motoda's physiological model was selected to estimate feeding rate and production of chaetognaths based on the body weight and water temperature in the present study, which has been applied in other several studies (Nagasawa 1991). Table 5 showed proportions of biomass and production of chaetognaths to total biomass and production of zooplankton (Huo et al. 2012) in the Yellow Sea. Even though chaetognaths had highest biomass in May 2007, but only contributed 6.63% to total biomass of zooplankton in the Yellow Sea. The biomass of chaetognaths occupied as high as 13.61% of total zooplankton biomass even though they had relatively low biomass in December 2006. And in other months, chaetognaths contributed 6.73 -12.73% to total zooplankton biomass. Contributions of chaetognaths to total zooplankton production were lower compared with those to total zooplankton biomass in the Yellow Sea. It was indicated that S. crassa and S. nagae dominated the chaetognath community whole year based on their biomass and production in the Yellow Sea. Sagitta crassa has five generations in the Yellow Sea and in adjacent sea areas (Nagasawa 1991;Huo et al. 2010). And these five generations of S. crassa overlapped during whole year in the Yellow Sea and in Tokyo Bay (Nagasawa 1991;Huo et al. 2010), which indicated that S. crassa can reproduce and grow under environmental conditions of whole year in the Yellow Sea. This was the main reason that S. crassa had relative high biomass compared with other Sagitta species in the Yellow Sea, especially from December to next June. Sagitta nagae had six cohorts throughout the year in Suruga Bay and peak numbers of all stages occurred between May and August (Nagasawa and Marumo 1978). In the present study, S. nagae also had relative high biomass during the period from May to August.

Biomass and Production of Chaetognaths in the Yellow Sea
Carbon content was considered as 36.7% of dry weight in chaetognaths group (Kiørboe 2013). So average production rate (P/B) of chaetognaths can be calculated based on the average biomass and production in different investigated months in the Yellow Sea (Fig. 6). Average production of chaetognaths was highest with 64.39 μg C m -3 d -1 in May 2007, but the average production rate was only 3.15% d -1 because of relatively low water temperature (Huo et al. 2012). The average production rate of chaetognaths was as high as 4.79 and 4.73% d -1 in September 2006 and August 2007, respectively, because of relatively high water temperature even though their low standing stock in the Yellow Sea. Chaetognath production occupied 6.58% of total zooplankton production in December 2006, and only contributed 3.17% to total zooplankton production in May 2007 (Table 5).
The YSCBW had a vital role in zooplankton biomass and production distributions from June to September in the Yellow Sea (Huo et al. 2008(Huo et al. , 2012. Sagitta enflata, S. nagae, and S. bedoti were not impacted by the YSCBW because they mostly distributed in the upper layer, but S. crassa was impacted because it inhabited mostly inside the thermocline in the Yellow Sea . Therefore, S. crassa production was strongly impacted by the YSCBW in September 2006, June and August 2007. In these sampling months, the production rate of S. crassa ranged from 3.57 -4.69% d -1 inside the YSCBW sea area, while 5.09 -6.74% d -1 outside the cold water mass sea area (data not shown).
The distribution patterns of chaetognaths were mainly impacted by some environmental factors in the Yellow Sea. Sagitta crassa was considered as a coastal species which had abilities to suit to wild temperature and salinity ranges (Nagai et al. 2006), and the spatial-geographical distribution patterns of S. crassa were also affected by food availability. Liu et al. (2003) reported that the high biomass of every development stages of Calanus sinicus was found at the same area of S. crassa distribution, and Zuo et al. (2006) also reported that the as much as 1.80 -3.52 × 10 5 ind m -2 of small copepod distributed at these sea areas. It was showed that the relatively high biomass of S. nagae was distributed in the central and southern part of the study area in the Yellow Sea. Xiao (2004) renamed S. nagae as Sagitta sinica which was a new species in the Yellow Sea. Sagitta sinica was described as a warm-temperate neritic species and was wildly distributed in the Yellow Sea and East China Sea (Xiao 2004). It is necessary to confirm whether it is a new species based on combinations of morphological, molecular and other methods in the future. The distribution mechanism of S. enflata and S. bedoti was different between warm seasons and cold seasons in the Yellow Sea. In warm seasons, the expansion of these two Sagitta species was mainly caused by water temperature rise and was not by current transportation . While in cold seasons, they were mainly impacted by physical transportation and could be transported up to 36 degree north latitude into northern Yellow Sea by the Yellow Sea Warm Current .

Feeding Pressure on the Zooplankton in the Yellow Sea
Feeding pressure on biomass and production of zooplankton by specific species of chaetognaths in different sea areas was summarized in Table 6. Feeding pressure of chaetognaths on zooplankton biomass in the Yellow Sea was lower than that in Kaneohe Bay (Kimmerer 1984), in Aegean Sea (Kehayias et al. 2005) and in Mediterranean (Kehayias 2003), but was similar with that in Southern Ocean . Sagitta friderici, Sagitta setosa, Sagitta tenuis, and S. enflata exerted the high feeding pressure on zooplankton biomass in South Afirica, Black Sea, Chesapeake Bay, and Chile Sea than those of Sagitta species in the Yellow sea (Canino and Grant 1985;Drits and Utkina 1988;Stuart and Verheye 1991;Giesecke and González 2004). Feeding pressure on zooplankton production by chaetognaths was different from those on biomass (Table 6). Sagitta elegans and Sagitta hispida feed 36 and 100% on zooplankton production in Bedford Basin and Card Sound, respectively, which were higher than those of chaetognaths in the Yellow Sea (Sameoto 1973;Reeve and Baker 1975). While the feeding pressure of S. crassa on zooplankton production in the Yellow Sea was higher than that of S. crassa in Tokyo Bay (Nagasawa 1991), and higher than other Sagitta species in other sea areas (Sameoto 1972;Feigenbaum 1977;Canino and Grant 1985;. Copepod is commonly reported as being the most abundant food item found in chaetognath guts because of its abundance in the marine habitat (Feigenbaum 1991). It was reported that the feeding of chaetognaths was more serious for the copepod community in their lower production period (Baier and Purcell 1997b). Sameoto (1973) indicated that the average feeding pressure of S. elegans on copepod production was 1%, but increased up to 50% during winter time in Bedford Basin. Øresland (1990) discovered that Eukrohnia hamata averagely feed on 0.03 -0.06% of copepod biomass, but 12% of copepod biomass was fed during their diapause stages during winter time in Southern Ocean. In the present study, the feeding pressure of chaetognaths on zooplankton production was also highest during cold seasons in December 2006 and March 2007 with 21.21 and 26.90%, respectively, which indicated that the feeding of chaetognaths had important impacts on zooplankton community structure and production, especially on copepod.

Feeding Impacts of Chaetognaths on Different Body Length of Zooplankton
Body size of zooplankton was an important factor on determining the composition of food item fed by chaetognaths (Rakuza-Suszczewski 1969;Sullivan 1980), which indicated that feeding pressure of chaetognaths on different  Table 5. Contributions of chaetognaths to total the biomass (mg DM m -3 d -1 ) and production (mg C m -3 d -1 ) of zooplankton in different months in the Yellow Sea. body length of zooplankton was obvious distinct. There was a positive relationship between body length of chaetognaths and their food: H = aP b (Pearre 1980), but Sullivan (1980) indicated that chaetognaths did not have directly food selection. S. nagae did not showed food selection when the body width of copepod was in the range of 0.2 -1.2 mm in Suruga Bay (Nagasawa and Marumo 1976). Yang and Li (1995) reported that the body length of zooplankton from 0.7 mm (e.g., Paracalanus parvus) to 2.3 mm (e.g., Calanus sinicus) was the main food of S. crassa in the Yellow Sea. Although adults of chaetognaths can feed on wide size spectrum of zooplankton, there exactly occurred difference on food size ranges at their different development stages (Sullivan 1980). Saito and Kiørboe (2001) demonstrated that feeding rates of S. elegans were affected by prey size, prey swimming behavior and small-scale turbulence, and it had highest feeding rates when the prey size was 6 -10% of their body length. So in the present study, effects of feeding on different size spectrum of zooplankton by chaetognaths were evaluated in the Yellow Sea based on 6 -10% of prey size to body length of chaetognaths. Based on this assumption, chaetognaths had highest feeding rate on the body length with 0.6 -1.6 mm of zooplankton in March, May, and June 2007, which indicated that chaetognaths mainly feed on small copepod, such as Paracalanus parvus, Cen-tropages mcmurrichi, Oithona similis, and individuals of large copepod at their development stages, such as Calanus sinicus Huo et al. 2012), and in September 2006, December 2006, and August 2007 mainly fed on small copepod and nauplius and copepodite of large copepod due to the highest feeding rate on the body length with 0.5 -1.1 mm of zooplankton in the Yellow Sea. Therefore, the biomass and production of small copepods, and nauplius and copepodite of large copepods would be impacted during whole year in the Yellow Sea. Calanus sinicus was the absolute dominant species in the Yellow Sea (Sun 2005;Sun and Zhang 2005). Calanus sinicus was in the diapause stage in the summer time in the Yellow Sea (Pu et al. 2004), and had lowest production in this period (Zhang et al. , 2007Huo et al. 2008), which indicated that C. sinicus would be impacted seriously by the feeding of chaetognaths in the summer time in the Yellow Sea. are acknowledged. Thanks are also due to the anonymous reviewers for their valuable comments and suggestions.  Table 6. Feeding pressure by the different species of chaetognaths on biomass and production of zooplankton in different seas.