Seasonal Variations in Abundance and Size Composition of the Lobate Ctenophore Bolinopsis mikado (Moser) in Tokyo Bay, Central Japan

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1 Journal of Oceanography, Vol. 56, pp. 419 to 427, 2000 Seasonal Variations in Abundance and Size Composition of the Lobate Ctenophore Bolinopsis mikado (Moser) in Tokyo Bay, Central Japan TOMOYUKI KASUYA*, TAKASHI ISHIMARU and MASAAKI MURANO Department of Ocean Sciences, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo , Japan (Received 29 October 1998; in revised form 1 November 1999; accepted 22 December 1999) Surveys of the abundance and size composition of the ctenophore Bolinopsis mikado were conducted in Tokyo Bay over a 5-year period from 1990 to B. mikado appeared throughout the year, and its mass occurrence was observed between August and November; annual maximum abundance ranged from 19 to 91 ind. m 2. Water temperature seems to influence the seasonal variation of B. mikado abundance. Environmental conditions (e.g. rough waters due to a typhoon) and predation by the beroid ctenophore Beroe cucumis appear to affect annual variations of B. mikado abundance. Size frequency distributions of B. mikado indicated that its reproduction was most active in summer and fall but occurred throughout the year in Tokyo Bay. A sharp decline of the copepod population in August 1990 was probably due to predation by B. mikado which was very abundant at that time; its predatory impact was estimated to be 24 % day 1. Keywords: Ctenophora, Bolinopsis mikado, abundance, Tokyo Bay, copepod. 1. Introduction Lobate and cydippid ctenophores are voracious predators on mesozooplankton such as copepods (Chandy and Greene, 1995) and are considered to be important regulators of these populations in marine coastal waters (Hirota, 1974; Deason and Smayda, 1982; Suthers and Frank, 1990; Båmstedt, 1998). Many quantitative studies have been conducted on the growth, seasonal abundance and distribution of ctenophores, especially considering the lobate species Mnemiopsis leidyi and M. mccradyi (Deason, 1982; Kremer and Reeve, 1989; cf. Kremer, 1994; Mutlu et al., 1994) and the cydippids Pleurobrachia pileus (van der Veer and Sadée, 1984; Frank, 1986; Mutlu et al., 1994; Wang et al., 1995) and P. bachei (Hirota, 1974; Larson, 1986). In Tokyo Bay, central Japan, the lobate ctenophore Bolinopsis mikado (Moser) is a common species and oc- * Corresponding author. ad91203@tokyo-u-fish.ac.jp * Present address: Asian Natural Environmental Science Center, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo , Japan. Present address: Institute of Environmental Ecology, Shin-Nippon Meteorological and Oceanographical Consultant Co. Ltd., Riuemon, Ooigawa-cho, Sida-gun, Shizuoka , Japan. Copyright The Oceanographic Society of Japan. curs in high abundance during late summer (Komai, 1915). For the genus Bolinopsis, some quantitative information on abundance and distribution has been obtained for a tropical species, B. vitrea (Harbison et al., 1978; Kremer et al., 1986), and a boreal-arctic species, B. infundibulum (Siferd and Conover, 1992; Bailey et al., 1994; Falkenhaug, 1996). Ecological studies of B. mikado are scarce (Kasuya et al., 1994): however only Kanashiro and Senta (1985) and Nomura and Ishimaru (1998) have presented studies of its population dynamics. As a first step in clarifying the life history of B. mikado and understanding its role in coastal marine ecosystems, this paper describes the seasonal variations of its abundance and size composition in Tokyo Bay. We discuss factors regulating the seasonal abundance of B. mikado in Tokyo Bay, including the interaction with its predator, the beroid ctenophore Beroe cucumis Fabricius. Potential grazing pressure of B. mikado on copepods is also discussed. 2. Materials and Methods 2.1 Morphology As lobate ctenophores develop, they pass through a stage of cydippid larva with a pair of tentacles after hatching. Bolinopsis mikado loses its tentacles at a total length (TL, mm) of ca. 15 mm and assumes a morphology similar to the adult (Kasuya, 1997). In this paper, organisms 419

2 Table 1. Number of a post-larval Bolinopsis mikado sampled by a vertical haul of a Norpac net at respective stations in Tokyo Bay. Numbers in parentheses are the number of Beroe cucumis. Date Station F1 F2 F3 F4 F5 F Jun. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 Jul. 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 0 (0) 21 Aug. 9 (0) 0 (12) 22 (4) 10 (2) 19 (6) 10 Sep. 0 (0) 0 (1) 0 (9) 0 (0) 0 (8) 0 (3) 22 Oct. 0 (0) 0 (1) 17 Nov. 8 (0) 5 (0) 2 (1) 3 (0) 0 (0) 0 (0) 14 Dec. 3 (0) 0 (0) 0 (1) 0 (0) 0 (0) 0 (0) Jan. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 13 Feb. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 12 Mar. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 14 Apr. 0 (0) 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 11 May 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 13 Jun. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 3 (0) 10 Jul. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 (0) 5 Aug. 0 (0) 1 (0) 4 (0) 4 (0) 2 (0) 5 (0) 11 Sep. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 28 Oct. 0 (0) 1 (1) 0 (0) 0 (0) 0 (0) 0 (0) 16 Nov. 2 (0) 8 (0) 4 (2) 3 (1) 0 (0) 0 (0) 9 Dec. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (1) , 17 Jan. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 13 Feb. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 16 Mar. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 14, 15 Apr. 0 (0) 0 (0) 0 (0) 0 (0) 1 (0) 0 (0) 12, 13 May 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 6 Jun. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 8 Jul. 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 27 Aug. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 15 Sep. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1 Oct. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 9, 10 Nov. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 Dec. 0 (0) 0 (0) 0 (1) 0 (1) 0 (0) 0 (0) of <15 mm TL are defined as larvae and those of 15 mm TL are defined as post-larvae. TL of larval Bolinopsis mikado is represented by the length from the aboral pole to the mouth. For larva with developing oral lobes and the post-larva, TL is the length from aboral pole to the end of the lobes (cf. Kasuya et al., 1994). 2.2 Sampling A series of samplings were conducted from June 1990 to December 1994 utilizing a routine survey carried out once a month by the plankton laboratory of Tokyo University of Fisheries in Tokyo Bay. Samples were collected at 2 to 6 stations from June 1990 to March 1993, with supplementary sampling at several stations until December 1994 (Table 1, Fig. 1). Approximate bottom depths at each station were: Stn. F1, 15 m; Stn. F2, 19 m; Stn. F3, 26 m; Stn. F4, 22 m; Stn. F5, 27 m; Stn. F6, 28 m. In the routine survey, a vertical haul of a Norpac net (45-cm mouth diameter, 0.33-mm mesh) from the near bottom to the surface was made once at each station, correcting for towing distance by the inclination of the towing wire (Table 1). As most ctenophore species are known to occur in patches of high abundance (Hirota, 1974; Kremer and Nixon, 1976), it is difficult to quantify their abundance using data obtained by a vertical haul of a Norpac net. To obtain reliable data on the appearance of Bolinopsis mikado, an 80-cm mouth diameter plankton 420 T. Kasuya et al.

3 Table 1. (continued). Date Station : no sampling made. F1 F2 F3 F4 F5 F Jan. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 8 Feb. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 Mar. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 11 Apr. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 19 May 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 16, 17, 20 Jun. 0 (0) 0 (0) 0 (0) 0 (0) 10 Jul. 0 (0) 0 (0) 0 (0) 0 (0) 26 Aug. 0 (0) 0 (0) 0 (0) 0 (0) 15 Sep. 0 (0) 0 (0) 0 (0) 0 (0) 21 Oct. 1 (0) 1 (0) 2 (0) 16 Nov. 5 (0) 4 (0) 15 (0) 20 (0) 14, 15 Dec. 1 (0) 0 (0) 9 (0) 5 (0) 0 (0) 1 (0) Jan. 0 (0) 1 (0) 3 (0) 1 (0) 2 (0) 0 (0) 20 Feb. 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 8 Mar. 0 (0) 0 (0) 15 Apr. 0 (0) 0 (0) 18 May 1 (0) 0 (0) 13 Jun. 0 (0) 0 (0) 9 Jul. 0 (0) 0 (0) 8 Aug. 0 (0) 1 (0) 16 Sep. 12 (0) 17 (0) 25 Oct. 0 (0) 0 (0) 8 Nov. 0 (0) 0 (0) 14 Dec. 2 (0) 1 (0) net (80-cm net) fitted with a 3-l cod end was towed horizontally in the surface layer for a towed distance of ca m at the same station as was used for the Norpac net sampling. To prevent clogging, the 80-cm net was used with a mesh size of 3 mm. B. mikado in an 80-cm net sample was counted and TL was measured. Although the sampling by a horizontal haul of an 80-cm net was continued until March 1993, data on TL were collected until August The Norpac and 80-cm nets were towed at an approximate speed of ca. 1 m s 1. A flowmeter (Rigosha Co. Ltd.) was attached at the middle point between the center and the rim of the mouth of either net. The volume of water filtered was 2 8 m 3 for a vertical haul and m 3 for a horizontal haul. Water temperature and salinity were measured at each station with a CTD recorder (Neil Brown Mark 3B). The sampling at each station was conducted at almost the same time of day between 0900 h and 1500 h. 2.3 Counting and measurements Each Norpac net sample was transferred into a cylindrical glass bottle of 950 ml capacity. The numbers of the post-larval Bolinopsis mikado and Beroe cucumis in the bottle were counted and converted into abundance per volume of water sampled. Ctenophores collected in a horizontal haul of an 80- cm net were gently transferred into a polycarbonate tank (30 l) containing approximately 10 liters of seawater. Bolinopsis mikado and Beroe cucumis in the tank were counted by dipping up with a plastic Petri dish for a small individual and with a plastic scoop (ca cm) for a large one. TL of B. mikado was subsequently measured to the nearest 1 mm only. If an individual had injured oral lobes, TL was estimated from body length (BL, mm) using the regression equation BL = 0.73TL (Kasuya et al., 1994). Larvae smaller than 5 mm TL were not counted since it was difficult to distinguish them from broken ctenophore tissue in the sample. Wet weight (WW, g) of a post-larval Bolinopsis mikado was estimated from TL using the regression equation WW = TL 2.72 (Kasuya et al., 1994). Bolinopsis mikado in Tokyo Bay 421

4 to June, with an abundance ranging from 0 to 3 ind. m 2. B. mikado usually became abundant from July. The seasonal occurrence and the maximum abundance of B. mikado varied annually. B. mikado was usually most abundant in summer, but in 1993 it was most abundant in November and December, with abundance ranging from 19 to 91 ind. m 2. By contrast, no mass occurrences of B. mikado were observed at any stations during summer and fall in The maximum abundance (138 ind. m 2 ) was recorded at Stn. F3 on 22 August 1990 with a biomass of 346 g WW m 2. Figure 3(b) shows the seasonal variations of the mean abundance of Bolinopsis mikado in unit volume of surface water based on 80-cm net samples. Because of the great number of B. mikado in the 80-cm net samples, we could not count individual numbers in August B. mikado appeared almost throughout the year. Although we could not obtained a sample with a horizontal haul of an 80-cm net every month, the data of B. mikado abundance we did obtain show a similar seasonal pattern to that based on Norpac net samples. Fig. 1. Monthly sampling stations of Bolinopsis mikado in Tokyo Bay. The dashed lines define the divisions of Tokyo Bay. 3. Results 3.1 Hydrography Seasonal variations of water temperature and salinity at the surface of Stations F3 and F6 are shown in Fig. 2. Water temperatures showed a similar seasonal pattern at all other stations, ranging from 8.7 to 29.1 C. The minimum and the maximum values were recorded in January or February and in August or September, respectively. Salinity ranged from 18.6 to 32.3 psu, decreasing in summer and fall. At Stn. F3, the influence of river discharge caused salinity to be more variable and lower than at other stations. 3.2 Seasonal and annual variations of Bolinopsis mikado abundance The number of the post-larval Bolinopsis mikado collected by a vertical haul of a Norpac net is shown in Table 1. B. mikado appeared under conditions in which the temperature ranged between 8.7 and 29.1 C and salinity between 24.9 and 32.6 psu. Although the number of B. mikado collected differed at each station, the seasonal change showed a similar pattern. Figure 3(a) shows the seasonal variations in the mean abundance of B. mikado in unit area of the water column, based on the data in Table 1. B. mikado was very scarce from January 3.3 Size frequency distributions Successive data on size frequency distributions of Bolinopsis mikado in Tokyo Bay were taken from June 1990 to August 1991 (Fig. 4). The individuals of the 5 15 and mm TL size classes occupied more than 65 95% of the B. mikado population in August and October 1990 and August B. mikado of the 5 15 mm TL size class also appeared from November 1990 to February 1991, occupying about 30 40% of the population. In May 1991, no larvae were collected, but individuals of >35 mm TL occupied over 80% of the population and large individuals of >65 mm TL occurred, with a maximum of 85 mm TL. 4. Discussion 4.1 Sampling problems In November 1990, the total number of Bolinopsis mikado collected by a Norpac net vertical haul was 18 individuals, and the mean abundance was 0.9 ind. m 3. This density is ca. four-fold larger than that based on 80- cm net samples collected at the same time. As ctenophores are known to occur in patches of high abundance (Hirota 1974; Kremer and Nixon, 1976), a large difference between the B. mikado abundance calculated from a vertical haul and that from a horizontal haul seems to be due to the patchiness of B. mikado distribution. Because of an increase in the volume of water filtered, a horizontal haul of an 80-cm net must be appropriate when B. mikado is very scarce and may minimize the under- or overestimation of B. mikado abundance due to its patchiness. It is difficult to estimate an accurate abundance of 422 T. Kasuya et al.

5 Fig. 2. Seasonal variations in water temperature ( C) and salinity (psu) at Stations F3 (dashed line) and F6 (solid line) in Tokyo Bay, Fig. 3. Seasonal variations in the mean abundance of Bolinopsis mikado and Beroe cucumis in Tokyo Bay. Individuals in unit area of water column based on Norpac net samples (a) and those in unit mass of surface water based on 80-cm net samples (b) from all stations. Abundance of B. cucumis in (b) was determined since October Vertical bars represent single standard deviations of abundance (n = 3 6). For the circled crosses in (b), the standard deviation was not calculated, due to recording the total number of B. mikado collected at 6 stations. ns = no sampling made. B. mikado by a vertical haul of a Norpac net. However, we have sampled B. mikado at a number of stations in a survey during its mass occurrence (Table 1), and the seasonal patterns of B. mikado abundance based on Norpac net samples are similar to those based on 80-cm net samples (Fig. 3). Therefore, it is concluded that the data based on Norpac net samples show a pattern of seasonal and annual variations in B. mikado abundance. Although Beroe cucumis was not present in the Norpac net samples in August and November 1992 (Table 1), a large number of B. cucumis were collected by an 80-cm net horizontal haul, and its mean abundance was calculated as ca. 0.2 ind. m 3 (Fig. 3(b)). As species of Beroe swim rapidly during the escape response Bolinopsis mikado in Tokyo Bay 423

6 Fig. 4. Size frequency distributions of Bolinopsis mikado from June 1990 to August 1991 in Tokyo Bay. The data are pooled from all stations. N is the number of ctenophores measured. (Matsumoto and Harbison, 1993), net avoidance might cause an underestimate of B. cucumis abundance, calculated according to the data from a vertical haul of a Norpac net the mouth area of which is smaller than 80 cm. 4.2 Factors affecting the seasonal and annual abundance of Bolinopsis mikado Temperature and food availability Although Bolinopsis mikado showed no distinct patterns in the seasonal abundance in Tokyo Bay, it was highly abundant mainly from August to November when the surface temperature exceeded 18 C (Fig. 2). A similar seasonal pattern of abundance has been shown for Mnemiopsis leidyi in Narragansett Bay, Rhode Island (Kremer and Nixon, 1976). In Saanich Inlet, British Columbia, food supply coupled with temperature seem to be the primary factors regulating population growth of Pleurobrachia bachei (Larson, 1986). In temperate waters, water temperature has a greater influence on the annual abundance cycle of ctenophores than food availability (cf. Kremer, 1994). Lobate and cydippid ctenophores predate many zooplankton species, such as copepods, molluscs and fish larvae (Burrell and Van Engel, 1976; Siferd and Conover, 1992). In the Inland Sea of Japan, an 424 T. Kasuya et al.

7 inverse relationship between the abundance of B. mikado and that of the cyclopoid copepod Oithona davisae and the appendicularian Oikopleura dioica has been reported (Uye and Ichino, 1995; Uye and Sano, 1995). Kasuya (1997) cultured B. mikado from larva to adult in the laboratory using the calanoid copepod Acartia spp. as food. B. mikado appears to consume these zooplankton species in the field. In Tokyo Bay, mesozooplankton abundance is generally high from January to June (Nomura and Murano, 1992), suggesting that the occurrence of B. mikado is closely linked to water temperature. In the laboratory and field experiments, the instantaneous growth rate and the WW specific egg production rate of Bolinopsis mikado were measured, with high Q 10 values of 5.9 at a temperature of C and 4.3 at a temperature of C, respectively (Kasuya, 1997), indicating that its growth and fecundity are very sensitive to temperature. The increase of growth and egg production rates of B. mikado with increase of water temperature appears to lead to its high population growth and mass occurrence in the summer/fall, when water temperatures are over 18 C, and vice versa in the winter/spring in Tokyo Bay. In the summer/fall of 1992, no Bolinopsis mikado was collected by a horizontal haul of an 80-cm net, suggesting that B. mikado did not occur with high abundance. Inter-annual variations of ctenophore abundance appear to be influenced by food availability (Kremer, 1976; cf. Kremer, 1994). In Tokyo Bay, mesozooplankton abundance in August 1992 was 6,700 ind. m 3 as compared with 700 ind. m 3 in August 1990 when B. mikado occurred abundantly (Nomura, unpublished data). The population growth of B. mikado might not have been limited by prey abundance during summer in Physical factors Physical factors could be important in controlling the abundance of Bolinopsis mikado. In September and October 1991, July 1992, and from July to September in 1993, a drastic decrease in salinity was observed not only at Stn. F3 but also at Stn. F6 in Tokyo Bay (Fig. 2), which was due to increased fresh water inflow due to a typhoon. After stormy weather, B. mikado with a mark of injury in the aboral region are very common (Komai, 1915; Kasuya, personal observation). It is suggested that the destructive effect of turbulence in rough water injures B. mikado s body or kills it, limiting its population increase for the respective periods in 1991, 1992 and 1993 as a consequence. Low salinity in the surface layer might enhance the advective transport of surface water from the inner to the outer part of Tokyo Bay. In the Malangen fjord, northern Norway, Bolinopsis infundibulum appears to be transported out of the fjord at a daily rate of 55% of its standing stock (Falkenhaug, 1996). Because the sampling interval and the number of sampling stations are not adequate in the present study, the effect of advection on Bolinopsis mikado population in Tokyo Bay is unknown Predation by Beroe cucumis Predation by the beroid ctenophore Beroe is considered to be a cause of the rapid decline of lobate and cydippid ctenophore populations (Hirota, 1974; van der Veer and Sadée, 1984; cf. Purcell, 1991; Swanberg and Båmstedt, 1991; Siferd and Conover, 1992; Falkenhaug, 1996). Beroe cucumis was present at many stations in August 1990 in Tokyo Bay (Table 1). At Stn. F2, while no Bolinopsis mikado was sampled, 12 individuals of B. cucumis were collected and its abundance reached 76 ind. m 2. A high abundance of B. cucumis was also observed in September 1990, and no B. mikado was collected at any stations in this month (Table 1, Fig. 3(a)). In August and November 1992, a large number of B. cucumis was collected by a horizontal haul of an 80-cm net, and no mass occurrences of B. mikado were found through the year (Fig. 3(b)). Predation by B. cucumis appears to be responsible for the sharp decline of B. mikado population in September 1990, leading to no mass occurrence of B. mikado in the summer/fall in 1992, working in concert with the above physical factors. In Narragansett Bay, predation by butterfish Peprilus triacanthus and parasitism by sea anemone Edwardsia leidyi have a significant impact on the population of Mnemiopsis leidyi (cf. Kremer, 1994). Psenopsis anomala, which is closely related to P. triacanthus (Abe, 1971) and Edwardsia japonica (Uchida, 1971), are distributed in Japanese coastal waters. However, the effects of these organisms on the Bolinopsis mikado population remain unclear in Tokyo Bay. 4.3 Reproduction and seed population of Bolinopsis mikado Larval Bolinopsis mikado was present almost throughout the year, being very abundant in August and October. B. mikado appears to reproduce throughout the year with its annual peak in summer and fall in Tokyo Bay. In size frequency distributions of Bolinopsis mikado in Tokyo Bay, individuals of >35 mm TL dominated the population in May. A similar seasonal pattern in size composition has been shown for Mnemiopsis leidyi in Narragansett Bay (Kremer and Nixon, 1976). Kremer (1976) created the population model and revealed that a small number of overwintering M. leidyi is sufficient to be a seed population for the dramatic summer time population increase in Narragansett Bay. In Tokyo Bay, an overwintering population, such as the large sized B. mikado population that occurred in May, seems to play the role of seed population for the mass occurrence in summer and fall, too. Bolinopsis mikado in Tokyo Bay 425

8 4.4 Predatory impact of Bolinopsis mikado population on mesozooplankton Trophic cascades were often initiated by ctenophores, suggesting that ctenophores are a key predator (Verity and Smetacek, 1996). In Tokyo Bay, a sharp decline of the copepod population was observed in August 1990 when Bolinopsis mikado occurred abundantly. For the representative species Oithona davisae and Acartia omorii, respective densities decreased from 210,000 to 110,000 ind. m 3 and from 193 to 4 ind. m 3 within a month (cf. Nomura, 1993). Hence, the effect of the B. mikado population on copepod populations is evaluated using data of August As Bolinopsis mikado is mainly present in the upper 5-m depth in summer (Kasuya, unpublished data), the abundance of post-larval B. mikado in August 1990 (shown in Fig. 3(a)) was recalculated assuming that all individuals are distributed evenly in the 0 5-m layer. The mean abundance of post-larval B. mikado is calculated 15 ind. m 3 with a biomass of 37.8 g WW m 3. As the size frequency distribution of the B. mikado population (cf. Fig. 4) shows, the frequency of larvae (<15 mm TL) was ca. 2-fold larger than that of post-larvae in August 1990; the abundance of larvae is estimated as 30 ind. m 3 at this time. At the water temperature of 28 C observed in August 1990 in Tokyo Bay (Fig. 2), the WW specific clearance rate of post-larval Bolinopsis mikado is estimated as 4.8 l day 1 at the food concentration of 10 Acartia l 1 using the Q 10 value of 1.8 (cf. Kasuya et al., 1994). For larval B. mikado (7 8 mm TL), its clearance rate at 28 C becomes 2 l day 1 at the same food concentration (cf. Kasuya, 1997). In Tokyo Bay, as the density of Acartia sized copepods has been reported to be ca. 10 ind. m 3 (Anakubo and Murano, 1991), clearance rates of larval and post-larval B. mikado estimated above are applied to calculate the predatory impact on the copepod population. The calculated clearance rate of B. mikado population is 60 l day 1 for larvae and 181 l day 1 for post-larvae. Thus, the total population could clear 240 l day 1, i.e. 24% of the water mass in August In this paper, we have shown that Bolinopsis mikado occurred throughout the year in Tokyo Bay, with an abundance that changes rapidly in a month or two. More frequent observations covering a wide area of the bay are required to show the population dynamics of B. mikado. However, because it is very fragile and cannot be preserved by formalin or by any preservatives known at present, it is difficult to conduct a more intensive field research by the present method. Well designed monitoring programs are necessary to understand or predict the kinds of ecological mechanisms and processes that are important for the survival and development of the B. mikado population. The submersible luminescence meter used for monitoring Mnemiopsis leidyi (Kremer and Nixon, 1976) is not applicable for B. mikado because it has no bioluminescence. We have been developing a video recording system with dark-field illumination attached to a plankton net instead of an ordinary cod end. This will be used for a future study of the population dynamics of B. mikado. Acknowledgements We are grateful to the captains, officers and crews of the T/S Seiyo Maru and T/B Hiyodori and the members of the Planktology Laboratory, Tokyo University of Fisheries, for their assistance and co-operation. We also thank three anonymous reviewers for their helpful comments on the manuscript. We appreciate the advice and encouragement given by Dr. Haruto Ishii and Dr. Yuji Tanaka. References Abe, T. (1971): Psenopsis anomala. p In New Illustrated Encyclopedia of the Fauna of Japan, Vol. 3, ed. by K. Okada, T. Uchida and S. Uchida, Hokuryu-Kan, Tokyo. Anakubo, T. and M. 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