Feeding by larvae of the mussel mytilus galloprovincialis on red-tide dinoflagellates.

ABSTRACT To investigate feeding by the larvae of the mussel Mytilus galloprovincialis on red-tide dinoflagellates, we measured grazing rates of M. galloprovincialis larvae as a function of larval age and prey concentration when feeding on several species of the red-tide dinoflagellates Alexandrium affine, Cochlodinium polykrikoides. Lingulodinium polyedrum, Prorocentrum minimum. Prorocentrum micans, and Scrippsiella trochoidea, as well as the flagellate Isochrysis galbana as a control species. The larvae were able to ingest all dinoflagellates offered in the current study; however, first feeding of the larvae on each species of the dinoflagellates occurred 9-13 days alter fertilization, whereas that for I. galbana occurred after 5 days. Ingestion rates of the larvae on unialgal diets of the dinoflagellates and I. galbana increased with increasing larval age up to 17-21 days, but were saturated or showed a continuous increase thereafter. Ingestion rates of 25-day-old larvae feeding on unialgal diets of the dinoflagellates increased rapidly with increasing prey concentration up to 1000-2200 ng C [mL.sup.-1], but were saturated at higher prey concentrations. The harmful alga C. polykrikoides. which has been responsible for great losses in the aquaculture industry, was the optimal prey. Maximum ingestion and clearance rates of the larvae on these dinoflagellates were 14-69 ng C [predator.sup.-1] [day.sup.-1] and 1.5-11.4 [micro]L [predator.sup.-1] [h.sup.-1], respectively. M. galloprovincialis larvae, one component of microzooplankters, exhibited higher maximum ingestion and clearance rates than previously reported for other microzooplankters, such as Fragilidium cf. mexicanum (mixotrophic dinoflagellate), Proroperidinium cf. divergens, Polykrikos kofoidii (heterotrophic dinoflagellates), or Tiarina fusus (small ciliate), but lower rates than Strombidinopsis sp. and Favella sp. (large ciliates). The results of the current study suggest that dinoflagellates sometimes can be primary prey for the Mytilus larvae, and the grazers compete with other microzooplankters for dinoflagellate prey. Also, red-tide dinoflagellates can be used for culturing the Mytilus larvae as prey in the aquaculture industry.

KEY WORDS: benthic-pelagic interaction, benthos, bivalve; HAB, mollusca, plankton. Mytilus

INTRODUCTION

Bivalves and dinoflagellates are major components of benthos and plankton in marine environments, respectively (Ruppert & Barnes 1994, Steidinger & Tangen 1997). Red tides and/or harmful algal blooms dominated by phototrophic dinoflagellates often have caused large-scale mortality of adult bivalves (e.g., ECOHAB 1995). As a consequence, them have been many studies on interactions between adult bivalves and red-tide dinoflagellates (Widdows et al. 1979, Nielsen & Stromgren 1991, Shumway & Cembella 1993. Shumway et al. 1997. Matsuyama et al. 1997, Bricelj & Shumway 1998). Bivalve larvae spend a certain period after hatching as plankton and need to feed on planktonic prey. Red-tide dinoflagellates often dominate phytoplankton assemblages in coastal waters. Thus, there is a high possibility that bivalve larvae frequently encounter red tide dinoflagellates. While there are some studies on the grazing by bivalve larvae on microflagellates and/or diatoms in the laboratory (Bayne 1965, Riisgard et al. 1980, Sprung 1984a, Sprung 1984b, Leonardos & Lucas 2000), there are a few studies on the interactions between bivalve larvae and red-tide dinoflagellates (Wikfors & Smolowitz 1995, Matsuyama et al. 2001); no data are available for bivalve larvae grazing rates as a function of red-tide dinoflagellate concentration and first feeding age for prey species.

Among bivalves, the genus Mytilus has a cosmopolitan distribution (e.g., Seed 1976). Some species are commercially important and cultivated at high densities in many countries (Hickman 1992). Mytilus galloprovincialis is a common bivalve in Europe (Morono et al. 1998, Tubaro et al. 1998), Asia (Matsuyama et al. 1997, NFRDI 1999), and Oceania (Gardner 2002) and is spreading to other areas as an invasive species (McQuaid & Phillips 2000).

To investigate interactions between bivalve larvae and red-tide dinoflagellates, we established cultures of M. galloprovincialis larvae and conducted experiments to examine their functional response when fed a variety of red-tide dinoflagellates. Our goal was to explore the predator-prey relationship between M. galloprovincialis larvae and red-tide dinoflagellates by determining the larval ingestion and clearance rates as functions of prey concentration and larval age.

The maximum ingestion and clearance rates of M. galloprovincialis larvae on unialgal diets of red-tide dinoflagellates were compared with those of other microzooplankters (heterotrophic dinoflagellates, and ciliates), which also are potential competitors, when feeding out the same prey species. Results of the current study provide a basis for understanding the interactions between bivalve larvae and red-tide dinoflagellates.

MATERIALS AND METHODS

Culture of Phytoplankton Prey

The dinoflagellates (Table 1) which have formed red tides in many coastal waters (Eppley & Harrison 1975, Jeong 1995, Ismael 2003) were grown at 19[degrees]C in enriched f/2 seawater media (Guillard & Ryther 1962) without silicate, under continuous illumination of 100 [micro]E [m.sup.-2] [s.sup.-1] provided by cool white fluorescent light. Only cultures in the exponential growth phase detected by cell count were used for the feeding experiments. Carbon contents for red-tide dinoflagellates were estimated from the volume of cells in batch cultures according to Strathmann (Strathmann 1967).

Preparation of M. galloprovincialis Larvae

Approximately 300 adults of the blue mussel M. galloprovincialis were collected from an aquafarm off Yeosu, Korea. in March 2001 when the seawater temperature was 12[degrees]C and the salinity was 33.4 psu. The shell length of the mussels ranged from 45 to 65 mm and gonads of most individuals were at late active or ripe stages. The mussels were transported to the laboratory within 6 h after collection and then acclimated to an experimental temperature (15[degrees]C) for two months. During acclimation, the mussels were maintained in two 200-L aquariums where seawater freshly filtered through 5 [micro]m GF/F filters and air from an air pump were supplied. The microflagellate Isochrysis galbana (final concentration = approximately [10.sup.3] cells [mL.sup.-1]) was provided as prey every day. The mussels were observed daily to check their condition, and dead mussels were removed immediately after they were found.

Spawning of the mussels was induced from May to June when natural spawning usually occurs in the Korean coastal waters. Approximately 10 individuals were used for each spawning period. Before spawning, the shell surface of the mussels was scraped to remove epibionts and rinsed with freshly filtered seawater. To induce spawning, the mussels were exposed to air for 1 h, put hack into a 10-L aquarium filled with freshly filtered seawater, and then the water temperature was increased gradually to 25[degrees]C. Most of these mussels released sperm and eggs within 30 min after the water temperature reached 25[degrees]C. As soon as any male first released sperm, the other males were removed to avoid possible polyspermy. One hour after spawning, all mussels were removed, and then aliquots of the water in the aquarium were taken to determine the fertilization rate. During the current study, we induced spawning 10 times, and the fertilization rates were always >95%. The mean diameter of the fertilized eggs was approximately 60 [micro]m. The egg suspension was passed through a 100-[micro]m mesh screen to remove focal material and other large particles, and a 35-[micro]m mesh screen was used to collect fertilized eggs without excessive numbers of sperm and smaller eggs. The eggs were rinsed three times with 5-[micro]m filtered and autoclaved seawater and were then incubated in a 20-L aquarium at 15[degrees]C in darkness without aeration. After 1-day incubation, most eggs had developed to the trochophore larval stage. From the first day after fertilization, Isochrysis galbana (final concentration = approximately 5 x [10.sup.3] cells [mL.sup.-1]) was provided to larvae as prey every day. Incubation water was wholly renewed every day.

Ingestion Rates as a Function of Larval Age

Experiments 1 to 7 were designed to measure ingestion and clearance rates of M. galloprovincialis larvae as a function of the larval age (elapsed time after fertilization), when feeding on unialgal diets of 6 red-tide dinoflagellate species anti Isochrysis galbana as a control species (Table 2). Feeding experiments were conducted when larvae were 1, 5, 9, 13, 17, 21, and 25 days old, because settlement of the larvae had started when they were 27 days old.

One day before these experiments were conducted, cultures of M. galloprovincialis larvae unfed (for the 1-day-old larvae) or growing on Isochrysis galbana (for the other aged larvae) were sieved through meshes of 80-150 [micro]m, and the larvae retained were transferred into I-L polycarbonate (PC) bottles. The bottles were filled to capacity with filtered seawater and placed on plankton wheels rotating at 0.9 rpm and incubated at 15[degrees]C under continuous illumination of 50 [micro]E [m.sup.-2] [s.sup.-1] from cool white fluorescent light. One day later, prey inside the stomach of the larvae was almost digested and indiscernible by microscope examination. The abundance of M. galloprovincialis larvae was determined by enumerating larvae in three 1-mL Sedgwick-Rafter counting chambers (hereafter SRCs).

Initial densities of M. galloprovincialis larvae and target prey were established using an autopipette to deliver predetermined volumes of known densities to the bottles. Triplicate 270-mL PC experiment bottles (mixtures of predator and prey) and triplicate control bottles (prey only) were set up at each predator-prey combination. Triplicate control bottles containing only M. galloprovincialis larvae also were established. Thirty milliliters of f/2 medium were added to each bottle. Each bottle then was filled to capacity with freshly filtered seawater and capped. The bottles were placed on plankton wheels under the environmental conditions described above. To determine actual predator and prey densities at the beginning of the experiment, and after 24, 48, and 72 h incubation, 10-mL aliquots were removed from each bottle and fixed with 5% Lugol's solution, and all larvae and all or >200 prey cells in three 1-mL SRCs were enumerated. Prior to taking subsamples, the condition of M. galloprovincialis larvae and prey was assessed by looking through the surface of each capped bottle using a dissecting microscope. The bottles were filled again to capacity with freshly filtered seawater, capped, and placed on plankton wheels as described above. Dilution of the cultures associated with refilling the bottles was considered in calculating ingestion rates.

Ingestion and clearance rates of M. galloprovincialis larvae on RTDs were calculated using the equations of Frost (1972). The incubation time for calculating ingestion and clearance rates was 48 h.

To examine the occurrence of ingestion by M. galloprovincialis larvae on each prey species, 2-mL aliquots from each experimental and control bottle, preserved with Lugol's solution after 48 h incubation, were transferred into wells of 12-well plate chambers. Subsequently, 0.3 mL thiosulfate ([Na.sub.2][S.sub.2][O.sub.3]) of a [10.sup.5] mg [L.sup.-1] concentration was added into each well to decolorize. One day later, the stomachs of the larvae were examined at 40-200x using Olympus compound and dissecting microscopes, and photographs were taken. Also, to find any undigested prey cells inside the stomachs of the larvae, approximately 20 M. galloprovincialis larvae fed on Prorocentrum micans were rinsed and carefully crushed using a very thin needle. We chose P. micans because it has an easily discernible shape. Photographs of the contents from the stomach were taken.

The shell lengths of M. galloprovincialis larvae at the beginning of each experiment and after 48 h incubation were measured using an image analyzing system; each individual larva was observed at a magnification of 20x, and its image was recorded using a Toshiba Model IK-642K CCD camera attached to a stereo zoom microscope (Nikon, SMZ-U). Measurements of the shell length (the distance between the anterior and posterior ends of a shell) were conducted using the UTHSCSA Image Tool program. The shell lengths of 30 larvae at each age were measured.

Ingestion Rates as a Function of Prey Concentration

Experiments 8 to 13 were designed to measure ingestion and clearance rates of 25-day-old M. galloprovincialis larvae, as a function of prey concentration, when feeding on unialgal diets of the red-tide dinoflagellates (Table 2).

The procedures for setting-up experiments, measuring predator and prey densities, and calculating ingestion and clearance rates were the same as described above, except that 80-mL PC bottles were used, 10 mL of f/2 medium were added into each bottle, and 5-mL aliquots were removed from each bottle and fixed with 5% Lugol's solution.

Ingestion rate data were fitted to a Michaelis-Menten equation:

(1) IR = [I.sub.max](x)/[[K.sub.IR] + (x)]

where [I.sub.max] = the maximum ingestion rate (cells [predator.sup.-1] [day.sup.-1] or ng C [predator.sup.-1] [day.sup.-1]); x = prey concentration (cells [mL.sup.-1] or ng C [mL.sup.-1]), and [K.sup.IR] = the prey concentration sustaining 1/2 [I.sub.max].

RESULTS

Feeding and Shell Length of Mytilus Larvae

M. galloprovincialis larvae were able to ingest all red-tide dinoflagellates offered in the current study. The larvae fed on RTD in feeding currents produced by the ciliated velum. The stomach areas of M. galloprovincialis fed red-tide dinoflagellates were 20-30% larger than those starved in the control bottles, and the color of the stomachs of fed larvae became dark brown, whereas that of unfed larvae was almost transparent (Figs. 1A and 1B). Undigested Prorocentrum micans cells from the crushed stomachs of the larvae also ascertained the ingestion of the prey species (Fig. 1C).

[FIGURE 1 OMITTED]

With increasing larval age (elapsed time after fertilization), the mean shell length of M. galloprovincialis larvae almost linearly increased from 69 to 204 [micro]m (Fig. 2).

[FIGURE 2 OMITTED]

Ingestion and Clearance Rates as a Function of Larval Age

The first feeding by M. galloprovincialis larvae on each red-tide dinoflagellate species occurred when the larvae were approximately 9-13 days old (Figs. 3-8); whereas that for Isochrysis galbana occurred at the larval age of 5 days (Fig. 9).

[FIGURES 3-9 OMITTED]

The ingestion rates of M. galloprovincialis larvae on P. minimum were undetectable when the larvae were 1-5 days old. However, they increased to 6-14 ng C [predator.sup.-1] [day.sup.-1] at the larval age of 9-21 days and reached 26 at 25 days (Fig. 3A). The clearance rates were 0.2 0.5 [micro]L [predator.sup.-1] [h.sup.-1] at the age of 9-21 days and reached 0.9 at 25 days (Fig. 3B).

The ingestion rates of M. galloprovincialis larvae on Cochlodinium polykrikoides were undetectable or very low when the larvae were 1-13 days old, but they rapidly increased to 34-56 ng C [predator.sup.-1] [day.sup.-1] at the larval age of 21-25 days (Fig. 4A). The clearance rates also were undetectable or very low at the larval age of 1-13 days, but increased to 1.3-2.3 [micro]L [predator.sup.-1] [h.sup.-1] at the larval age of 21-25 days (Fig. 4B).

The ingestion rates of M. galloprovincialis larvae on Alexandrium affine were undetectable or very low when the larvae were 1-17 days old, but they rapidly increased to 19 ng C [predator.sup.-1] [day.sup.-1] at the age of 25 days (Fig. 5A). The clearance rates also were undetectable or very low at the larval age of 1-17 days, but increased to 0.8 [micro]L [predator.sup.-1] [h.sup.-1] at the larval age of 25 days (Fig. 5B).

The ingestion rates of M. galloprovincialis larvae on Scrippsiella trochoidea were undetectable or very low when the larvae were 1-13 days old. but they increased rapidly to 19-20 ng C [predator.sup.-1] [day.sup.-1] at the age of 21-25 days (Fig. 6A). The clearance rates also were undetectable or very low at the larval age of 1-13 days, but increased to 0.3-0.7 [micro]L [predator.sup.-1] [h.sup.-1] at the larval age of 21-25 days (Fig. 6B).

The ingestion rates of M. galloprovincialis larvae on Prorocentrum micans were undetectable when the larvae were 1-9 days old, but they increased rapidly to 27-32 ng C [predator.sup.-1] [day.sup.-1] at the age of 17-25 days (Fig. 7A). The clearance rates were also undetectable or very low at the larval age of 1-9 days, but increased to 0.8-1.2 [micro]L [predator.sup.-1] [h.sup.-1] at the larval age of 17-25 days (Fig. 7B).

The ingestion rates of M. galloprovincialis larvae on Lingulodinium polyedrum were undetectable when the larvae were 1-9 days old, but they were 10-11 ng C [predator.sup.-1] [day.sup.-1] at the age of 13-21 days (Fig. 8A). The rates reached a maximum of 27 ng C [predator.sup.-1] [day.sup.-1] at the age of 25 days. The clearance rates were also undetectable when the larvae were 1-9 days old, but they were 0.1-0.4 [micro]L [predator.sup.-1] [h.sup.-1] at the age of 13-21 days (Fig. 8B). The rate reached a maximum of 1.1 [micro]L [predator.sup.-1] [h.sup.-1] at the age of 25 days.

The ingestion rates of M. galloprovincialis larvae on Isochrysis galbana were undetectable when the larvae were 1 day old, but they were 19-32 ng C [predator.sup.-1] [day.sup.-1] at the age of 5-17 days (Fig. 9A). The rates reached maximum of 111 ng C [predator.sup.-1] [day.sup.-1] at the age of 21 days. The clearance rates were also undetectable when the larvae were 1 day old, but they were 1.0-1.7 [micro]L [predator.sup.-1] [h.sup.-1] at the age of 5-17 days (Fig. 9B).The rate reached a maximum of 7.8 [micro]L [predator.sup.-1] [h.sup.-1] at the age of 25 days.

No dead M. galloprovincialis larvae were found upon examination with a dissecting microscope prior to taking subsamples in these experiments.

Ingestion and Clearance Rates as a Function of Prey Concentration

In Experiments 8 to 13, the ingestion rates of 25-day-old M. galloprovincialis larvae on unialgal diets of P. minimum, C. polykrikoides, A. affine, S. trochoidea, P. micans, and L. polyedrum increased rapidly with increasing prey concentration up to 1000-2200 ng C [mL.sup.-1], but were almost saturated at higher prey concentration (Figs. 10-15). When the data were fitted to Eq. 1, the maximum ingestion rates of M. galloprovincialis larvae in ng C [predator.sup.-1] [day.sup.-1] and (prey cells [predator.sup.-1] [day.sup.-1]) were 69 (99) for C. polykrikoides, 56 (60) for P. micans. 45 (18) for L. polyedrum. 26 (173) for P. minimum, 21 (25) for S. trochoidea, and 14 (18) for A. affine (Table 3). Maximum clearance rates of M. galloprovincialis larvae were 11.4 [micro]L [predator.sup.-1] [h.sup.-1] for L. polyedrum, 8.4 for P. micans, 3.8 for A. affine, 3.5 for S. trochoidea, 2.8 for C polykrikoides, and 1.5 for P. minimum.

[FIGURES 10-15 OMITTED]

DISCUSSION

Prey Species and Feeding Rates as a Function of Larval Age

There has been no report on the feeding by larvae in the genus Mytilus on red-tide dinoflagellates. M. galloprovincialis larvae were able to feed on all red-tide dinoflagellate prey offered in the current study. Thus, M. galloprovincialis larvae have diverse prey species. In the phylum mollusca, larvae of the oyster Crassostrea virginica have been shown to feed on the dinoflagellate P. minimum (Wikfors & Smolowitz, 1995), and larvae of the gastropod Philine aperta feed on the dinoflagellates Heterocapsa triquetra and Scrippsiella faroense (Hansen 1991). Further studies on feeding by other molluscan larvae on diverse red-tide dinoflagellates are necessary to better understand their interactions.

Mytilus edulis larvae have been known to ingest particles of 1-9 [micro]m (Riisgard et al. 1980, Sprung 1984b). However, M. galloprovincialis larvae are able to ingest red-tide dinoflagellates whose ESDs are 12-38 [micro]m (Table 1). The change in color of the stomachs of the larvae and undigested prey cells found in the crushed stomachs indicated their ingestion (Fig. 1). The mouths of the larvae may be very flexible for ingesting large prey cells or different species in the genus Mytilus may have different prey size limitations.

The first feeding by M. galloprovincialis larvae on each red-tide dinoflagellate species occurred when the larvae were approximately 9-13 days old (Figs. 3-8), whereas that for I. galbana occurred at the larval age of 5 days (Fig. 9). Much larger size of red-tide dinoflagellates might delay the larval first feeding.

The ingestion and clearance rates of M. galloprovincialis larvae L on red-tide dinoflagellates at the larval ages of 21-25 days (14-56 ng C [predator.sup.-1] [day.sup.-1] and 0.5-2.3 [micro]L [predator.sup.-1] [h.sup.-1], respectively) measured in Experiments 1 to 7 were lower than those on I. galbana (107-111 ng C [predator.sup.-1] [day.sup.-1] and 6.5-7.8 [micro]L [predator.sup.-1] [h.sup.-1]) (Figs. 3-9). Therefore, red-tide dinoflagellates are less preferred prey for M. galloprovincialis larvae than I. galbana.

Ingestion and Clearance

Data from this study show that the maximum ingestion rates of 25-day-old M. galloprovincialis larvae on each red-tide dinoflagellate species measured in Experiments 8 to 13 are poorly correlated with prey cell volume (Fig. 16). This relationship suggests that prey cell volume does not have an affect on ingestion by the larvae of red tide dinoflagellates. Thus, factors other than prey cell volume may be important to the feeding activity of the larvae. Maximum ingestion rates of the larvae Oil C. polykrikoides and P. micans were much higher than those for S. trochoidea and A. affine, even though these prey species are similar in cell volume. However, these results are difficult to interpret. The C:N ratios of C. polykrikoides (7.7) and P. micans (8.7) are similar to or higher than those for S. trochoidea (5.6) and A. affine (8.4) (Jeong et al., unpublished data). Thus, nutritional values of prey species may not be responsible for these different maximum ingestion rates. Trochophore larvae of the oyster Crassostrea gigas 17 h after fertilization have been shown to be killed when exposed to toxic dinoflagellates (Matsuyama et al. 2001). However, S. trochoidea and A. affine are nontoxic dinoflagellates (our data). Regarding cell shape, C. polykrikoides and P. micans cells are compressed, whereas S. trochoidea and A. affine are spherical. Therefore, the compressed cells may be easier for the larvae to ingest than the spherical cells. However, to determine the exact cause of this pattern, further study is necessary.

[FIGURE 16 OMITTED]

The maximum ingestion ([I.sub.max]) and clearance rates ([C.sub.max]) of 25-day-old M. galloprovincialis larvae on red-tide dinoflagellates obtained in this study (69 ng C [predator.sup.-1] [day.sup.-1] and 11.4 [micro]L [predator.sup.-1] [h.sup.-1], respectively) are comparable to or lower than those for M. edulis larvae on microflagellates (37-160 ng C [predator.sup.-1] [day.sup.-1] and 11-88 [micro]L [predator.sup.-1] [h.sup.-1], respectively) (Bayne 1965, Riisgard et al. 1980, Riisgard et al. 1981, Jespersen & Olsen 1982, Sprung 1984b), when corrected to 15[degrees]C using [Q.sub.10] = 3.4 (Hansen et al. 1997). The smaller size of M. galloprovincialis larvae (193 [micro]m) with respect to M. edulis larvae (250) might be responsible for the former predator's lower [C.sub.max].

The [I.sub.max] and [C.sub.max] of 25-day-old M. galloprovincialis larvae on each red-tide dinoflagellate species are higher than those previously reported for a mixotrophic dinoflagellate, beterotrophic dinoflagellates, or a small ciliate, but much lower than those for large ciliates on the same prey (Table 4). For example, the [I.sub.max] of M. galloprovincialis larvae on Lingulodinium polyedrum obtained in this study (45 ng C [predator.sup.-1] [day.sup.-1]) is higher than those of Polykrikos kofoidii (16 ng C [predator.sup.-1] [day.sup.-1]), Tiarina fusus (15), Protoperidinium cf. divergens (8), Fragilidium cf. mexicanum (5), and Protoperidinium crassipes (3), but much lower than that of Strombidinopsis sp. (147 ng C [predator.sup.-1] [day.sup.-1]), when corrected to 15[degrees]C using [Q.sub.10] = 2.8 (Hansen et al. 1997). The [C.sub.max] of M. galloprovincialis larvae on L. polyedrum (11.4 [micro]L [predator.sup.-1] [h.sup.-1]) is also higher than those of P. kofoidii (3.9), T. fusus (3.0), F. cf. mexicanum (2.6), P. cf. divergens (0.5), and P. crassipes (0.3), but much lower than that of Strombidinopsis sp. (73). This pattern is maintained in S. trochoidea, C. polykrikoides, P. minimum, and P. mican prey. This evidence suggests that engulfing prey in the feeding current produced by the ciliated velum near the mouth (Mytilus larvae) is a more effective feeding mechanism than engulfing prey captured by a tow filament (Polykrikos spp.) or pallium feeding on prey captured by a tow filament (Protoperidinium spp.), but less effective than engulfing prey using rows of cilia in the mouth (Strombidinopsis spp. and Favella spp.).

Ecological Importance

In the current study, M. galloprovincialis larvae fed on red-tide dinoflagellates without mortality after 72 h exposure to considerable high prey concentrations. Thus, the larvae are able to survive during and/or after red tides dominated by these dinoflagellates. Also, dinoflagellates are one of the most abundant phytoplankters in coastal waters, and thus the bivalve larvae may develop healthily by feeding on commonly distributed dinoflagellates. M. galloprovincialis larvae, one component of microzooplankters, exhibited higher maximum ingestion and clearance rates than previously reported for other microzooplankters such as the Fragilidium cf. mexicanum (mixotrophic dinoflagellate), the Protoperidinium cf. divergens, Polykrikos kofoidii (heterotrophic dinoflagellates), or Tiarina fusus (small ciliate), but lower rates than Strombidinopsis spp. and Favella spp. (large ciliates) when fed the same prey species. Thus, Mytilus larvae may compete with some microzooplankters for dinoflagellate prey.

TABLE 1.
Species of red-tide dinoflagellate prey and a flagellate Isochrysis
galbana used in the current study, listed in order or cell volume. *

                                                          Approximate
                                            ESD +           Volume
                                          Standard         ([micro]
              Species                 Error ([micro]m)    [m.sup.3])

Isochrysis galbana (IGKC99)           4.8 [+ or -] 0.2          57
Prorocentrum minimum (PMJH99)         12.9 [+ or -] 3.6       1100
Cochlodinium polykrikoides (CPKS00)   23.2 [+ or -] 3.1       6600
Alexandrium affine (AAJM00)           24.0 [+ or -] 1.1       7200
Scrippsiella trochoidea (STKP99)      25.1 [+ or -] 2.8       8300
Prorocentrum micans (PMCJH99)         26.0 [+ or -] 2.3       9200
Lingulodinium polyedrum (LPSD95)      37.9 [+ or -] 4.5     28,500

* Mean equivalent spherical diameter (ESD) was measured by the
PAMAS-SVSS particle counter. Volume was calculated according to
the equation: Volume = 4/3 [pi] [(ESD/2).sup.3]. The number of
cells measured, n, was >2000.

TABLE 2.
Experimental design: Values in prey and predator columns represent
actual initial densities (cells [mL.sup.-1] for prey and individuals
[mL.sup.-1] for predator) followed by calculated carbon biomass
(ng C [mL.sup.-1]) in parentheses.

                                 Prey                         Predator

Expe-
riment
 No.              Species                    Density          Density

  1      Isochrysis galbana           72,100-84,067             3-7
                                        (865-1009)
  2      Prorocentrum minimum         7227-9976 (1084-1496)     4-9
  3      Cochlodinium polykrikoides   1532-1883 (1072-1318)     3-9
  4      Alexandrium affine           1103-1560 (838-1185)      4-9
  5      Scrippsiella trochoidea      1245-1667 (1058-1417)     3-8
  6      Prorocentrum micans          947-1363 (890-1223)       3-5
  7      Lingulodinium polyedrum      439-582 (1098-1455)       4-9
  8      Prorocentrum minimum         255 (38), 723 (108),      1-5
                                        1443 (216), 7638
                                        (1146), 15483
                                        (2322), 32288
                                        (4843)
  9      Cochlodinium polykrikoides   27 (19), 151 (106),       1-5
                                        298 (208), 1492
                                        (1044), 3120
                                        (2184), 4147 (5925)
  10     Alexandrium affine           33 (25), 130 (99),        1-7
                                        266 (202), 1354
                                        (1029), 2178
                                        (1655), 4423 (3362)
  11     Scrippsiella trochoidea      20 (17), 113 (96),        1-7
                                        224 (191), 1126
                                        (957), 2433 (2068),
                                        4930 (4191)
  12     Prorocentrum micans          24 (23), 108 (102),       1-4
                                        236 (221), 1159
                                        (1089), 2249
                                        (2114), 4703 (4420)
  13     Lingulodinium polyedrum      10 (25), 43 (107), 93     1-4
                                        (234), 417 (1041),
                                        819 (2048), 1629
                                        (4072)

TABLE 3.
Grazing data for Mytilus galloprovincialis larvae. *

Figures         Species          [I.sub.max]   [K.sub.IR]   [r.sup.2]

  10      Prorocentrum minimum       26           577         0.72
  11      Cochlodinium               69          1510         0.73
            polykrikoides
  12      Alexandrium affine         14           539         0.48
  13      Scrippsiella               21           269         0.50
            trochoidea
  14      Prorocentrum micans        56           538         0.61
  15      Lingulodinium              45           590         0.73
            polyedrum

* Parameters are for functional response from Eq. 1 as presented in
Figs. 10-15. [I.sub.max] (maximum ingestion rate, ng C
[predator.sup.-1] [day.sup.-1]), [K.sub.IR] (prey concentration
sustaining 0.5 [I.sub.max], ng C [mL.sup.-1]).

TABLE 4.
Comparison of ingestion and clearance rates of Mytilus
galloprovincialis larvae and protistan predators on the
same red-tide algal prey. *

Prey Species             Predator             PV    [I.sub.max]

Lingulodinium   Mytilus galloprovincialis    4240        45
polyedrum         larvae (ML)
                Tiarina fusu (NC)              23        15
                Polykrikokos kofoidii (HD)     43        16
                Protoperidinium cf.           119         8
                  divergens (HD)
                Protoperidinium crassipes     204         3
                  (HD)
                Fragilidium cf. maxicanum      85         5
                  (MD)
                Strombidinopsis spp. (NC)     560       147

Scrippsiella    Mytilus galloprovincialis    4240        21
trochoidea        larvae (ML)
                Tiarina fusus (NC)             23         7
                Polykrikokos kofoidii (HD)     43        11
                Strombidinopsis spp. (NC)     560       137
                Favella spp. (TC)                       157

Cochlodinium    Mytilus galloprovincialis    4240        69
polykrikoides     larvae (ML)
                Strombidinopsis spp. (NC)     560       234

Prorocentrum    Mytilus galloprovincialis    4240        26
mininum           larvae (ML)
                Strombidinopsis spp. (NC)     560       177

Prorocentrum    Mytilus galloprovincialis    4240        26
micans            larvae (ML)
                Polykrikokos kofoidii (HD)     43         3

Prey Species             Predator            [C.sub.max]

Lingulodinium   Mytilus galloprovincialis       11.4
polyedrum         larvae (ML)
                Tiarina fusu (NC)                3.0
                Polykrikokos kofoidii (HD)       3.9
                Protoperidinium cf.              0.5
                  divergens (HD)
                Protoperidinium crassipes        0.3
                  (HD)
                Fragilidium cf. maxicanum        2.6
                  (MD)
                Strombidinopsis spp. (NC)       72.9

Scrippsiella    Mytilus galloprovincialis        3.5
trochoidea        larvae (ML)
                Tiarina fusus (NC)               0.1
                Polykrikokos kofoidii (HD)       0.7
                Strombidinopsis spp. (NC)       27.2
                Favella spp. (TC)               28.5

Cochlodinium    Mytilus galloprovincialis        2.8
polykrikoides     larvae (ML)
                Strombidinopsis spp. (NC)       33.0

Prorocentrum    Mytilus galloprovincialis        7.2
mininum           larvae (ML)
                Strombidinopsis spp. (NC)       73.0

Prorocentrum    Mytilus galloprovincialis        7.2
micans            larvae (ML)
                Polykrikokos kofoidii (HD)       1.5

Prey Species             Predator                  Reference

Lingulodinium   Mytilus galloprovincialis    This study
polyedrum         larvae (ML)
                Tiarina fusu (NC)            Jeong et al. (2002)
                Polykrikokos kofoidii (HD)   Jeong et al. (2001)
                Protoperidinium cf.          Jeong and Latz (1994)
                  divergens (HD)
                Protoperidinium crassipes    Jeong and Latz (1994)
                  (HD)
                Fragilidium cf. maxicanum    Jeong et al. (1999a)
                  (MD)
                Strombidinopsis spp. (NC)    Jeong et al. (1999b)

Scrippsiella    Mytilus galloprovincialis    This study
trochoidea        larvae (ML)
                Tiarina fusus (NC)           Jeong et al. (2002)
                Polykrikokos kofoidii (HD)   Jeong et al. (2001)
                Strombidinopsis spp. (NC)    Jeong et al. (1999b)
                Favella spp. (TC)            Stoecker et al. (1981)

Cochlodinium    Mytilus galloprovincialis    This study
polykrikoides     larvae (ML)
                Strombidinopsis spp. (NC)    Jeong et al. (1999b)

Prorocentrum    Mytilus galloprovincialis    This study
mininum           larvae (ML)
                Strombidinopsis spp. (NC)    Jeong et al. (1999b)

Prorocentrum    Mytilus galloprovincialis    This study
micans            larvae (ML)
                Polykrikokos kofoidii (HD)   Jeong et al. (2001)

* Rates are corrected to 15[degrees]C using [Q.sub.10] = 2.8 (Hansen et
al. 1997). PV (Predators' volume as x [10.sup.3] [micro][m.sup.3]);
[I.sub.max] (maximum ingestion rate in ng C [predator.sup.-1]
[day.sup.-1]); [C.sub.max] (maximum clearance rate as [micro]L
[predator.sup.-1] [h.sup.-1]); NC (naked ciliate): TC (tintinnid
ciliate); HD (heterotrophic dinoflagellate); MD (mixotrophic
dinoflagellate); ML (metazoan larvae).

ACKNOWLEDGMENTS

The authors thank Kwang Young Kim, Jae Seong Kim, Yeong Du Yoo, and Kyeong A Seong for technical support. This paper was funded by grants from MOST & KOSEF (R12-1999-027-12000-0) and from MOST & KISTEP (M1-0302-00-0068).

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HAE JIN JEONG, (1) * JAE YOON SONG, (2) CHANG HOON LEE, (3) AND SEONG TAEK KIM (2)

(1) School of Earth and Environmental Science, College of Natural Sciences, Seoul National University Seoul 151-747; (2) Department of Oceanography, College of Ocean Science and Technology. Kunsan National University, Kunsan 573-701; (3) Red Tide Research Center, Kunsan National University, Kunsan 573-701, Republic of Korea

* Corresponding author. E-mail: [e-mail]