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Laboratory expenments investigating predation by plankton on n~eroplanktonic invertebrate l a ~ v a e often use unnaturally h ~ g h densities of prey in filtered seawater Offering prey under these c o n d ~ t ~ o n s , however can alter predator behavior and capture success, potentially creating artifactual predator-prey relat~onships and predation rates We conducted laboratory expenments investigating the effect of a range of larval invertebrate densities on predat~on rates For the 4 predator-prey combinations examined, there was no predat~on at natural prey dens~t ies in filtered seawater LVc then conducted predator-prey experiments in the presence and absence of naturally occurring a m b ~ e n t plankton ('background plankton ) at densitles where predation had been observed In filtered seawater In most cxperiments, background plankton dramatically decreased 01 eliminated predation which had been observed w ~ t h unnaturally h ~ g h prey densities in filtered seawater K E Y WORDS: Meroplankton . Invertebrate larvae . Predat ion. Background plankton . Prey density Laboratory experiments mvestigating predation upon meroplanktonic invertebrate larvae are often conducted using unnaturally high densities of meroplanktonic prey in filtered seawater. Unnaturally high prey densitles can alter predator behavior, capture success, and food preference. These density effects have been observed in other predator-prey systems (e.g. Holling 1959, Krebs et al. 1977). To the best of our knowledge, however, this is the first study directly examining the influence of prey densities on predation of invertebrate larvae by planktonic predators. Using filtered seawater for laboratory predation experiments, like using unnaturally high prey densities, may also induce unnatural predation. Planktonic predators may be generalists, feeding upon all poten'E-mail: kbjmdarkwing uoregon edu tial prey, including the naturally occurring ambient plankton ('background plankton'). Background plankton, including protists and phytoplankton, are far more abundant than relatively rare meroplanktonic invertebrate larvae. By occupying or satiating the predator, or obscuring larvae from detection, background plankton may reduce larval predation Alternatively, predators may specialize in feeding on prey other than the type being offered. In either case, predators consun~ing prey in filtered seawateimay not do so in the presence of background plankton. We conducted predation experiments, observing predation rates, in filtered seawater over a range of prey densities, including near-natural and unnaturally high dens~ties. Using prey densities where predation was observed in filtered seawater, we then conducted predation experiments with and without background plankton. Methods. Three predators (the zoea of the mud shrimp Upogebia pugettensis, the leptoniedusa Obelia sp. , and an unidentified leptomedusa) and 3 prey types (blastulae and plutei of the purple sea urchin Strongylocentrotus purpuratus, and barnacle nauplii) were used to create 4 predator-prey combinations. Some zoeae and hydromedusae are known to be predatory (e .g . Runirill 1987), but no information is available on the natural prey of our selected predatory species. S. purpuratus were spawned and maintained using standard techniques (Strathmann 1987). Blastulae were approximately 120 pm long and plutei were 4-arm stage and approximately 200 pm in length. Barnacle nauplii (body length 200 to 250 pm) and all predators were collected at high tide from near the mouth of Coos Bay, Oregon, USA (43"21' 10" N, 124" 19'50" W) by slowly towing a plankton net equipped with a large blind cod-end (after Reeve 1981). Experiments began within 24 h of predator collection and were conducted on a roller table (Omori & Ikeda 1984, Larson & Shanks 0 Inter-Research 1997 Resale of full article not permitted Mar Ecol Prog Ser 158: 293-296. 1997 1996), which rolled 3 l cylindrical tanks at 1 rpm and prevented pl.ankton from and without predators to predation. When treatments are settling Though enclosed, plankton 30 25 mortality without a predator do not suffer oxygen depletion during the experimental time frame (Larson 2.0 & Shanks 1996). The roller table was 15 maintained at 12OC in a constant tem10 perature room with a 14:10 1ight:dark 3 1= 5 cycle for 24 h. Observations of preda0 tors and prey in roller tanks revealed 2 them to stay suspended in the water 35 l * is attributed Fig. 1. Predator-induced mortality as a function of prey density. (A) Mud shrimp zoea preying upon purple urchin plutei. (B) Mud shrimp zoea preying upon stated to be purple urchin blastulae. (C) Unidentified leptomedusa preying upon barnacle . . different, we refer to a = 0.05 with nauplii. (D) Obelia sp. medusa preying upon purple urchin-blastuiae. Columns the G~~~~ & ~ ~ ~ ~ l l (G&H) mean with zero mean and variance are indicated by a '0' Error bars represent the 95% confidence interval. Predator treatments that are significantly different significant difference method of a posfrom t h e ~ r predator-less controls at cl = 0 05 are marked with a star terion pairwise comparison of means (Sokal & Rohlf 1995), performed after a significant Kruskal-Wallis ANOVA (K-W ANOVA). represented in our experiments as densities of 1, 3, 5, The G&H method of comparing means is appropriate and 10 I-'. Our high density of 50 1-' exceeds published for heterogeneous variances and small sample sizes. observations and is intended to be unnaturally high. At Prey density experimenfs: Experiments investigatthe end of each experiment, predators and remaining ing the effect of variation in prey density on predation prey were collected and fixed. Counts of surviving were conducted in 1 pm filtered seawater with 4 differlarvae were made using a compound microscope. ent predator-prey combinations: mud shrimp zoea Background plankton experiments: Predation expreylng upon plutei, mud shrimp zoea preying upon periments with and without background plankton blastulae, unidentified leptomedusa preying upon were conducted with 3 of the same predator-prey barnacle nauplii, and Obelia sp. medusa preying combinations used in the previous experiments. upon blastulae. Predator density was 1 tank-'. Three Experiments were run at prey densities at which prereplicate treatments (predators present) and controls dation was observed In the above-described prey (predators absent) were run for each prey density. Prey density experiments (Fig. 2) . Experiments with the densities (Fig. 1) ranged from near-natural to unnatuObelia sp. medusa preying upon blastulae and the rally hlgh densities. Published observations oi larval unidentified leptornedusa preying upon barnacle nauurchin field densities (and, by extrapolation, conplii consisted of 5 treatments, 3 replicates each, at servative urchin blastula densities) range from 0.08 to each selected prey density. The 5 treatments were 0.39 1-I (Zimmerman 1972, Cameron & Rumrill 1982, prey alone in filtered seawater, prey with a predator Rumrill et al. 1985, Emlet 1986, Rumrill 1987) and the in flltered seawater, prey alone with background highest reported density is only 0.74 1-I (Miller 1995). plankton, prey with a predator and background Natural urchn densities are represented in our experiplankton, and larvae and background plankton fixed ments as a density of 1 1-' By contrast, densities of at the onset of the experiment (a control for retrieval echinopluteus larvae used in past laboratory predation artifacts in the presence of background plankton). experiments has often ranged from 25 to 500 1-I (e.g. The protocol for the experiment with the mud shrimp Rumrill et al. 1985, Pennlngton et al. 1986). Natural zoea preying upon plutei was the same as those densities for barnacle nauplii may be as high as 15 1-l described above, but lacked the background plankton (Zimmerman 1972). Natural nauplius densities are control. Background plankton were obtained by colcolumn and exhibit apparently normal 30 behavior. At the end of each experi25 ment, predators and remaining prey 20 were collected, fixed, and counts of 15 surviving larvae made using a com10 pound microscope. 'Mortality' is based 5 upon the lack of retrieval of whole, 0 unconsumed larvae and the difference 1 3 5 10 50 1 3 5 10 50 83 in mortality between treatments with Prey Density (# larvae I") -

The importance of prey densities and background plankton in studies of predation on invertebrate larvae