refuges from fish predation: experiments with phytal meiofauna from the new zealand rocky intertidal

Refuges from Fish Predation: Experiments with Phytal Meiofauna from the New ZealandRocky IntertidalAuthor(s): Bruce C. Coull and J. B. J. WellsSource: Ecology, Vol. 64, No. 6 (Dec., 1983), pp. 1599-1609Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1937513 .

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Ecology, 64(6), 1983, pp. 1599-1609 ? 1983 by the Ecological Society of America

REFUGES FROM FISH PREDATION: EXPERIMENTS WITH PHYTAL MEIOFAUNA FROM THE

NEW ZEALAND ROCKY INTERTIDAL'

BRUCE C. COULL Belle W. Baruch Institute for Marine Biology and Coastal Research

and Department of Biology, University of South Carolina, Columbia, South Carolina 29208 USA

AND

J. B. J. WELLS Zoology Department, Victoria University of Wellington,

Private Bag, Wellington, New Zealand

Abstract. Phytal meiofauna are numerically important members of rocky intertidal communities and known to be prey items for various small fishes. In a series of experiments we demonstrated that substrate complexity is an important factor in reducing predation. The alga Corallina officinalis, the most complex structure used, was the only refuge from blenny (Helcogramma medium) predation for total meiofauna, and the dominant taxon, copepods. This refuge effect was evident whether the prey fauna were originally from Corallina or from another alga. One harpacticoid copepod species (Amphiascus lobatus) was selectively preyed upon, with females preferentially taken over males. Our results and those from the literature suggest that fish predation on benthic invertebrates may not be linearly related to decreasing substrate complexity. Rather there appears to be a complexity threshold below which removal rate is not significantly affected by structure and above which removal rate is significantly reduced.

Key words: algae; complexity;fish; meiofauna; New Zealand; predation; rock pools; thresholds.

INTRODUCTION

Structural complexity of a habitat is known to affect abundance, diversity, and distribution of its associated fauna (e.g., Huffaker 1958, Cooper and Crowder 1979). Correlations between habitat complexity and increased diversity have traditionally been viewed as an expression of the outcome of competition (e.g., MacArthur and MacArthur 1961, MacArthur 1972, Woodin 1974). They may also reflect increased refuges from predation for a prey item (e.g., Glass 1971, Heck and Thoman 1981, Woodin 1981, Peterson 1982, Ston- er 1982, Crowder and Cooper 1982), increased refuges from disturbance (Woodin 1978) or increased attach- ment sites for the associated fauna's food (Abele 1974, Hicks 1980).

Experimental manipulations to test for structural effects have recently become a popular avenue of ecological research. In the past most evidence dealing with the relationship between structural complexity and abundance/diversity of an assemblage has been infer- ential, i.e., authors observe, correlate, and then state that as complexity increases, so does the measured variable (see reviews of Colwell and Fuentes 1975, Cooper and Crowder 1979). More recent experimental marine work has focused on providing the associated fauna with habitats of varied structural complexity and assessing its success in avoiding predation (Vince et

al. 1976, Van Dolah 1978, Brock 1979, Nelson 1979, Coen et al. 1981, Heck and Thoman 1981, Peterson 1982, Stoner 1982, Crowder and Cooper 1982), disturbance (Kohn and Leviten 1976, Woodin 198 1), or competition (Coen et al. 1981, Stoner 1982).

Meiofauna (micrometazoans that pass through a 0.5- mm mesh) are the most abundant metazoans in marine benthic habitats. They are metabolically important members of benthic ecosystems (Gerlach 1971) and are known to be prey items for a variety of larger organisms (Coull and Bell 1979), particularly juvenile fishes (Feller and Kaczynski 1975, Sibert et al. 1977, Sheridan 1979, Grossman et al. 1980, Hicks and Coull 1983). While the rocky intertidal has been a primary site for experimental ecology, there is but limited knowledge of the meiofauna in such habitats. The phytal meiofauna of the rocky intertidal at Island Bay, Wellington, New Zeland is one that has been well studied (Hicks 1977a, b, c). It is dominated by harpacticoid copepods, animals which are typically the most important food items of many small fish (Odum and Heald 1972, Roland 1978, Hicks and Coull 1983, and references listed above).

Large populations of the blenny Helcogramma medium (Gunther) inhabit mid-to-high intertidal rock pools at Island Bay. There are no published data on the feeding ecology of this blenny, but gut analysis of 32 fresh- ly collected Helcogramma indicated that fish longer than 35 mm (SL) fed primarily on macrofaunal amphipods, while those smaller than 35 mm fed almost

1 Manuscript received 12 August 1982; revised 10 Novem- ber 1982; accepted 15 November 1982.



1600 BRUCE C. COULL AND J. B. J. WELLS Ecology, Vol. 64, No. 6

exclusively on harpacticoid copepods (>85% of the gut contents by number). The sides and bottom of the pools inhabited by Helco gramma were covered with tufted mats of the alga Corallina officinalis, an alga Hicks (1977a, b) reports as having the highest density and diversity of harpacticoid copepods of the six phytal meiofaunal assemblages he studied.

In this paper we provide experimental evidence relating habitat structural complexity and meiofaunal predator avoidance. In addition, we noted whether there was species-, size-, or sex-selective predation by the blenny on the dominant harpacticoid copepods. Rock pool meiofaunal assemblages, particularly the dominant copepods, apparently use complex algae as a refuge from fish predation.

METHODS AND MATERIALS

Meiofauna used in all experiments were collected from algae living in tide pools in the rocky intertidal near the Marine Laboratory of Victoria University of Wellington at Island Bay, New Zealand (41'21.01'S, 174?45.88'E), the same area reported on by Hicks (1977a, b, c). Hicks (1977a) presents relevant physical data (air temperature, seawater temperature, rainfall, salinity, wind strength, and sea state) for 1973-1974; conditions appeared to be similar during our study pe- riod (January-April 1981).

Laboratory

Experiments were designed to test the effects of habitat structure on meiofauna predation by Helco- gramma medium. For each set of experiments (seven laboratory, one field), meiofauna were extracted from intertidal algae (Corallina officinalis L. in all but one set of experiments, see below) by first relaxing the associated meiofauna on the algae in a bucket of iso- tonic MgCl2 (73.2 g/L) for 10 min, shaking individual algal clumps in it, and rinsing each clump with running seawater onto 0.5- and 0.062-mm sieves. After each algal clump was washed, the remaining residue was passed through these two nested sieves. The fauna remaining on the 0.5-mm sieve were discarded and those retained on the 0.062-mm sieve (meiofauna) placed into clean seawater. This 0.062-mm fraction was stirred into suspension and split into 16 equal portions (rations) using a Folsom plankton splitter (Wickstead 1976). For each experiment four randomly selected rations were preserved immediately to establish the mean ration input (initial abundance) and to measure the efficiency of the plankton splitter. This technique was used because hand sorting to obtain a constant number (several thousand) of meiofauna for each experiment proved impractical.

The 12 live rations remaining from the plankton splitter were placed into 12 plastic aquaria (18 cm long x 12 cm wide x 10 cm high; 216 cm2 basal area) with rounded corners to eliminate a "corner effect." Each aquarium was fed by an individual seawater line

and fitted with an outlet valve (covered with 0.042-mm mesh) at 7 cm above the bottom. A constant water flow was maintained through the unit; outflow was discarded. Each experiment comprised four replicates of a control (empty aquaria = no structure) and four replicates of each of two structures. The "structure" covered 144 cm2 of aquarium floor, distributed as two strips, each 12 x 6 cm, separated by a 6 cm wide cen- tral open space, which was left clear to avoid clogging the outflow valve and to provide some swimming area for the blennies. Structures used were: (1) the alga Corallina officinalis L., dried for 1-2 mo; (2) dried stones of 25-30 cm diameter; (3) dried beach gravel, median diameter 2.9 mm; (4) plastic bottle brushes; and (5) fresh algae: Cystophora retroflexa (Labill.) J. Ag.; Zonaria turneriana J. Ag.; Caulerpa brownii Endl. var. selaginoides J. Ag., and Champia novalzealan- diae (Hook. f. Harv.) J. Ag. with meiofauna extracted. See Morton and Miller (1973) for description of the algae. All algae used in the experiments were retrieved from intertidal rock pools, even though Cystophora, Zonaria, Caulerpa, and Champia are more commonly found in the sublittoral fringe community (G. R. F. Hicks, personal communication). Corallina is an intertidal species.

Each aquarium was allowed to fill and the flow adjusted. With the flow off, one of the 12 meiofauna rations was added to each aquarium, allowed to settle and disperse for 15-20 min. Flow was then renewed and two starved (24 h) 25-30 mm blennies were added to each aquarium. For each experiment new blennies were collected by dip-netting from mid- to high-intertidal pools. The fish were allowed to remain in the aquaria 24 h, at which time all structure and animals remaining in the aquaria were re-treated with MgCl2, decanted onto a 0.062-mm sieve, preserved, counted, and identified.

Laboratory control experiments

Three additional sets of laboratory experiments were run to test particular aspects of the general experimental design: (1) To test if there was natural mortality of the meiofauna in the aquaria, four replicate aquaria with no structure and no fish were established and allowed to run for 24 h (as with all other experiments). There were no significant differences (t = 0.35, df = 6, P = .74) in meiofauna abundance between the initial concentration and that after 24 h, i.e., no significant natural mortality in the aquaria. (2) Dead dried Corallina was used in the Corallina experiments since we wanted to avoid the potential of exudate attraction of meiofauna to Corallina and limit our observations to structural complexity effects only. We did, however, test live Corallina vs. dead Corallina as a refuge by simultaneously running four replicates each of live and dead Corallina with the same treatment (meiofauna, two fish for 24 h). As there was no significant difference in total abundance of meiofauna (t = 0.16,



December 1983 MEIOFAUNA PREDATION REFUGES 1601

df= 6, P .88), major taxon abundance (t = 0.27, df = 6, P .79), or number of harpacticoid copepod species (t 0. 12, df = 6, P = .9 1), we concluded that live or dead Coralli/a provide an equivalent refuge. (3) Generally the experiments were conducted with meiofauna extracted from Coralli/a. To test if the Cora/lina-extracted fauna were unique in their response, one set of experiments was conducted with fauna extracted from the alga Champia novazealan- (ihe; extraction, splitting, and aquaria addition of this fauna was the same as outlined above. The "refuge" structures used were dried Coralli/a and live Cham- pia (see Results).

Structure meausui regents

Complexity was measured in three ways: (a) surface area; (b) volume; (c) surface-to-volume ratio. Quan- tification of the surface area of the specific structures was based on the Harrod and Hall (1962) detergent method as modified by Hicks (1977a), where the increase in mass of a structure dipped into commercial liquid dishwashing detergent is directly proportional to surface area (Hicks 1977a; r = 0.99, P < .005). We dried replicate samples of each of our structures ex- ternally by first dipping in acetone, then drip-drying and weighing. They were immersed in detergent, allowed to drip-dry, and reweighed. All values were standardized to a I g base mass and surface area cal- culated from Hicks' (1977a:451) regression equation (v = 0.033363 + 0.002518x) where y = increase in mass per gram of structure and x = surface area. Vol- ume of each structure was measured by displacement in a graduated cylinder.

Since Harrod and Hall (1962) and Hicks (1977() used only plants, they could safely assume relatively constant density of their measured structure; we, by using some nonbiological material (stones, gravel, bottle brushes), could not. When we compared gain in mass of detergent vs. mass, and gain in mass of detergent vs. volume, there was no difference in the complexity rankings of the nonbiological compared to the biolog- ical structures.

Field

A field test was conducted after completion of the laboratory experiments. Four intertidal rock pools (=30 cm diameter, 12-15 cm deep) were drained by siphon and filled with 90% ethyl alcohol, let stand for 15 min, then redrained. All structures, i.e., attached algae, snails, sediment, were removed by scraping the pools with wire brushes and chisels. The pools were then flushed with clean seawater for 15 min and filled to a standard volume with seawater that had been passed through a 0. 150-mm mesh. Two rock pool prawns (Pa- luemon uffinis Milne-Edwards) were placed in each pool as bioassay organisms. When the prawns were still alive after 30 min and we could neither smell nor taste ethyl alcohol in the seawater, we removed them

TABLE 1. Summary of variance component analysis com- paring source of variation in the ration inputs (initial abundance of meiofauna provided as food for predator blennies) for the laboratory experiments. In all cases the amount of variance explained by differences in ration input to each experiment was significant at P = .0001.

x ration input % vari- (no. organisms) ance due % vari-

for all to ance due experiments plankton to ration

Taxon (SD) splitter* input

Total meiofauna 1408 (109) 15.7 84.3 Copepods 801 (91) 20.0 80.0 Amphipods 202 (28) 8.4 91.6 Polychaetes 199 (28) 14.4 85.6 Other taxa 205 (31) 21.4 79.6

* The plankton splitter was intended to divide a meiofaunal sample into equal parts for replicate rations.

and placed an equal amount of dried dead Corallina into two of the pools. We then added equal amounts of Corallina-extracted meiofauna to all four pools (two pools with Corallina, two with bare rock) and four 24- h-starved blennies to each pool. The blennies were allowed to forage in the pools for 4 h (low tide), after which the pools were entirely drained, all structures removed and all fauna extracted.

Data analysis

Since predation rate is a function of initial prey density (e.g., Ivlev 1961, Ware 1972, Stoner 1982) and since analysis of variance indicated significant differences (P < .0001) between the meiofaunal rations used in each experimental set, initial abundance had to be considered, a posteriori, a covariable. Variance component analysis on the initial ration input to all experiments indicated that 8.4-21.3% of the variance was due to plankton splitting error; the remainder was due to differences in faunal abundance in the initial rations added to each experiment, i.e., the covariable (Table 1). While there was a significant covariate effect (P <

.0001), there was also a significant effect of structure (P < .001) (Table 2).

Analysis of covariance (ANCOVA) was used to test for differences in taxon abundance remaining after treatment with respect to the initial abundance (the covariable) of that taxon. As our experimental design was not originally established for covariance analysis we were forced to assume commonality of slopes for the ANCOVA. We used 8 of the I I refuge structures only once, providing a single datum (i.e., at one initial abundance) for each estimate (four within-experiment replicates); thus, testing for commonality of slopes was impossible. Our ANCOVA model was:

Yij = b + Ti + Bwithin (Xi - xi) + Eij

where: Yjj = abundance after predation on

structure i at initial abundance j




TABLE 2. Summary of analyses of covariance (model outlined in text) for the major groups of organisms. The covariable is initial abundance. Refuge structures are the habitats provided for the meiofauna. MSE = Mean Square Error for the model. The coefficient of determination (r2) indicates how well the data fit the model.

Taxon df MS F P of greater F

Total meiofauna (r-2 = 0.93) Initial abundance (covariable) 1 787 231 130.7 .0001 Refuge structures (treatment) 10 2 359 929 39.2 .0001

MSE 42 6023

Copepods (r-2 = 0.91)

Initial abundance (covariable) 1 232 573 78.9 .0001 Refuge structures (treatment) 10 869 528 29.5 .0001

MSE 42 2948

Amphipods (r2 = 0.83) Initial abundance (covariable) 1 485 0.7 .4079 Refuge structures (treatment) 10 135697 19.6 .0001

MSE 42 694

Polychaetes (r2 = 0.87) Initial abundance (covariable) 1 31 220 62.6 .0001 Refuge structures (treatment) 10 25 636 5. 1 .0007

MSE 42 499

Other taxa (r2 = 0.92) Initial abundance (covariable) 1 38 563 16.1 .0001 Refuge structures (treatment) 10 49 113 126.4 .0001

MSE 42 304

= overall mean

Ti fixed treatment effect (refuge structures)

BWithin (xij - i) = the effect explained by the difference of each independent replicate from the new initial abundance (i.e., the regression parameter associated with the covariate xij).

ij= random error associated with each measurement.

Our ANCOVAs then test for differences in elevation at the mean initial abundance (ration input) for each taxon in question. For example, the mean initial total meiofauna abundance (across all experiments) was 1408 (Table 1), and in Table 3 we list the adjusted mean for total meiofauna remaining after predation when all experiments were started with 1408 total meiofauna. For each taxon studied, we then tested for treatment differences due to structure in the adjusted (least square) mean abundance remaining after predation, using Scheff&'s Multiple Comparison Method (experiment- wise error rate [EWER] S 0.05).

Kendall's Tau correlation coefficients were calcu- lated for each structure measure (surface area, volume, surface-to-volume ratio) and mean abundance of five taxa (total meiofauna, copepods, amphipods, polychaetes, others) remaining after fish predation.

ANCOVA was used to test for changes in the number of each of the dominant copepod species in the experimental treatments with respect to the initial abundance of that species. While some species abundances changed, others did not. The numbers of fe-

males, males, and copepodites (population components) of the dominant harpacticoid species did not change with respect to initial density (two-way x2 test of independence; x2 = 8.3, df = 10, P > .1 for Am- phiascus lobatus; X2 = 4.7, df = 10, .95 > P > .9 for Ectinosoma australe; X2 = 9.2, df = 10, P > .1 for Paralaophonte meinerti); thus ANCOVA was not nec- essary. A one-way x2 test was employed to test if there were changes in relative abundance of the population components with treatments (structures), i.e.,

X2 _ (?i -E,)2 X-i -

where Oi = observed number of each component after treatment i; El = expected number after treatment i; where E, =

initial number of each component total of all stages

initial total of after treatment i all components

All statistical analyses were conducted using SAS (Statistical Analysis Systems) software (Helwig and Council 1979).

RESULTS

Harpacticoid copepods were the dominant taxon in all rations placed in the aquaria and the field; they comprised 58% of the total meiofauna extracted from Corallina and 48% of the fauna extracted from Cham- pia. Juvenile amphipods (i.e., those passing a 0.5-mm




TABLE 3. Adjusted mean abundance of meiofauna taxa remaining after treatments. Treatments included providing meiofauna with one of the structural refuges and allowing two starved blennies (Helcogramma medium) to feed for 24 h on the meiofauna in the refuge. For each taxon, structures arranged in order of prey numbers remaining. Those structures with common underlines are not significantly different in providing refuge from predation (Scheff6 Multiple Comparison Method; experiment-wise error ?.05). The fauna used in the experiments were extracted from Corallina except where noted.

Taxon (initial abundance + SD) Structural refuge and adjusted mean abundance remaining after predation by blennies for 24 h

Cortllina Champia

with with Champi/ Champia Bottle No

Total meiofauna Corallina fauna CYs.tophora Caulerpa Zonaria fauna Champia brushes Stones structure Gravel /1408 + 109) 1075 991 737 735 699 612 602 570 498 382 369

Corallina Champia

with with Champia Champia Bottle No

Copepods Corallina fauna Zonaria Cystophora Caulerpa fauna brushes Chumpia Gravel Stones structure (801 + 28) 668 579 402 398 377 361 339 307 280 238 204

Corallina Champia

with with ChainNia Champia Bottle No

Amphipods fauna Corallina CYstophora Zonaria fauna Caulerpa brushes Champia Stones structure Gravel (202 2 28) 182 176 123 82 75 69 57 55 42 27 25

Corallina Champia

with with Champia Bottle Champi/ No

Polychaetes Caulerpa Champia fauna Zonaria brushes fauna Cystophora Corallina Stones Gravel structure /199 2 8) 146 116 115 103 102 97 93 90 82 74 70

Coralli//am Chamnpia with with

Chamnpia Bottle Champia No Other taxa fauna Cs'tophora Zonaria( Corallina Caulerpa Stones brushes Chammpi/a fauna structure Gravel

(205 + 31) 155 138 120 116 111 102 100 92 89 87 16

sieve, see Methods) were second in abundance, with 15 and 30% from Corallina and Champia respectively; polychaetes third with 14 and 9%; other taxa (includ- ing mites, ostracods, and nematodes) comprised 13% of the fauna extracted from both algae.

Surface area and surface-to-volume ratio (Table 4) indicates Corallina was the most complex structure. Our Corallina mean values of surface area: 211 cm2/g were higher than Hicks' (1977a) 135 cm2/g, but the Zonaria values (the only other alga in common with Hicks) were similar; Hicks = 80 cm2/g; ours = 72 cm2/g. Differences in values could be due to a variety of factors, e.g., time of year algae were collected, location of collected algae (i.e., low intertidal, high intertidal), adherence properties of different deter- gents, etc.

Laboratory experiments

Analysis of covariance clearly indicated a significant effect of initial abundance for all taxa except amphipods as well as a significant structure effect for all meiofaunal taxa considered (Table 2). The adjusted mean abundances after treatment (Table 3) indicate that, in general, predation removal was greatest when structure was absent or of low complexity (Tables 4, 5; Fig. 1).

'We used the Scheff& Multiple Comparison Method to ask whether the abundances of meiofauna remain-

ing on a given structure were different from those on each other structure at the end of the experiments. For total meiofauna and copepods Corallina was the one structure significantly different (i.e., more prey remaining) from all others (Table 3). It did not matter if the meiofauna used with Corallina were extracted from Corallina or from Champia (Table 3). For amphipods, Cystophora was included with Corallina, but Cystophora as a refuge was not significantly different from several other structures (Table 3). For poly-

TABLE 4. Surface (S) and volume (V) measurements of structures used as refuges in the experiments (n = 2 in each case).

x detergent mass (g)

gained per x surface gram of area x volume i SIV

Structure structure (cm2/g) (mL/g) ratio

No structure . 0 0 0 Stones .049 6 0.36 17 Gravel .056 9 0.44 20 Zonaria .200 72 0.57 126 Cystophora .320 118 1.25 95 Champia .355 128 1.75 73 Caulerpa .375 137 1.24 110 Bottle brush .365 133 1.09 122 Corallina .565 211 1.06 199




TABLE 5. Kendall's Tau correlation coefficients between three measures of structural complexity and meiofaunal taxon abundance remaining after predation. * significant at P 'E .05, ** P S .01, *** P - .001.

Surface area Volume SIV ratio

Total meiofauna 0.57* 0.27 0.69** Copepods 0.65* 0.27 0.80*** Amphipods 0.54* 0.24 0.68** Polychaetes 0.38 0.54* 0.39 Other taxa 0.42 0.05 0.57*

chaetes and other taxa all structures overlapped (Table 3).

Since Coral/ina was the most complex structure (Table 4), we specifically tested the hypothesis that more meiofauna remained on Corallira after exposure to predatory fish than on any other structure. When so tested, Cortalli/a was a significantly better refuge for total meiofauna, copepods, and amphipods, but was not significantly different from other structures for polychaetes and other taxa (Scheffr Multiple Com- parison Method, EWER - 0.05, Corallina vs. other structures). Kendall's Tau correlation coefficients of taxa loss rate (i.e., adjusted mean abundance of each taxon remaining after predation) vs. the "complexity" measures, indicated that surface-to-volume ratio provided the most significant correlations (Table 5) and thus was the "best" measure of complexity. Note that polychaetes and other taxa were not highly correlated with any structure measured. Fig. I illustrates the relationship of the 'best" measure of complexity (i.e., surface-to-volume ratio) and the adjusted mean density of meiofauna remaining at each complexity level.

All copepods from all experiments were identified to assess if there was species-selective predation. There were 31 species from Corallina and 18 species from Champia. Three species, Paralaophonte meinerti (Brady) (35%), Amphiascius lobatus Hicks (21%) and Ectinosomra australe Brady sensu Hicks, 1977a (18%), dominated the Corallina fauna; no other species rep- resented >5% of total copepod abundance. Porcelli- dium ervthrum Hicks comprised 58% of the total Champia fauna, with three additional species each representing >5%: Coullia sp. (8.5%), Scutellidium spinatum Hicks (8.2%), and Parastenhelia spinosa (Fischer) (7.6%). Of the three dominant harpacticoids from Corallina, P. meinerti was not encountered in the Champia fauna and A. lobatus and E. australe made up only 0.6 and 0.3% of the Champia fauna, respectively. Thus, <3% of the fauna was shared between the two algae, which was not surprising since the algae primarily inhabit different tidal levels.

Of the Corallina fauna, only the abundance of Am- phiascus lobatus was significantly different after treatment when compared to the initial abundance (Table 6). Treatments in which Corallina was used as a refuge

had significantly higher abundances of A. lobatus remaining compared to all other treatments (Scheff6, EWER, S 0.05), and only on Corallina was there no difference between initial and postpredation abundance of A. lobatus. Of the four dominant species extracted from Chairnpia, three (Porcellidium erythrurm, Coullia sp., and Scutellidium spinatum) had significantly different (P - .05) postpredation abundances compared to the initial abundances (Table 6). The mean abundances and the results of the Scheff6 Multiple Comparison Method (EWER S 0.05) for these three species are listed in Table 7. Note that the dominant species of the fauna extracted from Champia avoided predation best on Champia. This is in contrast to the major-taxon analysis (Table 3) where Champia was not the "best" refuge for copepods extracted from Champia.

If there was predator selection for males, females, or copepodites, one would expect one or more of these population components to be disproportionately abun-

TABLE 6. Summary of ANCOVA for three dominant copepod species extracted from Corallina and one-way AN- OVA for four dominant copepod species extracted from Chainpia. ANCOVA explanation as in Table 2. Since the Champia fauna was used in only one experiment there was no covariate effect. Thus the ANOVA shows whether there were differences in abundance of a species remaining in three different refuges (Champia, Corallina, no structure) after exposure to predation.

P of greater

Species df MS F F

Corallina species Paralaophonte meinerti

Initial abundance 1 2752 22.69 .0001 Refuge structures 8 1766 1.82 .1075

MSE 34 121 ...

Amphiascus lobatus Initial abundance 1 56 1.07 .3084 Refuge structures 8 3078 7.39 .0001

MSE 34 52 ... ...

Ectinosoma australe Initial abundance 1 972 0.44 .0063 Refuge structures 8 405 8.49 .8870

MSE 34 114 * ...

Champia species Porcellidium erythrum

Refuge structures 2 725 8.1 .0097 MSE 9 89 ...

Coullia sp. Refuge structures 2 13.6 6.2 .0204

MSE 9 2.2 * -

Scutellidium spinatum Refuge structures 2 32.3 13.9 .0018

MSE 9 2.3 ...

Parastenh elia spinosa

Refuge structures 2 7.8 3. 1 .0965 MSE 9 2.5 ...




1100 - 800 A B

1000 rzO.93 700 r - 0.93

900 600 CD

800 T 500 0)1

E

700 400 EI C) 0

600 - 30 0

500 200

400 { 100

300 0 -

0 50 100 150 200

200 Surface to Volume Ratio

100

o 200

0 I , D 0 50 100 150 200

CY_ .90 50 1

Surface to Volume Ratio 100 0~

0.

E 0 50 100 150 200 FIG. 1 a t a d o y i f 4Surface to Volume Ratio

0) 0

200 ~ ~ ~ ~ ~ ~ ~~~~6200 SIV ratios (a measur=o.48 (D ra tw s s

e r u a t n o t s 1.0 10 a) 0~~~~~~~~~~~~~~

u 0

0: 0 10 10 200 5 100 150 20

CL ~~Surface to Volume Ratio Surface to Volume Ratio

FIG. 1. The adjusted mean abundance of prey remaining after 24 h predation by He/cogramma medium at different refuge S/V ratios (a measure of complexity). Error bars are ?1- standard error. Each S/V ratio is the mean of two measurements of each structure used as a refuge (Table 4). Initial abundance for each taxon is given in Table 1.

dant after predation. Chi-square values for population components of the three dominant species extracted from Corallina indicated that there was no selective removal of any Ectinosoma austrole or Paralao- phonte meinerti population component (not surprising, since there were no differences in total abundance of E. austrole or P. meinerti among treatments, Table 6). However, Amphioscus lobotus females were significantly less abundant after treatment (all treatments combined, df = 47) than expected, and copepodites significantly more abundant (P < .005); there was no difference in the males. All Corallin/ refuge experiments, however, had more A. lobatus females remaining at the end than expected (P = .05, df = 5).

Field experiment

Were the experimental results artifacts of the laboratory? In the field experiment the abundances of total

meiofauna (t = 10.3; df = 2; P = .009) and copepods (t = 13.4; df = 2; P = .006) were significantly less in the no-structure pools compared to the Corallina pools. There were no significant differences in the densities of amphipods (t = 0.84, df = 2, P = .49), polychaetes (t = 1.41, df = 2, P = .29) or other taxa (t = -0.21, df = 2, P = .85) between the Corallina and no-structure pools. With total meiofauna, for example, 84.1% of the original fauna remained in the Corallina pools after 4 h of fish predation, while only 31.6% remained in the no-structure pools. While these field experiments did not include all the structures used in the laboratory, the results are similar; i.e., with Corallina there was a small amount of removal, and without Corallina most of the fauna was eaten. There were no significant differences in abundances of each of the three dominant copepod species (A. lobatus, P. meinerti, and E. australe) remaining on Corallina vs. no




TABLE 7. Mean abundance of the four harpacticoid species extracted from Champia remaining after treatment. Treat- ments in this experiment included providing the fauna with three different refuges and allowing two starved blennies to feed for 24 h. Those structures with common underlines are not significantly different in providing refuge from predation (Scheffe Multiple Comparison Method, P -_ .05).

Refuge

Species No (initial abundance ? SD) Chaimpia Corallina structure

Remaining abundance Porcellidium ervthruin

(61.7 + 21.0) 53.0 27.5 32.7

Coullia sp. (9.0 ? 1.7) 5.5 2.7 2.0

Scutellidiumn spinatum (8.7 + 7.2) 6.0 2.0 0.5

Parastenh/lia spinosa (8.0 + 1.7) 2.8 1.0 0.3

structure, nor were there differences in the population components (female, male, copepodite). Recall that the field experiments lasted only 4 h, whereas the laboratory experiments lasted 24 h.

DiSCUSSION

The results of our experiments are consistent with the hypothesis that increased habitat complexity reduces predation on abundant prey items. The state- ment "complexity clearly reduces overall capture rates" (Cooper and Crowder 1979:267) is predictable, but we provide both field and laboratory data to illus- trate the relationship empirically. Other authors have demonstrated a similar relationship with other predators and prey (Table 8). In most instances previous investigators have manipulated the density (i.e., number per area) of one structure, and found reduced predator efficiency. In our study we used different natural structures, whose complexity was measured a posteriori to test a different aspect of the complexity/ refuge relationship. We deliberately selected structures suspected of varying in complexity, but we had no idea, a priori, what the absolute complexity rankings were. Thus, we did not pre-bias our experimental design nor did we expose the predator or the prey to unnatural refuges. Our experiments provide evidence that different structures of varying complexity also affect predator success. This is the first such demon- stration of these effects on meiofaunal organisms.

In our experiments, the alga Corallina officinalis, the most complex structure used (Table 3), consis- tently provided the best, and the only statistically significant, refuge for total meiofauna and the dominant prey taxon, harpacticoid copepods (Table 3, Fig. IA, B). Further, Corallina was the "best" refuge for copepods regardless of whether the fauna was originally extracted from Corallina or from another alga (Cham-

pin). Of the two points at the highest S/V ratio in Fig. IB, one is for fauna extracted from Corallinn on Cor- allinn and one for fauna extracted from Champin on Corallinn. However, while total meiofauna and total copepod abundance from Champia is higher on Cor- allinn as a refuge vs. Champin (Table 3), two of the four dominant Champia copepod species (Porcelli- dium erythrum and Scutellidium spinatum) were significantly less preyed upon on Champin than on Cor- allinn (Table 7). Thus, although Corallinn provided a more effective refuge than Champia for Champia copepods in toto (Table 3), two of the dominant species survived better on Champin. Fish predation on the copepod species extracted from Champin must there- fore have focused on the less abundant species.

Even though our measurements indicate that Cor- allinn was the most complex structure (Table 4), its rigidity may also have been important in reducing predation. Admittedly we did not test for a Corallinn rigidity effect. Ideally, we should have run a compara- tive experiment with a fleshy or bendable structure with the same complexity as Cornllina, but we could not find one! Excluding stones and gravel (also hard substrata), all other structures used in the experiments were fleshy (algae) or flexible (bottle brushes). Hel- cogramma medium is a "picker-type" fish sensu Bray and Ebeling (1975), and in our fleshy, flexible structures it pushed "fronds" apart to capture prey. In Cornllinn, however, H. medium did not push the cal- careous fronds apart but waited outside the algal clumps and thus only captured prey that moved onto the pe- riphery. Thus, we cannot separate complexity and rigidity effects on prey removal from Cornllinn.

If Cornllinn was the "best" refuge for copepods and total meiofauna in our experiments, how does this re- late to field abundances? Hicks (1977a) reported copepod density and diversity higher on Cornllina than on several other algal assemblages he studied at our sites. He suggested that the increased density/diversity was due to more available microhabitat in Cor- nllinn and the accumulation of sediments around the fronds. While these sediments may provide additional feeding sites for the fauna (Hicks 1980), it is very clear from our experiments that even without its associated sediment, Cornllina is a significant copepod refuge from fish predation. Whether additional feeding surfaces and predation refuges complement each other in the field to account for the high density/diversity of the New Zealand Cornllinn fauna is not known. Empirically at least, we know that refuges from predation are important in regulating this phytal assemblage.

Our results on polychaetes and other taxa (sensu Table 1) indicate few or no significant correlations with structural complexity (Tables 3, 5; Fig. IC, E). We envision three potential reasons for this lack of correlation: (1) Because polychaetes and other taxa were low in abundance, the lack of correlations may simply be the result of small sample sizes where internal vari-




TABLE 8. Summary of aquatic predator (primarily fish)/prey experiments where structural complexity was increased.

Threshold response* Author

Predator or disturber/prey Structural complexity Yes or No? Complexity level

Laboratory/microcosm studies

Largemouth bass/guppies 0, 85, 185, 370 dowels/m2 Yes 370 dowels Glass 1971 Rainbow trout/amphipods 0, 6%, 15%, 100% litter cover ?Yes 6-15%, and 100% litter cover Ware 1972

Rainbow trout/shrimp 0, 6%, 15%, 100% litter cover ?Yes >15% litter Ware 1972 Fundulus/amphipods 84 + 8 and 598 + 51 Spartina stalks Yes 598 stalks Vince et al. 1976

per core Fundulus/amphipods 0, 2, 10-14 Spartina culms per disk Yes 10-14 culms Van Dolah 1978

Parrotfish/biomass of benthos no, fine, medium and coarse (most Yes medium and coarse mesh Brock 1979 complex) mesh

Large pinfish/amphipods 0, 15, 35, 50 (plastic), 75 blades Yes 75 blades Nelson 1979 seagrass per tank

Small pinfish/amphipods 0, 15, 35, 50 (plastic), 75 blades ?Yes >15 blades Nelson 1979 seagrass per tank

FunduluslPalaeoinonetes 0, 274, 484, 674 shoots artificial sea- Yes 674 shoots Heck and Thoman 1981 grass/M2

Pinfish/shrimp 0, 20W, 100% red algae cover ?Yes >20W cover Coen et al. 1981 0 and 25 plastic seagrass per tank Yes 25 plastic seagrass

Small pinfish/amphipods 0, 5, 20, 40 g of three different sea- Stoner 1982 grasses per tank:

Syringodium Yes 40 g of Syringodiumn Thallasia and Thallasia Halodule No Halodule

Large pinfish/amphipods 0, 5, 20, 40 g of three different sea- No gradual decline Stoner 1982 grasses per tank

rockpool blennies/meiofaunal cope- no structure, through a variety of Yes only most complex algae Present study pods complexities to complex articulat-

ed Corallina officinulis

Field studies

'?/infauna 0, 1 (3-S. A. Woodin, personal Yes 6 Diopatra tubes S. A. Woodin 1978 and commnunication), 6 Diopatra personal co(mmflunifcationl (Polychaete) tubes

Pinfish/amphipods three densities of seagrass Yes high seagrass density Stoner 1979

'?/copepods increased sediment on Diopatra No with sediment, number of Bell and Coen 1982 tubes tubes

increased number of tubes Ulva added to tubes Yes with Ulva

Bluegills/invertebrates low, medium, high macrophyte den- No most prey removed at medi- Crowder and Cooper 1982 sity urm density

Rockpool blennies/copepods no structure and Corallina Yes Corallina Present study

* Lowest complexity level at which predation is reduced.

ance masked any effect. (2) Refuge from predation is not an important regulating force for the polychaete and other taxa assemblages in these phytal habitats. (3) H. medium does not normally eat polychaetes and other taxa, and thus prey removal was independent of structural complexity. In fact, of the 32 field-collected blennies whose gut contents we analyzed, there were only 18 "other taxa" and 1 polychaete among the 439 prey items in their guts. In any case, statements regarding the "value" of a refuge must be tempered to the individual taxon level, if not the species level (see below), and should not be made regarding meiofauna (or macrofauna or birds, etc.) in general.

If species-selective predation was operating, one or more species should have been disproportionately less abundant after predation. Of the copepods extracted from Corallina, only Amphiascus lobatus was significantly less abundant after predation (Table 6), indi- cating that it was selectively preyed upon. However, since it was not significantly lower in abundance on Corallina (Scheff6, P S .05), it appears that A. lobatus was able to avoid predation in its "home" struc-

ture. Predation on A. lobatus in the other structures was primarily on females (see Results, x2 test). Small fish often select harpacticoids preferentially from a suite of available prey items (Feller and Kaczynski 1975), but as a fish grows, it typically switches to larger prey items (e.g., Grossman et al. 1980; present study). A predator's choice of food is influenced by prey "size, abundance, and edibility, and the ease with which it is caught" (Brooks and Dodson 1965: 30). In the case of A. lobatus, females were the largest (>0.70 mm) population component of the three dominant species. Further, A. lobatus requires substrate to grasp, and our laboratory observations of its movement revealed it to be a slow, lumbering-crawling form (compared to the fast swimming-darting E. australe, and rapid crawling P. meinerti). This type of movement and the necessity to cling to a substrate probably made A. lobatus an easy catch, while the large size of the females may have made them either easier to catch or more attractive to the predator than smaller individ- uals.

A priori we expected there to be a gradient of de-




creasing predation as complexity increased. While there are linear relationships with complexity and prey remaining (Fig. 1, r values), the Scheff6 tests tell us that for total meiofauna and copepods, only Corallina provided a significant refuge (Table 3). The juvenile amphipod data were not as clear cut (since the alga Cys- tophora was included with Corallina) and we have already discussed the "no refuge" effect for polychaetes and other taxa. Obviously, the total meiofauna data reflect the response of the dominant copepods which were also the primary prey item of the predator.

Structures of intermediate complexity (between the extremes of no structure and Coralli/a) were regularly not significantly different from no structure or low complexity structures (stones, gravel) in affording refuge from predation (Tables 3 and 5). Does this mean that physical complexity must be at some threshold level before it becomes an important component in structuring communities? For the most part the available literature relating fish predation and complexity is consistent with such a proposal. In fact, Nelson (1979) suggested that predation was not a simple linear function but probably resembled a step function where there would also be a decrease in predation rate with intermediate complexity. Table 8 lists the available data on fish predation where different habitat complexities have been utilized. Even in field studies (e.g., Woodin 1978), where predations/disturbance could not be directly attributed to fish, refuge was not provided by the lack of structure (no Diopatra tubes) or intermediate structure ( 1, 3 Diopatra tubes); only when Woodin's "complexity" increased to 6 Diopatra tubes did polychaete (prey?) density increase.

None of the studies listed in Table 8 were designed to test the threshold hypothesis, and tempting as it is, we have not reinterpreted these results to mesh with our hypothesis. Had it been only our experimental data or only a few data sets that supported such a threshold relationship, we would have ignored it. However, we find it remarkable that from such a variety of habitats, predators, prey, and experimental designs, so much of the published data implies that there is a threshold complexity level at which predatory success is greatly reduced. Thus, the results are similar from two different kinds of complexity experiments, i.e., (1) the same structure at different densities (Glass 1971, Ware 1972, Vince et al. 1976, Van Dolah 1978, Woodin 1978, Nel- son 1979, Coen et al. 1981, Heck and Thoman 1981, Stoner 1982, Crowder and Cooper 1982), or (2) different structures of different shape or form covering the same area (present experiments); no structure or intermediate complexity provides negligible refuge. It appears that, with at least the dominant prey taxa, a complexity threshold must be reached before a refuge from predation or disturbance is achieved. Beyond that threshold, predation is significantly reduced. The question remains if this is merely an artifact of the experiments or is a general ecological principle.

ACKNOWLEDGMENTS

This research was conducted while the first author was a Fullbright-Hays Research Scholar at Victoria University of Wellington, New Zealand. Gratefully acknowledged is the New Zealand-United States Educational Foundation for sup- port. We express our sincere thanks to Brent Coull, Kendall Clements, George Grainger, G. R. F. Hicks, Peter Lawless, Hugh Packer, and R. G. Wear for assistance in the field and laboratory; to G. R. F. Hicks, D. F. Wethey, and S. A. Woodin for innumerable hours spent discussing this research with us, and to P. A. Montagna who taught the first author how to utilize the University of South Carolina computer facilities. S. S. Bell, R. J. Feller, C. Griffiths, K. L. Heck, G. R. F. Hicks, P. A. Montagna, M. A. Palmer, D. S. Weth- ey, and S. A. Woodin read earlier drafts of this manuscript and their comments have greatly improved it; our grateful thanks to them for so freely assisting. Contribution Number 475 from the Belle W. Baruch Institute of Marine Biology and Coastal Research, University of South Carolina.

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refuges from fish predation: experiments with phytal meiofauna from the new zealand rocky intertidal

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