presence of potential predators (rock crabs and lobsters) and/or food (kelp) on the behaviour of large juvenile (10 to 15 ram) and small adult sea urchins were ...
Marine Biology110, 105-116 (1991)
Marine BiOlOgy ..............
@ Springer-Verlag 1991
Interactions between sea urchins (Strongylocentrotus droebachiensis) and their predators in field and laboratory experiments R.E. Scheibling and J. Hamm
Department of Biology,Dalhousie University, Halifax, Nova Scotia B3H 4Ji, Canada Date of final manuscript acceptance: March 15, 1991. Communicatedby J.M. Lawrence,Tampa
Abstract. Field observations and manipulative experiments in a nearshore cobble bed (2 to 3 m below mean low water) at Eagle Head, Nova Scotia, Canada, between 1984 and 1986, showed that small juveniles of Strongylocentrotus droebachiensis (3 to 6 mm diam) sheltering beneath cobbles had a refuge from predators such as rock crabs, small lobsters, and fish. Sea urchins gradually outgrew these refuges and small adults (25 to 30 mm) required larger rocks as shelter from predators, particularly large cancrid crabs. Small juveniles were usually solitary and well dispersed beneath cobbles, whereas small adults tended to aggregate on the undersides and in the interstices of boulders. These aggregations may develop passively as sea urchins accumulate in suitablysized refuges. Chemotaxis experiments indicate that juvenile S. droebachiensis are repelled by waterborne stimuli from conspecifics. In a factorial experiment, effects of the presence of potential predators (rock crabs and lobsters) and/or food (kelp) on the behaviour of large juvenile (10 to 15 ram) and small adult sea urchins were examined in flowing seawater tanks. Both size classes formed exposed feeding aggregations when kelp was provided as food, irrespective of the presence or absence of predators. In the absence of kelp, each size class responded differently to the presence of a predator: juveniles became more cryptic, whereas adults aggregated on the tank sides. Increased movement to the sides of a tank in the presence of a predator may reflect a flight response, since chemotaxis experiments indicated that S. droebachiensis is repelled by waterborne chemical stimuli from predators. Observational and experimental data suggest that predation at the late juvenile and early adult stages may influence population structure, distribution and abundance of S. droebachiensis.
Introduction
The pivotal role of sea urchins in structuring subtidal communities has been widely documented, yet the factors
which control sea urchin abundance, or lead to population outbreaks and intensive grazing are poorly understood (see reviews by Lawrence 1975, Harrold and Pearse 1987). A reduction in predation pressure has been implicated as a causal factor in sea urchin outbreaks in some areas, but except for the case of sea otters in the Northwest Pacific, evidence for predatory control is inconclusive (Harrold and Pearse 1987). Predation on recentlysettled juveniles may be important in limiting recruitment of sea urchin populations (e.g. Highsmith 1982), but this has received little attention. Alternatively, selective predation on small individuals, which have outgrown spatial refuges but have not yet reached a size refuge, may influence population structure and abundance in some species (Tegner and Dayton 1977, 1981, Andrew and Choat 1982, 1985, Tegner and Levin 1983). On the Atlantic coast of Nova Scotia, Canada, large fluctuations in the abundance of the sea urchin Strongylocentrotus droebachiensis have caused major shifts in the structure of the rocky subtidal community (Scheibling 1984 a, 1986, Miller 1985 a). Sea urchin population outbreaks and intensive grazing of kelp beds in the 1960's and 1970's resulted in the formation of "urchin-dominated barren grounds" which persisted until the early 1980's (see reviews by Mann 1977, 1982). These events were linked initially to declines in the commercial catch of the lobster Hornarus americanus (Breen and Mann 1976, Wharton and Mann 1981), although the hypothesis that lobsters are key predators of sea urchins has not been supported by observational or experimental evidence (Pringle et al. 1982, Miller 1985 b, Vadas et al. 1986, Elner and Campbell 1987). Bernstein et al. (1981, 1983) advanced a more complicated scenario in which behavioural responses of S. droebachiensis to predators (mainly crabs and lobsters) triggered the formation of defensive aggregations within kelp beds, leading to intensive grazing. However, Vadas et al. questioned the experimental evidence for this scenario, and argued that aggregations ofS. droebachiensis form only in response to food or topographical features, and are not linked to predators.
R.E. Scheibling and J. Harem: Sea urchins and predators
106
Controversy over the role of predation in controlling p o p u l a t i o n s o f Strongylocentrotus droebachiensis has focussed largely on i n t e r a c t i o n s b e t w e e n a d u l t sea urchins a n d their p r e d a t o r s . Juvenile sea urchins ( < 20 m m a m bitus d i a m e t e r ) are susceptible to a g r e a t e r r a n g e o f p r e d a t o r s t h a n adults, i n c l u d i n g s m a l l - m o u t h e d fish such as cunner, o c e a n p o u t , a n d f l o u n d e r ( G r e e n et al. 1984, K e a t s et al. 1985, 1987, K e a t s 1990). H o w e v e r , the imp o r t a n c e o f p r e d a t i o n o n small sea urchins a n d its influence on r e c r u i t m e n t p a t t e r n s is u n k n o w n . Juveniles o f S. droebachiensis live c r y p t i c a l l y u n d e r rocks, in small crevices, o r in b i o g e n i c m i c r o h a b i t a t s such as u n d e r c u t coralline algae a n d mussel p a t c h e s (Bernstein et al. 1981, K e a t s et al. 1985, W i t m a n 1985, H i m m e l m a n 1986, R a y m o n d a n d Scheibling 1987). A s they increase in size, they m a y o u t g r o w these refuges o r c o m e o u t o f h i d i n g to actively f o r a g e on e x p o s e d r o c k surfaces, t h e r e b y b e c o m i n g m o r e v u l n e r a b l e to p r e d a t o r s such as crabs, lobsters, a n d fish (Bernstein e t a l . 1981, K e a t s e t a l . 1985, 1987, W i t m a n 1985, H i m m e l m a n 1986). L a r g e a d u l t sea urchins r e a c h a size refuge f r o m m o s t p r e d a t o r s except large c r a b s a n d lobsters ( H i m m e l m a n a n d Steele 1971, Elner 1980, Scheibling 1984b) a n d wolffish ( K e a t s et al. 1986). I n the early 1980's, m a s s m o r t a l i t i e s o f Strongylocentrotus droebachiensis due to disease resulted in the re-est a b l i s h m e n t o f k e l p beds off N o v a S c o t i a (Miller 1985 a, Scheibling 1986). These events p r o v i d e a n o p p o r t u n i t y to investigate processes, such as p r e d a t i o n , w h i c h m a y be i m p o r t a n t in d e t e r m i n i n g the r e c o v e r y rate o f p o p u l a tions o f S. droebachiensis a n d the persistence o f kelp beds. In this study, we m a n i p u l a t e d s p a t i a l refuges (cobbles a n d b o u l d e r s ) a n d e x a m i n e d the effects o f p o t e n t i a l p r e d a t o r s ( m a i n l y c r a b s a n d lobsters) o n the survival o f j u v e n i l e a n d small a d u l t sea u r c h i n s in field e x p e r i m e n t s . We also c o m p a r e d b e h a v i o u r a l responses (e.g. a g g r e g a tion, flight, hiding) o f these small size classes to p r e d a t o r s a n d / o r m a c r o a l g a l f o o d (kelp) in l a b o r a t o r y experiments. T h e e x p e r i m e n t a l results give insight into the interactive effects o f sea u r c h i n size, spatial refuges, a n d b e h a v i o u r in m e d i a t i n g p r e d a t i o n rates a n d influencing p a t t e r n s o f p o p u l a t i o n structure, d i s t r i b u t i o n a n d a b u n d a n c e .
Materials and methods
Study site Field experiments were conducted in a shallow (2 to 3 m below mean low water), nearshore cobble bed at Eagle Head (44°04'N; 64°36'W) on the southwestern coast of Nova Scotia, Canada. The cobbles range in length from 4 to 10 cm and are heavily entrusted with coralline red algae of the genera Lithothamnium, Clathromorphum and Phymatolithon. The bed is protected along its offshore extent by a ridge of boulders which dissipates much of the incoming wave energy. A more detailed description of the study site is given in Scheibling and Raymond (1990). Strongylocentrotus droebachiensis was abundant on the cobble bed and surrounding boulder fields prior to a mass mortality due to disease in September 1983 (Scheibling and Stephenson 1984, Scheibling and Raymond 1990). After the die-off, sea urchins recruited to these areas via their planktonic larvae. The density of juvenile sea urchins on the cobble bed increased in 1984 due to
cohorts which settled in fall 1983 and summer 1984 (Scheibling and Raymond 1990). The sea urchin population then declined precipitously over the next few months and gradually disappeared from the cobble bed by 1986, although sea urchins persisted on the adjacent boulder ridge. Crabs Cancer irroratus, C. borealis, Carcinus rnaenas and lobsters Homarus americanus were the most active and abundant predators on the cobble bed, particularly in the summer and fall. Whelks Buccinum undaturn and small sea stars Asterias vulgaris were among the smaller, more sedentary predators. Fish were uncommon, although small rock gunnels (Pholis gunnellus) and juveniles of demersal species such as sculpins (Myoxoeephalus octodecemspinosis) and winter flounder (Psuedopleuronectes americanus), were occasionally observed. For a detailed description of the macrobenthic community of the cobble bed see Scheibling and Raymond (1990).
Caging experiments with small juvenile sea urchins The effects of potential predators and spatial refuges on survival of small juveniles of Strongylocentrotus droebachiensis were tested in 12 cylindrical cages (32cm diam, 38 cm height, 1.1 mm mesh) placed at 2 to 3 m intervals in the cobble bed. The solid base of each cage (cut from a plastic bucket) was recessed 12 cm into the cobble bed and filled to the level of the bed surface either with cobbles (2 to 3 layers) from the surrounding area or sand from an adjacent subtidal habitat. The cobbles and sand were carefully searched to ensure that no macroscopic predators or juvenile sea urchins were introduced into cages prior to the experiments. Small herbivorous molluscs (limpets, Notoacrnaea testudinalis, and chitons, Tonicella rubra) were not removed from the cobbles. Treatments involved enclosing groups of 25 juvenile sea urchins (3 to 6 ram) and 1 to 3 predators in cages, either with cobbles (a spatial refuge for the sea urchins) or with sand (no refuge), for 2 wk. As controls for non-predatory mortality (or incomplete recovery of juveniles), groups of 25 sea urchins were placed in cages with cobbles or sand but without predators. Predators included juvenile sculpins, Myoxocephalus octodecemspinosis (50 to 60 mm body length, one per cage), whelks, Buccinum undatum (36 to 48 mm shell length, three per cage), green crabs, Carcinus rnaenas (40 to 58 mm carapace width, three per cage), rock crabs, Cancer irroratus (48 to 61 mm carapace width, three per cage), and juvenile lobsters, Homarus arnericanus (43 to 54 mm carapace length, one per cage). Predators were randomly allocated to two replicate cages for each treatment with or without cobbles. Two replicates of each control (with or without cobbles) were run during each 2 wk period (the same controls were used in concurrent experiments with two different predators). Sea urchins and predators were collected from the surrounding cobble bed immediately prior to each experiment. Predators were introduced to cages 4 to 6 h after the sea urchins. In cages with cobbles, sea urchins dispersed into the interstices within 1 h. After 2 wk, predators were removed and the remaining sea urchins were relocated. Cobbles were carefully removed from cages by divers and delivered in plastic buckets to the shore, where they were examined for sea urchins. The number of sea urchins on the sides or roof of the cage or on sand was recorded in situ. Seawater temperature ranged from 9.5 to 10°C during the experimental period (15 June to 13 July 1984),
Caging experiments with small adult sea urchins The effects of larger potential predators (cancrid crabs and lobsters) and spatial refuges on the survival of small adults of Strongylocentrotus droebachiensis were tested in six larger cages (1 m diam, 40 cm height, 16 mm mesh) placed at 5 m intervals in the cobble bed. The cages were bottomless and held in place with guy ropes attached to cement anchor-blocks. The bottom rim of each cage was sunk 10 to 15 cm into the cobble bed to completely enclose an area of
107
R.E. Scheibling and J. Hamm: Sea urchins and predators 0.785 m z. Boulders collected from the adjacent ridge (the same ones used in the boulder transplantation experiment described in following subsection) were placed in a single layer over the cobbles to cover the bottom of two randomly selected cages. Cobbles and boulders within caged areas were carefully checked for naturally occurring sea urchins, but none were found. Sea urchins were rare on the cobble bed during these experiments (3 October to 14 November 1986). Treatments involved enclosing groups of 40 small adult sea urchins (25 to 30 mm) and three crabs (rock crabs Cancer irroratus, 65 to 90 m m carapace width; J o n a h crabs C. borealis, 95 to 110 m m carapace width) or one lobster (60 to 85 m m carapace length) in cages, either with boulders (a spatial refuge) or without boulders, for 2 wk. Controls consisted of groups of 40 sea urchins in cages without predators. For each predator tested, there were two replicates of each treatment (with and without boulders) and a control (without boulders). The sea urchins, crabs and lobsters were collected from the cobble bed and/or boulder ridge prior to each experiment. Predators were added to cages 6 h after the sea urchins, which by that time had dispersed beneath the cobbles or boulders. After 2 wk, the number of surviving sea urchins and their distribution in the cage were recorded. Distributional categories included: on b o t t o m vs side/roof of cage, exposed vs cryptic (i.e., on undersides or in interstices of boulders or nestled in the cobbles), and solitary vs aggregated (i.e., contacting spines of one or more adjacent sea urchins). The number and size of aggregations also were recorded. Seawater temperature ranged from 8 to 10 °C during the experimental period.
Boulder transplantation experiment To examine the effects of boulders as a refuge for the natural population of Strongylocentrotus droebachiensis in the cobble bed, boulders from the adjacent ridge were collected on 25 November 1984 and placed in three, 0.5 m 2 circular clusters randomly positioned in the cobble bed. The boulders were about 20 to 30 cm in length, encrusted with coralline algae, and devoid of fleshy macroalgae. Any sea urchins found on the boulders were removed. On 27 November 1985, sea urchins in each boulder cluster, and in the cobble bed 1 m from each cluster, were sampled in 0.125 m 2 plots.
Laboratory experiments with juvenile and adult sea urchins: behavioural responses to potential predators and kelp The behaviour of juvenile and adult Strongylocentrotus droebaehiensis in the presence or absence of potential predators (rock crabs or lobsters) and kelp was investigated in six, 250-liter (91 x 61 x 45 cm) fiberglass seawater tanks. Each tank was supplied with flowing ( ~ 4 liters m i n - 1 ) sand-filtered seawater through a 12 m m diam hose at the center of the tank. The water drained through a 50 m m diam vertical standpipe at one end. Water temperature was maintained at 12 °C; fluorescent lighting was supplied on an 18 h light:6 h dark daily cycle. The b o t t o m of each tank was covered with cobbles (5 to 10 cm length) and small boulders (20 to 25 cm) collected from Eagle Head. The rocks were entrusted with coralline algae, but all other macroalgae and invertebrates were removed. Cobbles and boulders were interspersed to provide different scales of refuge space for sea urchins throughout the tanks. The rocks were similarly arranged among tanks to minimize betweentank variability. The cobble and boulder substratum was carefully mapped for each tank (Fig. i a). Sea urchins were collected from barren grounds at Half Island Cove, Chedabucto Bay, Nova Scotia (45°21'N; 61°12'W) in October 1986. They were maintained at ambient seawater temperatures in tanks like those described above, and fed kelp Laminaria longicruris and L. digitata at 2 to 4 wk intervals; 150 large juveniles (10 to
standpipe sm~l~l~ge
b
predator cage
¢,S-¢-,A--1' ,S,E-
~
B,S,C B,A,E B,A,C
Fig. 1. Diagram of experimental tank used to study behavioural responses of juveniles and adults of Strongylocentrotus droebachiensis to potential predators and kelp. (a) Top view showing arrangement of cobbles and boulders on bottom and kelp attachment points (kelp blades shown at one point); (b) cross-sectional view showing predator-enclosure cage and distributional categories for sea urchins: B, bottom; W, side; A, aggregated; S, solitary; C, cryptic; E, exposed 15 mm) and 150 small adults (25 to 30 mm) were selected for use in experiments and split into three groups of 50 sea urchins of each size class. These groups were randomly allocated to the six experimental tanks, giving three replicate tanks for each size class. The sea urchins fed on crustose coralline algae and acclimated to 12°C for 3 wk prior to experiments. Two experiments were conducted: the first used rock crabs as potential predators (17 to 25 February 1986), the second used lobsters (3 to 11 March 1986). Treatments were factorially designed with two levels of each of three factors: sea urchin size (juveniles, adults), predator (present, absent), and kelp (present, absent). Predators were presented in cages (16 m m mesh) suspended in the tanks to preclude any physical contact with the sea urchins (Fig. 1 b). The seawater inflow was through the top and center of these cages to ensure dispersal of any chemical stimuli released by predators. Each tank was supplied with a cage throughout the experimental period (regardless of whether or not a predator was present) to control for any cage effect. Two rock crabs (one 100 to 120 m m carapace width, the other 60 to 80 mm) or one lobster (100 to 120 m m carapace length) were used in treatments with predators present. Rock crabs were freshly collected from Halifax Harbour; freshly caught lobsters were obtained from a local seafood distributor. Kelp blades (Laminaria longieruris and L. digitata) were presented in cut strips ~ 100 x 150 m m attached to three evenly spaced anchor points on the bottom (stainless steel bolts cemented onto the tops of boulders). One strip of each species per anchor was used. The four predator and kelp treatment combinations were systematically allocated to each set of three tanks containing either juvenile or adult sea urchins, but staggered among tanks to ensure interspersion of treatment replicates (n = 3) during the experimental period. Thus, each tank of sea urchins received one replicate of each predator and kelp treatment combination. Treatments with kelp were alternated with treatments without kelp to maintain a regular feeding regime. Each treatment lasted 24 h. The distribution of individual sea urchins was recorded on maps of the tank immediately before and after each treatment. Distributional data were categorized as described above ("Caging experiments with small adult sea urchins"; see also Fig. 1 b), and the number and size of aggregations were recorded. After each treatment, water flow to a tank was shut off and the bottom was vacuumed to remove sea urchins feces; uningested kelp also was removed. Each tank was then rapidly drained by removing the standpipe and immediately refilled. The sea urchins were left undisturbed for 12 h before the next treatment was applied.
108 Laboratory chemotaxis experiments Chemotaxic responses of juveniles of Strongylocentrotus droebachiensis to potential predators and conspecifics were examined in "I"-shaped choice chambers (after Pratt 1974) similar to those used in previous studies with adults of this species (Garnick 1978, Mann et al. 1984). A header tank supplied heated (11 to 12°C), flowing seawater to two stimulus tanks, which in turn fed water into opposite ends of a clear Plexiglas chamber. The water flowed down sloping platforms to a central, perforated drainage plate. Four small chambers (9 x 3 cm) were used for small juvenile sea urchins (2 to 8 mm), two large chambers (71 x 15 cm) were used for large juveniles or small adults (15 to 25 mm). Water inflow was adjusted to 27 ml rain -1 and 2.5 liters min 1 in small and large chambers, respectively. Dye tests showed that the opposite flows converged to form a sharp boundary at the drainage plate. A stimulus (1 to 2 potential predators or 8 small adult sea urchins) was added to one of the two stimulus tanks (the same stimulus tanks supplied small and large chambers). A sea urchin was placed at the centre of a chamber for 15 min in the dark, after which its position was recorded. Individuals were scored as moving towards or away from a stimulus, or remaining inactive (moving < 1 and < 3 cm in small and large chambers, respectively). Controls consisted of runs with no added stimulus. Sea urchins were collected at Half Island Cove and potential predators (stimulus animals), including rock crabs (66 to 107 mm carapace width), green crabs (48 to 60 mm carapace width) and cunner Tautolgolabrus adspersus, a labrid fish (190 to 230 mm body length), were collected at Mill Cove (44°35'N; 64°04'W), St. Margaret's Bay, Nova Scotia. Lobsters (117 to 118 mm carapace length) were obtained from a seafood distributor. Each species was maintained in a separate flowing seawater tank at ambient seawater temperatures. Stimulus animals were fed live mussels, frozen squid, or Purina Lobster Chow twice weekly; sea urchins were fed kelp twice monthly. Experiments were conducted from May to June 1986 and February to April 1987. Sea urchins and stimulus animals were acclimated to 11 to 12°C for at least 7 d before use in experiments. Each sea urchin was tested only once with the same stimulus species. Stimulus tanks and choice chambers were thoroughly flushed with seawater between replicate runs, and drained and cleaned between runs with different stimuli. Each stimulus was randomly allocated to one of the two stimulus tanks.
R.E. Scheibling and J. Hamm: Sea urchins and predators urchins received all four predator/kelp treatment combinations, these groups are considered to be statistically independent. Differences in the pre-treatment distribution between juveniles and adults are reflected in the post-treatment distribution of the two size classes in the control group (predator and kelp absent): there were no significant differences between pre- and post-treatment counts in any distributional category for each size class in the control treatment (paired t-tests, P > 0.05). Thus, the distribution of sea urchins in tanks in the absence of predators or kelp was relatively stable. In chemotaxis experiments, the numbers of sea urchins moving towards or away from a given stimulus were compared to a hypothetical 1 : 1 distribution (no response to stimulus) using a Z 2 goodness of fit test with Yates' correction for continuity. Tests were conducted for each size class and for results pooled from both classes, which were not significantly heterogeneous in any experiment ( P > 0.25).
Results C a g i n g e x p e r i m e n t s w i t h s m a l l j u v e n i l e sea u r c h i n s S u r v i v a l r a t e s o f s m a l l j u v e n i l e s o f Strongylocentrotus droebachiensis v a r i e d a m o n g t r e a t m e n t s w i t h d i f f e r e n t p o t e n t i a l p r e d a t o r s a n d s u b s t r a t a (refuges), b u t w e r e c o n s i s t e n t l y h i g h (23 to 25 o u t o f 25 sea u r c h i n s w e r e r e c o v e r e d ) in c o n t r o l c a g e s w i t h o u t p r e d a t o r s (Fig. 2). T h e r e w e r e n o s i g n i f i c a n t d i f f e r e n c e s in s u r v i v a l / r e c o v e r y rates b e t w e e n t h e t w o t y p e s o f c o n t r o l (i.e., w i t h o r w i t h o u t c o b b l e s ) in a n y e x p e r i m e n t ( P > 0.475), a n d r e p l i c a t e s o f both types were pooled for comparisons with predator t r e a t m e n t s . S u r v i v a l o f sea u r c h i n s in c a g e s w i t h a j u v e nile s c u l p i n a n d w i t h c o b b l e s as a r e f u g e w a s s i g n i f i c a n t l y l o w e r t h a n in c o n t r o l c a g e s ( P = 0.004), a l t h o u g h s u r v i v a l in c a g e s w i t h a s c u l p i n b u t w i t h o u t c o b b l e s d i d n o t differ s i g n i f i c a n t l y f r o m c o n t r o l s ( P = 0.891). S u r v i v a l o f j u v e niles in c a g e s w i t h 3 w h e l k s o r 3 g r e e n c r a b s , w i t h o r without cobbles, did not differ significantly from controls ( P > 0.10). S u r v i v a l o f j u v e n i l e sea u r c h i n s in c a g e s w i t h 3 rock crabs or a lobster was significantly lower witho u t c o b b l e s t h a n w i t h c o b b l e s ( P < 0 . 0 0 1 ) , w h i c h in
Statistical analysis In caging experiments in the cobble bed, the number of surviving sea urchins was compared in contingency tables with Yates' continuity correction. Replicates of treatments or controls were tested for heterogeneity (Zar 1984), which was not significant in each case ( P > 0.05), and pooled to increase cell frequencies and accuracy of z2-estimates. In experiments with adult sea urchins, one of two replicate cages in some treatments or controls was dislodged by wave action over the 2 wk experimental period, creating gaps along the cage bottom through which sea urchins or predators could escape. These replicates were discarded. In laboratory experiments on the behaviour of juvenile and adult sea urchins in the presence or absence of a predator and/or kelp, counts of sea urchins in each behavioural/distributional category were analysed by three-factor ANOVA. Variances of counts for each category were not significantly heterogeneous, as indicated by Cochran's test for homogeneity of variance. In similar analyses of pre-treatment data, only the main effect of sea urchin size was significant (P < 0.05). The absence of significant pre-treatment main effects or interactions of the other two factors (presence of predator, presence of kelp) indicates that there were no residual effects of previous treatments; i.e., draining and refilling of the tanks after each treatment and the 12 h period between treatments was sufficient to redistribute sea urchins similarly among tanks for each size class. Therefore, although each group of juvenile or adult sea
+ Cobbles - Cobbles
100 -
80-
60-
40-
20-
0 Con~ol
Sculpin
Whelks
Green Crabs
1 Rock Crabs
Lobster
Fig. 2. Strongylocentrotus droebachiensis. Percentage survival of small juveniles after 2 wk in cages with different potential predators or in control cages without a predator, and with or without cobbles as a refuge. Data are pooled from two replicate cages for each predator treatment and from six replicate cages for each control. Pooled replicates are not significantly heterogeneous 0~2, P > 0.05)
R.E. Scheibling and J. Hamm: Sea urchins and predators I I I + Boulders[ [ ] - Boulders
m Control
Rock Crabs
Jonah Crabs
Lobster
Fig. 3. Strongylocentrotus droebachiensis. Percentage survival of small adults after 2 wk in cages with different potential predators and with or without boulders as an added refuge on the cobble bottom. Controls are cages without a predator and without boulders. Data are pooled from two replicate cages where available (see Table 1) for each predator treatment and control. Pooled replicates are not significantly heterogeneous 0/2, P> 0.05)
turn was significantly lower than survival in controls (rock crabs: P < 0 . 0 0 1 , lobster: P
