Effect of dietary history and algal traits on feeding rate and food preference in the green sea urchin Strongylocentrotus droebachiensis

Journal of Experimental Marine Biology and Ecology 349 (2007) 194 – 204 www.elsevier.com/locate/jembe

Effect of dietary history and algal traits on feeding rate and food preference in the green sea urchin Strongylocentrotus droebachiensis Devin A. Lyons ⁎, Robert E. Scheibling Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1 Received 27 October 2006; received in revised form 15 May 2007; accepted 22 May 2007

Abstract Feeding behaviour is influenced by a variety of factors, including nutritional requirements, the quality of available foods, and environmental conditions. We examined the effect of two factors, food morphology and dietary history, on the feeding rate and preference of the sea urchin Strongylocentrotus droebachiensis. Standardizing food shape and structure did not alter urchins' expected preference for the native kelp Laminaria longicruris over the invasive alga Codium fragile ssp. tomentosoides. However, when foods containing L. longicuris were shaped to mimic the algae, the C. fragile mimic was consumed more rapidly than the kelp mimic. Dietary history had no effect on single diet feeding rate. Urchins feeding on C. fragile consistently consumed twice as much (by mass) as those fed kelp, regardless of their previous diet. Despite higher feeding rates on C. fragile, urchins feeding on this alga were unable to compensate for its low energetic content and ingested less energy. Dietary history had a short-term effect on food preference, with urchins tending to prefer less familiar foods. Our findings suggest that urchins feed on C. fragile at a high rate, due to ease of handling and/or compensatory feeding, and that they do not a have strict preference hierarchy. Rather, food choice appears to reflect active maintenance of a mixed diet. © 2007 Elsevier B.V. All rights reserved. Keywords: Codium fragile ssp. tomentosoides; Diet mixing; Feeding behaviour; Herbivory; Laminaria longicruris; Strongylocentrotus droebachiensis

1. Introduction Foraging models predict consumers will feed in a way that optimizes their intake of energy, or limiting nutrients, so as to maximize their fitness (MacArthur and Pianka, 1966; Emlen, 1966; Krebs et al., 1974; Perry and Pianka, 1997). For herbivores, the challenge of obtaining a nutritious diet is complicated by the fact

⁎ Corresponding author. Tel.: +1 902 494 2296; fax: +1 902 494 3736. E-mail address: [email protected] (D.A. Lyons). 0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2007.05.012

that their food is of much poorer nutritional quality than their own bodies (Sterner and Hessen, 1994). As a result, the feeding strategies of herbivores reflect their need to optimize nutrition. For example, energy- or nutrient-rich foods are sometimes preferred to lower quality foods (Davidson, 1978; Berteaux et al., 1998) and herbivores can compensate for the nutritional value of their food by consuming poor quality foods more rapidly than high quality foods (Slansky and Feeny, 1977; Cruz-Rivera and Hay, 2000). Traits that influence palatability or handling time of food items, such as digestibility, anti-grazing defenses, and morphology, also are important determinants of a

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Table 1 Published values of energy content of Laminaria longicruris and Codium fragile and means (± SD) for each species

L. longicruris

Mean C. fragile

Mean a b

Energy content (kJ g− 1 dw)

Method

Reference

11.9 11.3 11.5 13.2 12.0 ± 0.8 11.0 8.04 7.63 8.9 ± 1.8

Bomb calorimetry Bomb calorimetry Wet oxidation Bomb calorimetry

Paine and Vadas (1969) a Miller and Mann (1973) Propp (1977) Larson et al. (1980) b

Calculated from proximate analysis of Cho et al. (1995) Bomb calorimetry

Paine and Vadas (1969) Scheibling and Anthony (2001) Lamare and Wing (2001)

Listed as L. saccharina, recent molecular evidence suggests L. saccharina and L. longicruris are conspecific (Cho et al., 2000). Average of summer (13.0) and winter (13.3) value.

herbivore's feeding strategy (Freeland and Janzen, 1974; Oates et al., 1980; Cooper and Owen-Smith, 1986). The influence of algal structure and morphology on herbivory has been addressed in comparative studies involving different functional groups. These have shown that “opportunistic” forms tend to be consumed more readily than “late successional” forms (Littler and Littler, 1980) and “minute” and “expansive” forms more readily than algae of intermediate size (Steneck and Watling, 1982). Other studies have examined specific structural features, such as algal “toughness” and calcification (Padilla, 1989; Pennings et al., 1998). A few studies have used standardized foods of different shapes to show that algal morphology can influence the rate at which herbivores feed (Klinger, 1982; Kennish and Williams, 1997). Feeding behaviour also is mediated by factors such as food availability, competition, and predation (Inouye, 1978; Abrams, 1984; Harrold and Reed, 1985; Denno et al., 1995; Schmitz et al., 1997). While the effects of these various factors on herbivore behaviour are well documented in marine systems, comparatively few studies have examined the effects of dietary history on plant–animal interactions. Several studies have shown that nutritional condition influences feeding rate as well as food choice. Hunger stress has been shown to reduce the selectivity of the urchin Arbacia punctulata and juvenile rabbitfish Siganus spinus fed artificial foods with and without defensive secondary metabolites (Cronin and Hay, 1996; Thacker et al., 1997). Sea urchins increase their feeding rate as the frequency of feeding decreases (Minor and Scheibling, 1997; Lawrence et al., 2003). Dietary history may also influence feeding behaviour through learning and ingestive conditioning. The ascoglossan sea slug Elysia viridis learns to feed on new hosts, with consequent reductions in handling time, but prefers the food on which it was maintained prior to

choice experiments, regardless of the host algal species from which it was originally collected (Jensen, 1989). Conditioning need not induce positive responses to algal foods. A diversity of terrestrial animals, including rats (Nachman, 1963), insects (Chapman and Bernays, 1989), and slugs (Gelperin, 1975), learn to avoid unpalatable or noxious foods, as do some fish (Gerhart, 1991; Long and Hay, 2006). Another mechanism whereby previous diet may influence feeding behaviour exists in cases where a consumer actively maintains a mixed diet. In such cases, a herbivore may consume the same ratio of two algae over a wide range of availabilities (Kitting, 1980), or select an alga over others that it had fed on most recently (Poore and Hill, 2006). Such diet mixing has been shown to improve performance in sea hares and mobile amphipods (Pennings et al., 1993; Cruz-Rivera and Hay, 2001). Thus, in situations where herbivores have a choice of algae, avoidance of a recently consumed food may reflect that alga's low preference rank or the herbivore's need for a mixed diet. In this study, we investigate the effect of algal traits and dietary history and on the feeding rate and food preference of the green sea urchin Strongylocentrotus droebachiensis using the native kelp Laminaria longicruris and the invasive alga Codium fragile ssp. tomentosoides. C. fragile arrived in Atlantic Canada in the late 1980s (Bird et al., 1993) and since that time, it has replaced L. longicruris as the dominant canopy-forming algae in some areas (Chapman et al., 2002; Scheibling and Gagnon, 2006). Although urchins consume C. fragile, it supports reduced urchin growth and gonad production compared to L. longicruris, and is normally considered a low preference food (Prince and LeBlanc, 1992; Scheibling and Anthony, 2001; Levin et al., 2002; Lyons and Scheibling, 2007). C. fragile produces dimethylsulfoniopropionate, the precursor molecule in a chemical defense known to deter urchin grazing (Van

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February 1, 2005. C. fragile has not established a population at this site. These urchins were maintained in 185 l flow-through seawater tanks and provided with L. longicruris until they were used in experiments. Seaweeds used in these experiments were collected from mixed stands of L. longicruris and C. fragile at Birchy Head (44° 35′ N, 64° 03′ W) and Cranberry Cove (44° 28′ N, 63° 56), Nova Scotia. 2.2. Food shape and structure experiments The effect of food shape and structure on urchin feeding was evaluated in two experiments using agarbased foods. In the first experiment, we investigated the effect of standardizing food shape and structure on the preference of urchins for L. longicruris and C. fragile by providing them with a choice of two disc-shaped agarbased foods containing dried, ground algal material, one composed of L. longicruris and the other, C. fragile. In the second experiment, we examined the effect of food shape on feeding preference by providing urchins with a choice of two agar-based foods of the same nutritional composition, one a flattened mimic of L. longicruris and the other a cylindrical mimic of C. fragile. Fig. 1. The effect of food composition and shape on urchin feeding. Bars represent the mass (±SE, n = 18) of agar-based food discs containing dried L. longicruris or C. fragile (a); and agar- and kelpbased foods of different shapes (discoid mimics of L. longicruris and cylindrical mimics of C. fragile) consumed by urchins provided with both simultaneously (b).

Alstyne et al., 2001, Van Alstyne and Houser, 2003), which may contribute to its low palatability. In addition, the low preference rank of C. fragile may be caused by a structural or morphological defense that makes it difficult for urchins to manipulate the alga (Sumi and Scheibling, 2005). Here we present the results of a series of experiments designed to examine the potential importance of plant traits (structural or morphological defenses, algal nutritional value) and dietary history (nutritional condition, ingestive conditioning, learned aversion, diet mixing) on urchin feeding behaviour. 2. Methods 2.1. Collection of experimental organisms Adult urchins (∼ 45 to 50 mm test diameter) were collected from a “grazing front” along the edge of a kelp bed at Gill Cove, Nova Scotia (44° 29′ N, 63° 32′ W) on

2.2.1. Food shape and structure experiments: preparation of agar-based foods In preparing foods for these experiments, freshly collected kelp and C. fragile were gently shaken to remove excess water, weighed, dried to a constant mass at 60 °C, and weighed again. Each species was ground separately in an electric coffee grinder. This dried seaweed was then used to create agar-based kelp and C. fragile foods containing as much dried algal material per gram (wet weight) as the original algae (C. fragile, 0.061 g; L. longicruris, 0.099 g). To prepare the agarbased C. fragile foods, 2.5 g of high gel-strength agar was dissolved in 44.5 ml of boiling water. The agar solution was cooled to 40 °C and 3.0 g of dried C. fragile was rapidly stirred into the mixture, which was immediately poured into one of two wells in a series of plastic molds. The molds consisted of 45 × 100 mm pieces of 2.5-mm thick plastic sheet with two circular wells (2 cm diameter) cut out of them. The molds were arranged on a second plastic sheet. After the liquid food was poured, a third plastic sheet was pressed firmly on top of the mold to flatten the food. The agar was allowed to cool and harden for 5 min and any excess food was cleaned from the mold. Kelp foods were prepared using the same technique, except that 42.55 ml of water and 4.95 g of dried algae were used. The kelp food was poured into the second well in each mold. Food pieces

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Table 2 Repeated measures ANOVA of the algal mass and energy intake of urchins in the four diet history treatments before, and after, the diet switch Test

Algal mass Days 1–3

Days 4–6

Energy Days 1–3

Days 4–6

Source

Between subjects

Diet Error

Within subjects

df

7.734 0.719

3 16

Day Diet  Day Error Diet Error

0.559 0.160 0.275 13.83 1.389

2 6 32 3 16

Within subjects

Day Diet  Day Error

0.057 0.169 0.190

Between subjects

Diet Error

Within subjects

Between subjects

Between subjects

Within subjects

a

Mean square

F

10.8

p-value

b0.001

2.03 2.62

0.15 0.74

9.96

b0.001

2 6 32

0.303 0.890

0.74 0.51

1.771 0.194

3 16

9.13

b0.001

Day a Diet  Day a Error Diet Error

0.080 0.058 0.104 1.282 0.234

1.4 4.2 22.3 3 16

0.769 0.561

0.43 0.70

5.48

0.009

Day Diet  Day Error

0.034 0.073 0.043

2 6 32

0.790 1.71

0.46 0.15

Tukey's test (diet) Comparison

p

CC vs. LL CC vs. LC CC vs. CL LL vs. LC LL vs. CL LC vs. CL CC vs. LL CC vs. LC CC vs. CL LL vs. LC LL vs. CL LC vs. CL

⁎ ⁎ ns ns ⁎ ⁎ ⁎ ns ⁎ ⁎ ns ⁎

CC vs. LL CC vs. LC CC vs. CL LL vs. LC LL vs. CL LC vs. CL CC vs. LL CC vs. LC CC vs. CL LL vs. LC LL vs. CL LC vs. CL

⁎ ⁎ ns ns ⁎ ⁎ ⁎ ns ns ⁎ ns ns

Indicates that Greenhouse–Geisser adjusted degrees of freedom were used for this test.

were removed from the molds, weighed and returned to the molds before being provided to the urchins. Both the flattened (L. longicruris) and cylindrical (C. fragile) mimics were prepared with 44.5 ml of boiling water, 2.5 g of agar, and 3 g of dried, ground kelp. The flattened foods were made using the molds described above. A metal washer was placed in each well prior to pouring. The washer became imbedded in the food piece and served to make it negatively buoyant. The agar was allowed to cool and harden for 5 min and the food pieces were gently removed from the molds. The cylindrical foods were made by pouring the liquid food into a length of Nalgene 180 PVC tubing (6.4 mm internal diameter), split down one side. Branch tip diameter of live C. fragile ranges from 2.5 to 10.4 mm (21 to 68 cm thallus height, n = 10, A.L. Schmidt, Dalhousie University, unpublished data). Once the food had hardened, the tubing was opened; the food was removed and cut into 25 mm lengths. A 17 mm length of 1.5 mm diameter baling wire was then imbedded into each piece to make them negatively buoyant. Both foods had similar volumes (∼ 0.79 cm3) but their surface areas differed

(L. longicruris mimic, 7.9 cm2; C. fragile mimic, 5.6 cm2). 2.2.2. Food shape and structure experiments: experimental design and statistical analysis Once the experimental foods were prepared, individual urchins were put into 12.5 × 9 × 7.5 cm (length × width × height) plastic containers. The containers were set in a 137 × 71 × 15 cm sea table with 7-cm deep flow-through seawater (∼ 4 l per min). The containers were perforated with 2 mm holes on all sides to allow water exchange. In the experiment with foods containing L. longicruris or C. fragile, the foods were left in the wells of the mold to ensure that they did not float out of reach of the urchins (n = 18). A mold containing the two foods was placed in the center of the container and the urchin was placed on the center of the mold, in contact with both foods. In the experiment with mimics of L. longicruris and C. fragile, single, preweighed pieces of each food were placed at opposite ends of the container and the urchin was placed in the center (n = 11). In both experiments, the direction of

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the first part of the experiment, we compared the feeding rate of urchins maintained on either C. fragile or L. longicruris for 10 weeks for 3 days before and 3 days after half of the urchins were switched from one diet to the other. We predicted that, if the feeding rate of urchins is wholly determined by plant traits, dietary history would have no effect on feeding rate and urchins would feed at the same rate on a given macroalga regardless of their previous diet. However, if dietary history had an effect, the feeding rate of urchins switched from one diet to another might increase or decrease relative to that of

Fig. 2. The effect of dietary history on urchin feeding rate. Symbols represent the mass (a) and energy (b) (±SE, n = 5) consumed by urchins in the aquaria assigned to the four diet treatments 3 days before and after the diet switch. Vertical dashed lines indicate the timing of the diet switch.

flow was perpendicular to the long axis of the container and the position of the foods along this axis was alternated. Urchins were allowed feed for 2 h. The food pieces were then removed and weighed to determine how much of each was consumed. Preliminary experiments revealed that mean autogenic mass changes of our agar-based foods (± 0.005 g) were negligible; therefore, we did not adjust for them. The masses of the two foods consumed in each experiment were compared with twotailed paired t-tests. 2.3. Dietary history experiment The effect of dietary history on feeding rate and food preference was examined in a two-part experiment. In

Fig. 3. The effect of dietary history on the food preferences of urchins. The bars represent the mean mass (± SE, n = 5) of L. longicruris (black) and C. fragile (white) consumed by urchins in the aquaria assigned to the four diet treatments on the first, second, and third day of choice. ⁎ Indicates a significant (p b 0.05) difference.

D.A. Lyons, R.E. Scheibling / Journal of Experimental Marine Biology and Ecology 349 (2007) 194–204 Table 3 Paired t-tests comparing the amount of Laminaria longicruris and Codium fragile ssp. tomentosoides consumed by urchins in each of the four dietary history treatments over 3 days Day

Treatment

Larger mean consumption

t-value

Two-tailed p-value

1

LL LC CL CC LL LC CL CC LL LC CL CC

C. fragile L. longicruris L. longicruris L. longicruris C. fragile L. longicruris L. longicruris L. longicruris C. fragile L. longicruris L. longicruris L. longicruris

5.10 6.80 0.04 6.91 0.72 2.17 1.72 7.58 0.12 2.95 4.42 3.93

0.007 0.002 0.97 0.002 0.51 0.096 0.16 0.002 0.91 0.042 0.012 0.017

2

3

Five replicates used in all tests.

un-switched urchins, depending on whether nutritional condition, ingestive conditioning, or learned aversion were important. If nutritional condition determines the urchins' response, we would expect that those switched to L. longicruris would feed at a faster rate than those fed kelp throughout the experiment, while those switched to C. fragile would feed at a slower rate than those that fed on this alga throughout the experiment. If ingestive conditioning occurs, we would expect urchins that switch diets to initially feed at a slower rate than those that accustomed to feeding on the same species. Finally, if urchins develop an aversion to C. fragile over time, we would expect urchins switched to a C. fragile diet to feed at a higher rate than those fed this alga throughout the experiment. In the second part of the experiment, the urchins were provided both L. longicruris and C. fragile so that we could compare the preferences of urchins with four different dietary histories. We predicted that, if preference was fixed and determined by plant traits, all urchins would prefer L. longicruris to C. fragile, as seen in previous studies. However, if nutritional condition, ingestive conditioning, learned aversion, or diet mixing were important influences on urchin preference, the preference of urchins with different dietary histories would vary. 2.3.1. Dietary history experiment: diet switching At the start of the experiment on March 29, 2005, urchins were habituated to one or the other of the two macroalgal species. Ten urchins were distributed to each of 20 aquaria (60 × 30 × 30 cm; 47 l) with flow-through seawater (∼ 0.6 l min− 1). Urchins were fed either L. longicruris or C. fragile for 10 weeks. The diet of half of

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the urchins was then switched, creating four treatment groups (LL — fed kelp throughout; LC — fed kelp then C. fragile; CC — fed C. fragile throughout; CL — fed C. fragile then kelp). L. longicruris was provided as strips of the blade not including the basal meristem. C. fragile was provided as whole plants or large branches (20–30 cm long). Urchins were furnished with sufficient algal material for them to feed ad libitum. Feeding rates were measured daily between June 12 and 17, 3 days before and after the diets of the LC and CL urchins were switched. Algal consumption was determined by weighing the amount of kelp or C. fragile in each aquarium at the start and at the end of each 24 h period. Algae were gently shaken to remove excess water before weighing on an electronic balance (0.01 g accuracy). All consumption measurements were adjusted by adding correction factors to the initial mass of the algae (C. fragile, 0.24%; L. longicruris, − 0.34%) to account for autogenic mass changes. Correction factors were determined from 30 replicates of each species, weighed before and after 24 h periods under the same conditions, and concurrently, with algae used in the experiments. 2.3.2. Dietary history experiment: food choice On June 20, urchins were provided with enough L. longicruris and C. fragile that they would be able to feed ad libitum on either diet. The amount of kelp and C. fragile consumed in each aquarium was measured daily for 3 days. Dry to wet weight ratios of L. longicruris (0.14 ± 0.02 SD) and C. fragile (0.060 ± 0.006 SD), were determined by weighing 15 samples of each species before, and after, they were dried to a constant weight at 60 °C. Energy intake was then calculated by multiplying these ratios by the amount of each alga consumed and then by the mean energetic content of each species (Table 1). 2.3.3. Dietary history experiment: statistical analysis Feeding rates for the different treatments were compared using repeated measures analysis of variance

Table 4 Repeated measures ANOVA of the difference in algal mass (Laminaria longicruris–Codium fragile ssp. tomentosoides) consumed by urchins in the four dietary history treatments Source Between subjects Within subjects

Diet Error Day Diet  Day Error

Mean square

df

F

p

1322 168.0 16.57 172.9 50.96

3 16 2 6 32

7.87

0.002

0.325 3.39

0.725 0.011

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Fig. 4. The decay of treatment effects on food preferences over time. The bars represent the difference in the mass of algae (kelp–C. fragile) (±SE, n = 5) consumed by urchins in the aquaria assigned to the four diet treatments during the 3 days of choice. Lines at top of figure indicate homogenous subsets within each day.

(ANOVA) and Tukey's tests, before and after the diet switch. Paired t-tests were used to compare the mass of kelp and C. fragile consumed in each treatment group and to determine preference in the choice experiment. The consumption differences (L. longicruris–C. fragile) of the treatment groups were then compared with repeated measures ANOVA, followed by Dunn–Sidak adjusted contrasts (Underwood, 1997) comparing all treatments within each day. Levene's test was used to check the assumption of homoscedasticity and tests were performed on transformed data when appropriate. Energy intake data were log-transformed and consumption difference data were transformed by adding four and squaring the result. Greenhouse–Geisser adjusted degrees of freedom were used when the Mauchly Criterion test indicated that the assumption of spherictiy was not met. Type I error rates for all tests were α = 0.05, except contrasts (adjusted to α = 0.003). 3. Results 3.1. Food shape and structure experiments In experiments where S. droebachiensis were given a choice of two agar-based foods of the same shape and structure, urchins preferred the food containing L. longicruris to the food containing C. fragile (t17 = 6.29, p b 0.001) (Fig. 1a). However, they consumed significantly more of the C. fragile mimic than the kelp mimic when provided with a choice between two

foods with the same composition but different shapes (t10 = 3.15, p = 0.010) (Fig. 1b). 3.2. Dietary history experiment: diet switching Prior to the diet switch, urchins feeding on C. fragile (CC and CL treatments) fed at the same rate regardless of treatment group, as did those feeding on L. longicruris (LL and LC treatments) (Table 2, Fig. 2a). Dietary history did not affect the rate at which urchins fed on either seaweed. Urchins switched from L. longicruris to C. fragile immediately began feeding at the same rate as those fed C. fragile throughout the experiment, and urchins switched from C. fragile to L. longicruris fed at the same rate as those fed kelp throughout the experiment (Table 2, Fig. 2a). Urchins fed C. fragile consumed a significantly greater mass than those fed L. longicruris both before and after the diet switch. However, the high feeding rate on C. fragile did not compensate for the alga's lower energetic content compared to kelp: urchins feeding on C. fragile tended to obtain less energy than those feeding on kelp. Prior to the diet switch, urchins in the LL and LC treatments ingested energy at a higher rate than urchins in the CC and CL treatments (Table 2, Fig. 2b). After the diet switch, mean energy intake in the LL group was higher than in the CC and LC groups (Table 2, Fig. 2b,). Although energy intake by the CL group also tended to be higher than the CC and LC groups, it was not significantly so.

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3.3. Dietary history experiment: food choice The preference of urchins for L. longicruris and C. fragile differed between dietary history treatments. On all 3 days of the choice experiment, urchins that had been feeding on C. fragile (CC and LC treatments) consumed more kelp than C. fragile, although the consumption difference was not statistically significant for the LC treatment on day 2 (Fig. 3, Table 3). On the first day of the experiment, CL urchins consumed equal amounts of both species and LL urchins consumed significantly more C. fragile (Fig. 3, Table 3). However, by the second day, urchins in the CL group had begun to consume slightly, but not significantly, more L. longicruris than C. fragile, and consumption of C. fragile and kelp was no longer significantly different for the LL treatment. On the third day, urchins in the LL group consumed similar amounts of both seaweeds, and all other groups preferred L. longicruris. The LL, CL, LC, and CC treatments can be thought of as a gradient of increasing recent experience with C. fragile and decreasing recent experience with kelp. As familiarity with Codium increases, there appears to be a trend for decreasing C. fragile consumption (Fig. 3). This inverse relationship appears to persist, but weaken over time. Similarly, decreasing recent experience with kelp appeared to correlate with an increase in relative kelp consumption, but only on the first day of the experiment. Examining the difference in consumption of the two algae (L. longicruris–C. fragile) revealed a significant interaction between treatment and time (Table 4). Although there were significant differences between treatments on the first and second days, the differences were damped over time and no significant difference was detected on the third day (Fig. 4). 4. Discussion Algal size, toughness, calcification, and resistance to digestion, as well as morphological defenses like spines or colony formation, protect algae against grazing (Littler and Littler, 1980; Steneck and Watling, 1982; Hessen and Van Donk, 1993; Van Donk et al., 1997). We found no evidence to suggest that C. fragile is protected from S. droebachiensis by such defenses. When we standardized the structure and morphology of L. longicruris and C. fragile by drying, grinding, and incorporating them into agar-based foods, urchins consumed more kelp, as we expected. However, when food composition was standardized and food shape differed, urchins fed on cylindrical mimics of C. fragile more rapidly than they did on the flattened, kelp mimics. Similarly, Klinger

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(1982) found that the urchin Lytechinus variegatus consumes terete forms more rapidly than flattened, bladelike forms. S. droebachiensis tended to consume the C. fragile mimics end-on, as they do C. fragile branches (Scheibling and Anthony, 2001), while with the kelp mimics, they either scraped the flat surfaces or took small bites from the thin edge. It appears that urchins ‘normally’ prefer kelp to C. fragile, despite the fact that it may be more difficult (time consuming) to handle. The single-diet feeding rates of urchins fed L. longicruris and C. fragile in the dietary history experiment support the above assertion. Although several field and laboratory experiments have demonstrated that S. droebachiensis prefers laminarian kelps over C. fragile (Scheibling and Anthony, 2001; Levin et al., 2002; Sumi and Scheibling, 2005), we found that urchins consistently consumed C. fragile at a higher rate than L. longicruris when the algae were provided individually. For herbivorous marine invertebrates, such single-diet feeding rates have often been considered a measure of preference (e.g. Vadas, 1977; Himmelman and Nédélec, 1990; Granado and Caballero, 2001). However, feeding rates in single-diet situations reflect the ability of herbivores to manipulate a food item, not preference (Nicotri, 1980); and choice experiments often are flawed in that they do not consider differences in the ability animals to catch and handle prey as potential confounding factors (Underwood et al., 2004). Compensatory feeding may be another factor contributing to the high feeding rate of urchins on C. fragile. Several marine herbivores have been shown to feed more rapidly on less nutritious foods (Stachowicz and Hay, 1996; Cruz-Rivera and Hay, 2000). Our results demonstrate that although urchins feed on C. fragile at a faster rate, they obtain more energy by feeding on kelp. The low nutritional value of C. fragile, and the urchins' inability to fully compensate for it by increasing their feeding rate, may explain why growth and reproduction are impaired in urchins fed C. fragile (Scheibling and Anthony, 2001, Lyons and Scheibling, 2007). Dietary history had no effect on feeding rate. Urchins switched from one diet to the other immediately began feeding at the same rate as those that had been fed the same diet for several months. Although it appears that plant traits were of primary importance in determining feeding rate in this experiment, previous diet undoubtedly influences feeding rate in other situations. S. droebachiensis been shown to compensate for reduced feeding frequency by increasing feeding rate (Minor and Scheibling, 1997). Urchins fed mussels 1 day out of seven reduce their rate of feeding on L. longicruris (continuously available) for 2 days following the provision of

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mussels (Meidel and Scheibling, 1999), suggesting that dietary conditioning may occur under some conditions. Long-term laboratory feeding experiments have shown that the single-diet rate of consumption of L. longicruris and C. fragile can vary greatly throughout the year (Scheibling and Anthony, 2001; Lyons and Scheibling, 2007), suggesting that additional factors, such as the urchins' reproductive stage or variable plant quality, influence feeding rate on these algae. Although dietary history did not influence the rate at which urchins fed on L. longicruris and C. fragile in single diet situations, it did have a short-term effect on food preference. Urchins that had been fed kelp throughout the single-diet experiment, and that were naïve to C. fragile, reversed the expected preference and consumed significantly more C. fragile than kelp on the first day of the choice experiment, and consistently consumed more C. fragile than urchins in the other diet history groups. In contrast, urchins that had been fed C. fragile for several months essentially consumed only L. longicruris on the first day of the choice experiment, but began to consume small amounts of C. fragile thereafter. Urchins that had been switched from one diet to the other displayed intermediate responses. Although the food preference hierarchy of urchins is clearly not fixed, our results were not consistent with the expected effects of habituation, learned aversion or reduced selectivity associated with poorer nutritional condition. Urchin feeding behaviour is often viewed as an optimization strategy for energy or nutrients, and algal preferences are correlated with caloric intake and urchin performance (Vadas, 1977; Larson et al., 1980; Himmelman and Nédélec, 1990; Lemire and Himmelman, 1996). Urchins feeding on a single diet of C. fragile take in less energy and have reduced performance compared to those feeding on L. longicruris (Scheibling and Anthony 2001, our results). Therefore, viewed from the standpoint of diet optimization, it is somewhat surprising that some urchins would prefer C. fragile to L. longicruris, or even consume C. fragile at all, unless they do so in order to obtain a mixed diet. Our results are similar to those of Pennings et al. (1993), who found that sea hares preferred novel foods over foods on which they had been maintained. Urchins that had been maintained on a single alga preferred the novel alga. However, by examining the choice of urchins with more complex dietary histories (diet switches) we were able to show that the strength of this behavioral response increases along a gradient of increasing novelty. By monitoring the preference of the four groups over a period of several days, we were also able to show

that differences between groups diminished over time. Because urchins in all groups had continued access to and consumed both foods, differences in their recent dietary histories also diminished. Diet mixing is common feeding strategy among animals, and it is often assumed to be adaptive. Indeed, several studies have shown increased performance by consumers fed mixed diets compared to single-species diets (e.g. Pennings et al., 1993; Cruz-Rivera and Hay, 2001). The benefit of diet mixing may result from consumers being able to obtain a balance of complementary nutrients that may be lacking in individual foods (Westoby, 1974, 1978; Pulliam, 1975; Rapport, 1980) or because consumption of a mixed diet improves digestion, and thus resource acquisition (Bjorndal, 1991). Alternatively, diet mixing may benefit herbivores by allowing them to increase their consumption, yet avoid acquiring too much of any one noxious chemical by consuming foods defended by different chemicals (Freeland and Janzen, 1974). The purpose of diet mixing in S. droebachiensis is not clear. In nature, the availability of algal foods changes in time and space. The inclusion of sub-optimal foods, such as C. fragile, in the diet is likely to be advantageous for times when better foods are not available. This may be particularly important for a species like S. droebachiensis that is capable of total depletion of their preferred food, kelp, from large areas of seabed (Scheibling and Hatcher, 2007). A generalist strategy may also reduce search costs and exposure to risks such as predation. While such factors may explain why S. droebachiensis is a generalist, they do not explain why urchins with different dietary histories would prefer different foods when both are abundant, such as in our experiment. Mixed animal and algal diets significantly enhance urchin growth and gonad production (Nestler and Harris, 1994; Meidel and Scheibling, 1999), and some combinations of algal foods may confer a similar nutritional benefit. Thus, although benefits of mixed L. longicruris and C. fragile diets have not been demonstrated (Scheibling and Anthony, 2001), diet mixing may be a good general strategy for urchins. Acknowledgements We are indebted to John Lindley, Megan Saunders and Allison Schmidt for their assistance with diving. The manuscript was improved thanks to comments from four anonymous reviewers. The research was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) to

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