Mar Biol DOI 10.1007/s00227-007-0668-2
R ES EA R C H A R TI CLE
Seasonal variation in movement, aggregation and destructive grazing of the green sea urchin (Strongylocentrotus droebachiensis) in relation to wave action and sea temperature Jean-Sébastien Lauzon-Guay · Robert E. Scheibling
Received: 20 July 2006 / Accepted: 5 February 2007 © Springer-Verlag 2007
Abstract Hydrodynamic forces are an important determinant of subtidal community structure, particularly when they limit the distribution and foraging ability of mobile consumers. We examined the eVect of wave action on the rate of movement and destructive grazing of a kelp bed by the green sea urchin (Strongylocentrotus droebachiensis) under Weld conditions. We measured density and rate of advance at Wxed intervals along »100 m of a grazing front over 1 year, and quantiWed individual movement rates in the barrens 5–10 m behind the urchin front using a timelapse videography. Seasonal variation in the mean rate of advance of the front (range: 0–4 m month¡1) was explained by changes in urchin density at the front (120–360 individuals m¡2), which in turn varied inversely with signiWcant wave height (0.5–2 m). Water temperature (0.8–17.6°C) had no eVect on the rate of advance or on urchin density (aggregation) at the front, except when temperature exceeded 17°C. Movement of individual urchins also was aVected by wave action: we observed a signiWcant decrease in speed and displacement of urchins with increasing signiWcant wave height. Wave action had no eVect on the proportion of urchins moving or the degree of linearity of their movements. We propose that the decrease in urchin density at the front associated with increased wave action, results
Communicated by R.J. Thompson. J.-S. Lauzon-Guay (&) Biology Department, University of New Brunswick, Bag Service 45111, Fredericton, NB, Canada, E3B 6E1 e-mail:
[email protected] Present Address: J.-S. Lauzon-Guay · R. E. Scheibling Department of Biology, Dalhousie University, Halifax, NS, Canada, B3H 4J1
from de-aggregation, which reduces the risk of dislodgement, combined with a reduction in urchin movement in barrens, which supplies new urchins to the front.
Introduction The impact of hydrodynamics on body size (Sebens 2002), morphology (Rogers-Bennett et al. 1995) and distribution (Lissner 1983) of organisms is well documented, especially for sessile organisms. Exposure to oceanic swell, and the frequency and intensity of storms, can have direct structuring eVects on communities by removing resident species (Sousa 1979) or preventing the establishment of new colonists. Mobile animals are capable of adjusting their behaviour to minimize wave impact in high-energy environments, although this can aVect the rate at which they exploit resources (Menge 1978a, b). Thus, wave action can have indirect eVects on community structure by modifying interspeciWc interactions, such as predation and herbivory (Menge and Sutherland 1987; Underwood 1999). Sea urchins are dominant herbivores in subtidal habitats; when abundant they can have a profound eVect on community structure (Lawrence 1975). In the Northwest Atlantic, Xuctuations in abundance of the green sea urchin Strongylocentrotus droebachiensis mediate the transition between two community states: kelp beds and urchin barrens (Chapman and Johnson 1990). The shift to urchin barrens occurs through the formation of urchin feeding aggregations (or fronts) at the lower margin of kelp beds (Breen and Mann 1976; Scheibling et al. 1999). In these fronts, urchin densities can reach 100s to 1,000 individuals m¡2 in 3-dimensional aggregations spanning 1–2 m in width and 100s of meters to kilometers in length (Foreman 1977; Scheibling et al. 1999). Urchins in the front destructively graze kelps
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Mar Biol
and other erect macroalgae, climbing on stipes and weighing down fronds, leaving barrens dominated only by crustose coralline algae in their wake (Lauzon-Guay and Scheibling 2007). The rate of destructive grazing depends on the density of urchins at the front and the biomass of the kelp bed it is consuming (Lauzon-Guay and Scheibling 2007). Previous studies have reported seasonal variation in urchin density at the front with a winter minimum, suggesting a causal relationship between the frequency of storm events, which is greatest in winter, and decreased urchin density (Scheibling et al. 1999; Gagnon et al. 2004). This is consistent with recent short-term observations (1 month in summer) indicating urchin density at the front was inversely correlated with signiWcant wave height (Lauzon-Guay and Scheibling 2007). However, water temperature also shows a strong seasonal signal that is reciprocal to that of wave action. The eVect of temperature on the feeding rate of urchins is unclear. Laboratory studies have found both positive (Scheibling and Anthony 2001) and negative (Larson et al. 1980) eVects of temperature on feeding rate of S. droebachiensis, and temperature may be confounded with reproductive stage in feeding experiments (Scheibling and Hatcher 2001). In this study, we examine the role of wave action and temperature in the seasonal dynamics of an urchin front by correlating variation in both urchin density and rate of advance of the front with environmental measures. Also, we use time-lapse videography to measure individual urchin movement under varying wave and temperature conditions. These Wndings expand our understanding of physical factors that aVect urchin movement and foraging ability in variable environments, and in turn the rate of transition between alternative community states.
Materials and methods Study site Our study was conducted between 5 July 2005 and 12 July 2006 at Splitnose Point (44°28.609⬘ N, 63°32.741⬘ W) on the Atlantic coast near Halifax, NS, Canada. This site is situated at the mouth of a long inlet (Ketch Harbour) and exposed to ocean swell from south to southeast. The seabed is granite bedrock with ledges and crevices that slope gradually to about 35 m (below chart datum), where it grades to sand. At the start of the study (July 2005), a kelp bed extended from the low intertidal zone to 12 m depth. Laminaria longicruris (=Saccharina longicruris) and Laminaria digitata were the dominant kelps below »6 m; Alaria esculenta was the dominant kelp in shallower waters. A dense front of Strongylocentrotus droebachiensis occurred along the lower margin of the kelp bed, destructively graz-
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ing kelps and understory seaweeds and creating barrens dominated by encrusting coralline algae. For a more detailed description of the sea urchin front and kelp bed at this site see Lauzon-Guay and Scheibling (2007). Environmental variables Wave data from 1 July 2005 to 12 July 2006 were retrieved from a meteorological buoy (http://www.meds-sdmm.dfompo.gc.ca) located at the mouth of Halifax Harbour (buoy ID# C44258, 44°30⬘ N, 63°24⬘ W) 11.9 km from our study site. We also recorded wave height on site from 28 November 2005 to 3 January 2006 using a pressure sensor (TDR2050, RBR Ltd., Ottawa, ON, Canada). Water temperature was recorded hourly using a temperature logger (StowAway TidbiT Temp Logger, Onset Computer Corporation, Cape Cod, MA, USA). Both instruments were deployed at a depth of 12 m in the barrens. We used linear correlation and a paired t-test to examine concordance between the pressure sensor output and daily signiWcant wave height (SWH, the mean of the largest one third of waves measured) from buoy records. Urchin movement Individual urchin movement was recorded on 12 dates between 28 September 2005 and 8 May 2006 using timelapse videography. All video sequences were taken at the same location (Xat bedrock at 12 m depth) as the urchin front advanced. A camera (Sony Handycam DCR-TRV25) in an underwater housing (Amphibico Dive Buddy TRV18) was mounted on a tripod »1 m above bottom (giving a Weld of view of »0.70 m2) and set to record 0.5 s of video every minute. The video records were downloaded onto a computer and 1 frame per minute was selected for analysis. A sample of 20 urchins per video sequence was randomly selected from the Wrst frame of each video. Position of these urchins was recorded every minute for the length of the video or until urchins left the Weld of view. From these positions, we calculated individual urchin speed (total distance travelled divided by total time of observation), individual displacement (distance between starting position and Wnal position divided by time of observation), and proportion of time spent moving (number of time intervals during which an urchin moved divided by the total number of intervals). The presence of directional bias was evaluated using Rayleigh’s test for uniformity (Zar 1999). Urchin density was estimated for each video sequence by averaging counts of urchins in the Wrst and last frame and dividing by the area of the Weld of view. We examined the relationship between movement variables (speed, displacement, and proportion of time spent moving) and urchin density, wave height and water temperature
Mar Biol
by stepwise multiple regression. Variables were entered ( · 0.05) in or removed ( ¸ 0.10) from the model according to their signiWcance. Homoscedasticity and linearity assumptions were assessed by visual inspection of residuals (Draper and Smith 1998). Front dynamics We measured the density and rate of advance of urchins at the front along 100 m of the kelp-barrens interface. Our study area was divided into four blocks, each spanning 12– 20 m of relatively Xat sea Xoor without large crevices or ledges. Each block was subdivided into 6 plots, each 2 m wide. We used numbered stainless-steel eyebolts anchored to the substratum using marine epoxy (Z-Spar A-788 Splash zone compound, Kop-Coat Inc.) to delineate plots, and measured the rate of advance of the front relative to these benchmarks. Initially, eyebolts were located in the barrens »1 m behind the leading edge of the urchin front. On two occasions (23 August 2005 and 17 March 2006), the increased distance between the benchmarks and the front necessitated deployment of a new series of eyebolts, 1 m behind the front. The distance between each series of benchmarks was measured so that distances from the front to the second and third series could be converted to distance from initial series. On each sampling date, we used a plastic tape to measure (1 cm accuracy) the distance of the leading edge of the front (at the plot center) to each of the two benchmarks delimiting a plot. These 2 measures were then converted, by triangulation, into a perpendicular distance between the front and the 2 m line connecting the benchmarks. The leading edge of the front was deWned as the shoreward edge of the urchin aggregation at the kelp–barrens interface. Therefore negative values of front advance indicate that the urchin front has moved away from the kelp bed (towards the barrens). On each sampling date we measured urchin density at the front by manually counting urchins in a 0.25 m2 quadrat at the leading edge of the front and in two contiguous quadrats sequentially positioned in the kelp bed immediately in advance of the front. We also recorded a video transect in each plot extending from the leading edge of the front to 3 m into the barrens. The camera (same as above) was propelled by a diver at 0.75 m above bottom along the center of the plot. For spatial reference, a 0.25 m2 quadrat was embedded in the transect at the leading edge of front. Video stitching software (PanoraGen.DV V1.0, http://www. fml-home.de/panoragen) was used to transform video sequences into still images. A linear series of 5 contiguous 0.25 m2 quadrats was juxtaposed with the reference quadrat. Counts of urchins in each quadrat were obtained with the aid of image enhancement software (Photoshop v.9.0, Adobe Systems Inc.).
Results Environmental variables SigniWcant wave height (SWH) recorded at the Halifax Harbour buoy indicate that waves are typically largest (up to 6 m) during fall and winter months (September to March) and smallest (usually 17°C), each lasting 2–3 days (Fig. 1a). Urchin movement The density of urchins in video sequences varied from 31 to 86 urchins m¡2 (Table 1). There is no apparent temporal pattern in density among sequences and variation is not explained by wave action (r2 = 0.02, P = 0.646) or temperature (r2 = 0.02, P = 0.692). Urchins did not exhibit directionality, except in one video sequence, and there was no common directionality among sequences (Table 1). Wave action was the only signiWcant predictor of speed (range: 0.1–33.5 cm h¡1) and displacement (0.1–22.5 cm h¡1) (Fig. 2). Little of the variation in movement parameters was explained by temperature (speed: r2 = 0.023, P = 0.659; displacement: r2 = 0.01, P = 0.746) or urchin density (speed: r2 = 0.103, P = 0.310; displacement: r2 = 0.13, P = 0.254), and these variables did not enter the regression model. Proportion of time spent moving ranged from 1.5 to 33.8%. Although a trend is apparent (r2 = 0.39) (Fig. 2), the regression with SWH was marginally non-signiWcant (P = 0.056), while temperature (r2 = 0.13, P = 0.255) and urchin density (r2 = 0.01, P = 0.713) had no eVect on time spent moving. Front dynamics Front advance varied between ¡3.6 and 12.3 cm day¡1 over the year. The front advanced most rapidly between July and August 2005 (2.4–12.3 cm day¡1) and then slowed in September 2005 to a relatively constant rate (-3.6 to 6.9 cm day¡1) that was sustained throughout the rest of the study (Fig. 3a). SigniWcant wave height averaged between
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Mar Biol Fig. 1 a SigniWcant wave height (SWH, m) at the Halifax Harbour meteorological buoy and mean daily water temperature (°C) at 12 m at Splitnose Point from July 2005 to July 2006. Arrows indicate peaks in water temperature associated with a decrease in rate of advance of the urchin front. b Mean daily SWH (m) at the Halifax Harbour buoy between 29 November and 31 December 2005 and from a pressure transducer deployed at 12 m (below chart datum) at Splitnose Point
Table 1 Date, urchin density (ind. m¡2), mean angle of displacement (0° indicates a shoreward direction), length of mean vector r, Rayleigh’s Z value, and level of signiWcance for each time-lapse video sequence Date
Urchin density (ind. m¡2)
Angle (°) r
Z
28-Sep-2005
50
362
0.033
0.022
>0.500
05-Oct-2005
86
288
0.507
5.138
0.050
27-Nov-2005
31
199
0.018
0.003
>0.500
26-Feb-2006
76
33
0.128
0.098
>0.500
02-Mar-2006
76
327
0.269
0.435
>0.500
17-Mar-2006
39
144
0.384
2.658
>0.050
18-Mar-2006
49
353
0.085
0.131
>0.500
22-Mar-2006
58
181
0.354
2.007
>0.100
30-Mar-2006
33
325
0.386
2.683
>0.050
28-Apr-2006
32
297
0.298
1.778
>0.100
08-May-2006 54
335
0.248
1.226
>0.200
Overall
308
0.327
1.280
>0.200
P
An overall test of directionality between sequences is also given. Bold values indicate a signiWcant result (P < 0.05)
two sampling intervals explained 44% of the variation in front advance (Fig. 3b) and was the Wrst variable to enter the model. Independently, water temperature had no signiW-
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cant eVect on front advance (r2 = 0.015, P = 0.600), but when SWH was already included in the model, water temperature became signiWcant (partial r2 = 0.14, P = 0.017) and entered the model with a negative slope coeYcient (slope = ¡0.45 § 0.17 SE). This statistically signiWcant result is attributable to two sampling dates (28 August and 20 September 2005) when the rate of advance of the front decreased sharply (the front had actually receded from the kelp bed margin on the latter date, Fig. 3a) and temperature exceeded 17°C (Fig. 1a). At these times, we found that urchins were not feeding on kelp and some showed signs of stress and morbidity (e.g. loss of spines, weak attachment of tube feet to the substratum). We also observed a high mortality of urchins in Xow-through seawater tanks in the laboratory (at about ambient sea temperature) during this period. If data for these sampling dates are removed from the analysis, water temperature does not emerge as a signiWcant variable (P = 0.241) and does not enter the model and SWH explains 56% of the variation in front advance (Fig. 3b). Urchin density at the leading edge of the front peaked at 90.6 urchins 0.25 m¡2 in July 2005, and then progressively decreased over fall and winter to a low of 34.5 urchins 0.25 m¡2 in March 2006 before increasing again in spring and summer, although densities in 2006 did not reach those observed in summer 2005 (Fig. 4a). SigniWcant wave height
Mar Biol
P < 0.001) in rate of advance of the front (Fig. 5), it did not enter the model as a signiWcant variable when SWH was included. We frequently observed urchins climbing on kelp stipes during calm conditions, but none were observed on stipes or holding down fronds during periods of high SWH. Also, during or after storm events, the urchin front would retreat from the kelp-barrens interface, leaving a »1 m wide band free of urchins between the kelp bed margin and the leading edge of the front. Variation in the density of urchins ahead and behind the leading edge of the front also was largely explained by SWH (Fig. 6). SWH had a signiWcant eVect on urchin density at all but three sampling locations: 1 m into the kelp bed and 2 and 2.5 m into the barrens. This eVect was most pronounced at the leading edge (indicated by a steeper slope). The proportion of variance in urchin density explained by SWH decreased with distance from the leading edge (Fig. 7). Beyond 2 m into the barrens, urchin density does not appear to be aVected by SWH.
Discussion
Fig. 2 Linear regression of a urchin speed (cm h¡1), b displacement (cm h¡1), and c proportion of time moving (%) per video sample on signiWcant wave height (SWH, m) at the Halifax Harbour meteorological buoy
explained 56% of the variation in urchin density (P < 0.001) (Fig. 4b) and water temperature did not enter into the model as a signiWcant variable. Although urchin density explained much of the variation (r2 = 0.43,
We observed seasonal variation in movement of urchins with little or no movement during winter and greater individual movement in summer. Our analysis indicates that this variation in movement is largely explained by changes in wave action throughout the year. Although temperature usually has a strong inXuence on the activity level of invertebrates (Cossins and Bowler 1987), the movement of urchins was not generally aVected by temperature. Percy (1974) demonstrated that Strongylocentrotus droebachiensis can adjust its metabolism to compensate for variations in temperature over the natural range. Although slightly increased feeding rates of urchins have been observed at warmer temperatures in some laboratory studies (Larson et al. 1980), this is not always the case (Scheibling and Anthony 2001). Furthermore, Weld observations suggest feeding rates remain high at low temperatures prior to spawning (Himmelman 1984), which supports our Wnding that cold winter temperature is not a key factor limiting urchin foraging movement. Warm water temperature events may have a greater eVect on activity level of S. droebachiensis. The upper tolerance limit for S. droebachiensis is about 22°C in the laboratory (Percy 1973; Scheibling and Stephenson 1984) and we observed a marked decrease in urchin activity when water temperature exceeded 17°C in the Weld. On these occasions, the rate of advance of the front was lower than what could be expected based on SWH alone. As previously reported, Strongylocentrotus droebachiensis in barrens exhibit little directionality in individual movement (Duggan and Miller 2001; Dumont et al. 2004;
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Mar Biol Fig. 3 a Weekly signiWcant wave height (SWH, m) at the Halifax Harbour meteorological buoy and rate of advance (cm day¡1) of the urchin front over each sampling interval. b Linear regression of rate of advance of the front on mean SWH recorded during a sampling interval using the entire data set (solid line) and excluding two (open circles) high temperature events (dashed line)
Lauzon-Guay et al. 2006). Urchins moved randomly in seven out of our eight video sequences, and mean directions varied among video sequences. S. droebachiensis can be induced to move in response to water-borne cues emanating from food or predators (Garnick 1978; Scheibling and Hamm 1991), and the presence of a chemical stimulus in the vicinity of the camera (but not in the Weld of view) may explain the single case of directional movement. The decreased movement rate under strong wave conditions, coupled with random movement, could explain the low density of urchins at the front during the winter months, as well as the protracted period of low density during winter. The rate of replenishment of urchins at the front would be greatly reduced during the stormy winter period compared to calmer summer months. Sea urchins, like other benthic species, are aVected by turbulent Xow conditions. Strongylocentrotus nudus reduces its movement and feeding under oscillatory Xow in the laboratory (Kawamata 1998). Similarly, Centrostephanus coronatus is less likely to leave its burrow at night to for-
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age in turbulent conditions (Lissner 1980) and Diadema antillarum tends to be less active during storms (Ogden et al. 1973). Heavy wave action may limit the ability of sea urchins to forage on kelp (Himmelman and Steele 1971; Scheibling et al. 1999), and wave-driven algal whiplash can repulse sea urchins (Velimirov and GriYths 1979; Himmelman and Steele 1971; Himmelman 1984; Dayton 1985; Konar 2000; Konar and Estes 2003) and sea stars (Gagnon et al. 2003). Throughout this study, urchins were never observed climbing on kelp stipes and fronds during periods of heavy wave action (SWH ¸ 2 m during the previous 48 h), and the front sometimes receded from the kelp bed margin under such conditions). The level of aggregation in an urchin front is also related to wave action, with less dense, two-dimensional aggregations associated with strong wave surge, compared to the three-dimensional aggregations that characterize an actively grazing front under calmer conditions (Scheibling et al. 1999; Lauzon-Guay and Scheibling 2007, this study). Through time-lapse videography we have shown that individual urchins are more
Mar Biol Fig. 4 a Weekly signiWcant wave height (SWH, m) at the Halifax Harbour meteorological buoy and urchin density (ind. 0.25 m¡2) at the leading edge of the front during sampling intervals. b Linear regression of urchin density (ind. 0.25 m¡2) at the leading edge of the front on mean SWH (m) recorded during a sampling interval
Fig. 5 Linear regression of the rate of advance of the urchin front (cm day¡1) on the mean urchin density (ind. 0.25 m¡2) at the leading edge of the front during a sampling interval
sedentary in strong Xow. This corroborates our previous Wndings at a short time-scale (Lauzon-Guay and Scheibling 2007) and provides a causal link between wave action and individual movement patterns. These behavioural responses
to increased wave action are likely adaptations to reduce the risk of dislodgement (Lissner 1978). A reduction in the rate of advance of the urchin front during winter is partly explained by a decrease in individual movement with increased wave action, but also by a decrease in urchin density. Density at the front determines the rate at which urchins destructively graze seaweeds and advance through a kelp bed (Breen and Mann 1976; Scheibling et al. 1999; Lauzon-Guay and Scheibling 2007). Front density generally is lower during winter, which has been attributed to an increased frequency of heavy wave action during this period (Scheibling et al. 1999; Gagnon et al. 2004). The extent to which this decrease in density is caused by urchins moving away from the kelp bed (or from one another) or by dislodgement of individuals as waves increase in strength is unknown, but strong storms can generate water velocities suYcient to dislodge urchins (50% dislodgement at 5 m s¡1; Siddon and Witman 2003). It also is unclear whether the slow movement of the front in heavy wave conditions is a direct consequence of the inability of urchins to feed, or an indirect result of the lower density of individuals at the front. Likely, both factors are important.
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Mar Biol Fig. 6 Linear regression of urchin density (ind. 0.25 m¡2) at 0.5 m intervals from the leading edge of the front towards the kelp bed and barrens, on mean signiWcant wave height (SWH, m) recorded during a sampling interval
The eVect of SWH on urchin density was only apparent in quadrats extending less than 2 m into the barrens and less than 0.5 m into the kelp bed. This suggests that SWH aVects the density of urchins in the front only, and not in the adjacent barrens or kelp bed. We previously estimated that the eVective width of the urchin front (the band over which urchins participate in destructive grazing) is 2 m, on average, at this site (Lauzon-Guay and Scheibling 2007). This corresponds to the distance beyond which there is no statistically detectable eVect of SWH on urchin density. Urchins at or near the kelp-barrens interface are likely to be most aVected by wave action through kelp whiplash. Away from the kelp bed margin (towards the barrens), urchin density is relatively low and urchins typically occur as noncontiguous individuals. A further reduction in density may not confer any additional advantage to these individuals (in terms of attachment) when wave forces increase. Urchins
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just inside the kelp bed (0.5 m), which are usually at very low density, may be driven from this area by kelp whiplash under heavy wave surge. In the Mingan Islands (Quebec), Gagnon et al. (2004) reported a mean extension of a kelp bed of 13.7 m during the winter months, associated with low urchin density (not measured). Although we observed a marked decrease in the rate of kelp destruction by urchins during periods of strong wave action, and their retreat from the kelp–barrens interface in some instances, there was no recolonization of barrens by kelp. Even during winter, urchin density along the interface was >150 urchins m¡2 (=5.5 kg m¡2; LauzonGuay and Scheibling 2007). This greatly exceeds the proposed threshold biomass to maintain barrens (0.15 kg m¡2) (Breen and Mann 1976). On the Atlantic coast of Nova Scotia, mass mortality appears to be the only natural mechanism capable of suYciently reducing urchin density to a
Mar Biol
Fig. 7 Mean urchin density (ind. 0.25 m¡2 § SD) over 15 sampling dates and proportion of variance explained (%) by linear regression of urchin density (along a 3 m transect across the front) on signiWcant wave height (m) during a sampling interval. The leading edge of the front is located at 0 m. Negative values indicate distances into the kelp bed (shaded area); positive values are towards the barrens. Closed circles indicate signiWcant regression results (P < 0.02); open circles are non-signiWcant results (P > 0.10)
level that allows expansion of kelp beds (Scheibling et al. 1999), although intensive Wshing can potentially have the same eVect (Steneck et al. 2002). We have shown that wave action (and not sea temperature) signiWcantly aVects the rate of destructive grazing by sea urchin fronts, resulting in pronounced seasonal variation in the progression between alternative states of the rocky subtidal ecosystem. We have linked this pattern to the direct and indirect (through algal whiplash) eVects of wave action on the foraging movements and behaviour of individual urchins. We predict that spatial variation in grazing rate also is related the degree of wave exposure. Thus the advance of fronts is expected to be faster in more sheltered areas and at greater depths, where wave disturbance is lower or less frequent. Our study underscores the importance of understanding variation in the physical environment that can mediate trophic interactions that regulate the structure and dynamics of marine ecosystems. Acknowledgments We are indebted to J. Lindley for is invaluable assistance with Weldwork. We also thank D. Lyons, M. Saunders, P. Gagnon, D. Knip, A. Schmidt, and Dr. A. Pinder for diving assistance. We are grateful to Dr. M. Barbeau for providing the underwater video camera. The research was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) to RES. J-SL-G was supported by scholarships from Fonds Québécois de la Recherche sur la Nature et les Technologies and NSERC.
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