Effects of food ration and feeding regime on growth and reproduction of the sea urchin Strongylocentrotus droebachiensis

To determine the effects food ration and feeding regime on growth and reproduction of Strongylocentrotus droebachiensis (Müller), sea urchins in laboratory aquaria were fed kelp (Laminaria longicruris) supplied at either a high (H, ad libidum daily) or a low (L, ad libidum 1 d wk⁻¹) ration in two successive 12-wk intervals during the reproductive period. After 24 wk, urchins fed the high ration continuously (HH) or for the last 12 wk only (LH) had a significantly greater mean gonad index [(gonad weight/total body weight) × 100] and body weight than urchins fed the low ration continuously (LL) or for the last 12 wk only (HL). Urchins in the HL treatment had a significantly greater gonad index than those in the LL treatment; there was no significant difference in gonad index between the LH and HH treatments. Females had a greater gonad index than males in the low ration (LL and HL) treatments at the end of the experiment; there was no significant difference between sexes in the high ration (LH, HH) treatments. Gametogenesis proceeded to maturation in all treatments and some individuals spawned at the end of the experiment. Females in the high ration (HH and LH) treatments had a greater proportion of nutritive phagocytes in their ovaries than females in the low ration treatments, but there was no effect of feeding treatment on oocyte or ovum size. Feeding treatment had no effect on the relative abundance of nutritive phagocytes in the testes, although the proportion of spermatocytes was higher (and that of spermatozoa lower) in the high ration than in the low ration treatments. Urchins in the high ration treatments had a lower mean jaw height index [(jaw height/test diameter) × 100] and greater mean test diameter than those in the low ration treatments at the end of the experiment, although these differences were not statistically significant. Feeding rate on kelp at the end of the experiment was significantly greater for urchins in the low ration than in the high ration treatments. Our experimental results show that even relatively low rations of kelp support somatic and gonadal growth in S. droebachiensis. Increasing the supply of kelp, particularly during the period of active gametogenesis, results in maximal rates of growth and reproduction. These results suggest that populations of S. droebachiensis in barrens may derive a substantial proportion of their nutrition from drift kelp, which may contribute to their persistence in these habitats. They also explain the large body size, high reproductive effort and fecundity of urchins grazing on kelp beds. These findings have important implications for understanding the dynamics of natural populations of S. droebachiensis and for development of effective aquacultural practices. Received: 17 February 1997 / Accepted: 5 March 1997     

Selective accumulation of mycosporine-like amino acids in ovaries of the green sea urchin Strongylocentrotus droebachiensis is not affected by ultraviolet radiation

Field sampling and laboratory experiments examined whether ultraviolet radiation (UVR) affects the reproduction or the accumulation of mycosporine-like amino acids (MAAs) and ascorbic acid in ovaries of the green sea urchin Strongylocentrotus droebachiensis (Müller). Ovaries of sea urchins sampled across a depth gradient (0.5–10 m) in March 1998 did not differ in their gonadal index, or in concentrations of MAAs, or ascorbic acid. Concentrations of MAAs and ascorbic acid in ovaries were higher in sea urchins collected from a kelp bed compared with those collected from a community of crustose coralline algae. The concentrations of MAAs in ovaries varied seasonally, peaking in March, when sea urchins had high gonadal indices just before spawning. Ovaries of sea urchins maintained on controlled diets from October 1997 to April 1998 accumulated significantly higher concentrations of MAAs when fed a diet rich in MAAs than did ovaries of sea urchins fed an alga lacking MAAs, but the gonadal indices did not differ between diets. Sea urchins accumulated principally one MAA, shinorine, but not others that were available in high concentrations in their diet. Neither the gonadal index nor the ovarian concentrations of MAAs were affected by daily exposure of adult urchins to UVR for 6 months. Concentrations of ascorbic acid in ovaries did not differ among diets or UV-treatments. The percentages of nutritive phagocytes and gametic cells were not affected by diet or UVR, and did not co-vary with concentrations of MAAs or ascorbic acid in ovaries. These data support previous demonstrations that female sea urchins accumulate MAAs from their diet of macroalgae, but further show that the accumulation is selective for specific MAAs, particularly shinorine, and that adult S. droebachiensis do not accumulate MAAs in their ovaries or eggs in response to UV-exposure. These are also the first experimental studies to address whether MAAs are affected by or regulate gametogenesis, and indicate that they do not. Received: 5 May 2000 / Accepted: 29 September 2000     

Annual reproductive cycle of the green sea urchin, Strongylocentrotus droebachiensis, in differing habitats in Nova Scotia, Canada

We monitored the reproductive cycle of Strongylocentrotus droebachiensis (OF Müller) between April 1993 and August 1995 in kelp beds, barren grounds and grazing fronts at both a wave-exposed and a sheltered site along the Atlantic coast of Nova Scotia. Gonad index and histological analyses showed that S. droebachiensis has an annual reproductive cycle that is synchronous across sites and habitats, and between females and males. Spawning occurs in March/April of each year but a small proportion of sea urchins in the study populations also spawned in fall 1995. During most of the year, sea urchins in kelp beds and grazing fronts have a higher gonad index than those in barren grounds. Gonad indices also tended to be higher at the wave-exposed than the sheltered site. Interannual variability in peak gonad index was significant in the barren grounds at the wave-exposed site and in the grazing front at the sheltered site. The gametogenic cycle is characterized by six stages based on the abundance of nutritive and germinal/gametic cells. Nutritive phagocytes are abundant after spawning and replaced by increasing numbers of germinal and gametic cells as the gametogenic cycle progresses. The temporal patterns of abundance of each cell type were similar among habitats indicating that the gonads were qualitatively similar despite large differences in gonadal mass. The quantity of gut contents (ratio of food volume to body volume) was similar among habitats, but the quality (percentage of organic material) tended to be higher in kelp beds and grazing fronts than in barren grounds suggesting that differences in gonad index of S. droebachiensis in different habitats are related to differences in diet. The high density of sea urchins in grazing fronts combined with their high fecundity suggests that they make the greatest contribution, per unit area, to the overall larval pool. Received: 20 May 1997 / Accepted: 21 January 1998     

Feeding aggregations of sea stars (<Emphasis Type="Italic">Asterias</Emphasis> spp. and <Emphasis Type="Italic">Henricia sanguinolenta</Emphasis>) associated with sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) grazing fronts in Nova Scotia

Migrating feeding aggregations (or fronts) of sea urchins can dramatically alter subtidal seascapes by destructively grazing macrophytes. While direct effects of urchin fronts on macrophytes (particularly kelps) are well documented, indirect effects on associated fauna are largely unknown. Secondary aggregations of predators and scavengers form around fronts of Strongylocentrotus droebachiensis in Nova Scotia. We recorded mean densities of the sea stars Asterias spp. (mainly A. rubens) and Henricia sanguinolenta of up to 11.6 and 1.7 individuals 0.25 m⁻² along an urchin front over 1 year. For Asterias, mean density at the front was 7 and 15 times greater than in the kelp bed and adjacent barrens, respectively. There was strong concordance between locations of peak density of urchins and sea stars (Asterias r = 0.98; H. sanguinolenta r = 0.97) along transects across the kelp–barrens interface, indicating that sea star aggregations migrated along with the urchin front at rates of up to 2.5 m per month. Size–frequency distributions suggest that Asterias at the front were drawn from both the barrens (smaller individuals) and the kelp bed (larger individuals). These sea stars fed intensively on mussels on kelp holdfasts and in adjacent patches. Urchin grazing may precipitate aggregations of sea stars and other predators or scavengers by incidentally consuming or damaging mussels and other small invertebrates, and thereby releasing a strong odor cue. Consumption of protective holdfasts and turf algae by urchins could facilitate feeding by these consumers, which may obtain a substantial energy subsidy during destructive grazing events.     

Differences in somatic and gonadic growth of sea urchins ( Stronglyocentrotus droebachiensis ) fed kelp ( Laminaria longicruris ) or the invasive alga Codium fragile ssp. tomentosoides are related to energy acquisition

The rocky subtidal community off the Atlantic coast of Nova Scotia has historically undergone a cyclical transition between Laminaria-dominated kelp beds and sea urchin-dominated barrens. Since the introduction of the invasive alga Codium fragile ssp. tomentosoides, a third community state has emerged: Codium-dominated algal beds. We conducted a 42-week feeding experiment in the laboratory, which mimicked the quantity and quality of food available to urchins (Strongylocentrotus droebachiensis) in each of these community states. Feeding rate, growth, reproduction, and survival of urchins fed either Laminaria longicruris or C. fragile ad libidum, or L. longicruris 2 days per month, were measured. Although the ad libidum feeding rate on C. fragile was higher than that on kelp, energy intake was lower. Urchins in the ad libidum kelp treatment were larger and had larger gonads than those in the C. fragile treatment. Urchins fed kelp infrequently exhibited little somatic and gonadic growth over the course of the experiment. Regression analysis revealed that urchin performance on these diets was strongly related to energy intake. Diet treatment had no effect on survival or gonad maturation. Although urchins can consume substantial amounts of C. fragile, it appears that they cannot, or do not, feed quickly enough to compensate for its lower nutritional value. Our results suggest that, although urchins feeding on C. fragile are capable of surviving, growing, and reproducing, the replacement of kelp by C. fragile in some areas might negatively affect urchin populations as they continue to repopulate the shallow subtidal zone.     

Effects of temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis

To assess the effects of both temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), individuals collected December 1996 (winter experiment) and June 1997 (summer experiment) were maintained for 3 months in one of four experimental treatments: (1) 3 °C and fed ad libitum (high ration), (2) 3 °C and fed one-seventh of the maximum ration (low ration), and (3) 12 °C and fed the high ration; (4) 12 °C and fed the low ration. All individuals were fed an artificial diet and exposed to only 1 h of light every day. At the end of both experiments, mean gonad indices of sea urchins fed the high ration had increased significantly (11–24% and 6–19% in the winter and summer experiments, respectively), while the gonad indices of individuals fed the low ration did not change. At the high ration (both experiments), the increase in gonad index of sea urchins occurred primarily as the result of a significant increase in the mass of nutritive phagocytes, as revealed by histological analyses. Primary oocytes were significantly larger in individuals held at 3 °C than at 12 °C throughout the winter experiment, regardless of food ration; during the summer experiment, primary oocytes were significantly larger in individuals receiving the high ration, regardless of the temperature at which they were held. These results suggest that: (1) food availability is the most important factor regulating energy storage and the relative size of gonads throughout the year, (2) temperature affects the rate of growth and maturation of primary oocytes during the later stages of oogenesis, and (3) once gametogenesis has been initiated, mature ova will be produced, even under conditions of low food availability. Conditions of high food availability in summer and low temperature in winter would thus favor reproductive output in sea urchin populations. Received: 1 March 2000 / Accepted: 4 October 2000     

Density-related reproductive trade-offs in the green sea urchin, Strongylocentrotus droebachiensis

Green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), populations are being depleted rapidly in the Gulf of Maine and there is justified concern that the potential of this free-spawner to produce larvae may be severely inhibited. We evaluated the opposing effects of different population densities on gonad development and fertilization success, using population surveys and fertilization experiments. We determined gonad indices (gonad mass/body mass) over a range of population densities (0.1 to 250 ind. m⁻²) at seven sites in coastal Maine, USA, sampled at two depths (5 and 15 m). At shallow sites, we found that gonad indices declined by 50% over the 1500-fold range in adult population density. At 15 m deep locations, gonad mass was consistently low and did not vary significantly with density. Patterns of macroalgal abundance suggest urchins at high density and in deeper water were food limited. Because macroalgal cover co-varies inversely with sea urchin density, we designed field experiments to determine the interaction between sea urchin density and kelp canopy on fertilization success. On square arrays we manipulated the spacing of simulated urchins, but held their numbers constant (five sperm-filled syringes interspersed with four Nitex mesh egg containers permeable to sperm). These experiments, simulating the observed range of natural density, suggested that (1) fertilization rates decreased many times faster than individual gamete production increased over the same range in density, and (2) kelp increased fertilization success at high density when eggs were within 25 cm of a sperm source, but not when spaced 1 m apart. Additional laboratory fertilization experiments at ambient temperatures (3 to 5 °C) indicated that diluted sperm were viable for <1 h, but egg viability was virtually unchanged for >8 h. In short, to the individual the reproductive benefits of aggregating appear to outweigh the costs; and while sperm may be limiting at low population density, eggs may remain viable long enough to be fertilized by sperm from more distant males. Received: 10 February 1998 / Accepted: 22 January 1999     

Changes in sea urchin populations after the destruction of kelp beds

In St. Margaret's Bay, Nova Scotia, Canada, there are large areas in which sea urchins (Strongylocentrotus droebachiensis) have eliminated beds of kelp (Laminaria spp.). Sites were identified where destruction of kelp beds had taken place 1, 2, 3, 3.5 and 4 years ago. With increase of time since kelp disappearance, the sea urchins showed decreased growth rate, reduced gonad size, but an increase in numbers resulting from high recruitment rates in the first two years after kelp bed destruction. These sea urchin populations, by their browsing, effectively prevent the regeneration of kelp. There is, as yet, no evidence of the sea urchin populations being starved out to allow the kelp to return.     

The association of N2-fixing bacteria with sea urchins

Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C₂H₂ reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6g N fixed g wet wt^-1 d^-1, with a yearly mean activity of 1.36 g N fixed g wet wt^-1 d^-1. Studies with ¹⁵N confirmed the C₂H₂ reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N₂ fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity.     

Seasonal variation in movement,aggregation and destructive grazing of the green sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) in relation to wave action and sea temperature

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 effect of wave action on the rate of movement and destructive grazing of a kelp bed by the green sea urchin (Strongylocentrotus droebachiensis) under field conditions. We measured density and rate of advance at fixed intervals along ∼100 m of a grazing front over 1 year, and quantified individual movement rates in the barrens 5–10 m behind the urchin front using a time-lapse 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 significant wave height (0.5–2 m). Water temperature (0.8–17.6°C) had no effect 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 affected by wave action: we observed a significant decrease in speed and displacement of urchins with increasing significant wave height. Wave action had no effect 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 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.     

Mycosporine-like amino acids prevent UVB-induced abnormalities during early development of the green sea urchin Strongylocentrotus droebachiensis

Experiments were performed to determine how ultraviolet radiation (UVR) in the environmentally relevant range affects development of the sea urchin Strongylocentrotus droebachiensis (Müller) and whether mycosporine-like amino acids (MAAs), present in the early life stages, reduce UV-induced damage. Eggs, embryos, and larvae contained five MAAs having absorption maxima ranging from 320 to 334 nm. Eggs contained principally shinorine and porphyra-334, which absorb maximally at 334 nm and half-maximally at 312 and 348 nm, spanning much of the environmental range of biologically effective UVR. Concentrations of MAAs remained constant in unirradiated embryos through the gastrula stage, but decreased significantly in two-armed pluteus larvae. Daily exposure to combined photosynthetically active radiation (PAR, 400–700 nm) and UVR did not affect the concentration of MAAs in these embryos up to the two-armed pluteus stage. Prism larvae of sea urchins and the sand dollar Echinarachnius parma (Lamarck) did not accumulate shinorine from the surrounding seawater. Daily exposure of embryos to UVA (320–400 nm) and UVB (295–320 nm) radiation in the presence of PAR induced delays and abnormalities during development, and removing UVB eliminated this effect. Abnormalities in embryos included thickening of the blastoderm wall, filling of the blastocoel by abnormal cells, exogastrulation, and formation of abnormal spicules. The percentage of embryos that developed normally was lower in batches of embryos exposed to PAR + UVA + UVB, except in embryos from urchins maintained on MAA-rich diets. In all cases, the percentage of PAR + UVA + UVB-exposed embryos that developed normally was positively related to the concentration of MAAs in eggs from which the embryos developed. Thus, the MAAs found in S. droebachiensis embryos protect them against UVB-induced abnormalities during their development to at least the four-armed pluteus larval stage. Received: 8 May 2000 / Accepted: 29 September 2000     

Heat waves,baby booms,and the destruction of kelp beds by sea urchins

Large populations of sea urchins, Strongylocentrotus droebachiensis (Müller), destroyed kelp beds along the Atlantic coast of Nova Scotia in the 1960's and 1970's. The origin of these large sea urchin populations is not understood. We have investigated the potential influence of variable growth and development of the planktonic larvae of sea urchins (in response to temperature and food abundance) on recruitment of benthic juveniles. The adult sea urchins were collected at Sandy Cove, Digby County, Nova Scotia, Canada, in December 1986. Temperature strongly affected larval size and the growth of the echinus rudiment within the range 3° to 9°C, and larvae grew most rapidly at 14°C. Food abundance had a smaller effect on larval growth, and these effects were apparent only at high temperature. Larvae fed the same concentration of two different algal food species grew and developed similarly. Correspondence between spring temperature variation and qualitative variation in sea urchin recruitment, as well as strong temperature effects on larval growth in culture, and the occurrence of a large, positive temperature anomaly in June 1960, all suggest that temperature effects on larval growth and development may have led to intense sea urchin recruitment in 1960 and the appearance of large adult populations 4 to 6 yr later. This result invites further research.     

Manipulation of food and photoperiod promotes out-of-season gametogenesis in the green sea urchin, Strongylocentrotus droebachiensis: implications for aquaculture

Green sea urchins, Strongylocentrotus droebachiensis OF Müller, collected off the coast of New Hampshire, USA, in late February 1995, were brought into the laboratory and fed an artificial diet ad libitum, and subjected to a photoperiod advanced by 4 months. During this study, temperatures and salinities for experimental urchins mirrored those recorded at the collection site. We examined the effects of changes in feeding regime and photoperiod on gametogenesis and compared the experimental urchins with those from the source population. During the 7-month period, experimental urchins showed no detectable changes in mean test height or diameter. Experimental urchins had a significantly higher gonad index (GI) in March, April and May (18 ± 6%) compared with field urchins in March (11 ± 3%). Subsequently, experimental urchins had a mean monthly GI of 25 to 30%, while the mean GI for field urchins was 11 to 13%. Gonial cell mitosis and gametogenesis occurred earlier in experimental male and female urchins compared with field urchins. Stereological and histological observations and stage–frequency data showed that the ovaries of experimental urchins were large because of the accelerated development of nutritive phagocytes, the volume fraction (V _v) of which was 89 to 90% of the gonad, while new vitellogenic primary oocytes occupied <1% V _v. In males, stereological and histological observations and stage–frequency data suggested a mobilization of materials from the nutritive phagocytes beginning between June and August, i.e. earlier than in females, and, by September, new gametes occupied a V _v of 49 ± 3% of the testes. Oocyte size–frequency distributions demonstrated that most primary oocytes were <80 μm in diameter between March and September, suggesting that cold temperatures may be needed for completion of vitellogenesis. We describe changes in the two principal cell types in the germinal epithelium of urchin gonads and indicate how knowledge of their population dynamics may be useful in aquaculture applications. Received: 21 February 1997 / Accepted: 3 June 1998     

Interactions between sea urchins (Strongylocentrotus droebachiensis) and their predators in field and laboratory experiments

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 ofStrongylocentrotus 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 juvenileS. 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 mm) 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 thatS. 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 ofS. droebachiensis.     

Feeding, growth and reproduction of sea urchins (Strongylocentrotus droebachiensis) on single and mixed diets of kelp (Laminaria spp.) and the invasive alga Codium fragile ssp. tomentosoides

Since its introduction to Nova Scotia in the early 1990s, the invasive green alga Codium fragile ssp. tomentosoides has spread rapidly to become a dominant and persistent component of seaweed assemblages in the rocky subtidal zone. To examine the potential of sea urchins (Strongylocentrotus droebachiensis) to regulate Codium, and the potential of the alga to support urchin growth and reproduction, we fed urchins Codium and/or kelp (Laminaria spp., a high quality and preferred native food) in the laboratory for 11 months. Urchins showed a strong and active preference for kelp: they consumed more Laminaria than Codium (as dry weight) in single and mixed diet treatments. Urchins fed only Codium consumed 3.5 times more of the alga than those on a mixed diet, but did not increase their feeding rate in winter when kelp consumption was greatest. Laminaria was consumed at the same rate in single and mixed diets, indicating that the presence of an alternative food had no effect on kelp consumption. Survival and growth (change in test diameter) were lowest on the Codium diet, although the differences among diets were not statistically significant. Urchins on the Codium diet showed no gonadal production over the annual reproductive cycle, compared to a marked rise in gonad index on the Laminaria and mixed diets. Our results indicate that Codium is an unattractive, unpalatable and poor quality food, which is not readily consumed by urchins. Thus we predict that urchins at moderate densities will be much more likely to graze kelp than Codium in mixed stands, creating a mosaic of Codium patches and barren areas. At high densities, however, urchins are likely to destructively graze all seaweeds, although dense stands of the invasive alga may prolong the widespread formation of barrens.     

Effects of grazing by two species of sea urchins (Strongylocentrotus franciscanus and Lytechinus anamesus) on recruitment and survival of two species of kelp (Macrocystis pyrifera and Pterygophora californica)

We studied the effects of grazing by two species of sea urchins on two species of kelp (Macrocystis pyrifera and Pterygophora californica) in the San Onofre kelp bed in southern California from 1978 through 1981. Both red sea urchins, Strongylocentrotus franciscanus, and white sea urchins, Lytechinus anamesus, were abundant and lived in aggregations. The purple sea urchin (S. purpuratus) was rare at the study site and was not studied. The aggregations of red urchins were either relatively small and stationary (for over 3 yr) or relatively large and motile (advancing at about 2 m mo^–1). Both stationary and moving aggregations were observed at the same time, and within 100 m of one another. Stationary aggregations of red urchins probably subsisted mainly on drift kelp and had no effect on kelp recruitment or on adult kelp abundance. In contrast, red sea urchins in large, motile aggregations or fronts ate almost all the macroalgae in their path. The condition of their gonalds indicated that red urchins in fronts were starved relative to red urchins in the small, stationary aggregations. Large, motile aggregations developed after 2 yr of declining kelp abundance (probably due largely to storms). We propose that a scarcity of drift algae for food results in a change in the behavior pattern of the red urchins and thus leads to the formation of large, motile aggregations. The aggregations of white urchins, which occurred along the offshore margin of the kelp bed, were large, but relatively stationary. The white urchins rarely ate adult kelps, but grazed extensively on early developmental stages of kelps and evidently prevented seaward expansion of the bed. The spatial distribution of both types of red urchin aggregations appeared to be unrelated to predation pressure from fishes or lobsters.Please address all requests for reprints to the senior author at his present address.     

Survival, growth and reproduction of the salt-marsh amphipod Uhlorchestia spartinophila reared on natural diets of senescent and dead Spartina alterniflora leaves

The talitrid amphipod Uhlorchestia spartinophila lives in close association with standing-dead leaves of the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the Atlantic coast of North America. This study probed the strength of the trophic link between the amphipod population and the decomposition process in this detrital-based ecosystem. We measured survival, growth and reproductive output in groups of amphipods reared for 6 wk on five diets derived from sheath and blade portions of S. alterniflora leaves just prior to (senescent) and during (dead) decomposition. In unfed treatments, the daily specific mortality rate was 0.391 and starved amphipods survived no longer than 11 d. Among the fed treatments, a diet of senescent sheaths resulted in the lowest survival (20%) and yielded no offspring. Groups fed senescent blades, dead sheaths, dead blades and unwashed dead sheaths had survival rates of 56 to 84% and produced 5.0 to 12.5 offspring replicate⁻¹. Sex ratio usually favored females, but approached unity in treatments with high overall survival, suggesting that quality of available food resources may influence sex ratio in this species. Mean specific growth rates (mm mm⁻¹ d⁻¹) ranged from 0.013 to 0.016, and matched previous estimates of growth from field populations. Overall ecological performance (survival + growth + reproduction) was similar for all food treatments, except senescent sheaths, which yielded a final mean (±SD) dry biomass (0.4 ± 0.42 mg replicate⁻¹) of amphipods significantly lower than that of other diets (1.7 ± 0.81 to 2.6 ± 0.69 mg replicate⁻¹). Natural diets derived from decomposing cordgrass leaves can fulfill the nutritional requirements of U. spartinophila populations, but variation in initial amounts of living fungal biomass among the five experimental diets only partially explained the responses of amphipods in our experiment. Structural characteristics and variation in rates of fungal occupation within different portions of cordgrass leaves may affect the amphipod's ability to access plant production made available by decomposers. Received: 12 December 1996 / Accepted: 18 December 1996     

Enlarged lantern size in similar-sized,sympatric, sibling species of Strongylocentrotid sea urchins: from phenotypic accommodation to functional adaptation for durophagy

Strongylocentrotus droebachiensis is a well known omnivorous sea urchin with an unrivalled capacity to destroy North Atlantic kelp forests. S. pallidus is a lesser known, morphologically similar, and closely related species with no record of destructive grazing, despite its larger lantern size. I quantify the lantern size of both species using bivariate allometric analysis, and test the hypothesis that enlarged lantern size facilitates durophagy, the consumption of hard prey, by measuring the feeding capacity of urchins with different lantern sizes when offered a hard-shelled prey, the blue mussel, Mytilus edulis. The results suggest that S. droebachiensis has a limited capacity for durophagous feeding irrespective of lantern size, whereas in S. pallidus the ability to exploit hard shelled prey is positively related to lantern size. This is apparently the first evidence of a relationship between trophic morphology and diet in regular sea urchins. The hypothesis of systematic latitudinal variation in the lantern size of S. pallidus is reappraised and rejected. S. droebachiensis had larger gonads than S. pallidus in field samples, confirming that its small lantern is not impeding nutrient acquisition in shallow habitats.     

Stability of sea urchin dominated barren grounds following destructive grazing of kelp in St. Margaret's Bay,Eastern Canada

Kelp regeneration was observed for the first time in St. Margaret's Bay, Nova Scotia, Canada in an area known to have been devoid of macroalgae for several years. The regeneration was destroyed by sea urchins within 10 months. Experimentally induced kelp regeneration met a similar fate under normal grazing pressure. At the lowest sea urchin biomass and density encountered re-establishment of mature kelp stands seems highly unlikely. The sea urchin population required to suppress kelp regeneration is fed by benthic microalgae. Diatoms and other pioneer algal community species were found in the guts of sea urchins. The mean standing crop of benthic microalgae was found to be, 2.2 g C m^-2 and production estimated as ca 15 g C m^-2yr^-1 at 8m depth. Most of the primary production of St. Margaret's Bay has been lost with the disappearing kelp populations.     

Temporal variation in community interfaces: kelp-bed boundary dynamics adjacent to persistent urchin barrens

We examined, over 2 years, factors affecting the temporal stability of the lower limit of kelp beds (Alaria esculenta) at five subtidal sites in the Mingan Islands, northern Gulf of St. Lawrence. The position of the lower limit of the beds varied markedly among sites and over time and was largely controlled by the green sea urchin, Strongylocentrotus droebachiensis, which formed dense (up to 500 individuals m^–2) feeding fronts at the lower edge of the beds. These aggregations advanced over the kelp most rapidly during the summer (at rates as high as 2.5 m month^–1), and there appeared to be a threshold urchin biomass of ~5 kg m^–2 below which the fronts could not substantially reduce the limit of the beds. The fronts consisted mainly of large individuals, whereas smaller urchins predominated in the barrens zone below the kelp beds. At one site, we recorded large seasonal shifts in overall urchin densities, with large increases and decreases during the summer and winter, respectively. An urchin exclusion experiment indicated that algal recruitment in the barrens was two orders of magnitude greater in the absence than in the presence of urchins. The kelp Agarum cribrosum greatly restricted urchin movements, and the greater temporal stability of the kelp bed at one site appeared related to the gradual replacement of Alaria esculenta in the lower kelp bed by a large stand of Agarum cribrosum. We propose that perturbations by abiotic factors (e.g., ice scouring and water motion) trigger important but localized changes in urchin densities that, in turn, largely determine the limits of kelp bed distribution in this region of the Atlantic where urchin barrens are a persistent community state.Communicated by R.J. Thompson, St. Johns