Robert Eric Scheibling

The Influence of Larval Supply and Settlement on the Population Structure of Barnacles

The abundance and distribution of settlers, recruits, and adults of the barnacle Semibalanus balanoides were monitored in the high, mid, and low intertidal zones at two adjacent, but physically and biologically different sites (A and B) on a rocky intertidal shore in Nova Scotia following a rare occurrence of ice scouring. Availability of planktonic cyprid larvae also was measured at site A. There was a strong concordance between cyprid availability and settler density over the main settlement period among intertidal heights, where each was inversely related to intertidal height. Differences in cyprid availability with intertidal height were related to immersion time as the larvae, in general, were uniformly distributed at the water—substratum interface. Settlement rate was strongly correlated with cyprid availability in the low intertidal zone at site A, where most settlement occurred. In all zones at both sites, recruit density at the end of the main settlement period was a positive function of settler density over this period: on average, 79% of the variation in recruit density was explained by settler density over the main settlement period. Adult density and total settler density also tended to be strongly related: 86% (mid zone at site A) and 73% (high zone at site B) of the variation in adult density was explained by total settler density, except where postrecruitment mortality (i.e., predation) was high (e.g., only 1% of the variation in adult density was explained by total settler density in the low zone at site A). Local hydrodynamics, which influences supply and distribution of planktonic larvae, may dictate the distribution of settlers and, ultimately, adults on this shore.

Increased macroalgal abundance following mass mortalities of sea urchins (Strongylocentrotus droebachiensis) along the Atlantic coast of Nova Scotia

SummaryRecurrent outbreaks of disease between 1980 and 1983 caused catastrophic mortality of sea urchins (>260,000 t fresh weight) along 280 km (straight line distance) of the Atlantic coast of Nova Scotia. The complete elimination of sea urchins and concomitant development of fleshy macroalgal communities have occurred along different parts of this coast in different years. Macroalgal communities in areas where sea urchins died off 1, 3 and 4 years previously are compared to existing sea urchin-dominated barren grounds and to a mature kelp bed without sea urchins. Changes in macroalgal cover and species composition, and increases in biomass, density and size of kelp (Laminaria) species, characterize the succession from barren grounds to 3- and 4-year-old kelp beds. The greatest change occurred between one and three years following sea urchin mass mortality. Within 3 years, kelp beds attained a level of biomass (7.6 kg m-2) comparable to that of mature beds. Recovery of sea urchin populations via recruitment of planktonic larvae has been slow and spatially variable. Large-scale reciprocal fluctuations in kelp and sea urchin biomass may characterize the trajectory of a dynamic system which cycles between two alternate community states: kelp beds and sea urchin-dominated barren grounds. Periodic decimation of sea urchin populations by disease may be an important mechanism underlying this cyclicity.

Mass Mortality of Strongylocentrotus-Droebachiensis (Echinodermata, Echinoidea) Off Nova-Scotia,Canada

A mass mortality of Strongylocentrotus droebachiensis, attributed to disease, was monitored in an echinoiddominated barren ground at Eagle Head on the south-western coast of Nova Scotia, Canada, in 1982. Mortality was 70% in a shallow (3 m) nearshore area, resulting in a loss of echinoid biomass of 2 042 g fresh weight m-2, and 6% in deeper (7 m, 10 m) offshore areas. Echinoid density, size and nutritional condition (gonad index) were highest in the nearshore area. Survivorship was higher in juveniles (<15 mm diameter) than in adults resulting in the formation of a bimodal size distribution in the nearshore area. Mortality began around early October, near the peak of the annual cycle of seawater temperature (∼15°C), and was arrested by early December (seawater temperature ∼7°C) when morbid echinoids appeared to recover. In laboratory experiments, time to morbidity of S. droebachiensis exposed to morbid conspecifics increased exponentially with decreasing temperature (20° to 8°C). There was no survival at 20° and 16°C, 20% survival at 12°C and 100% survival at 8°C after 60 d; suggesting a lower temperature limit (between 12° and 8°C) for possible transmission of a pathogenic agent. Morbid laboratory echinoids from experiments at 16°C, and recovering echinoids collected in the nearshore area in early December, showed 100 and 85% survival respectively at <=8°C, and 0 and 15% survival respectively at 16°C, after 30 d. Time to morbidity was not affected significantly by nutritional condition and was similar for juvenile and adult echinoids. Time to morbidity was greater in echinoids exposed to one or three morbid individuals continuously, or seven morbid individuals for 1 h, relative to higher levels of exposure (up to seven morbid individuals continuously). Recent mass mortalities in S. droebachiensis have occurred in years of record high sea surface temperatures. The extent of mortality is correlated with the magnitude and duration of temperatures above a lower limit.

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 1960s and 1970s. 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.

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.

Behavioral mechanisms of prey size selection by sea stars (Asterias vulgaris Verrill) and crabs(Cancer irroratus Say) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin))

Abstract Predation upon different size classes of juvenile sea scallops Placopecten magellanicus (Gmelin) (5–25 mm shell height) by different size classes of predatory sea stars Asterias vulgaris Verill (30–150 mm diameter) and crabs Cancer irroratus Say (45–120 mm carapace width) was studied in laboratory experiments using single prey size (no choice) and multiple prey size (choice) designs. Components of predation rate were quantified to assess the relative importance of prey vulnerability and active predator choice in determining observed prey selection. All sizes of sea stars consumed more small scallops than medium or large ones. Encounter rates between predator and prey were similar, irrespective of sea star and scallop size, and did not differ from encounter rates calculated on the basis of body sizes and movement velocities. Scallops assumed a ready-to-swim position when contacted by sea stars, and often actively escaped. The probability of capture by sea stars upon encounter was generally low ( 0.2) than that observed with sea stars. Therefore, prey size selection by crabs appears to be determined by both size-related differences in prey vulnerability, due to differential encounter rates, and active selection of larger prey.

Effect of macroalgae, microbial films, and conspecifics on the induction of metamorphosis of the green sea urchin Strongylocentrotus droebachiensis (Mu¨ller)

Abstract Various macroalgae, microbial films, and conspecifics were tested for their potential to induce metamorphosis of larvae of the green sea urchin Strongylocentrotus droebachiensis (Mu¨ller) in the laboratory. The percentage of larvae that metamorphosed varied with macroalgal species: coralline and noncoralline red algae induced the greatest percent metamorphosis, fucoid and laminarian algae the lowest. Percent metamorphosis in response to acrylic plastic plates with marine microbial films was higher when films were developed in light than in dark and increased with the age of the film. Filmed substrata taken from the intertidal zone of a rocky shore induced a high percentage of larvae to metamorphose, sand collected subtidally did not. Adult urchins, adult-conditioned seawater, adult faecal matter, and recently settled juveniles did not induce metamophosis of larvae, suggesting that a conspecific cue was not involved. These laboratory findings suggest that settlement of S. droebachiensis in the field may not be very selective, although other factors influencing larval supply and transport in the water column and along the boundary layer may result in differential settlement among different micro- and macrohabitats.

Induction of Metamorphosis of Larvae of the Green Sea Urchin, Strongylocentrotus droebachiensis, by Coralline Red Algae

The coralline red algae, Lithothamnion glaciale, Phymatolithon laevigatum, P. rugulosum, and Corallina officinalis, induced >85% of laboratory-reared larvae of Strongylocentrotus droebachiensis to metamorphose. Larvae must contact live L. glaciale or its spores for metamorphosis to occur; the inducer is not sensed in the water column. However, aqueous extracts of L. glaciale can induce metamorphosis, suggesting that the inducing factor is chemical. Neither ashed nor boiled L. glaciale induces metamorphosis, indicating that the factor is heat-labile and that thigmotaxis, per se, is not important in the response. The amino-acid, γ-aminobutyric acid (GABA), which induces settlement of other marine invertebrate larvae, also induces significant rates of metamorphosis of S. droebachiensis at concentrations ≥ 10-4 M. A reduction (with antibiotics) in the number of live bacteria on the surface of L. glaciale does not affect the rate of metamorphosis of larvae.

The ecology of Strongylocentrotus droebachiensis

This chapter discusses about the ecology of Strongylocentrotus droebachiensis; the green sea urchin is the most widely distributed member of the family Strongylocentrotidae. Throughout its range it plays a key ecological role determining the distribution and abundance of benthic macroalgae, particularly kelps. More recently, Strongylocentrotus droebachiensis has become a valued commodity and it is now extensively fished and cultured for its roe. All of these factors have combined to make Strongylocentrotus droebachiensis one of the most studied, if not the most well known, sea urchin species on the planet. The chapter also gives an insight of its geographical range, population density, food preferences and nutrition, feeding behavior, growth rates, reproductive timings, Larval development and settlement behavior, behavioral responses to predators, microbial pathogens, macroparasitic infections, mortality factor, and ecological role of Strongylocentrotus droebachiensis.

Recruitment and growth of the sea urchin Strongylocentrotus droebachiensis (Muller) following mass mortalities off Nova Scotia, Canada

Recruitment of Strongylocentrotus droebachiensis (Muller) was monitored for four successive years following a mass mortality of this species on a subtidal boulder bottom in St. Margarets Bay, Nova Scotia. Recently recruited juveniles (1–2 mm diameter) were observed in the fall (September–November) between (1982) and (1984), but not in (1985). Juveniles grew to 6–8 mm diameter at 1-yr post-settlement and 19 mm diameter ((1983) cohort) at 2-yr post-settlement. Individuals > 18 mm diameter had macroscopic gonads in the spring of their third year and spawning probably occurred at 2.75-yr post-settlement. The growth rate of juveniles (3–6 mm initial diameter) maintained in cages with coralline-covered rocks was similar to that of the natural population. Growth and gonad index increased significantly when kelp (Laminaria longicruris (Pylaie)) was added to cages, suggesting that juveniles were food limited in St. Margarets Bay. Despite an abundance of kelp and other fleshy algae, they remained cryptic under rocks and were not observed grazing on attached plants. The effects of temperature and food supply on juvenile growth and survival were examined in laboratory experiments using factorial designs. Recently-metamorphosed juveniles (from laboratory cultures), maintained at three temperature regimes (5°C, ambient, 16 °C) and two feeding regimes (no added food, coralline algae added), had similar growth rates (over 9 mth) among treatment combinations. However, growth was asymptotic at 16 °C and ambient temperatures, suggesting food limitation. Survival of these juveniles was higher in treatments with coralline algae, but survival was not affected by temperature. Field-collected juveniles (3–6 mm diameter), maintained at three temperature regimes (5°C, ambient, 16 °C) and three feeding regimes (no added food, coralline algae added, and kelp and coralline algae added), grew only in treatments with kelp added as food. In these treatments, growth rates increased with temperature. However, the growth curve at variable ambient temperature regimes was linear, suggesting seasonal acclimation. Survival of the field-collected juveniles was highest in treatments at 5 °C and/or with added kelp.