Population structure and energetics of the shallow-water antarctic sea star Odontaster validus in contrasting habitats

Individuals and populations of Odontaster validus Koehler differed markedly among different habitats, as revealed in a study from October 1984 through January 1986 in McMurdo Sound, Antarctica. At McMurdo Station, individual sizes (wet weight) and population biomass (g wet wt m^-2 and kJ m^-2) decreased significantly with increasing depth. Individuals from shallow (10 to 20 m) habitats were in superior nutritional condition to those from deeper water (30 and 165 m), as shown by higher gonad and pyloric cecum indexes, and by higher lipid and energetic levels in the pyloric ceca. Moreover, gonadal output (reproductive output) was higher in shallow-water individuals. Higher levels of chlorophyll in the pyloric ceca and richer yellow to red coloration of the body wall in the shallow-water individuals indicate that they utilize the higher levels of primary production at shallow depths. At East Cape Armitage, where nearly permanent, thick, snow-covered ice most of the year resulted in very low levels of benthic primary production, the lowdensity sea stars were all very small and nutritionally similar to the deep-water individuals at McMurdo Station. At Cape Evans, where the generally snow-free sea-ice that broke up in mid-summer resulted in a luxurient benthic cover of diatoms and macroalgae, the sea stars were smaller than at McMurdo Station at comparable depths, but population densities were higher, resulting in 4 to 9 times greater biomass. Growth rates of sea stars fed in the laboratory were very low, especially compared to laboratory-reared temperate and tropical species; well-fed individuals need about 9 yr to reach 30 g wet weight, near the mean size of shallowwater individuals at McMurdo Station. No growth was detected in individuals caged at McMurdo Station for one year, suggesting even lower growth rates in the field. The stable size-frequency distributions at the different sites and depths throughout the year-long study suggest highly stable populations with low temporal variability in recruitment, migration and mortality. These data indicate that individuals and populations of O. validus quantitatively and qualitatively reflect the general level of productivity of a habitat. Differences noted in size, coloration, nutrition, and reproductive effort may be the result of long-term integration of local levels of primary production. These ubiquitous sea stars may serve as a biotic indicator of productivity in localized habitats around the continental shelf of Antarctica.     

Patterns of abundance, population size structure and microhabitat usage of Paracentrotus lividus (Echinodermata: Echinoidea) in SW Portugal and NW Italy

In coastal habitats, wave exposure influences several aspects of the life history of marine organisms. Here, we assess how hydrodynamic conditions can generate variation in density, size structure and microhabitat usage of Paracentrotus lividus and whether these effects are consistent between regions that are markedly different for oceanic climate, such as the coasts of SW Portugal and NW Italy. The abundance of P. lividus was ~4 times higher in SW Portugal than in NW Italy, but within each region, there was no effect of wave exposure. In SW Portugal, higher urchin abundances were found at shallower depths, while no effect of depth on urchin abundance emerged in NW Italy. Most of the variation in urchin abundance occurred at small spatial scales (metres), and our results suggest that habitat complexity, that is, the presence of cracks and crevices, is an important determinant of patterns of distribution of this species. The population in NW Italy was characterized by a unimodal size distribution, with a higher proportion of medium-sized individuals. In contrast, in SW Portugal, smaller individuals represented a large proportion of the populations. Size structure varied between exposed and sheltered habitats in SW Portugal, suggesting that the proportion of individuals from different size cohorts may vary along wave-exposure gradients as a result of direct or indirect effects of hydrodynamic forces. In SW Portugal, most urchins occurred in burrows, while in NW Italy, urchins were mainly observed in crevices. These results suggest that creating/occupying burrows might be an adaptive behaviour that allows sea urchins to better withstand stressful hydrodynamic conditions and, therefore, are more common on exposed Atlantic coasts. Overall, our study suggests that the effects of hydrodynamic forces on sea urchin populations are context dependent and vary according to background oceanic climate.     

Mass spawning by the sea urchin Evechinus chloroticus (Echinodermata: Echinoidea) in a New Zealand fiord

A mass synchronous spawning of the sea urchin Evechinus chloroticus (Valenciennes) was observed in situ in Doubtful Sound, a large New Zealand fiord. Spawning occurred between 17:30 hrs and 18:30 hrs on 27 January 1994 and coincided with a full moon, spring tides and a period of decreasing sea temperatures. During spawning, the sea urchins formed a dense spawning aggregation of both sexes, with >90% of the urchins observed spawning at the time. Spawned gametes clouded the water column, and some were eaten by small labrid fish species. The spawning, which may have been as widespread as 40 km, marked a 42 to 50% decrease in gonad indices and resulted in a widespread, dense cohort of  E. chloroticus larvae within the fiord. Received: 25 September 1997 / Accepted: 6 March 1998     

The effects of predator abundance and habitat structural complexity on survival of juvenile sea urchins

We studied the effect of the abundance of predatory fishes and structural complexity of algal assemblages on the survival of juveniles of the sea urchin Paracentrotus lividus on Mediterranean infralittoral rocky bottoms. Post-settlement juveniles (2–10 mm) were placed on four distinct natural substrates with increasing structural complexity (coralline barren, algal turf, erect fleshy algal assemblages and small crevices) inside and outside the Medes Islands Marine Reserve. Predation on these sea urchins increased at greater abundance of predatory fishes, and decreased with greater structural complexity. The refuge provided by structural complexity, however, decreased with increasing size of sea urchin recruits. Predation on the smallest post-settlers was carried out almost exclusively by small fishes (<20 cm), mainly the labrid Coris julis, while the dominant predator of larger juveniles was the sparid Diplodus sargus. Our results demonstrate the cascading effects caused by the prohibition of fishing in marine reserves, and highlight the potential role of small predatory fishes in the control of sea urchin populations.Communicated by O. Kinne, Oldendorf/Luhe     

Incidence,occurrence and distribution of the nematode Echinomermella matsi in its echinoid host,Strongylocentrotus droebachiensis,in northern Norway

The incidence, occurrence and distribution of the nematode Echinomermella matsi (Jones and Hagen) in its host, the echinoid Strongylocentrotus droebachiensis Müller, were examined in Norwegian waters from Nordmre (63° N) to the Russian border (70° N) in a range of habitats (e.g. kelp cover and community type) from 1982 to 1992. The results are used to examine the likely influence of the parasite on fitness (individual growth and gonad production) and its potential in regulating the population size of its echinoid host. The nematode infections are restricted to areas between Vikna (Vega) and middle Troms, with the highest prevalence (40 to 88% infected) in the Bodø area, the focus of the distribution. The prevalence rates are similar on kelp beds and barren grounds. No difference in growth rates between infected and noninfected sea urchins was found. Nematode-infected sea urchins in kelp bed populations had gonads weighing about one-sixth of those of equivalent uninfected individuals. This indicates that nematodes are really parasites causing reduced reproductive capacity. Areas with high prevalence (>40% infected) of nematodes had low sea urchin densities. High nematode intensities were found in the areas with high prevalence. The mean abundance in high prevalence areas was highest (eight nematodes) in the 4-to 7-yr-old age group and decreased to zero among the oldest age groups. The ratio of variance/mean abundance was also highest among 5-to 8-yr-old sea urchins. At highly and slightly infected locations, the relationships between the log of the variance and the log of the mean abundance of parasites were 1.35 and 1.69, respectively. Measurements of the frequency distribution of nematodes in sea urchins and the corresponding estimates of the negative binomial distribution showed significantly higher counts in estimated than observed subpopulations exceeding 20 nematodes per sea urchin. These results may explain nematode-induced sea urchin mortality. The low density of the host in the Bodø area, which had high prevalence, may be a result of increased host mortality caused by nematodes, by lower recruitment because of reduced gonad production, or by other factors. Low prevalence among older sea urchins may be caused by nematodeinduced host mortality or by cycles in infection. However, studies on the life cycle of the nematode and long term investigations of host density and nematode intensity/prevalence will be needed before any true conclusions can can be drawn.     

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.     

Age structure,growth, and morphometric variations in the Atlantic surf clam,Spisula solidissima,from estuarine and inshore waters

The surf clam, Spisula solidissima (Dillwyn), population in the estuarine waters of Long Island Sound, New York, USA, was characterized in 1984 and again in 1988 by an age structure restricted to just two age-classes, and an apparent lifespan of only about 10 yr. In the inshore coastal waters off Fire Island, New York, a wide age range from 2 to 22 yr old was present. The age structure at Long Beach, New York, a third geographic region which is coastal but influenced by the Hudson River estuary, was similar to Long Island Sound. Juvenile surf clams grew at similar rates in all three geographic regions. However, adults from the Long Island Sound population grew significantly slower and reached an asymptotic maximum size which was 37% smaller than Fire Island adults. Long Beach adults had intermediate growth rates and maximum sizes. The shells of Long Island Sound clams were also 25% thinner than those from the other two regions. Density dependent effects on growth, evaluated over abundances ranging from 0.5 to 294 ind. m^–2, were present but were too small to account for observed regional differences. Results suggest that adult surf clams may be physiologically stressed by the reduced salinity and more extreme temperatures found in estuarine waters.     

Comparison of larval duration and pre- and post-settlement growth in two species of damselfish,Chromis atripectoralis andPomacentrus coelestis (Pisces: Pomacentridae), from the Great Barrier Reef

Light traps were used to capture larval fishes, immediately before settlement, at two localities 500 km apart on the Great Barrier Reef (GBR) in December, 1987. Samples from Lizard Island, in the northern GBR, and Davies Reef, in the central GBR, were dominated by two species of damselfish:Chromis atripectralis andPomacentrus coelestis. Analysis of otoliths revealed significant differences in both size and age at settlement between the two localities forP. coelestis, but not forC. atripectoralis. Growth rates determined for pre- and post-settlementP. coelestis suggested a sigmoidal growth trajectory through the larval life, with growth slowing as fishes approached the time of settlement. Post-settlement growth rates were faster than growth prior to settlement in both species. Growth in both species was, however, similar between localities. The relationship between fish size and otolith size was complex, varying both between pre- and post-settlement fishes, and among localities. This emphasizes the need to validate the relationship between fish size and otolith size before otoliths may be used to back-calculate individual growth trajectories.Contribution No. 500 from the Australian Institute of Marine Science     

Natural growth bands and growth variability in the sea urchin Echinus esculentus: results from tetracycline tagging

Growth of the European edible sea urchin Echinus esculentus L. was studied in a population held for 2 yr in cages on the sea bed, after labelling with the skeletal growth marker tetracycline. The final position of the tetracycline tag on the genital plates agreed with an annual periodicity in natural growth zones; two such growth zones appeared beyond the position of the tag on the ground surface of the plate as light-reflecting bands separated by narrow dark lines in the middle layer. Individual and group (pooled data) growth parameters were estimated from the growth increment shown in the genital plate, whose lateral growth displayed a linear relationship to the diameter of the urchin test within the size range of these measurements. Von Bertalanffy growth parameters (asymptotic size and growth-rate function K) fitted to the growth increment on each individual were highly significantly correlated to those fitted to the natural growth lines, assuming an annual periodicity. The inferred growth pattern agrees well with conclusions based on H. B. Moore's growth-band data. The caged urchins can be assumed to have experienced exactly similar conditions, yet the growth curves fitted to individuals showed considerable variability. The good agreement between estimated growth function parameters of individuals obtained by the two methods indicate that this reflects real variability in growth between individuals that probably is genetically rather than environmentally determined. The growth of E. esculentus, and the adaptational significance of high growth variability in the population is briefly discussed.     

Human impact on <Emphasis Type="Italic">Paracentrotus lividus</Emphasis>: the result of harvest restrictions and accessibility of locations

The sea urchin Paracentrotus lividus is common in the Mediterranean Sea in shallow subtidal rocky habitats, and it is intensely harvested for commercial and recreational purposes. This study is aimed at investigating whether the effects of harvest restrictions of P. lividus in rocky reef habitats interact with the accessibility of locations in structuring sea urchin population (total and commercial-sized individuals). These results are important for generating hypotheses about the influence of human harvesting on P. lividus and for addressing suitable measures of conservation. Paracentrotus lividus was sampled after the end of the sea urchin harvesting period (May–July 2007) within the Gulf of Alghero (North West Sardinia), where the Capo Caccia–Isola Piana Marine Protected Area (MPA) was established since 2002. Paracentrotus lividus was sampled at sixteen locations and attributed in groups of four to 4 combinations of harvest restrictions (Restricted Harvest, RH, vs. Unlimited Harvest, UH) and accessibility (Boat vs. Car), which correspond to a gradient of potential human activity on P. lividus in the ranked order of very low (RHBoatfar), low (RHBoatclose), moderate (RHCar) and high (UHCar). At each location, two depth ranges of 3–7 and 8–12 m were considered. At each of these depths, two areas of about 100-m² size were chosen. The density of P. lividus was assessed in ten quadrats of 1 × 1 m, and the size of 100 individuals (test diameter) was considered. Human activity has been found to significantly affect population structure of P. lividus influencing the proportion of individuals larger than 50 mm. Although harvest was restricted by MPA regulations, a significantly lower abundance of large individuals was found at sites accessible by car. This result highlights that there is an effect of harvest restrictions in relation to accessibility and emphasizes the need to carefully address the enforcement of the MPA toward easily accessible sites. Thus, surveillance and investment in enforcement of marine reserves seem crucial points that may provide the greatest return on maintaining the ecological benefits to the fishery activities.     

Reproductive variability over a four-year period in the sea urchin Evechinus chloroticus (Echinoidea: Echinodermata) from differing habitats in New Zealand

 The endemic New Zealand echinoid, Evechinus chloroticus (Valenciennes), was sampled approximately monthly from September 1990 to October 1994 at three sites in Tory Channel, Marlborough Sounds, New Zealand. These channel sites (outer, mid and inner) were up to 20 km from the open ocean and differed in their shore type, exposure to wave action and macrophyte abundance. E. chloroticus showed an annual reproductive cycle at each site, with gametogenesis commencing in the late austral winter and spawning in summer. Maximum gonad indices (reproductive potential) varied spatially, with the outer site generally having higher maximum gonad indices than the inner site, and temporally, with maximum gonad indices occurring earlier in 3 of the 4 seasons at the outer site than the inner site. The mid and inner sites showed much greater variation in maximum gonad indices (range 15.83 to 26.99% and 11.87 to 20.90%, respectively) than the outer site (range 19.31 to 22.95%). Reproductive output (weight of gametes released per gram echinoid) also varied, with the different sites showing significantly different outputs in the different years. A regression of maximum reproductive potential against reproductive output was significant ( p < 0.001), and had a positive slope with an r ² of 0.79. While, the initiation of gametogenesis was relatively synchronous between sites and years, and is possibly cued by increasing daylength, it progressed at different rates among populations. Spawning did show spatial and temporal variability, occurring near the time of highest sea-surface temperatures (∼15 °C). The observed variations in reproductive cycle may be related to small-scale variability in diet and environmental conditions. Furthermore, asynchronous spawning, variable spawning duration, and variable reproductive output are likely to strongly influence annual recruitment variability in E. chloroticus, with different larval subpopulations contributing unequally in different years. The ecological consequences of this, both for the ability of E. chloroticus to propagate itself in space and time and for the management of the developing E. chloroticus fishery in New Zealand, are discussed. Received: 9 December 1998 / Accepted: 13 June 2000     

Succession in sea urchin and seaweed abundance in Nova Scotia,Canada

From 1980 through 1983 sea urchin mass mortalities were at least 245 000 t. The habitat released to seaweed is expected to support a standing crop of 1.8 million tons and an annual production of 7 million tons. An extensive survey of 2 900 km of Nova Scotia shoreline revealed that 511 km² of habitat (rock bottom less than 15 m deep) was available to seaweeds. Areas most exposed to ocean swells bordered 26% of the shoreline length but included 82% of the seaweed habitat. Before mass mortalities, seaweed refuges from sea urchin grazing included very sheltered habitats, very exposed habitats, and boulders set among shifting sand; however, these were less than 10% of the total habitat. In the 1 to 2 years following mass mortalities seaweed cover and occurrence increased significantly for all of five categories of wave exposure and for six of seven algal taxa respectively. In at least three locations the sea urchin population began to recover as a result of larval recruitment but in 1983 was again reduced by disease. Sea urchins appear to lack natural resistance to disease, at least at warm temperatures. In the laboratory all sea urchins collected from the Bay of Fundy, southern Nova Scotia, eastern Newfoundland, and an area recovering from mass mortalities died following exposure to diseased animals. Also, field mortalities included a wide variety of habitats. Interviews of Nova Scotia lobster fishermen revealed that sea urchin mass mortalities have probably occurred before in this century, but infrequently.     

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m^-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.     

Ecotypic differentiation in the kelpLaminaria saccharina: Phase-specific adaptation in a complex life cycle

Comparison of cultured sporophytes and gametophytes in common-garden experiments confirmed the existence of ecotypic differences in light-related traits among populations ofLaminaria saccharina (L.) Lamour. Cultured sporophytes from the turbid habitat in Long Island Sound, New York, USA, grew faster under both limiting and saturating daily irradiances than sporophytes from shallow and deep habitats along the Atlantic coast of Maine. Rapid growth of turbid plants was attributable to several factors, including high photosynthetic capacity and efficiency [due to differences in photosynthetic unit (PSU) number and size], low respiration rates, and high surface area:weight ratios. In contrast to sporophytes, microscopic gametophytes from the three kelp populations grew at similar rates under limiting and saturating daily irradiances. Biomass-specific photosynthesis vs irradiance (PI) parameters were similar for gametophytes from the shallow, deep, and turbid sites, despite population differences in chlorophyll-specific PI parameters and PSU characteristics. However, turbid gametophytes produced microscopic sporophytes more rapidly than gametophytes from the shallow and deep sites, apparently due to a lower blue-light requirement for gametogenesis. Ecotypic differences in sporophytes and gametophytes ofL. saccharina from shallow, deep, and turbid habitats can be understood as phase-specific adaptations.     

Population biology and antipredator defenses of the shallow-water Antarctic nudibranch Tritoniella belli

The circumpolar nudibranch Tritoniella belli Eliot occurs in abundance in shallow-water benthic communities of McMurdo Sound, Antarctica. Density estimates based on belt transects averaged collectively 0.46 individuals m⁻² at three study sites between depths of 6 and 30 m in November 1996. At two of the sites, population densities increased linearly between 18 and 30 m depth (up to 0.7 and 1.15 individuals m⁻² at 30 m depth). Individuals at all sites were rare or absent at depths shallower than 12 m. Size frequencies of individuals at the sites were similar, and a pooled analysis revealed a unimodal distribution skewed highly towards juvenile size classes. This suggests both recent recruitment and constant rates of mortality across size classes. The relationship between foot length and wet weight best fits an exponential growth equation, indicative of an allometric growth pattern. Distribution of T. belli in the field suggests that it is a habitat and diet generalist. Potential invertebrate predators include sea anemones and seastars, both of which co-occur in abundance in McMurdo Sound. Laboratory experiments indicate that the sea anemone Isotealia antarctica can capture and ingest T. belli. However, 70% of T. belli that are captured escape from the tentacles or, following ingestion, are rejected from the gastrovascular cavity. The seastars Odontaster validus, Perknaster fuscus, and Acodontaster conspicuus, avoid contact with T. belli, but if forced into contact with mantle tissues, retract their tube-feet. Mucus secreted from the mantle tissues, coated on to the tips of glass rods, and presented to seastar tube-feet, causes significantly longer tube-foot retraction times than control rods. Moreover, pieces of freeze-dried krill coated with mantle mucus are consumed significantly less often than untreated control pieces of krill by a benthic scavenging fish (Pseudotrematomas bernacchi). Employing seastar tube-foot retractions as a bioassay, we found the bioactive compound(s) are soluble in ethyl acetate, indicating they are lipophilic or moderately hydrophilic in nature. Chemical defenses in the mucus of T. belli probably contribute to its high abundance in Antarctic benthic communities. Received: 6 October 1997 / Accepted: 24 March 1998     

Reproductive cycle of Centrostephanus rodgersii (Echinoidea), with recommendations for the establishment of a sea urchin fishery in New South Wales

The reproductive cycle of the sea urchin Centrostephanus rodgersii (Agassiz) was investigated in two populations, at Clovelly and Little Bay, in Sydney, New South Wales, Australia. C. rodgersii were collected at monthly intervals from February 1992 through January 1993. The reproductive cycle was determined by histological examination of oogenesis and spermatogenesis, monthly measurements of gonad index (GI), and induction of spawning by KCl injection. C. rodgersii has an annual reproductive cycle that was highly synchronous in both populations. From February to June, gametogenesis was accompanied by a decline in the amount of nutritive tissue in the gonads. The urchins were mature from June to September, with peak spawning between July and August, as indicated by a significant drop in GI. The breeding season of C. rodgersii therefore coincides with the lowest sea temperatures and the shortest days of the year. The gonads returned to the recovering condition within a month of spawning, with a substantial thickening of the nutritive layer along the gonad wall, and the GI returned to near pre-spawning levels. As a result, the spent phase was rarely found in C. rodgersii. With the exception of a significant decrease in the GI following spawning of urchins from the Clovelly population, the GI measurements did not show any distinct pattern through time. Specimens spawned in response to KCl injection from mid-May to early October, with the maximum response in July. Although all individuals sampled were at a similar stage of maturity at any one time, inter-site differences were seen with all of the methods used. Gonad indices from Little Bay were consistently higher and less variable than those from Clovelly for most of the year. The Little Bay population could also be induced to spawn for a longer period of time than could the Clovelly population. The breeding season of the Little Bay population appears to be longer than that of the Clovelly population. The relationship between size and sexual maturity was also examined. All C. rodgersii with a test diameter of >60 mm could be induced to spawn and produced viable gametes. C. rodgersii has been nominated for commercial exploitation in New South Wales, and the results of this investigation are used to make recommendations on the timing and size limits for a fishery utilising this species.     

Interactions among sea otters, sea stars, and suspension-feeding invertebrates in the western Aleutian archipelago

The structuring and organizing effects of apex predators on ecosystems are becoming increasingly well documented. The enhancement of kelp forests via sea otter predation on herbivorous sea urchins is among the earliest and best known examples. This study provides evidence for direct and indirect trophic interactions among sea otters, predatory sea stars, and filter-feeding mussels (Mytilus trossulus) and barnacles (Semibalanus cariosis). In western Massacre Bay at Attu Island (173°E, 53°N), subtidal transects showed sea star body size and biomass density declined markedly between 1983 and 1994 as sea otters reinhabited this area. Mussels and barnacles translocated from the rocky intertidal zone to shallow subtidal habitats to assess loss rates from sea star predation showed lower mortality rates after the arrival of sea otters. Prey mortality rates in subtidal caged controls were consistently low and similar to those of intertidal controls in both years. These findings elucidate a trophic pathway by which sea otters can influence ecosystems separate from the well-known sea otter/sea urchin/macroalgae cascade.     

Matching spatial distributions of the sea star Echinaster sepositus and crustose coralline algae in shallow rocky Mediterranean communities

Understanding why a species is present in a particular location and the consequences of its presence is complex but necessary to identify the mechanisms that generate and maintain ecological diversity. The common sea star Echinaster sepositus can be either very abundant or non-existing in nearby localities of the western Mediterranean. Yet, the factors that shape its distribution and the impact of the sea star on natural communities remain uninvestigated. Here, we quantified multiple biotic and abiotic factors that may affect the distribution of E. sepositus and tested whether this sea star can shape the organization of the community it inhabits. Our results showed that the distribution of this sea star was highly contagious and positively correlated with the abundance and distribution of crustose coralline algae from tens of meters to tens of kilometers. Despite significant differences in community composition between localities with high or low abundance of the sea star, experimental addition of E. sepositus to natural communities failed to shift the composition of the algal community in 4 months. Overall, our results suggest that within habitat variability in the abundance of crustose coralline algae may explain the abundance of E. sepositus at multiple geographic scales, emphasizing the need to investigate small-scale processes at larger geographic scales.     

Foods and predators of the green sea urchin Strongylocentrotus droebachiensis in Newfoundland waters

Gut analyses of the green sea urchin Strongylocentrotus droebachiensis (O. F. Müller) demonstrated that perennial phaeophytes, mostly fucoids and Alaria esculenta, were predominant in the diet. Ephemeral species, coralline algae and animals, were consumed in smaller amounts when available. Grazing by the urchins is evidently responsible for the dearth of non-coralline sublittoral algae in Newfoundland waters. Lobsters, rock crabs, purple sea stars, other urchins, and a variety of fishes and birds feed on S. droebachiensis, but predation is apparently not effective in limiting the abundance of the urchin.Studies in Biology from the Memorial University of Newfoundland No. 234.Contribution from the Marine Sciences Research Laboratory No. 66.     

Morpho-functional defences of Mediterranean sea urchins,<Emphasis Type="Italic"> Paracentrotus lividus</Emphasis> and<Emphasis Type="Italic"> Arbacia lixula</Emphasis>, against fish predators

Morpho-functional features potentially involved in defence mechanisms against fish predators (i.e. attachment tenacity, spine length, and test robustness and thickness) have been assessed in two Mediterranean sea urchins, Paracentrotus lividus and Arbacia lixula. All four morpho-functional features were significantly and positively related to individual size for both species of sea urchins. Test robustness (i.e. static load needed to break sea urchin tests) was significantly greater for A. lixula (from 3,450 to 15,000 g depending on size) than for P. lividus (1,180–11,180 g). Attachment tenacity (i.e. force needed to dislodge sea urchins from the rocky substrate) was greater in A. lixula (280–3,300 g) than in P. lividus (110–1,450 g), and the difference tended to decrease in relation to smaller sea urchin size. Spine length was greater in A. lixula (1.5–2.9 cm) than in P. lividus (0.5–2.3 cm), but the difference decreased for larger sea urchin size. Test thickness was slightly greater (but not significantly) in A. lixula (0.35–1.10 mm) than in P. lividus (0.12–0.90 mm). These results provide evidence that morpho-functional features of sea urchins could be involved in affecting predation rates by fishes upon P. lividus and A. lixula, with potential implications for the population structure and distribution patterns of the two sea urchins in shallow rocky reefs.Communicated by R. Cattaneo-Vietti, Genova