Sea Urchin Abundance Research Articles

Is there a link between the type of habitat and the patterns of abundance of holothurians in shallow rocky reefs?

The presence of a mosaic of habitats, largely determined by sea urchin grazing, across shallow rocky reefs may potentially influence in differences in the distribution patterns of invertebrates. The aim of this paper was to assess, using a correlative approach, whether the type of habitat influences the abundance patterns of holothurians in the eastern Atlantic. We hypothesized that abundances of large (> 10 cm) holothurians varied among four types of habitat (3 vegetated habitats with low abundances of the sea urchin D. antillarum vs. ‘barrens’ with hyperabundances of sea urchins), and that these differences were consistent at a hierarchy of spatial scales, including two islands and several replicated sites within each type of habitat and island. Three species of large holothurians were found, accounting for a total of 300 specimens. We found remarkable differences in abundances of holothurians between the ‘barrens’ and the three vegetated habitats. This pattern was strongest for the numerically dominant species, Holothuria sanctorii. Total abundances of holothurians were between 5 and 46 times more abundant in ‘barrens’ compared with the vegetated habitats. Inter-habitat differences were species-specific with some inconsistent patterns from one island to the other. The total abundances of holothurians tended to increase with the abundance of sea urchins within ‘barrens’. Our study suggests that there may be a link, at least for the dominant species Holothuria sanctorii, between the distribution and abundances of large holothurians and the habitat across shallow-waters of the eastern Atlantic.

Variations morphologiques des Toxaster (Echinoida : Spatangoida) en regard des fluctuations spatiales (Arc de Castellane, SE France) et temporelles (Valanginien-Hauterivien) du milieu sédimentaire : expression d'un potentiel adaptatif restreint

The impact of sedimentary conditions on the settlement, growth, and morphology of primitive spatangoid sea urchins of the genus Toxaster from the Lower Cretaceous (Valanginian and Hauterivian of the Provencal platform) is analyzed. The analyses concern spatial and temporal patterns of variation. The spatial approach is based on six isochronous populations of Toxaster granosus kiliani (424 specimens in total) dated from the Verrucosum biozone, and distributed along a proximo-distal gradient. The temporal approach concerns a reference section (Carajuan) ranging from the Lower Valanginian to the Upper Hauterivian, and involving the species T. granosus kiliani and T. gibbus. Sedimentary environments have been characterized by a factorial analysis of five characteristics of facies. Sea urchin morphology has been described with seven measures allowing the computation of two angles (α and β), and of six shape indices analyzed through a principal components analyses. The spatial approach shows that the adult mean size (> 20 mm) increases distally, and that morphology changes significantly across localities. However, only variations in α and β are sequentially organized along the proximo-distal gradient (they correspond to a forward migration of the peristome and to an expansion of the plastronal area). The stratigraphic survey of facies variation along the 369 beds analyzed in Carajuan shows a clear change in sedimentary conditions: from unstable with bioclastic events (Valanginian), to homogeneous with a low content in bioclasts (Hauterivian). Most Toxaster specimens are concentrated in three stratigraphic units (Campylotoxus and Verrucosum biozones for T. granosus kiliani; Sayni biozone for T. gibbus), their abundance being related to the macrofaunal presence and diversity and to burrowing activity. The adult mean size is only significantly related to facies variation, for the T. granosus kiliani from the Verrucosum biozone, increasing with textural gradient and bioclastic content. Shape – facies relationships have been identified for T. granosus kiliani in both Campylotoxus and Verrucosum intervals. However, the morphological variables and facies characteristics involved differ between the two biozones. The synthesis of all significant relationships between size or shape and facies characteristics demonstrates that the response of Toxaster to environmental changes was limited, and varied in space as well as for the different periods of time considered. The only recurring impact of sedimentary changes involved sea urchin abundance, that is, their ability to settle. The low adaptive potential of Toxaster is discussed in the context of the early radiation of the Spatangoida.

Population density of four species of sea urchin (Echinodermata: Echinoidea) at La Entrega Reef, Oaxaca.Population density of four species of sea urchin (Echinodermata: Echinoidea) at La Entrega Reef, Oaxaca.

Bimonthly samples of sea urchins fauna inhabiting the La Entrega Reef, Oaxaca, Mexico, were obtained from October 2002 to August 2003. The reef area was divided in two sampling zones, corresponding to shallow (0-6 m) and deep (6-12 m) water. Six transects of 50 m length by 1 m wide were sampled in each zone. Greatest annual density corresponded to Diadema mexicanum (5.96 ± 0.85 ind/m2), followed by Eucidaris thouarsii (0.46 ± 0.14 ind/m2); Echinometra vanbrunti and Toxopneustes roseuspresented low densities (0.03 ± 0.01 indv/m2 and 0.02 ± 0.01 ind/m2, respectively). Density of the four populations varied considerably and higher densities were observed in the deep zone. Total abundance of sea urchins (shallow and deep samples) did not vary significantly over time. Abundance per depth range, however, was different over time for all species except for E. thouarsii. In December densities of D. mexicanum and E. thouarsii decreased in the deep zone and increased in the shallow zone, due to massive mortality of coral in the shallow part of the reef and to subsequent increase of algal coverage attracting these species to the shallowest part of the reef. The La Entrega reef is presently in an advanced state of erosionthat could favor an increase of sea urchins abundance and of their bioerosion activity.

Indirect food web interactions: sea otters and kelp forest fishes in the Aleutian archipelago.

Although trophic cascades-the effect of apex predators on progressively lower trophic level species through top-down forcing-have been demonstrated in diverse ecosystems, the broader potential influences of trophic cascades on other species and ecosystem processes are not well studied. We used the overexploitation, recovery and subsequent collapse of sea otter (Enhydra lutris) populations in the Aleutian archipelago to explore if and how the abundance and diet of kelp forest fishes are influenced by a trophic cascade linking sea otters with sea urchins and fleshy macroalgae. We measured the abundance of sea urchins (biomass density), kelp (numerical density) and fish (Catch per unit effort) at four islands in the mid-1980s (when otters were abundant at two of the islands and rare at the two others) and in 2000 (after otters had become rare at all four islands). Our fish studies focused on rock greenling (Hexagrammos lagocephalus), the numerically dominant species in this region. In the mid-1980s, the two islands with high-density otter populations supported dense kelp forests, relatively few urchins, and abundant rock greenling whereas the opposite pattern (abundant urchins, sparse kelp forests, and relatively few rock greenling) occurred at islands where otters were rare. In the 2000, the abundances of urchins, kelp and greenling were grossly unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant. Significant changes in greenling diet occurred between the mid-1980s and the 2000 although the reasons for these changes were difficult to assess because of strong island-specific effects. Whereas urchin-dominated communities supported more diverse fish assemblages than kelp-dominated communities, this was not a simple effect of the otter-induced trophic cascade because all islands supported more diverse fish assemblages in 2000 than in the mid-1980s.

Variation in abundances of herbivorous invertebrates in temperate subtidal rocky reef habitats

The present study assessed variation in the abundances of large herbivorous invertebrates in south-western Australia. There was some habitat partitioning between different parts of the reef: of the most frequently recorded species, the sea urchins Phyllacanthus irregularis and Centrostephanus tenuispinus were found primarily at the base of steep rock faces, whereas the gastropods Turbo torquatus and Australium squamifera were found primarily on open sections of reef. The sea urchin Heliocidaris erythrogramma was evenly distributed between these two habitats. For C. tenuispinus and H. erythrogramma, differences among locations (separated by tens to hundreds of kilometers) were the main source of variation in abundances. Phyllacanthus irregularis was more evenly distributed among locations. Abundances of sea urchins at each reef varied little over 26 months, suggesting low mortality and low recruitment. Turbo torquatus and A. squamifera varied significantly in abundance among reefs separated by < 10 km, although these differences were influenced by fluctuations over time. Broad patterns in abundances were evident: overall, abundances of herbivorous invertebrates were low, but certain areas supported high abundances. This suggests that herbivory may be a minor process in this region; however, the importance of herbivory at reefs with and without high abundances of herbivores deserves further attention.

Temporal and spatial variability in settlement of the sea urchin Paracentrotus lividus in the NW Mediterranean

Settlement into the benthic habitat may be an important process in regulating sea urchin abundance, which potentially modifies the structure of benthic communities. Strong settlement events may increase sea urchin abundance beyond a certain threshold, leading to the formation of coralline barrens (overgrazed communities with a dominance of encrusting coralline algae). To understand the role of settlement in regulating sea urchin populations we first need to determine settlement variability. Temporal variation in settlement of the sea urchin Paracentrotus lividus was monitored at three sites in the Medes Islands, NW Mediterranean, during three settlement seasons (March 1998 through October 2000). Spatial variation in settlement was studied in 1999 at 50 sites along a gradient of exposures to waves and currents, inside and outside the archipelago, and separated by distances from tens to thousands of meters. Bathymetric distribution of settlement was also studied in 2000 at six sites at 5, 10, 15 and 20 m depths. Settlement of P. lividus occurred in a single annual peak within 3 weeks in May–June. Differences in settlement between years were more than two orders of magnitude. Spatial variability was found at all scales investigated, showing strong patchiness at the smallest spatial scales (tens of meters). Sea urchins settled preferentially at depths between 5 and 10 m. Substratum type, level of protection, and adult population densities were not significant in determining settlement. However, settlement was found to be related to the degree of exposure to waves and currents, indicating that physical processes are very important at the spatial scales investigated. This greatly variable settlement is a necessary, although not sufficient, condition to create gradients of adult P. lividus abundance. Further studies should be designed to investigate the interaction between settlement strength and post-settlement mortality.

Reproductive sources and sinks within a sea urchin, Evechinus chloroticus, population of a New Zealand fjord

We examined population structure of New Zealand sea urchins Evechinus chloroticus across the Doubtful-Thompson Sound complex, a fjord with a large influx of freshwater, a low salin- ity surface layer and a mean estuarine circulation. Measurements of abundance, growth, gonad development and larval settlement were collected at 9 sites. We observed large differences in each of these vital population parameters among sites. The highest growth rates and gamete production occurred at the entrances of the fjord. We used 3 alternative growth models to test whether the spa- tial pattern in growth rates was robust. Variability in growth among sites likely reflected differences in nutritional history, as indicated by the Aristotle's lantern index. Size distributions taken 2 yr apart at each of the 9 sites showed that the adult mode was relatively stable in time. As a consequence much of the variance in mean size among sites could be explained by differences in growth rather than in mortality or recruitment. The highest abundance and aggregation of sea urchins occurred at mid-fjord and entrance sites, which coincided with the highest rates of larval settlement and sug- gested limited larval supply elsewhere. The observed spatial patterns in vital rates were used to eval- uate the likely influences of spatial heterogeneity in larval production on larval dispersal patterns within the fjord, based on particle movements within a validated hydrodynamic model. We then tested the sensitivity of these model results to magnitude of wind and rainfall events and duration of larval development. Results from our field observations and modeling studies suggest that the sea urchin population in Doubtful-Thompson Sound is likely composed of distinct areas of reproductive source and sink.

A Novel Method of Assessing Bioerosion by the Sea Urchin Echinometra sp. A on a Fijian Reef

Sea urchins are major substratum eroders of coral reefs and the calcium carbonate condition of coral reefs may therefore reflect the abundance of sea urchins. The omnivorous urchins, Echinometra spp. are known as ?rock -borers? and are agents of large-scale bioerosion, particularly in the Caribbean, Eastern Pacific and Western Indian Ocean. By burrowing into coral rocks they protect themselves from predators, wave and current action, and to some extent from desiccation at low tide. Bioerosion estimates are known for a few specific reefs only. Although the Echinometra sp. A have not been named yet, they were mistaken for Echinometra mathaei for long in Fiji. This novel study attempted to estimate bioerosion rates of the low but consistent numbers of sea urchins, Echinometra sp. A (green-white-tipped) on the Nukubuco reef, Fiji by calculation of the net carbonate accumulation. Cage experiments reported bioerosion rates (kg CaCO3/m2/urchin/d) of 35-37 x 10-3 at the reef crest and 30-43 x 10-3 at the reef flat. This method of assessing bioerosion showed both strengths and weaknesses. However, the Nukubuco reef balance between reef growth and reef destruction is shifting, with bioerosion becoming the dominant process.

Can low sea urchin densities control macro-epiphytic biomass in a north-east Atlantic maerl bed ecosystem (Bay of Brest, Brittany, France)?

The distribution, abundance and feeding behaviour of sea urchins (Paracentrotus lividus, Psammechinus miliaris, Sphaerechinus granularis) and the macro-algal biomass were assessed in two maerl beds within a north-east Atlantic coastal ecosystem (Bay of Brest, Brittany, France). To study the possible control of herbivorous sea urchins on the macrophytic algae, one maerl bed under the influence of urban sewage (northern site) was compared to one less influenced by urban and industrial outlets (southern site). Macrophytic biomass in the northern site was estimated as 40-fold higher than in the southern site where sea urchins were, on average, 38-times more numerous. Preliminary results support the hypothesis that grazing of sea urchin, even in low densities, can be a factor regulating the macrophytic biomass on maerl beds except in too nutrient enriched environments. So in the northern basin of the Bay, data pointed out the role of anthropogenic impacts on macrophytic biomass increase which was concomitant with the progressive disappearance of sensitive herbivorous species like sea urchins, both processes result in change in the ecosystem.

Effects of the 1998 Coral Morality Event on Kenyan Coral Reefs and Fisheries

Data were collected in southern Kenya on coral reef ecosystems and fisheries to assess the influence of the 1998 coral bleaching and mortality event. We compared benthic cover, sea urchin and fish abundance in unfished marine parks and fished reefs and the reef-associated fisheries 3 years before and after 1998. Hard and soft coral decreased while coralline algae increased in both management areas. Turf increased in marine parks and sponge and fleshy algae increased in the fished reefs. Sea urchin grazer biomass was unchanged over this period and the fish community changed less than benthic cover. In general, butterflyfish, damselfish and wrasses were negatively influenced while surgeonfish and a few uncommon families were positively influenced by the substratum change. There was a 17% increase in fishing effort as measured by fishermen per day at each landing site and the total demersal catch declined by 8% and the catch per man declined by 21% after 1998. The decline in the total catch and CPUE combined with the increase in effort suggest an overexploited fishery and this makes it difficult to distinguish changes caused by coral mortality or fishing effort. The price of fish increased over this period and this caused an 18% increase in the total value of the fishery but no difference in the net income of individual fishermen.

THE EFFECT OF MARINE RESERVES AND HABITAT ON POPULATIONS OF EAST AFRICAN CORAL REEF FISHES

The effects of fishing, the duration of protection from fishing, features of the reef habitat, including benthic cover and sea urchin abundance, and the distance between reefs were examined to determine the ability of these factors to predict ecological aspects of fish communities. Population density, species richness, and rarity were estimated for 127 species of coral reef fish on 22 patch and fringing reefs along ∼400 km of East African coastline. The reefs included five protected areas, of which three study sites were protected for more than 25 years, and four sites were protected less than 10 years. Habitat variables were often significantly associated with fish community variables in fringing reefs, but not in patch reefs. Fish diversity was positively correlated with hard coral and coralline algal cover, and negatively correlated with sea urchin and algal turf abundance. However, multiple regression analysis suggests that protection from fishing was the single strongest factor affecting fish abundan...

Spatial distribution of the most abundant sea urchin populations on the southeast coast of Sao Paulo (Brazil)

The spatial variability of Echinometra lucunter, Lytechinus variegatus and Arbacia lixula was studied in relation to four spatial scales (105, 104, 103 and 102 m) and two depths (0–3 and 3–8 m), along 100 km of the São Paulo coastline (August to October 1996). Echinometra lucunter was the most abundant species, with preference for shallow substrates subject to wave action. An abundance gradient was identified, increasing from southwest to northeast. Arbacia lixula showed the same pattern of spatial variation, but was more abundant in the deeper areas. Both species showed significant differences on a spatial scale of hundreds of meters, and E. lucunter also on a scale of kilometers. Lytechinus variegates presented a patchy distribution, being more abundant at certain sites. Variations in the water quality and natural heterogeneity of the habitat may explain the spatial distribution of these populations.

The effects of sea urchin grazing and drift algal blooms on a subtropical seagrass bed community.

Subtropical seagrass beds can be subject to relatively high levels of direct herbivory and large blooms of drift algae, both of which can have important effects on the floral and faunal components of the community. Caging experiments were used to investigate these factors in a Thalassia testudinum bed in Biscayne Bay, Florida. Abundance of sea urchins, Lytechinus variegatus, and drift algae was manipulated within the cages. Naturally occurring levels of urchin grazing do not appear to affect the T. testudinum population. With experimentally increased urchin densities in the winter, seagrass shoot density and aboveground biomass decreased significantly. Similar effects were not detected in the summer, indicating that the impact of grazing on T. testudinum is lessened during this time of year. Shoot density was more vulnerable to grazing than aboveground biomass. This may be a result of grazing-induced increases in seagrass productivity, in which the remaining shoots produce more or longer leaves. In the winter, drift algal blooms form large mats that cover the seagrass canopy. Under the normal grazing regime these algal blooms do not have significant negative effects on the seagrass. With increased grazing pressure, however, there is a synergistic effect of grazing and drift algae on seagrass shoot density. At intermediate urchin density (10 per m(-2)), cages without algae did not undergo significant decreases in shoot density, while those with algae did. At the high density of urchins, the number of seagrass shoots in cages both with and without algae decreased, but the effect was more pronounced for cages with algae. Invertebrate abundance at the field site was low relative to other seagrass beds. There were no discernible effects, either positive or negative, of urchin and algae manipulations on the sampled invertebrate community.

Predation and the Control of the Sea Urchin Echinometra viridis and Fleshy Algae in the Patch Reefs of Glovers Reef, Belize

The massive reduction in sea urchin Diadema antillarum populations since the mid-1980s has been associated with large increases in the abundance of fleshy algae on many Caribbean reefs despite the availability of other sea urchin and finfish grazers. This study examined the ecology of a grazer living sympatrically with D. antillarum, the common and abundant sea urchin Echinometra viridis. I examined the role that finfish and invertebrate predators play in controlling the distribution of E. viridis as well as the ability of this sea urchin to control exposed fleshy algae on the patch reefs of the Glovers Reef Atoll lagoon. I found that the major predators of this sea urchin were Calamus bajonado (jolthead porgy), Balistes vetula and Canthidermis sufflamen (queen and ocean triggerfish), Lachnolaimus maximus (hogfish), and a gastropod, probably Cassis madagascariensis. The abundance of E. viridis is constrained by predation, which restricts E. viridis to cryptic locations, such as crevices. Sea urchins bit a smaller percentage of experimental algal assays than finfish. Finfish herbivory was associated positively with patch reef topographic complexity. Unexpectedly, E. viridis abundance was positively correlated with fleshy algal abundance, but negatively correlated with the frequency of finfish bites. Predators restrict E. viridis to crevices and therefore reduce their influence on exposed fleshy algae, even at moderately high population densities (up to 10 per square meter). Since net benthic primary production of coral reefs is most strongly associated with herbivory on exposed surfaces, it would appear that E. viridis is unable to maintain the same production as reefs dominated by D. antillarum.

The effects of marine parks and fishing on coral reefs of northern Tanzania

The macrobenthic (coral, algae, and sea urchins) and fish communities in 15 back-reef sites on the patch and rock-island reefs of southern Kenya and northern Tanzania (∼250 km of coastline) were studied in order to (1) test an overfishing model developed in Kenya’s fringing reef ( McClanahan, 1995a, A coral reef ecosystem-fisheries model: impacts of fishing intensity and catch selection on reef structure and processes. Ecol. Model. 80, 1–19.), (2) develop a baseline of information on Tanzanian coral reef ecosystems, and (3) determine if some of the government gazetted but unprotected marine reserves were still deserving of protective management. The overfishing model was tested by comparing five sites in two fully protected reefs—one in southern Kenya (Kisite Marine National Park) and the other in Zanzibar (Chumbe Island Coral Park)—with 10 sites in eight fished reefs, and by comparing coral surveys conducted in reefs off of Dar es Salaam in 1974 with present-day studies. These comparisons suggest that fishing is primarily reducing the abundance of angelfish, butterflyfish, parrotfish, scavengers, surgeonfish, and triggerfish groups while some species of small-bodied damselfish and wrasse appear to have benefited. The total fish wet weight estimate was 3.5 times higher in protected than unprotected sites. Sea urchin abundance was six times higher, and predation rates on tethered sea urchin Echinometra mathaei were two times lower, in unprotected compared to protected sites. This is largely attributable to the reduction of the red-lined triggerfish Balistapus undulatus and other sea urchin predators by fishing. Loss of coral cover and changes in coral generic composition had occurred in four of the five sites visited in the Dar es Salaam area after the 22-year period. There was no evidence for species losses. One site appeared to be severely damaged over this time. Some reefs were dominated by fleshy brown algae, such as Sargassum and Dictyota, which may result from a loss of grazers and coral cover. Reduced fishing effort, elimination of destructive gear (dynamite and beach seines), protection of vulnerable species and, in some cases, sea urchin reductions could rectify the problems of overfishing. Despite the damage, the gazetted but unprotected reefs of Mbudya and Bongoyo still have high potential as marine protected areas due to the persistence of species and reef structure.

Temporal variability in abundance of the sea urchins Paracentrotus lividus and Arbacia lixula in the northwestern Mediterranean:comparison between a marine reserve and an unprotected area

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 168:135-145 (1998) - doi:10.3354/meps168135 Temporal variability in abundance of the sea urchins Paracentrotus lividus and Arbacia lixula in the northwestern Mediterranean: comparison between a marine reserve and an unprotected area Enric Sala1,*, Marta Ribes2, Bernat Hereu1, Mikel Zabala1, Victor Alvà2, Rafel Coma2, Joaquim Garrabou1 1Departament d¹Ecologia, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain 2Institut de Ciencies del Mar (CSIC), Passeig Joan de Borbó s/n, E-08039 Barcelona, Spain *Present address and address for correspondence: Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0201, USA. E-mail: esala@coast.ucsd.edu ABSTRACT: Sea urchin populations were monitored in the Medes Islands Marine Reserve (NW Mediterranean) and an adjacent unprotected area in order to (1) describe temporal variability in abundance and population size-structure of sea urchins within each area (from 1991 to 1997), and (2) to compare these areas to investigate the role of fish predation level in determining sea urchin population structure over time (from 1995 to 1997). Abundance of the sea urchin Paracentrotus lividus (Lamarck) was monitored at 7 sites (4 inside and 3 outside the reserve), and 2 distinct habitats (fields of big boulders and vertical walls). P. lividus exhibited significant variability in density over time on boulders, both inside and outside the reserve, whereas its density did not vary significantly on vertical walls. P. lividus populations differed significantly in size-structure across sites, varying from unimodal to bimodal among sites and years. Density and mean size of P. lividus were not significantly different between the protected and the unprotected area in either of the 2 habitats. Abundance of the sea urchin Arbacia lixula (L.) was monitored from 1995 to 1997 on vertical walls and boulders (4 sites inside and 3 sites outside the reserve). Density of A. lixula differed significantly over time on boulders, but it did not on walls. On vertical walls, density and mean size of A. lixula were not significantly different between areas in both habitats. This study shows that in the Medes Islands region, sea urchins exhibit striking short-term fluctuations in abundance, which can lead to misinterpretation of larger-scale temporal patterns. The comparison between the protected and the unprotected area does not support the hypothesis of fish predation as the most important factor affecting P. lividus populations in the Medes Islands, as patterns of lower sea urchin density relative to the unprotected area nearby were not maintained over time. These facts indicate that factors other than fish predation are very important in determining sea urchin population structure in the northwestern Mediterranean. KEY WORDS: Sea urchins · Temporal variability · Marine reserves · Fish predation · Mediterranean · Paracentrotus · Arbacia Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 168. Publication date: July 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Echinoid Bioerosion as a Major Structuring Force of Red Sea Coral Reefs.

Echinoid bioerosion is ecologically important as a limiting factor for reef growth. Diadema setosum and Echinometra mathaei are the most abundant sea urchins in the Gulf of Eilat, Red Sea. Bioerosion by these urchins was estimated from field experiments with urchins placed in underwater chambers. A novel methodology was developed for measuring the CaCO3 content of fecal pellets and total gut contents. This method is based on measurement of the amount of gas formed by total acidification of a mixed sample. The population density of the urchins was determined for the reef flat (depth 0.6-1 m) and the reef slope (depth 6-8 m). D. setosum, which erodes 310 mg individual-1 d-1, is much less abundant on the reef flat (0.1 individuals m-2) than on the reef slope (6.4 individuals m-2). In contrast, E. mathaei, which erodes 120 mg individual-1 d-1, is more dominant on the reef flat (10.5 individuals m-2) than on the reef slope (3.7 individuals m--2). We estimate that echinoid bioerosion converts to carbonate sediments 7%-11% of the total reef flat calcification and 13%-22% of the total reef slope calcification. These findings emphasize the importance of echinoid bioerosion as a structuring force in the coral reef, and its potential importance to the dynamics of reef development. However, they imply that some previous studies may have overestimated the role of echinoid bioerosion, thus possibly underestimating internal bioerosion.

Effect of Sea Urchin Reductions on Algae, Coral, and Fish Populations

Many heavily fished coral reefs have a high abundance of sea urchins that may suppress the recovery of fish and coral populations once fishing effort is reduced or eliminated. Restoration of these reefs may be accelerated by intervening and reducing sea urchin populations—particularly those of long‐lived species. We studied three Kenyan coral‐reef lagoons to determine the influence of reductions in sea urchin populations on coral, algae, and fish populations. Populations were monitored seven times over a 1‐year period in 50 × 50 m unmanipulated control plots and experimental sea urchin reduction plots where sea urchin populations were reduced by about 85%. Census of the most‐abundant fish families found the greatest positive population responses to sea urchin reduction in plots protected from fishing. The wet weights of fish nearly tripled, population density increased by 65%, and species richness increased by 30% compared with adjacent control plots. Parrotfish, wrasse, scavenger, and snapper families showed the greatest population and wet‐weight increases. In the two fished reefs, fish populations also increased but to a lesser degree than in the unfished reef—particularly when comparing the wet‐weight estimates of fish. Small‐bodied species of the dam‐selfish and wrasse families and juvenile parrotfish exhibited the largest population increases in these fished reefs. In fished reefs algae and seagrass cover exhibited the greatest increases following sea urchin reduction. Seagrass colonized bare sand, whereas fleshy brown algae colonized hard substrate (dead coral). The tall canopy‐forming alga Sargassum latifolium became the dominant alga in the fished reefs, whereas the more prostrate genus Padina dominated in the unfished reef. Fleshy algae cover in the unfished reef was about half that of the two fished reefs—attributable to the greater abundance of parrotfish in the unfished reef. This suggests that greater herbivory in the unfished reef kept the algae from reaching an algal forest climax typified by Sargassum dominance. In the fished reefs the high S. latifolium cover and the increased thickness of the algal turf reduced hard coral cover by around 30%. In the unfished reef coral cover was reduced by around 13%, but by the end of the experiment coral cover and genera richness were the same in both the control and the sea urchin reduction plots. We conclude that, primarily, fishing and, secondarily and indirectly, high sea urchin abundance are reducing fish numbers and diversity in Kenyan reefs and that sea urchin reduction has the potential to increase reef fisheries production and recovery from overfishing. Sea urchin reduction is not recommended, however, on moderately fished reefs due to the possible loss of coral cover and diversity. If reefs are either severely degraded or if fishing effort is reduced or eliminated, then sea urchin reduction is recommended for the restoration of fish numbers, feeding importance, species diversity, and fisheries production.

Effects of kelp forests Macrocystis pyrifera on the larval distribution and settlement of red and purple sea urchins Strongylocentrotus franciscanus and S. purpuratus

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 133:125-134 (1996) - doi:10.3354/meps133125 Effects of kelp forests Macrocystis pyrifera on the larval distribution and settlement of red and purple sea urchins Strongylocentrotus franciscanus and S. purpuratus Schroeter SC, Dixon JD, Ebert TA, Rankin JV It has often been observed that the abundance of sea urchins is lower inside kelp forests and along their nearshore edges than outside their offshore boundaries. We tested the hypothesis that this distributional pattern is a reflection of settlement patterns by monitoring settlement on artificial surfaces at 3 kelp forests near San Diego, California, USA. We estimated settlement at paired sites under the canopy and outside the offshore edge of the kelp forests and at paired sites located 20 m outside the canopy near the inshore and offshore boundaries of each kelp forest. There was no evidence of an effect of dense stands of giant kelp on the settlement of purple sea urchins. The results for red sea urchins were ambiguous; in 3 of 7 comparisons, average settlement was higher or observations of higher settlement were more frequent offshore from the kelp forest than elsewhere. Although this could result from different effects on the 2 species, we think it probably was due to accidents of sampling. We conclude that the distributional patterns of purple and red sea urchins relative to kelp forests are unlikely to reflect larval availability or settlement, but are more probably a function of post-settlement events. We qualify our conclusions because of the tremendous temporal variability in the physical and biological factors that can potentially affect the local distribution of sea urchin larvae. The 2 years of this study may be short relative to natural cycles. Echinoidea . Kelp . Larval filter hypothesis . Sea urchin . Settlement Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 133. Publication date: March 28, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Fish predators and scavengers of the sea urchinEchinometra mathaei in Kenyan coral-reef marine parks

Predation on 120 adult sea urchins of the speciesEchinometra mathaei was observed during daylight in shallow-water coral reefs (0.5 to 3 m deep) in a variety of sites in 3 Kenyan marine parks. The predators were few (8 species) and dominated by the triggerfishBalistapus undulatus (65 % of all observations) followed by terminal-male wrassesCoris formosa, C. aygula andCheilinus trilobatus, and lastly the scavengerLethrinus mahsena. Those species that attempted, but failed, to prey onE. mathaei were slightly more numerous (11 species), while scavengers of opened carcasses were the most speciose (20 species). Based on these observations, it is suggested thatB. undulatus is a ‘keystone predator’ and that fishery regulations that protect this species may be necessary in order to reduce the detrimental consequences of high sea urchin abundance — such as high reef substrate erosion and competitive exclusion of fishes.