On the dynamics of the mass mortality of Diadema antillarum in Barbados

Widespread mortality of the black sea urchin Diadema antillarum occurred in the Caribbean in 1983; beginning in Panama in January, and having its major impact at Barbados in September. Mortality on ten reefs surveyed in Barbados was 93.2%, with the highest being 99.9% and the lowest 86.9%. Mortality on each reef was independent of the pre-mortality density on the reef. Urchins with test diameters between 20 and 40 mm were more severely affected than smaller or larger urchins. Populations on reefs exposed to incoming oceanic water suffered heavier mortality than those on protected reefs. Mean size of urchins was smallest on high density reefs. This may indicate a negative effect of density on urchin growth. At post-mortality densities, urchins may grow faster and reach sexual maturity sooner.     

The commercial potential of the common sea urchin Echinus esculentus from the west coast of Scotland

Sea urchin roe is a valuable food product. The commercial potential of Echinus esculentusL. (Echinodermata: Echinoidea) was assessed both by surveying field populations and gonad growth in response to artificial feeds. Urchins were collected from four locations on the Scottish west coast between June and August of 1998. The roe content was found to be of variable quality. A relatively low percentage of urchins (34.9%) had roe which was considered marketable both in terms of yield and colour. Urchins with a marketable yield of roe were found where algal growth or encrusting organisms were most prolific. Good roe colours and high roe yields were not necessarily coincident. The data suggest an urchin fishery is likely to be impractical economically. In tank-based trials large E. esculentus were fed either commercially available salmon feed, a pelletised urchin diet or the macroalgae Laminaria saccharina. Urchins fed the artificial diets increased in test diameter during the 24-week experiment. At the end of the trial urchins fed salmon feed had a significantly higher gonad and alimentary index than those in the other treatments. E. esculentusis able to successfully exploit a diet rich in animal-origin oils. The diets also influenced gonad colour. The proportion of urchins with marketable gonad colours was highest (85%) in the treatment receiving salmon feed. As this species appears robust in culture and as artificial diets promote somatic and gonadal growth, an investigation into the practicalities of a fully farmed approach to gonad production may be worthwhile.     

The effect of the predator-avoidance behavior of the sea urchin,Centrostephanus coronatus,on the breadth of its diet

Summary The sea urchin, Centrostephanus coronatus, exhibits a relatively broad diet. Evidence presented in this paper suggests that evolution of dietary breadth is favored by two mechanisms. First, individual urchins occur in habitat types that support somewhat different foods. Urchins consume principally the most preferred food where it is common but are forced to eat other food types where it is rare. Second, overgrazing by individual urchins reduces the local abundance of the most preferred food, thereby forcing them to add less preferred items to their diet. Previous work indicates that the escape behavior of Centrostephanus from its fish predator, Pimelometopon pulchrum, results in different individuals being exposed to different foods and in reduction of the size of urchin grazing ranges to the point that overgrazing occurs. Hence, by the two mechanisms discussed in this paper, the presence of the fish predator favors the broad diet of its sea urchin prey.     

Changes in growth and reproduction of green sea urchins, Strongylocentrotus droebachiensis (Müller), during repopulation of the shallow subtidal zone after mass mortality

Following a disease outbreak that caused mass mortality of green sea urchins, Strongylocentrotus droebachiensis, along the Atlantic coast of Nova Scotia in September 1999, changes in growth and reproduction were monitored over 3.75 years as surviving individuals migrated from deep water to repopulate the shallow subtidal zone at a wave-exposed site. Urchins were sampled at 4 depth strata: at 24 m on a boulder field where the population was unaffected by the disease, at 12 and 16 m on a steeply sloping bedrock ramp, and at 8-10 m along the lower margin of a kelp bed (Laminaria digitata) where urchins formed a grazing front by January 2002. Urchins migrating shoreward from the deep-water refuge responded rapidly to increased algal productivity in the shallows through increased growth and reproduction. Measures of annual increments of skeletal elements (rotules) from urchins across the depth gradient indicated that the fastest growing individuals from the source population formed the grazing front. Urchins in the front reached a larger asymptotic size and produced larger gonads than urchins lower on the ramp. The annual cycle in gonad index showed a pronounced spring spawning period across all depths; a secondary fall spawning was evident at the front and 12 m. The presence of mature, fertilizable ova and short response time to spawning induction in both spring and fall supported the occurrence of two spawning periods.     

The contribution of single and double cones to spectral sensitivity in budgerigars during changing light conditions

Bird colour vision is mediated by single cones, while double cones and rods mediate luminance vision in bright and dim light, respectively. In daylight conditions, birds use colour vision to discriminate large objects such as fruit and plumage patches, and luminance vision to detect fine spatial detail and motion. However, decreasing light intensity favours achromatic mechanisms and eventually, in dim light, luminance vision outperforms colour vision in all visual tasks. We have used behavioural tests in budgerigars (Melopsittacus undulatus) to investigate how single cones, double cones and rods contribute to spectral sensitivity for large (3.4°) static monochromatic stimuli at light intensities ranging from 0.08 to 63.5 cd/m². We found no influences of rods at any intensity level. Single cones dominate the spectral sensitivity function at intensities above 1.1 cd/m², as predicted by a receptor noise-limited colour discrimination model. Below 1.1 cd/m², spectral sensitivity is lower than expected at all wavelengths except 575 nm, which corresponds to double cone function. We suggest that luminance vision mediated by double cones restores visual sensitivity when single cone sensitivity quickly decreases at light intensities close to the absolute threshold of colour vision.     

The larvae of Diadema setosum (Leske, 1778) (Camarodonta: Diadematidae) from South China Sea

Abstract

Diadema setosum (Leske, 1778) develops from small isolecithal eggs with a diameter of 84 ± 3 μm. Embryonic development took about 6.5–7 h and finished when a blastula left the fertilization envelope and became a larva. At this stage, the first pigment cells had appeared. At 23 h a prism developed; at 44 h a pluteus with one pair of arms had appeared; at 45 h of development plutei had two pairs of arms. The pigment cells colour the pluteus of D. setosum dark red. When 20-day-old larvae were mechanically stimulated, they flared their arms which may be defensive behaviour. During further development, the post oral arms of plutei grew to 1900 μm or more. Metamorphosis took place at about 40–45 days. At this time, five primary ambulacral podia were visible within the larval body. The duration of metamorphosis from the moment of larval settlement until the juvenile sea urchins began to move along the bottom was 40–60 min. The diameter of the test of the newly metamorphosed juvenile sea urchins was about 500 μm.     

Responses of the black sea urchin Tetrapygus niger to its sea-star predators Heliaster helianthus and Meyenaster gelatinosus under field conditions

We ran field experiments to examine the responses of the black sea urchin Tetrapygus niger to predatory sea stars. Trials involving simulated attacks (one or several arms of a sea star being placed on top of half the urchin) showed that the urchin differentiated between the predatory sea stars, Heliaster helianthus and Meyenaster gelatinosus, and a non-predatory sea star, Stichaster striatus, and showed almost no response to a sea star mimic. We further compared the responses of the urchin to different threat levels presented by the two predatory sea stars. The highest threat level was a simulated attack, then mere contact, and subsequently sea stars being placed at different distances from the urchin. All urchins responded to simulated attacks and contact with both sea stars. The proportion responding decreased with distance and more rapidly in trials with H. helianthus (0% at a distance of 30 cm) than with M. gelatinosus (33% at a distance of 50 cm). At each of the threat levels where there was a response to both sea stars, the urchins responded more rapidly to M. gelatinosus than to H. helianthus. In a third experiment where a predatory sea star was added to a circular area (1-m diameter) in which either 4-8 or 11-19 undisturbed urchins were present, the urchins fled the area more rapidly when the added sea star was M. gelatinosus, but the rate of fleeing did not vary with density, as might occur if there was communication among urchins using alarm signals. Our observations suggest that M. gelatinosus presents a stronger predatory threat than H. helianthus. This corresponds to field observations showing that the urchins are more frequently consumed by M. gelatinosus. These are the first field experiments demonstrating distance chemodetection by a marine invertebrate under back-and-forth water flow from wave activity.     

Crown-of-thorns starfish have true image forming vision

Background

Photoreceptors have evolved numerous times giving organisms the ability to detect light and respond to specific visual stimuli. Studies into the visual abilities of the Asteroidea (Echinodermata) have recently shown that species within this class have a more developed visual sense than previously thought and it has been demonstrated that starfish use visual information for orientation within their habitat. Whereas image forming eyes have been suggested for starfish, direct experimental proof of true spatial vision has not yet been obtained.

Results

The behavioural response of the coral reef inhabiting crown-of-thorns starfish (Acanthaster planci) was tested in controlled aquarium experiments using an array of stimuli to examine their visual performance. We presented starfish with various black-and-white shapes against a mid-intensity grey background, designed such that the animals would need to possess true spatial vision to detect these shapes. Starfish responded to black-and-white rectangles, but no directional response was found to black-and-white circles, despite equal areas of black and white. Additionally, we confirmed that starfish were attracted to black circles on a white background when the visual angle is larger than 14°. When changing the grey tone of the largest circle from black to white, we found responses to contrasts of 0.5 and up. The starfish were attracted to the dark area’s of the visual stimuli and were found to be both attracted and repelled by the visual targets.

Conclusions

For crown-of-thorns starfish, visual cues are essential for close range orientation towards objects, such as coral boulders, in the wild. These visually guided behaviours can be replicated in aquarium conditions. Our observation that crown-of-thorns starfish respond to black-and-white shapes on a mid-intensity grey background is the first direct proof of true spatial vision in starfish and in the phylum Echinodermata.

     

Early development and neurogenesis of Temnopleurus reevesii

Sea urchins are model non‐chordate deuterostomes, and studying the nervous system of their embryos can aid in the understanding of the universal mechanisms of neurogenesis. However, despite the long history of sea urchin embryology research, the molecular mechanisms of their neurogenesis have not been well investigated, in part because neurons appear relatively late during embryogenesis. In this study, we used the species Temnopleurus reevesii as a new sea urchin model and investigated the detail of its development and neurogenesis during early embryogenesis. We found that the embryos of T. reevesii were tolerant of high temperatures and could be cultured successfully at 15–30°C during early embryogenesis. At 30°C, the embryos developed rapidly enough that the neurons appeared at just after 24 h. This is faster than the development of other model urchins, such as Hemicentrotus pulcherrimus or Strongylocentrotus purpuratus. In addition, the body of the embryo was highly transparent, allowing the details of the neural network to be easily captured by ordinary epifluorescent and confocal microscopy without any additional treatments. Because of its rapid development and high transparency during embryogenesis, T. reevesii may be a suitable sea urchin model for studying neurogenesis. Moreover, the males and females are easily distinguishable, and the style of early cleavages is intriguingly unusual, suggesting that this sea urchin might be a good candidate for addressing not only neurology but also cell and developmental biology.     

Testing for the Effects of Seasonal and Lunar Periodicity on the Reproduction of the Edible Sea Urchin <Emphasis Type="Italic">Tripneustes gratilla</Emphasis> (L) in Kenyan Coral Reef Lagoons

The annual and lunar reproductive cycle of the widely distributed edible sea urchin Tripneustes gratilla (L) was examined through measurements of gonad index, histological examination of gametogenesis, and induction of spawning with KCl injections. The population density and morphological characteristics of urchins at Diani, Kanamai, and Vipingo reef lagoons were also studied as well as the effects of seawater temperature and light on reproduction. Gonad growth started early during the northeast monsoon and reached a peak in June at the beginning of the southeast monsoon followed by a sharp decrease in gonad size of 50% in July and August towards the end of the southeast monsoons. Histological examination of gonads, revealed many different stages of gametogenesis with gametes present throughout the year, indicating continuous reproduction. There was a significant relationship between gonad index and lunar day with spawning occurring between lunar day 7 and 21, but spawning was not in perfect synchrony in the population. The population density of urchins at each reef is variable from year to year and was highest on average at Vipingo. Urchins at Kanamai had the lowest gonad indices, the largest jaws and smallest individuals an indication of food limitation. The gonads (roe) of T. gratilla at all three sites, were perpetually ‘runny’ an attribute that is not suitable for urchin fisheries. Studies to develop techniques to improve roe quality are recommended.     

A laboratory study of the correlation between texture and the speed of locomotion by the sea urchin Hemicentrotus pulcherrimus

This laboratory study examined the relationship between substrate texture and movement speed of the sea urchin Hemicentrotus pulcherrimus. We assessed the movement speed of 14 sea urchins placed on either acrylic, or three types of waterproof sandpaper, on the bottom of a water tank. Images were taken at regular intervals and analyzed to determine the speed of each sea urchin. Light intensity was stronger at one end of the tank. Our analysis showed sea urchins moved away from light, at a speed that was negatively correlated with the roughness of the substrate, with slower movement on rougher surfaces. This result has implications for the design of equipment for capturing sea urchins in areas where their explosive population growth presents a threat to algal growth and reef environments.     

Evaluation of indicators of nitrogen limitation in deep prairie lakes with laboratory bioassays and limnocorrals

A comparison of Kenyan reefs of different historical and observed levels of fishing exploitation showed that more exploited reef lagoons had greater sea urchin densities and sizes, fewer and smaller fish and less coral cover. In the most exploited lagoon the biomass of the burrowing sea urchin Echinometra mathaei increased five fold during the previous 15 years. An ecological study of the three most common omnivorous sea urchin species inhabiting hard substrate within these reef lagoons (E. mathaei, Diadema savignyi and D. setosum) suggests that they are ecologically separated by predation and avoid predators and competitors by occupying different size burrows or crevices within the lagoon. Predator removal through fishing activities may result in ecological release of the sea urchins and result in competitive exclusion of weaker competitors. The most exploited reef had a nearly monospecific barren of E. mathaei living outside burrows suggesting that E. mathaei may be the top competitor. Its ecological release appears to lead to a decrease in live coral cover, increased substrate bioerosion and eventually a loss of topographic complexity, species diversity, fish biomass and utilizable fisheries productivity. Data from the outer reef edge were more difficult to interpret but may indicate similar patterns. Within this area, physical stresses such as waves and currents may be a greater controlling force in regulating fishing activities and coral reef community structure.     

Hyalomma truncatum (Acari; Ixodidae): evidence for the inability of adult ticks to discriminate between colours

Behavioural investigations into the perception and differentiation of coloured objects by unfed adult Hyalomma truncatum ticks revealed that silhouettes of blue, green, red and yellow colour, under illumination by a sun-simulating waveband spectrum, are perceived by the ticks and responded to equally by a directed response. Two green or dark grey rectangles each with a luminance contrast ratio of 5 : 1 against the white wall of the test arena in combination with an overlapping, equally sized dark grey or green target were consistently reached by ticks in a ratio of 2 : 1. Since the outer targets were occupied by the double number of ticks compared with the central silhouette this shows that the response is independent of the colour of the object. Investigations into target perception under monochromatic radiation of different wavelength ranges which were evenly adjusted in their irradiances revealed that ticks responded equally to a black target irradiated by blue, green, yellow and red light of wavelengths 428–472, 517–563, 549–591 and 606–654 nm, respectively. These results indicate the lack of true colour vision in H. truncatum.     

Can multitrophic interactions and ocean warming influence large‐scale kelp recovery?

Ongoing changes along the northeastern Atlantic coastline provide an opportunity to explore the influence of climate change and multitrophic interactions on the recovery of kelp. Here, vast areas of sea urchin‐dominated barren grounds have shifted back to kelp forests, in parallel with changes in sea temperature and predator abundances. We have compiled data from studies covering more than 1,500‐km coastline in northern Norway. The dataset has been used to identify regional patterns in kelp recovery and sea urchin recruitment, and to relate these to abiotic and biotic factors, including structurally complex substrates functioning as refuge for sea urchins. The study area covers a latitudinal gradient of temperature and different levels of predator pressure from the edible crab (Cancer pagurus) and the red king crab (Paralithodes camtschaticus). The population development of these two sea urchin predators and a possible predator on crabs, the coastal cod (Gadus morhua), were analyzed. In the southernmost and warmest region, kelp forests recovery and sea urchin recruitment are mainly low, although sea urchins might also be locally abundant. Further north, sea urchin barrens still dominate, and juvenile sea urchin densities are high. In the northernmost and cold region, kelp forests are recovering, despite high recruitment and densities of sea urchins. Here, sea urchins were found only in refuge habitats, whereas kelp recovery occurred mainly on open bedrock. The ocean warming, the increase in the abundance of edible crab in the south, and the increase in invasive red king crab in the north may explain the observed changes in kelp recovery and sea urchin distribution. The expansion of both crab species coincided with a population decline in the top‐predator coastal cod. The role of key species (sea urchins, kelp, cod, and crabs) and processes involved in structuring the community are hypothesized in a conceptual model, and the knowledge behind the suggested links and interactions is explored.     

Factors leading to cannibalism in Lytechinus variegatus (Echinodermata: Echinoidia) held in intensive culture

Ecologically, sea urchins are an important species in marine habitats around the world. Economically, sea urchins are prized for their gonads (uni). With fisheries declining worldwide, intensive sea urchin culture has been proposed. For urchins in commercial culture, any factor that affects survivorship negatively should be addressed to maximize cost benefit. One potential obstacle to optimizing culture of sea urchins is cannibalism. Approximately 2000 adult and juvenile Lytechinus variegatus (1 g-45 g) were collected from Port Saint Joseph Peninsula State Park, FL between June and September 2009. Urchins were held in recirculating tanks at different sizes, densities, and feeding regimes for 4 weeks. Starvation and high density contributed to the highest level of cannibalism among small (12-21 g) urchins (percent cannibalism = 18.8%), whereas fed, high density conditions contributed to the highest level of cannibalism among large (32-37 g) urchins (percent cannibalism = 18.4%). These results suggest that (1) small urchins cannibalize at higher rates than large urchins, and (2) increased density is an important contributing factor leading to cannibalism. We quantified stress, defined as a decrease in production as a result of environmental conditions, by evaluating weight gain within each treatment and suggest that weight loss or minimal weight gain is an indicator of stress. We hypothesize increased stress caused by competitive interference can lead to increased cannibalism and decreased growth rates, even when food is not limiting. Ecologically, there are no reports of cannibalism of urchins in wild populations. Consequently, the role of cannibalism in regulating sea urchin community structure is not known. However, factors affecting cannibalism of L. variegatus in the laboratory may provide insight into the conditions that could result in cannibalism in wild populations. From an aquaculture perspective, it is important to determine those factors that contribute to the incidence of cannibalism in sea urchins so that the appropriate culture conditions can be maintained to reduce the incidence of cannibalism.     

How bees detect coloured targets using different regions of their compound eyes

Honeybees Apis mellifera detect coloured targets presented to the frontal region of their compound eyes using their colour vision system at larger visual angles (α > 15°), and an achromatic visual system based on the long-wave photoreceptor type at smaller visual angles (5° < α < 15°). Here we examine the capability of the dorsal, ventral and frontal regions of the eye for colour detection. The minimum visual angle α_min at which the bees detect a stimulus providing both chromatic contrast and receptor-specific contrasts to the three receptor types varies for the different regions of the eye: 7.1 ± 0.5° for the ventral region, 8.2 ± 0.6° for the dorsal region and 4.0 ± 0.5° for the frontal region. Flight trajectories show that when the target was presented in the horizontal plane, bees used only the ventral region of their eyes to make their choices. When the targets appeared dorsally, bees used the frontodorsal region. This finding suggests that pure dorsal detection of coloured targets is difficult in this context. Furthermore, α_min in the ventral plane depends on receptor-specific contrasts. The absence of S-receptor contrast does not affect the performance (α_min = 5.9 ± 0.5°), whilst the absence of M- and L-receptor contrast significantly impairs the detection task. Minimal visual angles of 10.3 ± 0.9° and 17.6 ± 3°, respectively, are obtained in these cases. Thus, as for many visual tasks, the compound eye of the honeybee shows a regionalisation of colour detection that might be related to peripheral or central specialisations. Accepted: 28 September 1999     

Global patterns in the effects of predator declines on sea urchins

Latitudinal gradients in the strength of biotic interactions have long been proposed, but empirical evidence for the expectation of more intense predation, herbivory and competition at low latitudes has been mixed. Here, we use a meta‐analysis to test the prediction that predation pressure on sea urchins, a group of consumers with a particularly strong influence on community structure in the world's oceans, is strongest in the tropics. We then examine which biotic and abiotic factors best correlate with biogeographic and within habitat patterns in sea urchin responses to predation. Consistent with expectations, predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reef habitats (> 25° absolute latitude). However, latitude and temperature were weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. traits including behaviour, physical, and chemical defences) rather than large scale abiotic factors in determining variation in interaction strengths. Ecosystem‐shaping sea urchins such as Tripneustes gratilla, Diadema savignyi and Centrostephanus rodgersii were strongly impacted by consumers, indicating a tight coupling between predators of these species and their boom and bust prey. Anthropogenic activities such as over‐fishing, climate change and habitat destruction are causing rapid environmental change, and understanding how predation pressure varies with temperature, across habitats and among prey species, will aid in predicting the likelihood of ecosystem wide effects (via trophic cascades).     

Resistance of the heart sea urchin Echinocardium cordatum (Echinoidea: Spatangoida) to extreme environmental changes

The ability to survive under extreme environmental conditions was studied in the adults of the heart sea urchin Echinocardium cordatum (Pennant). At seawater temperatures of 13.3 to 14.8°C and salinity of 33.2–33.4‰, being devoid of the possibility to burrow into the sand or eat, some sea urchins died on day 5 and all individuals had perished by the end of day 8. At a temperature of 19°C, the salinity tolerance range of adults was limited to 33–28‰. Only 30 to 20% of sea urchins transferred to a solid substrate survived for 7 days at a salinity of 33 to 24‰, but all of them perished toward the end of day 8.     

Reduced density of the herbivorous urchin Diadema?antillarum inside a Caribbean marine reserve linked to increased predation pressure by fishes

Disease has dramatically reduced populations of the herbivorous urchin Diadema antillarum Philippi on Caribbean reefs, contributing to an increased abundance of macroalgae and reduction of coral cover. Therefore, recovery of D. antillarum populations is critically important, but densities are still low on many reefs. Among the many potential factors limiting these densities, the focus of this study is on predation pressure by fishes. Marine reserves provide opportunities to examine large-scale manipulations of predator–prey interactions and, therefore, D. antillarum densities were compared inside and outside a reserve in The Bahamas (Exuma Cays Land and Sea Park; ECLSP). Urchins and their fish predators were surveyed at nine sites inside and outside the ECLSP. Because of lower fishing effort, the total biomass of urchin predators, weighted by their dietary preferences for urchins, was significantly higher inside the ECLSP. Furthermore, fish community structure was significantly different inside the Park because of the increased biomass of the majority of species. No urchins were seen inside the ECLSP and this was significantly lower than the density of 0.04 urchin m⁻² outside the Park. Regression analysis indicated that the relationship between the biomass of urchin predators and the proportion of transects containing urchins was non-linear, suggesting that small increases in fish biomass dramatically reduce urchin abundances. The link between lower density of urchins and higher density of their predators inside the ECLSP is strengthened by discounting five alternative primary mechanisms (variations in macroalgal cover, larval supply, environmental setting, density of other urchin species and abundance of predators not surveyed). Caribbean marine reserves have an important conservation role, but increased fish predation appears to reduce densities of D. antillarum. Urchins currently have limited functional significance on Bahamian reefs, but any future recovery of D. antillarum is likely to be limited in reserves, with potentially important ecological consequences. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Environmental factors influencing urchin spatial distributions on disturbed coral reefs (New Caledonia, South Pacific)

Few works have examined the relative contributions of habitat variables to the distribution of coral reef urchins. In the present study, the spatial distribution of two common urchin species (Diadema setosum and Echinometra mathaei) was studied in the fringing reefs of two urban bays in New Caledonia (South Pacific). Urchins were surveyed at 105 stations with contrasted habitat structure/anthropic disturbance levels; 32 environmental variables (water/sediment characteristics, reef structuring species) were considered. Moderate densities were generally observed at station scale (mean 0.5 individuals m^− 2). The combination of univariate and multivariate techniques highlighted patchy distributions for Diadema as well as Echinometra, with distinct species/habitat associations; environmental gradients occurring within the bays did not seem to influence the species patterns. For Diadema, the spatial variability was better explained by sediment type than by biotic cover; increasing densities occurred across habitats with larger sediment sizes and decreasing coral complexity/macrophytes cover. In contrast, the distribution of E. mathaei exhibited weak relationships with habitat variables. In coral reefs, small-scale heterogeneity may thus be responsible for most of urchins spatial variability.