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.     

Predation of the sea urchin Heliocidaris erythrogramma by rock lobsters (Jasus edwardsii) in no-take marine reserves

The formation of sea urchin ‘barrens’ on shallow temperate rocky reefs is well documented. However there has been much conjecture about the underlying mechanisms leading to sea urchin barrens, and relatively little experimentation to test these ideas critically. We conducted a series of manipulative experiments to determine whether predation mortality is an important mechanism structuring populations of the sea urchin Heliocidaris erythrogramma in Tasmania. Tethered juvenile and adult sea urchins experienced much higher rates of mortality inside no-take marine reserves where sea urchin predators were abundant compared to adjacent fished areas where predators were fewer. Mortality of tagged (but not tethered) sea urchins was also notably higher in marine reserves than in adjacent areas open to fishing. When a range of sizes of sea urchins was exposed to three sizes of rock lobsters in a caging experiment, juvenile sea urchins were eaten more frequently than larger sea urchins by all sizes of rock lobster, but only the largest rock lobsters (> 120 mm CL) were able to consume large adult sea urchins. Tagging (but not tethering) juvenile and adult sea urchins in two separate marine reserves indicated that adult sea urchins experience higher predation mortality than juveniles, probably because juveniles can shelter in cryptic microhabitat more effectively. In a field experiment in which exposure of sea urchins to rock lobster (Jasus edwardsii) and demersal reef fish predators was manipulated, rock lobsters were shown to be more important than fish as predators of adult sea urchins in a marine reserve. We conclude that predators, and particularly rock lobsters, exert significant predation mortality on H. erythrogramma in Tasmanian marine reserves, and that adult sea urchins are more vulnerable than smaller cryptic individuals. Fishing of rock lobsters is likely to reduce an important component of mortality in H. erythrogramma populations.     

How diet influences energy partitioning in the regular echinoid Psammechinus miliaris; constructing an energy budget

The effect of diet (made from either animal or plant material) on the partitioning of energy in small (8-16 mm test diameter (td)) and large (29-37 mm td) Psammechinus miliaris was examined. Diet significantly affected ingestion, digestion and growth of both size groups. Assimilation rates of the different types of feed varied and they supported differential development of the body parts. Urchins fed on an algal diet showed poorer absorption efficiency, assimilation, gonadal and somatic growth than individuals fed on an artificial sea urchin diet (mixed plant and animal material) or on the animal based diets. Small urchins fed on a diet of salmon food utilised proteins as the primary energy source rather than carbohydrates or lipids as indicated by low O/N atomic ratios, and salmon diet promoted a higher energetic investment in reproductive development in both size classes. A negative energy balance resulted when large urchins were fed on the algal diet. Energy losses due to ammonia excretion were negligible and the metabolic losses of assimilated energy, measured as oxygen consumption, were between 0.2% and 1.5% with the different diets. A diet made of mussel flesh stimulated ingestion, gave the highest assimilation rates and best overall growth performance. Such detailed information should assist in the design of diets for the cultivation of sea urchins.     

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.     

Impact of CO2-acidified seawater on the extracellular acid-base balance of the northern sea urchin Strongylocentrotus dröebachiensis

We investigated the effect of five day exposure to CO₂-acidified sea water treatments (pH_NBS = 7.89 [control], 7.44, 7.16 and 6.78, T = 9.5 °C) on the extracellular acid-base balance of the northern sea urchin Strongylocentrotus dröebachiensis. In each case there was an uncompensated respiratory acidosis which increased in intensity with decreasing environmental pH. This was very similar to results for another sea urchin species, Psammechinus miliaris (8 d exposure, T = 15 °C). However, there were some important differences in the response to low seawater pH between the two urchin species S. dröebachiensis and P. miliaris. At the lowest pH tested (6.78) there was a metabolic component to this acidosis recorded (correlated with a significant increase in l-lactate) in S. dröebachiensis but not P. miliaris. The acidosis was accompanied by a very small, but significant increase in coelomic fluid calcium. Also the water used in our study was (controlling for pH) markedly undersaturated with respect to carbonate compared with that used in the Psammechinus study, highlighting the need for the environmental context to be assessed in future comparative studies.     

遮蔽行为对海刺猬摄食、生长和性腺性状的影响

海胆遮蔽行为对于海胆的生长和发育的影响目前还不清楚,在实验室条件下,研究了遮蔽行为对海刺猬(Glyptocidariscrenularis)摄食、生长和性腺性状的影响。实验设两个对照组和一个处理组:对照组为无任何遮蔽条件的空白组和以堆积砖块创造的黑暗空间来满足海胆的掩蔽行为的掩蔽组,遮蔽组则以菲律宾蛤仔(Ruditapes philippinarum)的贝壳作为遮蔽物来满足海胆的遮蔽行为。结果显示:遮蔽组的海胆周摄食量要显著大于掩蔽组(P<0.05),而空白组与遮蔽组和掩蔽组之间的差异不显著(P>0.05);各组海胆的存活率没有显著差异(P>0.05);空白组和遮蔽组海胆的壳径、壳高、体重要显著大于掩蔽组(P<0.05),而空白组和遮蔽组之间没有显著性的差异(P>0.05);空白组和遮蔽组的性腺湿重、性腺干重、性腺指数、壳湿重、壳干重、亚氏提灯湿重、亚氏提灯干重要显著大于掩蔽组(P<0.05),而空白组和遮蔽组之间没有显著性的差异(P>0.05);空白组的提灯指数要显著小于遮蔽组和掩蔽组(P<0.05),而遮蔽组和掩蔽组之间没有显著的差异(P>0.05)。结果证明,遮蔽和掩蔽行为对海刺猬生长及发育的影响是有差别的,掩蔽行为对海刺猬的生长与发育造成了负面影响,而遮蔽行为在一定的生活环境条件下不会对其生长造成影响。研究表明,海刺猬的遮蔽行为反映了其对外界不利环境条件的一种适应性,且这种行为对其生长和发育的影响与掩蔽行为有所不同。至于海刺猬遮蔽行为的进化选择压力,目前的研究还不足以下定论,需要进一步深入的研究。     

不同实验生态环境对海刺猬遮蔽行为的影响

研究了较长时间生活在3种不同实验遮蔽条件下海刺猬(Glyptocidaris crenularis)的遮蔽行为特点。结果表明:不同生活环境下海刺猬都保持着遮蔽行为。生活在以贝壳作为遮蔽材料环境下(遮蔽组)和以砖块作为掩蔽材料环境下(掩蔽组)的海刺猬初次遮蔽耗时要显著短于生活在无遮蔽(或掩蔽)材料环境下(空白组)的海刺猬(N=3,P<0.05)。3组海刺猬用于遮蔽的贝壳总数和有遮蔽行为的海刺猬总数都呈现先增加后趋于平稳的趋势。掩蔽组有遮蔽行为的海刺猬总数要显著多于遮蔽组和空白组(P<0.05),后两者差异不显著。3组海刺猬用于遮蔽的贝壳总数差异不显著(P>0.05)。海刺猬遮蔽时对两种贝壳(菲律宾蛤仔和贻贝)存在显著的选择差异(P<0.05)。生活环境中一段时间内遮蔽物的缺失并不会使其失去这种行为,但是会在一定程度上影响该行为的强度。因此,光照很可能是海刺猬遮蔽行为的一个进化压力,该行为也许只是作为一种避光策略。海刺猬对遮蔽材料具有显著的选择性,这可能与遮蔽材料自身特征和海刺猬的生理状态相关。     

Organismal defenses versus environmentally mediated protection from herbivores: Unraveling the puzzling case of Desmarestia viridis (Phaeophyta)

The role of anti-herbivore organismal defenses in algae-herbivore interaction is frequently investigated without taking into account the potential role of environmental factors in mediating the interaction. Here we reexamine the interaction between the highly acidic, brown alga Desmarestia viridis and the green sea urchin, Strongylocentrotus droebachiensis, by incorporating a previously overlooked facet, the effect of changes in the wave environment on the ability of the urchin to establish contact with the alga. Factorial experiments in a wave tank (presence versus absence of waves; real versus mimic algae) showed that the aggregation of urchins on D. viridis was more than 2-fold greater in the absence than in the presence of waves. Similar numbers of urchins made contact with natural and mimic D. viridis plants, both with and without waves, indicating that any external release of chemicals (acid) from the alga had no perceptible repulsive effect on the urchin. The ability of the urchins to climb onto D. viridis increased markedly when urchin density attained a critical level. These results were consistent with field observations that urchins readily attack D. viridis under conditions of low wave action but do not under conditions of moderate wave action. We conclude (1) that the chemical makeup of D. viridis alone is neither necessary nor sufficient to limit contacts by the urchins and that (2) wave action is a major factor explaining the survival of D. viridis on urchin barrens, because waves limit the movements of the urchins towards the alga. We recommend that studies addressing marine algal defenses against herbivores be more comprehensive and examine interactions between algal traits, the physical environment, and the abundance and behavioral repertoire of herbivores.     

Random movement pattern of the sea urchin Strongylocentrotus droebachiensis

We describe the fine-scale movement of the sea urchin Strongylocentrotus droebachiensis based on analyses of video recordings of undisturbed individuals in the two habitats which mainly differed in food availability, urchin barrens and grazing front. Urchin activity decreased as urchin density increased. Individuals alternated between moving and being stationary and their behaviour did not appear to be affected by either current velocity (within the range from 0 to 15 cm s^− 1) and temperature (2.3 to 6.0 °C). Movement of individuals at each location was compared to that predicted by a random walk model. Mean move length (linear distance between two stationary periods), turning angle and net squared displacement were calculated for each individual. The distribution of turning angles was uniform at each location and there was no evidence of a relationship between urchin density and either move length or urchin velocity. The random model predicted a higher dispersal rate at locations with low urchin densities, such as barrens habitats. However, the movement was sometimes greater or less than predicted by the model, suggesting the influence of local environmental factors. The deviation of individual paths from the model revealed that urchins can be stationary or adopt a local (displacement less than random), random or directional movement. The net daily distance displaced on the barrens, predicted by a random walk model, was similar to the observed movement recorded in our previous study of tagged urchins at one site, but less than that observed at a second site. We postulate that the random dispersal of urchins allows individuals on barrens to reach the kelp zone where food is more abundant although the time required to reach the kelp zone may be considerable (months to years). Urchins decrease their rate of dispersal once they reach the kelp zone so that they likely remain close to this abundant food sources for long periods.     

Absorption efficiencies of Lytechinus variegatus (Lamarck) (Echinodermata: Echinoidea) for selected marine plants

The absorption efficiencies of the sea urchin Lytechinus variegatus (Lamarck) were calculated by an indicator method. Absorption efficiencies for total organic material, protein, lipid, and carbohydrate were highest with Halimeda incrassata Ellis as food (43 %, 65 %, 71 % and 35 %, respectively), and lowest with Eucheuma isiforme C. Agardh as food (?35%, ?42%, ?56%and ?34%, respectively). Moderately positive absorption efficiencies for total organic material were found with Thalassia testudinum König & Sims and Ulva lactuca L. as food, and low positive absorption efficiencies for total organic material are found with Syringodium filiformis Kützing and Sargassum sp. as food. Absorption efficiencies of the sea urchin Strongylocentrotus purpuratus (Stimpson) (calculated from the literature) were much greater than those of Lytechinus for carbohydrate, but less for protein and lipid. Both sea urchins showed negative efficiencies for some nutrient fractions of their foods. The absorption index (mg nutrient fraction absorbed/g dry wt ingested) showed that Thalassia supplied the most protein and total organic material (43 mg/g dry wt ingested and 142 mg/g dry wt ingested, respectively). In general, the absorption indices indicate that Lytechinus obtains more protein and lipid, but less carbohydrate from its food than does Strongylocentrotus. In the field Lytechinus has primarily a detrital diet.     

Identification and characterization of PlAlix,the Alix homologue from the Mediterranean sea urchin Paracentrotus lividus

The sea urchin provides a relatively simple and tractable system for analyzing the early stages of embryo development. Here, we use the sea urchin species, Paracentrotus lividus, to investigate the role of Alix in key stages of embryogenesis, namely the egg fertilization and the first cleavage division. Alix is a multifunctional protein involved in different cellular processes including endocytic membrane trafficking, filamentous (F)‐actin remodeling, and cytokinesis. Alix homologues have been identified in different metazoans; in these organisms, Alix is involved in oogenesis and in determination/differentiation events during embryo development. Herein, we describe the identification of the sea urchin homologue of Alix, PlAlix. The deduced amino acid sequence shows that Alix is highly conserved in sea urchins. Accordingly, we detect the PlAlix protein cross‐reacting with monoclonal Alix antibodies in extracts from P. lividus, at different developmental stages. Focusing on the role of PlAlix during early embryogenesis we found that PlAlix is a maternal protein that is expressed at increasingly higher levels from fertilization to the 2‐cell stage embryo. In sea urchin eggs, PlAlix localizes throughout the cytoplasm with a punctuated pattern and, soon after fertilization, accumulates in larger puncta in the cytosol, and in microvilli‐like protrusions. Together our data show that PlAlix is structurally conserved from sea urchin to mammals and may open new lines of inquiry into the role of Alix during the early stages of embryo development.     

Consumers of sea urchins,<Emphasis Type="Italic"> Paracentrotus lividus</Emphasis> and<Emphasis Type="Italic"> Arbacia lixula</Emphasis>, in shallow Mediterranean rocky reefs

Underwater observations on fish and asteroid consumers (i.e. predators and scavengers) of sea urchins, Paracentrotus lividus and Arbacia lixula, were carried out at several locations in shallow Mediterranean rocky reefs. Observations conducted in the marine reserve of Torre Guaceto (Adriatic Sea) revealed that sparid fishes, Diplodus sargus and D. vulgaris, are the main fish predators of small (<1 cm in test diameter) and medium (1–4 cm) sea urchins, whereas the labrids Coris julis and Thalassoma pavo preyed only upon small sea urchins. Large D. sargus were able to prey upon small and medium, and occasionally large (>4 cm) sea urchins, whereas medium and small Diplodus preyed mainly upon small sea urchins. The number of sea urchins preyed upon by fishes was negatively related to sea urchin size for both species. P. lividus appeared to be subject to higher predation levels than A. lixula. The scavenger guild comprised 11 fish species, with D. sargus, D. vulgaris, Coris julis and Chromis chromis accounting for about 80% of scavenger fishes. Observations performed at several locations in the Mediterranean on the predatory asteroid Marthasterias glacialis revealed that only 3% of the detected individuals were preying upon sea urchins. Due to the importance of sea urchins for assemblage structure and functioning of Mediterranean rocky reef ecosystems, these results may have also important implications for management of fishing activities.Communicated by H.-D. Franke     

The effect of dietary protein and carbohydrate concentration on the biochemical composition and gametogenic condition of the sea urchin Lytechinus variegatus

Previously starved urchins, Lytechinus variegatus, (36.0 ± 0.8 (SE) mm test diameter) were held in replicated (3) 10-L aquaria with artificial seawater at 22 ± 2  °C and 32‰ salinity and fed three diet treatments. Urchins were fed diets containing 9 : 35, 20 : 23 or 31 : 12% dry protein: % dry carbohydrate (P : C) ad libitum for a 65-day period. Gonads from urchins fed the 9 : 35 P : C diet had similar organic, lower ash, and lower water content than urchins fed the 31 : 12 P : C protein diet. Water content varied with both diet and nutritional history; consequently, water content may have limited value as a predictor of gonad nutritional status. Protein and carbohydrate concentrations in the gonad were directly related to the dietary composition of these nutrients; gonad lipids did not vary with diet. Excess carbohydrates are frequently stored as fats in fish and mammals but this does not appear to be the case in L. variegatus. Test carbohydrate storage and gut protein storage also reflected dietary composition. Image analysis of ovaries indicated decreased nutritive phagocyte volume, increased germinal epithelium volume and larger oocyte diameters in urchins fed high protein, low carbohydrate diets. Analysis of testes also indicated decreased nutritive phagocyte volume and increased gamete volume with urchins fed high protein, low carbohydrate diets, but differences among treatments were less obvious than in ovaries. This study suggests that high protein, low carbohydrate diets promote gamete growth and development. In addition, the biochemical and gametic composition of gonads can be altered by manipulating dietary composition. This could affect the quality and value of sea urchin roe for human consumption.     

Long-Term Changes in the Gonad Condition of the Sea Urchin Strongylocentrotus intermediusfrom Alekseev Bight (Amurskii Bay) under Polluted Conditions

The gonad condition of the sea urchin Strongylocentrotus intermediuscollected in August 1997 at two stations in Peter the Great Bay was examined. One of the stations was located in a polluted area (Alekseev Bight, Popov Island) and the other, in a relatively clean area (the Verkhovskii Islands). The results were compared with analogous data for 1984, 1985, and 1989. In 1997, the gonad condition of sea urchins inhabiting the two areas differed significantly. The mean value of the gonad index (GI) for sea urchins from Alekseev Bight was less than half and the maturity index was about twice that of sea urchins from the Verkhovskii Islands. The GI of sea urchins from Alekseev Bight decreased by a factor of 1.5 between 1984 and 1997. Pronounced histopathological changes were found in sea urchin gonads at this station: granular and hydropic dystrophy of oocytes, resorption and a sharp decrease in the number of gametes (in about 20% of the sea urchins, hardly any gametes were found in the gonads), changes in the morphology of accessory cells (hypertrophy, atrophy, and necrosis), and accumulation of lipofuscin in the cytoplasm of accessory cells and oocytes, in the hemal sinuses and mesentery. The suppressed gonad condition of the sea urchin S. intermediusin Alekseev Bight may be a consequence of the unfavorable environmental situation that formed in the bight in the 1980s–1990s. The main negative factor is anthropogenic pollution of Amurskii Bay.     

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.     

Age determination in the sea urchin paracentrotus lividus (Lamarck), with notes on the reproductive cycle

Evidence from other species of sea urchins indicated that the growth rings found in the skeletal plates of Paracentrotus lividus (Lamarck) were probably formed annually, and could therefore be used as a means of determining age. In this species, annual formation of the rings has been proved by the similarity of growth curves derived from ring counts and size-frequency analyses; there is a good agreement between the annual increments predicted from these growth curves and those observed in experimental populations. Determinations of the cycles of ring deposition and reproduction gave results which were consistent with this interpretation.Similarities in the growth curves did not always extend to the mean values determined for each ring-class and the equivalent size-frequency groups, and the size-frequency distributions of single ring-classes were found to be much wider than those of the equivalent groups determined by sizefrequency analysis. This is attributed to a wide variation in the season of ring deposition in different individuals, and this limits the accuracy of ring count ageing to about ± one year.There were no significant differences between the reproductive cycles of male and female urchins; the gonad indices rose to peak values in late winter and late summer. Gonad condition was very variable within each population sample. The animals grew to a mean diameter of 35–50 mm in about four years, after which growth rates slowed down, and it is probable that most of the urchins died of senescence after 6–9 years.     

Functional variants of 5S rRNA in the ribosomes of common sea urchin Paracentrotus lividus

We have previously reported a molecular and cytogenetic characterization of three different 5S rDNA clusters in the sea urchin Paracentrotus lividus; this study, performed at DNA level only, lends itself as starting point to verify that these clusters could contain transcribed genes, then, to demonstrate the presence of heterogeneity at functional RNA level, also. In the present work we report in P. lividus ribosomes the existence of several transcribed variants of the 5S rRNA and we associate all transcribed variants to the cluster to which belong.     

Benthic assemblages and temperature effects on Paracentrotus lividus and Arbacia lixula larvae and settlement

Recruitment is a principal controlling factor in population dynamics of marine species. In marine invertebrates with a planktonic larval stage, such as echinoids, recruitment is assured by larval supply, settlement and juvenile survival. Larval supply and juvenile survival are affected by a wide range of factors, including temperature, presence of predators, quality and quantity of food. Echinoid larval settlement is mainly conditioned by the finding of a suitable substrate to metamorphose. The sea urchins Arbacia lixula and Paracentrotus lividus are considered key species of the Mediterranean infralittoral rocky shores. At high densities, the grazing activity of both species can produce and maintain barren grounds, a particular habitat condition characterized by extremely low cover values of erect algae with high presence of naked substrates and encrusting corallinales, poor in biodiversity and ecosystem functions. We tested the role of different settlement substrates on the metamorphosis competent larvae of the two species. Furthermore, from our larval rearing trails we were able to identify strong temperature effects on larval development of the two species. P. lividus and A. lixula larvae have been reared at 18 °C but for the second species it was necessary to use higher temperatures (22 °C) to perform settlement experiments, as in the 18 °C set all larvae died in the first week. Both species larvae have been fed Cricosphaera elongata. Metamorphosis of competent larvae has been induced using different substrates: naked stones, Lithophyllum incrustans, Stypocaulon scoparium, Corallina elongata, turf forming algae and Posidonia oceanica. For each species, two larval batches were used for settlement experiments; for each larval batch two replicates/substrates were set up. No differences in the rate of metamorphosis on any of the tested substrates were observed for P. lividus, while A. lixula showed to prefer naked stones and encrusting coralline algae Considering that A. lixula population growth may trigger barren extension on rocky shores, this may lead to a positive feedback between barren extension and A. lixula population density. Furthermore, our results suggest that the predicted rise in seawater temperature may favor A. lixula larval survival and inhibit P. lividus. Combining information on temperature tolerance with other sources of information for these species in the Mediterranean, it is possible to develop a conceptual model of the interaction between the two species and the alternative state of their habitats.     

Prior residence and body size influence interactions between black sea urchins

Body size and prior residence can modulate agonistic interaction in several animal species, but scientists know little about these relationships in echinoderms. In this study, we tested the effects of these traits on interactions in the black sea urchin (Echinometra lucunter). After a sea urchin was isolated for 24-h in a glass tank to establish prior residence, we introduced an intruder animal adjacent to the resident in the tank and observed interactions for 30 min. The intruder animal was larger, smaller, or size-matched to the resident. We found body size and prior residence concomitantly modulated interactions among black sea urchins, with prior residence as the major determinant. Black sea urchins mainly exhibited opponent inspection and fleeing responses during interaction to avoid fights, especially when a fight could be seriously disadvantageous (small intruder vs. large resident).