Rates of fission, somatic growth and gonadal development of a fissiparous sea star, Allostichaster insignis, in New Zealand

Seasonal changes in population structure and incidence of fission were measured in intertidal and subtidal populations of Allostichaster insignis, a fissiparous sea star. Population size structure was stable over the course of the 1-year study. Sea stars in the subtidal zone attained greater maximum size (mean arm length, R = 35 mm) than those in the intertidal population (20 mm). Fission rates were greatest among small individuals (R < 20 mm). The frequency of fission ranged from 5 to 32% with peaks in early austral summer in the intertidal zone, and in autumn and winter in the subtidal zone. Sexual reproduction occurred in early spring in sea stars larger than 12 mm. The populations were heavily biased toward males. In the laboratory, A. insignis of three size classes (small, R = 9–13 mm; medium, 19–21 mm; and large, 29–31 mm) were fed mussels ad libitum or starved (not fed macroscopic food) for ∼1 year in a 3 × 2 factorial experiment. Small and medium-sized sea stars divided throughout the experiment and the ramets of most individuals regenerated sufficiently to divide again after 6–9 months. Unfed sea stars did not undergo fission (with one exception), had a higher mortality rate, and did not grow. Small, fed sea stars grew significantly faster than medium-sized or large individuals. At the end of the experiment, the pyloric caeca index (a measure of nutritional condition) was greater in fed than in unfed animals. Gonads (only testes were observed) developed in medium-sized and large, fed sea stars. Our field and laboratory results indicate that asexual reproduction in A. insignis predominantly occurs in small, well-nourished individuals. Ramets grow gradually through repeated fission and regeneration to a size (mean length of regenerating arms, R _r ∼ 20 mm) at which they begin to switch to sexual reproduction as the dominant reproductive mode.     

Population dynamics,reproduction and growth of the Indo-Pacific horned sea star, <Emphasis Type="Italic">Protoreaster nodosus</Emphasis> (Echinodermata; Asteroidea)

The horned sea star (Protoreaster nodosus) is relatively common in the Indo-Pacific region, but there is little information about its biology. This study of the population biology of P. nodosus was carried out in Davao Gulf, The Philippines (7°5′N, 125°45′E) between September 2006 and May 2008. Protoreaster nodosus was found in sand and seagrass dominated habitats at a mean density of 29 specimens per 100 m² and a mean biomass of 7.4 kg per 100 m², whereas a significantly lower density and biomass was found in coral and rock dominated habitats. Adult specimens (mean radius R = 10.0 cm) were found at depths of 0–37 m, whereas juveniles (R < 8 cm) were only found in shallow sandy habitats with abundant seagrass (water depth ≤2 m). Increased gonad weights were found from March to May (spawning period), which coincided with an increasing water temperature and a decreasing salinity. Density and biomass did not change significantly during reproduction, but sea stars avoided intertidal habitats. All specimens with R > 8 cm had well developed gonads and their sex ratio was 1:1. Protoreaster nodosus grew relatively slowly in an enclosure as described by the exponential function G = 7.433 e^{−0.257 × R} . Maturing specimens (R = 6–8 cm) were estimated to have an age of 2–3 years. Specimens with a radius of 10 cm (population mean) were calculated to have an age of 5–6 years, while the maximum age (R = 14 cm) was estimated as 17 years. Potential effects of ornamental collection on the sea star populations are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Biochemical composition,energy content and chemical antifeedant and antifoulant defenses of the colonial Antarctic ascidian<Emphasis Type="Italic"> Distaplia cylindrica</Emphasis>

The colonial ascidian Distaplia cylindrica occurs both as scattered individual colonies or in gardens of colonies in fine-grained soft substrata below 20 m depths off Anvers Island along the Antarctic Peninsula. Individual colonies, shaped as tall rod-like cylinders and anchored in the sediments by a bulbous base, may measure up to 7 m in height. D. cylindrica represent a considerable source of materials and energy for prospective predators, as well as potential surface area for fouling organisms. Nonetheless, qualitative in situ observations provided no evidence of predation by sympatric predators such as abundant sea stars, nor obvious biofouling of colony surfaces. Mean energy content of whole-colony tissue of D. cylindrica was relatively high for an ascidian (14.7 kJ g^–1 dry wt), with most of this energy attributable to protein (12.7 kJ g^–1 dry wt). The sympatric omnivorous sea star Odontaster validus consistently rejected pieces of D. cylindrica colonies in laboratory feeding assays, while readily ingesting similarly sized alginate food pellets. Feeding deterrence was determined to be attributable to defensive chemistry, as colonies of D. cylindrica are nutritionally attractive and lack physical protection (conspicuous skeletal elements or a tough outer tunic), and O. validus display significant feeding-deterrent responses to alginate food pellets containing tissue-level concentrations of organic extracts. In addition, high acidity measured on outer colony surfaces (pH 1.5) as well as homogenized whole-colony tissues (pH 2.5) are indicative of surface sequestration of inorganic acids. Agar food pellets prepared at tissue levels of acidity resulted in significant feeding deterrence in sea stars. Thus, both inorganic acids and secondary metabolites contribute to chemical feeding defenses. D. cylindrica also possesses potent antifoulant secondary metabolites. Tissue-level concentrations of hydrophilic and lipophilic extracts caused significant mortality in a sympatric pennate diatom. Chemical feeding deterrents and antifoulants are likely to contribute to the abundance of D. cylindrica and, in turn, play a role in regulating energy transfer and community structure in benthic marine environments surrounding Antarctica.Communicated by P.W. Sammarco, Chauvin     

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

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

Discrimination of ocean wave features by hatchling loggerhead sea turtles, Caretta caretta

At the beginning of their offshore migration, hatchling sea turtles orient directly into oceanic waves as they swim away from land. Recent experiments have demonstrated that hatchlings swimming underwater can determine the propagation direction of waves by monitoring the circular movements they experience as waves pass above. During July and August 1993, we studied how loggerhead sea turtle hatchlings (Caretta caretta L.) from the east coast of Florida, USA, responded to a range of wave parameters. We constructed a wave simulator to reproduce in air the circular movements that normally occur beneath small ocean waves. Hatchlings suspended in air and subjected to these orbital movements attempted to orient into simulated waves when periods and amplitudes were similar to those found near the Florida coast. Orbital movements with longer periods (greater than 10 s), however, failed to elicit responses. The results demonstrate that hatchling loggerheads can distinguish between waves with different periods and amplitudes, and that Florida hatchlings respond most strongly to orbital movements closely resembling those of waves that occur near their natal beach. Received: 28 May 1996 / Accepted: 17 September 1996     

Feeding aggregations of sea stars (<Emphasis Type="Italic">Asterias</Emphasis> spp. and <Emphasis Type="Italic">Henricia sanguinolenta</Emphasis>) associated with sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) grazing fronts in Nova Scotia

Migrating feeding aggregations (or fronts) of sea urchins can dramatically alter subtidal seascapes by destructively grazing macrophytes. While direct effects of urchin fronts on macrophytes (particularly kelps) are well documented, indirect effects on associated fauna are largely unknown. Secondary aggregations of predators and scavengers form around fronts of Strongylocentrotus droebachiensis in Nova Scotia. We recorded mean densities of the sea stars Asterias spp. (mainly A. rubens) and Henricia sanguinolenta of up to 11.6 and 1.7 individuals 0.25 m⁻² along an urchin front over 1 year. For Asterias, mean density at the front was 7 and 15 times greater than in the kelp bed and adjacent barrens, respectively. There was strong concordance between locations of peak density of urchins and sea stars (Asterias r = 0.98; H. sanguinolenta r = 0.97) along transects across the kelp–barrens interface, indicating that sea star aggregations migrated along with the urchin front at rates of up to 2.5 m per month. Size–frequency distributions suggest that Asterias at the front were drawn from both the barrens (smaller individuals) and the kelp bed (larger individuals). These sea stars fed intensively on mussels on kelp holdfasts and in adjacent patches. Urchin grazing may precipitate aggregations of sea stars and other predators or scavengers by incidentally consuming or damaging mussels and other small invertebrates, and thereby releasing a strong odor cue. Consumption of protective holdfasts and turf algae by urchins could facilitate feeding by these consumers, which may obtain a substantial energy subsidy during destructive grazing events.     

The nature of the symbiosis between Indo-Pacific anemone fishes and sea anemones

Under the general heading of symbiosis, defined originally to mean a living together of two dissimilar species, exist the sub-categories of mutualism (where both partners benefit), commensalism (where one partner benefits and the other is neutral) and parasitism (where one partner benefits and the other is harmed). The sea anemone-fish (mainly of the genus Amphiprion) symbiosis has generally been considered to benefit only the fish, and thus has been called commensal in nature. Recent field and laboratory observations, however, suggest that this symbiosis more closely approaches mutualism in which both partners benefit to some degree. The fishes benefit by receiving protection from predators among the nematocyst-laden tentacles of the sea anemone host, perhaps by receiving some form of tactile stimulation, by being less susceptible to various diseases and by feeding on anemone tissue, prey, waste material and perhaps crustacean symbionts. The sea anemones benefit by receiving protection from various predators, removal of necrotic tissue, perhaps some form of tactile stimulation, removal of inorganic and organic material from on and around the anemone, possible removal of anemone parasites, and by being provided food by some species of Amphiprion.     

Chemical defense in the eggs and embryos of antarctic sea stars (Echinodermata)

Fifteen species of reproductively mature echinoderms (11 sea stars, 3 sea urchins, 1 sea cucumber) were collected from McMurdo Sound, Antarctica, during austral spring and summer of 1985 and 1986; eggs and embryos were obtained, and were tested for ichthyonoxicity using the common marine killifishFundulus grandis as a model predator. Chemical deterrents occurred in the large, yolky eggs of the pelagic lecithotrophic sea starPerknaster fuscus and the planktotrophic sea starPorania antarctica. Brooded embryos of the sea starsDiplasterias brucei andNotasterias armata were also noxious. Significant ichthyonoxicity was not detected in the remaining 7 species of sea stars, 3 sea urchins, and 1 sea cucumber. Chemical deterrents were generally effective at concentrations below a single egg or embryo per agar test-pellet. Although chemicals found in these eggs and embryos are noxious to an allopatric fish, they may not be noxious to sympatric fish.     

Cryptic speciation in the recently discovered American cycliophoran <Emphasis Type="Italic">Symbion americanus</Emphasis>; genetic structure and population expansion

Symbion americanus was recently described as the second species in the phylum Cycliophora, living commensally on the American commercial lobster Homarus americanus. A previous genetic analysis of American and European populations of cycliophorans suggested that haplotype divergence in S. americanus was much greater than in its European counterpart S. pandora. This study examined the population structure and demographics of 169 individuals thought to belong to S. americanus collected from lobsters over 13 North American localities (Nova Scotia, Canada to Maryland, USA) between October 2003 and January 2006. Cytochrome c oxidase subunit I sequence data clearly suggested the presence of three cryptic lineages in a species complex, often co-occurring in the same lobster specimens. One of these lineages, named the “G” lineage, was represented by very few individuals and therefore was excluded from subsequent statistical analyses. The other two sympatric lineages, named the “T” and “C” lineages, showed different population structure and demography. Although limited geographic structure was found in the T lineage, the C lineage showed higher nucleotide and haplotype diversity values, as well as more variation between localities. The data also indicated that the T lineage underwent a recent population expansion, suggesting that the C and T lineages may have speciated in allopatry but a subsequent population expansion may have been responsible for their current sympatric distribution. Studies on the anatomy and ecology of the sympatric lineages of this species complex should provide further information on the identity of the holotype of S. americanus, which currently cannot be ascribed to any of the three cryptic lineages.     

Encounters between Antarctic limpets, <Emphasis Type="Italic">Nacella concinna</Emphasis>, and predatory sea stars, <Emphasis Type="Italic">Lysasterias</Emphasis> sp., in laboratory and field experiments

Antarctic limpets, Nacella concinna, from the Admiralty Bay (King George Island, South Shetlands) for at least part of the year (austral winter) co-exist with predatory sea stars Lysasterias sp. Our laboratory and field experiments established that the presence of Lysasterias sp. or its odour had considerable influence upon their behaviour. Limpets’ responses, consisting of shell mushrooming, shell rotation and flight, were distinctly different from their reaction to other stimuli, such as food and conspecific odours, or mechanical stimulation. Moreover, a significant impact of sea star presence on limpets’ activity was observed, with limpets fleeing to a distance of 60 cm from the predator. Such reactions allow limpets to lower the incidence of sea star predation, but at the cost of presumptive disrupting of foraging and an additional energy expended for locomotion. A visible difference was noted between two limpet populations, with the rockpool limpets responding only after physical contact with being touched by a sea star, and the subtidal ones responding at a distance of up to 20 cm.     

Ontogenetic habitat shift,population growth,and burrowing behavior of the Indo-Pacific beach star, <Emphasis Type="Italic">Archaster typicus</Emphasis> (Echinodermata; Asteroidea)

Archaster typicus, a common sea star in Indo-Pacific regions, has been a target for the ornamental trade, even though little is known about its population biology. Spatial and temporal patterns of abundance and size structure of A. typicus were studied in the Davao Gulf, the Philippines (125°42.7′E, 7°0.6′N), from February 2008 to December 2009. Specimens of A. typicus were associated with intertidal mangrove prop roots, seagrass meadows, sandy beaches, and shoals. Among prop roots, specimens were significantly smaller and had highest densities (131 ind. m⁻²) between November and March. High organic matter in sediment and a relatively low predation rate seemed to support juvenile life among mangroves. Size and density analyses provided evidence that individuals gradually move to seagrass, sandy habitats, and shoals as they age. Specimens were significantly larger at a shoal (maximum radius R = 81 mm). New recruits were found between August and November in both 2008 and 2009. Timing of recruitment and population size frequencies confirmed a seasonal reproductive cycle. Juveniles had relatively high growth rates (2–7 mm month⁻¹) and may reach an R of 20–25 mm after 1 year. Growth rates of larger specimens (R > 30 mm) were generally <2 mm month⁻¹. The activity pattern of A. typicus was related to the tidal phase and not to time of day: Specimens moved over the sediment surface during low tides and were burrowed during high tides possibly avoiding predation. This is one of the first studies to document an ontogenetic habitat shift for sea stars and provides new biological information as a basis for management of harvested A. typicus populations.     

Chemical mediation of invertebrate defensive behaviors and the ability to distinguish between foraging and inactive predators

When the predatory sea star Pycnopodia helianthoides was placed upstream, the sea urchin Strongylocentrotus purpuratus responded defensively by extending and opening its globiferous pedicellariae. No pedicellaria response was given in control seawater or when the sea star was downstream. The snail Tegula funebralis responded by moving up vertical surfaces when Pycnopodia helianthoides or when Pisaster ochraceus were placed upstream. When these sea stars were introduced downstream, the snail's response was not significantly different from that in control seawater. Water collected from an aquarium containing a single sea star was sufficient to trigger the response of S. purpuratus and T. funebralis; the physical presence of the sea star was not essential. This indicated that a chemical stimulus was involved, and the lack of responses when sea stars were downstream argued strongly against the possible additional involvement of visual or vibrational stimuli. S. purpuratus gave stronger pedicellaria responses to water flowing over an active Pycnopodia helianthoides than to water flowing over the same sea star when it was inactive. The significance of the ability to distinguish between actively foraging and inactive predators is discussed, and a mechanism is proposed to explain differences in the amount of stimulatory chemicals released by active and inactive sea stars.     

Dislocation experiments in the migrating sea star Astropecten jonstoni

Movements of tagged specimens of the sea star Astropecten jonstoni (D. Ch.), both spontaneous and after displacement, were studied by means of SCUBA diving in shallow coastal waters of Sardinia (Mediterranean Sea). In spring, the entire population nigrates towards the shore. Displaced sea stars were able to return to their original depth zone. Both migration and return to the original depth zone occur in a unidirectional way.     

Alimentary niche partitioning in the Galapagos sea lion, Zalophus wollebaeki

Sea lions are generally considered opportunistic feeders. However, studies from different areas suggest their diet consists mostly of four to five types of prey. Previous studies in Galapagos sea lions have identified at least three feeding strategies for this species, suggesting diversification of their diet. Diet diversification is favored in organisms with relatively high trophic position and subject to high intra-specific and low inter-specific competition. Zalophus wollebaeki meet these criteria as the only pinniped on San Cristobal Island, where three sea lion rookeries are located within 11 km: a distance considerably shorter than their 41 km foraging range. To measure the degree of diet diversification, we used scats and stable isotope analyses. A total of 270 scat samples from lactating females and 142 fur samples from sea lion pups were collected during the breeding season 2006. The scat analysis identified distinct diets among rookeries, with minimal trophic overlap (Cλ = 0.19), a trophic level TL = 4.5 (secondary–tertiary carnivore), and trophic breadth of a specialist predator (B _i  = 0.37). The mean δ¹⁵N and δ¹³C values were 13.07 ± 0.52 and −16.34 ± 0.37, respectively. No significant difference was found in the δ¹⁵N values from the sea lion rookeries, but differences were found inter- and intra-population in δ¹³C values for pups from different groups (ANOVA P < 0.05). Our results indicate that diet diversification is present in the Galapagos sea lion and may play important role to the survival of the species in a habitat where pinniped populations are limited.     

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

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

In situ net primary productivity and photosynthesis of Antarctic sea ice algal, phytoplankton and benthic algal communities

Primary production at Antarctic coastal sites is contributed from sea ice algae, phytoplankton and benthic algae. Oxygen microelectrodes were used to estimate sea ice and benthic primary production at several sites around Casey, a coastal area in eastern Antarctica. Maximum oxygen export from sea ice was 0.95 mmol O₂ m⁻² h⁻¹ (~11.7 mg C m⁻² h⁻¹) while from the sediment it was 6.08 mmol O₂ m⁻² h⁻¹ (~70.8 mg C m⁻² h⁻¹). When the ice was present O₂ export from the benthos was either low or negative. Sea ice algae assimilation rates were up to 3.77 mg C (mg Chl-a)⁻¹ h⁻¹ while those from the benthos were up to 1.53 mg C (mg Chl-a)⁻¹ h⁻¹. The contribution of the major components of primary productivity was assessed using fluorometric techniques. When the ice was present approximately 55–65% of total daily primary production occurred in the sea ice with the remainder unequally partitioned between the sediment and the water column. When the ice was absent, the benthos contributed nearly 90% of the primary production.     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus franciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographically separated populations, seven from northern California and five from southern California (including Santa Rosa Island). Significant population heterogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring populations were often more genetically differentiated than distant populations. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variation and patterns of recruitment, large samples were collected from several northern California populations in 1996 and 1997, and were divided into three size classes, roughly representing large adults (>60 mm), medium-sized individuals (31 to 60 mm, “subadults”) and individuals <2 yr of age (≤30 mm test diam, referred to as “recruits”). Comparisons of allelic counts revealed significant spatial and temporal differentiation among size-stratified population samples. Recruit samples differed significantly from adult samples collected at the same locale, and showed extensive between-year variation. Genetic differentiation among recruit samples was much higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations for the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or “sweepstakes” recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, such differentiation across northern California populations indicates that the larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration. Received: 6 July 1999 / Accepted: 25 January 2000     

Dissolved histamine: a potential habitat marker promoting settlement and metamorphosis in sea urchin larvae

Many species of marine invertebrate larvae settle and metamorphose in response to chemicals produced by organisms associated with the adult habitat, and histamine is a cue for larvae of the sea urchin Holopneustes purpurascens. This study investigated the effect of histamine on larval metamorphosis of six sea urchin species. Histamine induced metamorphosis in larvae of three lecithotrophic species (H. purpurascens, Holopneustes inflatus and Heliocidaris erythrogramma) and in one planktotrophic species (Centrostephanus rodgersii). Direct comparisons of metamorphic rates of lecithotrophic and planktotrophic larvae in assays cannot be made due to different proportions of larvae being competent. Histamine (10 μM) induced metamorphosis in 95% of larvae of H. purpurascens and H. inflatus after 1 h, while the coralline alga Amphiroa anceps induced metamorphosis in 40–50% of these larvae. Histamine (10 μM) and A. anceps induced 40 and 80% metamorphosis, respectively, in the larvae of H. erythrogramma after 24 h. Histamine (10 μM) and the coralline alga Corallina sp. induced 30 and 70% metamorphosis, respectively, in the larvae of C. rodgersii after 24 h. No metamorphosis of any larval species occurred in seawater controls. Larvae of two planktotrophic species (Tripneustes gratilla and Heliocidaris tuberculata) did not metamorphose in response to histamine. Seagrasses, the host plants of H. inflatus, induced rapid metamorphosis in larvae of the two Holopneustes species, and several algae induced metamorphosis in C. rodgersii larvae. Histamine leaching from algae and seagrasses may act as a habitat marker and metamorphic cue for larvae of several ecologically important sea urchin species.     

Sea anemones and brittle stars: unexpected predatory interactions during induced in situ oxygen crises

During oxygen crises, benthic faunas exhibit a series of behavioural patterns that reflect the duration and severity of the event. During artificially induced oxygen deficiencies at 24 m depth in the Northern Adriatic Sea, we photographically documented predation by the sea anemones Cereus pedunculatus (Pennant, 1777) and Calliactis parasitica (Couch, 1842) on the brittle star Ophiothrix quinquemaculata (DelleChiaje, 1828). Five predatory events were recorded with four anemones during nine deployments totalling 817 h of observation. Under near-anoxic conditions, individuals of both actinians made contact with, pulled in and consumed the brittle stars. The duration of each predatory event was 1.5–7.5 h. In three of the five events, brittle star remains were regurgitated after an additional 2.0–12.5 h of digestion by the anemones. Our time-lapse sequences demonstrate that oxygen deficiency, beyond eliciting a series of specific behaviours in members of each species, also promotes previously unobserved interspecific interactions. Our results show that sea anemones are not only highly resistant to anoxia, but may also benefit by taking advantage of prey that are more vulnerable to anoxic conditions.