Short-term stability of genetic structure in populations of the sea anemone Metridium senile

Populations of the sea anemone Metridium senile (L.) were sampled from several locations in eastern North America in two series, one collected from 1977–1979 and the other from 1981–1985. Fourteen populations were sampled twice at one- to six-year intervals. Samples were analyzed for temporal differences in genetic composition at both the single locus and multiple locus levels. Overall patterns of geographic variation in allele frequency did not change between series. Regressions describing clines did not differ significantly, and loci not showing clinal variation in the first series remained similar in the second. Analysis of populations sampled twice produced no systematic evidence of change in allele frequency with time for any of four polymorphic loci. Comparisons with computer simulations of repeated sampling of multiple locus genotypes from panmictic populations with free recombination also revealed little temporal change at this level. One population showed possible evidence of recruitment from a different gene pool. Other significant departures from expectations reflected more reproducibility of genotype distributions between samples than expected for sexual populations. This excess stability likely results jointly from clonal reproduction and little sexual recruitment. Despite these indications of genetic stability in adult populations, newly settled juveniles were genetically different from resistant adults in one population, demonstrating the potential for genetic change by immigration. Successful sexual recruitment seems to be rare, even though larvae regularly settle from the plankton. Although interpretation of these results is somewhat limited by lack of knowledge of longevities and generation times, to the extent that they reflect longer-term trends, they suggest that at least some of the observed patterns of geographic variation in allele frequency probably result from natural selection.     

The fluffy sea anemone Metridium senile in periodically oxygen depleted surroundings

In SCUBA-diving monitored field experiments (Flensburg Fjord, 15 m, 1981) and parallel aquarium tests,the behavior of Metridium senile under anoxic conditions and its oxygen-deficiency resistance were studied. When oxygen is lacking the fluffy sea anemone diminishes body surface, and then, successively, shuts off most energy consuming activities. By means of this strategy, 50% of the tested individuals survived 3 weeks of total anoxia.     

Palatability of invertebrate larvae to corals and sea anemones

Risk of larval mortality is an underlying theme in debates and models concerning the ecology and evolution of the differing reproductive characteristics among marine benthic invertebrates. In these discussions, predation is often assumed to be a major source of larval mortality. Previous studies, focused primarily on planktotrophic larvae, suggested that marine larvae generally were susceptible to, and poorly defended against, planktivorous fishes and invertebrates. Larval-planktivore interactions involving larger and more conspicuous lecithotrophic larvae that are typical of many brooding sessile invertebrates have not been well studied. This lack of data for diverse larval types has hindered testing broad generalities about marine larvae and planktivore prey-preferences. This study demonstrates that lecithotrophic larvae of many Caribbean and temperate western Atlantic invertebrates are distasteful to co-occurring corals and anemones. These larval predators frequently rejected larvae of sponges (6 of 9 species), gorgonians (7 of 9 species), corals (3 of 3 species), hydroids (2 of 2 species) and a bryozoan. Larvae of three temperate colonial ascidians were readily consumed. Frequencies of survivorship for larvae captured but rejected by corals and anemones were generally high and, in 20 of 24 assays, were not statistically different from those of unattacked control larvae. Levels of metamorphosis (when it occurred) of rejected larvae also rarely differed significantly from those of unattacked controls. These results provide further evidence that larval palatability to predators may not be as high as once thought, particularly for brooded larvae of sessile colonial invertebrates. The means by which larvae may avoid or deter predators, and the demographic consequences for marine invertebrates and for the evolution of invertebrate life-history patterns, need to be assessed.     

Genetic variability and relationships among geographically widely separated populations of Petitia amphophthalma (Polychaeta: Syllidae). Results from RAPD-PCR investigations

An analysis of the population genetics of the meiofaunal polychaete Petitia amphophthalma Siewing, 1956, in which the RAPD-PCR method was applied to 103 individuals from eight populations, some of them very far apart (Atlantic: Florida, Tenerife, France; Mediterranean: two Greek islands, Tunisia; Red Sea: Egypt), gave closely reproducible results. In the band patterns produced with 13 decamer primers, a total of 195 genetic characters was detected. The data were evaluated by a number of methods, including the cluster programs UPGMA, WPGMA and neighbour-joining. The detected genetic distances between the populations vary between 58.9 and 66.6, but 97% of the genetic characters, although polymorphic, are found in at least two populations and usually in all the others as well. Phenograms of the analyses find four population clusters [Florida, France (Atlantic), the Mediterranean and Tenerife]. They are, however, not completely congruent and show low bootstrap values at the junction points of the clusters (with the exception of the Tenerife cluster). Mediterranean P. amphophthalma form a cohesive population, although within it the genetic distances are graded in parallel with the geographic distances between the sites. The colonization of Tenerife, an island of relatively recent volcanic origin, can be taken as evidence that this meiofaunal species can become dispersed not only along coastlines but also across expanses of open water. However, the severely restricted variability of these populations implies that in this case a founder effect has operated, and that transport over open water is not a routine event but extremely rare. The absence of the species on the Australian coast and, for instance, on the Galapagos Islands indicates that there has been no continuous gene flow across the oceans. The idea that all the populations investigated belong to one cosmopolitan species is discussed. Received: 17 October 1997 / Accepted: 15 April 1998     

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.     

Genetic divergence between colour morphs in populations of the common intertidal sea anemones Actinia equina and A. prasina (Anthozoa: Actiniaria) in the Isle of Man

A distinctive morph ofActinia equina (L.) is found at low frequency among populations on intertidal hard substrata on some shores on the Isle of Man. This morph is red with rows of green oval spots or elongate markings running longitudinally down the column. Starch gel electrophoresis of 21 allozyme loci was used to compare samples of this morph from two localities with other sympatricActinia spp. Collections were made in early summer 1986. For one sampling site the comparison was with redA. equina and greenA. prasina Gosse, whilst at the other site with redA. equina only. At both sites significant genetic differences in allele frequencies at several loci were found between the new morph and sympatricA. equina andA. prasina. The results also confirm the reproductive isolation ofA. prasina. As with much previous work on sea anemones, levels of mean heterozygosity per locus (H) were found to be high (H=0.157 to 0.342). A surprising feature of the results, although bearing out earlier unpublished data, is that high levels of genetic differentiation are found between populations of what appear to be the same morphs collected from shores only a few km apart. Among theActinia spp. studied, differentiation between populations of the same morph on different shores was of the same order as between sympatric, reproductively isolated populations.     

Nutrient transfer in a marine mutualism: patterns of ammonia excretion by anemonefish and uptake by giant sea anemones

Many symbioses involve multiple partners in complex, multi-level associations, yet little is known concerning patterns of nutrient transfer in multi-level marine mutualisms. We used the anemonefish symbiosis as a model system to create a balance sheet for nitrogen production and transfer within a three-way symbiotic system. We quantified diel patterns in excretion of ammonia by anemonefish and subsequent absorption by host sea anemones and zooxanthellae under laboratory conditions. Rates of ammonia excretion by the anemonefish Amphiprion bicinctus varied from a high of 1.84 μmole g⁻¹ h⁻¹ at 2 h after feeding, to a basal rate of 0.50 μmole g⁻¹ h⁻¹ at 24–36 h since the last meal. Conversely, host sea anemones Entacmaea quadricolor absorbed ammonia at a rate of 0.10 μmole g⁻¹ h⁻¹ during the daytime in ammonia-enriched seawater, but during the night reduced their absorption rate to near zero, indicating that ammonia uptake was driven by zooxanthella photosynthesis. When incubated together, net ammonia excretion was virturally zero, indicating that host anemones absorbed most of the ammonia produced by resident fish. Adult anemonefish weighed about 11 g under laboratory conditions, but on the coral reef may reach up to 64 g, resulting in a maximal potential ammonia load of >200 μmole h⁻¹ produced by two adult fish during daylight hours. In contrast, host sea anemones weighed about 47 g in the laboratory, but under field conditions, large individuals may reach 680 g, so their maximal ammonia clearance rates may reach about 70 μmole h⁻¹ during the daytime. As such, the ammonia load produced by adult anemonefish far exceeds the clearance rate of host anemones and zooxanthellae. Ammonia transfer likely occurs mainly during the daytime, when anemonefish consume zooplankton and excrete rapidly, and in turn the zooxanthellae are photosynthetically active and drive rapid ammonia uptake. We conclude that zooplanktivorous fishes that form mutualisms with coral reef cnidarians may serve as an important link between open water and benthic ecosystems, through the transfer of large quantities of nutrients to zooxanthellate hosts, thus enhancing coral reef productivity.     

Physiological energetics of cultivated mussel (Mytilus edulis) populations in two Scottish west coast sea lochs

Physiological responses of suspended cultured mussels, Mytilus edulis L., in two Scottish sea lochs (Lochs Etive and Leven) were investigated in their native and transplanted environments, after 15 d, 4.5 mo and 1 yr acclimatization, during the main growing season of May to September 1992. These measurements were integrated by means of the balanced energy equation, and scope for growth was calculated to assess the performance of each stock. Transplanted mussels showed clear signs of stress during the first 15 d after transfer, with low clearance rates and energy retention and high rates of respiration and nitrogen excretion. There were significant differences in some of the physiological responses (clearance, respiration and excretion rates), scope for growth and growth efficiency between the native populations, with the responses of mussels in Loch Etive being more favourable than those in Loch Leven. With newly transplanted mussels after 15 d acclimatization, almost all these measurements also differed significantly from native mussels in their host site and the original stocks but, with the exception of ammonia excretion rates, all the variables of cross-transplanted mussels after 4.5 mo acclimatization were the same as those of the native stock at the host site. Good agreement between observed long-term growth rates and estimated scope for growth suggests that, like growth rate, differences in physiological responses are mainly controlled by environmental factors; i.e., stock or origin had no significant influence on variations in physiological response (except ammonia excretion). The results further indicate that scope for growth estimated during the main growing season can be used to assess the actual growth rate and to compare sites for on-growing.     

Functional morphology and zonation of three species of sea anemones from rocky shores in southern Chile

Three species of sea anemones, clearly exhibiting zonation on the shore, are grouped into intertidal [Phymactis clematis (Drayton, 1846), Anthothoe chilensis (Lesson, 1830)] and infralittoral [Antholoba achates (Drayton, 1846)] forms. A series of observations revealed that intertidal forms have the ability to retain water in the coelenteron during exposure to air,/while the infralittoral form cannot do so. This different behaviour is attributed to morphological characteristics, such as the specific body shape and the structure of the sphincter and retractor muscles. These features, in combination with specific abiotic factors of the various habitats under consideration, determine the upper limits of distribution of each species, while zonation below these limits seems to be affected in addition by biological interactions.     

Genetic divergence between east and west Atlantic populations of<Emphasis Type="Italic"> Actinia</Emphasis> spp. sea anemones (Cnidaria: Actiniidae)

The sea anemone Actinia equina was considered a highly variable species with a wide geographical distribution, but molecular systematic studies have shown that this wide distribution may be the result of the lumping of cryptic species. In this work enzyme electrophoresis was used to analyse the genetic variability of A. equina from the Atlantic coasts of Europe and Africa, as well as the relationships between those populations and other species of the genus. Samples of A. equina from the United Kingdom and France were compared with supposedly conspecific populations from South Africa and a recently described species from Madeira, Actinia nigropunctata. The South African and Madeiran populations were genetically very divergent from each other (genetic identity, I=0.15), as well as from the A. equina population from the United Kingdom and from the other species studied (I between 0.23 and 0.78), indicating that the African population of A. equina belongs to a different species. High genetic similarities were found between west Atlantic A. bermudensis and east Atlantic A. sali, in spite of the great geographical distance between them, and between the European A. equina and A. prasina. The results confirm the presence of the new species A. nigropunctata from Madeira and suggest a new species within the A. equina species group, from South Africa.Communicated by O. Kinne, Oldendorf/Luhe     

Predation on zooplankton by the benthic anthozoans Alcyonium siderium (Alcyonacea) and Metridium senile (Actiniaria) in the New England subtidal

The Alcyonacean octocoral Alcyonium siderium Verrill and the sea anemone Metridium senile (L.), the only common perennial zooplanktivores on shallow (16-m depth) subtidal rock walls in much of northern New England, USA, are of similar heights and overlap in their habitat and microhabit distributions. The coelenteron contents of both species were sampled at four-hour intervals over a diel cycle and were compared to zooplankton available in the water at 1 to 5 cm from the rock wall, the height at which the cnidarians held their feeding tentacles. Prey in coelenterons of A. siderium were significantly smaller (means of 256 to 345 m), and those in coelenterons of M. senile were equal to or slightly greater in length (means of 415 to 1006 m) than the available zooplankton. The diets of A. siderium and M. senile differed significantly from each other and from the available zooplankton. A. siderium showed strong positive electivites for ascidian larvae and for foraminiferans, and strongly negative electivities for most crustaceans. M. senile had strong positive electivities for barnacle cyprids, ascidian larvae, and gammarid amphipods, and strong negative electivities for invertebrate eggs, foraminiferans, calanoid and harpacticoid copepods, and ostracods. Electivities may reflect tentacle avoidance or escape by motile prey as well as predator preference. Substratum-associated organisms (e.g. demersal crustaceans, larvae of benthic invertebrates) were the most common items in the diets of both species, suggesting a tight benthic food web, similar to the situation for coral reef anthrozoans which rely on reef-generated zooplankton. A. siderium ate large numbers of ascidian larvae which, as benthic adults, compete for space with A. siderium and can overgrow small colonies. Predation on the larvae of a competing species may alleviate competition by decreasing the competitor's recruitment.     

Population structure and genetic variation in two tropical sea anemones (Cnidaria,Actinidae) with different reproductive strategies

Genetic variation and population structure of two tropical sea anemones, Bunodosoma caissarum Correa and Actinia bermudensis McMurrich were related to their different dispersal capabilities and reproductive modes. B. caissarum reproduces sexually and has a long-lived planktotrophic larva; A. bermudensis can reproduce both sexually and asexually, supposedly with short-range dispersal. Both species were sampled along 1150 km of Brazilian coastline between 1990 and 1991 and analyzed by horizontal starch gel electrophoresis for 16 enzyme loci in B. caissarum and 19 in A. bermudensis. B. caissarum had higher levels of heterozygosity (H=0.35) and lower levels of population structuring (F _ST=0.042) than A. bermudensis (H=0.17; F _ST=0.262). At one locality large genetic differences between two sympatric colour morphs of A. bermudensis provide evidence of possible cryptic speciation. Overall levels of genetic variation and heterozygosity in the two species are compatible with their known modes of reproduction.     

Short-term variability of photosynthesis in natural marine phytoplankton populations

Harmonic regression analysis has been used to determine the short-term variability in the photosynthetic rate (mgC/mg chlorophyll a/h) of phytoplankton in three inlets of Japan. In natural water without large zooplankton present, the photosynthetic rate [log P=log (100xmgC/mg chlorophyll a/h)] can be expressed as (B+A cos T). Factor B represents the average photosynthetic rate, of which the maximum is usually designated as P _max, and Factor A corresponds to the slope of the regression line. The phase of the periodicity, represented by T, is adjusted to give the highest correlation: usually T is expressed as [360/24 x (local time + 4)] in degrees. The correlation between Factors A and B is very high (r=0.95, P<0.001), indicating that Factor A may depend upon Factor B (potential activity of chlorophyll a). Both Factors A and B decrease with decreasing irradiance, but the slope of each regression between Factor A and irradiance varies with season. Continuous darkness reduces the phase of the periodicity to one cycle a day when phytoplankton has multiple cycles of photosynthetic rate per day. Adequate nutrient supply from zooplankton regeneration may cause an increase in Factor B; however, excess density of zooplankton decreases Factor A.     

On the distribution and variability of the Indo-Pacific sea star Asterina wega (Echinodermata: Asteroidea) in the Mediterranean sea

The distribution of the Mediterranean populations of Asterina wega Perrier is described. Four populations were found to differ in size of animals and regeneration rate. The possible migratory pattern is duscussed.     

Temperature effects on mortality and household adaptation: Evidence from China

This paper examines the effects of extreme temperatures on mortality rates, using random year-to-year variation in temperature based on county-level panel data from China. The analysis finds a robust, U-shaped relationship between temperature and mortality rates, indicating that extremely cold or hot temperatures lead to excess deaths. The heat-related (cold-related) effect is 3.5 times (3.2 times) as large as previous findings that used U.S. data, and it is especially large for the elderly population, mainly due to excess deaths caused by cardiovascular diseases. Applying these results to climate change predictions from Hadley Centre Global Environmental Model shows that by 2061–2080 the annual mortality rate is likely to increase by 14.2% if global greenhouse gas emissions continue to rise throughout the 21st century, the estimated health cost of which is around 0.98 trillion Chinese Yuan per year. The paper also explores households’ adaptation behaviors to extreme temperatures. It finds that although urban households adaptively increase energy consumption when they are exposed to cold temperatures and purchase more air conditioners on hot and cold days, rural households are unresponsive to temperature fluctuations. This finding implies that rural people may be more resource constrained and suffer more when extreme temperatures occur.     

Restricted gene flow and evolutionary divergence between geographically separated populations of the Antarctic octopus Pareledone turqueti

Samples of the Antarctic octopus Pareledone turqueti were taken from three locations on the Scotia Ridge in the Southern Ocean. The genetic homogeneity of these populations was investigated using isozyme electrophoresis. Whilst panmixia appeared to be maintained around South Georgia (F _ST = 0) gene flow between this island and Shag Rocks, an island only 150 km away but separated by great depths, was extremely limited (F _ST = 0.74). These results are examined with respect to the discontinuous distribution of P. turqueti throughout Antarctica. An estimate of effective population size was also calculated (N _e = 3600). Received: 7 March 1997 / Accepted: 27 March 1997     

Effects of starvation,and light and dark on the energy metabolism of symbiotic and aposymbiotic sea anemones,Anthopleura elegantissima

Rates of oxygen and carbon-dioxide exhange were measured in symbiotic and aposymbiotic specimens of the sea anemone Anthopleura elegantissima while fed and starved under light or dark conditions. Respiratory quotients indicated that fed anemones switched from a carbohydrate to a fat catabolism when starved, with the exception that symbiotic individuals starved in the light showed a pronounced carbohydrate catabolism for over 1 month. The source of the carbohydrate was probably photosynthate translocated by the dinoflagellate Symbiodinium (=Gymnodinium) microadriaticum (Freudenthal) living in the anemones' tissues. The starved symbiotic anemones maintained in the light had lipid levels not significantly different from fed controls and 44 to 61% higher than starved aposymbiotic anemones after 1 month. Thus, the quality and quantity of the metabolic flux from the symbionts to the sea anemone were sufficient to conserve the host's lipid reserves.     

Development of north sea coastal plankton communities in separate plastic bags under identical conditions

In two experiments lasting 4 to 6 weeks, communities of North Sea coastal plankton kept in separate plastic bags (of about 1400 l) and exposed to the same environmental conditions showed very similar patterns of growth and decline. This result means that the method is suitable for the evaluation of toxic effects of environmental pollutants at low concentrations on complex plankton systems. The phytoplankton in the bags produced a succession of blooms, which were probably limited by shortage of nutrients. The dominant zooplankton organisms were various species of copepods which can develop in the bags from egg to adult. Strong indications were found that mineralization of organic matter occurs in the bags. Chemical parameters and phytoplankton biomass were found not to be stratified, indicating that the contents of the bags were well mixed.Work carried out under Contract No. 110-75-1 ENVN of the E.C. Environmental Research Programme.     

Reproductive cycle,development and recruitment of two geographically separated populations of the dendrochirote holothurian Aslia lefevrei

The boreo-mediterranean holothurian Aslia lefevrei (Barrois) occupies cryptic habitats from low-water mark to ca. 20 m. During the present work specimens were collected from the west coast of Ireland (February 1979 to August 1980), where A. lefevreiis a prominent element in two discrete epibenthic assemblages, and from the north-west of Spain (September 1981 to August 1982), where this holothurian is densely aggregated in association with raft cultivation of the mussel Mytilus edulis. Its reproductive cycle, spawning process, embryological development, settlement, and early growth rates have been documented by means of laboratory and field studies. Depending on geographic location, spawning in the wild occurs between January and May. Development to the gastrula takes some 3 to 4 d. Uniformly ciliated dipleurula/auricularia hatch 12 to 24 h after gastrulation. The pentactula stage is attained approximately 7 d after fertilization, following which the larva settles. Newly settled individuals utilize the five primary tentacles for both feeding and locomotion. With the advent of small numbers of podia in the ambulacral regions, two of the primary tentacles prolong their locomotory and anchorage capabilities while the remaining tentacles develop adult characteristics and function solely in feeding. Growth was followed for an estimated 30 mo period using length-frequency analyses. In the first year of life, A. lefevrei attain an average length of 3.4 cm; individuals on the sea floor can attain up to 18 cm, indicating a possible life expectancy of 10 yr or more.Contribution No. 301 from the School of Marine Sciences, University College Galway, Ireland