Feeding factors for the sea anemone Anthopleura midorii

The carnivorous feeding behavior of the sea anemone Anthopleura midorii was found to be a sequence of successive feeding motions, which consisted of several separate actions: (1) tentaculation on any solid matter; (2) retention of prey by tentacles; (3) mouth opening; (4) ingestion of food; (5) digestion of food; (6) extrusion of indigestible waste material. Hot water extract of a natural prey induced the entire feeding sequence when extract was given in the form of an artificial food by mixing with -potato starch and water. The fractionated extract induced only a limited feeding action. Several amino acids induced feeding. Alanine, glycine or histidine stimulated the retention of artificial food by tentacles, and proline evoked mouth opening. Cysteine or reduced glutathione (GSH) induced food ingestion. Unnatural food stuff such as -potato starch was ingestible, if an appropriate amount of a chemical substance was added.     

Feeding behavior and acquisition of zooxanthellae by planula larvae of the sea anemone Anthopleura elegantissima

Symbiotic associations between cnidarians and photosynthetic dinoflagellates (i.e., zooxanthellae) are common in the marine environment. Many symbiotic cnidarians produce offspring that are initially nonsymbiotic. These new hosts must acquire symbiotic algae from environmental sources. We examined zooxanthella acquisition by laboratory-reared planula larvae of the temperate sea anemone Anthopleura elegantissima. Larvae ingested zooxanthellae while they were feeding. However, the signal that prompted larval feeding behavior did not originate from the symbiotic algae; the addition of algal cells to larval cultures never elicited a feeding response. In contrast, the addition of macerated animal tissue from several sources invariably generated a strong feeding response, which resulted in the larvae indiscriminately ingesting any particulate matter that was present, including zooxanthellae or other unicellular algae. Ingested zooxanthellae were incorporated into endodermal cells, where they remained undigested, while all other ingested material was digested or expelled within 24 h. Our results provide evidence that one source of zooxanthellae likely to serve as a route of infection in the natural environment is zooxanthella-laden mucus egested by anemones. This egested material fulfilled both of the criteria necessary for successful infection: it prompted larvae to begin feeding and provided an abundant supply of zooxanthellae that were ingested and taken up into endodermal cells of the new host.     

Performance level and efficiency of two differing predator-avoidance strategies depend on nutritional state of the prey fish

Animal prey has developed a variety of behavioural strategies to avoid predation. Many fish species form shoals in the open water or seek refuge in structurally complex habitats. Since anti-predator strategies bear costs and are energy-demanding, we hypothesised that the nutritional state of prey should modify the performance level and efficiency of such strategies. In aquaria either containing or lacking a structured refuge habitat, well-fed or food-deprived juvenile roach (Rutilus rutilus) were exposed to an open-water predator (pikeperch, Sander lucioperca). Controls were run without predators. In the presence of the predator, roach enhanced the performance of the anti-predator strategy and increased the use of the refuge habitat whereby food-deprived roach were encountered more often in the structure than well-fed roach. Nonetheless more starved than well-fed roach were fed upon by the predator. In the treatments offering only open-water areas, roach always formed dense shoals in the presence of the predator. The shoal density, however, was lower in starved roach. Starving fish in shoals experienced the highest predation mortality across all experimental treatments. The experiment confirmed the plasticity of the anti-predator behaviour in roach and demonstrated that food deprivation diminished the efficiency of shoaling more strongly than the efficiency of hiding. The findings may be relevant to spatial distribution of prey and predator–prey interactions under natural conditions because when prey are confronted with phases of reduced resource availability, flexible anti-predator strategies may lead to dynamic habitat use patterns.     

Mating systems in the sea anemone genus Epiactis

Four morphologically similar species in the sea anemone genus Epiactis exhibit overlapping distributions on the Pacific coast of North America; E. prolifera, E. lisbethae, E. ritteri and E. fernaldi. All brood their offspring up to the juvenile stage, but each has a different combination of internal versus external brooding and hermaphroditism versus gonochory (separate sexes). Specimens were collected from sites ranging from British Columbia to southern California between December 1988 and July 1992. Mating systems were inferred from genetic comparisons of mothers and offspring histological analyses of sex expression and observations on brooding and spawning behavior. Allozyme and multilocus DNA fingerprint analyses of the gynodioecious hermaphrodite E. prolifera showed that offspring were all identical to their mothers, a result consistent with either asexual reproduction, self-fertilization or extreme biparental inbreeding. In the gonochore E. lisbethae, mothers and offspring were also electrophoretically identical, but variation in DNA fingerprints indicated cross-fertilization. Similar DNA fingerprint differences between mother and offspring in the gonochore E. ritteri implied that cross-fertilization also occurs in this species. No mother-offspring comparisons were performed on E. fernaldi, as this species was not observed brooding offspring during this study. Although incomplete, the results of this study increase our knowledge of the very unusual combination of reproductive modes in the genus Epiactis, and argue for further investigations of the evolution and genetic consequences of mating systems in these species.     

Physiological energetics of the intertidal sea anemone Anthopleura elegantissima

Energy budgets were calculated for individuals of the sea anemone Anthopleura elegantissima (Brandt), collected in 1981 and 1982 from Bodega Harbor, California, USA. Rates of ammonium excretion were measured in high-and low-intertidal, symbiotic and aposymbiotic sea anemones within 24 h of collection. Among symbiotic and aposymbiotic individuals, no differences in excretion rate were found on the basis of intertidal height. However, rates of ammonium excretion in aposymbiotic anemones (2.14 mol NH ₊ ⁴ g^-1 h^-1) were significantly higher than in symbiotic ones (0.288 mol NH ₊ ⁴ g^-1 h^-1). Rates of excretion were used with estimated rates of oxygen uptake to calculate nitrogen quotients (NQ). NQ and RQ values from the literature were used to calculate an oxyenthalpic equivalent [501 kJ (mol O₂)^-1 for R+U], and mass proportions of protein (54%), carbohydrate (44%) and lipid (2%) catabolized during routine metabolism in this species 24 h after feeding. Integrated energy budgets of these experimental anemones were calculated from data on ingestion, absorption and growth, and estimates of translocated energy from the symbiotic algae. Contribution of zooxanthellae to animal respiration based on translocation=90% and RQ=0.97 are 41 and 79% in high-and low-intertidal anemones, respectively. Calculated scope for growth is greater than directly measured growth in both high-and low-intertidal individuals. The deficit, estimated as 30% of assimilated energy in high-intertidal anemones, is attributed to unmeasured costs (specific dynamic effect) or production (mucus). Low-intertidal anemones lost mass during the experiment, implying that the magnitude of the deficit was greater in these anemones than in upper intertidal individuals. Anemones from both shore levels lost zooxanthellae during the experiment, which contributed to energy loss since the contribution of the zooxanthellae is greater in low-intertidal anemones. Scope for growth is preserved in high-intertidal anemones (29% of assimilated energy) because metabolic demands are lower due to aerial exposure, and prey capture rate is higher compared to lowshore anemones. Although possibly underestimated, lower scope for growth in low-shore anemones may result from continuous feeding and digestion processes that are less efficient than those of periodically feeding high-intertidal anemones.     

Inter-genotype aggression in the solitary sea anemone Actinia tenebrosa

The entire process of development from eggs to juveniles was observed in the sea-star Ctenopleura fisheri Hayashi. The breeding season of this sea-star in Toyama Bay, the Sea of Japan, occurs in the winter. The eggs are 465 in diameter, semitranslucent and pale brown in color. They develop into a barrel-shaped larva, neither bipinnaria nor brachiolaria, through a wrinkled blastula stage by holoblastic, radial cleavage. Larvae are free-swimming and do not feed during the larval stage. At metamorphosis the stalk, a larval organ, disappears by one of either 2 different processes; absorption into future body of the juveniles, or rupture and collapse. Fifteen days after insemination, metamorphosis is completed and the resulting juveniles, about 1 000 m in diameter, bear 2 pairs of tube-feet and a terminal tentacle in each arm.     

Physiological energetics of the intertidal sea anemone Anthopleura elegantissima

High-intertidal (H) individuals of the sea anemone Anthopleura elegantissima (Brandt) are exposed aerially up to 18 h each day, unlike low-intertidal (L) individuals which may be continuously immersed over many days. Thus, H anemones experience shorter feeding periods compared to L anemones. From 1980 to 1982, H and L anemones were observed and collected at the mouth of Bodega Harbor in North Central California (USA) to determine whether any physiological adaptations mitigate the energetic effects of reduced feeding time in H anemones. Weight of prey in coelenterons of H anemones was three times more than that of L anemones following a single immersion period. This difference is not due to slower digestion rates in H anemones. Prey residence time in coelenterons (4h) was equivalent in both groups. Different prey weights imply that ingestion rates were greater in H individuals. However, all anemones had similar weight-specific feeding-surface areas. Different prey-capture rates result from increased receptivity to prey in H anemones, rather than from increases in feeding surface. Absorption efficiency was inversely related to ration size in anemones from both shore positions. H individuals absorbed food more efficiently than L individuals fed equivalent rations. Ration, not exposure conditions, affected absorption efficiency. Daily growth rates were 1.5 to 1.8% and 1.2 to 1.4% of dry body weight in H and L anemones fed large rations (4.0 to 5.6% of dry body weight), respectively. H anemones fed smaller daily rations, approximating amounts of zooplankton captured naturally (1% of anemone dry weight), had higher growth rates and growth efficiencies than L anemones, which lost mass. Higher growth rates in H anemones, which are supported by higher prey-capture rates, result in attainment of minimum body size for reproduction in a relatively short period of time despite reduction in time available for feeding, thus improving relative fitness of these anemones in the upper intertidal zone.     

Feeding enhancement by social attraction in the Sandwich Tern

Summary A controversy exists over the relative roles of feeding enhancement and predation in shaping the flocking behavior of birds. We examined social attraction among foraging Sandwich Terns, Sterna sandvicensis, in the Gulf of Mexico. The prediction that foraging terns would coalesce at a spot where another bird dove was supported. Approach took place within seconds, occurred over a distance of at least 500 m, and lasted about 10 to 20 s after the food source dissipated. Although social attraction appears to play a major role in the feeding of the terns, this need not be construed as evidence against a similarly important role for predation in shaping social foraging behavior.     

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.     

Amino acid synthesis in the symbiotic sea anemone Aiptasia pulchella

Symbiotic Aiptasia pulchella and freshly isolated zooxanthellae were incubated in NaH¹⁴CO₃ and NH₄Cl for 1 to 240 min, and samples were analysed by reverse-phase high-performance liquid chromatography (HPLC) and an online radiochemical detector. NH₄ ⁺ was first assimilated into ¹⁴C-glutamate and ¹⁴C-glutamine in the zooxanthellae residing in A. pulchella. The specific activities (dpm nmol⁻¹) of ¹⁴C-glutamate and ¹⁴C-glutamine in vivo, were far greater in the zooxanthellae than in the host tissue, indicating that NH₄ ⁺ was principally incorporated into the glutamate and glutamine pools of the zooxanthellae. ¹⁴C-α-ketoglutarate was taken up from the medium by intact A. pulchella and assimilated into a small amount of ¹⁴C-glutamate in the host tissue, but no ¹⁴C-glutamine was detected in the host fraction. The ¹⁴C-glutamate that was synthesized was most likely produced from transamination reactions as opposed to the direct assimilation of NH₄ ⁺. The free amino acid composition of the host tissue and zooxanthellae of A. pulchella was also measured. The results presented here demonstrate that NH₄ ⁺ was initially assimilated by the zooxanthellae of A. pulchella. Received: 3 February 1997 / Accepted: 24 October 1997     

Population structure in the sexually reproducing sea anemone Oulactis muscosa

The intertidal sea anemone Oulactis muscosa (Drayton) is dioecious and most individuals are sexually mature throughout the year. Biochemical genetic evidence was used to determine the genetic structure of populations and to infer the relative contributions of sexual and asexual reproduction to recruitment. Data were collected for six enzyme-encoding loci from local populations spread along 735 km of the south east coast of Australia. The genetic structure of each of the nine local populations studied was consistent with recruitment by sexually produced individuals. In almost all cases, the observed single-locus genotypic frequencies closely matched those expected for hardy-Weinberg equilibria, however, consistent deficits of heterozygotes were detected for all loci. No apparent subdivision of the population was detected within the sampling area. Low levels of genetic differentiation were found between local populations and standardised variance (F _ST ) values were similar to those for other species with widespread planktonic dispersal of larvae.Contribution No. 60 from the Ecology and Genetics Group of the University of Wollongong     

Genetic evidence of self-fertilization in the sea anemone Epiactis prolifera

Biochemical genetic variation provided evidence for the mode of reproduction of brooded young in the sea anemone Epiactis prolifera Verrill, 1869. Individuals of E. prolifera are female when small but hermaphroditic when large (i.e., gynodioecious); juveniles are brooded externally on the column. Brooding individuals collected from 6 intertidal sites (5 in central California and 1 in Washington State, USA) in the spring and summer of 1980 were assayed for gene-enzyme variation by starch-gel electrophoresis. Three of 12 enzyme loci were polymorphic; phosphoglucose isomerase appeared to be encoded by two, closely linked loci. Genotypic frequencies deviated markedly from expected random mating proportions. Only three heterozygotes were found; two were heterozygous at all three polymorphic loci, and the other was polymorphic at the two PGI loci. All 158 juveniles from 25 brooding individuals were assayed (2–19 juveniles per parent). Juveniles on homozygous adults were always identical to their parent. However, brooded young of heterozygous individuals were not identical to their parent. but showed 1:2:1 phenotypic segregation ratios consistent with reproduction by self-fertilization. This genetic evidence together with findings of marked heterozygote deficiencies and genetic identity of homozygous adults and their brooded young supports the conclusion that E. prolifera usually reproduces by self-fertilization, and cross-fertilization is rate.     

Acetate incorporation into the lipids of the anemone Anthopleura elegantissima and its associated zooxanthellae

Anthopleura elegantissima containing zooxanthellae, as well as isolated zooxanthellae, incubated with acetate-1-¹⁴C under both light and dark conditions readily incorporate radioactivity into their total lipid pools. In both cases, the specific activity was greatly increased in the light. Dark-incubated anemones and isolated zooxanthellae incorporate activity predominantly into polar lipid; the remainder being present principally in the triglyceride moiety. Light-incubated organisms, however, show a dramatic redistribution of isotope towards greatly increased triglyceride and was ester incorporation, with a concomitant drop in polar lipid. onder light conditions, 70 to 75% of the radioactivity found in the fatty acids of the total zooxanthellae lipid was present in hexadecanoic (16:0) and octadecenoic (18:1) fatty acids. These are also the two major fatty acids by mass. Octadecanoic acid (18:0) is less than 5% by mass. Isotope incorporation patterns suggest that octadecenoic acids arise by elongation of hexadecenoic acids and that this conversion is blocked in the dark. Isotope incorporation patterns for anemones suggest that fatty acids, primarily in the form of saturated or monoenoic fatty acids, are translocated from algal to animal cells. No activity was found in either octadecadienoic (18:2) or octadecatrienoic (18:3) acids. The significance of these data is discussed.     

Host feeding and nutrient sufficiency for zooxanthellae in the sea anemone Aiptasia pallida

Nutrient sufficiency of zooxanthellae in the sea anemone Aiptasia pallida cultured in low nutrient seawater depends on the availability of particulate food to the host. Zooxanthellae in anemones unfed for 20 to 30 d exhibited the following characteristics of nutrient deficiency: cell division rates decreased; chlorophyll a content gradually decreased from 2 to <1 pg cell^–1; and C:N ratios increased from 7.5 to 16. Over a 3-mo period, algal populations in unfed anemones gradually decreased, indicating that zooxanthellae were lost faster than they were replaced by division. The mitotic index of zooxanthellae in unfed anemones was stimulated either by feeding the host or by the addition of inorganic N and P to the medium. Whether algae are nutrient-limited in hosts under field conditions has not been examined fully; however, C:N ratios in zooxanthellae from field-collected hosts are slightly higher (9.4 vs 7.5) than in hosts fed to repletion in laboratory cultures. This observation might indicate N limitation in the field.     

Symbiont shuffling during thermal bleaching and recovery in the sea anemone Entacmaea quadricolor

The sea anemone Entacmaea quadricolor simultaneously harbours multiple symbiont types from the genus Symbiodinium, while providing essential habitat for anemonefish. This anemone lives close to its upper thermal threshold and experiences bleaching under elevated temperature and light stress. Here, we determine whether E. quadricolor experienced a shuffling in the abundance of two genetically distinct symbiont types (Symbiodinium C25 and C3.25) during bleaching and recovery. Anemones were exposed to control (22.9 °C) or elevated temperature (28.5 °C) for 42 days, whereas for the following 75 days, all anemones were exposed to 22.9 °C. By day 47, a more pronounced bleaching occurred via symbiont expulsion in the elevated temperature treatment than the control, and the proportion of C25 to C25 + C3.25 increased by 6.2 and 13.2 % in the control and bleached anemones, respectively. The increased relative abundance of C25 to C3.25 after exposure to thermal stress may indicate that C3.25 performs poorly when temperature is elevated. Although no significant recovery in symbiont density was detected, a revival of the C3.25 genotype was found at day 117, which may indicate that it is either more competitive or has qualities that are beneficial to the symbiosis when thermal stress is no longer apparent. This work demonstrates the potential for this anemone species to shuffle its symbiont types in response to environmental change and could provide resilience during times of stress.     

Photosynthesis-irradiance responses and photosynthetic periodicity in the sea anemone Aiptasia pulchella and its zooxanthellae

Sea anemones (Aiptasia pulchella) containing zooxanthellae (Symbiodinium microadriaticum) were maintained in a long-term laboratory culture on a 12 h light (100 E m^-2 s^-1):12 h dark cycle. Photosynthetic oxygen production was measured for the symbiotic association and for freshlyisolated zooxanthellae. Light utilization efficiencies () were similar for both sets of zooxanthellae, suggesting negligible shading of zooxanthellae by animal tissue in this association. Whereas freshly-isolated zooxanthellae were photoinhibited at high irradiances (800 to 1 800 E m^-2 s^-1), zooxanthellae in the host continued to function at photosynthetic capacity. Time of day may influence photosynthetic measurements in symbiotic organisms, as it was found that photosynthesis in A. pulchella followed a diel periodicity at both light-saturating (1 200 E m^-2 s^-1) and subsaturating (150 E m^-2 s^-1) irradiances. There was a peak period of photosynthesis between 12.00 and 14.00 hrs. Light stimulated dark respiration rates of A. pulchella. Dark respiration of sea anemones increased somewhat towards the end of the light cycle and was always greater after exposure to high irradiances.     

Genetic subdivision in the subtidal,clonal sea anemone Anthothoe albocincta

Anthothoe albocincta, a common subtidal anemone along south-eastern Australia, reproduces both sexually through broadcast spawning and clonally through fission. Clones may be distinguished both by their electrophoretic genotypes and the colour of their tentacles and oral discs. Local populations typically consist of many, dense clonal aggregations. However, some clones appear to have locally extensive distributions, forming a series of separate aggregations. The capacity of clones to disperse among local populations is unknown. In this study we used an electrophoretic survey at six allozyme loci to quantify levels of variation among samples from each of 13 local populations and four geographic regions sampled between April 1992 and June 1993. These data revealed that populations of A. albocincta, separated by up to 930 km, were at least moderately subdivided. Levels of geographic variation were high and the average F _ST value (standardised genetic variation) was 0.27 based on genotypes of all individual polyps. This value reflects substantial variation both within (F _PR =0.13) and among (F _RT =0.16) regions. Estimates of gene flow among both neighbouring populations and regions are therefore low (N _e m=1.7 and 1.3, respectively). UPGMA (unweighted pair-group method using arithmetic averages) dendrograms suggest that a genetic discontinuity occurs at the very south-east corner of Australia, paralleling reports for two other south-eastern Australian marine invertebrates. In addition, our analyses and theoretical predictions imply that localised proliferation of clonal genotypes may have caused us to underestimate the potential importance of gene flow via larval dispersal. Moreover, the abundance and vast geographic range of this species suggests that widespread dispersal does occur. Collections from three populations covering the peroid December 1992 to June 1993 were examined by crude dissection along with histological sectioning, and showed A. albocincta to be dioecious, with unisexual clones. Eggs within the ovaries of six females sampled over a 3 mo period were small (96±4 m) and similar to those of related species that produce planktotrophic larvae. In contrast, we found no evidence that clones were dispersed (shared) among neighbouring local populations. An average of only 6% of six-locus genotypes were common to pairs of local populations separated by up to 125 km, this being equal to the percentage expected through sexual reproduction alone. In addition, the percentage of shared genotypes did not decline with increasing geographic separation. These data imply that although asexual reproduction may be used to maintain local populations, the sexual production of genotypically diverse larvae is the primary source of widely dispersed colonists and hence of new clones.     

Dynamics of external brooding in the sea anemone Epictis Epiactis prolifera

The sexually produced young of the externally brooding actinian Epiactis prolifera Verrill, 1869 are attached to the parent's column just above the base. A transitory brood groove may be formed around the limbus when the parent contracts. In the population studied on the coast of Sonoma County, California, USA, from 27 to 49% of the adult anemones were brooding at any time, the proportion being inversely related to seawater temperature and directly related to size of the anemones. A brood usually consisted of young of various sizes. Overall, number of young being brooded was directly related to parent size and inversely related to size of the brooded juveniles. The growth rate of juvenile anemones was inferred to average about a millimeter in pedal disc diameter per month. Juveniles dislodged from the parent when smaller than 4 mm in basal diameter apparently cannot survive in the intertidal zone. Juveniles which successfully make the transition to adulthood have, therefore, been brooded for at least 3 months. Fewer than 50% of the smallest juveniles survived to a size at which they could live independently. The transition to adulthood was accompanied by a mortality of 80%. The survival rate, from egg to adult, was calculated to be about 1% per season.This work was done in partial fulfillment of the requirements for the Ph.D. degree in the Department of Zoology, University of California, Berkeley, and was supported by a U.S. National Science Foundation Traineeship.     

Are there physiological races of the sea anemone Diadumene luciae?

The chemical activators of feeding behaviour for coelenterates have been generally regarded as species specific. Recently, however, and English (North Sea) population of the sea anemone Diadumene (Haliplanella) luciae (Verrill) and an American (Californian) population of the same species were found to respond to different chemical activators. The results of further examination of this apparent anomaly show that a third population (from the English Channel, selected because of differences in colour variety and type of habitat from the North Sea population) shared the same activators as the other English population. There was no correlation between the identities of the feeding-behaviour activators and either the type of habitat or coloration of the three populations. Since the only physical factor shared by anemones with similar activators (i.e., the two English populations) was geographical, it is tentatively concluded that physiological races of D. luciae may be evolving, independently of morphological or differently coloured races, in different parts of the world.