The influence of nectar secretion rates on the responses of bumblebees (Bombus spp.) to previously visited flowers

Bumblebees can avoid recently depleted flowers by responding to repellent scent-marks deposited on flower corollas by previous visitors. It has previously been suggested that avoidance of visited flowers for a fixed period would be a poor strategy, since different plant species vary greatly in the rate at which they replenish floral rewards. In this study, we examined the duration of flower repellency after an initial bumblebee visit, using wild bumblebees (Bombus lapidarius, B. pascuorum and B. terrestris) foraging on four different plant species (Lotus corniculatus, Melilotus officinalis, Phacelia tanacetifolia and Symphytum officinale). We constructed a model to predict flower visitation following an initial visit, based on the nectar secretion pattern of the different plant species, the insect visitation rate per flower, and the search and handling times of bumblebees foraging on the plant species in question. The model predicts an optimal duration of flower avoidance which maximises the rate of reward acquisition for all bees. However, this optimum may be open to cheating. For two plant species, the evolutionary stable strategy (ESS) is a shorter duration of flower avoidance than the optimum. We found the duration of flower avoidance was markedly different among flower species and was inversely related to nectar secretion rates. The predicted ESSs for each plant species were close to those observed, suggesting that the key parameters influencing bumblebee behaviour are those included in the model. We discuss how bees may alter the duration of their response to repellent scents, and other factors that affect flower re-visitation.     

Responses of the flea beetles Phyllotreta nemorum and P. cruciferae to metabolically engineered Arabidopsis thaliana with an altered glucosinolate profile

Summary. Insects feeding on Cruciferae recognize their host plants at least partially by means of specific responses to glucosinolates. However, the effects of variations in glucosinolate levels on the acceptability of plants for specialized insects are not well understood. A survey of the literature demonstrated positive, no, as well as negative correlations between plant acceptability and glucosinolate levels. The present study took advantage of the presence of transgenic Arabidopsis thaliana plants with increased glucosinolate levels. Transgenic A. thaliana contain the CYP79A1 gene from Sorghum bicolor. This gene encodes an enzyme which converts L-tyrosine into p-hydroxyphenylacetaldoxime in the biosynthesis of cyanogenic glycosides in S. bicolor. In transgenic A. thaliana plants, endogenous enzymes convert p-hydroxyphenylacetaldoxime into p-hydroxybenzylglucosinolate (sinalbin), which is not found naturally in this plant. The introduction of CYP79A1 resulted in a four-fold increase in total glucosinolate levels in transgenic A. thaliana plants. Although these changes in glucosinolate levels were rather dramatic, they did not have any effects on the acceptability of A. thaliana for the two flea beetle species, Phyllotreta nemorum and P. cruciferae. The flea beetles did not discriminate between transgenic and wildtype plants. Furthermore, they did not discriminate between leaf discs of wildtype plants where different concentrations of p-hydroxybenzylglucosinolate had been applied topically on the leaf surface. Feeding in P. nemorum was stimulated by extremely high levels of allylglucosinolate while this compound had no effect on P. cruciferae. It is concluded that the effect of glucosinolates on adapted insects depends on the chemical or physical environment in which the glucosinolates are found.     

Nematode assemblages from <Emphasis Type="Italic">Avicennia</Emphasis><Emphasis Type="Italic">marina</Emphasis> leaf litter in a temperate mangrove forest in south-eastern Australia

Meiofauna from Avicennia marina leaf litter in a temperate mangrove forest was enumerated, and the nematode assemblages compared on the bases of leaf colour (used as a guide to leaf age) and shore horizon where samples were collected. Twenty-one putative nematode species were collected from 48 leaf litter samples. Univariate analyses indicated that neither the colour of the leaf nor the shore horizon significantly affected abundance of nematodes. However, of the four (2Ƕ) treatment groups, rarefaction curves revealed highest diversity on brown leaves from under the shade of the tree canopy (H'=0.751ǂ.126 SE, n=17). Species diversity of leaf litter nematodes was lower in this temperate mangrove system than reported from tropical mangrove studies. ANOSIM tests confirmed a significant effect of shore horizon on nematode assemblages. The dominant feeding group among nematodes was non-selective deposit feeders (7/21 species, but 77% of all nematodes). Epigrowth grazers were represented by 8/21 species of nematodes, but only 19% of the total number. Excised leaves became skeletonised by about 15 weeks. Shorter temporal scales of life cycles of nematodes compared with leaf degradation, and the dynamic nature of epibiontic assemblages, probably explain the similar assemblage structure on yellow and brown leaves.     

Studies on pigments,proteins and photosynthetic rates in some mangroves and mangrove associates from Bhitarkanika,Orissa

Pigment contents, proteins and net photosynthesis were investigated in fully developed leaf of 1-year-old seedlings of six mangroves (Bruguiera gymnorrhiza, Rhizophora apiculata) and mangrove associates (Caesalpinia bonduc, Cerbera manghas, Derris heterophylla, Thespesia populnea), collected from Bhitarkanika, located on the east coast of India. Large variations in the photosynthetic rates (P_N) among the six species were observed, ranging from 10.16 µmol CO₂ m^-2 s^-1 in C. bonduc to 15.28 µmol CO₂ m^-2 s^-1 in R. apiculata. The total leaf protein content ranged from 12.09 mg g^-1 dry wt in T. populnea to 51.89 mg g^-1 dry wt in B. gymnorrhiza. The chlorophyll a/b ratio was typically about 3.0 in all the studied species, except C. bonduc (2.8). Photosynthetic rates and chl a/b ratio in the leaves were found to be correlated. Analysis of chlorophyll and xanthophyll spectra suggested: (1) variations in different forms and amounts of carotenes as well as xanthophylls and (2) the presence of high amounts of near-UV-absorbing substances in leaves, particularly in the two mangroves (B. gymnorrhiza, R. apiculata) and a mangrove associate (T. populnea), which appears to be an adaptive feature. Estimation of the chl a/b ratios in isolated thylakoids yielded a low value of 1.8 for R. apiculata and >2.6 for other species. The total protein/chlorophyll ratios in thylakoids varied considerably from 3.14 (D. heterophylla) to 10.88 (T. populnea) among the mangrove associates and from 16.09 to 18.88 between the members of the Rhizophoraceae. The chlorophyll/carotenoid ratios in thylakoids of the six species were more or less similar. The absorption spectra for washed thylakoids of C. manghas and D. heterophylla exhibited absorption characteristics typical for C₃-plant thylakoids. However, thylakoids isolated from R. apiculata, B. gymnorrhiza, C. bonduc and T. populnea exhibited an unusual increase in absorption in the blue region (380-410 nm) of the absorption spectrum. The presence of high-absorbing (in the short-wavelength, near-UV region) pigments appears to be closely associated with the thylakoids in R. apiculata and T. populnea. Our results, therefore, suggest a wide range of variation, not only in protein and pigment contents of photosynthetic tissues, but also in the spectral characteristics and composition of the pigments in mangrove species. An understanding of the nature of these pigments in mangroves and their associates, under their natural conditions and especially in relation to eco-physiological adaptations, is necessary, not only in relation to conservation, but also to allow propagation under different salinity conditions.     

D-Pulegone: A feeding deterrent to deer mice (Peromyscus maniculatus) and prairie voles (Microtus ochrogaster)

Summary D-Pulegone, a monoterpene present in pennyroyålMentha sp. is repellent to a number of vertebrates, decreasing consumption of feed and granular particles. In the present study, several tests were used to determine the reliability of this substance as a feeding deterrent. D-Pulegone (10mM—1M) applied as a coating to apple pieces significantly (P < 0.001) decreased ingestion by deer mice,Peromyscus maniculatus, and prairie voles,Microtus ochrogaster. The deterrent effect of d-pulegone was not diminished following multiple exposures suggesting that this compound may be a promising candidate for use a a general vertebrate repellent. The inhibitory effect of 1M d-pulegone on apple ingestion in voles was abolished when contact was prevented by encasing the stimulus solution in plastic mesh capsules. We conclude that while d-pulegone has repellent properties following direct contact, volatile exposure alone may not be sufficient to elicit avoidance behavior.     

Alarm responses of the green sea urchin, Strongylocentrotus droebachiensis, induced by chemically labelled durophagous predators and simulated acts of predation

The green sea urchin, Strongylocentrotus droebachiensis, exhibited immediate behavioural responses to waterborne chemosensory cues from two durophagous predators, the Atlantic wolffish, Anarhichas lupus, and the edible crab, Cancer pagurus; as well as from crushed conspecifics and crushed blue mussels, Mytilus edulis. The response patterns were dose dependent and diet dependent. Strong responses were elicited by water conditioned by echinivorous wolffish (97.5%), undiluted urchin extract (73%), undiluted mussel extract (52.5%), and by water conditioned by echinivorous crab (45%). In contrast, urchin extract diluted to 1%, mussel extract diluted to 10%, and water conditioned by predators on a mussel diet elicited weak responses (~20%). Infection by the endoparasitic nematode Echinomermella matsi had no significant effect on the response pattern of S. droebachiensis. There was no conclusive evidence of an alarm response to the predators per se, as the weak response to stimuli from non-echinivorous wolffish and crab, as well as from the seastar Asterias rubens, may have been caused by chemical cues transmitted from their mussel diet. The diet-dependent response to predators suggests that active predators were labelled by chemical cues from their echinoid prey. The chemical cue from echinivorous wolffish acted as both an arrestant and as a repellent, whereas the response to other cues was predominantly or exclusively repellent. The response to echinivorous wolffish was quantitatively stronger than, and qualitatively different from, the response to other stimuli, including undiluted urchin extract. The wolffish is apparently being labelled by a latent chemical cue which derives its potency from activation by, or interaction with, substances in the digestive system of the wolffish. We interpret this novel phenomenon as evidence of alarm signal magnification. The induced behavioural modifications demonstrate the green sea urchins' ability to detect chemical cues associated with active durophagous predators, and would therefore seem to have adaptive potential as a predator defence mechanism.     

Combined effects of dissolved oxygen concentration and water temperature on embryonic development and larval shell secretion in the marine snail <Emphasis Type="Italic">Chorus</Emphasis><Emphasis Type="Italic">giganteus</Emphasis> (Gastropoda: Muricidae)

The present study was undertaken to determine the effects of both extracapsular oxygen concentration and temperature on embryonic development in Chorus giganteus. In normoxia increasing water temperature from 12°C to 18°C reduced by 15 days the median time required for the capsules to hatch. Hypoxia (oxygen content at 50% of air saturation) generated a low development rate and totally prevented both shell secretion and larval hatching from the egg capsule. Experimental transfer at weekly intervals, from normoxia to hypoxia and vice versa, induced a decrease and increase in the embryonic ash content, respectively, but did not affect the number of hatched larvae. Such an effect was more pronounced at 12°C than at 15°C or 18°C. The embryonic inability to produce a shell under hypoxia is likely to be a result of the low intracapsular oxygen concentration (IPO₂) generated as the combined effect of a low extracapsular oxygen concentration (environmental) added to the intracapsular embryonic oxygen demands, which lowers the IPO₂ still further. Under such conditions, a decrease in intracapsular pH is likely to take place, and, if so, embryos might divert carbonates away from shell calcification to balance such changes in pH.     

Nutritional quality of phloem sap in relation to host plant-alternation in the bird cherry-oat aphid

Summary. Host alternation in aphids has been attributed to complementary growth of host plants, or more specifically to seasonal changes in the nitrogen quality of the phloem sap. In this report, seasonal fluctuation of free amino acids in phloem of the winter and summer host plants (Prunus padus, bird cherry and Hordeum vulgare, barley) of Rhopalosiphum padi (the bird cherry-oat aphid) were investigated in the context of aphid growth and behaviour. Phloem was collected from the cut stylets of aphids taken from plants that were grown outdoors. The total concentration of amino acids in P. padus phloem increased between bud break and late flush (spring), decreased in mature leaves (summer), and increased again in early senescent leaves (autumn). In H. vulgare, however, amino acid concentration fluctuated less from seedlings to flowering. Spring aphids from P. padus grew rapidly on this host from bud break to late flush, but died on mature and early senescent leaves. Summer aphids from H. vulgare grew as fast on this host as spring aphids did on flush leaves of P. padus. Sexual females grew more slowly than other generations and nearly as well on mature as on early senescent P. padus leaves. As judged by aphid growth and phloem nitrogen quality, P. padus during spring equals the summer host H. vulgare. However, the lower growth rates of R. padi on mature and senescent leaves of P. padus appear only loosely correlated with phloem amino acid concentrations. Therefore, factors influencing aphid nutrition, or ecology, other than seasonal changes in phloem sap amino acid concentration may explain the existence of host alternation in R. padi.     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.     

Okadaic acid: a potential defense molecule for the sponge <Emphasis Type="Italic">Suberites domuncula</Emphasis>

Sponges are sessile filter feeders that are exposed to toxic compounds/xenobiotics and a huge number of diverse pro- and eukaryotic organisms occurring in the aqueous milieu. To cope with these threats, sponges have developed a variety of chemical and immunological defense systems. Among them are potent secondary metabolites or toxins. The demosponge Suberites domuncula contains the bioactive compound okadaic acid; concentrations of ᅢ ng/g (ᅬ nM) of wet tissue have been quantified by LC/MS. In the present study, we sought to determine whether okadaic acid is beneficial for this sponge. Antibodies were raised against okadaic acid and used to localize this compound in situ. It could be demonstrated that the polyclonal antibodies/antiserum stained the sponge cells (endopinacocytes) which form the epithelium of the lacunae and the water channels. They also reacted with bacteria present in sponge tissue. Okadaic acid was found to augment the immune response of S. domuncula against the bacterial endotoxin lipopolysaccharide (LPS). If primmorphs, a special form of 3-D aggregates, are incubated with LPS at a concentration of 3 µg/ml, a reduction in the incorporation rate of [³H] phenylalanine is measured. In the presence of okadaic acid, this effect is measured already at a concentration of 1 µg/ml, suggesting that okadaic acid sensitizes this (immune) response. Okadaic acid is a known inhibitor of protein phosphatases which, as shown here, cause an increased phosphorylation of p38, a central kinase of the MAP kinase pathway; this may explain the sensitive response of the sponge cells to LPS. At higher concentrations, okadaic acid induces apoptosis, as was demonstrated here by a differential expression of the Bcl-2 homolog and the caspase gene. It is concluded that okadaic acid has two functions in the sponge; first, at low concentrations (<100 nM) a stimulation of the defense system against bacteria, and second, at concentrations above 500 nM, a differential effect on the pro-/anti-apoptotic genes.     

Algal products as naturally occurring substrates for p-glycoprotein in Mytilus californianus

Mytilus californianus is a filter feeder that removes seaweed particulates, phytoplankton, and their byproducts from the water. The gills of this animal express high multixenobiotic resistance (MXR) and multixenobiotic transport activity that is related to the mammalian p-glycoprotein (p-gp). The high p-gp observed in mussel gills may provide the mussel protection from natural toxins in the diet. To test this hypothesis, extracts of various seaweeds and phytoplankton found in and around the mussel habitat were examined for presence of p-gp substrates by competition assays with a fluorescent dye substrate, rhodamine B. Accumulation of rhodamine was increased whereas dye efflux was slowed in the presence of algal extracts, indicating that p-gp substrates are present in the seaweeds Macrocystis pyrifera, Egregia menziesii, and Phyllospadix scouleri and the phytoplankton Alexandrium catanella and Pseudonitzchia australis. Initial fractionation of E. menziesii extracts by HPLC showed several distinct peaks of moderate hydrophobicity with p-gp-modulating ability. Additionally, Egregia extract showed potential as a chemosensitizer in tests with mussel (Mytilus edulis) and sea urchin (Lytechinus pictus) embryos. These data indicate that marine algae contain compounds that are substrates and/or chemosensitizers for the p-gp transporter in marine bivalves, thus providing evidence that MXR may have evolved in response to dietary pressures.     

Influence of host plants on sexual communication in the herbivorous bug Lygocoris pabulinus

Summary. Host plant volatiles may be involved in the sexual communication of insects in several ways. In the pheromone-producing sex, these volatiles may affect pheromone production or release and, in the receptive sex, plant volatiles may have a synergistic effect on the attraction to sex pheromone. We conducted three types of experiments to determine if and how plant volatiles are involved in the sexual communication of Lygocoris pabulinus (L.) (Heteroptera: Miridae), the females of which attract males. In a one-choice cylinder bioassay, females were offered to males on two different plants, belonging to different plant families, i.e. potato leaves and goosefoot leaves, to determine if specific plant volatiles were involved in the attraction of males towards females. Females on potato leaves were as attractive as females on goosefoot leaves, but significantly more attractive than females without plant material. The latter result suggests an interaction between females and potato leaves. However, in two-choice flying and walking bioassays, using delta traps in a wind tunnel and a vertical Y-track olfactometer, males were attracted to females irrespective of the presence of potato leaves. This difference in result is probably due to the fact that in the latter assays females were confined with pollen as an alternative food source, while females in the one-choice assay had access to water only, so that they may have suffered from malnutrition. Males in the one-choice assay were also attracted to potato leaves from which females had been removed, indicating that attractive components from females are deposited and adsorbed to the substrate. Plants are probably only indirectly involved in sexual communication, their surface merely functioning as a substrate from which pheromone is released. Males may subsequently be attracted to such plants or substrates. Clean plant material was not attractive to L. pabulinus males, hence plant volatiles alone do not seem to be used by these males as possible mate location cues.     

Possible involvement of the glycocalyx in the ichthyotoxicity of Chattonella marina (Raphidophyceae): immunological approach using antiserum against cell surface structures of the flagellate

Although the ichthyotoxic mechanism of Chattonella marina is still unknown, several lines of evidence suggest that the reactive oxygen species (ROS), such as superoxide anion (O₂^-), hydrogen peroxide (H₂O₂)_, and hydroxyl radical (·OH), produced by C. marina are involved in the mortality of fish exposed to this flagellate. Recently, we found that the cell-free supernatant prepared from C. marina, which is considered to contain the glycocalyx, showed NADPH-dependent O₂^- generation. In this study, we prepared antiserum against the crude glycocalyx of C. marina. Using indirect immunofluorescence, it was confirmed that the antiserum specifically reacted with C. marina cells. In addition to C. marina, the antiserum also reacted with other raphidophycean flagellates such as Heterosigma akashiwo, Olisthodiscus luteus, and Fibrocapsa japonica, whereas no reactivity was observed against six other flagellate species tested. These results suggest that raphidophycean flagellates have common epitopes recognized by the antiserum. Interestingly, immunohistochemical analysis of paraformaldehyde-fixed gill lamellae from yellowtail exposed to C. marina revealed that the antiserum stained the surface of gill lamellae, while no such staining pattern was observed in control gill lamellae. These results suggest that the glycocalyx may be discharged when C. marina cells are inhaled into the fishes' mouths and then come into contact with the gill surface. Based on the present results, together with our previous findings, we propose that continuous accumulation of the discharged glycocalyx on the gill surface occurs during C. marina exposure, which may be responsible for the ROS-mediated severe gill tissue damage leading to fish death.     

Seasonal variation in the degree of heterospecific association of two syntopic hyraxes (<Emphasis Type="Italic">Heterohyrax</Emphasis><Emphasis Type="Italic">brucei </Emphasis>and <Emphasis Type="Italic">Procavia capensis</Emphasis>) exhibiting synchronous parturition

Adult rock hyrax (Procavia capensis) and bush hyrax (Heterohyrax brucei) allocate much of their surface activity to being vigilant for vertebrate predators. Individuals of the two species associate for thermoregulatory basking on rock outcrops (koppies) and, in the Matobo National Park, Zimbabwe, frequently constitute heterospecific groups that produce offspring synchronously. Exposure to predators during basking is thought to put a high premium on vigilance, particularly during the presence of dependent offspring in nurseries. We predicted, therefore, an increase in the association of the two species of hyraxes when offspring were present because this would lead to larger groups, thus enhancing the ability to detect predators. Koppie aggregations of hyraxes were classified as homospecific or heterospecific according to basking group composition. Heterospecific association increased when offspring were present, and heterospecific groups were larger than homospecific ones. Fifty-one percent of nurseries were heterospecific for adults/subadults. During the pre-weaning phase (<2 months after birth), 85% of P. capensis and 65% of H. brucei juveniles were in nurseries of heterospecific basking groups. Heterospecific groups had more offspring than homospecific groups. Juvenile mortality is most evident after young are weaned and heterospecific associations break down. Heterospecific aggregation, through increased group size, may result in a higher probability of avoiding predators, thereby reducing the vulnerability of young.     

Elysia timida (Risso, 1818) (Gastropoda, Opisthobranchia): relationship and feeding deterrence to a potential predator on the south-western Mediterranean coast

The relationship between the sacoglossan Elysia timida and the ornate wrasse Thalassoma pavo was studied in two laboratory experiments using artificial models. A feeding-preference experiment was conducted to determine whether mollusc extract deterred feeding by T. pavo, by using a "realistic" mollusc model (imitating the colour pattern of E. timida) coated with mollusc extract, and a reference model (without imitation or extract), and fishes collected from two locations. It was observed that fish approached, attacked and ate more reference models than mollusc models. A second feeding-preference experiment was designed with four different coloured models: "realistic" (W), green (G), red (R) and chequered (S) pattern. Both fish populations clearly rejected the S model, and differed in their colour preferences. Although both populations seemed to prefer the R and G models equally, the W model was clearly rejected by the fish that coexist with the mollusc at one site (Mazarrón), but was not rejected by the other population of fish which does not coexist with it (Cabo de Palos). Mazarrón fishes would identify the W model with the presence of a toxic compound during their coexistence, and therefore avoid attacking conspicuously coloured E. timida models as a response to their visual signals. Therefore, it was concluded that extract of E. timida is a deterrent for T. pavo, and its effect is sufficiently noxious that the fish tend to avoid it, so that the ability of the fishes to learn to recognise colours and identify certain colour patterns associated with obnoxious prey provides the molluscs the opportunity to survive by exhibiting a conspicuous coloration.     

Evolutionary relationships of deep-sea vent and cold seep clams (Mollusca: Vesicomyidae) of the "<Emphasis Type="Italic">pacifica/lepta</Emphasis>" species complex

Vesicomyid clams are among the dominant invertebrates of chemosynthesis-based communities found at deep-sea cold seeps and hydrothermal vents, yet there is considerable taxonomic confusion within the family Vesicomyidae. The present study examined phylogenetic relationships among vesicomyid clams belonging to a cryptic-species complex that includes Vesicomya (previously Calyptogena) pacifica (Dall, 1891) and Vesicomya lepta (Dall, 1896). Mitochondrial (mt) COI sequences from clams collected between 1991 and 2001 along the western margin of North America from the San Clemente Basin (32°N latitude) to the Juan de Fuca Ridge (48°N latitude) revealed five discrete evolutionary lineages: V. pacifica (mt-type I), V. lepta (mt-type IV), and three undescribed species defined by mt-types II, III, and V. Separation of the three lineages defined by mt-types I, II, and III was also revealed by an independent nuclear gene (ribosomal ITS-1), and by DNA sequences from vertically transmitted bacterial endosymbionts found in the gills of these clams. Lineages I, II, and III occur in Monterey Bay, California, and the data suggest that bathymetric segregation may have played a role in their isolation and divergence.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Lack of phlorotannin induction in the kelp Laminaria hyperborea in response to grazing by two gastropod herbivores

Phlorotannins (polyphenolic compounds) are found exclusively in brown algae and are commonly regarded as an important component of algal defense against herbivores. The variation in phlorotannin concentration within and among individual seaweed specimens of the same species can be substantial, and seaweeds may respond to herbivore damage by an increased production of phlorotannins in order to prevent further herbivory. In the present study, we investigated the natural variation of phlorotannins in different parts of the kelp Laminaria hyperborea in an observational field survey. We also performed a manipulative induction experiment in which we studied the effect of natural grazing by the patellid limpet Ansates (=Helcion) pellucida and the littorinid snail Lacuna vincta, as well as artificial damage on the production of phlorotannins in L. hyperborea. There was a large variation in phlorotannin content both among and within L. hyperborea individuals and populations. The control plants in the induction experiment had a higher phlorotannin content compared to plants in natural populations, but grazing by A. pellucida or L. vincta, or artificial damage did not induce a higher phlorotannin concentration in the kelps. Instead, the phlorotannin content in wounded kelps was significantly lower than in the control plants, possibly due to removal of phlorotannin-rich outer layers of the kelp thallus, or to an induced compensatory growth response in the L. hyperborea plants. These results imply that L. hyperborea phlorotannins probably do not function as an inducible chemical defense against A. pellucida and L. vincta, instead grazing by these herbivores decreases the phlorotannin content of the kelps.     

Cardiac output during exposure to <Emphasis Type="Italic">Chattonella</Emphasis><Emphasis Type="Italic">marina</Emphasis> and environmental hypoxia in yellowtail (<Emphasis Type="Italic">Seriola</Emphasis><Emphasis Type="Italic">quinqueradiata</Emphasis>)

The experiments were undertaken to measure, for the first time, cardiac output in yellowtail (Seriola quinqueradiata) during exposure to a harmful red tide flagellate (Chattonella marina). The responses were compared with those during exposure to environmental hypoxia to evaluate the significance of the drop of arterial oxygen partial pressure (PaO₂) in the fish-kill mechanisms by C. marina. PaO₂ immediately decreased, whereas heart rate (HR) was maintained until shortly before death during exposure to C. marina. Suffocation developing during the exposure resulted from a decrease in blood oxygen content, but not from lowered blood flow to the tissue. Although exposure to both C. marina and hypoxia immediately decreased PaO₂, arterial oxygen content (CaO₂) and pH (pHa) were significantly lower, whereas HR and cardiac output (&Qdot;) remained significantly higher, for the C. marina-exposed fish than for hypoxia-exposed fish. Although the drop in PaO₂ appears to play a pivotal role in the mechanisms of fish death by C. marina, other physiological response(s) should also be considered.     

Ecological,genetic, and morphological differences among three <Emphasis Type="Italic">Pavona</Emphasis> (Cnidaria: Anthozoa) species from the Pacific coast of Panama

Ecological, genetic, and morphological differences among three Panamanian Pacific Pavona species with strongly developed collines (Pavona varians, P. frondifera, and P. chiriquiensis) were examined. Ecological factors included geographical distributions of species, habitat preferences, interspecific interactions, reproductive ecology, and tolerance to bleaching. Genetic differences were based on the electrophoretic analysis of ten allozyme loci. Morphological analyses consisted of tissue coloration, colony morphology, and measurements and counts of ten macro- and micro-skeletal characters. P. varians, present on reefs or in coral communities, is the most widely distributed and shows considerable morphological variation. P. chiriquiensis, a recently described species, encrusts basalt rock and has little morphological variation. P. frondifera is a reef dweller with a compact foliose morphology. Tissue coloration varies from light to dark brown in P. varians, from pink to brown in P. frondifera, and from brick red to brown or silvery in Pavona chiriquiensis. Also, the white to silvery polyp mouths of the latter species are a diagnostic feature that allows an easy identification in the field. Aggressive dominance during short-term interspecific interactions were as follows: Pavona chiriquiensis>P. varians>P. frondifera. P. chiriquiensis and P. varians showed contrasting responses to sea warming during the 1997-1998 El Niño Southern Oscillation. Whereas entire P. chiriquiensis bleached and died within 4 weeks of exposure to 30-31°C, colonies of P. varians did so only on their upper surfaces. The response of P. frondifera to elevated temperatures was not observed because it is mainly present in the Gulf of Panama where coral bleaching was absent in 1997-1998. The genetic data indicated that P. chiriquiensis differed strongly from both P. varians and P. frondifera, with Nei's unbiased genetic distances of 0.434 and 0.379, respectively. A fixed difference between P. varians and P. frondifera, and P. chiriquiensis exists at the triose phosphate isomerase (TPI-2) locus. A nearly fixed difference between P. chiriquiensis and P. frondifera and between P. chiriquiensis and P. varians was found at the hexokinase (HK) locus. P. varians differed slightly from P. frondifera with Nei's unbiased genetic distance of 0.068. No fixed difference was found between P. varians and P. frondifera. There were strong differences between P. chiriquiensis and P. varians in spawning times and gamete characteristics. Spawning in P. varians and P. chiriquiensis is 12 h out of phase. Also, eggs of the former species are white to beige and positively buoyant whereas those of the latter species are dark green and neutrally to negatively buoyant. No reproductive data are yet available for P. frondifera. Calicular diameters are significantly greater in P. chiriquiensis than in the other two species. In contrast, corallum thickness is greater in P. varians and P. frondifera than in P. chiriquiensis. Canonical discriminant function analysis readily separated the three species.