Shelter preference in captive juveniles of European spiny lobster <Emphasis Type="Italic">Palinurus elephas</Emphasis> (Fabricius, 1787)

Physical shelter features (e.g. shape, size and substrate slope) were tested in the laboratory to evaluate the preferences of juveniles of European spiny lobster, Palinurus elephas. Tethering experiments to assess whether substrate slope affects the vulnerability of juveniles to predation were also performed. Our results showed that: (1) semi-circular dens were significantly preferred over square and circular shapes; (2) when not disturbed by a predator, lobsters sheltered in holes with a diameter closely related to their own size, while in the presence of a potential predator lobsters showed no significant preference for a particular shelter size; (3) lobsters significantly preferred dens excavated on sub-vertical (35°) substrates over those excavated on vertical ones (90°); (4) individual lobsters tethered on vertical substrates were subject to greater predation activity than those tethered on horizontal structures. In conclusion, the present study contributes to the understanding of how physical properties of shelters affect the choice of P. elephas juveniles, enhancing their protection and survival rate.     

Improving escape responses of hatchery-reared scallops <Emphasis Type="Italic">Argopecten purpuratus</Emphasis>

Hatchery rearing of the scallop Argopecten purpuratus has resulted in successive generations of scallops not exposed to predators that are less sensitive to and escape more slowly from predators than wild scallops. The present study examined whether conditioning hatchery-reared A. purpuratus to its natural predator, the sea star Meyenaster gelatinosus, improved its escape responses. Both juvenile and adult A. purpuratus from Tongoy Bay, Chile, were exposed for 7 days to different conditions: (1) continuous predator odor, (2) predator contact for 30 min three times a day, (3) a combination of the two previous conditions, and (4) no exposure to the predator (control). After conditioning, we evaluated scallop’s escape responses: reaction time, total clap number, duration of the clapping response, clapping rate, and the time scallops spent closed when exhausted. Conditioning with contact and odor plus contact (i.e., high predation risk) resulted in 25 and 50% shorter reaction times of juveniles and adults, respectively. Further, these stimuli caused juveniles to increase the number of claps and clapping rate. For adults, the time spent closed after exhaustion decreased by 50 and 63% after conditioning with contact and odor plus contact, respectively. Therefore, it is shown for the first time that exposure of scallops to increasing predator stimuli enhances escape responses, evidence of threat-sensitive predator avoidance.     

Circadian shelter occupancy patterns and predator–prey interactions of juvenile Caribbean spiny lobsters in a reef lagoon

The spiny lobster, Panulirus argus, is predominantly nocturnal, remaining inside shelters during the day and foraging outside at night, presumably to minimize predation risk. Predation risk generally decreases with increasing lobster size. Therefore, this study examined the hypothesis that size would influence this basic circadian pattern. Video cameras continuously recorded the shelter occupancy of juvenile lobsters (n = 72) having a carapace length (CL) of 30–62 mm that were tethered to shelters in a shallow reef lagoon. The lobsters’ shelter occupancy was 100% during the day, but declined linearly from shortly before sunset to a minimum of 50% shortly after midnight and then increased linearly, reaching 100% by 1 h after sunrise. The percent time the lobsters spent in the shelters followed a similar trend, but there was wide variability at night (0–100%) for individual lobsters. Lobsters left their shelters 2–30 times night⁻¹, with a majority of excursions lasting <10 min. These results suggest that juvenile P. argus minimize predation risk by remaining in their shelters as long as possible but offset the energetic cost of this behavior by foraging close to their shelters for several short periods at night. This emergence pattern contrasts with those of early benthic phase lobsters (<15 mm CL), which seldom leave their shelters, and adults (>80 mm CL), which have a dusk/early evening peak in activity and leave the shelter for extended periods of time during the night. Furthermore, a minimum shelter occupancy in the middle of the night appears especially well adapted to avoid exposure to daytime predators. Videotaped observations also included interactions between lobsters and two dominant lobster predators, the triggerfish, Balistes capriscus, and the octopus Octopus cf. vulgaris. Lobsters responded differently to these predators: remaining in the shelter when attacked by a triggerfish and fleeing the shelter when attacked by an octopus. Triggerfish were nearly twice as likely to attack a lobster that was outside of the shelter than inside. Once under attack, however, a lobster had nearly the same chance of surviving if it was inside or outside. Results suggest that the patterns of shelter use and emergence change as lobsters grow, probably reflecting the interplay between perception of predation risk and the need to forage. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predation risk assessment by green frog (<Emphasis Type="Italic">Rana clamitans</Emphasis>) tadpoles through chemical cues produced by multiple prey

Many prey assess predation risk through predator chemical cues. Numerous studies have shown that (1) prey sometimes respond to chemical cues produced by heterospecifics and (2) that many species are capable of associative learning. This study extends this research by focusing on predation risk assessment and antipredator behavior in environments containing chemical cues produced by multiple prey species. The results show that green frog (Rana clamitans) tadpoles (1) assess risk from the chemical cue produced during predation by a heterospecific (gray tree frog, Hyla versicolor, tadpoles) and (2) can exhibit similarly strong behavioral responses to a mix of conspecific and heterospecific cues compared to conspecific cue alone, depending on their conditioning environment. I then discuss how the prey choice of the predators and the relative abundances of the prey species should influence the informational value of heterospecific cues.     

Disease avoidance influences shelter use and predation in Caribbean spiny lobster

Shelter competition is uncommon among social animals, as is the case among normally gregarious Caribbean spiny lobsters (Panulirus argus). However, healthy lobsters avoid sheltering with conspecifics infected by a lethal pathogenic virus, PaV1. These contradictory behaviors have implications for shelter use and survival, especially in areas where shelter is limited. In laboratory experiments, we tested shelter competition between paired healthy and diseased juvenile lobsters in shelter-limited mesocosms. Neither healthy nor diseased lobsters dominated access to shelters, but lobsters shared shelter less often when diseased lobsters were present relative to controls with two healthy lobsters. We hypothesized that exclusion of juvenile lobsters from shelter results in increased mortality from predation, especially for the more lethargic, infected individuals. Field tethering trials revealed that predation was indeed higher on infected individuals and on all tethered lobsters deprived of shelter. We then tested in mesocosm experiments how the contrasting risks of predation versus infection by a lethal pathogen influence shelter use. Lobsters were offered a choice of an empty shelter or one containing a diseased lobster in the presence of a predator (i.e., caged octopus) whose presence normally elicits shelter-seeking behavior, and these data were compared with a previous study where the predator was absent. Lobsters selected the empty shelter significantly more often despite the threat of predation, foregoing the protection of group defense in favor of reduced infection risk. These results offer striking evidence of how pathogenic diseases shape not only the behavior of social animals but also their use of shelters and risk of predation.     

Ontogenetic changes in digestive enzyme activity of the spiny lobster,<Emphasis Type="Italic"> Jasus edwardsii</Emphasis> (Decapoda; Palinuridae)

Digestive enzyme profiles of puerulus, post-puerulus, juvenile and adult stages of the spiny lobster Jasus edwardsii Hutton were determined in order to identify ontogenetic changes in digestive capabilities and assess capacity to use dietary components and to exploit diets to meet nutritional requirements. Juvenile and adult lobsters exhibited a diverse range of enzymes, suggesting they can exploit varied diets. Proteolytic activity of trypsin, chymotrypsin and carboxypeptidase A (1.86–3.70 U mg^–1 digestive gland protein) was significantly higher than carbohydrase activity of amylase, -glucosidase, cellulase and chitinase (0.0014–0.38 U mg^–1 digestive gland protein), thus showing that dietary protein was more important than carbohydrate and that the lobster is carnivorous. These conclusions are consistent with other studies that found spiny lobsters to feed predominantly on crabs, bivalves, ophiuroids and sponges. Lipase activity (0.371 U mg^–1 digestive gland protein) was also relatively high, thus showing the importance of dietary lipid. Total activities (units per digestive gland) of every enzyme assayed increased significantly with lobster carapace length. There were few significant ontogenetic trends in specific enzyme activity (U mg^–1 digestive gland protein). Amylase and laminarinase specific activity was significantly higher in small lobsters than large lobsters (regression analyses, respectively, F_(1,23)=9.84, P=0.005; F_(1,11)=19.65, P=0.001), suggesting that carbohydrates including laminarin are more important in the diet of small juveniles. Trypsin, amylase and lipase activities were detected in all non-feeding puerulus stages, suggesting that feeding is not a cue for digestive enzyme production in J. edwardsii. Significant variations in total and specific activities of amylase (F_(1,3)=14.2, P=0.00; F_(1,3)=14.2, P=0.00) and trypsin (F_(1,3)=8.8, P=0.00; F_(1,3)=21.41, P=0.00) and a declining trend in lipase total activity between non-feeding puerulus stages suggests that they may be hydrolysing endogenous energy reserves to sustain their onshore swimming activity prior to settlement. Profiles suggest carbohydrate and lipid are utilised first, followed by protein. Consistently high levels of lipase in all puerulus stages (0.24–0.7 U digestive gland^–1; P>0.05) confirm the importance of lipid as a major energy substrate.Communicated by G.F. Humphrey, Sydney     

Predation on egg capsules of <Emphasis Type="Italic">Zidona</Emphasis><Emphasis Type="Italic">dufresnei</Emphasis> (Volutidae): ecological implications

Among the diverse patterns of energy allocation to the offspring of gastropods, the presence of egg capsules to protect embryos is common. Females of the edible snail Zidona dufresnei attach egg capsules to hard substrates in shallow Argentine Patagonian waters (40°45′S, 64°56′W) during spring-summer. Embryonic development takes about 30 days at 22°C. In this study, three likely capsule predator species and the marks left by each on egg capsule walls were identified in laboratory experiments in February 2010. Abundances of predators and egg capsules with evidence of predation were assessed in the field in the summers of 2010 and 2011. Under laboratory conditions (N = 10 replicates per treatment and control), the predation rate by the chiton Chaetopleura isabellei was the highest (up to 90%), followed by the gastropod Tegula patagonica and the crab Neohelice granulata (~20% each). Nearly 60% of 41 capsules found in the field showed signs of predation. According to the marks identified in the laboratory, C. isabellei was responsible for 79% of this predation, and T. patagonica for the rest. Predation appears to be important during the encapsulated early life and could be an agent for selecting for resistant capsule walls and a relatively shorter development time.     

Threat-sensitive learning of predators by larval mosquitoes <Emphasis Type="Italic">Culex restuans</Emphasis>

A prerequisite for prey to show adaptive behavioural responses to predators is that the prey has the ability to recognise predators as threats. While predator recognition can be innate in many situations, learning is often essential. For many aquatic species, one common way to learn about predators is through the pairing of a novel predator odour with alarm cues released from injured conspecifics. One study with fish demonstrated that this mode of learning not only allows the prey to recognise the predatory cues as a threat, but also mediates the level of threat associated with the predator cues (i.e. threat-sensitive learning). When the prey is exposed to the novel predator with a high concentration of alarm cues, they subsequently show a high intensity of antipredator response to the predator cues alone. When exposed to the predator with a low concentration of alarm cues, they subsequently show a low-intensity response to the predator cues. Here, we investigated whether larval mosquitoes Culex restuans have the ability to learn to recognise salamanders as a threat through a single pairing of alarm cues and salamander odour and also whether they would learn to respond to salamander cues in a threat-sensitive manner. We conditioned individual mosquitoes with water or a low, medium or high concentration of crushed conspecific cues (alarm cues) paired with salamander odour. Mosquitoes exposed to salamander odour paired with alarm cues and subsequently exposed to salamander odour alone responded to the salamander as a threat. Moreover, the intensity of antipredator response displayed during the conditioning phase matched the response intensity during the testing phase. This is the first demonstration of threat-sensitive learning in an aquatic invertebrate.     

Use of chemical cues in detection of conspecific predators and prey by newts, <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis>

Summary. Detection of heterospecific predators and prey via chemical cues is well known, but only a few studies have examined the potential for such discrimination in cannibalistic systems. In newts, Notophthalmus viridescens, adults are opportunistic predators of conspecific larvae. I used a laboratory bioassay to determine whether larvae and adults distinguish between chemical stimuli from members of the different age classes. Larvae distinguished between chemical stimuli from larvae and adults by decreasing their activity only following exposure to stimuli from adults. Decreased activity is consistent with an antipredator response in many prey species, including larval newts. In contrast, adults increased their activity and increased time spent in open areas in response to stimuli from larvae, but not to stimuli from adults. Increased activity is consistent with a feeding response; adults also showed increased activity and increased time in open areas in response to chemical stimuli from familiar heterospecific prey (brine shrimp). The proximate cue that allowed the newts to distinguish between the different age classes is not associated with short-term dietary differences because all stimulus animals were fed the same prey. Stimulus strength was controlled by diluting the stimulus solutions according to the volume of the stimulus animal. Therefore, there appear to be intrinsic differences in the chemical signatures of larval and adult newts.     

Control of larval release in the Caribbean spiny lobster, <Emphasis Type="Italic">Panulirus argus</Emphasis>: role of chemical cues

The current model for larval release in subtidal crustaceans suggests that hatching time is controlled by the embryos, which release a pheromone that stimulates the parent female to undergo behaviors that synchronize larval release. Alternatively, hatching could be controlled by the females. Ovigerous spiny lobsters Panulirus argus (Latreille) exhibit stereotypic behaviors during larval release, including rapid abdominal extensions and pleopod-pumping activity. Ovigerous P. argus were collected from coral reefs in the Florida Keys, USA during the summers of 2005 and 2006. Pleopod-pumping activity was quantified to determine if a female’s pumping activity correlates with the developmental state of the embryos. The role of pheromones released by developing and hatching embryos in controlling pumping behaviors was tested by measuring the pumping response of ovigerous lobsters to (1) hatch water, (2) homogenized embryo water, (3) embryo-conditioned water (unhatched late-stage embryos soaked for 20 h), and (4) water containing homogenized post-hatch embryo cases. Bioassays were conducted under constant conditions (dim-red light) in the laboratory at random times during the day to control for any possible rhythm in pumping activity. Spontaneous pleopod-pumping activity increased significantly with increasing embryo development. Upon exposure to hatch water, ovigerous lobsters with late-stage embryos displayed increased pleopod pumping with increased treatment concentration. Water individually conditioned with homogenized late-stage embryos, intact late-stage embryos, and homogenized post-hatch embryo cases all induced larval release behaviors in females with late-stage embryos. Ovigerous females with early-stage embryos did not respond to water conditioned with homogenized early- or late-stage embryos. These results suggest that active substances are released by embryos at the time of hatching and induce the stereotypical pumping behaviors of the female that synchronizes larval release. The results support the model that larval release in subtidal crustaceans is controlled by pheromones released from hatching embryos.     

Larval metamorphosis of the mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> in response to <Emphasis Type="Italic">Alteromonas</Emphasis> sp. 1: evidence for two chemical cues?

Bacterial isolates from multi-species biofilms were identified by 16S rDNA gene sequences and investigated for their inductive effects as monospecific biofilms on larval metamorphosis of Mytilus galloprovincialis. Alteromonas sp. 1 biofilm was found to have inductive activity, which increased with increasing cell density. The cue(s) of Alteromonas sp. 1 biofilm responsible for inducing larval metamorphosis was further investigated. Treatment of the biofilm with formalin, ethanol, heat or ultraviolet irradiation resulted in a significant reduction in the inductive activity of Alteromonas sp. 1, and the crude extract of surface-bound products of the biofilm showed no activity. These results indicated that if the cue was a surface-bound chemical cue, it was unstable, or susceptible to the treatments or the extraction process. On the other hand, the inductive activity of treated biofilms had a linear regression to the cell survival of bacteria, indicating a metabolically active biofilm was a requirement for larval metamorphosis. Conditioned water of the biofilm did not induce larvae to metamorphose. However, larval crawling behavior in the conditioned water was the same as that in the biofilm prior to larval metamorphosis, and significantly different to larval behavior in seawater. This indicated that a potential or partial waterborne cue existed, but remained inactive when alone. A synergistic effect of the conditioned water with formalin-fixed Alteromonas sp. 1 biofilm resulted in a significant increase in larval metamorphosis. Heat treatment and fractionation of the conditioned water demonstrated that the waterborne cue was heat-stable and <3,000 Da in molecular weight. Platinum-coating, Lentil Lectin and Wheat Germ Agglutinin treatments of the formalin-fixed biofilm significantly reduced its synergistic effect with the conditioned water, suggesting that a surface-bound cue was present on the biofilm and that the cue might be associated with the bacterial exopolysaccharide or glycoprotein. Evidence presented here suggests that two chemical cues derived from bacteria act synergistically on larval metamorphosis of Mytilus galloprovincialis.     

Mortality is associated with social rank in the clown anemonefish (<Emphasis Type="Italic">Amphiprion percula</Emphasis>)

Elucidating the causes of post-recruitment mortality is a vital step toward understanding the population dynamics of coral reef fishes. Predation is often considered to be the primary proximate cause of mortality. It has, however, proven difficult to discern the relative contributions of predation and other processes, such as competition for food, shelter, or mates, to patterns of mortality. To determine which other processes might be important drivers of mortality patterns, factors related to mortality in the clown anemonefish Amphiprion percula (Lacepède, 1802) were examined. Patterns of mortality will not be driven by predation in A. percula, because these fish are well protected from predators by their close association with sea anemones. Mortality rates were based on the disappearance of known individuals from a population of 201, in 57 groups, during a 1-year field study (in 1997), in Madang Lagoon, Papua New Guinea. Mortality rate of A. percula was low (14% per annum) compared to other coral reef fish, probably due to the protection from predators afforded by the anemone. Six factors (reef, depth, anemone diameter, number of individuals, density, and standard length) showed no association with the probability of mortality (P>0.05). Rank was the only factor associated with the probability of mortality (P<0.03); low-rank individuals (ranks 4–6) suffered a higher mortality rate than high-rank individuals (ranks 1–3) (P<0.01). The most likely explanation for this pattern was that competition for rank, amongst individuals within an anemone, resulted in some individuals evicting their subordinates. Individuals probably competed for rank because it conferred access to reproduction, and not because it conferred access to food or shelter. Such competition for reproduction will be intense whenever some individuals obtain a greater share of reproduction than others do, and it may be an important process influencing the dynamics of coral reef fish populations.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Communicated by J.P. Grassle, New Brunswick     

Astaxanthin deposition in the cuticle of juvenile American lobster (<Emphasis Type="Italic">Homarus americanus</Emphasis>): implications for phenotypic and genotypic coloration

The phenotypic brownish-green color of an American lobster (Homarus americanus Milne-Edwards, 1837) is determined by genetic and dietary mechanisms. Diet influences color through the carotenoid astaxanthin. This pigment can appear red or blue depending if it is bound to protein, and the relative amounts of each form influence lobster color. Carotenoprotein formation was examined in sibling American lobsters rendered white by a diet low in astaxanthin, by feeding a diet containing 0, 55, 110, or 220 g astaxanthin g^–1 for 110 days beginning on 7 February 2004. The relative levels of red and blue in these lobsters were assessed through the analysis of digital photographs of the lobsters, which were taken monthly. The red/blue ratio was used to assess if free (red) or protein-bound astaxanthin (blue) was the dominant form in the lobsters, or if the two forms occurred in ratios allowing for natural coloration. Naturally colored lobsters developed only in the highest astaxanthin diet group, while lobsters fed the 55-g diet were blue. Differences were observed among different parts of the body, and were presumed to be a function of cuticle thickness. However, within each diet treatment, some lobsters initially became red, suggesting an accumulation of free astaxanthin, while other lobsters initially became blue, suggesting an accumulation of protein-bound astaxanthin. This variation may be a result of differences in the uptake of astaxanthin, the rate of carotenoid-protein complexing, or in the total amount of protein available for binding. This variability is likely to be the key to understanding the underlying basis of genetically determined color in American lobsters.Communicated by J.P. Grassle, New Brunswick     

Size at maturity,fecundity and reproductive potential of a protected population of the spiny lobster<Emphasis Type="Italic"> Palinurus elephas</Emphasis> (Fabricius, 1787) from the western Mediterranean

Protected lobster populations are expected to contribute to the replenishment of fished populations through increased egg production. We studied the reproductive biology and egg production potential of a population of the spiny lobster Palinurus elephas protected from fishing since 1990 in the Columbretes Islands Marine Reserve (western Mediterranean). An index of spawning potential was derived to compare egg production potential in the Reserve and in western Mediterranean exploited populations. Females' physiological maturity (ability to reproduce) and functional maturity (ability to mate and bear eggs) occurred at a carapace length (CL) of 76–77 mm. Males' physiological maturity occurred at a slightly larger size, 82.5 mm CL. In the Reserve, P. elephas' individual fecundity increases linearly with body size up to the females' maximum size, although maximum reproductive yield (eggs per body gram) was reached at intermediate sizes. Size-specific fecundity in the protected population was similar to that of lightly fished populations off Ireland and greater than that of western Mediterranean exploited populations. The female size class of 105–110 mm CL contributed most to egg production in the protected population and is well above the minimum landing size (MLS) for western Mediterranean fisheries. Newly mature females (below MLS) generate a very small fraction (1%) of the egg production from the Reserve. Given the pattern of exploitation in western Mediterranean fisheries, egg production potential depends more on the quantity than on the mean size and fecundity of the available females. The role of the greater availability of large males for mating in unfished populations is discussed in terms of the females' individual fecundity and mating success.     

Predator avoidance behaviour of Baltic littoral mysids<Emphasis Type="Italic"> Neomysis integer</Emphasis> and<Emphasis Type="Italic"> Praunus flexuosus</Emphasis>

The predator avoidance behaviours of two littoral mysid species, Neomysis integer (Leach) and Praunus flexuosus (Müller), were studied experimentally. In ingestion experiments, mysids responded to a combination of chemical and visual signals of perch (Perca fluviatilis), but not to each stimulus alone. In the presence of the combined visual and chemical predator signal the swimming activity and choice of habitat (open vs. artificial vegetation, the Charophyte Chara tomentosa or the brown alga Fucus vesiculosus) were also influenced. The two mysid species behaved differently when perceiving predation risk: N. integer reduced swimming activity, whereas P. flexuosus increased their use of the vegetation. The different antipredator strategies of the two mysid species reflect their different lifestyles, N. integer being a swarm-forming species and P. flexuosus living in association with aquatic macrophyte vegetation.Communicated by L. Hagerman, Helsingør     

Auditory defence in the peacock butterfly (<Emphasis Type="Italic">Inachis io</Emphasis>) against mice (<Emphasis Type="Italic">Apodemus flavicollis</Emphasis> and <Emphasis Type="Italic">A. sylvaticus</Emphasis>)

Morphological and behavioural traits can serve as anti-predator defence either by reducing detection or recognition risks, or by thwarting initiated attacks. The latter defence is secondary and often involves a ‘startle display’ comprising a sudden release of signals targeting more than one sensory modality. A suggested candidate for employing a multimodal defence is the peacock butterfly, Inachis io, which, by wing-flicking suddenly, produces sonic and ultrasonic sounds and displays four large eyespots when attacked. The eyespots make small birds retreat, but whether the sounds produced thwart predator attacks is largely unknown. Peacocks hibernate as adults in dark wintering sites and employ their secondary defence upon encounter with small rodent predators during this period. In this study, we staged predator–prey encounters in complete darkness in the laboratory between wild mice, Apodemus flavicollis and Apodemus sylvaticus, and peacocks which had their sound production intact or disabled. Results show that mice were more likely to flee from sound-producing butterflies than from butterflies which had their sound production disabled. Our study presents experimental evidence that the peacock butterfly truly employs a multimodal defence with different traits targeting different predator groups; the eyespots target birds and the sound production targets small rodent predators.     

How the social parasitic bumblebee <Emphasis Type="Italic">Bombus bohemicus</Emphasis> sneaks into power of reproduction

Social parasitism is widespread in many groups of social living hymenopteran species and has also evolved in the genus Bombus. Cuckoo bumblebees (subgenus Psithyrus) are obligate brood parasites in nests of other bumblebee species. After nest usurpation and the killing of the host queen, the parasite female has to control worker reproduction in order to accomplish and maintain reproductive dominance and to ensure her reproductive success. The aim of our study was to examine whether the generalist parasitic bumblebee Bombus bohemicus monopolizes and prevents worker reproduction by physical or chemical means and to identify possible odor compounds involved therein. We performed bioassays with callow workers of the host Bombus terrestris and have shown that B. bohemicus females are able to suppress host worker ovarian development, when these host workers are under the direct influence of the parasite female. Furthermore, by chemical analyses, we have demonstrated that the parasite females adjust to the odor profiles of their host queens in order to maintain the level of fertility signaling inside the host colony although the host queen is absent. We also found that host workers change their odor profile after nest usurpation by the parasite female and consequently, we suggest that the host and parasite are caught up in a chemical arms race.     

Roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>) as an important predator on blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) populations in a brackish water environment,the northern Baltic Sea

Suspension-feeding bivalves are organisms of major functional importance in several aquatic environments around the world. They are also important food items for many fish and benthivorous seabirds. It has commonly been thought that predation pressure on blue mussel (Mytilus edulis) populations is negligible in the Baltic Sea, owing to the scarcity of major invertebrate predators such as starfish and crabs. It has recently been shown, however, that the blue mussel is the main food item for roach (Rutilus rutilus) in the archipelago areas of the western Gulf of Finland, where this freshwater fish species has become increasingly abundant, mainly due to increased eutrophication. To quantify the influence of roach predation on blue mussel populations we measured the standing biomass and size structure of the local blue mussel population and used a bioenergetic model to estimate mussel consumption by individual roach during two consecutive summers, 1997 and 1998. The results of the model were combined with existing data on roach abundance, giving annual consumption estimates of 75–105 kg blue mussel dry weight ha^–1 in the study area, approximately two-thirds of these consumed mussels being >10 mm. This corresponds to approximately one-third of the standing population of mussels >10 mm in the area. Our results suggest that the predation effects of vertebrates on Baltic blue mussel populations are not insignificant, as previously believed. Predation by roach and other predators may have an important structuring effect on unstable blue mussel communities within the Gulf of Finland, where the species lives at the edge of its range.Communicated by M. Kühl, Helsingør     

Response of the grasshopper <Emphasis Type="Italic">Myrmeleotettix maculatus</Emphasis> (Orthoptera: Acrididae) to invasion by the exotic moss <Emphasis Type="Italic">Campylopus introflexus</Emphasis> in acidic coastal dunes

In Europe, acidic coastal dunes are threatened by the invasion of the exotic moss Campylopus introflexus. While the effect of the moss encroachment on the vegetation is well analysed, knowledge of possible impact on arthropods is lacking. Thus, an experiment was conducted in acidic coastal dunes on the Baltic island of Hiddensee, Germany. Myrmeleotettix maculatus, a common Orthoptera species of open and dry habitats, was sampled by pitfall trapping in eleven plots invaded by C. introflexus and in eleven native, non-invaded plots rich in lichens. Overall, 826 individuals of M. maculatus were captured (266 nymphs, 560 adults). Mean number of adults was significantly higher in native plots. This maybe explained by a higher proportion of grasses (food supply), a higher availability of shelter (from predators, weather), or more favourable microclimate conditions in native plots and a higher mortality rate in invaded plots. However, mean number of both young and old nymphs did not differ significantly between both types. This could imply that invaded areas at least serve as favourable oviposition sites and larval habitats. The observed negative effect of the moss invasion on M. maculatus remains to be studied on other arthropods.     

Food web analysis in intertidal <Emphasis Type="Italic">Zostera marina</Emphasis> and <Emphasis Type="Italic">Zostera noltii</Emphasis> communities in winter and summer

The food web of two intertidal seagrass (Zostera marina and Zostera noltii) beds that may be influenced by the seasonal variation in food source abundance was studied in winter and in summer with δ¹³C and δ¹⁵N analysis. In spite of high relative variation of abundance of main primary producers at the two sites, the food web did not vary between winter and summer. The δ¹³C range of primary producers was wide. Zostera leaves, the most ¹³C-enriched source, were not consumed directly by grazers. Deposit and filter feeders have a similar δ¹³C and could use a mix of suspended and sedimented organic particulate matter, largely composed of detritus from macroalgae to seagrass. This trophic pathway allows the local incorporation of the high biomass produced by seagrasses. The wide δ¹⁵N range of predators was linked either to a large variety from omnivore to carnivore predators or to the also wide ranges of δ¹⁵N of primary consumers.