Hard rock versus soft bottom: the fauna associated with intertidal mussel beds on hard bottoms along the coast of Chile,and considerations on the functional role of mussel beds

The fauna associated with hard bottom mussel beds along the exposed Pacific coast of Chile was examined. The abundance of adult (>10 mm body length) purple mussels Perumytilus purpuratus varied between 32 and 75 individuals per 50 cm², and their biomass between 4.8 and 8.6 g AFDW per 50 cm² at eight sampling sites between Arica (18°S) and Chiloé (42°S). At all sampling sites, the associated fauna was dominated by suspension-feeding organisms (cirripeds, spionid and sabellid polychaetes, a small bivalve) followed by grazing peracarids and gastropods. Predators and scavengers also reached high abundances while deposit- and detritus-feeding organisms were of minor importance. The majority of organisms associated with these hard bottom mussel beds feed on resources obtained from the water column or growing on the mussels rather than on materials deposited by the mussels. This is in contrast to the fauna associated with mussel beds on soft bottoms, which comprises many species feeding on material accumulated by mussels (faeces and pseudofaeces) and deposited within the mussel bed. Many of the organisms dwelling between mussels both on hard bottoms and on soft bottoms have direct development, but organisms with pelagic development also occur abundantly within mussel beds. We propose that species with direct development are disproportionately favoured by the structurally complex habitat with diverse interstitial spaces between the mussels, which provides ample shelter for small organisms. We conclude that mussels on hard-bottoms primarily provide substratum for associated fauna while mussels on soft bottoms provide both substratum and food resources. Electronic Publication     

Attachment strength of an adhesive nuisance mussel,limnoperna fortunei,against water flow

The attachment strength of the freshwater mussel Limnoperna fortunei against water flow was studied. Newton's expression successfully described the hydrodynamic drag force acting on the mussel with a drag coefficient value of 1.03. The drag‐resistant force (defined as hydrodynamic drag force at mussel detachment) was smaller than the detachment force measured using a tensile load test. A fairly good correlation was obtained between the drag‐resistant force and the number of secreted threads. The drag‐resistant force divided by the number of threads increased with shell size, suggesting that byssal thread strength increased with mussel growth. For the mussel specimens obtained from a water transmission pipe, thread width increased with shell size. However, thread width was not dependent on current velocity. There was no correlation between the number of secreted threads and shell length, which indicated that the number of secreted threads did not change with mussel size. Therefore, the water velocity needed to detach mussels increases with shell size of the mussel when the number of secreted threads is constant. The increases in the water velocity to detach mussels with larger shells suggests that the mussel becomes more resistant to water flow as it grows. It is estimated that a flow velocity of around lms^‐1 is critical for attachment/detachment of a juvenile mussel with a shell length of a few millimeters and one hundred byssal threads.     

Factors influencing the attachment strength of Dreissena polymorpha (Bivalvia)

Abstract

The effects of several factors (shell length, exposure time, substratum orientation in space, illumination, temperature, conspecifics) upon the attachment strength (measured with a digital dynamometer) of the freshwater, gregarious bivalve Dreissena polymorpha were studied under laboratory conditions. A rapid increase in attachment strength was observed on resocart (a thermosetting polymer based on phenol-formaldehyde resin, with paper as filler) substrata during the first 4-d exposure, after which it stabilised at ca 1 N. The attachment strength increased also with mussel size. Mussel adhesion on variously oriented surfaces (vertical, upper horizontal and lower horizontal) was similar. Illumination inhibited attachment strength, as expected for a photophobic species, but only after a 2-d exposure. After 6 d, no effects of light were detected. Thus, illumination seemed to influence the attachment rate, rather than the final strength. The optimum temperature for mussel attachment was 20 – 25°C. At lower and higher temperatures (5 – 15°C and 30°C), their adhesion strength decreased. The presence of conspecifics stimulated mussel attachment strength.     

Inhibition of the reattachment of young adult zebra mussels by single-species biofilms and associated exopolymers

AIMS: To determine the effects of single-species bacterial films and their associated extracellular products on the reattachment of young adult zebra mussels. MATERIALS AND RESULTS: Ten strains of bacteria were isolated from surfaces where adult zebra mussels can be found attached in nature. Single-species biofilms were developed on both glass and polystyrene using these bacteria. The reattachment of zebra mussels (i.e. with byssal threads) was compared between surfaces with and without films. Although no differences were observed in mussel reattachment between glass surfaces with and without films (P > 0.05, anova), a reduction in mussel reattachment between polystyrene surfaces with and without films was observed for seven of the 10 strains (P < or = 0.05 to <0.001, anova). Bacterial extracellular products (BEP) were isolated from five bacterial films and tested for their effects on mussel reattachment. Four of the five sets of isolated extracellular products evoked the same effects as their respective intact biofilms. CONCLUSIONS: We conclude that depending on the substratum, individual strains of bacteria in biofilms can inhibit the reattachment of adult zebra mussels. In some cases, BEP were the source of the inhibitory effects. SIGNIFICANCE AND IMPACT OF THE STUDY: The nature of the substratum on which the biofilms develop affects properties of the biofilm and its extracellular components, which subsequently influences zebra mussel reattachment.     

Population dynamics of the ribbed mussel,Geukensia demissa: The costs and benefits of an aggregated distribution

Summary Although mussel beds are common in many intertidal habitats, the ecological significance of the aggregated distribution of mussels has not been examined. The ribbed mussel, Geukensia demissa, is found in dense aggregations on the seaward margin of many salt marshes in New England. Here, we examine the population structure of G. demissa in a New England salt marsh and investigate experimentally the costs and benefits of aggregation.Size, growth rate, and settlement rates of mussels decrease with increasing tidal height, whereas survivorship and longevity increase with increasing tidal height. Winter ice dislodges mussels from the substratum, resulting in mortality over all size classes, whereas crab predation results in the mortality of smaller mussels. The intensity of each of these mortality agents decreases with increasing tidal height. Effects of intraspecific competition on individual growth and mortality also decrease with increasing tidal height.At high densities, individual growth rates were reduced, with depression of growth rates most pronounced on smaller individuals. Mortality from sources other than intraspecific crowding, however, was reduced at high mussel densities, including mortality due to winter ice and crab predators. As a result, our data suggest that the mussel population at our study site would be reduced by 90% in only five years and no juveniles would survive through their second year without an aggregated distribution.Juveniles settle gregariously with or without adults present. The aggregated distribution of settlers and the postsettlement movement of smaller mussels to favorable microhabitats result in size and age class segregation within the population. This probably reduces intraspecific competition for food, while maintaining the survivorship advantages of an aggregated distribution.     

Reach- and catchment-scale determinants of the distribution of freshwater mussels (Bivalvia: Unionidae) in south-eastern Michigan, U.S.A.

1. We investigated the diversity and distribution of freshwater mussels at 40 sites in an agricultural catchment, the River Raisin in south‐eastern Michigan, to relate mussel assemblages and individual taxa to reach and catchment‐scale variables. Unionids were surveyed by timed searches in 100‐m reaches, and in‐stream and riparian habitat were quantified as well as flow, water chemistry and channel morphology. Land use/cover and surficial geology were determined for site subcatchments and riparian buffers. 2. Some 21 mussel species were found overall; richness ranged from 0 to 12 living species per site. From the upper to middle to lower catchment, the number of individuals, number of species, Shannon–Weaver diversity and relative abundance of intolerant unionids all declined significantly. 3. Four groupings based on overall mussel diversity and abundance were significantly related to reach‐scale habitat variables. The richest mussel assemblages were associated with sites with higher overall habitat quality, greater flow stability, less fine substratum, and lower specific conductance. 4. Stepwise multiple regressions revealed that the distribution and abundance of the total mussel assemblage, as well as the most common species, could be predicted from a combination of reach‐ and catchment‐scale variables (R² = 0.63 for total mussels, R² = 0.51–0.86 for individual species). 5. Flow stability, substratum composition and overall reach habitat quality were the most commonly identified reach‐scale variables, and measures of surficial geology were the most effective catchment‐scale variables. The spatial pattern of geology is likely to be responsible for the diversity gradient from the upper to the lower catchment. 6. Prior studies, attempting to explain mussel distributions from local habitat features alone, have found relatively weak relationships. By employing a combination of reach‐ and catchment‐scale habitat variables, this study was able to account for a substantial amount of the spatial variability in mussel distributions.     

Mechanisms and consequences of competition between an alien mussel, Mytilus galloprovincialis, and an indigenous limpet, Scutellastra argenvillei

The influence of an alien mussel Mytilus galloprovincialis (Lamarck) on an indigenous limpet Scutellastra argenvillei (Krauss) was investigated on the west coast of South Africa by comparing four situations in which limpets occupied rock patches surrounded by mussels: (1) ‘established patches’ of high-density adult limpets; (2) ‘experimentally cleared patches’ from which mussels were manually removed; (3) ‘naturally disturbed patches’ where mussels had been eliminated by wave action; and (4) ‘half-density’ patches formed by thinning established patches. Limpets in established patches were most effective in retarding settlement and lateral invasion by mussels, but all patches shrank due to mussel encroachment. As patches shrank, limpet density fell in established and naturally disturbed patches due to emigration. Limpets living on mussel beds were small, sparse and never achieved sexual maturity; 60% shifted to rock, whereas only 7% moved from rock to mussels. The limpets showed highest fidelity to established patches (79% after 12 months) and lowest fidelity in naturally disturbed patches (20%). Thinning of established patches reduced fidelity to 26%. Mussel beds did not provide a suitable alternative substratum for S. argenvillei after they displaced the limpets from rocks. Moreover, elimination of mussels by waves failed to allow S. argenvillei to re-establish dense, adult populations resembling those prior to arrival of the mussel. Adults of S. argenvillei feed collectively by trapping kelp blades. On the topographically complex mussel beds limpets cannot feed this way. In sum, on exposed shores where M. galloprovincialis achieves maximal recruitment and growth, S. argenvillei is incapable of preventing mussel encroachment and is likely to become completely displaced. Semi-exposed shores do, however, offer a refuge preventing global extinction of the limpet.     

LIGHT, GRAVITY AND CONSPECIFICS AS CUES TO SITE SELECTION AND ATTACHMENT BEHAVIOUR OF JUVENILE AND ADULT DREISSENA POLYMORPHA PALLAS, 1771

Dreissena polymorpha, the zebra mussel, is one of the mostimportant components of the aquatic ecosystems it inhabits,due to high densities and filtration rates of this fouling organism.This laboratory study deals with several physical and biologicalfactors influencing zebra mussel juveniles (<10 mm) and adults(>10 mm) site selection behaviour and their byssal attachmentto substrate. Mussels preferred black substrate rather thanwhite. This preference was found to be stronger in smaller individuals.Furthermore, all mussels, independently of their size, selectedshadowed sites and avoided illuminated ones. Large mussels attachedmore often in darkness than in light, while for small specimenslight conditions made no difference. Large mussels tended toattach to lower parts of vertical test-tubes, while the numbersof small specimens in both halves of the test-tubes were similar.If initial conditions, used by mussels to select their attachmentsite, were changed by turning the test-tubes upside down, smallindividuals detached and looked for a new site more frequentlythan large ones. Large mussels attached more often in higherdensities of conspecifics, but physical contacts among conspecificsand their size composition seemed to have no impact on attachment.This study shows that many environmental stimuli influence locomotionand attachment of post-metamorphic zebra mussels. Therefore,the behaviour of this group may impact upon adult distributionobserved in field. Preferences exhibited by mussels lead tothe creation of dense aggregations in dark, deep places, providing protectionagainst dessication, predators and excessive water flow. (Received 18 April 2000; accepted 5 October 2000)     

PATTERNS OF ORIENTATION IN UNIONIDS AS A FUNCTION OF RIVERS WITH DIFFERING HYDROLOGICAL VARIABILITY

As unionids can become dislodged with high flows, it may provebeneficial for an individual to minimize its exposure to theflow. This can be accomplished by either burrowing as deep aspossible or orienting in a way that effectively reduces thedrag exerted on the mussel by the flow. The patterns of orientationwere examined in unionids with respect to hydrological variability.The orientation of mussels to flow was measured at four sitesalong an event river (hydro-logically variable) and a stableriver (hydrologically stable). Burrowing depth was measuredat a reference site in each river. Most individuals in bothriver types were oriented with their siphons pointing upstream.Mussel orientation differed significantly between the two rivertypes with mussels in the event river orienting more parallelto the flow than those in the stable river. Mussels in the eventriver were significantly larger than those in the stable riverbut the size of a mussel did not determine its orientation withina river. Burrowing depth did not differ for mussels betweenevent and stable rivers. The observed differences in orientationsamong river types are likely a function of differences in thepattern of orientation of the mussel community as a whole, withineach drainage. This burrowing behaviour may be an attributethat enhances the adaptations mussels have for remaining burrowedin the sediment. (Received 2 December 1996; accepted 24 February 1997)     

Strong impact of wintering waterbirds on zebra mussel (Dreissena polymorpha) populations at Lake Constance, Germany

1. Since zebra mussel invaded Lake Constance in the 1960s the number of wintering waterbirds increased fourfold. We studied the impact of predation by waterbirds (tufted duck Aythya fuligula, pochard Aythya ferina and coot Fulica atra) on the population of Dreissena polymorpha in winter 2001/2002. These three species, with monthly peak numbers of approximately 230 000 individuals, currently comprise up to 80% of the waterbird population wintering at Lake Constance. 2. Four different study sites and four depths, that represent typical and characteristic habitats of mussels in Upper Lake Constance, were chosen. 3. Zebra mussels were sampled before, during and after predation by waterbirds. Their biomass in shallow areas decreased by >90%; the biomass reduction in deeper areas was highly variable and dependent on the substratum. With one exception, no changes could be detected at the greatest depth (11 m). 4. Concurrent exclosure studies revealed that the decrease in zebra mussels was caused by waterbird predation. A GIS‐based approximation revealed that in an area of 1 km² a total of approximately 750 t mussel fresh mass was removed by birds, which is equivalent to 1390 g mussels per bird per day. 5. Wintering waterbirds have a strong structural impact on the littoral community of Lake Constance and could be the key predator of zebra mussels.     

Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

Group living and the effects of spatial position in aggregations of Mytilus edulis

Summary The mussel Mytilus edulis typically occurs in aggregations and several consequences of living in groups were studied. Isolated individuals and individuals associated in relatively small groups (6–9 mussles/group) grew more and therefore had greater reproductive output than mussels associated with relatively large groups of 21–28 individuals. Mussels located in the centers of groups exhibited reduced growth and thus lower reproduction relative to mussels located on the edges of groups whose growth and reproduction was similar to that of isolated individuals. Sampling from natural populations indicated that most mussels grow within the matrix of very large groups and hence will experience reduced growth and reproduction. Patterns of growth exhibited by mussels in association with living and model mussels showed that the adverse effects on growth exhibited by mussels in relatively large groups are not a function of the mere physical relief of a mussel clump, but are caused by some property of living neighbors.Laboratory experiments on mussel predation by the crab Pachygrapsus crassipes indicated that crabs prey disproportionately on mussels growing on the edges of groups.The consequences of group living in mobile and nonmobile organisms are considered, and it is suggested that a greater number of negative effects will arise in groups as mobility decreases. In addition, the noted ecological similarity between groups of sessile organisms and spreading clones and its evolutionary implications are discussed.This is contribution no 171 from the Smithsonian Marine Station at Link Port     

Biotic resistance to invasion along an estuarine gradient

Biotic resistance is the ability of native communities to repel the establishment of invasive species. Predation by native species may confer biotic resistance to communities, but the environmental context under which this form of biotic resistance occurs is not well understood. We evaluated several factors that influence the distribution of invasive Asian mussels (Musculista senhousia) in Mission Bay, a southern California estuary containing an extensive eelgrass (Zostera marina) habitat. Asian mussels exhibit a distinct spatial pattern of invasion, with extremely high densities towards the back of Mission Bay (up to 4,000 m⁻²) in contrast with near-complete absence at sites towards the front of the bay. We established that recruits arrived at sites where adult mussels were absent and found that dense eelgrass does not appear to preclude Asian mussel growth and survival. Mussel survival and growth were high in predator-exclusion plots throughout the bay, but mussel survival was low in the front of the bay when plots were open to predators. Additional experiments revealed that consumption by spiny lobsters (Panulirus interruptus) and a gastropod (Pteropurpura festiva) likely are the primary factors responsible for resistance to Asian mussel invasion. However, biotic resistance was dependent on location within the estuary (for both species) and also on the availability of a hard substratum (for P. festiva). Our findings indicate that biotic resistance in the form of predation may be conferred by higher order predators, but that the strength of resistance may strongly vary across estuarine gradients and depend on the nature of the locally available habitat.     

Testing a typology system of running waters for conservation planning in Hungary

The zebra mussel (Dreissena polymorpha) and its congener the quagga mussel (Dreissena rostriformis bugensis) are both invaders in freshwater, but have very different invasion histories, with zebra mussels attaining substantially faster rates of spread at virtually all spatial scales. However, in waterbodies where they co-occur, D. r. bugensis can displace D. polymorpha. To determine if the mechanisms for this displacement are associated with different survival and growth, we kept mussels in flow-through tanks for 289 days with two temperature regimes that mimicked the natural surface water (littoral zone) and hypolimnion conditions of Lake Erie. For the littoral zone regime, we used water directly from the surface of Lake Erie (range 4–25°C, average 11.9 ± 0.6°C). For the profundal zone treatment, Lake Erie surface water was chilled to about 6°C (range 5–8°C, average 6.2 ± 0.6°C) for the full duration of the experiment. For each of these temperature regimes, we used three replicate tanks with only zebra mussels present and three replicate tanks with only quagga mussels (150 ind./tank each), and three replicate tanks with both species (75 ind./tank of each species). Quagga mussels had higher survivorship and grew more than zebra mussels in all treatments. For both species, the size of the mussel entering the winter was critical for survivorship. Larger mussels had a higher survival over the winter in all treatments. For both species, there was a survivorship and growth tradeoff. In the warmer littoral zone treatment both species had higher growth, but lower survival than in the colder profundal zone treatment. Surprisingly, although quagga mussels outperformed zebra mussels, zebra mussel survivorship was better when they were faced with competition by quagga mussels than with just intraspecific competition. In addition, quagga mussels suffered size-specific mortality during the growing season only when facing interspecific competition with zebra mussels. Further experiments are needed to determine the possible mechanisms for these interspecific effects.     

The impact of the sea anemone Actinothoe sphyrodeta on Mytilus galloprovincialis mussel cultivation (Galicia,Spain)

Abstract

Marine mussel aggregations act as a substratum and refuge for many fouling species. Mussel cultivation in Galicia, Spain, is carried out on hanging ropes in subtidal systems. The fauna associated with this cultivation includes a large number of invertebrates that compete for space or food with the mussels, or use their clusters as a refuge from predators or water turbulence. Outbreaks of the epibiont anemone Actinothoe sphyrodeta have been reported in cultivated Galician mussels since 2013, but their impact has not been investigated rigorously. Here, the temporal and spatial variability of Actinothoe sphyrodeta on mussel shells throughout one year is presented. Sampling of mussel size, weight and byssus attachment strength allowed mussel tenacity (attachment strength relative to size) to be calculated. A higher presence of Actinothoe sphyrodeta correlated with lower mussel tenacity and greater biomass losses, suggesting that this species could be an economically important biofouling component.     

Preferences of the Ponto-Caspian amphipod Dikerogammarus haemobaphes for living zebra mussels

A Ponto-Caspian amphipod Dikerogammarus haemobaphes has recently invaded European waters. In the recipient area, it encountered Dreissena polymorpha , a habitat-forming bivalve, co-occurring with the gammarids in their native range. We assumed that interspecific interactions between these two species, which could develop during their long-term co-evolution, may affect the gammarid behaviour in novel areas. We examined the gammarid ability to select a habitat containing living mussels and searched for cues used in that selection. We hypothesized that they may respond to such traits of a living mussel as byssal threads, activity (e.g. valve movements, filtration) and/or shell surface properties. We conducted the pairwise habitat-choice experiments in which we offered various objects to single gammarids in the following combinations: (1) living mussels versus empty shells (the general effect of living Dreissena ); (2) living mussels versus shells with added byssal threads and shells with byssus versus shells without it (the effect of byssus); (3) living mussels versus shells, both coated with nail varnish to neutralize the shell surface (the effect of mussel activity); (4) varnished versus clean living mussels (the effect of shell surface); (5) varnished versus clean stones (the effect of varnish). We checked the gammarid positions in the experimental tanks after 24 h. The gammarids preferred clean living mussels over clean shells, regardless of the presence of byssal threads under the latter. They responded to the shell surface, exhibiting preferences for clean mussels over varnished individuals. They were neither affected by the presence of byssus nor by mussel activity. The ability to detect and actively select zebra mussel habitats may be beneficial for D. haemobaphes and help it establish stable populations in newly invaded areas.     

Reciprocal Effects between Burying Behavior of a Larval Dragonfly (Odonata: Macromia illinoiensis) and Zebra Mussel Colonization

Invasive zebra mussels (Dreissena polymorpha) often colonize dragonfly larvae, especially spawling species whose survivorship to emergence as terrestrial predators is consequently reduced. Using individuals of the sprawler, Macromia illinoiensis, as their own controls, we compared the burying behavior of penultimate instar larvae before (i.e. baseline) and after their colonization by zebra mussels under ambient conditions. Individuals that took longer to bury themselves when mussel-free had a higher rate of colonization by mussels over a five-day period compared to those that buried faster. In contrast, the depth at which individuals buried when mussel-free was not predictive of subsequent colonization rate. Although mean bury time did not differ between baseline and when an individual carried one or more mussels, colonized larvae buried more shallowly than when mussel-free. Moreover, attached mussels increased the risk of subsequent colonization by zebra mussels. After naturally losing all of their attached mussels, bury time and depth of individuals did not differ from their baseline behavior, indicating that the changes in the behavior of colonized individuals were due to mussel loads and not their time in captivity. Under natural conditions, the positive feed-back between mussel attachment and increasing vulnerability to colonization helps explain how mussel loads, which are lost at molting, can accumulate quickly over the duration of the final larval stadium. Because zebra mussel attachment decreases the crypsis that that a M. illinoiensis gains from burying, the invasive mussel may also make dragonfly larvae more detectable to visual predators.     

Effects ofFucus vesiculosus covering intertidal mussel beds in the Wadden Sea

The brown algaFucus vesiculosus formamytili (Nienburg) Nienhuis covered about 70% of mussel bed (Mytilus edulis) surface area in the lower intertidal zone of Königshafen, a sheltered sandy bay near the island of Sylt in the North Sea. Mean biomass in dense patches was 584 g ash-free dry weight m^?2 in summer. On experimental mussel beds, fucoid cover enhanced mud accumulation and decreased mussel density. The position of mussels underneath algal canopy was mainly endobenthic (87% of mussels with >1/3 of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with <1/3 of shell buried in mud). Mussel density underneath fucoid cover was 40 to 73% of mussel density without algae. On natural beds, barnacles (Balanidae), periwinkles (Littorina littorea) and crabs (particularly juveniles ofCarcinus maenas) were significantly less abundant in the presence of fucoids, presumably because most of the mussels were covered with sediment, whereas in the absence of fucoids, epibenthic mussel clumps provided substratum as well as interstitial hiding places. The endobenthic macrofauna showed little difference between covered and uncovered mussel beds. On the other hand, grazing herbivores — the flat periwinkleLittorina mariae, the isopodJaera albifrons and the amphipodsGammarus spp. — were more abundant at equivalent sites with fucoid cover. The patchy growth ofFucus vesiculosus on mussel beds in the intertidal Wadden Sea affects mussels and their epibionts negatively, but supports various herbivores and increases overall benthic diversity.     

Desiccation,predation, and mussel-barnacle interactions in the northern Gulf of California

Summary Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.     

Zebra mussels decrease burrowing ability and growth of a native snail, <Emphasis Type="Italic">Campeloma decisum</Emphasis>

Invasive species can drive native organisms to extinction by limiting movement and accessibility to essential resources. The purpose of this study was to determine if zebra mussels (Dreissena polymorpha) affect the burrowing ability and growth rate of a native snail, Campeloma decisum. Snails with and without zebra mussels were collected from Douglas Lake, MI, and burrowing depths were studied in both the laboratory and Douglas Lake. Growth rates were calculated as the amount of shell growth from 2004 to 2005. Both the tendency of snails to burrow and the depth to which they burrowed into the substrate were significantly decreased by the presence of zebra mussels on snail shells in both laboratory and lake experiments. There was no difference in the percentage of snails that exhibited growth as a function of zebra mussel presence. However, for those snails that grew, there was a 50% higher growth rate for snails without zebra mussels compared to snails with zebra mussels. These negative effects of zebra mussels on growth and burrowing ability will likely lead to decreases in snail population densities in the future. Handling editor: S. Wellekens