Mussel Recruitment Research Articles

Genetic structure of Irish freshwater pearl mussels (Margaritifera margaritifera and Margaritifera durrovensis): Validity of subspecies, roles of host fish, and conservation implications

Abstract European freshwater pearl mussels are among the most critically threatened bivalve molluscs. Margaritifera margaritifera and Margaritifera durrovensis are endangered, and both occur in Ireland and are currently listed separately in Annex II of the European Habitats Directive. This study had the objective of guiding the conservation of freshwater pearl mussels in Ireland based on a genetic characterization of the most important Irish populations of both species using microsatellite and mitochondrial DNA (mtDNA) analyses. Margaritifera durrovensis from the Nore was found to have the highest genetic divergence from all the populations studied; however, there was substantial relatedness between the genetic constitution of the Nore and of central–eastern M. margaritifera populations, placing the Nore pearl mussel within the M. margaritifera taxon (Habitats Directive species code 1029). Two main conservation units of pearl mussel were identified: a mostly salmon‐dependent western cluster and a trout‐dependent central–eastern cluster. The genetic diversity of western Irish freshwater pearl mussels, as expressed by allelic richness and observed heterozygosity, was greater by a factor of two than in central European and central–eastern Irish populations, suggesting a genetic diversity hot spot and low effects of genetic drift and selection. The trout‐dependent central and eastern populations had much lower genetic variability, but had the greatest differentiation and uniqueness. Conservation of freshwater pearl mussels in Ireland should recognize the existence of a minimum of two conservation units (western and central–eastern) that differ in their use of host fish and in geographic isolation. Low levels of genetic drift and inbreeding in western populations should be secured by sustaining optimal habitat conditions favourable for the recruitment of mussels and their migratory salmonid hosts. The small population sizes of central–eastern populations and problems with recruitment require urgent action, e.g. by captive breeding and augmentation, to prevent any further erosion of their genetic variability.

Benthic–pelagic coupling and bottom‐up forcing in rocky intertidal communities along the Atlantic Canadian coast

AbstractBenthic species from rocky intertidal systems are irregularly distributed along marine coastlines. Nearshore pelagic conditions often help to explain such variation, but most such studies have been done on eastern ocean boundary coasts. We investigated possible benthic–pelagic coupling along the Atlantic coast of Nova Scotia, a western ocean boundary coast. In 2014, we surveyed high‐intertidal habitats from nine wave‐exposed bedrock locations spanning 415 km of coastline. At each location in the spring, we measured the recruitment of barnacles and mussels, the two main filter‐feeders. Recruitment varied irregularly along the coast. Satellite data on coastal phytoplankton and particulate organic carbon (food for intertidal filter‐feeders and their pelagic larvae) and in‐situ data on sea surface temperature explained, to varying degrees, the geographic structure of recruitment. In turn, the summer abundance of barnacles and mussels was positively related to their spring recruitment. Ultimately, intertidal predator (dogwhelk) abundance was positively related to the recruitment and/or abundance of barnacles and mussels (the main prey of dogwhelks). Sea ice may also have influenced this predator–prey interaction. Drift ice leaving the Gulf of St. Lawrence in late winter strongly disturbed the northern surveyed locations, making barnacles (through high spring recruitment) the only food source for dogwhelks (which survived ice scour in crevices) in such places. Overall, this study supports the occurrence of benthic–pelagic coupling and bottom‐up forcing on this coast. Investigating the oceanographic drivers of pelagic food supply and seawater temperature should help to further understand how this large metacommunity is organized.

Nonconsumptive predator effects on prey demography: dogwhelk cues decrease benthic mussel recruitment

AbstractPredators often have nonconsumptive effects (NCEs) on prey behaviour, but the demographic consequences for prey remain poorly known. This is important to understand because demography influences the functional role of a species in its community. We used an intertidal predator–prey system to investigate predator NCEs on prey recruitment, a key demographic process for population persistence. Pelagic mussel larvae are known to avoid waterborne cues from dogwhelks, which prey on intertidal mussels. Through a field experiment done in Atlantic Canada, we manipulated the presence of dogwhelks in intertidal habitats during the mussel (Mytilus spp.) recruitment season. We measured mussel recruitment in collectors that could be reached by waterborne dogwhelk cues but not by dogwhelks themselves. We found that the nearby presence of dogwhelks significantly decreased mussel recruit density. A previous study done in the same habitats under the same experimental conditions showed that dogwhelk cues also limit the recruitment of barnacles, another prey item for dogwhelks. However, such NCEs were stronger than those observed for mussel recruitment, possibly because mussels have more opportunities to escape predation during their benthic existence. Thus, basic features of natural history might be useful to predict the intensity of predator NCEs on prey recruitment.

Effects of turbulence exposure on zebra mussel (Dreissena polymorpha) larval survival

To more accurately predict recruitment of zebra mussels (Dreissena polymorpha) in waterbodies downstream of a source population, factors that control the successful transport of zebra mussel larvae need to be quantified. Turbulence has been identified in previous studies as a likely factor in larval morality in rivers and streams where turbulent energy dissipation can be orders of magnitude greater than in lakes. To investigate the impact of turbulent energy dissipation on zebra mussel larval mortality, we conducted a series of experiments using lake water collected from Lake Minnetonka, Minnesota containing veligers in a jar test device. Results indicate that zebra mussel larval mortality is a function of veliger size, turbulent energy dissipation, and exposure time. The mortality at 24 h of turbulence exposure was fit to a function of d*, the ratio of shell size to Kolmogorov length scale, to develop a dose–response curve. Mortality rate constants were estimated by fitting mortality data from specified turbulence regimes to a first-order model. The mortality rates ranged from 0.09 to 1.71 day−1 and were correlated to energy dissipation.

Testing the intermittent upwelling hypothesis: upwelling, downwelling, and subsidies to the intertidal zone

AbstractThe Intermittent Upwelling Hypothesis (IUH) posits that subsidies of larvae and phytoplankton to intertidal communities should vary unimodally along a gradient of upwelling from persistent upwelling to persistent downwelling with most subsidies occurring where upwelling is of intermediate strength and intermittent. Furthermore, the hypothesis states that larvae and phytoplankton are transported far offshore by strong, persistent upwelling and fail to subsidize nearshore communities, whereas weak upwelling or downwelling reduces nutrients for phytoplankton production limiting food for larvae and nearshore communities. We review studies conducted at sea and onshore and reanalyze published data to test the IUH and evaluate alternative hypotheses. To test the hypothesis, we examine five predictions that must hold if the IUH is true. (1) Larvae should inhabit the surface Ekman layer where they are transported offshore during upwelling. Larvae of many intertidal taxa occur deeper in the water column where currents flow shoreward during upwelling. (2) Larvae of nearshore species should occur farther offshore during upwelling than during relaxation or downwelling. Larvae of many nearshore species remain within several kilometers of shore during both conditions. (3) Larval settlement in intertidal communities should be lower during upwelling than relaxation or downwelling. Daily larval settlement has not observed to be higher during relaxation or downwelling events; settlement has most often been seen to vary with the fortnightly tidal cycle likely due to onshore larval transport by internal tides. (4) Larval settlement and recruitment in intertidal communities should be lower in areas of strong, persistent upwelling than where upwelling is weaker and less persistent. Recruitment of mussels and barnacles to artificial and natural substrates did not vary with the strength of upwelling, but did vary inversely with two measures of desiccation potential, and directly with indicators of surf zone hydrodynamics; larval recruitment was higher where surf zones were more dissipative with rip currents. (5) Phytoplankton subsidies to nearshore communities should be highest where upwelling is moderate and intermittent. Like larval subsidies, phytoplankton subsidies varied spatially with surf zone hydrodynamics rather than upwelling. This reconsideration of the evidence for the IUH finds the hypothesis unsupported.

Nonconsumptive predator effects on prey: dogwhelk cues limit mussel recruitment and trigger mussel shell plasticity that hinders predation

Nonconsumptive predator effects on prey: dogwhelk cues limit mussel recruitment and trigger mussel shell plasticity that hinders predation

Estimating the impact of consumers in ecological communities: Manual removals identify the complex role of individual consumers in the Gulf of Maine

In intertidal communities, consumers (especially carnivorous gastropods) have historically been thought to exert strong top-down control on community composition by regulating competitively dominant mussels and barnacles. This paradigm was formulated based on wire mesh cage exclusion experiments, which have well-known artifacts such as altering hydrodynamics and excluding non-target, but potentially important, consumer species. Recent research highlights the potential importance of multiple consumers that are often subtle and transient, as well as the modifications of consumer pressure by spatial and temporal environmental variability and bottom-up processes. Manipulative experiments that target individual taxa will be essential to more clearly identify their roles in complex ecological communities.The predatory gastropod Nucella lapillus has long been considered an important consumer controlling the structure and dynamics of intertidal communities in the Gulf of Maine. To test the role of N. lapillus in shaping community structure, we manually reduced its densities for 2.33years. Species composition, stable stage community composition (based on a Markov model), and transition probabilities (as measures of ecological pathway strength) were compared between treatments (control vs. density reductions), and among seasons (spring, summer, fall, winter). In contrast to previous studies, exclusions had no effect on community composition or long-term Markov model predictions of stable stage community structure. Reducing N. lapillus abundance increased the persistence and reduced the mortality of the barnacle Semibalanus balanoides in the model, but did not affect blue mussels, Mytilus edulis, in a similar way. Reducing N. lapillus abundance had additional indirect effects of increasing Ralfsia spp. mortality and increasing hydroid persistence. Despite differences in transition probabilities among treatments, compensatory changes in direct and indirect pathways led to communities that converged over the long-term. Our results contrast previous estimates of the role of N. lapillus in intertidal communities and might reflect low mussel recruitment, predation by alternate consumers (e.g., C. maenas, C. borealis, Tautogolabrus adspersus, birds), or spatial and/or temporal environmental variation that influenced the role of N. lapillus in structuring these communities. The role of N. lapillus may be more limited, or variable, than often assumed in the Gulf of Maine, and the methods manipulating broad functional groups such as “consumers”, though useful in developing basic conceptual models, blur the effects of individual species in community assembly and dynamics.

Factors affecting the recruitment of Amphibalanus improvisus and Dreissena polymorpha in a highly eutrophic brackish bay

Species invasions are modifying ecosystems worldwide. Coexistence of invasive species of no common evolutionary history in their new ranges enables the study of ecological rules shaping novel communities at their initial stages. In oligohaline parts of the Baltic Sea, the most dominant and widespread epifaunal suspension feeders are invasive mussels Dreissena polymorpha and cirripeds Amphibalanus improvisus. This study experimentally evaluated recruitment and microhabitat use in response to environmental forcing in these two species in a eutrophic bay. Recruitment was structured by different microhabitat exploitation patterns coupled with both individual and interactive effects of several environmental gradients. Despite functional similarity, the importance of environmental gradients differed between these species: mussel recruitment was best explained by temperature while barnacle recruitment was best explained by wave exposure. Zebra mussels were more fastidious about surface orientation than barnacles. The preferred orientations also differed between species, as mussels strongly preferred horizontal surfaces, while barnacles were more abundant on vertical surfaces. Increase in one species also predicted well the abundance of the other, especially for barnacles, as mussel abundance was the best predictor of barnacle abundance over all the other factors. The extent of microhabitat segregation correlated weakly over the studied range of the most important environmental gradients. The study shows that coexistence of functionally similar non-native D. polymorpha and A. improvisus in their sympatric range is likely determined by complex interactions between these species, their different microhabitat exploitation patterns, environmental limitations and optima.

Recruitment and zonation in a sub-Antarctic rocky intertidal community

This study presents for the first time the factors governing the recruitment in a rocky intertidal community of the Beagle Channel, Tierra del Fuego (54°51′S 68°29′W), Argentina. The aim of this study was to examine the effect of grazers and predators, free substrate availability and crustose coralline algae on the recruitment of the main sessile components of the intertidal:Notochthamalus scabrosus, Notobalanus flosculus, Mytilus chilensis, Perumytilus purpuratusandAulacomya atraat three intertidal levels. For barnacles, the probability of recruitment was higher with grazers, while the contrary was observed for bivalves. The number ofN. flosculusrecruits was higher with increased substrate availability, whileN. scabrosusrecruited more with reduced free substrate in the first sampling. Mussel recruitment was higher with reduced free substrate. The highest probability of recruitment ofN. scabrosuswas observed at the upper level. Notably, this probability and the recruits per plot were higher at the mid level under uncaged-ORP treatment than expected for the mid level. The probability of bivalve andN. flosculusrecruitment was higher at upper and lower levels, respectively. At the lower level, barnacle recruitment was higher on bare rock than on crustose coralline algae. Our results suggest that grazers increase the probability of barnacle recruitment, while the presence of sessile organisms enhances the density of mussel recruits. Almost no recruitment of bivalves was observed in ORPs over one year, showing that the secondary succession is slow in this environment.

Field-based experimental acidification alters fouling community structure and reduces diversity.

Increasing levels of CO2 in the atmosphere are affecting ocean chemistry, leading to increased acidification (i.e. decreased pH) and reductions in calcium carbonate saturation state. Many species are likely to respond to acidification, but the direction and magnitude of these responses will be based on interspecific and ontogenetic variation in physiology and the relative importance of calcification. Differential responses to ocean acidification (OA) among species will likely result in important changes in community structure and diversity. To characterize the potential impacts of OA on community composition and structure, we examined the response of a marine fouling community to experimental CO2 enrichment in field-deployed flow-through mesocosm systems. Acidification significantly altered the community structure by altering the relative abundance of species and reduced community variability, resulting in more homogenous biofouling communities from one experimental tile to the next both among and within the acidified mesocosms. Mussel (Mytilus trossulus) recruitment was reduced by over 30% in the elevated CO2 treatment compared to the ambient treatment by the end of the experiment. Strong differences in mussel cover (up to 40% lower in acidified conditions) developed over the second half of the 10-week experiment. Acidification did not appear to affect the mussel growth, as average mussel sizes were similar between treatments at the end of the experiment. Hydroid (Obelia dichotoma) cover was significantly reduced in the elevated CO2 treatment after 8weeks. Conversely, the percentage cover of bryozoan colonies (Mebranipora membranacea) was higher under acidified conditions with differences becoming apparent after 6weeks. Neither recruitment nor final size of barnacles (Balanus crenatus) was affected by acidification. By the end of the experiment, diversity was 41% lower in the acidified treatment relative to ambient conditions. Overall, our findings support the general expectation that OA will simplify marine communities by acting on important ecological processes that ultimately determine the community structure and diversity.

Genetic decoupling of spat origin from hatchery to harvest of Mytilus galloprovincialis cultured in suspension

Productivity of Mediterranean mussel aquaculture in suspension could be enhanced with selected hatchery strains for a variety of candidate traits. While selective breeding and reproduction are well established technologies, several key parameters required to grow selected strains outdoors still remain elusive. For instance, external recruitment dynamics on suspended devices is poorly known and therefore it is uncertain whether a harvested strain comes from the one initially seeded or has been partially replaced by wild recruits during the pre-fattening period. The incorporation of genetic technologies to the management of mussel stocks makes feasible to disentangle the genetic contribution of different seed sources, provided that an appropriate type of marker and a suitable experimental design are properly combined. Using multiplexed microsatellites we have traced back the individual origin of a batch of harvested mussels, which grew in suspension ropes that were initially seeded with full-sib juveniles. We show that mean heterozygosity decreased by 28% between months 8 and 13 in suspension, a phenomenon thought to be caused by fall-offs of the more exposed heterozygous individuals of the cohort. Bayesian tests allowed to ascertaining that 98.3% of the adult harvest came from the original hatchery full-sib family while only 1.7% mussels were recruited from the wild. Such low external recruitment on unprotected ropes allows to being confident on the feasibility of culturing selected strains, provided that both, seed density and retention timing are adequately managed. Statement of relevanceGenetic assessment of mussel recruitment in suspension.

Loss of reproductive output caused by an invasive species.

We investigated whether Neogobius melanostomus, an invader of biodiversity ‘hot-spots’ in the Laurentian Great Lakes region, facilitates or inhibits unionid mussel recruitment by serving as a host or sink for their parasitic larvae (glochidia). Infestation and metamorphosis rates of four mussel species with at-risk (conservation) status (Epioblasma torulosa rangiana, Epioblasma triquetra, Lampsilis fasciola and Villosa iris) and one common species (Actinonaias ligamentina) on N. melanostomus were compared with rates on known primary and marginal hosts in the laboratory. All species successfully infested N. melanostomus, but only E. triquetra, V. iris and A. ligamentina successfully metamorphosed into juveniles, albeit at very low rates well below those seen on even the marginal hosts. Neogobius melanostomus collected from areas of unionid occurrence in the Grand and Sydenham rivers (Ontario, Canada) exhibited glochidial infection rates of 39.4% and 5.1%, respectively, with up to 30 glochidia representing as many as six unionid species per fish. A mathematical model suggests that N. melanostomus serve more as a sink for glochidia than as a host for unionids, thereby limiting recruitment success. This represents a novel method by which an invasive species affects a native species.

Annual variation in recruitment of freshwater mussels and its relationship with river discharge

Abstract Vital rates such as mortality, growth, and recruitment are important tools to evaluate the status of threatened populations and identify their vulnerabilities, leading to enhanced conservation strategies. Native freshwater mussels are a guild of largely sedentary, filter‐feeding bivalves currently facing worldwide declines. Lack of recruitment has been identified as a major threat to mussel populations. A mussel bed in the Upper Mississippi River was sampled for 5 years (2008–2012). A trend analysis showed a significant decline in the percentage of species with juvenile representatives. Species were grouped into equilibrium and periodic life history strategies to assess past recruitment. Residuals from catch‐curve regressions quantified past year‐class strength of both strategists and Amblema plicata over a 13‐year period (1994–2006), and identified strong and weak year‐classes. Generalized linear regression models containing July maximum discharge and April minimum discharge explained 64% of the variation in recruitment strength of A. plicata. The best model for the equilibrium strategists explained 86% of the variation in recruitment and contained the same variables as A. plicata, but also incorporated the 7‐day minimum discharge. For the periodic strategists, the model containing the number of low‐flow pulses and the mean duration of high‐flow pulses explained 56% of the variation in recruitment strength. Understanding variation in recruitment dynamics of native mussels and its relationship to river discharge will be useful in designing effective management strategies to enhance conservation of this imperilled fauna. Copyright © 2015 John Wiley & Sons, Ltd.

Population ecology of the gulf ribbed mussel across a salinity gradient: recruitment, growth and density

Benthic intertidal bivalves play an essential role in estuarine ecosystems by contributing to habitat provision, water filtration, and promoting productivity. As such, changes that impact population distributions and persistence of local bivalve populations may have large ecosystem level consequences. Recruitment, growth, mortality, population size structure and density of the gulf coast ribbed mussel, Geukensia granosissima, were examined across a salinity gradient in southeastern Louisiana. Data were collected along 100‐m transects at interior and edge marsh plots located at duplicate sites in upper (salinity ~4 psu), central (salinity ~8 psu) and lower (salinity ~15 psu) Barataria Bay, Louisiana, U.S.A. Growth, mortality and recruitment were measured in established plots from April through November 2012. Mussel densities were greatest within the middle bay (salinity ~8) regardless of flooding regime, but strongly associated with highest stem densities of Juncus roemerianus vegetation. Mussel recruitment, growth, size and survival were significantly higher at mid and high salinity marsh edge sites as compared to all interior marsh and low salinity sites. The observed patterns of density, growth and mortality in Barataria Bay may reflect detrital food resource availability, host vegetation community distribution along the salinity gradient, salinity tolerance of the mussel, and reduced predation at higher salinity edge sites.

Monitoring Recruitment Patterns of Mussels and Fouling Tunicates in Mariculture

ABSTRACT Methods to increase the precision of spat collection and strategies to mitigate fouling are greatly needed in aquaculture production. As such, larval recruitment of mussels and a common tunicate species was investigated. Recruitment was measured in shallow (1–2 m) and deeper (4–5 m) water at three sites during the summer of 2012. In addition, to evaluate the importance of timing in deployment of mussel ropes, differences in mussel yield were examined. The settlement plates provided a good description of the settling community with high temporal resolution. Peaks in recruitment were observed for both mussels and tunicates but recruitment rates and the timing of peaks differed among sites. Although mussel larvae preferred shaded substrates at some sites and times, these substrates were consistently preferred by tunicates. Mussels preferred to settle at shallow depths, whereas tunicates were consistently more abundant deeper. In contrast to predictions, there was no positive relationship between the...

Temporal variation in intertidal community recruitment and its relationships to physical forcings, chlorophyll-a concentration and sea surface temperature

We investigated the recruitment of intertidal barnacles and mussels at three temporal scales (months, weeks and days), and its relationships to physical forcings, chlorophyll-a concentration (Chla) and sea surface temperature (SST), at both a local (km) and a regional (10–100 km) resolution. The study was conducted in the South Brazilian Bight, a subtropical region influenced by upwelling and meteorological fronts, where recruitment rates were measured monthly, biweekly and daily, from 2012 to 2013 using artificial substrates fixed in the intertidal zone. The strength of the relationship between recruitment and physical forcings, Chla and SST depended on the temporal scale, with different trends observed for barnacles and mussels. Barnacle recruitment was positively correlated with wind speed and SST and negatively related to the wind direction, cold front events and Chla. Wind direction was positively correlated with mussel recruitment and negatively covaried with SST. We calculated net recruitment (NR) to estimate the differences in recruitment rates observed at longer time scales (months and weeks), with recruitment rates observed at shorter time scales (weeks and days), and found that NR varied in time and among taxa. These results suggest that wind-driven oceanographic processes might affect onshore abundance of barnacle larvae, causing the observed variation in recruitment. This study highlights the importance of oceanic–climatic variables as predictors of intertidal invertebrate recruitment and shows that climatic fluctuations might have different effects on rocky shore communities.

Impacts of climate change and land-use scenarios on Margaritifera margaritifera, an environmental indicator and endangered species

In this study, we assess the impacts of future climate and land-use in the Beça River (northern Portugal) under different scenarios and how this will translate into the conservation status of the endangered pearl mussel Margaritifera margaritifera (Linnaeus, 1758). This species is currently present in several stretches of the Beça River that still hold adequate ecological conditions. However, the species is threatened by projected declines in precipitation for the 21st century, with implication on the river flows and water depths that might decrease below the species requisites. This situation could be especially critical during summer conditions since the ecological flows may not be assured and several river stretches may be converted into stagnant isolated pools. The habitat connectivity will also be affected with reverberating effects on the mobility of Salmo trutta, the host of M. margaritifera, with consequences in the reproduction and recruitment of pearl mussels. In addition, human-related threats mostly associated with the presence of dams and an predicted increases in wildfires in the future. While the presence of dams may decrease even further the connectivity and river flow, with wildfires the major threat will be related to the wash out of burned areas during storms, eventually causing the disappearance of the mussels, especially the juveniles. In view of future climate and land-use change scenarios, conservation strategies are proposed, including the negotiation of ecological flows with the dam promoters, the replanting of riparian vegetation along the water course and the reintroduction of native tree species throughout the catchment.

Recruitment, abundance, and predation on the blue mussel (Mytilus edulis) on northeastern estuarine rocky shores

We report on patterns of abundance, recruitment, and predation on the blue mussel (Mytilus edulis) in three human‐dominated estuaries in the northeastern United States. Through replicate field experiments and observational studies at multiple sites nested within each of the three estuaries, we investigated the relative influences of local and regional scale variation in select bottom‐up and top‐down factors on blue mussel populations on wave‐protected rocky shores. The most striking result was the decoupling between adult abundance and recruitment: mussel recruitment rates were highest in the most northern estuary, Casco Bay, while adult abundances were highest in the most southern estuary, Long Island Sound. We detected evidence of top‐down forcing on adult abundance by consumers in the two more southern estuaries, Narragansett Bay and Long Island Sound, but not in Casco Bay. Finally, we observed some indications of bottom‐up forcing on mussel abundance and recruitment at the within‐estuary scale, but these signals were not consistent among estuaries or across the responses measured (e.g., adult abundances and recruitment rates). Our results support previous work demonstrating the importance of both top‐down and bottom‐up influences on rocky shore populations, and also highlight how future research—particularly integrating studies of the different ontogenetic stages of mussels—could further advance understanding of biological population dynamics in this and other systems.

Mammal carcasses attract a swarm of mussels in the deep Atlantic: insights into colonization and biogeography of a chemosymbiotic species

AbstractFive mammal carcasses were experimentally deployed at 1000 m depth in the Setubal Canyon (NE Atlantic) in March 2011 and the remaining bones were collected after 18 and 28 months. High numbers (1.92–6.27 individuals·cm2) of small mussels were found among a diverse invertebrate assemblage colonizing surfaces, crevices and cavities in the trabecular matrix of bones. In this study we characterized the mussel population and their associated bacteria and investigated population structure and settlement patterns. The results of molecular analysis indicated that the mussels are conspecific with the widely distributed bathymodiolin species ‘Idas’ simpsoni and harbor sulfur‐oxidizing bacterial symbionts. Modal decomposition of length‐frequency distributions and estimated shell growth rate suggested that settlement of ‘I.’ simpsoni is continuous and that post‐settlement mortality affects the population structure. This study reports the highest densities and fastest growth rates within bathymodiolin mussels occurring on organic substrates yet recorded and provides evidence for effective mussel recruitment in organic falls in the NE Atlantic Ocean. These results highlight the importance of ephemeral habitats on the biogeographic distribution and dispersal of chemosymbiotic species in deep‐sea ecosystems.

Recruitment facilitation and spatial pattern formation in soft‐bottom mussel beds

Mussels (Mytilus edulis) build massive, spatially complex, biogenic structures that alter the biotic and abiotic environment and provide a variety of ecosystem services. Unlike rocky shores, where mussels can attach to the primary substrate, soft sediments are unsuitable for mussel attachment. We used a simple lattice model, field sampling, and field and laboratory experiments to examine facilitation of recruitment (i.e., preferential larval, juvenile, and adult attachment to mussel biogenic structure) and its role in the development of power‐law spatial patterns observed in Maine, USA, soft‐bottom mussel beds. The model demonstrated that recruitment facilitation produces power‐law spatial structure similar to that in natural beds. Field results provided strong evidence for facilitation of recruitment to other mussels—they do not simply map onto a hard‐substrate template of gravel and shell hash. Mussels were spatially decoupled from non‐mussel hard substrates to which they can potentially recruit. Recent larval recruits were positively correlated with adult mussels, but not with other hard substrates. Mussels made byssal thread attachments to other mussels in much higher proportions than to other hard substrates. In a field experiment, mussel recruitment was highest to live mussels, followed by mussel shell hash and gravel, with almost no recruitment to muddy sand. In a laboratory experiment, evenly dispersed mussels rapidly self‐organized into power‐law clusters similar to those observed in nature. Collectively, the results indicate that facilitation of recruitment to existing mussels plays a major role in soft‐bottom spatial pattern development. The interaction between large‐scale resource availability (hard substrate) and local‐scale recruitment facilitation may be responsible for creating complex power‐law spatial structure in soft‐bottom mussel beds.