Molluscan Assemblages Research Articles

Concordant Spatial Gradients in Predation Intensity and Productivity Archived in Surficial Mollusk Shell Accumulations in Seagrass Meadows Along the Gulf Coast of Florida

Seagrass meadows are highly structured habitats of great socioeconomic value but are declining globally due to human impacts. The northern Gulf Coast of Florida contains one of the largest relatively unaltered seagrass habitats (~3,000 km2), making it a model system for acquiring baselines to better manage and monitor seagrass meadows. This project investigated spatial gradients in ecological and taphonomic attributes of surficial seagrass-associated benthic mollusk death assemblages. An updated analysis of water parameters (based on project COAST data) indicated that total dissolved phosphorous (TDP) and chlorophyll-a (CHL-a) concentrations increased steadily northward. To analyze the historical ecology of local seagrass meadows, mollusk assemblages were bulk sampled at 12 stations across multiple estuaries along the gradient. Radiocarbon dating of valves (n = 90) from several estuaries indicated that the assemblages represented a multi-millennial accumulation with a median shell age of 1760 AD. Focusing on two abundant bivalves found within all estuaries, Transennella spp and Crassostrea virginica, the study evaluated spatial trends in body size, traces of predation, and post-mortem shell alterations. The results, primarily focused on Transennella spp., indicated that ecological and taphonomic characteristics varied notably, both among and within estuaries. Relative abundance, body size, and taphonomic attributes did not appear to correlate significantly with productivity or drilling frequency. However, drilling frequency correlated significantly with TDP and CHL-a, suggesting that predator-prey interactions may co-vary tightly with productivity. These results suggest that spatial gradients in predation can be archived by drilling frequencies in death assemblages. Moreover, the consistency between the long-term record of predation and modern gradient conditions suggests that long-term spatial dynamics of predator-prey interactions may be hydrologically controlled.

Live-Dead Shifts in Molluscan Functional Diversity in Coastal Alabama

Abiotic and biotic environmental factors shape species behavioral, physiological, and morphological traits. Shifts in the diversity of these functional traits therefore can serve as an indicator of spatial and temporal changes in environmental conditions. In recent centuries, the delivery of nutrients to coastal ecosystems has increased markedly through changing waste management and land-use practices. Nutrient enrichment has increased rates of primary productivity, resulting in greater organic accumulation on the seafloor and the development of hypoxic conditions in some areas, due to the aerobic decomposition of phytoplankton blooms. Anthropogenic eutrophication and associated hypoxia have been monitored annually in the northern Gulf of Mexico for over four decades, with an emphasis on areas surrounding the Mississippi River Delta. To further understand how anthropogenic eutrophication affects functional diversity in benthic marine communities, we collected live and dead assemblages of bivalve mollusks from surficial sediments at five stations along the -20 meters isobath in coastal Alabama. Bivalves were categorized into functional groups using information about their mobility, fixation, feeding type, substrate preference, and body size. Consistent with our hypotheses, preliminary results indicate a temporal shift from benthic communities dominated primarily by epifaunal and infaunal suspension-feeding species to communities characterized primarily by deposit feeders, many of which dwell on the seafloor surface. Ongoing analyses will help identify the functional traits that are most sensitive to these changing environmental conditions. Understanding functional diversity shifts in the recent past can provide insight into how anthropogenic eutrophication may further impact benthic marine ecosystems in the future.

Impacts of Ocean Deoxygenation on Marine Benthos in the Gulf of Mexico

Ocean deoxygenation is a growing concern globally. Oxygen is less soluble in warm water, and warming temperatures also result in the slowdown of ocean circulation which limits oxygen delivery to deeper waters. Anthropogenic eutrophication has also contributed to the development of hypoxic conditions in many coastal areas. Here we investigate biodiversity structure along an environmental gradient in the Gulf of Mexico to see how patterns of spatiotemporal turnover can inform future biotic response of benthos to ocean deoxygenation. Live and dead assemblages of bivalve mollusks were collected at 15 stations offshore Louisiana, Alabama, and Florida. Abundance and body size data were collected, and specimens were classified functionally using information about feeding, attachment, life position, and body size. Environmental conditions were characterized using multi-decadal mean sea surface temperature (SST), dissolved oxygen (DO), and net primary productivity (NPP), and grain size data from our field samples. Stations in the north-central Gulf affected by Mississippi River discharge are characterized by higher NPP, lower DO, and higher percentages of silt and clay than stations in the northeastern Gulf. Both taxonomic and functional diversity significantly covary with this environmental gradient, with the lowest diversities observed at stations in the core of Louisiana’s “dead zone.” Analyses of spatiotemporal turnover patterns reveal shifts in the dominant feeding mode, with hypoxic environments containing a greater abundance of deposit and mixed feeders, compared with more oxygenated environments that host an abundance of suspension feeders and are characterized by a greater variety of feeding ecologies. Live-dead analyses reveal a shift in taxonomic and functional diversity in coastal Louisiana, that appears to coincide with the onset of anthropogenic eutrophication in these coastal settings.

Paleo-Biological Approaches to Present Day Wetland Ecosystem Restoration Problems

Establishing historic conditions of a site is frequently the first step in ecosystem restoration. Whether restoration to historic conditions is possible or not, recognition of change and the cause of change is critical information. Three different wetland restoration problems are addressed by paleo-biological methods. Chrono-stratigraphy was instrumental in all three cases, 210Pb at the decadal and radiocarbon for the century to millennial scale questions. In order to document salinity, change in the Southeast Saline Everglades molluscan assemblages were utilized as a proxy for salinity. Vertical changes in cores established that beginning in ~ 1900 salinity increased with the Anthropocene Marine Transgression. The freshwater-marine contact in all cores was identified and the contacts dated. The differences in time between two adjacent cores and the distance between them was utilized to determine the rate of saltwater encroachment, documenting that not all coastal basins exhibited the same rate and that the rate increased over time in response to the accelerating rate of sea-level rise, increasing from the pre-1960 rate of ~ 20 m yr-1 to > 80 m yr-1 between 1995 and 2017. This shift in regime suggests that present restoration activities are inadequate. Soon after Audubon acquired Corkscrew Swamp to preserve the largest remaining wood stork rookery, the swamp was diked to hold surface water because it was believed that surrounding land development was adversely impacting swamp hydroperiod. However, by 1970 cypress regeneration was severely reduced and hydrology and sediment core studies were initiated in an attempt to understand the driver of the change. Core analysis revealed that hydroperiod increased upwards terminating in patchy open-water peats, based upon pollen analysis, previous peat analysis and tissue recognition. Open-water peats indicate no drawdown, eliminating cypress germination. Cypress regeneration began soon after removal of the dike.

Molluscs Across Space and Through Time in a Hypersaline Coastal Lagoon, Mexico

Biotic remains have been used in paleoecological studies to provide information on past environmental conditions. Death assemblages can be used to make inferences about past environmental conditions if modern optima and ranges of taxa are known. Gaps in knowledge about historical biotic changes often stem from a paucity of information regarding species distributions, community species richness and evenness, community structure, ecological interactions, and possible factors that caused past biotic shifts. We studied mollusc assemblages in sediment cores from Río Lagartos coastal lagoon (Mexico) and compared them to present-day mollusc communities to gain insights into environmental changes that occurred in the lagoon throughout the last century. A total of 18,779 mollusc specimens, representing 20 bivalve and 45 gastropod species, and belonging to 32 families and 48 genera, were identified in three short sediment cores collected from the coastal lagoon in 2017. Molluscs in the sediment cores were compared to an inventory of modern fauna from the lagoon, which was collected along a salinity gradient in 2017 and 2018 to link species distributions with environmental variables. Mollusc communities from the sediment cores and present-day datasets possess the same ubiquitous species and feeding guilds. Nearly twice as many species, however, were identified in the sediment cores as in the present-day inventory. We report differences in mollusc abundance and taxonomic composition in the cores across space and time, which may be related to the salinity gradient in the lagoon, temporal shift in salinity, and recent human-mediated modifications of the nearby terrestrial environment. Biotic changes driven by shifts in salinity could have been reduced salinity. Such inputs may have been driven by hurricanes, along with associated high wind velocities and geomorphologic transformations. This paleobiology study will be of use for future conservation efforts in the coastal lagoon.

Mollusk Response Under Ocean Acidification in Shallow Marine Settings of Sicily (Central Mediterranean)

Anthropogenic CO2 is a major driver of change in most marine ecosystems, as the consequent ocean acidification is threatening marine calcifying organisms. In this respect, long-term analyses on ocean acidification effects on marine ecosystems acclimated to high pCO2, as found around CO2 vents, are needed. Here we tackle mollusk assemblages from acidified shallow marine settings off the Aeolian archipelago (Central Mediterranean). The detected gradient manifests along a 34 m long transect (9.6 m and 11.4 m water depth), mostly in a Posidonia oceanica matte from normal (site 1) to high levels (site 3) of pCO2 (405 μatm, pH 8.1 and 715 μatm, pH 7.8; respectively). The strongest acidified condition at the vent crater (site 4, pCO2 1110 μatm, pH 7.7). At the vent site gaseous emissions are characterized by ~99% in volume of CO2 and ~0.6% of H2S. However, water dissolved H2S was below detection limit and the sulphate content along the transect does not show significant variations with respect to normal seawater values. Preliminary paleoecologic surveys on diversity structure (diversity profiles) and taphonomic degradation (NMDS, z scored % values) were conducted on mollusk remains collected along the natural pH gradient (sites 1-4). Along the P. oceanica matte (sites 1-3), overall mollusk taxon diversity (alpha and beta) decreased, mollusk in site 3 were mostly juveniles and had higher overall taphonomic damages than those retrieved at normal pH conditions. Within the vent crater only fewer and highly taphonomically altered gastropod specimens were retrieved on the pebbly seafloor, suggesting a very short residence time of shell material and rapid dissolution. Even if vents are not exact predictors of the anthropogenic-designed future of marine settings, due to their limited spatial and temporal extent, they can act as natural laboratories where to evaluate the output of ecosystem processes under rising pCO2 and the effects on the creation of the future fossil record.

Biodiversity and assemblages of molluscs (Gastropoda and Bivalvia) from coastal and deep-sea soft-bottoms off Mauritania

The collections as well as quantitative faunistic and environmental data obtained from 384 bottom-trawling stations during the four Spanish Maurit and two FAO–CCLME expeditions have allowed us, for the first time, to establish the biodiversity distribution patterns and assemblage structures of Mauritanian molluscs. The six surveys prospected the soft-bottoms of the shelf and continental slope between Cape Blanc (20°50′N) and the Senegalese border (16°04′N), from a depth of 19 to 1860 m. Almost 13,000 live specimens collected from 150 stations (40% of the total) belonged to 104 species (37 bivalves and 67 gastropods). Nassariidae and Xenophoridae were the most frequently found families, and the gastropod Nassarius wolffi (Knudsen, 1956) was the most frequent mollusc species in the Mauritanian soft-bottoms. Naticidae and Marginellidae, followed by Muricidae and Pectinidae, were the most diverse families.Depth and depth-related variables, besides the carbonate content of the sediments, are the major environmental factors responsible for the distribution patterns of the ecological descriptors as well as separation of the three main assemblages identified in the shelf, slope and deep-slope. Besides their dissimilarities (β-diversity), the three faunistic assemblages showed significant quantitative differences linked to similar and marked bathymetric trends of diversity, abundance and biomass of both bivalves and gastropods, which strongly decline from the shelf along the slope. Although the mean value of species richness seems uniform along the coast, abundances and biomass are the lowest in the southernmost zone (17°N–16°N), with the highest values, particularly for gastropods, in the Banc d’Arguin region.

Lake trophic status and hydrological connectivity modify mechanisms underlying mollusc assemblage structuring

Lake trophic status and hydrological connectivity modify mechanisms underlying mollusc assemblage structuring

Miocene instead of Jurassic: the importance of sound fieldwork for paleontological data analysis

AbstractA diverse molluscan assemblage dominated by turritellid gastropods found in Kachchh, western India, has been interpreted in the past as Late Jurassic (Oxfordian) in age, based on associated undoubtedly Oxfordian ammonites. Recently, several investigations focused on the assemblage dealing with taxonomic, paleoecological, and evolutionary aspects. An analysis of the associated bivalve fauna, foraminiferal assemblage, and the geological context strongly suggests a Miocene rather than a Jurassic age and invalidates several conclusions drawn from the alleged Jurassic age of the fossils.

New marine warm-temperate molluscan assemblage demonstrates warm conditions during the Middle Pleistocene of the North Sea Basin

Abstract We report a marine Middle Pleistocene mollusc fauna from a borehole near Luxwoude (Friesland, northern Netherlands). The fauna contains several species, including Bittium species, Acanthocardia paucicostata and Polititapes senescens, that hitherto have been used as indicative for warm (lusitanian) conditions in the southern North Sea Basin during the Late Pleistocene Eemian (MIS5e) interglacial. However, the stratigraphic context of the Luxwoude marine fauna indicates a MIS11 or older age for this new fauna. This thermophilous fauna demonstrates very warm-temperate conditions and probably an open marine connection through the Dover Strait towards the south, during this Middle Pleistocene interglacial. In the North Sea basin, this distinctive lusitanian fauna with Bittium-dominated assemblages can therefore no longer be presumed to be of Eemian age without additional evidence.

The latitudinal gradient of functional diversity of Miocene marine mollusks from Chile

Understanding latitudinal variations in biodiversity is central for biogeography. Along the coasts of the Southeast Pacific, several taxa show inverse latitudinal patterns of biodiversity, i.e. increasing species numbers from lower to higher latitudes. A plausible explanation for these patterns is that fjords, formed during the Pleistocene glaciations, increased the diversity of available biotopes that allowed for higher diversity in high latitudes. Assessing this hypothesis requires us to analyze latitudinal patterns of functional diversity (which is intimately related to niche use) in the absence of fjords, i.e. earlier than their formation. Here we test if the fossil record before the generation of fjords shows higher functional diversity at lower than at higher latitudes (a ‘regular' functional diversity gradient).We analyzed four components of functional diversity (functional richness, functional divergence, functional dispersion and functional evenness) for a fossil marine mollusk fauna from the lower Miocene (~18 million years ago) across four regions spanning more than 10 latitudinal degrees of the Chilean coast (between ~34°S and 45°S).The functional richness of gastropods and bivalves decreased non‐linearly from low to high latitudes. Contrarily, the functional evenness of gastropods remained relatively constant across the region and that of bivalves peaked at 45°S. The functional divergence and functional dispersion of taxonomic groups did not exhibit clear patterns of latitudinal variation. The multivariate analysis uncovered non‐linear latitudinal patterns in the abundance of functional groups (gastropods and bivalves), with the largest abundance towards lower latitudes.These results suggest that the niche breadth of mollusk assemblages might have decreased with latitude in the lower Miocene. This work can shed light on the mechanisms underlying the latitudinal variation of diversity observed today, offering a better understanding of how these patterns changed over time.

Late Pleistocene and Holocene malacological and theriological faunas from the Tetyukhinskaya Cave (southern Far East, Russia) and their palaeoecological implications

Assemblages of terrestrial molluscs and small vertebrates were recovered from the Tetyukhinskaya Cave, located in the vicinity of the Dalnegorsk city, Primorsky Krai (south of the Far East, Russia), from which we identified characteristic zones of molluscs and small mammals. Based on analysis of the malacofauna and theriofauna, we interpret the molluscs as indicators of the biotopes at the cave entrance, and the mammals as indicators of the cave surroundings, and during late Late Pleistocene and Holocene, forests existed near the area surrounding the cave. The small mammal fauna suggests the development of open spaces of a pasture-like ecosystem during the warm periods of the Late Pleistocene with the development of the forest vegetation. This study demonstrates the importance of complex faunal investigations for the reconstruction of the palaeoenvironment in the Late Pleistocene and Holocene.

Vodní měkkýši CHKO Brdy [Aquatic molluscs of the Brdy PLA

Although the Brdy Protected Landscape Area is located not far from Prague on the border between central and western Bohemia, there was very little published and unpublished data on its aquatic malacofauna. For this reason, a more detailed survey of aquatic molluscs was carried out in 2020–2022. Altogether with previous data, 27 species were found at 108 studied sites, including springs, small streams, fishponds, pools, and wetlands. The low species diversity is related to not very suitable natural conditions. The occurrences of only 1–2 species of aquatic molluscs were found at 72 out of 108 studied sites. Two species (Radix labiata, Pisidium casertanum) are typical for nutrient-poor habitats. The more rich molluscan assemblages were detected only in the peripheral parts of the Brdy Mts. and/or in the Padrť region with numerous fishponds and associated wetlands. Most (18) of the detected species belong to common and widespread molluscs. Three non-native species (Physa acuta, Ferrissia californica, Sinanodonta woodiana) were found.

Early Miocene Sr-geochronology of Patagoniense marine units from southern Argentina: Implications for molluscan assemblages and interbasinal correlation

Early geological investigations in southern South America addressed the Oligocene-Miocene Patagoniense deposits, a fossiliferous marine succession distributed in several basins that had been flooded by three different transgressive events. Chronological calibration and correlation of these transgressive events, lithological units and fossil assemblages through the Patagonian basins still need further definitions in some areas. To tackle this problem, new Sr-isotope ages of pectinid shells from four early Miocene key sections located at the coastal sector of Patagonia, were obtained to improve correlations among the Magallanes-Austral (MAB), San Jorge (SJB) and Mazarredo (MB) basins. A bed in the middle part of the Punta Guanacos section (MAB) was dated in a mean age of 19.8 Ma allowing its correlation with the lower section of the Monte Leon Formation. In the SJB, the lowermost bed of the Chenque Formation at Punta Borja has an age of 19.6 Ma, while lower bed of Sequence 2 of this unit has an age of 17.7 Ma at Punta Delgada. Patagoniense deposits exposed along the cliffs of the southern San Jorge Gulf (MB), and overlying the continental Sarmiento Formation, are recognized herein as the Mazarredo Formation with a mean age of 20.9 Ma at Canadón El Lobo. It is younger than the San Julián Formation and correlates with the lower part of the Monte León Formation at Playa La Mina (MAB). The radiometric age of the Mazarredo Formation is of outstanding importance because the Mazarredo area constitutes a geographic link between the MAB and SJB and precludes the presence of the “Juliense” transgression in central Patagonia as had been proposed, supporting the early Miocene age previously provided by the authors for the Jorgechlamys centralis –Reticulochlamys borjasiensis Molluscan Assemblage. This assemblage is recorded in the Mazarredo Formation and in the lower section of the Chenque Formation (SJB). Moreover, faunal similarity with the Monte León Formation allows the identification of the Reticulochlamys zinsmeisteri - Struthiolarella patagoniensis - Pleuromeris cruzensis Assemblage at Punta Guanacos in the northeastern sector of the MAB.

The molluscan assemblage of a pristine Posidonia oceanica meadow in the eastern Mediterranean

The seagrass Posidonia oceanica forms extensive meadows in the Mediterranean Sea. Studies on their associated highly diverse invertebrate assemblages are limited to the western Mediterranean. The eastern Mediterranean, however, is a basin undergoing rapid change due to the synergistic effects of climate warming, biological invasions and other human stressors that are driving native biodiversity to regional-scale collapses. We here surveyed the shelled molluscan assemblage of a Posidonia oceanica meadow in Plakias, south-western Crete, the first such study in the eastern Mediterranean Sea. This area has increased its yearly mean temperature by 1 °C in the last 20 years and is under heavy pressure by Lessepsian species. We sampled across a 5- to 20-m depth gradient, in two seasons to capture intra-annual variation and the leaf and rhizome strata separately. Against our expectations, the molluscan assemblage proved to be highly diverse, with species richness, dominant species and trophic guilds comparable to healthy western Mediterranean ones, and with a negligible non-indigenous component. The diversity of the native community (following the biotic resistance hypothesis) and oxygen supersaturation in the meadow may cause greater resistance to biological invasions and warming, respectively, suggesting that Posidonia oceanica meadows may act as a precious refugium for native biodiversity in the fast changing eastern Mediterranean Sea.

Mollusk death assemblages from the deep slope off northwestern Cuba (Gulf of Mexico)

We evaluated the mollusk death assemblages along seven sites in a deepsea expedition off northwestern (NW) Cuba. We aimed to (1) provide the first baseline of mollusk diversity in sediments of the NW region, and (2) test if there were west-east gradients of α- and β-diversities. We found 141 species in sediments: 68 deepsea benthic, 43 shelf benthic, and 30 pelagic species. The richness of pelagic species was similar along the NW region indicating significant settlement of pteropod shells to the seabed. The occurrence of shelf species in the deepsea suggested a downslope transport of shells enhanced by the slope steepness and dredging in two ports. β-diversity of deepsea benthic mollusks showed a high turnover at local scales explained by habitat heterogeneity. β-diversity of pelagic species was extremely low suggesting that the planktonic assemblage was evenly distributed. In contrast, high β-diversity of shelf benthic species occurred likely due to downslope transportation of materials. Deepsea gastropods were mostly detritivores, while bivalves were mostly suspension feeders. These dominant biological traits indicated oligotrophic conditions in sediments likely coupled with an intense hydrodynamic regime in the deepsea. The NW Cuban region seems to be a hotspot for mollusk diversity because of the discovery in a single expedition of seven new species, 12 new records, and 36% endemism relative to the entire Gulf of Mexico. Our findings revealed a complex and diverse mix of deep/pelagic/shelf species but did not indicate a west-east gradient in the mollusk assemblage structure.

Deep anthropogenic impacts on benthic marine diversity of the Humboldt Current Marine Ecosystem: Insights from a Quaternary fossil baseline

The Humboldt Current Marine Ecosystem (HCE) is one of the most productive areas in the global ocean, but current anthropogenic stressors, particularly overfishing, pose a significant threat to marine biodiversity. Moreover, the limited time scale of modern assessments may underestimate the magnitude of human alterations to marine biodiversity. Here we use the rich Quaternary fossil record present along the HCE coast, encompassing the last ca. 500 kyr, to build a baseline to evaluate the impact of human activities on the diversity of mollusk assemblages. We compiled an extensive database of >13,000 occurrences and ca. 370,000 individuals of 164 species of gastropods and bivalves from modern and fossiliferous outcrops from southern Peru to northern Chile (15-30°S). We tested for changes in coverage-based species richness, species dominance, species composition (Chao dissimilarity, unweighted and weighted by abundance), and the relative abundance (i.e., the proportion of individuals) of species exploited by the artisanal fisheries. Comparisons between fossil and modern assemblages were carried out at different scales of spatial aggregation to buffer against inherent differences in spatial and temporal averaging. Species composition shows remarkable stability in fossil assemblages, from Middle Pleistocene to Holocene, at most scales of spatial aggregation. Modern assemblages showed drastic alterations compared to fossil counterparts when analyses considered spatial aggregation scales, i.e., significant changes in species composition, and a 3 to 6-fold reduction in the relative abundance of exploited species, but not changes in species richness and dominance. Results suggest that contemporaneous anthropogenic activities disrupted a long-term stability in the species composition. The diversity of modern mollusk assemblages is unseen in the past 500 kyr and seems deeply perturbated by overfishing. Our synthesis sets the foundations for a conservation paleobiology approach to robustly understand the impacts of anthropogenic stressors at the HCE.

The Holocene temperature conundrum answered by mollusk records from East Asia

Seasonal biases (the warm-season contribution) of Holocene mean annual temperature (MAT) reconstructions from geological records were proposed as a possible cause of the mismatch with climate simulated temperature. Here we analyze terrestrial mollusk assemblages that best reflect seasonal signals and provide quantitative MAT and four-season temperature records for northern China during the past 20,000 years. The MAT estimated from the seasonal temperatures of a four-season-mean based on mollusks shows a peak during ~9000–4000 years ago, followed by a cooling trend. In general, the contribution of summer and winter temperature to MAT is significantly greater than that of spring and autumn temperatures. The relative contribution of each season varies over time and corresponds roughly with the seasonal insolation in each season. This independent evidence from mollusk records from the mid-latitudes of East Asia does not support the Holocene long-term warming trend observed in climate simulations and the seasonal bias explanation.

Diversity of Molluscan Assemblage in Relation to Biotic and Abiotic Variables in Brown Algal Forests.

Canopy-forming macroalgae, mainly those belonging to the order Fucales, form the so-called brown algal forests, which are among the most productive assemblages in shallow coastal zones. Their vertical, branching canopies increase nearshore primary production, provide nursery areas for juvenile fish, and sustain understory assemblages of smaller algae and both sessile and vagile fauna. The majority of benthic invertebrates inhabiting these forests have larval stages that spend some time floating freely or swimming in the plankton. Therefore, canopy-forming macroalgae play an important role as species collectors related to larval supply and hydrodynamic processes. During the past several decades, brown algal forests have significantly reduced their extension and coverage in the Mediterranean basin, due to multiple interacting natural and anthropogenic pressures, with negative consequences also for the related fauna. The aim of this research was to examine how differences in macrophyte abundance and structure, as well as environmental variables, affect the associated molluscan communities in the shallow northern Adriatic Sea. Sampling sites with well-developed vegetation cover dominated by different canopy-forming species were selected in the shallow infralittoral belt of the northern Adriatic Sea in the spring–summer period of the years 2019 and 2020. Our results confirm the importance of algal forests for molluscan assemblage, with a total of 68 taxa of molluscs found associated with macrophytes. Gastropods showed the highest richness and abundance, followed by bivalves. Mollusc richness and diversity (in terms of biotic indices) were not related with the degree of development of canopy-forming species (in terms of total cover and total volume), nor with the ecological status of benthic macroalgae at different depths. On the contrary, the variability in molluscan taxa abundances was explained by some environmental variables, such as temperature, pH, light, and nitrates concentration.

Loss of lateral hydrological connectivity impacts multiple facets of molluscan biodiversity in floodplain lakes

Lateral hydrological connectivity (LHC) is a key process in maintaining aquatic biodiversity in river floodplain ecosystems. However, the effects of LHC loss on aquatic biodiversity are rarely studied. Here, we evaluated, for the first time, the responses of multiple facets (i.e., taxonomic, functional and phylogenetic) of alpha and beta diversity of freshwater molluscs to the LHC loss in 23 floodplain lakes in the Yangtze River Basin in China. Our results showed that taxonomic and functional alpha diversities were all significantly higher in connected lakes (CLs) than in disconnected lakes (DLs), whereas phylogenetic alpha diversity (Δ+) was lower in CLs than in DLs. For beta diversity facets, taxonomic (Tβsor) and phylogenetic (Pβsor) dissimilarities were slightly more contributed by the turnover component or equally contributed by the turnover and nestedness-resultant components both in CLs and DLs. Instead, functional beta diversity (Fβsor), generally showing much lower values than Tβsor and Pβsor, was mainly contributed by the nestedness-resultant component (76.6–84.0%), especially in DLs. We found that only functional dissimilarities were significantly higher in DLs than CLs, indicating a high level of functional diversity loss without replacement of species possessing traits sensitive to hydrological disconnection (i.e., large body size, lamellibranch body form, filter feeding, ovoviviparity and burrowing habits). In general, lake area, hydrological connectivity, aquatic vegetation coverage and nutrient levels (TN and TP) played important roles in structuring variation in molluscan alpha and beta diversities, although the three diversity facets responded to different environmental factors. Our results suggest that loss of connectivity to the mainstem river has negative impacts on molluscan assemblages in floodplain lakes. More importantly, as taxonomic, functional and phylogenetic diversities responded somewhat differently to the loss of hydrological connectivity, all of these biodiversity facets should be better incorporated into aquatic biodiversity assessment and conservation programs in large river floodplains.