Mytilus Californianus Research Articles

The influence of stochastic temperature fluctuations in shaping the physiological performance of the California mussel, Mytilus californianus.

Climate change is forecasted to increase temperature variability and stochasticity. Most of our understanding of thermal physiology of intertidal organisms has come from laboratory experiments that acclimate organisms to submerged conditions and steady-state increases in temperatures. For organisms experiencing the ebb and flow of tides with unpredictable low tide aerial temperatures, the reliability of reported tolerances and thus predicted responses to climate change requires incorporation of environmental complexity into empirical studies. Using the mussel Mytilus californianus, our study examined how stochasticity of the thermal regime influences physiological performance. Mussels were acclimated to either submerged conditions or a tidal cycle that included either predictable, unpredictable or no thermal stress during daytime low tide. Physiological performance was measured through anaerobic metabolism, energy stores and cellular stress mechanisms just before low tide, and cardiac responses during a thermal ramp. Both air exposure and stochasticity of temperature change were important in determining thermal performance. Glycogen content was highest in the mussels from the unpredictable treatment, but there was no difference in the expression of heat shock proteins between thermal treatments, suggesting that mussels prioritise energy reserves to deal with unpredictable low tide conditions. Mussels exposed to fluctuating thermal regimes had lower gill anaerobic metabolism, which could reflect increased metabolic capacity. Our results suggest that although thermal magnitude plays an important role in shaping physiological performance, other key elements of the intertidal environment complexity such as stochasticity, thermal variability and thermal history are also important considerations for determining how species will respond to climate warming.

Unpredictability is key for mussels' seashore resilience

Scientists like their experiments to be well defined: tightly regulating temperature fluctuations, precisely positioning heat sources and painstakingly defining exposures. Conditions in the real world are rarely this meticulous. But just how much of an impact does the pure randomness of the natural world have on the creatures that inhabit some of the planet's most peripheral environments, such as mussels clinging to the seashore? Immersed in cool, refreshing water twice daily, these molluscs are left high and dry once the waves recede, in sometimes searing conditions, which can vary dramatically from one day to the next – some are overcast while others drenched in sun. So, how do these seaside residents cope with and prepare for the shear unpredictability of their seashore homes? Sarah Nancollas and Anne Todgham from the University of California, Davis, USA, decided to put Mytilus californianus mussels through their paces in the lab, exposing the stalwart molluscs to reconstructions of genuine seashore conditions, to find out how well prepared the mussels are for extreme circumstances.After gathering mussels from the seashore just north of the University of California, Davis, Bodega Marine Laboratory, Bodega Bay, USA, Nancollas and Todgham rehomed the coastline residents in tanks that they could inundate twice a day with water, to simulate high tide, as well as turning on heat lamps to simulate the sun when the water level fell. Then, they recreated the conditions experienced by mussels over a 2-week period in late winter 2019, when the sea receded and the mussels experienced overcast and sunny days, with their top body temperatures ranging from 15 to 28°C. In addition, other mussels experienced relatively balmy conditions, as the researchers warmed them twice a day to a dependably constant 20°C, while others were exposed continually to air at 13°C – the same temperature as the water. Then, after 2 weeks, the duo tested the mussels’ heart rates, to find out how resilient they were when temperatures climbed critically, how much glycogen they were carrying in preparation for deprivation when the tide went out and how well they would cope during low tide, when they could no longer breathe oxygen from the water and had to respire anaerobically until the simulated tide returned. ‘Both air exposure and stochasticity [the randomness] of temperature change were important in determining thermal performance’, says Nancollas, who discovered that the mussels that had experienced the natural wildly varying seashore temperature fluctuations had plentiful glycogen stores in place to survive low tide when their food supply vanishes. However, the cells of the hardy seaside residents were no better prepared to withstand sizzling temperatures than mussels that had experienced mild predictable warmth when their water level dropped each day, although their hearts were more resilient, continuing to beat at temperatures over 40°C when the hearts of mussels that had experienced predictable warmth failed. ‘The mussels prioritise energy reserves to deal with unpredictable low tide conditions’, says Nancollas. And it seemed that the metabolisms of the mussels living in the real-world simulation were more efficient than those of mussels living in the more artificial laboratory conditions, although the real-world simulation mussels grew more slowly, suggesting that they had less spare energy to invest in growth.‘Our results suggest that while thermal magnitude plays an important role in shaping physiological performances, other key elements of the intertidal environment complexity such as stochasticity, thermal variability and thermal history are also important considerations for determining how species will respond to climate warming’, says Nancollas.

Return to the Shellfish Beds: New Insights on Use of California Sea Mussels (Mytilus californianus) and Turban Snails (Tegula spp.) Based on Harvesting Experiments

ABSTRACT Sea mussels and turban snails are among the most abundant mollusks recovered from central California middens. Experimental harvests of these and other shellfish have been conducted to help interpret archaeological findings, but methodological complications have plagued many of the early experiments. Here we report results of harvest experiments of mussels and turban snails completed between 2016 and 2022 on the Pecho coast of San Luis Obispo County. We developed a local formula to convert umbo thickness measurements into whole mussel valve length, and this article reports new return rates, including one of over 1,500 kcal/hour for mussels obtained by an experienced shellfish collector. This rate demonstrates the importance of accumulated knowledge in subsistence experiments, and suggests that mussels have higher dietary potential than previously thought. Archaeological mussel size profiles suggest that a selective (plucking-like) strategy was associated with sustainable harvest for at least the last 5,000 years on the Pecho coast.

Bivalves maintain repair when faced with chronically repeated mechanical stress.

Even though mollusks' capacity to repair shell damage is usually studied in response to a single event, their shells have to defend them against predatory and environmental threats throughout their potentially multi-decadal life. We measured whether and how mollusks respond to chronic mechanical stress. Once a week for 7 months, we compressed whole live California mussels (Mytilus californianus) for 15 cycles at ∼55% of their predicted one-time breaking force, a treatment known to cause fatigue damage in shells. We found mussels repaired their shells. Shells of experimentally stressed mussels were just as strong at the end of the experiment as those of control mussels that had not been experimentally loaded, and they were more heavily patched internally. Additionally, stressed shells differed in morphology; they were heavier and thicker at the end of the experiment than control shells but they had increased less in width, resulting in a flatter, less domed shape. Finally, the chronic mechanical stress and repair came at a cost, with stressed mussels having higher mortality and less soft tissue than the control group. Although associated with significant cost, mussels' ability to maintain repair in response to ongoing mechanical stress may be vital to their survival in harsh and predator-filled environments.

Effects of heat acclimation on cardiac function in the intertidal mussel Mytilus californianus: can laboratory-based indices predict survival in the field?

ABSTRACTThermal performance curves are commonly used to investigate the effects of heat acclimation on thermal tolerance and physiological performance. However, recent work indicates that the metrics of these curves heavily depend on experimental design and may be poor predictors of animal survival during heat events in the field. In intertidal mussels, cardiac thermal performance (CTP) tests have been widely used as indicators of animals' acclimation or acclimatization state, providing two indices of thermal responses: critical temperature (Tcrit; the temperature above which heart rate abruptly declines) and flatline temperature (Tflat; the temperature where heart rate ceases). Despite the wide use of CTP tests, it remains largely unknown how Tcrit and Tflat change within a single individual after heat acclimation, and whether changes in these indices can predict altered survival in the field. Here, we addressed these issues by evaluating changes in CTP indices in the same individuals before and after heat acclimation. For control mussels, merely reaching Tcrit was not lethal, whereas remaining at Tcrit for ≥10 min was lethal. Heat acclimation significantly increased Tcrit only in mussels with an initially low Tcrit (<35°C), but improved their survival time above Tcrit by 20 min on average. Tflat increased by ∼1.6°C with heat acclimation, but it is unlikely that increased Tflat improves survival in the field. In summary, Tcrit and Tflat per se may fall short of providing quantitative indices of thermal tolerance in mussels; instead, a combination of Tcrit and tolerance time at temperatures ≥Tcrit better defines changes in thermal tolerance with heat acclimation.

Environment-driven shifts in interindividual variation and phenotypic integration within subnetworks of the mussel transcriptome and proteome.

The environment can alter the magnitude of phenotypic variation among individuals, potentially influencing evolutionary trajectories. However, environmental influences on variation are complex and remain understudied. Populations in heterogeneous environments might exhibit more variation, the amount of variation could differ between benign and stressful conditions, and/or variation might manifest in different ways among stages of the gene‐to‐protein expression cascade or among physiological functions. Here, we explore these three issues by quantifying patterns of inter‐individual variation in both transcript and protein expression levels among California mussels, Mytilus californianus Conrad. Mussels were exposed to five ecologically relevant treatments that varied in the mean and interindividual heterogeneity of body temperature. To target a diverse set of physiological functions, we assessed variation within 19 expression subnetworks, including canonical stress‐response pathways and empirically derived coexpression clusters that represent a diffuse set of cellular processes. Variation in expression was particularly pronounced in the treatments with high mean and heterogeneous body temperatures. However, with few exceptions, environment‐dependent shifts of variation in the transcriptome were not reflected in the proteome. A metric of phenotypic integration provided evidence for a greater degree of constraint on relative expression levels (i.e., stronger correlation) within expression subnetworks in benign, homogeneous environments. Our results suggest that environments that are more stressful on average – and which also tend to be more heterogeneous – can relax these expression constraints and reduce phenotypic integration within biochemical subnetworks. Context‐dependent “unmasking” of functional variation may contribute to interindividual differences in physiological phenotype and performance in stressful environments.

Investigating controls of shell growth features in a foundation bivalve species: seasonal trends and decadal changes in the California mussel

Abstract. Marine bivalve mollusk shells can offer valuable insights into past oceanographic variability and seasonality. Given its ecological and archeological significance, Mytilus californianus (California mussel) presents the opportunity to examine seasonal and decadal changes recorded in its shell over centuries to millennia. While dark–light growth bands in M. californianus shells could be advantageous for reconstructing past environments, uncertainties remain regarding shell structure, environmental controls of dark–light-band formation, and the amount of time represented by a dark–light pair. By analyzing a suite of M. californianus shells collected in 2002, 2003, 2019, and 2020 from Bodega Bay, California, we describe the mineralogical composition; establish relationships among the growth band pattern, micro-environment, and collection season; and compare shell structure and growth band expression between the archival (2002–2003) and modern (2019–2020) shells. We identified three mineralogical layers in M. californianus: an outer prismatic calcite layer, a middle aragonite layer, and a secondary inner prismatic calcite layer, which makes M. californianus the only Mytilus species to precipitate a secondary calcite layer. Within the inner calcite layer, light bands are strongly correlated with winter collection months and could be used to reconstruct periods with moderate, stable temperatures and minimal upwelling. Additionally, modern shells have significantly thinner inner calcite layers and more poorly expressed growth bands than the archival shells, although we also show that growth band contrast is strongly influenced by the micro-environment. Mytilus californianus from northern California is calcifying differently, and apparently more slowly, than it was 20 years ago.

Too Hot to Handle: Negative Carry‐Over Effects on Offspring Tolerances to Temperature

Photo 1. Author, Heidi R. Waite, collecting California mussels (Mytilus californianus) at Corona del Mar Beach (Corona del Mar, California, USA) for preliminary studies. Photograph credit: Cascade Sorte. Photo 2. Measuring California mussels for collection at Crystal Cove State Park (Laguna Beach, California). Photograph credit: Heidi R. Waite. Photo 3. Study site at Reef Point at Crystal Cove State Park, chosen for its tidal height gradient across the reef. Photograph credit: Heidi R. Waite. These photographs illustrate the article “Negative carry-over effects on larval thermal tolerances across a natural thermal gradient” by Heidi R. Waite and Cascade J.B. Sorte published in Ecology. https://doi.org/10.1002/ecy.3565

Foundation species loss alters multiple ecosystem functions within temperate tidepool communities

Foundation species, which help maintain habitat and ecosystem functioning, are declining due to anthropogenic impacts. Within the rocky intertidal ecosystem, studies have investigated the effects of foundation species on community structure and some resource fluxes; however, how intertidal foundation species loss will affect multiple facets of ecosystem functioning in concert remains unknown. We studied the direct and indirect effects of foundation species loss of mussels Mytilus californianus and surfgrass Phyllospadix spp. on community structure, fluxes (light, temperature, dissolved oxygen [DO], dissolved inorganic nutrients, pHT), and ecosystem metabolism (net ecosystem calcification [NEC] and net ecosystem production [NEP]) in central Oregon using in situ tide pool manipulations. Surfgrass loss increased microalgae cover, increased average maximum light by 142% and average maximum temperature by 3.8°C, increased DO and pHT values, and indirectly increased NEP and NEC via increased maximum temperature and pHT respectively. Mussel loss increased microalgae cover, increased average maximum light by 5.8% and average maximum temperature by 1.3°C, increased DO and pHT values, and indirectly increased NEP via increased producer cover. Shifts in baseline nutrient concentrations and temperature values from coastal upwelling influenced ecosystem metabolism in pools with intact foundation species. Our results indicate that as communities respond to foundation species loss, ecosystem functioning depends on the dominant community present and biologically or physically driven shifts in biogeochemistry. This study highlights the importance of the connection between community and ecosystem ecology in understanding the magnitude of changes occurring with anthropogenically-driven intertidal foundation species loss.

Single-Larva RNA Sequencing Identifies Markers of Copper Toxicity and Exposure in Early Mytilus californianus Larvae.

One of the challenges facing efforts to generate molecular biomarkers for toxins is distinguishing between markers that are indicative of exposure and markers that provide evidence of the effects of toxicity. Phenotypic anchoring provides an approach to help segregate markers into these categories based on some phenotypic index of toxicity. Here we leveraged the mussel embryo-larval toxicity assay in which toxicity is estimated by the fraction of larvae that exhibit an abnormal morphology, to isolate subsets of larvae that were abnormal and thus showed evidence of copper-toxicity, versus others that while exposed to copper exhibited normal morphology. Mussel larvae reared under control conditions or in the presence of increasing levels of copper (3–15 μg/L Cu2+) were physically sorted according to whether their morphology was normal or abnormal, and then profiled using RNAseq. Supervised differential expression analysis identified sets of genes whose differential expression was specific to the pools of abnormal larvae versus normal larvae, providing putative markers of copper toxicity versus exposure. Markers of copper exposure and copper-induced abnormality were involved in many of the same pathways, including development, shell formation, cell adhesion, and oxidative stress, yet unique markers were detected in each gene set. Markers of effect appeared to be more resolving between phenotypes at the lower copper concentration, while markers of exposure were informative at both copper concentrations.

Contaminant concentrations and risks associated with the Pacific oyster in the highly urbanized San Diego Bay

Contaminant concentrations in filter-feeding shellfish may indicate the health of coastal waters and consumption risks. Widespread expansion of the Pacific oyster (Crassostrea gigas) and its popularity as food make it a useful sentinel. We surveyed intertidal Pacific oysters in San Diego Bay, California for contaminants during summer 2018 and winter 2019. We compared contaminants in Pacific oyster to California mussel from California's State Mussel Watch Program (1993–2003) and human consumption thresholds. Contaminants such as neonicotinoid and chlorinated pesticides, selenium, and several metals were higher in Pacific oysters in summer, while PBDEs, benzylbutyl phthalate, and plastics were higher in winter. Contaminant levels were generally lower in Pacific oyster than mussel except for copper and zinc. Bay-wide PCB concentrations in oysters exceeded thresholds but individual samples (locations) also met or surpassed chlordane, PCB and PAH thresholds. Monitoring and risk assessments that consider species' biology, season, location, effects of multiple contaminants, and human consumption patterns will contribute to more effective consumption guidelines.

Negative carry-over effects on larval thermal tolerances across a natural thermal gradient.

Under climate change, marine organisms will need to tolerate or adapt to increasing temperatures to persist. The ability of populations to cope with thermal stress may be influenced by conditions experienced by parents, by both genetic changes and transgenerational phenotypic plasticity through epigenetics or maternal provisioning. In organisms with complex life cycles, larval stages are particularly vulnerable to stress. Positive parental carry-over effects occur if more stressful parental environments yield more tolerant offspring while the opposite pattern leads to negative carry-over effects. This study evaluated the role of parental effects in determining larval thermal tolerances for the intertidal mussel, Mytilus californianus. We tested whether thermal environments across a natural gradient (shoreline elevation) impacted mussel temperature tolerances. Lethal thermal limits were compared for field-collected adults and their larvae. We observed parental effects across one generation, in which adult mussels exposed to warmer habitats yielded less tolerant offspring. Interestingly, although parental environments influenced offspring tolerances, we found no clear effects of habitat conditions on adult phenotypes (tolerances). We found indicators of trade-offs in energy investment, with higher reproductive condition and larger egg diameters in low stress environments. These results suggest that parental effects are negative, leading to possible adverse effects of thermal stress on the next generation.

Miraculous mussels heal in days

For many of us, breaking a bone is rare and hopefully no more than a once in a lifetime experience. It's excruciating and recovery can take weeks. But the shells of mussels (Mytilus californianus) clinging to the shoreline are continually at risk of harm, battered by waves and debris hurled by the surf. Even when the tide is out, mussel shells are vulnerable to hungry crustaceans taking a nip with their powerful claws. ‘The intertidal zone can be a particularly dangerous place. It is a harsh environment’, says Rachel Crane from Stanford University, USA. So how much damage do these resilient structures sustain after a substantial assault and how quickly can they recover? Crane, along with José Diaz Reyes and Mark Denny, both from the same university, headed down to the seashore below the Hopkins Marine Station, USA, to find out.Back in the lab with a bucket full of the molluscs, Crane and Diaz Reyes repeatedly compressed the mussels between two plates to stress them, adjusting the level of force depending on their size. Then, the duo checked the shells for damage by testing how much total force bearing down they could withstand. Sure enough, the shells of the squashed mussels were significantly weaker than shells that had not been compressed; they could only withstand forces that were 54 N less (equivalent to the weight of 5.4 kg) than unstressed mussels of the same size. And when Crane and Diaz Reyes opened up the mussels and looked at the surface of the shells within, they sometimes saw networks of cracks radiating across the dome of the shell or larger individual cracks. Repeatedly pounding the mussels had taken a toll on their shells, but how well and how quickly could the molluscs recover?Allowing the mussels 1, 2 and 4 weeks to recuperate, the team was astonished to see that the squashed shells were every bit as tough as the unharmed shells within a week. ‘We were amazed when we saw the data. We were expecting the mussels to take weeks or maybe even months to repair their shells; I never thought the response would be so fast’, says Crane. And, when she and Diaz Reyes rechecked the mussels 4 weeks later, they were impressed that the shells dealt the hardest blows were the most resilient, coming back even stronger. However, the effort of recovery had taken a toll on the molluscs, with the soft internal part of the mussels weighing 7% less after 4 weeks of recovery than the soft tissue of the mussels that had remained unassailed.In addition, when the team scrutinised the shells for signs of recovery, although some of the shells showed evidence of regrowth in the area near and surrounding cracks, many of the fissures remained unhealed, even though the shells were restored to full strength. ‘Mussels may have a self-healing or compensatory mechanism that does not involve shell deposition’ says Crane. And when the team compared the mollusc's healing rates with those of other armoured surf residents, the mussels were streaks ahead, recuperating in a matter of days, whereas snails take weeks and even months to repair damage. Crane also suggests that the mussels’ ability to recover rapidly could account for their undisputed success in one of the world's most turbulent environments. ‘Molluscs protect themselves with this amazing dynamic armour – these animals not only can resist repeated insults, but they can respond, repair and strengthen so quickly’, says Crane.

Self-assembling of chimeric mussel-inspired bio-adhesives originated from Mytilus californianus and Anabaena flos-aquae: A new approach to develop underwater adhesion

Bio-adhesives play a pivotal role in a wide range of medical applications. However, there are some problems about their application in different pH values and low adhesion force under wet conditions. Here, we report new recombinant fusion protein achieved by mussel foot proteins (Mfps) of Mytilus Californianus and gas vesicle protein A (GvpA) of Anabaena flos-aquae by genetic engineering methods. Transmission electron microscopy verified that these chimeric proteins self-assembled into β-sheet rich fibres because of GvpA amyloid structure. Also, in comparison to other studies, their adhesion forces were significantly increased, especially in the alkaline environments based on Mfp-3 and Mfp-5, confirmed by the colloidal probe atomic force microscope. This study illustrates that copolymer of Mfp-5-GvpA:GvpA-Mfp-3 can be used as a sturdy underwater adhesive with tolerance to auto-oxidation, especially at basic conditions.

Bivalves rapidly repair shells damaged by fatigue and bolster strength.

ABSTRACTHard external armors have to defend against a lifetime of threats yet are traditionally understood by their ability to withstand a single attack. Survival of bivalve mollusks thus can depend on the ability to repair shell damage between encounters. We studied the capacity for repair in the intertidal mussel Mytilus californianus by compressing live mussels for 15 cycles at ∼79% of their predicted strength (critically fracturing 46% of shells), then allowing the survivors 0, 1, 2 or 4 weeks to repair. Immediately after fatigue loading, mussel shells were 20% weaker than control shells that had not experienced repetitive loading. However, mussels restored full shell strength within 1 week, and after 4 weeks shells that had experienced greater fatiguing forces were stronger than those repetitively loaded at lower forces. Microscopy supported the hypothesis that crack propagation is a mechanism of fatigue-caused weakening. However, the mechanism of repair was only partially explained, as epifluorescence microscopy of calcein staining for shell deposition showed that only half of the mussels that experienced repetitive loading had initiated direct repair via shell growth around fractures. Our findings document repair weeks to months faster than demonstrated in other mollusks. This rapid repair may be important for the mussels’ success contending with predatory and environmental threats in the harsh environment of wave-swept rocky coasts, allowing them to address non-critical but weakening damage and to initiate plastic changes to shell strength. We highlight the significant insight gained by studying biological armors not as static structures but, instead, as dynamic systems that accumulate, repair and respond to damage.

Do host‐associated microbes show a contrarian latitudinal diversity gradient? Insights from Mytilus californianus, an intertidal foundation host

AbstractAimThe latitudinal diversity gradient (LDG) is one of the most pervasive large‐scale trends in biodiversity. However, whether microbial taxa also conform to this pattern remains poorly known. This study uses the gill and shell‐surface microbiomes of the marine bivalve Mytilus californianus to test whether the diversity of host‐associated microbial communities declines with increasing latitude, as predicted by the LDG.LocationNortheastern Pacific coast, ranging from Sitka, AK to La Jolla, CA, USA (spanning 24.12 degrees of latitude).TaxonBacteria and Archaea associated with M. californianus gills and shell‐surfaces.MethodsWe amplified and sequenced the 16S rRNA gene from M. californianus gill and shell‐surface samples. We used linear and quadratic regressions, accounting for spatial autocorrelation when needed, of average alpha and gamma diversities of the whole microbiome, as well as individual microbial clades, to determine whether these taxa conformed to the traditional LDG. We also used permutational multivariate analysis of variance to determine the level of compositional differentiation among sampling sites.ResultsGill and shell‐surface microbiota show differing latitudinal diversity patterns, with both micro‐environments exhibiting high levels of compositional differentiation along latitude. Individual microbial clades within each micro‐environment also show different latitudinal trends, most likely due to differing ecologies and life histories.Main conclusionsMytilus californianus‐associated microbial taxa show a flat, hump‐shaped or contrarian LDG, suggesting that a steep decline in diversity with increasing latitude may not be a universal pattern across different domains of life.

Circadian and circatidal rhythms of protein abundance in the California mussel (Mytilus californianus).

Coastal habitats fluctuate with the 12.4h tidal and 24h light/dark cycle to predictably alter conditions such as air exposure, temperature, and food availability. Intertidal sessile bivalves exhibit behavioural and physiological adjustments to minimize the challenges of this environment. We investigated a high-resolution time course of the changes in protein abundance in the gill tissue of the intertidal mussel Mytilus californianus in a simulated tidal environment of 12:12h light:dark cycles and a matching 6:6h high:low tide cycle within each 12h period. Approximately 38% of detected proteins showed significant rhythms in their abundances, with diversity in the phases of rhythmic isoforms. The circadian rhythm was dominant in protein abundance changes, particularly with oxidative metabolism. A tidal cycle elicited changes within functional groups, including in cytoskeletal proteins, chaperones, and oxidative stress proteins. In addition to protein abundance changes, we found the possibility for post-translational modifications driving rhythms, including methylation, mitochondrial peptide processing (proteolysis), and acylation. Dynamic changes in the proteome across functional categories demonstrate the importance of the tidal environment in entraining cellular processes, confirming that differential expression studies should not assume a static baseline of cellular conditions in intertidal organisms.

Archaeological and Contemporary Evidence Indicates Low Sea Otter Prevalence on the Pacific Northwest Coast During the Late Holocene

The historic extirpation and subsequent recovery of sea otters (Enhydra lutris) have profoundly changed coastal social-ecological systems across the northeastern Pacific. Today, the conservation status of sea otters is informed by estimates of population carrying capacity or growth rates independent of human impacts. However, archaeological and ethnographic evidence suggests that for millennia, complex hunting and management protocols by Indigenous communities limited sea otter abundance near human settlements to reduce the negative impacts of this keystone predator on shared shellfish prey. To assess relative sea otter prevalence in the Holocene, we compared the size structure of ancient California mussels (Mytilus californianus) from six archaeological sites in two regions on the Pacific Northwest Coast, to modern California mussels at locations with and without sea otters. We also quantified modern mussel size distributions from eight locations on the Central Coast of British Columbia, Canada, varying in sea otter occupation time. Comparisons of mussel size spectra revealed that ancient mussel size distributions are consistently more similar to modern size distributions at locations with a prolonged absence of sea otters. This indicates that late Holocene sea otters were maintained well below carrying capacity near human settlements as a result of human intervention. These findings illuminate the conditions under which sea otters and humans persisted over millennia prior to the Pacific maritime fur trade and raise important questions about contemporary conservation objectives for an iconic marine mammal and the social-ecological system in which it is embedded.

Bio-inspired adhesive joint with improved interlaminar fracture toughness

Herein, a novel adhesive joint that mimics the structure of the bondline of two biological systems that have excellent adhesion, gecko and mytilus californianus, was designed. Sacrificial cracks were embedded inside the adhesive layers to activate new dissipative mechanisms and thus increase the effective interlaminar fracture toughness. A finite element model (FEM) that considered the progressive damage in the adhesive layer and decohesion at both adhesive/adherend interfaces was used to understand the damage mechanisms of the bio-inspired adhesives. Contrary to classical joints that experience interfacial failure, wherein failure occurs at one of the substrate/adherend interfaces, our bio-inspired joints are characterized by crack bifurcation between both sides of the bondline. Thus the developed toughening ligaments cause the propagation of secondary and backward cracks at the lower and upper interfaces under and over the sacrificial cracks, respectively. Such crack branching and fragmentation together with the energy required to break these ligaments improve the effective toughness of the adhesive. Increasing the sacrificial crack width and gap between two successive cracks reduces the toughness through reduction in the allowed surface for secondary and backward crack propagation. The adhesive properties significantly affect this toughening effect that increases with adhesive strength and failure strain.

Total mercury content in the California ribbed sea mussel Mytilus californianus from the west coast of Baja California, México: Levels of contamination and human health risk

We analyzed spatial and temporal variations in total mercury concentration (THg) in Mytilus californianus from the west coast of Baja California, México, and assessed the potential risk for human health. The sites from the northern zone showed the highest levels of THg over the entire three years of study, however, no significant differences among years were found. The highest level of THg (0.110 μg/g d.w.) was recorded in 2010 at Bajamar (SS2), and the lowest (0.011 μg/g d.w.) in 2007 and 2008 at Eréndira (SS4) and Los Ojitos (SS7), respectively. The estimated daily intake (EDI) values for adults through mussel consumption were lower than the oral reference dose (RfDo) and the acceptable daily intake (ADI) values established by the USEPA and the FAO/WHO, respectively. The target hazard quotient (THQ) values were <1.0, indicating that mercury concentrations in M. californianus are not likely to pose a risk for human health.