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Synopsis Organisms rely on the integrity of the structural materials they produce to maintain a broad range of processes, such as acquiring food, resisting predators, or withstanding extreme environmental forces. The production and maintenance of these biomaterials, which are often modulated by environmental conditions, can therefore have important consequences for fitness in changing climates. One well-known example of such a biomaterial is mussel byssus, an array of collagen-like fibers (byssal threads) that tethers a bivalve mollusk securely to benthic marine substrates. Byssus strength directly influences mortality from dislodgement, predation, or competition and depends on the quantity and quality of byssal threads produced. We compared the temperature sensitivity of byssal attachment strength of two mussel species common to the west coast of North America, Mytilus trossulus and M. galloprovincialis, when exposed to seawater temperatures ranging from 10 to 24°C in the laboratory. We found that the two species attached equally strong in seawater ≤18°C, but higher temperatures caused byssal thread production rate and quality (break force and extensibility) to be greatly reduced in M. trossulus and increased in M. galloprovincialis, leading to a 2–10-fold difference in overall byssus strength between the two species. Using this threshold value (18°C), we mapped habitat for each species along the west coast of North America based on annual patterns in sea surface temperature. Estimated ranges are consistent with the current distribution of the two species and suggest a potential mechanism by which ocean warming could facilitate the northern expansion of M. galloprovincialis and displacement of native M. trossulus populations. 

Synopsis The mechanical properties of intestinal tissues determine how a thin-walled structure exerts forces on food and absorbs the force of food as it enters and travels down the gut. These properties are critically important in durophagous and stomachless fish, which must resist the potential damage to foreign bodies (e.g., shells fragments) in their diet. We test the hypothesis that the mechanical properties of the alimentary tract will differ along its length. We predict that the proximal region of the gut should be the strongest and most extensible to handle the large influx of prey often associated with stomachless fish that lack a storage depot. We developed a custom inflation technique to measure the passive mechanical properties of the whole intestine of the stomachless shiner perch, Cymatogaster aggregata. We show that mechanical properties differ significantly along the length of the alimentary tract when inflated to structural failure, with 25–46% greater maximal stress, strain, extension ratio, and toughness at the proximal (25%) position. We also find that the alimentary tissues (excluding the heavily muscular rectum) are generally highly extensible and anisotropic, and do not differ in wall circumference or thickness along the alimentary tract. These findings contribute to our knowledge of the mechanical properties of fish intestinal tissues and guide future studies of factors influencing the evolution of fish alimentary systems. 

Anthropogenic warming and natural climate variability affect global patterns of seawater temperature and marine primary productivity and affect organism survival, growth, and physiology. Mussels are ecosystem engineers that utilize byssal thread structures to attach to hard substrate, a strategy key to survival in wave-swept rocky shore environments. Byssal thread production varies according to season and environmental conditions, and temperature and food availability may influence the production of these structures by affecting energy limitation. Mytilus trossulus and M. galloprovincialis are congeneric mussel species in the Northeast Pacific with cold- and warm-adapted thermal tolerances, respectively. First, we hypothesized that temperature has opposing effects on growth rates of the 2 species. Second, we hypothesized that either (1) byssal thread production is positively correlated with growth rate (the ‘production’ hypothesis), or (2) there is a trade-off between growth and byssal thread production, and resources are allocated first to byssal thread production rather than growth. Under this ‘trade-off’ hypothesis, we predicted no relationship between growth rate and byssal thread production. We manipulated seawater temperature and food availability and quantified mussel performance in terms of survival, growth, and byssus attachment. Across all treatment combinations, we found that M. galloprovincialis had positive shell and tissue growth and M. trossulus had minimal shell growth and a loss in tissue mass. Temperature had opposing effects on each species; temperature increased shell growth of M. galloprovincialis but increased tissue loss of M. trossulus . Temperature did not affect byssal thread production, and there was no significant relationship between byssal thread quality or quantity and shell or tissue growth across the temperature and food gradient for either species. Our results suggest that energy allocation is prioritized towards byssal thread production over growth. 

Ocean warming and acidification are predicted to impact the physiology of marine organisms, especially marine calcifiers that must deposit calcium carbonate and resist dissolution. Of particular concern are articulated coralline algae, which must maintain both calcified segments (intergenicula) and uncalcified joints (genicula) in order to thrive along wave-swept rocky coastlines. We examined the effect of pH and temperature, both individually and in combination, on the growth, calcification, and biomechanical properties of 2 species of articulated coralline algae, Corallina vancouveriensis and Calliarthron tuberculosum , common on wave-exposed shores in the NE Pacific. Increased temperature and reduced pH were found to reduce growth rates in both species (30-89% lower) but had little influence on the amount of intergenicular calcium carbonate or on the genicular biomechanical properties of these species. Results suggest that although growth rates may decline, these 2 coralline species will maintain the integrity of their tissues and continue to persist under future climate stress. 

Bivalves frequently withstand shell damage that must be quickly repaired to ensure survival. While the processes that underlie larval shell development have been extensively studied within the context of ocean acidification (OA), it remains unclear whether shell repair is impacted by elevated pCO2. To better understand the stereotypical shell repair process, we monitored mussels (Mytilus edulis) with sublethal shell damage that breached the mantle cavity within both field and laboratory conditions to characterize the deposition rate, composition, and integrity of repaired shell. Results were then compared with a laboratory experiment wherein mussels (Mytilus trossulus) repaired shell damage in one of seven pCO2 treatments (400–2500 µatm). Shell repair proceeded through distinct stages; an organic membrane first covered the damaged area (days 1–15), followed by the deposition of calcite crystals (days 22–43) and aragonite tablets (days 51–69). OA did not impact the ability of mussels to close drill holes, nor the microstructure, composition, or integrity of end-point repaired shell after 10 weeks, as measured by µCT and SEM imaging, energy-dispersive X-ray (EDX) analysis, and mechanical testing. However, significant interactions between pCO2, the length of exposure to treatment conditions, the strength and inorganic content of shell, and the physiological condition of mussels within OA treatments were observed. These results suggest that while OA does not prevent adult mussels from repairing or mineralizing shell, both OA and shell damage may elicit stress responses that impose energetic constraints on mussel physiology.