Strategies for life in flow: tenacity, morphometry, and probability of dislodgment of two \u3cem\u3eMytilus\u3c/em\u3e species

The attachment strength of sessile intertidal organisms is continuously challenged by the hydrodynamic forces generated by breaking waves. This study explores mechanisms by which the attachment strength, or tenacity, can vary for one of the dominant competitors for space in this environment, the marine mussel. Tenacity was measured for 2 co-existing mussel species, Mytilus californianus and Mytilus trossulus, either solitary or within a bed (= bed mussels). The tenacity of M. californianus was higher than M. trossulus, due to increased byssal thread thickness, and the tenacity of solitary mussels was higher than bed mussels, due to the presence of more byssal threads per mussel. These tenacity measurements were coupled with modeled hydrodynamic forces to predict the probability of dislodgment due to wave action. For a given water velocity, the predicted probability of dislodgment of M. californianus was lower than that of M. trossulus because the latter produces relatively thinner threads (reducing tenacity) and a relatively more voluminous shell (increasing hydrodynamic loading). Compared to solitary mussels, bed mussels had a lower probability of dislodgment for a given water velocity (despite their lower tenacity) because they are subjected to relatively smaller hydrodynamic forces. These predictions are consistent with field observations that mussels typically form dense aggregations and that M. trossulus rarely inhabits highly wave-exposed shores

Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes

One of the most remarkable examples of convergent evolution among vertebrates is illustrated by the independent origins of an active electric sense in South American and African weakly electric fishes, the Gymnotiformes and Mormyroidea, respectively. These groups independently evolved similar complex systems for object localization and communication via the generation and reception of weak electric fields. While good estimates of divergence times are critical to understanding the temporal context for the evolution and diversification of these two groups, their respective ages have been difficult to estimate due to the absence of an informative fossil record, use of strict molecular clock models in previous studies, and/or incomplete taxonomic sampling. Here, we examine the timing of the origins of the Gymnotiformes and the Mormyroidea using complete mitogenome sequences and a parametric Bayesian method for divergence time reconstruction. Under two different fossil-based calibration methods, we estimated similar ages for the independent origins of the Mormyroidea and Gymnotiformes. Our absolute estimates for the origins of these groups either slightly postdate, or just predate, the final separation of Africa and South America by continental drift. The most recent common ancestor of the Mormyroidea and Gymnotiformes was found to be a non-electrogenic basal teleost living more than 85 millions years earlier. For both electric fish lineages, we also estimated similar intervals (16–19 or 22–26 million years, depending on calibration method) between the appearance of electroreception and the origin of myogenic electric organs, providing rough upper estimates for the time periods during which these complex electric organs evolved de novo from skeletal muscle precursors. The fact that the Gymnotiformes and Mormyroidea are of similar age enhances the comparative value of the weakly electric fish system for investigating pathways to evolutionary novelty, as well as the influences of key innovations in communication on the process of species radiation

Mechanical design of mussell byssus: Load cycle and strain rate dependence

The ability to produce a strong byssal attachment is one key to the competitive dominance of mussels on many rocky shores. The byssus is composed of numerous extracellular collagenous threads, which in turn can be divided into proximal and distal regions that are distinct in ultrastructure and chemical composition. Our current understanding of the mechanical design of mussel byssus is largely based on quasi-static testing, where a fiber is slowly extended to failure. Mussels in nature, however, inhabit a dynamic environment where repetitive loads can be applied on short time scales. This study evaluates the mechanical properties of the threads of Mytilus californianus subjected to repeated subcritical loads and a range of strain rates. A subset of these mechanical tests was also performed on the threads of three other mytilid species. Results indicate that subcritical loading alters the mechanical properties of a thread in a manner that is dependent on the extension applied, and that thread stiffness and damping increase with increasing strain rate. Overall, this study provides insight into the mechanical design of a byssus that is subjected to dynamic loading

The Biomechanics of the Arteries of Nautilus, Nototodarus, and Sepia

The mechanical properties of the dorsal aorta of three cephalopod mollusks, Nautilus pompilius, Nototodarus sloani, and Sepia latimanus, were investigated by in vitro inflations of isolated arterial segments. As expected, all three arteries exhibit nonlinear, J -shaped stress-extension curves, and all are highly extensible in the circumferential direction. Differences in longitudinal extensibility appear to be correlated to specific features of the tissue architecture. The squid, Nototodarus, and to a lesser extent the cuttlefish, Sepia, arteries are reinforced longitudinally with a dense layer of longitudinally oriented elastic fibers. Analysis of the form of the incremental wall stiffness data for Nautilus and Nototodarus suggests that the in vivo blood pressures for these animals fall in the ranges 20-60 cm H20 and 100-200 cm H20, respectively. Nautilus has a thinwalled, low-pressure arterial system that is in keeping with its relatively limited locomotory capabilities. Nototodarus has a high-pressure, thick-walled circulation that is required to support the high-speed, aerobic locomotion generally common in squid. Analysis of pressure wave velocities for these arteries indicates that the Nautilus circulatory system contains a true Windkessel whereas it appears possible that wave propogation effects may make a relatively minor contribution to the hemodynamics of Nototodarus

Molecular Design of the α-keratin Composite: Insights from a Matrix-free Model, Hagfish Slime Threads

We performed mechanical tests on a matrix-free keratin model—hagfish slime threads—to test the hypothesis that intermediate filaments (IFs) in hydrated hard α-keratins are maintained in a partly dehydrated state. This hypothesis predicts that dry IFs should possess mechanical properties similar to the properties of hydrated hard α-keratins, and should swell more than hard α-keratins in water. Mechanical and swelling measurements of hagfish threads were consistent with both of these predictions, suggesting that an elastomeric keratin matrix resists IF swelling and keeps IF stiffness and yield stress high. The elastomeric nature of the matrix is indirectly supported by the inability of matrix-free IFs (i.e. slime threads) to recover from post-yield deformation. We propose a general conceptual model of the structural mechanics of IF-based materials that predicts the effects of hydration and cross-linking on stiffness, yield stress and extensibilit

α-Helical Protein Based Materials and Methods for Making Same

The invention relates to a method of producing useful materials from filament-forming α-helical proteins or filaments made of such proteins. The method comprises allowing filament-forming α-helical proteins to self-assemble into α-helix containing filaments and forming fibres, films or bulk materials from the filaments. The materials are stretched to strain the filaments so that the α-helices substantially irreversibly change to β-sheet forms. The filament-forming α-helical proteins can comprise intermediate filament proteins. In a specific embodiment, the filament-forming proteins comprise hagfish slime thread IF proteins