Mechanical basis of morphogenesis and convergent evolution of spiny seashells

Convergent evolution is a phenomenon whereby similar traits evolved independently in not closely related species, and is often interpreted in functional terms. Spines in mollusk seashells are classically interpreted as having repeatedly evolved as a defense in response to shell-crushing predators. Here we consider the morphogenetic process that shapes these structures and underlies their repeated emergence. We develop a mathematical model for spine morphogenesis based on the mechanical interaction between the secreting mantle edge and the calcified shell edge to which the mantle adheres during shell growth. It is demonstrated that a large diversity of spine structures can be accounted for through small variations in control parameters of this natural mechanical process. This physical mechanism suggests that convergent evolution of spines can be understood through a generic morphogenetic process, and provides unique perspectives in understanding the phenotypic evolution of this second largest phylum in the animal kingdom.\ud \ud Homoplasy, the appearance of similar traits in separate evolutionary lineages as a result of convergence, parallelism, or evolutionary reversals, is a major concern in phylogenetic analysis for which it is viewed as noise. However, over the past two decades, homoplasy has also become a subject of increasing interest, stimulated by the rise of evolutionary developmental biology (evo devo) and the wish to uncover the developmental basis of this phenomenon (1⇓–3). Spines constitute the most prominent ornamentation of mollusk shells and have evolved in many distantly related fossil and current mollusk species (at least 55 genera and 21 families of current gastropods; 10 genera and 8 families of current bivalves; 11 genera and 8 families of ammonoids; and 6 fossil nautiloid genera; see Fig. 1 for examples). Convergent evolution of spines in mollusks has been addressed in functional terms, these structures being interpreted as having evolved as a defense in response to shell-crushing predators (4⇓–6). This hypothesis is itself the basis of the widely cited “escalation hypothesis,” according to which long-term trends in the fossil record were caused by the evolutionary response of prey to predation pressure (7). The idea that convergent evolution of similar mollusk ornamentations might be fully explained in functional terms is based on the premise that similar characters, perceived as well designed for a presumed function, cannot conceivably have independently evolved fortuitously. Therefore, natural selection is thought to have repeatedly shaped similar functional traits out of random variations

Effect of hot calendering on physical properties and water vapor transfer resistance of bacterial cellulose films

This work investigates the effect of hot calendering on bacterial cellulose (BC) films properties, aiming the achievement of good transparency and barrier property. A comparison was made using vegetal cellulose (VC) films on a similar basis weight of around 40 g.m-2. The optical-structural, mechanical and barrier property of BC films were studied and compared with those of highly beaten VC films. The Youngs moduli and tensile index of the BC films are much higher than those obtained for VC (14.5 16.2 GPa vs 10.8 8.7 GPa and 146.7 64.8 N.m.g-1 vs 82.8 40.5 N.m.g-1), respectively. Calendering increased significantly the transparency of BC films from 53.0 % to 73.0 %. The effect of BC ozonation was also studied. Oxidation with ozone somewhat enhanced the brightness and transparency of the BC films, but at the expenses of slightly lower mechanical properties. BC films exhibited a low water vapor transfer rate, when compared to VC films and this property decreased by around 70 % following calendering, for all films tested. These results show that calendering could be used as a process to obtain films suitable for food packaging applications, where transparency, good mechanical performance and barrier properties are important. The BC films obtained herein are valuable products that could be a good alternative to the highly used plastics in this industry.The authors thank FCT (Fundação para a Ciência e Tecnologia) and FEDER (Fundo Europeu de Desenvolvimento Regional) for the financial support of the project FCT PTDC/AGR-FOR/3090/2012— FCOMP-01-0124-FEDER-027948 and the awarding of a research grant for Vera Costa

Observations dans les semiconducteurs à l'aide du mode induit

Short discussion of an experiment to detect an intrinsic dislocation contrast in planar structure. Unfortunately, a negative result do not give a clear cut answer to the question to know if the contrast generally observed is due to a decoration in the diffusion process.On discute de difficultés rencontrées dans un essai de mise en évidence d'un contraste intrinsèque dû aux dislocations d'une structure planar. Il semble bien en effet que les contrastes généralement observés soient dus à une décoration des dislocations lors du processus de dopage par diffusion

The physical basis of mollusk shell chiral coiling

Snails are model organisms for studying the genetic, molecular, and developmental bases of left–right asymmetry in Bilateria. However, the development of their typical helicospiral shell, present for the last 540 million years in environments as different as the abyss or our gardens, remains poorly understood. Conversely, ammonites typically have a bilaterally symmetric, planispiraly coiled shell, with only 1% of 3,000 genera displaying either a helicospiral or a meandering asymmetric shell. A comparative analysis suggests that the development of chiral shells in these mollusks is different and that, unlike snails, ammonites with asymmetric shells probably had a bilaterally symmetric body diagnostic of cephalopods. We propose a mathematical model for the growth of shells, taking into account the physical interaction during development between the soft mollusk body and its hard shell. Our model shows that a growth mismatch between the secreted shell tube and a bilaterally symmetric body in ammonites can generate mechanical forces that are balanced by a twist of the body, breaking shell symmetry. In gastropods, where a twist is intrinsic to the body, the same model predicts that helicospiral shells are the most likely shell forms. Our model explains a large diversity of forms and shows that, although molluscan shells are incrementally secreted at their opening, the path followed by the shell edge and the resulting form are partly governed by the mechanics of the body inside the shell, a perspective that explains many aspects of their development and evolution

How seashells take shape

The article explores the precise development of the spines, ribs and spirals of mollusks. Topics include the independent transformation of the Byzantine features in most gastropods which is the largest group of mollusks, the basic rules on the development of the spirals found in the shells of the mollusks and the influence of environmental factors on the growth rate of the shells

The morpho-mechanical basis of ammonite form.

Ammonites are a group of extinct cephalopods that garner tremendous interest over a range of scientific fields and have been a paradigm for biochronology, palaeobiology, and evolutionary theories. Their defining feature is the spiral geometry and ribbing pattern through which palaeontologists infer phylogenetic relationships and evolutionary trends. Here, we develop a morpho-mechanical model for ammonite morphogenesis. While a wealth of observations have been compiled on ammonite form, and several functional interpretations may be found, this study presents the first quantitative model to explain rib formation. Our approach, based on fundamental principles of growth and mechanics, gives a natural explanation for the morphogenesis and diversity of ribs, uncovers intrinsic laws linking ribbing and shell geometry, and provides new opportunities to interpret ammonites' and other mollusks' evolution

How seashells take shape

The article explores the precise development of the spines, ribs and spirals of mollusks. Topics include the independent transformation of the Byzantine features in most gastropods which is the largest group of mollusks, the basic rules on the development of the spirals found in the shells of the mollusks and the influence of environmental factors on the growth rate of the shells

Mechanics unlocks the morphogenetic puzzle of interlocking bivalved shells

Brachiopods and mollusks are 2 shell-bearing phyla that diverged from a common shell-less ancestor more than 540 million years ago. Brachiopods and bivalve mollusks have also convergently evolved a bivalved shell that displays an apparently mundane, yet striking feature from a developmental point of view: When the shell is closed, the 2 valve edges meet each other in a commissure that forms a continuum with no gaps or overlaps despite the fact that each valve, secreted by 2 mantle lobes, may present antisymmetric ornamental patterns of varying regularity and size. Interlocking is maintained throughout the entirety of development, even when the shell edge exhibits significant irregularity due to injury or other environmental influences, which suggests a dynamic physical process of pattern formation that cannot be genetically specified. Here, we derive a mathematical framework, based on the physics of shell growth, to explain how this interlocking pattern is created and regulated by mechanical instabilities. By close consideration of the geometry and mechanics of 2 lobes of the mantle, constrained both by the rigid shell that they secrete and by each other, we uncover the mechanistic basis for the interlocking pattern. Our modeling framework recovers and explains a large diversity of shell forms and highlights how parametric variations in the growth process result in morphological variation. Beyond the basic interlocking mechanism, we also consider the intricate and striking multiscale-patterned edge in certain brachiopods. We show that this pattern can be explained as a secondary instability that matches morphological trends and data

Mechanical growth and morphogenesis of seashells

Seashells grow through the local deposition of mass along the aperture. Many mathematical descriptions of the shapes of shells have been provided over the years, and the basic logarithmic coiling seen in mollusks can be simulated with few parameters. However, the developmental mechanisms underlying shell coiling are largely not understood and the ubiquitous presence of ornamentation such as ribs, tubercles, or spines presents yet another level of difficulty. Here we develop a general model for shell growth based entirely on the local geometry and mechanics of the aperture and mantle. This local description enables us to efficiently describe both arbitrary growth velocities and the evolution of the shell aperture itself. We demonstrate how most shells can be simulated within this framework. We then turn to the mechanics underlying the shell morphogenesis, and develop models for the evolution of the aperture. We demonstrate that the elastic response of the mantle during shell deposition provides a natural mechanism for the formation of three-dimensional ornamentation in shells