Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Adaptive design of locomotion and foot form in prosobranch gastropods

A comparison of the locomotor types, speed, tenacity, and foot form of nearly 300 species in 52 families of marine prosobranchs has revealed that foot size and shape and even subtle variations of locomotion affect the speed and strength of adhesion to the substratum.Gastropods inhabiting soft substrata move primarily by pedal cilia or by discontinuous locomotion in which shell and foot move alternately. Both types of movement are accompanied by low tenacity. A specialized type of discontinuous locomotion, namely, leaping, surpasses all other methods of movement in speed. Species with ciliary locomotion have a very large foot while those with discontinuous movement have an exceedingly small foot relative to shell size.The majority of prosobranchs inhabit hard substrata, move by continuous pedal muscular gliding, and have moderately high tenacity during movement. Arhythmic pedal locomotion yields lower maximum speeds and tenacities than do rhythmic pedal waves. Foot size and shape relative to shell length in species with arhythmic locomotion vary from very short and broad to long and narrow. Studies of transects at several temperate and tropical marine littoral stations showed that these species are confined to low littoral or sublittoral habitats that are sheltered from heavy wave action. High speed and tenacity are simultaneously attained only by species with rhythmic pedal waves.Speed and tenacity do not represent competing selective pressures on the size and shape of the foot. Speed increases among species as the foot approaches or exceeds shell length and is highest if the foot is also broad; the greatest tenacities are attained by species with a long, broad foot whose dimensions do not exceed that of the shell. The optimal shape for both high tenacity and speed is a broad foot somewhat shorter than the shell; neither speed nor tenacity are much compromised by this form. In general, only species with rhythmic pedal waves whose foot size and shape approximate the optimal form for high tenacity and speed are found in habitats exposed to much wave action. Long rhythmic waves, moving a large proportion of foot area at once, are in theory energetically more economical than small, very rapid waves resulting in the same overall speed, but experiments showed that tenacity is significantly reduced in gastropods which increase speed by enlarging the waves. The optimal wave pattern of a species should be a balance between the demand for speed with the least expenditure of energy, favored by a pattern of many large waves at once, and the demand for tenacity, favored by a pattern of few and small waves.Retrograde ditaxic waves of elongation are the most common pattern encountered among prosobranchs, and are associated with a large range of foot sizes and shapes. Such waves are at least one third as long as the foot, while direct waves and other waves of compression are frequently much smaller. The range of foot forms of species with waves of compression is restricted, tending to be optimal for high tenacity or to be long and narrow. Waves of compression appear to be a specialization with the potential for maintaining high tenacity even at high speeds since the waves can be very small, and for giving superior speed since they can travel very rapidly.     

Effects of foliaceous varices on the mechanical properties of Chicoreus dilectus (Gastropoda: Muricidae)

Removal of the foliaceous varices from the shells of Chicoreus dilectus had no significant effect on the crushing strength (load at failure) of the shells, but decreased the total work necessary to break the shells by 35–50%. This effect was due to the removal of varices as an architectural feature of the shell rather than simply to the removal of shell material per se ; varicate shells had a mean work to failure per unit shell mass 1–6–2–3 times greater than avaricate shells. Because the strength of gastropod shells often exceeds the maximum forces that durophagous predators may bring to bear on them, work to failure may be a more important factor in the design of gastropod shells than breaking load.     

The tenacity of the limpet, Patella vulgata L.: An experimental approach

It has been shown that adhesion of the limpet, Patella vulgata L. is influenced by both physical and physiological factors. The tenacity is sensitive to surface properties of the substratum, varying inversely with the contact angle which water makes with a substratum. This can be explained in terms of thermodynamics. Surface roughness also affects tenacity and this is explained in the same manner. Different angles of detachment were tested and it was clearly shown that when a strong peeling component was introduced, a much reduced force was needed to detach a limpet. Contrary to a normal pull, when a shear pull is exerted the force is not proportional to the surface area of the foot. It has also been shown that the speed of separation affects the measured tenacity; there is a speed at which tenacity will be maximum. The effect of water temperature on tenacity has been tested, tenacity increasing with rising temperature (7, 13, 20 °C). At the higher temperatures limpets are able to contract the foot muscles more powerfully, indicating that increased foot rigidity increases tenacity. By measuring the tenacity of limpets left out of water for different periods of time it has been shown that desiccation has no effect on tenacity, but a change from aquatic to aerial respiration increases tenacity. Tenacity has also been measured when the limpets have been subjected to a reduction in metabolic rate. The effect of both anoxia and narcotization shows that reduced muscle tonus, especially in the foot, results in decreased tenacity. These results further demonstrate that foot rigidity is essential for efficient adhesion. Eimpets from different habitats (exposed and sheltered) and vertical distribution (high and low level on shore) exhibited no differences in tenacity. During locomotion limpets leave a mucous trail, most of the mucus being confined to the edge of the trail. Water is incorporated anteriorly under each new locomotory wave and these pockets of water are used to release the mucus from the substratum during locomotion. It is concluded from this study that limpet adhesion can be explained solely by the tackiness of the pedal mucus, tack being due to the stored elastic energy within the mucous layer itself.     

A comparison of the strengths of gastropod shells with forces generated by potential crab predators

Carcinus manenas, Liocarcinus puber and Cancer pagurs are thought to be three likely crab predators of the gastropod Calliostoma Zizyphinum. In order to compare the strenghts of predators and their prey, the whole shell and aperture lip strengh of white and pink Calliostoma morphotypes and the maximum forces exerted by the chelipeds of three crab species were measured. Although white shells were thicker than pink shells, Calliostoma colour morphotyes did not differ significantly in either the force required to break the shell lip or the whole shell. Both Liocarcinus puber and Carcinus maenas have dimorphic chelipeds and their "crusher" chelipeds deliver almost double the forces generated by the'cutter'chelipeds. In constrast, Cancer pagurus has monomorphic chelipeds both delivering similar forces.
When compared with Calliostoma shell strenght, the forces generated by the'crusher'chelipeds of most L. puber tested were, in general, sufficient to break the shell lip of Calliostoma shells, whereas forces generated by the'cutter'chelipeds of only the larger individuals were sufficient to break the shell lip. In C. manenas , forces generated by both the'cutter'and'crusher'chelipeds often exceeded the minimum recorded force required to break the shell lip and the'crusher'cheliped reached the minimum force required to break whole Calliostoma shells. Both chelipeds of all C. pagurus tested generated forces in excess of the minimum required to break the shell lip, and the proportion of individuals capable of generating the minimum force required to break the whole shell increased with the size of the size of the crab. Carcinus maenas and Cancer pagurus were capable of breaking both the shell lips and the whole shells of a wider range of shell sizes than L. puber.     

What makes a good home for hermits? Assessing gastropod shell density and relative strength

The survival and reproductive success of hermit crabs is intrinsically linked to the quality of their domicile shells. Because damaged or eroded shells can result in greater predation, evaluating shell structure may aid our understanding of population dynamics. We assessed the structural attributes of Cerithium atratum shells through assessments of (a) density using a novel approach involving computed tomography and (b) tolerance to compressive force. Our goal was to investigate factors that may influence decision making in hermit crabs, specifically those that balance the degree of protection afforded by a shell (i.e. density and strength) with the energetic costs of carrying such resources. We compared the density and relative strength (i.e. using compression tests) of shells inhabited by live gastropods, hermit crabs (Pagurus criniticornis) and those found empty in the environment. Results failed to show any relationship between density and shell size, but there was a notable effect of shell density among treatment groups (gastropod/empty/hermit crab). There was also a predictable effect of shell size on maximum compressive force, which was consistent among occupants. Our results suggest that hermit crabs integrate multiple sources of information, selecting homes that while less dense (i.e. reducing the energy costs of carrying these resources), still offer sufficient resistance to compressive forces (e.g. such as those inflicted by shell-breaking predators). Lastly, we show that shell size generally reflects shell strength, thus explaining the motivation of hermit crabs to search for and indeed fight over the larger homes.     

Tenacity and shell shape in six Patella species: Adaptive features

Measurements were made of te forces necessary to detach various South African Patella spp. These ranged from 5.18 to 1.95 kg/cm², with significant differences between the species. P. cochlear Born. had the highest value, followed in sequence by P. argenvillei Kr., longicosta Lam., P. granularis L., P. granatina L., and P. oculus Born. Adhesion is the only mechanism capable of providing forces of this magnitude.Differences between the species are related to differences in their morphology, high tenacity being associated with low mucus secretion, small number of mucocytes, and inflexibility of the foot. This is associated with a large area of muscle attachment on the shell and the size of basal haemocoelic spaces.Features favouring high tenacity conflict with those favouring mobility, and the limpets fall into two groups. P. granularis, P. oculus, and P. granatina occur on the upper shore and the last two usually occur in areas which are sheltered from strong wave action. They a rapid growth and high gonadal output, demanding extensive foraging and hence mobility at the cost of tenacity. P. cochlear and P. longicosta are territorial and together with P. argenvillei, remain low on the shore, grow slowly, and have a low reproductive output. Feeding is localized and mobility sacrificed for high tenacity which is essential in P. cochlear and P. argenvillei as they occur in areas of strong wave action.Shell height is not correlated with tenacity nor with the intensity of wave action normally experienced by each species, but P. cochlear, P. argenvillei and to a lesser extent P. granularis, are subjected to strong wave action and have proportionally narrower shells (increasing streamlining) and low coefficients of drag. The latter are low due to the rough but regular texture of the shells creating a turbulent boundary layer and hence reducing drag.     

Alternation between attachment mechanisms by limpets in the field

The attachment mechanism used by limpets in the rocky, wave-swept intertidal zone of California was determined during high tide and low tide. The two mechanisms that limpets are known to use, suction and glue-like adhesion, were distinguished by measuring the limpets' attachment forces in shear and by staining for glue-like residues where the limpets had been attached. The results show that ≈ 73% of limpets at high tide use suction, while the rest use glue-like adhesion. Conversely, ≈ 75% of limpets at low tide use glue-like adhesion, while the rest use suction. The normal tenacity of limpets was also measured at high and low tide. The mean tenacity at high tide was significantly less than that at low tide. From these data it was estimated that the mean tenacity of glue-like adhesion is ≈ 0.23 MN·m⁻² and the mean tenacity of suction adhesion is ≈ 0.09 MN·m⁻². It is hypothesized that the cycle of alternating attachment mechanisms is linked to the limpets foraging cycles.     

On the Action of Limpets (Patella) in sinking Pits in and Abrading the Surface of the Chalk at Dover

The surface of the chalk which is exposed between high‐ and low‐water mark on the foreshore to the east of Dover is covered by a series of small and finely grooved hollows made in the substance of the chalk. These abrasions of the surface are made by the limpets when feeding on the coating of delicate seaweed which covers the surface of the chalk. When the rock has a good coating of this seaweed, the proceedings of any single limpet may be well seen. The lingual teeth make a small scoop or groove in the chalk; and as the animal makes a number of grooves one beside the other, a line is produced. After the limpet has completed a line, which is curved with the concave side towards the animal, it reverses its action and makes another curved line, in which each new groove is made to the left of the last one. The first and second lines meet at a more or less acute angle; so the limpet moves over the ground making curved lines in alternate directions, which form a zigzag. Sometimes the angle which the curved lines make with one another is so small, and the lines are consequently so close together, that all, or nearly all, the surface of the chalk is subjected to the grooving. In such cases patches of freshly abraded chalk more than an inch square in area represent the work of a limpet probably in one tide. In other cases, when the animal had moved more rapidly over the ground, the result of such an excursion appeared in an open zigzag line. In these cases the length of the path of the animal was sometimes more than 12 inches—the length of the curved lines forming the zigzag being 1/34 of an inch, and the width 1/16 of an inch, but varying from that downwards, according to the size of the animal by which they were made. On the part of the chalk foreshore immediately to the east of Dover, which is generally free from great inequalities or débris, limpets are very abundant, almost to the exclusion of other shellfish; and down to near low‐water mark there is little or no seaweed, excepting the young growth, which appears to be removed with part of the surface on which it grows soon after it appears. The number of limpets to a square foot varied, in the few cases in which I had time to count them, from 5 to 9, omitting small ones less than about half an inch. Further to the east along the shore, where there has recently been a fall of the cliff, the shore is encumbered with blocks of chalk. Many of these blocks were covered with a matted coating of fine, semitransparent, ribbon‐like seaweed. The limpets had not yet obtained a footing here; but I found one or two, conspicuous by the little clearing they had made in the midst of the seaweed. It was here possible to ascertain the area of surface which one limpet could abrade and keep clear of any but the youngest growth of seaweed. I measured some of these bare patches, and found them to vary from 8 to 14 square inches in area. The whole surface of these patches was closely grooved, the less recent work being covered with an incipient growth of seaweed. If one limpet could keep clear 14 square inches, it would require ten to keep clear a square foot, which agrees with my former estimate (small ones being omitted) of nine to a square foot where the rock was grooved all over. It is not easy to estimate the amount of chalk removed by limpets in the course of a year; but they must repeat the abrading process many times if they can, as some do, confine their operations to a few square inches of surface. Some of the best‐defined grooves which I measured were 1/50 of an inch in depth; but I think that the limpets in grazing over a surface which has been previously grooved have a tendency to deepen the first‐made grooves in the centre; and if so, the above depth might be the result of several operations. As nearly as I can estimate it, the depth of chalk removed on a fresh surface is about ·006 of an inch; so that if we suppose the limpets to feed over the same area of surface ten times in a year, the total depth of chalk removed will be ·06, or about 1/17 of an inch. In any case they do more to destroy the rock‐surface than the sea ordinarily does. If this were not the case, the action of the sea would obliterate the marks made by the limpets, which it does not; for the surface of the chalk is free from the marks or grooves only along the base of the cliffs where the shingle is washed about by the waves, and in a few holes and gullies where loose pebbles are rolled to and fro. The limpets do a great deal of apparently unnecessary work in rasping away so much chalk; but it may be beneficial to them in preventing the settlement of sedentary rivals, such as Balani or the larger seaweeds, and so enabling them to keep a large surface of pasture‐ground to themselves. The rasped surface seems to be soon covered again by the fine green coating on which, I presume, they feed. They rasp close round any hard object, such as a piece of shell or flint imbedded in the chalk; so that any Balanus or other sedentary growth would be left on an exposed pedestal of chalk, and, as the chalk is soft on the surface, would be liable to be washed off by the waves. On a large block of chalk which was tenanted by a quantity of limpets, so that every part of the surface was rasped over by them, I noticed one or two solitary Balani. The raspings extended close round the base of the shells of the Balani, and must have tended to weaken their hold on the rock. Yet a large proportion of the shells of the limpets had five or six large Balani on them. It would appear probable from this that there was something which made the chalk an unsuitable resting‐place for Balani; and the action of the limpets may not unlikely be the cause. The limpets certainly had the foreshore almost entirely to themselves down to low‐water mark. These comparatively large areas of rock‐surface covered only by a short vegetable growth, and browsed over by limpets, remind one, in a small way, of the llanos or pampas on the land, where arboreal vegetation is kept down by herbivorous animals. Yet the limpets appear to do their work more effectually, as they uproot all alien growths. The holes in the chalk, in which the limpets are often to be found, are, I believe, excavated in a great measure by rasping with the lingual teeth, though I doubt whether the object is to form a cavity to shelter in, though the cavities, when formed, may be of use for that purpose. It must be of the greatest importance to a limpet that, in order that it may ensure a firm adherence to the rock, its shell should fit the rock accurately; when the shell does fit the rock accurately, a small amount of muscular contraction of the animal would cause the shell to adhere so firmly to a smooth surface as to be practically immovable without fracture. As the shells cannot be adapted daily to different forms of surface, the limpets generally return to the same places of attachment. I am sure this is the case with many; for I found shells perfectly adjusted to the uneven surfaces of flints, the growth of the shells being in some parts distorted and indented to suit inequalities in the surface of the flints. As the edges of the shells, especially those of the younger animals, are very sharp, the effect of pressure brought to bear on the edge, either by the contraction of the animal or by the shock of the waves, would, if there is the least sideway movement, be to cut into the chalk round the edge of the shell. The muscles of the animal are generally relaxed when reposing; for if the point of a knife be quickly inserted beneath the edge of the shell, it may be detached from the rock without difficulty; but if the least warning by a touch be given to the animal, its muscles contract, and it adheres so firmly that it is impossible to detach it without breaking the edge of the shell*. These alternate relaxations and contractions on sudden alarms would tend to increase the effect of the cutting action of the edge of the shell. I saw the fine indentations round the edge of some of the shells exactly reproduced upon the surface of the chalk; and this could only result from pressure on the shell forcing its sharp edge into the chalk. A very little pressure, as may be found by trial, will suffice to force the edge of the shell into the chalk. The effect of the formation of a groove in the chalk corresponding with the edge of the shell would be to diminish the internal capacity of the shell, and possibly to cause discomfort to the animal, or prevent its obtaining a firm bold on the rock. As all the surface of the chalk outside the shell becomes covered with the fine growth of seaweed, the outer side of the groove round the edge of the shell, which forms the side of the pit, becomes in like manner covered with seaweed, and is pared away to a slope. This assists the cutting effect of the edge of the shell, as it is more effective against the foot of a slope than it would be if the face of the pit were perpendicular. I noticed one case in which a limpet appeared to have pared away one side of the pit, that opposite the head of the animal, as fast as the pit had been sunk. The animal had begun to browse from the edge of the shell outwards. The above appears to me to be an explanation of the manner in which the habit of sinking pits may have been acquired by limpets. But in many cases they now appear to excavate deeper pits than would be required for the removal of the protuberance, extending the excavation below the plane of the rim of the shell. For what purpose this is done I do not know, unless it be to get a clean surface of chalk to adhere to, as their slimy bodies would detach pieces of chalk in time, and possibly render their hold less secure. Small pieces of chalk do adhere to the animals when you remove them from the rock. These hollows which they excavate below the plane of the rim of the shell are, when completed, basin‐shaped, sloping away from the edge of the shell. At first they are begun beneath the head of the animal, and a considerable hollow is often made there before the excavation is extended round the sides backwards. During the process of excavation a lump is left in one stage in the centre. When a limpet has sunk some distance into the chalk by the above processes combined, the pits are further enlarged by smaller limpets sinking secondary ones and browsing on the seaweed which grows on the sides of the pits. I noticed signs that limpets prefer a hard smooth surface to a pit in the chalk. On one face of a large block, over all sides of which limpets were regularly and plentifully distributed, there were two flat fragments of a fossil shell about 3 inches by 4 inches, each imbedded in the chalk. The chalk all round these fragments was free from limpets; but on the smooth surface of the pieces of shell they were packed as closely as they could be. I noticed another case which almost amounts, to my mind, to a proof that they prefer a smooth surface to a hole. A limpet had formed a clearing on one of the seaweed‐covered blocks before referred to. In the midst of this clearing was a pedestal of flint rather more than 1 inch in diameter, standing up above the surface of the chalk: it projected so much that a tap from my hammer broke it off. On the top of the smooth fractured surface of this flint the occupant of the clearing had taken up its abode. The shell was closely adapted to the uneven surface, which it would only fit in one position. The cleared surface was in a hollow with several small natural cavities, where the limpet could have found a pit ready made to shelter in; yet it preferred, after each excursion, to climb up on to the top of the flint, the most exposed point in all its domain. In South America our limpets have, I believe, representatives with shells a foot in diameter. If the proceedings of these South‐American giants are at all the same as those of the limpets of our own shores and are in proportion to their size, they must materially aid in the encroachment of the sea on the land when the rock happens to be soft*.     

Latitudinal and depth gradients in marine predation pressure

Aim There is a general paradigm that marine predation pressure increases towards the tropics and decreases with depth. However, data demonstrating global trends are generally lacking. Rhynchonelliform brachiopods inhabit all the oceans and often survive shell‐crushing predator attacks. We investigate shell repair in brachiopods across a range of Southern Hemisphere and tropical Northern Hemisphere latitudes and depths. Location The Southern Hemisphere and tropical Northern Hemisphere. Methods We analysed the frequency of shell repair in 112 bulk samples, over 70% of which showed traces of shell damage and repair. Results The pattern of shell repair frequency (RF) was more complicated than the anticipated increase with decreasing latitude, with low levels at both polar and tropical sites but high levels at temperate latitudes. This pattern is only evident, however, in shallow water assemblages; and there is no latitudinal trend in water depths greater than 200 m, where shell RF is systematically low. There was a significant logarithmic relationship between RF and depth. Low polar repair rates reflect reduced predation pressure, directly supporting the global paradigm. Low rates in the tropics appears counter to the paradigm. However, tropical brachiopods are generally very small (micromorphic) in shallow water and below the minimum size at which damage is recorded anywhere. Main conclusions Predation pressure decreased logarithmically with depth. At shallow depths (< 200 m) RF showed its highest levels in the mid temperate latitudes with decreasing frequency towards both the tropics and the poles. Low levels of shell repair at high latitudes are likely to be due to a lack of crushing predators, but in the tropics it is suggested that the low frequency is a result of the small size of tropical brachiopods. We hypothesize that micromorphy in this region may be an outcome of high predation pressure.     

Optimal growth pattern of defensive organs: the diversity of shell growth among mollusks

Mollusks show a diversity of shell growth patterns. We develop a model for the dynamic resource allocation to defense organs and analyze it with the Pontryagin maximum principle. A typical optimal growth schedule is composed of the initial phase of soft-body growth without shell followed by a simultaneous growth of shell and soft body and finally the reproductive phase without growth (simultaneous shell growth). If the defensible predation risk is low or if the cost of defense is high, the optimal strategy is to have no shell (shell-less growth). If defensible predation pressure or general mortality differs before and after maturation, an additional three strategies, characteristic of the exclusive growth of shell or soft body, can be optimal (sequential shell growth, additional body-expansion growth, and additional callus-building growth). These optimal strategies are in accord with the patterns observed for mollusks. In particular, the growth strategies with exclusive growth phase of external shells are preferred when durophagous predation pressure after maturation is higher than that before maturation. This result explains the observation that many tropical gastropods with thickened shell lips spend their vulnerable juvenile phase in sheltered habitats.     

Depth gradients in shell morphology correlate with thermal limits for activity and ice disturbance in Antarctic limpets

To fully understand how species distributions will respond to changing environments it is essential to understand the mechanisms underlying variation in animal performance and the relative importance of different ecological and environmental factors. A performance measure that has previously been used as an indicator of thermal capacity of the Antarctic limpet (Nacella concinna) to cope with regional warming is the ability to right if removed from the substratum and turned upside down. As part of an on-going study into limpet genetics and phenotypic plasticity, we tested the temperature limits for 50% righting of limpets from 6 and 30 m depth. The 50% threshold for limpets collected from 6 m (4.7 °C) was higher than for those collected from 30 m (0.7 °C). This compares with a previously published limit of 2.2 °C for limpets collected from 12-15 m at the same location. These thermal limits positively correlated with a depth gradient in shell height to length ratio; thickness and strength. Flatter limpets, had a reduced thermal limit for righting than taller limpets which we hypothesise is related to increased energy requirements of flat limpets, which have to turn through a greater angle to right than tall limpets. Of the factors that cause morphological plasticity of gastropod shells, iceberg disturbance is the most likely cause of the sub-tidal gradient in N. concinna shell shape, and therefore the thermal limit for righting of limpets from 6 to 30 m depth, rather than environmental temperature.     

Avian eggshell thickness: scaling and maximum body mass in birds

The avian eggshell represents a highly evolved structure adapted to the physiological requirements of the embryo and the potential fracturing forces it is exposed to during incubation. Given its many roles, it is not surprising that the eggshell is also central to the current hypothesis about maximum avian body mass. Eggshell thickness ( L ) and strength has historically been scaled as a function of initial egg mass (IEM). However, maximum incubator mass (IM) is likely a better indicator of the forces the shell must be selected to withstand during incubation. We compare the results of analyses of L ² (an indicator of shell strength) as a function of IEM and IM. We conclude from IM scaling that megapode and kiwi eggshells are not thin but rather are thicker than expected and in general birds with a clutch size of 1 have thicker shells, and further, that reversed sexual dimorphism in the large, particularly extinct birds may be a strategy to avoid shell breakage during incubation of the largest eggs without creating a shell so thick as to inhibit hatching.     

A two-stage model for Cepaea polymorphism

The history of the study of snails in the genus Cepaea is briefly outlined. Cepaea nemoralis and C. hortensis are polymorphic for genetically controlled shell colour and banding, which has been the main interest of the work covered. Random drift, selective predation and climatic selection, both at a macro- and micro-scale, all affect gene frequency. The usual approach to understanding maintenance of the polymorphism, has been to look for centripetal effects on frequency. Possible processes include balance of mutation pressure and drift, heterozygote advantage, relational balance heterosis, frequency-dependent predation, multi-niche selective balance, or some combination of these. Mutational balance is overlaid by more substantial forces. There is some evidence for heterosis. Predation by birds may protect the polymorphism, and act apostatically to favour distinct morphs. Although not substantiated for Cepaea, many studies show that predators behave in the appropriate manner, while shell colour polymorphisms in molluscs occur most commonly in species exposed to visually searching predators. It is not known whether different thermal properties of the shells help to generate equilibria. Migration between colonies is probably greater than originally thought. The present geographical range has been occupied for less than 5000 generations. Climatic and human modification alter snail habitats relatively rapidly, which in turn changes selection pressures. A simple simulation shows that migration coupled with selection which fluctuates but is not centripetal, may retain polymorphism for sufficiently long to account for the patterns we see today. There may therefore be a two-stage basis to the polymorphism, comprising long-term but weak balancing forces coupled with fluctuating selection which does not necessarily balance but results in very slow elimination. Persistence of genetic variants in this way may provide the conditions for evolution of a balanced genome.     

The defensive role of scutes in juvenile fluted giant clams (Tridacna squamosa)

This study tests the hypothesis that the scaly projections (scutes) on the shells of juvenile giant fluted clams, Tridacna squamosa, are an adaptation against crushing predators such as crabs. The forces required to crush scutes and clams were measured with a universal testing machine whereas crab chela strength was measured with a digital force gauge connected to a set of lever arms. Results for shell properties and chela strength are used to create two, non-mutually exclusive, predator–defense models. In Model 1, scutes increase the overall shell size, consequently reducing the number of crab predators with chelae that are large enough to seize and crush the prey. In Model 2, the chela has to open more to grasp a prey with these projecting structures which leads to a loss of claw-closing force such that crabs fail to crush the scutes, and consequently the clam. Clam scutes may also deter crab predators by increasing the risk of claw damage and/or handling time.     

Predator discrimination in the hermit crab Calcinus californiensis: tight for shell breakers,loose for shell peelers

Prey exposed to predators with different hunting and feeding modes are under different selective pressures, therefore it is expected that they should exhibit plastic and adaptive antipredator responses according to current risks. The hermit crab Calcinus californiensis faces two contrasting predators, the shell peeler Arenaeus mexicanus that hunts by active searching and the shell breaker Eriphia squamata that hunts by ambush. In order to discover whether C. californiensis displays plastic responses depending on the type of predatory challenge, we examined the shell size preference, the hiding time, and the escape velocity of hermit crabs in the presence of chemical cues from a shell peeler, a shell breaker, and a control. We also examined the role of shell fit on the escape velocity of the hermit crabs in natural tidal pools. Crabs chose shells with a loose fit (relatively large shells) in the presence of chemical cues from the shell peeler Arenaeus and shells with a tight fit when exposed to cues from the shell breaker Eriphia. The hermit crabs hid for shorter times and moved away faster from Eriphia than from Arenaeus stimulus. The use of a tight shell favours faster movement away from the shell breaker (pre‐capture strategy), but prevents the crab retracting deeper inside the shell, increasing the risk of be eaten by the shell peeler once captured. Hence, the use of loose shells that protect the crab from the shell peeler hinders fast escape. This study shows specific and plastic antipredatory responses to contrasting predators, each bringing adaptive benefits at different levels of the predator sequence.     

Evidence for divergent natural selection of a Lake Tanganyika cichlid inferred from repeated radiations in body size

Divergent natural selection is thought to play a vital role in speciation, but clear, measurable examples from nature are still few. Among the many possible sources of divergent natural selection, predation pressure may be important because predators are ubiquitous in food webs. Here, we show evidence for divergent natural selection in a Lake Tanganyika cichlid, Telmatochromis temporalis , which uses burrows under stones or empty snail shells as shelters. This species contains normal and dwarf morphs at several localities. The normal morph inhabits rocky shorelines, whereas the dwarf morph invariably inhabits shell beds, where empty snail shells densely cover the lake bottom. Genetic evidence suggested that the dwarf morph evolved independently from the normal morph at two areas, and morphological analysis and evaluation of habitat structure revealed that the body sizes of morphs closely matched the available shelter sizes in their habitats. These findings suggest that the two morphs repeatedly evolved through divergent natural selection associated with the strategy for sheltering from predators.     

Biogeography of sponge chemical ecology: comparisons of tropical and temperate defenses

Examples from both marine and terrestrial systems have supported the hypothesis that predation is higher in tropical than in temperate habitats and that, as a consequence, tropical species have evolved more effective defenses to deter predators. Although this hypothesis was first proposed for marine sponges over 25 years ago, our study provides the first experimental test of latitudinal differences in the effectiveness of sponge chemical defenses. We collected 20 common sponge species belonging to 14 genera from tropical Guam and temperate Northeast Spanish coasts (Indo-Pacific and Mediterranean biogeographic areas) and conducted field-based feeding experiments with large and small fish predators in both geographic areas. We use the term global deterrence to describe the deterrent activity of a sponge extract against all of the predators used in our experiments and to test the hypothesis that sponges from Guam are chemically better defended than their Mediterranean counterparts. Sympatric and allopatric deterrence refer to the average deterrent activity of a sponge against sympatric or allopatric predators. All of the sponges investigated in this study showed deterrent properties against some predators. However, 35% of the sponge species were deterrent in at least one but not in all the experiments, supporting the idea that predators can respond to chemical defenses in a species-specific manner. Tropical and temperate sponges have comparable global, sympatric, and allopatric deterrence, suggesting not only that chemical defenses from tropical and temperate sponges are equally strong but also that they are equally effective against sympatric and allopatric predators. Rather than supporting geographic trends in the production of chemical defenses, our data suggest a recurrent selection for chemical defenses in sponges as a general life-history strategy.     

The effects of exposure and predation on the shell of two British winkles

Changes in shell size and shell shape of the two British winkles Littorina nigrolineata and L. rudis were studied in relation to exposure and to crab-size. In both species, shells from exposed shores are smaller and more globose than those from sheltered shores. Also, in rudis of exposed shores the mouth is relatively wider. In shores of equally sheltered conditions, shells are bigger at those localities where crabs are large than at those localities where they are small. The largest shells are found in those localities where it is extremely sheltered, and the crabs are very large.
It is argued that on exposed shores, small shells are favoured because they have more possibilities than large ones to shelter in crevices and in barnacle interspaces, from the impact of winds and waves. A globose shell could accommodate more foot muscle and thus enable a stronger adherence to the rock; and an increased mouth diameter would increase the area of foot adherence to the rock. On sheltered shores, on the other hand, large, narrow-mouthed shells are favoured because they discourage crab predation, large crabs being abundant mainly on sheltered shores.
The possible significance of shell size and shape in relation to zonation is discussed, in view of the different predatory and physical conditions which prevail in different zones of the shore, and the different shell specializations which these conditions would require.     

Gene regulation of shell banding in a land snail from Israel

The Israeli land snail, Xeropicta vestalis, offers a particularly clear example of gene regulation in relation to natural selection, in that within each population the appropriate phenotype is generated only at the correct part of the animals life cycle, and a contrasting phenotype develops when the forces of natural selection change. In the mountains of Jerusalem, where the winter is cold, the shells are dark. Westwards, towards the coastal plain where the winter is warmer, the shells gradually become paler. As dark shells absorb more radiation than pale ones, this clinal variation in morph frequencies can be explained in thermal terms. (Banded shells are also more cryptic than non-banded shells, so that in the mountains visual selection by predators may be an additional force which favours dark shells.) Xeropicta vestalis is an annual, semelparous species: the snails hatch in winter, become mature within one year, reproduce, and then die. In the coastal plain the snails are active throughout most of the year, and they have a long period in which to grow to reach adult size. In the mountains and hills, on the other hand, the snails are active for only a very short period. They spend most of their lives as small snails, in a state of aestivation. Xeropicta vestalis must be dark in mountains because when it finally awakens, it must very rapidly and hastily reach reproduction size. A dark shell, by speeding up temperature-dependent processes in this critical stage, assists the snail to mature rapidly. Shell darkness varies with age: in the mountains and hills the shells are moderately dark when they hatch, but become darker whilst growing in early winter. In the coastal plain also, the snails are moderately dark when they hatch; but here they become paler, whilst growing in winter and spring. In both cases, each snail is darker in the colder months and paler in the hot ones. A strategy of gene regulation of shell colour is thus favoured when the subsequent forces of the environment are very contrasting in their direction, very severe—yet also very predictable.