title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
  Foods, feeding, & growth
  Topics in this secion include factors in diet selection, considered here, and
, and
HATCHLINGS AS PREDATORS considered in other sections.
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  Factors in diet selection
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Nucella spp.

  This sub-section is divided into studies on Nucella spp., considered here, and studies on OTHER WHELKS, considered elsewhere.
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Research study 1

A predator contemplating a prey item to eat may: 1) selective and eat anything that is edible, 2) select a food item in accordance with its relative abundance in the habitat, or 3) select a diet that gives best return on energy and nutrition for the least outlay of time and energy.  This last is the optimal-foraging theory of diet selection.  How does this relate to whelks?  A cost-benefit analysis applied to Nucella spp. and their barnacle prey must take into account several features including: 1) the thickness of the preferred drilling area on the prey in relation to the size of the prey, 2) photograph of whelk Nucella lamellosa attacking a barnacle Semibalanus cariosusdrilling efficiencies of different sizes of predator, 3) food value of different prey species, and 4) time required to locate, drill, and ingest different sizes and types of barnacle prey.  When this is done with Nucella lamellosa and N. ostrina and their prey barnacles Balanus glandula and Semibalanus cariosus, a predator/prey size relationship is predicted, and can be confirmed in field studies.  Thus, the smaller-sized whelk N. ostrina prefers the smaller barnacle species B. glandula, while the larger-sized whelk N. lamellosa switches its preference from B. glandula to the larger-sized S. cariosus as it increases in size.  Emlen 1966 Time, energy and risk in two species of carnivorous gastropods PhD Thesis, University of Washington, Seattle.


Nucella lamellosa attacks a thatched
barnacle Semibalanus cariosus 1.6X

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Research study 2

In theory, a predator that attacks one prey disproportionately more when the prey is abundant relative to other prey, and disproportionately less when the prey is relatively rare, will act to stabilise the numbers in the prey population.  This is tested at University of California Santa Barbara in laboratory simulations using photograph of whelk Acanthinucella spirata courtesy Kaustuv Roy, predatory whelks1 Nucella emarginata feeding on mussels Mytilus trossulus and M. californianus (a strong2 preference for the former), and whelks Acanthinucella (Acanthina) spirata feeding on M. trossulus and barnacles Balanus glandula (a weak3 preference for the latter).  Generally, the results do not support an hypothesis of switching and are sometimes contrary to expectation.  For example, in the instance of strong preference for one prey species, the ratio of prey in the diet is proportional to the ratio that is presented to the whelks and is not biased towards the favoured prey. In the second instance of weak prey preferences, the results are even less clear.  The study is valuable in its theoretical approach, but the experiments are complex and the results too variable for a tidy summary to be presented here.  Murdoch 1969 Ecol Monogr 39: 335. Photograph courtesy Kaustuv Roy, University of California San Diego.

NOTE1  whelks are 25-30mm in length, and prey are 14-25mm in length for mussels and 9-16mm basal diameter for barnacles.  The experiments are run for several weeks

NOTE2  proportions of trossulus:californianus eaten in experiments where both prey are offered in equal numbers range from 89:11 for N. emarginata and 94:6 for A. spirata

NOTE3 in comparison with the strong preference shown by Nucella for M. trossulus, the whelk Acanthinucella exhibits a “weak preference” for barnacles B. glandula over M. trossulus: 52:48. However, as this ratio is not likely to be significant, no preference exists at all

Whelk Acanthinucella spirata 2X

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Research study 3

schematic showing vertical distributions of Nucella prey in Torch Bay, AlaskaStudies at Friday Harbor Laboratories, Washington confirm that 2 prey species of Nucella, the sea mussel Mytilus californianus and the thatched barnacle Semibalanus cariosus, are able to coexist with their predator by reaching a size too large to be eaten.  They are in size refuge.  However, 2 other prey species, namely, the bay mussel Mytilus trossulus and barnacle Balanus glandula, do not reach refuge in size, but can be preyed upon by all sizes of Nucella greater than 10mm in length.  The author notes that these 2 species are the ones with greatest food value to the predator. 

In southeastern Alaska, where densities of Nucella may reach 300 . m-2, the 2 species Mytilus trossulus and Balanus glandula only survive if higher on the shore.  In the zone of Nucella, they are consumed.  Palmer 1983 J Exp Mar Biol Ecol 73: 95.

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Research study 4

histogram showing how whelks are able to discriminate high-value prey items in the field from low-value prey items Whelks Nucella lamellosa, N. canaliculata, and N. ostrina in northwestern Washington not only discriminate between different prey species, but also discriminate between different sizes of a given prey species. The study starts with a nice premise, that is, that the whelks themselves are best able to define what is their optimal diet.  The whelks are caged with 4 different sizes of their 3 principal prey species for 30d in the field.  The results provide 4 different categories of food value1 of the prey.  These are, based on rates of growth, and ranging from best to worst:  medium-large barnacles Balanus glandula > small- or medium-sized mussels Mytilus trossulus >  large M. trossulus = small barnacles Semibalanus cariosus > large S. cariosus

The histogram shows what the whelks actually eat in the field2 . Note that most whelks feed on higher quality prey, ones that give them highest energy and nutrition yield, as determined from the growth3  study.  In other words, they are feeding optimally.  In some way they are able to predict the best food value of a prey, either by species, by size of individual, or both. 

Now, what happens if whelks4  are conditioned for several weeks on diets that provide different “food values”, that is, different levels of growth per unit time, and then allowed to select freely from a variety of different prey items (small M. trossulus, and small, medium, and large B. glandula)?  The graph on the Right shows that in all instances snails conditioned on the poorest diets eat the most prey items on release from their restricted diets.  Presumably, these snails are the hungriest ones.  Palmer 1984 Oecologia 62: 162. graph showing effect of "pre-conditioning" whelks on diets of low and high food value on subsequent numbers of prey eaten

NOTE1   defined as "yield", equivalent to % body-mass gain per unit time

NOTE2   most observations are from whelks at sites in Washington, but 6/44 are from a site in Alaska

NOTE3   the author elsewhere describes a non-destructive method for determining the shell-less live mass of a Nucella snail, yielding less than 11% error.  Palmer 1982 Malacologia 23: 63.

NOTE4   3 species of whelks are used, N. lamellosa, N.ostrina, and N. canaliculata.  The whelks are conditioned on the different diets for 26d, then released into “free-choice” cages in the field or, in one case, into an “arena”, a fenced area, in the field

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Research study 5

schematic showing the relationship between prey availability in the habitat and prey eaten by whelks Nucella emarginataDiets of individual Nucella emarginata in Pacific Grove, California are remarkably variable, even when the same types (barnacles, mussels, and limpets) and numbers of prey are present to individuals in a population.  During a 4-mo study at one site, for example, 104 marked snails attack 7 prey species, but no individual snail is observed to eat more than 3 species of prey.  Moreover, prey selection by an individual snail does not simply reflect the relative abundance of the prey species in the individual’s microhabitat (see graph). The study is interesting in that it documents aspects of prey selection, handling, and feeding for individual snails during the entire period of study.  The author suggests that the diet patterns of N. emarginata are likely to have several causes, and ingestive conditioning and learning may be included. West 1986 Ecology 67: 798. 

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Research study 6

collecting sites in Puget Sound and environs for study on feeding preferences in whelks Nucella lamellosaHow adaptable is a whelk to a novel prey species?  This is tested at Friday Harbor Laboratories, Washington with Nucella lamellosa accustomed to feeding on barnacles Balanus glandula and Semibalanus cariosus, and presented with a novel species Semibalanus balanoides collected from Denman Island, British Columbia.  Results from experiments in both field and laboratory show that the whelks immediately add the histogram showing growth of whelks Nucella lamellosa on 3 species of barnaclesnovel prey to their diets and eat it as much or more than the other 2 barnacle species.  The authors credit the active prey choice to the high quality, or profitability, of the new prey. Note in the graph that whelks fed on B. glandula and S. balanoides roughly double their tissue mass in 66d. Other experiments show that handling costs are equivalent to the best of the other prey barnacles, namely, B. glandula, while handling costs on S. cariosus are almost 3 times as much. This may explain why whelks feeding on S. cariosus actually lose weight. The authors view the switch in diets as being consistent with the predictions of optimal foraging models that energy intake will be maximised.  Although not mentioned specifically by the authors, prey novelty may also be involved.  Carroll & Wethey 1990 J Exp Mar Biol Ecol 139: 101.

NOTE  this Atlantic-coast species occurs in Alaska and also in British Columbia, at least to Denman Island, mid-way down the coast of Vancouver Island

NOTE  "handling costs" represent combined drilling and ingestion times for each prey species, as measured in the laboratory. The handling times for same-sized (4.2mm opercular diameter) B. glandula and Semibalanus balanoides are comparable (6.6 and 6.3h, respectively), versus 17.4h for S. cariosus.

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Research study 7

photograph showing 3 species of whelks Nucella from British Columbia: L-R: N. lamellosa, N. canaliculata, N. ostrinaPreference for one prey species over another by whelks Nucella canaliculata and N. ostrina is greatly influenced by past experience.  Thus, if a test group of whelks is taken from a site with barnacles but no mussels as food they will, when presented with both types of prey in mesh-enclosed arenas, largely ignore the mussels and feed on the barnacles instead.  This type of ingestive-conditioning is seen in many marine invertebrates, including other populations of Nucella spp., and is thought to lead to more efficient feeding mainly through decreased prey-handling time.  Wieters & Navarrete 1998 J Exp Mar Biol Ecol 222: 133.




Three species of whelks Nucella.
L-R: lamellosa, canaliculata, and ostrina 1X

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Research study 8

histogram showing numbers of mussel Mytilus californianus drilled by whelks Nucella canaliculata in sites along the west coastAlong the California coast the whelk Nucella canaliculata is a strong predator on sea mussels Mytilus californianus, but not in areas of Oregon and Washington where it is only a weak predator of these species.  On wave-exposed coasts in California there are relatively fewer bay mussels M. trossulus from which a whelk can choose.  The reverse is true on comparable wave-exposed coasts in Oregon and Washington, where bay mussels are more prevalent.  Do drilling preferences differ in the different areas and, if so, can the differences be correlated with genetic differences in the whelk numbers of mussels Mytilus californianus drilled by whelks Nucells canaliculata after the whelks have been conditioned to this prey for 1yr in the laboratorypopulations? The researchers first collect all whelks, and all dead and drilled M. californianus shells from 0.25m quadrats at 16 sites along a 1500km stretch of coastline from Piedras Blancas, California to Tatoosh Island, Washington.  The quadrats are placed randomly in the most wave-exposed mussel beds, but at the same tidal heights. The data indicate that while drilled M. californianus are abundant along the California coast, they are almost or entirely absent from Oregon and Washington coasts (see figure upper Left).  This is despite the fact that densities of whelks in the last 2 sites are as high or higher than at California sites.  If whelks from each area are maintained in a common laboratory environment with only M. californianus as prey for 1yr, similar predation results are obtained (see figure on Right).  Note in the histogram that while California whelks drill intensely in the laboratory, the Oregon and Washington whelks drill only desultorily or not at all. 

percentage whelks Nucella canaliculata from different areas of the west coast preferring to drill mussels Mytilus californianus when given a choice between this species and bay mussels M. trossulusTo test if the whelks from the different areas are dietarily conditioned to a certain prey species, that is, if they have formed “search” images for prey to which they have become accustomed, another set of experiments is performed.  The researchers collect egg capsules from 8 of the 16 sites along the coast, hatch them, and grow the juveniles to adult size (2cm shell length) over 9-10mo on a strict diet of M. trossulus from a common Oregon source.  The propensity of these whelks to drill M. californianus is then tested. The results show that that 76% of California graph showing genetic differentiation in whelks Nucella canaliculata along the west coast of North Americawhelks, 9% of Oregon whelks, and 4% of Washington whelks opreferentially drill M. californianus (see histogram on Left). These data provide strong support for the hypothesis that variation in drilling behaviour has a genetic basis. 

To test for this, base pairs of a metabolic enzyme are assayed in whelks from each site.  Results show that while a distinct break between geographic populations is absent, there is a significant correlation of increased genetic differentiation with increasing geographic distance between the populations, which would be characteristic of a species with limited dispersal and low gene flow.  Sanford et al. 2003 Science 300: 1135.

NOTE  because the drilled shells are embedded within the matrix of living mussels, the mussel beds contain a record of past predation

NOTE  870 base pairs of the mitochondrial gene encoding cytochrome c oxidase (subunit 1).  This enzyme is commonly used in evolutionary studies because it is not only common to most or all organisms, but is easy to detect.  It was once believed that the mitochondrial genome evolved, that is, mutated more quickly than the nuclear genome and thus was especially useful for examining closely related species or groups of organisms.  We now know that this is true for some organisms, but not for others.  Nonetheless, it is an easy gene with which to work, as sets of mitochondrial probes are available “off the shelf”.  It is unlikely that this particular enzyme has any direct relationship with drilling or with propensity to drill into mussels; rather, the authors use the gene to look at the relatedness of the different populations and then assess the correlation of degree of relatedness with propensity to drill into mussels

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Research study 9

map showing collection sites for whelks Nucella canaliculata in a study of drilling into mussels Mytilus californianusphotograph of whelk Nucella canaliculata with egg capsules courtesy Sanford & Worth, Stanford University, CaliforniaAs we have seen in Research Study 8 above, whelks Nucella canaliculata show striking geographic variation in their ability to drill mussels Mytilus californianus in different areas of the west coast. For example, while drilled mussels are common on Californian coasts and include many large individuals (>10cm long), they are rare on central Oregon coasts, and consist mainly of small mussels (<3cm).  Are these differences based on phenotypic plasticity or fixed genetic differences among the snail populations?  This is tested in a 3-yr study at the Bodega Marine Laboratory, California, involving collections of snails from 8 populations along the coast (see map on Left) and rearing them in the laboratory over 2 generations on a common diet of bay mussels Mytilus trossulus. The offspring of the 8 populations are then tested for their ability to drill M. californianus. 

True to prediction based on the foregoing Research Study 8, few of the F2-generation snails from Oregon sites are able to drill mid-sized M. californianus, 5-7cm in length, while ones from California have no trouble doing so (see histogram on Right). Note that histogram comparing the ability of whelks Nucella canaliculata from California and Oregon to drill into mussels Mytilus californianus after being reared for 2 years on diets of bay mussels Mytilus trossulus the progeny of a cross between central California and central Oregon whelks are proficient at drilling M. californianus.

 Further tests on these Oregon F2 whelks show that they are indeed capable of drilling into mussels, but only small-sized ones (1-2cm; see histogram below Left).  However, because differences in drilling capability persist through 2 generations in the laboratory, the authors rule out  transgenerational phenotypic plasticity. Overall, the data point to the liklihood of a genetic basis for the difference.  The different populations readily interbreed in the laboratory and the offspring produced are reproductively viable.  The researchers hypothesise that persistent variation in mussel recruitment along the coast has selected for strong histogram comparing the ability of whelks Nucella canaliculata from California and Oregon to drill small-sized mussels Mytilus californianusinterpopulation differences in drilling capability of whelks N. canaliculata, with potential consequences for the size structure of mussel beds.  At present, the functional basis for the differences in drilling capability is unknown.  Sanford & Worth 2009 Ecology 90: 3108; for more on this topic see Sanford & Worth 2010 Ecology 91 (3): 891. Photograph courtesy authors and Stanford University, California.

NOTE  in the earlier study (Research Study 8 above) egg capsules are collected from the field, and the researchers comment that maternal effects may have played a role in influencing the phenotypes of the hatchling snails.  The authors are referring here to a phenomenon known mainly from studies of plants termed adaptive transgenerational phenotypic plasticity, where environmental conditions experienced by a female parent may alter her offspring’s phenotype, thus leading to increased fitness

NOTE  the small letters above the bars in the histogram indicate statistical differences

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Research study 10

graphs showing strength of shells and susceptibility to predation by crabs Cancer productus in 3 species of west-coast whelks NucellaTo what extent does the presence of a predator, such as a crab Cancer productus, influence feeding behaviour in west-coast whelks Nucella spp.?  The influence in question is a nonconsumptive one, generated by the scent of the crab in the immediate vicinity of the whelks.  The study is done at Friday Harbor Laboratories and involves the relationship between morphological vulnerability (shell thickness) and predator-induced feeding suppression among 3 whelk-guild members N. lamellosa, N. ostrina, and N. canaliculata.  The first species has the strongest shell and is potentially least vulnerable to predation by C. productus, while the other 2 species are weaker-shelled and potentially more vulnerable (see graphs).  This corresponds with the extent to which each species’ feeding is affected by crab scent.  Thus, in laboratory experiments where crabs are caged upstream of whelks and their barnacle Balanus glandula food, it is the thinnest-shelled N. ostrina whose feeding is absolutely (i.e., among-species comparisons) most suppressed and the thickest-shelled N. lamellosa whose feeding is least affected. However, when a statistical adjustment is made that corrects for relative differences between test and control feeding rates (i.e., proportional differences), the results become non-significant.  The author comments that when interspecific studies such as these are done, it is important to assess and compare both absolute and proportional predator-induced changes in behaviour.  Bourdeau 2013 Behav Ecol 24 (2): 505.

NOTE  such influences are termed here nonconsumptive predator effects (other names are behaviorally transmitted indirect effects and interaction modifications, and more) and in west-coast molluscan taxa include such things as suppression of feeding and growth, and modification of shells for greater defense: see LEARNABOUT/WINKLE and LEARNABOUT/WHELK

NOTE  unless the significance of the researcher’s data has been misinterpreted in this synopsis, there is little originality to the study.  Would a study that included second-order interactions have made a more useful contribution?  That is, does a snail that perceives the scent of an upstream predator communicate this via pheromones or behaviour to other conspecifics that are not exposed? Or possibly to other congeneric species?

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Research study 11

Predatory whelks are thought to hunt their prey using a combination of long-distance perception of odour (osphradium) and close-in touch-and-taste perception (oral tentacles). In the wave-turbulent environments frequented by whelks and their barnacle prey, one would think only the second method would work. In an innovative, original, and challenging study done by American and Australian researchers in Malibu, California, whelks Acanthinucella spirata are tested in the field for responsiveness to seawater drawn directly from above large aggregations of barnacles Balanus glandula and pumped through tubes containing individual snails located in the high intertidal zone. As the whelks are known to move upstream in response to barnacle-scented water in laboratory situations, a positive response is tallied if the test individual moves 15cm in the direction of the flow within 15min. Control tests involve a snail being exposed to clean seawater pumped from over a sandy area not occupied by barnacles. An individual snail is used only once, for either experimental or control tests, then placed back on the shore. All tests are done drawing of a whelk with mantle cavity opened to show morphologyduring calm-water days. Results show no evidence that water-borne chemical cues under the circumstances described are used by the whelks for locating prey (see histogram). In contrast, “up-close and personal” chemotactile contact with both live barnacles and “faux” barnacles (cleaned shells stuffed with gels containing a purified glycoprotein in physiological concentrations), elicited immediate attack and feeding. Later tests on different components of the gels show the active ingredient to be protein. Although by no means conclusive, the study is suggestive, and we look forward to seeing more on this topic in the future. Ferrier et al. 2016 Biol Bull 231: 207. Drawing of whelk courtesy Richard Fox, Lander University, South Carolina.

histogram showing responses of whelks Acanthinucella spirata to water-borne scent of prey barnacles Balanus glandulaNOTE the sensory parts mentioned are shown here. Water flow enters the mantle cavity from the Left, passes over the sensory osphradium, irrigates the ctenidium (gill), then exits on the Right after picking up fecal and urinary products for discharge to the outside

NOTE the prey patches sampled (5 replicates) averaged about 11,000 barnacles . m-2


Left bars: snails show no significant difference in response to barnacle-scented or
-unscented (sand-flat) water. Right bars: snails that did not respond initially are retested
with the same seawater solutions, but with the addition of minced barnacle flesh
(filtered) to stimulate them to crawl towards the seawater flow. These whelks as well
show no significant responses to the two treatments (one could argue that these are
not useful data, as the presence of the extract is likely to cause sensory overload)

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Research study 12

In a concurrently published research paper, the same group as above reports on the operative glycoprotein molecule in Balanus glandula, which they name multifuncin, describing its molecular makeup, sites of production, and control of expression. Multifuncin and related proteins are upregulated in barnacles before and after an individual moults. Its genetic makeup is similar to that of a settlement-inducing pheromone in the tropical/subtropical barnacle Amphibalanus amphitrite (78% nucleotide-sequence homology), suggesting that multifuncin may similarly act to attract and induce settlement of conspecific cyprid larvae (research underway) to adult aggregations. Such aggregations, so typical of B. glandula and other species, promote fitness through greater survival and reproductive success (recall that mating in barnacles requires that individuals be within penis length of one another). Multifuncin in the cuticle of the penis may be involved in species/species recognition during micrograph of cuticle of cirri of acorn barnacle stained for presence of the glycoprotein multifuncinreproduction. The “catch-22” of multifuncin’s low solubility in water, of course, is that predatory species of whelks and possibly sea stars can use it as a handy tool to identify their prey. Ferrier et al. 2016 Int Comp Biol 56 (5): 901.

NOTE multifuncin is 199kDa in size, is produced in the epidermis, cuticle, and new shell, and specifies the presence of a live barnacle to its predators

The reddish/brown coloration in the outer cuticle of two adjacent cirri
of Balanus glandula indicates the presence of multifuncin. The colour
owes to a pigment attached to a monoclonal antibody raised against the
glycoprotein. Blue coloration is hematoxylin staining of muscle protein

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  Research study 13

The contribution made by above consortium of U.S. and Australian scientists is enormous, as it seems to pinpoint settlement induction in barnacles to a single glycoprotein molecule (multifuncin) contained in barnacle cuticle. A series of laboratory and field experiments with Balanus glandula shows a strong positive settling response by conspecific cyprid larvae to the purified glycoprotein. The work appears to have answered the age-old question of settlement induction in barnacles and, in a wider perspective, may give us a hint of how it might work in other marine-invertebrate species with similar identification molecules. Similarly, In a series of preference/attraction experiments using“facsimile” prey (sterile barnacle/mussel/turban snail shells packed with gels flavoured with emulsified tissues of these organisms, and also ones containing multifuncin), the researchers show that predatory whelks Acanthinucella spirata prefer barnacle flavours over other prey, and are strongly attracted to gels of purified multifuncin. Identification of suitable “prey” by the whelks in these tests appears to be primarily chemotactile, with little or no reliance on waterborne chemical cues (this needs further investigation by the researchers). Although the authors are themselves "cued in" to multifuncin being the primary or only factor involved in prey (barnacle) recognition by whelks, one guesses that there are a host of other signals emanating from a live barnacle such as metabolic and other exudates (urine, pheromones, feces) that could be involved in close-in perception of waterborne or chemotactile recognition. Multifuncin is likely to be just one component of a"package" of chemical and physical stimuli. Much of what is reported in this publication is repetitive on the researchers’ previous studies, but with the addition of a new twist. Around the premises of multifuncin promoting increased fitness of barnacle diagram illustrating the concept of contradicting convergences relating to whelks Acanthinucella spirata and barnacles Balanus glandulapopulations through immigration on the one hand, and enhancing predation on the other, the authors construct an hypothesis of contradicting convergences: in their words “…multifuncin drives opposing demographic processes toward structuring predator and prey populations.” The conceptual model (see diagram) shows opposing selective forces orchestrated bya single molecular entity, multifuncin, converging over evolutionary time to produce the intertidal barnacle community that we are familiar with. Now, being simple (and of course elegant) is good, but being simplistic, not so good. There are many forces, both biological and physical, that operate to shape community structure on the shore, and predation is only one of them. The whelk in question is only one type of predator of settled barnacles; there are many others, including fishes, limpets, sea stars, insects, and crabs, to name a few, each with its own diverse and specialised means of prey identification that may not necessarily include multifuncin. Construction of an all-inclusive model on such tentative grounds is misleading and too simplistic. The authors’ boldness is, however, to be admired, and it will be interesting to see how their ideas are received by other west-coast ecologists. Zimmer et al. 2016 Ecology 97 (9): 2232.

NOTE the authors criticise use of urine in this regard as being too non-specific, as in their view ammonia released in urine is “produced by all heterotrophic organisms”. In reality, heterotrophs release their nitrogenous excretions in a variety of different ways, such as guanine (by spiders), urea (mammals, fishes), uric acid (birds, lizards), and certain snails also employ urea and uric acid. Additionally, urine of primarily ammoniotelic organisms may also contain amino acids and purines, and sometimes pheromones, any combination of which could give signature odours and tastes to the urine of a barnacle

Diagram illustrating the authors' concept of opposing demographic
processes acting through evolutionary time to structure intertidal
barnacle communities as we see them today. The “convergence” relates
to the same chemical entity being recognised by the larvae and by the
whelks. The first leads to enhanced fitness through immigration; the
second, to death. It's not clear how "constraint" fits into the scheme

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