title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
  Predators & defenses
 

Adaptive strategies of whelks and other snails to avoid being crushed by crabs or fishes is to have a thicker shell, a narrow/small aperture, a thick outer lip, strong sculpture, and a low spire. These features act to strengthen the shell and limit the ability of a crab to get a grip on it.  They also effectively reduce the critical size of the prey, in other words, permit refuge from crushing at a smaller size.  For review see Zipser & Vermeij 1978 31: 155.

The section on predators & defenses is divided into topics of shell size & thickness, considered here, and
DURING EARLY DEVELOPMENT

ESCAPE BY BURIAL
ESCAPE BY SWIMMING, CRAWLING, HIDING, OR BITING,
SHELL COLOURS & CAMOUFLAGE
SHELL SCULPTURING, and
NOXIOUS SECRETIONS
, considered in other sections.

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  Shell size & thickness
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Research study 1
 

photograph showing a crab (Cancer sp.) holding a whelk Lirabuccinum dirumDoes extra shell thickness provide enhanced defense to whelks?  This is tested in a study using mixed sizes of Nucella lamellosa and N. ostrina from Puget Sound, Washington offered to large (12cm carapace width) and small (8cm) Cancer productus.  While the small crabs prefer N. ostrina, perhaps because it has a thinner shell than N. lamellosa, and, to some extent, smaller-sized N. lamellosa, the larger crabs eat all sizes of both prey species.  The results imply that there is a maximum size of snail that can be eaten by sub-adult crabs.  Bertness 1977 Ecology 58: 86.

 

 

A crab Cancer oregonensis holds a
whelk Lirabuccinum dirum - possibly too big
a prey for the little crab to handle 1.3X

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Research study 2
 

histogram showing sizes of whelks Nucella lamellosa selected by northwestern crows to eatdiagram showing release of prey whelks by crows at the top of their flight pathShells of whelks Nucella lamellosa are characteristically thick and strong, and in some environments bear additional, possibly strengthening, sculptural adornments.  Despite this, northwestern crows Corvus caurinus find them quite easy to break by flying up and dropping them onto rocks.  The birds select a specimen, then fly up and either release the prey at the apex of their flight or shortly after they start down.  The latter is preferred, perhaps because it allows the crow to monitor where the shell lands. 

This seems straightforward enough, but there are a surprising number of decisions required of the bird if it is to be successful. For example, what size of whelk is best to select? Studies on Mandarte Island, British Columbia show that the crows prefer the largest specimens to drop, based on what appears to be an initial estimation by eye and, then, an estimation by mass (see histogram upper Right).  This is not a palatability issue, as both old and young snails are equally tasty to the crows.  Rather, it is a combination of breakability and meal size at the end.  Large snails break quicker with fewer drops.  To illustrate this, experimental drops of snails by the author from a graph showing flight heights of successive droppings of whelks Nucella lamellosa by northwestern crowsplatform show that a large snail will break after 10-50 drops from 3m height, but after only 1 drop from 15m.  In histogram showing heights of flights of northwestern crows when dropping whelks to crack themcomparison, a small snail will require 20-50 drops from 3m, and 10 drops from 15m.  The birds know this, and they know also that energy yield is greater from a large snail.

Next, what is the optimum height from which to drop a snail? The flight path of the crows is comparatively low, averaging about 5m (see histogram lower Left).  At this height it takes many more drops than at, say, 15m, but thievery is easier to monitor from the lower height, and with enough drops the shell will eventually break.  An average-sized large whelk breaks after about 4 drops from this height, but some require up to 20 drops. 

In face of this, does the crow adjust its flight pattern? No, flight heights change little over time and 5-6m seems to be the norm (see graph bottom Right). The crows seem to be remarkably patient, trading off a low flight height for many drops. A crow rarely leaves a shell unbroken, even after a dozen or more attempts.  Can they count?  Probably not, and it seems that their experience in knowing that the shell will eventually break (less than 1% are left unbroken) is what keeps them going. Zach 1978 Behaviour 67: 134; Zack 1979 Behaviour 68: 106.

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Research study 2.1
 

photograph of snails Alia carinata with varying degrees of keel formationIn Bodega Bay, California the small snail Alia carinata exhibits 2 morphs, one characterised by a distinctive keel on the shell and found in a protected harbour; the other, possessing a smooth shell and found in a nearby rocky cove.  The keel is located on the shoulder of the body whorl and is associated with a more crenulated outer apertural lip (see photographs).  Based on more extensive scarring and other damage on  harbour shells than on cove shells (36% vs. 15% scarring, respectively), researchers at the Bodega Marine Laboratory suggest that the keels may be predator-induced phenotypic adaptations that act to reduce mortality from shell-crushing invertebrates, such as crabs.  Alia hatch directly to crawl-away juveniles, so extensive mixing of the harbour and cove populations is unlikely.  Although the authors present no actual data that the keels significantly strengthen the shells, cafeteria-style preference tests in the laboratory with crabs Cancer antennarius, a known shell-crushing predator that inhabits both harbour and cove, reveal that almost 3-fold more unkeeled snails are eaten or damaged than keeled snails.  Bergman et al. 1983 The Veliger 26 (2): 116.

NOTE of course, what is needed now is a long-term growth experiment where snails are grown in the presence and absence of crabs C. antennarius or other shell-crushing species

 

 

Dorsal (above) and ventral (below) views of snails Alia
carinata
with varying degrees of keel-formation 2.5X

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Research study 3
 

graph comparing cumulative consumption of whelks Nucella lamellosa by medium-sized red-rock crabs Cancer productusPredation by crabs on whelks is investigated at Friday Harbor Laboratories, Washington.  In experiments where medium-sized red-rock crabs Cancer productus are given a choice of thick- and thin-shelled morphs of Nucella lamellosa over 55d in the lab, they select the latter to eat (see graph).  There is no refuge in size for the thin-shelled morph from even medium-sized crabs, and even the thick-shelled morph falls prey to largest-sized crabsPalmer 1985 Veliger 27: 349.

NOTE as, for example, 16.5cm carapace width; data for this size of crab are not included here

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Research study 4
 

graph shell masses of thin- and thick-shelled morphs of whelks Nucella ostrinagraph comparing egg-capsule production in thick- and thin-shelled morphs of whelks Nucella ostrinaAnother whelk species with thin- and thick-shelled morphs is Nucella ostrina.  Similar to N. lamellosa, the thin-shelled morphs are found on wave-exposed outer coasts and the thick-shelled morphs on protected shores (see graph on Left). What costs might there be to a whelk in maintaining a thicker shell?  This is investigated in a study at Bodega Marine Laboratory, California in which both thick- and thin-shelled morphs are damaged in a standardised way, fed on barnacles for 34d, then examined later for efficacy of shell repair.  Other thick- and thin-shelled snails are monitored for differences in reproductive output.  Results show that there is no apparent difference between the morphs in their ability to repair damage to their shells.  However, thin-shelled females produce more egg capsules and thus more offspring than thick-shelled ones (see graph on Right), but with no difference in mean number of offspring per capsule.  The capsules of thick-shelled females are also larger than those of thin-shelled females, and thus more resources are additionally allocated per embryo.  The author suggests that because of increased resource allocation to their shells, thick-shelled morphs are less reproductively fit than thin-shelled ones.  Geller 1990 J Exp Mar Biol Ecol 144: 173.

NOTE  the shells are ground away from the aperture edge to the level of the withdrawn operculum, representing a loss ≈ 6% of total live mass

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Research study 5
 

photograph of mud snails Cerithidea californica and a major predator Pachygrapsus crassipes, courtesy of predation by grapsid crabs Pachygrapsus crassipes on mud snails Cerithidea californica in Bolinas Lagoon, California appears to dependent upon the size of the prey.  This is determined by a researcher from the University of California Berkeley who marks snails of different sizes and records predation rates over a 10wk summer period in the field, as well as conducting laboratory experiments.  Results from  the laboratory experiments show that size of prey eaten generally increases with increasing crab size.  This size-dependent predation, however, occurs to a lesser extent in field animals, and the author is unable to show evidence of an absolutely invulnerable snail size.  Cerithidea is slow-moving and relies mostly on its shell for protection.  Crabs are easily able to crush small snails or peel back the shell from the aperture of medium-sized snails.  However, larger snails with exaggerated shell sculpturing in the form of thickened aperture lip or raised varix further back on the shell appear to suffer less mortality than the smaller individuals with less sculpted shells.  Crabs can circumvent these defenses in large snails by creating a hole behind the thickened shell lip and working backwards from there, or simply cutting the shell in two by exerting pressure with its chela along the suture line between whorls. Sousa 1993 J Exp Mar Biol Ecol 166: 19. Photographs courtesy EPA, University of California Davis, and Kevin Lafferty.

NOTE  the author also provides information on sizes of Cerithidea eaten by willets Catoptrophorus semipalmatus, the only one of 8 bird species livingin the marsh known to prey on mud snails.  Examination of regurgitation pellets show that, in general, the birds eat smaller-sized snails than the crabs.  Interestingly, the presence of living snails in the pellets (29 of 80, or 36%) suggests a potential bird-vectored mechanism of transport of Cerithidea among coastal mud lagoons

NOTE  tags used have a floating end, making recovery of tagged snails easier and more effective

 

Mud snails Cerithidea californica (0.3X) on a California mudflat
and a predatory shore crab Pachygrapsus crassipes 1X

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Research study 6
 

graph comparing effect of effluents from predatory crabs Cancer productus and Carcinus maenas on shell thickness in whelks Nucella lamellosaWhelks are known to grow thicker shells in the presence of predatory crabs. Is the shell-thickening response specific to a crab species that the whelks are familiar with, or is the response a general one to all snail-eating crab species?  This is tested at the Bamfield Marine Sciences Centre, British Columbia using whelks Nucella lamellosa exposed to seawater effluent of caged predatory crabs Cancer productus and invasive European green crabs Carcinus maenas.  West-coast Nucella spp. have a long shared history with Cancer spp., including C. productus, but potentially have had contact with C. maenas for only the last decade. 

Results show that after 50d exposure to the different seawater effluents, Nucella significantly increases its shell thickness to C. productus, but not to C. maenas (see photographs of whelks Nucella lamellosa showing attack sites on older, thinner whorls by green crabs Carcinus maenasaccompanying graph). Note that exposure to large Cancer leads to a 3-fold increase in shell thickness.  Growth is significantly decreased in the presence of large or small Cancer, but not to Carcinus of any size (see inset histogram).

When these thick-shelled whelks are provided as potential prey to the 2 crab species, they are less likely to be eaten by C. maenas than by C. productus. However, those that are eaten by C. maenas are most often attacked at the old, thinner shell whorls (see photographs). The study provides interesting insight into the way in which latent plasticity of shell-thickness in whelks couples with chemical cues from predatory crabs based possibly on an evolved risk-threat. The authors discuss whether associative learning may be involved in the response.  Edgell & Neufeld 2008 Biol Lett 4: 385.

NOTE  the authors use both large and small crabs of each species in order to vary the amount of chemical cues present in the effluent water

NOTE  small and large letters, respectively, indicate statistical separation of the data sets. A "low-food treatment, #2 in the author's original design, is removed from the presentation here for reason of little relevance

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Research study 7
 

drawings showing shell dimensions and morphometric landmarks used in study of predator effect on shell growth in whelks Nucella lamellosahistogram showing effects of single and multiple predators on shell mass and volume in whelks Nucella lamellosaMost studies on shell-growth responses of whelks to predators involve a single predator type, but what happens if a whelk is simultaneously exposed to 2 predatory species each having its own unique attack mode?  This is tested with Nucella lamellosa1 at Friday Harbor Laboratories, Washington using a shell-breaking predatory crab Cancer productus and a shell-entry predatory sea-star Pisaster ochraceus.  The experimental design includes 4 treatments on whelks (18 replicates for each): 1) no predator, 2) crab, 3) sea star, and 4) crab + sea star, all maintained for 70d.  In each treatment the test snails are exposed to seawater effluent from a separately-housed predator being fed on whelks2After 70d each whelk's shell length, width, mass, and withdrawal depth are measured, and the positions of 10 landmark sites on each shell recorded for use in analyses3 (see drawings upper Left).

Results show that exposure to a certain predator-type leads to morphometric changes reflecting the attack mode of the predator.  Thus, whelks exposed to crabs produce heavy, rotund shells (see 3rd bar from Left in upper histogram), while ones exposed to sea stars produce more elongate shells that provide more space for retraction (see 2nd bar from Left in lower histogram). The various photos show the heavy, squat-shape shell that is produced in the presence of crabs, regardless of whether a sea star is also presentl.

Later tests on the efficacy of these particular shapes  in reducing predation show that sea star-induced changes reduce predation by sea stars more than crab-induced changes reduce predation by crabs.  Thus, there is an asymmetrical functional trade-off.  A combination of predators leads to a morphological phenotype similar to that expressed to the predator type likely to be most important as a cause of mortality in the field.  In this regard, results of laboratory4 tests show that crabs are by far the most dangerous predator, by a factor of 13 on a per-individual predator basis.  Thus, predator-induced morphology is “prioritised” according to degree of predation risk likely to be present in the habitat.  Bourdeau 2009 Ecology 90: 1659. Photographs courtesy P.E. Bourdeau.

NOTE1  the whelks are collected as juveniles and fed on barnacles Balanus glandula during their stay in the laboratory

NOTE2  the whelks N. lamellosa are cracked to ensure their consumption by the predators.  Use of conspecific whelks in this way provides a combination of predator and alarm cues for maximal effect

NOTE3  in addition to length/width/mass analyses, these include use of landmark-based geometric morphometrics in a relative-warp analysis of shape variation, similar to principal-components analysis

NOTE4  this is tested by offering crabs and sea stars both morphological types of whelks in 50:50 ratio and then assessing which is eaten preferentially by each predator type

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Research study 8
 

photograph showing apertural teeth of the whelk Nucella lamellosaThe same researcher further investigates phenotypic plasticity in whelks at Friday Harbor Laboratories, Washington, again using shell-breaking predatory crabs Cancer productus and Nucella lamellosa, but with a focus on water-borne cues.  The inducible morphological features are thickness of the apertural lip, height of the associated apertural teeth1, and mass of shell and tissues. The author uses 3 experimental treatments2 to test the efficacy of water-borne chemical cues from upstream feeding of crabs to induce changes in these features: 1) crabs feeding on fish, a macerated Dover sole, representing a non-snail prey), 2) crabs feeding on winkles Littorina sitkana, representing a non-whelk prey), and 3) crabs feeding on whelks N. lamellosa.  Another parallel set of experiments involves whelks with no upstream crabs, representing a no-crab control, but with the same damaged-prey cues.  The test N. lamellosa are fed ad libitum histogram showing effect of water-borne cues from predatory crabs and damaged prey on apertural lip thickness in whelks Nucella lamellosaon a preferred barnacle prey Balanus glandula and kept in the treatments for 48d before being measured. 

Results show non-significant effects of upstream fish and winkles, with or without crabs, and upstream whelks without crabs, on apertural lip thickness, but do show a significant increase in apertural lip thickness in the presence of crabs eating whelks (see histogram3). Similar results are obtained for apertural teeth (results not shown here).  Finally, results for shell and tissue mass show significant effects of crab presence, as expected, but not for type of prey.

Thus, cues from damaged conspecific snails N. lamellosa alone do not trigger a response.  However, in combination with predator cues, these “damaged conspecific” cues strongly signal predation risk and induce an apertural-thickening defense.  The non-additivity of these cues is a surprising outcome of the study, and the author discusses possible explanations for it. The author suggests that the ability of whelks to assess the “danger level” of upstream predators is potentially beneficial owing to the liklihood of decreased avoidance costs.  Bourdeau 2010 Oecologia 162: 987.

NOTE1  the function of the teeth is not known but, because they are an integral part of the aperture, researchers conjecture that any increase in thickness/height in this area will reduce a whelk’s vulnerability to having its shell aperture probed, or chipped away, by a predatory crab

NOTE2  a fourth treatment, an upstream unfed crab, is originally included in the design but, as the results do not differ significantly from Treatment 1 (and introduces a certain statistical “asymmetry”), the author omits these data from the final analysis

NOTE3  the author presents the results in the form of a Y/X plot but, since there are no dependent or independent factors, the data are presented here more appropriately in histogram form

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Research study 9
 

photograph of whelk Nucella lamellosa showing a "whisker" of new shell growth, courtesy Nate Charbonneau and Dave Cowles, Walla Walla University, WashingtonIs it possible that a whelk’s sensitivity to a predatory crab like Cancer productus could increase following an initial exposure to the crab’s scent?  The potential for such adaptive phenotypic plasticity is tested with Nucella lamellosa at the Bamfield Marine Sciences Centre, British Columbia by measuring shell deposition over an 8d period in groups of snails exposed to serial dilution of crab-effluent seawater.  Half of the snails have previously been treated for 30d with effluent seawater flowing from a header aquarium containing C. productus feeding ad libitum on live N. lamellosa, and the other half with clean seawater not containing predator effluent.  The snails are given a 2wk “rest” period between the 30d and 8d treatments.  Results show that whelks "pre-conditioned" to the scent of feeding crabs lay down additional shell defenses at significantly lower concentrations of crab effluent (50%) than crab-naïve control whelks (100%).  Thus, "pre-conditioning" appears to have reduced the whelks’ risk-response threshold.  Edgell 2010 Mar Biol 157: 215. Photograph courtesy Nate Charbonneau and Dave Cowles, Walla Walla University, Washington rosario.wallawalla.


NOTE  the whelks are fed ad libitum on barnacles prior to, but not during, the 8d test period.  However, during this period the whelks on average will deposit almost 4mg of shell matter per day – an easily measurable amount

NOTE  the 8d treatment “dilutions” are obtained by mixing the header-tank effluent seawater (“100% crab-effluent seawater”) with appropriate amounts of clean seawater to produce flows of 100, 50, 25, 12.5, 6.25, and 0% of its original concentration.  These flows lead to replicate experimental chambers, half containing crab-conditioned whelks and half containing crab-naïve whelks.  During this part of the experiment the crabs are fed on chunks of fish rather than whelks to eliminate the possibility that the test whelks may respond to conspecific alarm chemicals that may be released were whelks to be used as food for the crabs    

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Research study 10
 

histogram showing shell-thickening induction in 3 species of whelks Nucella in presence of predatory crabsFurther investigation by the same researcher featured in 2 of the studies above, also done at Friday Harbor Laboratories, reveals that 2 species of whelks Nucella living lower on the shore and thus exposed for longer periods to predatory crabs exhibit greater inducible increases in shell thickening than an upper-shore species.  The author finds that the 2 mid-to-lower-shore species N. lamellosa (0-0.75m tidal level) and N. canaliculata (0.25-1.5m) allocate relatively more energy to shell mass than to soft-tissue mass, thus producing stronger, more crab-resistant shells when in the presence of crabs than the higher shore-photographs of whelks Nucells spp. courtesy Bourdreau2011inhabiting N. ostrina (0.75-1.75m).  The experimental part of the study is done in laboratory aquaria and involves exposing juveniles of each species to chemical cues emanating from upstream predatory crabs Cancer productus over a 76d period in order to assess degree of inducible shell-thickening.  At the end of the exposure period, morphological measurements are taken and compressive strength determined for the shelsl of all snails in an Instron testing device.  Results show that all species actally grow more in the presence of risk cues than in their absence but, as noted above, with greater relative shell-thickening in the 2 lower-shore species in the presence of risk cues than in the upper-shore species (see graphs).  Bourdeau 2011 Functional Ecol 25: 177; see also Bourdeau 2012 J Anim Ecol 81: 849.

NOTE  crab presence is not measured directly for field animals; rather, is simply correlated with the relative times of submersion of the 3 species in the field

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Research study 11
 

photographs showing damage to olive shells Callianax biplicataLike other molluscan species, olive shells Callianax (Olivella) biplicata1 are subject to crushing-type predators as well as to drilling-type.  A problem for investigators of predation in any mollusc is how to identify the kind and intensity of crushing-type predation after the fact, that is, after the shell has been broken to pieces, or otherwise damaged in the waves.  One commonly used method is the frequency of shell repair, as number of repair scars can act as a proxy for the intensity of crushing-type predation.  Thus, populations with higher frequency of repair are assumed to have an associated higher frequency of mortality from crushing-type predators.  This works better for extant populations of molluscs than for fossil populations.  This is because a higher repair frequency in the fossil record may reflect an evolution of greater defensive ability in the prey that results in a higher proportion of attacked prey surviving to repair the damage.  Also, although shell damage from crushing predators is often characteristic and recognisable (see images on Left), it may still be difficult to separate such predator-caused damage from damage caused by post-mortem wave-impact and other types of abrasive damage.  So, if only shells with characteristic crushing damage are counted, then the true frequency of mortality from crushing-type predators is likely underestimated because of lost shell fragments.  Conversely, the counting of all shells with any damage may lead to overestimates.

Researchers in southern California attempt to address these questions for a population of olive shells in Del Mar, California using a method developed by another researcher2 for tropical shells.  The method uses shells drilled by moon snails Euspira spp. to establish a baseline of post-mortem damage frequency, and then this level of damage is essentially “subtracted” from other dead but undrilled shells, producing an estimate for damage caused by shell-crushing predators, in the study area represented mainly by spiny lobsters and several species of crabs.  So, a snail is drilled and killed, and later incurs environmentally induced damage3.  Other  snails are killed by, say a crushing crab, and their shells also incur a similar amount of later environmentally induced damage.  Now, subtract the first level of damage from the second, and what is left is theoretically the level of crushing damage.  The method seems a bit “iffy”, but appears to have been successfully applied in the past, and is tested again histogram showing frequency of occurrence of different types of shell damage to olive shells Callianax biplicatahere.  An example of how it works is shown in the histogram, which plots damage of several sorts to C. biplicata in a collection of 146 drilled and 849 undrilled shells. The difference between damage to drilled shells and undrilled shells for each category of damage represents the estimated crushing mortality for that category, with the largest difference being considered the “estimated minimum crushing mortality” (see histogram; why this is considered the minimum crushing mortality is not made clear by the authors). The work is statistically interesting, but readers will need to peruse the paper for themselves to assess the potential usefulness of the technique.  Stafford & Leighton 2011 Palaeogeography, Palaeoclimatology, Palaeoecology 305: 123. 

NOTE1  although this species is emphasized in the study, 3 other species are also included, including the related olive shell C. baetica and the shell-boring naticid snail Euspira lewisii

NOTE2   the researcher is Gerrat J. Vermeij of the University of California, Davis.  He is a well-known evolutionary biologist and paleontologist who has contributed much to the study of functional morphology of molluscs.  He has written many well-acclaimed books on the subjects of malacology, biogeography, and evolution

NOTE3   this type of post-mortem damage is termed taphonomic, that is, a part of the process of a shell becoming fossilised

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Research study 12
 

graph showing mortality of whelks Nucella lamellosa to sea-star predation in relation to extent of retractability into the shellgraph showing effect of sea stars Pisaster ochraceus on retractability into the shell of whelks Nucella lamellosaAlmost all snails are able to retract1 (withdraw) fully into their shells when danger threatens.  An interesting question asked by a group of researchers at Shannon Point Marine Center, Washington relates to the phenotypic plasticity of the behaviour in whelks Nucella lamellosa in response to predatory sea stars Pisaster ochraceus - specifically, whether the response is an inducible defense.  The researchers first collect whelks from 16 locations2 in the San Juan Islands group, then expose replicate groups of 6 individuals in laboratory aquaria to seawater bathing sea stars that are themselves consuming3 conspecific whelks in adjacent screened-off sections of the same aquaria.  Another equal number of treatment groups are in aquarium tanks without sea stars (termed a “control” group by the researchers).  Results after 30d show significantly greater retractability (by an average of 1mm depth, equal to 10% of the distance) in the sea-star treatment as compared with the no-sea-star treatment (see graph on Left).  When these individuals are later given to hungry sea stars as prey, significantly fewer whelks from the sea-star treatment are eaten as compared with the no-sea-star treatment, survival being directly dependent upon the degree of retraction relative to shell length (see graph on Right).  Note that a 50% retraction distance appears to offer almost complete immunity from sea-star predation.  The authors conclude that the response is an inducible defense.  Interestingly, freshly collected field individuals are even more retractable than the post-induction experimental ones and, when tested for tenacity (attachment strength to the substratum), those individuals with greatest retractability are least able to hang4 on. Thus, greater retractability comes with an unexpected cost.  Miner et al. 2013 Mar Ecol Progr Ser 493: 195.

NOTE1   retractability in this study is measured as the depth into the shell that a whelk can withdraw its operculum.  To enable comparisons to be made of “standard” retractability for whelk populations at different sites, the authors plot a regression of retraction depth against shell length for individuals from each site.  They then read off the value for a “standard” individual of 26.2mm shell length from each regression and use these baseline values for comparisons

NOTE2  all locations sampled apparently have resident populations of ochre stars P. ochraceus, but no estimates of their densities are made.  The authors know that these data could have been a useful addition to the study

NOTE3   this particular design likely introduced an additional factor to the inducibility experiment in the form of possible fright cues from the whelks being consumed.  The authors are aware of this. However, in that it would have simply enhanced the induction process, it should not have affected how the results turned out

NOTE4   this suggests that attachment points for the foot retractor muscles may actually change during induction, shifting posteriorad and providing more retraction distance, but at the cost of lengthening the distance the foot has to reach out of the shell to attach to the substratum.  Alternatively, as noted by the authors, body volume could be reduced, perhaps by assimilation of gonadal materials

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