Predators & defenses
  The main defenses that adult clams have against predators are refuge in burial and sturdy shells.  Neither defense is perfect, but both effectively increase the finding and handling costs to predators.  Additionally, some clams are able to sequester toxins from their phytoplankton food and other species employ various escape behaviours. 
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  Invertebrate predators
  Defenses are described for different predators.  The topic of invertebrate predators:snails & crabs is considered in this section, while INVERTEBRATE PREDATORS: OCTOPUSES & SEA STARS and VERTEBRATE PREDATORS are dealt with in other sections.
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Snails

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Research study 1
 

photograph of hard clam Mercenaria mercenaria releasing visible plume of fluorescein dye from its exhalent siphon courtesy Zimmer&Butman 2000 Biol Bull 198: 168Of all the vertebrate and invertebrate predators that hunt down clams in subtidal locations, only sea otters are thought not to use distance chemoreception. All invertebrate predators can sense bivalve prey from a distance. Zimmer & Butman 2000 Biol Bull 198: 168.

 

 

 

Odour plume released from excurrent siphon of a hard clam
Mercenaria mercenaria made visible by fluorescein dye.
Such plumes are readily discerned by invertebrate predators

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Research study 2
  A study on gaper clams Tresus capax in Humboldt Bay, California provides information on recruitment, distribution, and common predators.  Predators are mainly moon snails Polinices lewisii, Dungeness crabs Cancer magister, and ochre stars Pisaster ochraceusWendell et al. 1976 Cal Fish Game 62: 41.
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Research study 3
 

photograph of a moon snail Euspira lewisiigraph showing numbers of clams eaten daily by moon snails Euspira lewisii in study sites in British ColumbiaA common predator of west-coast burrowing clams is the moon snail, Euspira (Polinices) lewisii.  Its powerful, wedge-shaped foot is well adapted to burrowing deeply into sand.  Perhaps only geoducs Panopea abrupta burrow deeply enough to be in spatial refuge from the predator. Studies on predation on littleneck clams Protothaca staminea by moon snailsin British Columbia show that feeding is mainly in summer when surface water temperatures are warmer (see graph).  Burial depth provides little refuge for the clams as the predator can easily push through the sediments to the 15-cm maximum depth of its prey. 

photograph of a clam Saxidomus gigantea drilled by moon snail Euspira lewisiiCharacteristic features of boreholes in clams caused by moon snails is their location in the umbo region and their counter-sunk appearance (see photograph lower Left of a drilled butter clam Saxidomus gigantea2X).  While the umbo location appears to be fixed for the snail immediately after it metamorphoses, as it gets more experienced with age its ability to position the borehole becomes more precise. 

In an experiment in Ladysmith Harbour, British Columbia in which Euspira kill 286 littleneck clams Protothaca photograph of a clam-shell Protothaca staminea with locations of boreholes from predation by moon snails Euspira lewisii indicated on the shellstaminea in subtidal cages, the researchers note that 97% of the boreholes are located on the umbo (simulated dispositions shown in photo on Right). In another sample of 435 Protothaca collected from the field, 56% of the holes are located on the left shell valve and 44% on the right one. Location of the borehole on the umbo region may proximally relate to how the snail gets best grip on the prey, but ultimately may relate to the fact that beneath the umbo lies the main body mass of the clam.  This would reduce time and effort on the part of the predator to eat its prey.  Peitso et al. 1994 Can J Zool 72: 319

NOTE both gigantea and giganteus seem to be accepted species names

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Research study 4
 

Optimal strategy for a moon snail Euspira lewisii preying on a clam would be to select thinner-shelled prey over thicker-shelled ones.  As an hypothesis, this would be difficult to test, were it not for the occurrence naturally of thick- and thin-shelled morphs1 of littleneck clams Protothaca staminea.  When these are offered side-by-side to Euspira in feeding trials2 at the Bamfield Marine Sciences Centre, British Columbia thin-shelled forms are eaten over thick-shelled forms in a ratio of 35:7.  In addition to shell thickness at the umbo, the authors measure length, height, and volume (estimated by filling the shells with sand and weighing same) of the eaten clams and, also an additional measure of “inflation”.  However, of 5 variables tested (2 dimension parameters, shell thickness, volume, and inflation3), only shell thickness is found to have an effect on survival.  Actual shell thickness of drilled and eaten Protothaca is 1.07mm, as compared with 1.35mm for undrilled ones.  The results are quite surprising and lead to the obvious question:  how does the moon snail do it?  One obvious explanation, that increased handling time for thick-shelled morphs causes the predator to abandon its drilling, does not apply here because no clam is abandoned in the experiments, i.e., no partially drilled clams are found.  Is it possible that the snails can sense metabolic activity in their prey?  If so, the choice might be for more metabolically active, faster-growing prey - i.e., ones with thinner shells.  The authors discuss these ideas and others, and suggest that there may be a correlated trait other than those included in the study that acts as a tactile clue as to thickness of the prey, or perhaps behavioural differences between the 2 morphs. Grey et al. 2007 Veliger 48: 317.

photograph of littleneck clam Protothaca staminea showing umbo and hingeDEFN1  thin-shelled P. staminea occur in a lagoon area where, for whatever reason, growth is faster leading presumably to thinner shells.  Mean thickness, presumably measured at the umbo region where Euspira invariably drills, is 1.1mm for the thin-, and 1.3mm for the thick-shelled morph

DEFN2  2 prey of each morph are presented to single Euspira in each feeding trial.  A trial lasts until 2 of the 4 clams are eaten. Overall, 6 moon snails eat 42 clams, with each snail being used 3-5 times – a potential pseudoreplication bias that is noted and commented on by the authors

DEFN3  this appears to be an estimate of breadth of the shells, although it is not clear from the authors' explanation.  It is included as a variable in the multivariate analysis applied to the data, but is found to be non-significant


Littleneck clam Protothaca staminea
showing umbo, hinge, and growth lines 2X

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Crabs

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Research study 1
  photograph of crab Cancer anthonyiA 2yr study at Mugu Lagoon, California shows that principal predators of Protothaca staminea in mud habitats are crabs Cancer anthonyi and, in sand habitats, moon snails Glausaulax (Polinices) reclusianusPeterson 1982 Ecol Monogr 52: 437.
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Research study 2
 

Numerous species of crabs prey on clams in mud-, sand-, and gravelly/mud-flat areas including Telmessus cheiragonus, Cancer productus, Cancer magister, Hemigrapsus oregonensis, and H. nudus.  The crabs can sense their buried prey at a distance and are able to dig them up if not buried too deeply.  If small enough to fit between the claws, the clam is crushed outright.  Larger clams are attacked at the ventral margins where the shell photograph of chela of Dungeness crab Cancer magistervalves are thinner.  The crabs apply crushing pulses with their claws until the edges are chipped enough to insert the claws or 3rd maxillipeds to remove the soft tissues.  Boulding 1984 J Exper Mar Biol Ecol 76: 201.

NOTE  both species of Hemigrapsus are known to eat juveniles of bivalves Saxidomus giganteus, Tapes japonica (Venerupis philippinarum), and Mytilus trossulus.  Bourne & Lee 1973 Proc Natl Shellf Assoc 64: 10.


Right claw or cheliped of a Dungeness crab Cancer magister showing
the proximal dentition for crunching and the distal shell-chipping "pick" 1X

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Research study 3
 

histogram showing burial depths of 2 clam species Venerupis philippinarum and Protothaca staminea with and without their siphons cropped to shorter lengthsClams whose siphons are nipped off by fishes must bury themselves less deeply in order to maintain contact with the sediment surface.  However, at these new shallower depths are they at greater risk of being dug up and eaten by other kinds of predators?  This is tested with littleneck clams Protothaca staminea and Manila clams Venerupis philippinarum in San Juan Island, Washington in an area inhabited by Cancer spp. crabs.  First, the authors cut off 40% of the length of each clam’s siphons, consistent with the proportion of siphon missing from cropped clams in their field surveys.  After a 24-h recovery period the researchers place these experimental clams along with equal numbers of intact clams in mesh-walled enclosures on a sand flat rich in crab predators.  After 1mo, the clams are dug up, and burial depths and number surviving are recorded. 

First, burial depth is 35-50% shallower in the siphon-cropped clams (graph on Left). The effect is proportionately less for Protothaca because its siphons are twice the length of Venerupis siphons for comparable-sized individuals.

histogram showing comparative mortality of clams Venerupis philippinarum and Protothaca staminea to crabs with and without their siphons cropped
Second, mortality from crab predators is almost 2 times greater in siphon-cropped Venerupis than in siphon-intact individuals, but is not significantly different between the comparable treatment groups for Protothaca (graph on Right). The difference between the species again relates to the 2-4cm deeper depths inhabited by Protothaca, placing them in spatial refuge from the predators. 

graph showing mortallity of clams from crab predation versus burial depth of the clams
In fact, if mortality is plotted against burial depth for all 4 treatment groups a convincing relationship appears, with potential refuge against crab predation being suggested at about 4cm depth (graph on Left). Meyer & Byers 2005 Ecol Letters 8: 160.

NOTE  enclosures are 0.3 x 0.3m in size, with walls mostly buried, and with 12 controls or 12 cropped clams in each

NOTE  after 1mo some regeneration of siphons has occurred, but no cropped clam has regenerated more than 65% of the siphon mass lost by the cropping procedure

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What is the chief “take-home” message from this study? Consider the answers given below, then CLICK HERE to see explanations.

Short-siphoned clams are subject to more predation than long-siphoned ones. 

Manila clams are more tasty than littleneck clams to crabs. 

Siphon-nipped clams are more vulnerable to predators. 

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Research study 4
  graph showing preferred prey size (clams Venerupis philippinarum) of shore crabs Hemigrapsus oregonensisSince their importation to the west coast in the late 1930’s along with oyster seed Crassostrea gigas, Manila clams Venerupis philippinarum have been extensively cultured and in some areas have come to dominate commercial clam sales.  To reduce predation by crabs and other predators, culturists use nets and cages, with the latter being more successful despite their greater cost.  In the Yaquina area of Oregon, shore crabs Hemigrapsus oregonensis and Dungeness crabs Cancer magister are serious pests both under netting and within cages, especially the former because of their greater numbers and their small, hard-to-exclude, size.  Results of laboratory studies at the Hatfield Marine Science Centre, Newport, Oregon underscore the magnitude of potential predation by shore crabs on Manila clams, with a single adult Hemigrapsus oregonensis (17-22mm carapace width) consuming in excess of 6 juvenile clams per day of various sizes (see graph on Right). Based on these results the clams reach size refuge from this predator at about 17mm carapace width.  Smith & Langdon photograph of shore crab Hemigrapsus oregonensis with a shell of a manila clam Venerupis philippinarum1998 J Shellf Res 17: 223.


NOTE  comparable consumption rates for Dungeness crabs Cancer magister of similar size are also given by the authors but not reported here



Shore crab Hemigrapsus oregonensis of 17mm carapace width
with a 17mm shell of a Manila clam to illustrate relative sizes of
the two when a clam has reached its size refuge. The crab has
regurgitated some gut contents in defense of what it perceives
to be a predator (the photographer) 1.6X
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Several physical features in addition to distance of siphon-extension above the substrate surface make clams more vulnerable to being attacked and eaten.  Let's review what these are before we see the further Research Studies. Features "IMPORTANT TO VULNERABILITY" are mixed with ones "NOT IMPORTANT TO VULNERABILITY" in the list below. Try to separate them into the 2 categories, then CLICK HERE to see how you've done.

degree of inflation (how "globular" the shell-shape is)
colour
shell ridging
size
taste
shell thickness
depth of burial
armoured siphons
gape

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Research study 5
 

schematic series of photos of clams showing burial depthsAny feature of a clam that increases the handling time of an attacking predator will decrease the clam's vulnerability. A study at the Bamfield Marine Sciences Centre, British Columbia on 8 species of sympatric clams shows that deeper-burrowing clams such as horse clams Tresus capax, soft-shell clams Mya arenaria, and butter clams Saxidomus gigantea are generally larger in size and have more pronounced gaping than shallow-burrowing clams.  A larger gape will allow easier access by shell-chipping predatory crabs. These large species of clams also generally have relatively weaker shell-closing muscles than do the smaller species. Boulding 1984 J Exper Mar Biol Ecol 76: 201.

NOTE another species of deep-burrowing clam, the geoduc Panope generosa, really spills out of its shell

NOTE a trick in keeping large clams like gaper or geoducs more healthy in aquarium-holding situations, is to fasten a large rubber band around the shell

The blue-highlighted species names are ones used in Research Study 5 to follow

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Research study 6
 

photograph of 2 littleneck clams Protothaca stamineaphotograph of red-rock crab Cancer productusAnother part of Research Study 4 (above) on vulnerability of clams to predation assesses the feeding preference of red-rock crabs Cancer productus for 4 different morphological types of clams simultaneously presented to the crabs : 1) a thick-shelled butter clam Saxidomus gigantea with typical siphonal gape, 2) a thin-shelled Mya arenaria with both siphonal and pedal gape, 3) a thin-shelled variety of littleneck clam Protothaca staminea and, 4) a thick-shelled variety of the same species.  The graph showing cumulative number of various clams eaten by red-rock crabs Cancer productusstudy lasts over several days and shows that although all treatment clams are smaller than refuge size, that is, the size at which they are no longer vulnerable to being crushed by a certain predator, only Mya is killed by being crushed outright.  All other types are chipped away from the shell margins, as shown in the photo of butter clams Saxidomus gigantea shown above Left (one individual displays the typical valve-chipping pattern resulting from red-rock crab predation; the other is normal).

The graph shows that Mya is eaten most quickly by the crabs, followed by Saxidomus. Both of these are characterised by gaping.  In contrast, only one individual of the two types of littlenecks Protothaca - a thin-shelled one - is eaten. Neither type of littleneck exhibits gapiing. Boulding 1984 J Exper Mar Biol Ecol 76: 201; photo of butter clams courtesy Liz Boulding, Univ Guelph and photo of red-rock crab courtesy Iain McGaw, Univ Nevada.

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Research study 7
 

graph showing sizes of littleneck clams Protothaca staminea and Venerupis philippinarum, and varnish clams Nuttallia obscuratahistogram showing proportion of Cancer-crab diets made up of varnish and littleneck clamsWhich defense is more effective against crabs: thicker shells, narrower shape ("breadth"), or deeper burial?  Littleneck clams (Pacific or Native littleneck Protothaca staminea and Japanese littleneck clams Venerupis philippinarum) have thicker shells than varnish clams Nuttallia obscurata, but bury themselves less deeply (varnish: 30cm vs. littleneck: 10-15cm.  Additionally, littleneck clams have a more inflated shape than varnish clams, which are harder for crabs to grab graph showing degree of "profitability" of different clams to crabs Cancer spp.hold of. A more narrow (less breadth) shape is easier for the crabs.

Cafeteria-style feeding studies at the Bamfield Marine Sciences Centre, British Columbia show that littleneck and varnish clams are eaten readily by large-sized Dungeness crabs Cancer magister and red rock crabs C. productus, but with a preference for varnish clams, likely owing to their flatter more easily manipulated shape, and thinner more easily crushable shells (graph on upper Right).

In fact, when all aspects of handling, including opening and feeding times, are taken into account, it is more profitable for the crabs to eat varnish clams (graph on lower Right).  Dudas et al. 2005 J Exp Mar Biol Ecol 325: 8. 

NOTE profitability is defined here as mass of flesh eaten per unit handling time minus opening time (picking up and crushing) + feeding time

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Research study 8
 

photograph of soft-shell clam Mya arenariahistogram showing excavation success of red rock crabs Cancer productus when attacking clams Mya arenaria buried at different depths
What about refuge in depth?  Experiments conducted at the Bamfield Marine Sciences Centre, British Columbia in which adult male red rock crabs Cancer productus are allowed to prey on soft-shelled clams Mya arenaria (7cm shell length) buried at different depths show that crabs have much less success at excavating deeper clams (graph on Right).

 

A comparison of time budgets for each component of "handling" (namely, digging, breaking, and eating) shows also that the time required to excavate deep clams is almost 10 times greater than for shallow clams (graph on Left).  Once excavated, though, breaking times (orange-coloured portion of bars) and eating times (white) are similar regardless of depth.

histogram showing profitability for a red rock crab Cancer productus eating clams Mya arenaria buried at different depthsHowever, because of the extra time required to dig up deep-buried clams, overall profitability for the crabs is nearly 4 times less than when they attack shallow-buried prey (graph on Right).  Smith et al. 1999 J Exper Mar Biol Ecol 238: 185.

NOTE  defined as energy intake from the prey divided by handling time, where handling time includes digging, breaking, and eating the prey (expressed as Joules per sec). Three sizes of Mya are used in this part of the study (6, 7, and 8cm shell length), but data are averaged for 7cm shell length in the presentation shown here

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What more have we learned about clam defenses against predatory crabs from the previous Research Study 8? Consider the answers given below then CLICK HERE for explanations.

Burial depth affords an important refuge from crab predation. 

Crabs are better off if they attack and eat shallow-buried clams. 

Refuge in depth for a clam affords more defense than refuge in shell thickness. 

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Research study 9
 

histogram showing burial depths of clams Mya arenaria when exposed to different types of stimuli relating to presence of red rock crabs on the sand surface aboveAnother study at the Bamfield Marine Sciences Centre, British Columbia of softshell clams Mya arenaria shows that the clams when buried are not passive; rather, they appear to be able to assess and respond to threat of predation.  For example, laboratory studies on individuals of 4-7cm shell length show that they will increase their burial depth by over 100% over a 2-wk period when a red rock crab Cancer productus is present for part of each day or when a glass rod is used to mimic the mechanical disturbance produced by a crab’s walking.   Zaklan & Ydenberg 1997 J Exper Mar Biol Ecol 215: 1.

NOTE  the crab is positioned (in a cage) such that seawater passing over it flows into the aquarium holding the clams, thus providing chemical stimulus (2nd treatment from Right in graph). In another treatment the effluent from a crushed conspecific clam is placed in a sieve through which seawater flows when the experimental aquaria are filled each day (2nd treatment from Left in graph). Of the 5 treatments, the data from CONTROL, effluent from crushed clams, and touching glass rod to sediment form statistically distinct subgroupings; the 2 treatments involving the crab have degrees of statistical overlap with the other treatments. However, the treatment involving effluent from live crab differs significantly from CONTROL, which supports the most important of the authors' hypotheses

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Research study 10
 

Ten years after its arrival in San Francisco Bay in 1989 (and perhaps in other areas of the west coast at about the same time) the European green crab Carcinas maenas has spread into Oregon, Washington, and British Columbia.  Even as crabs go, it is an especially voracious predator - apparently much more so than equal-sized local species Cancer magister and C. productus.  It preys upon clams of several species and has become a nuisance to mariculturists, especially those rearing Manila clams Venerupis philippinarum.  Published Abstr from 2000 Ann Meeting Nat Shellf Assoc. Photo courtesy Citizen Scientist Initiative: Marine Invasive Species Monitoring Organization, Maine.


European green crab Carcinas maenas 0.25X.
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