Predators & defenses
  photograph of pediveliger larval stage of a clamLittle is known about predators of clams during their larval life. However, as with most marine invertebrates, mortality is high in clams during the several-week-long larval stage.  Open-water predators include medusae, jelly-fishes, siphonophores, chaetognaths, and filter-feeding fishes such as herrings.The tiny veliger larvae have shells, but these are thin and the larvae likely fall prey to all but the smallest planktonic predators. The subsequent developmental stage, the pediveliger (shown in the photo) has paired shells and swims with its velum or crawls with its foot. Pediveligers are most vulnerable during settling, when an array of benthic filter-feeding invertebrates such as clams, mussels, barnacles, sea squirts, anemones, and tubeworms take their fill.
 

photograph of a mink Mustela vison stealing a butter clam Saxidomus gigantea from wildlife biologist Dave Hatler
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. 


Coastal mink Mustela vison raids the larder of wildlife biologist Dave
Hatler, who provided this amusing photo. Butter clams Saxidomus
gigantea
would otherwise be inaccessible to the mink, although
their siphons might be vulnerable to being nipped off

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  Vertebrate predators
  Defenses are described for different predators.  For convenience, vertebrate predators are considered in this section in the order mammals, fishes, and birds. Invertebrate predators are found in 2 other sections: SNAILS & CRABS and OCTOPUSES & SEA STARS.
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Mammals

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Sea otters

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Research study 1
 

One tends to think of sea otters Enhydra lutris eating sea urchins, crabs, abalone, and other organisms that they wrench, pry, or pick up from the sea bottom, but in the Kodiak Archipelago, Alaska studies show that the otters preferentially dig for clams, with a special taste for butter clams Saxidomus gigantea.  As shown in the accompanying list, these Alaskan sea otters eat several other types of invertebrates. Doroff & DeGange 1994 Fish Bull 92: 704.

% frequency of occurrence of prey items eaten by sea otters:
clams (of which 98% are butter clams): 60
mussels (Mytilus spp.): 20
crabs (mostly Telmessus spp.): 3
sea urchins: 3
miscellaneous: 14

NOTE includes cockles, sea cucumbers, echinoids, snails, octopuses, sea stars, giant barnacles, chitons, and tunicates

photograph of sea otter Enhydra lutris eating a clam at the Vancouver Aquarium
Sea otter Enhydra lutris eats a clam at the Vancouver Aquarium
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Research study 2
 

drawing of sea otters diving down to dig up clams from different depths An animal forages optimally when it spends least time and energy finding its food.  Sea otters Enhydra lutris preying on clams in Elkhorn Slough, California exemplify this strategy.  Here, otters preferentially attack gaper clams Tresus nuttallii and Washington clams Saxidomus nuttallii in an area where burrow depths in a sand/silt sediment layer are restricted by a deeper well-consolidated layer of clay (20-28cm depth), over clams in a nearby population where the clay layer is covered by deeper sediments (28-40cm).  Clams comprise over 60% of the otters’ diet in this area. 

To catch a clam the otters excavate conical pits measuring up to 6m2 in area and 30-50cm deep.  Even though the clams are much smaller than ones living in the nearby deeper sediments, the otters prefer the shallow-dwelling clams because they are quicker and easier to dig up. By feeding on the shallow-dwelling population, the otters gain almost twice the clam biomass per volume of sediment excavated, thus maximising prey yield per time and effort expended.  Based on counts of prey remains in the Elkhorn Slough area during the period of their study, the authors calculate that 10 sea otters each of 25kg live mass could eat 25,000 clams over a 2-5mo period.  Although this seems a large figure, at the time of the study the otters were actually not making much of an impact on the clam populations because of the enormous density of the bivalves.  Kvitek et al. 1988 Mar Biol 98: 157.

NOTE over the 1-yr study period the sea-otter population varies from about 10-20 individuals in summer, to 0 individials in winter

NOTE an earlier report of sea otters feeding on clams in the Monterey Bay area (Moss Landing) shows that pismo clams Tivela stultorum are also eaten.  In one study, densities of large clams (>4cm shell length) are reported to decrease from 6 individuals . m-2 to 0.5 individuals . m-2 over an 18-mo period.  One individual otter is noted as eating 24 clams (average size 10cm) and 2 mole crabs in 2h 15min.  Based on dietary requirements and counts of sea otters in the area, Enhydra could eat as many as 700,000 pismo clams annually in the Monterey Bay area.  Stephenson 1977 Cal Fish Game 63: 117

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Research study 3
 

map of Alaska showing durations of occupation of different areas by sea otters graph showing edible meat mass of clams Tresus and Protothaca/MacomaA later study by the same research group on sea otters Enhydra lutris preying in soft-bottom communities in the Kodiak Archipelago, Alaska shows also that bivalves are favoured prey.  The accompanying map shows how otters have been spreading from the northeast, down the archipelago, to the southwest. At the time the study was published in 1992, otters had yet to colonise the southwesterly parts of Kodiak Island, whereas they had been present for >25y in the northeasterly parts.  Bivalve prey vary in accessibility and “meal value”, with horse clams Tresus spp. being hardest for an otter to catch, but yielding the greatest measure of edible flesh, and Protothaca staminea and Macoma spp. being easiest to catch, but providing only small flesh yields (see graph on Right). Other species are intermediate (not shown here).

 

histogram showing sizes of clams being eaten by otters in areas where the otters have been around for greater than 25yrThe effects of Kodiak otters on bivalve abundance, biomass, and size are inversely related to duration of sea-otter occupancy.  Thus, at sites long occupied by otters the only live clams are small, overlooked ones (graph on lower Right). Remains of broken shells show that the large bivalves have been eaten. 

histogram showing clam sizes eaten by sea otters in areas where the otters have only been present for 5yr or less

In comparison, at sites where otters have been present for <5y, large bivalves are being eaten, but are still abundant (graph on Left). 

Finally, at sites where otters have been around for intermediate durations (5-15y), bivalve availability and mortality records are intermediate. The otters first exploit butter clams Saxidomus giganteus in shallow-water areas (<10m), then switch to Macoma spp. in deep areas (>20m).  Few Tresus shells are found, indicating that they may exist in partial “burrow-depth” refuge. 

The otters influence soft-bottom community structure in ways other than directly by predation.  For example, sea stars Pycnopodia helianthoides are attracted to otter foraging pits, where they feed on small clams overlooked by the otters.  Also, clam shells discarded by the otters are colonised by sea anemones and kelp.  Kvitek et al. 1992 Ecology 73: 413.

NOTE  similar results to those reported here for the Kodiak Archipelago are reported by the same research group in a companion study in the Alaska Panhandle.  Numbers of butter clams S. giganteus and sea urchins Strongylocentrotus spp. are dramatically reduced in the presence of sea otters.  In this area, as in the Kodiak Archipelago, deep-burrowing bivalve species such as horse clams Tresus capax and geoducs Panope generosa are rarely attacked, even at sites where they account for most of the available prey biomass.  Kvitek & Oliver 1992 Mar Ecol Progr Ser 82: 103.

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Research study 4
 

photograph of butter clam Saxidomus gigantea
Studies in Kodiak Island, Alaska show that clams are not completely defenseless against sea-otter depredation.  When fed ad libitum in the lab on butter clams Saxidomus gigantea containing low levels of saxitoxin, then switched to clams containing 6 times higher levels, the otters either reduce their consumption of clams or continue to eat them, but discard the highly toxic siphons and kidneys.  When switched back to the low-toxicity clams, feeding rates return to pre-treatment levels, but the otters continue to discard siphons.  The results suggest that sea otters are susceptible to paralytic shellfish-poisoning toxins (PSPTs) and that they have the ability to detect and avoid consumption of lethal amounts of the toxin.  A comparison of sea-otter distributions with those of toxic butter clams in southeastern Alaska suggests that the sequestering of PSPs may protect some populations from sea otters and restrict the otters’ distribution to outer coast locations where toxic clams are rare.  Kvitek 1988 Am Zool 28: 188A; Kvitek et al. 1991 Limnol & Oceanogr 36: 393; for a summary of PSP contents of clams in Puget Sound, Washington see Trainer et al. 2003 J Shellf Res 22: 213.

NOTE  a common type of paralytic shellfish toxin in clams derived from consumption of saxitoxin-containing dinoflagellates Protogonyaulax spp. (related to toxic red tides));  see Costa et al. 2009 Toxicon 54: 313 for a comparison of the efficacy of 5 different methods for determining PSPs in several different shellfish species in the Aleutian Islands, Alaska

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Research study 5
 

diagram showing dive depths of sea otters when foragingdiet makeup of sea otters in Prince William Sound, AlaskaResearchers from Texas A & M University provide data on food resources of sea otters Enhydra lutris kenyoni in Prince William Sound, Alaska.  The area studied has a soft, mixed-sediment bottom with an average depth of 30m.  During 1816 dives observed, most of them occurring in shallow water (<15m) and lasting 4min or less, prey items identified include clams (75%), mussels (9%), crabs (6%), and others (10%).  Males, females, and juveniles appear to eat the same types and proportions of foods.  Wolt et al. 2012 Mammalian Biol 77: 271.

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Walruses

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Research study 1
 

drawing of walrus engaged in hydraulic jettingWalruses Odobenus rosmarus root up infaunal invertebrate prey by snuffling with their snouts (see drawing top Left).  Deeper-burrowing species such as geoducs Panope generosa are excavated by hydraulic jetting, a behaviour that liquifies the bottom sediments down the burrow track until the clam’s body is exposed (see photograph bottom Left).  Traces of these behaviours in the form of sea-floor furrows and pits have long been known from the Bering Sea, photograph of sedimentary rock outcropping in Willapa Bay, Washington showing fossil evidence of hydraulilc jetting by walrusesbut not until recently have they been identified in Pleistocene sedimentary rocks near Willapa Bay, Washington (see photographs on Right).  Researchers primarily from the University of Calgary, Alberta present geological evidence of photograph in situ of a walrus feeding in the sea-bottom sedimentspast walrus feeding on clams that are the first such examples known from the geologic record (see photograph on Right).  Gingras et al. 2007 Palaios 22 (5): 539. Award-winning photograph of walrus in feeding plume courtesy Goren Ehlme, Sweden; other photos and drawing courtesy authors.

same photograph of sedimentary rock outcropping in Willapa Bay, Washington with jetting depressions highlightedNOTE  an epoch in the Quaternary Period beginning about 2.6mya and lasting until about 12,000ya when the last glaciation cycle finished

Top: walrus Odobenus rosmarus engaged in hydraulic
jetting, with excavation cavity highlighted;
Bottom: in situ photograph of
walrus face appearing out of feeding plume

 

Top: face of sedimentary-rock outcropping with
fossilised excavations indicated with arrows;
Bottom: same photo with coloured overlays
showing exact locations of fossilised
depressions in relation to a few clam burrows

 

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Bears

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Research study 1
 

photograph of a brown bear Ursus arctosFemale brown bears Ursus arctos (especially ones with cubs) in southwestern Alaska appear to forage routinely for clams, including soft-shelled Mya arenaria and Pacific razor clams Siliqua patula.  Harvest rate by an adult bear in sand-beach areas of Katmai National Park, Alaska is just less than 1 clam per minute, with 77% of the prey being soft-shelled and 18% being razor clams.  The authors note that the 2 prey species are not consumed together during a single foraging session, perhaps because of the different zones of the beach that they inhabit.  Interestingly, a bear’s success per attempt at excavating the 2 clam species does not differ significantly (59 vs. 66% for soft-shell and razor clam, respectively), which is somewhat surprising in view of the ease that a human would have in digging the former species over the latter, with a shovel.  Smith & Partridge 2004 J Wildlife Managem 68 (2): 233. Photograph courtesy Peter Romanow, Moscow BioLib.


Related Urus arctos in Kamtchatka, Russia
after a good day at the beach

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Fishes

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Research study 1
 

photograph of gaper clam Tresus sp. showing protective plates on the siphon tipA clam’s siphons, to varying extents, extend in full view of fishes passing by.  A few species, such as the gaper clam Tresus capax, have armoured plates on their siphons, perhaps as defense against siphon-nipping predators.  Studies in Humboldt Bay, California show that over 50 species of sessile invertebrates and plants commonly grow on the plates, including tubeworms, hydroids, protists, mussels, barnacles, and various algae, suggesting an element of camouflage as well as physical defense for the function of the plates.  Stout 1970 The Veliger 13: 67.

 

Protruding siphons of a gaper clam Tresus sp. The siphons
are mostly closed in this photo. The armoured plates
are the leathery-looking surfaces on either side 0.8X

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Research study 2
  photo composite of a sculpin studying the burrow holes of a clam
With siphons that lie in open sight of predators, it is no surprise that siphon-nipping or siphon-cropping is fairly common, especially in areas co-inhabited by sculpins, sole, flounder, and certain halibut species.  In that cropping is usually not fatal to the bivalve, the process can be likened to a terrestrial herbivore grazing on grass.  Studies on littleneck clams Protothaca staminea in California show that growth within cages that exclude the fish-croppers may be more than double that without, so croppers can have large effects on productivity of bivalves.
  Peterson & Quammen 1982 J Exp Mar Biol Ecol 63: 249.
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Why is growth of Protothaca less in the presence of the siphon-cropping fishes?  Although regeneration costs would seem to be the obvious answer, there is at least one other good explanation?  Think of what it could be, then CLICK HERE for the answer.

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Research study 3
 

Photo of Leptocottus armatus  courtesy ARS Consulting, California
Clam siphons may comprise up to 25-30% of the stomach contents of staghorn sculpins Leptocottus armatus and great sculpins Myoxocephalus polyacanthocephalus. Meyer & Byers 2005 Ecol Letters 8: 160.

Photo of Leptocottus armatus
courtesy ARS Consulting, California

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Research study 4
 

Several species of bivalves, most notably the butter clam Saxidomus giganteus, sequester saxitoxin from their phytoplankton foods.  Studies on butter clams from Puget Sound show that their nerve cells are up to 100X more resistant to the toxin than nerve cells of other bivalves living in the same habitat (Mya arenaria, M. truncata, Tresus capax, Protothaca staminea).  Butter clams sequester the toxin mainly in the epithelium of the siphons (60-80%, mostly in the distal region) and the toxin may persist for more than a year following the seasonal disappearance of the dinoflagellate from the plankton.  In most other bivalves, such as littlenecks Protothaca staminea, the toxins, if present at all, are restricted to the digestive gland or gills and are lost in a few weeks. The resistance of Saxidomus's own nerves to the toxin seems to be innate, rather than one acquired with increased exposure.  Kvitek & Beitler 1991 Mar Ecol Progr Ser 69: 47.

Chemical formula for saxitoxin from a clamNOTE  the toxin, also known as Paralytic Shellfish Toxin (PST) or Paralytic Shellfish Poison (PSP) is a potent Na-channel blocker, and is derived from a dinoflagellate Protogonyaulax consumed as food by the clams. The "saxi" part of the name comes from its original description in butter clams Saxidomus

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Research study 5
 
Let’s look at how behaviour of siphon-nipping sculpins Leptocottus armatus is affected by consuming toxic siphons of butter clams a different times during a 56d study period.  The experiment starts with young-of-year fishes with no previous exposure to PST-containing clams (i.e., toxic clams). Each experiment is accompanied by a histogram that shows the results obtained: Photograph of a sculpin showing its relatively large mouth
The fishes are given toxin-free strips of siphon to eat. All pieces are eaten: The fishes are then given toxic strips of siphon. Half of what is eaten is regurgitated. The fishes that swallow the strips jerk, flop, lie upside-down or on their sides, but all recover: After a few days the fishes are given toxic strips once again. None is consumed. The fishes spit out what they take in their mouths: The next time the fishes are given toxin-free strips, 100% are eaten:
First in a series of histograms describing results of experiments to show effects of saxitoxin on behaviour of a sculpin              <td><img src="../../IMAGES/CLAM/Kvitek1.gif" alt="Second in a series of histograms describing results of experiments to show effects of saxitoxin on behaviour of a sculpin              <td><img src="../../IMAGES/CLAM/Kvitek1.gif" alt="Third in a series of histograms describing results of experiments to show effects of saxitoxin on behaviour of a sculpin              <td><img src="../../IMAGES/CLAM/Kvitek1.gif" alt="Fourth in a series of histograms describing results of experiments to show effects of saxitoxin on behaviour of a sculpin

The author concludes that sculpins can develop aversion to siphons from toxic clams after a single feeding, whether they actually consume the flesh or not. Because the toxic siphons are often rejected on contact or, if taken in are spit out, experienced fishes would cause little or no siphon damage to clams bearing the toxin.  The toxin thus acts as an effective deterrent to siphon-nipping behaviour in these sculpins. Kvitek 1991 Mar Biol 111: 369.

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Research study 6
 

histogram showing height of siphon extension in butter clams Saxidomus gigantea: effects of light and presence or absence of saxitoxin in the siphonsLaboratory studies at Moss Landing Marine Laboratories, California on butter clams Saxidomus giganteus collected in Washington state show that siphons are extended more at night than during the day (see graph). This should be a useful strategy to minimise loss to siphon-nipping fishes that hunt by sight.  Moreover, during nighttime, clams that are toxic actually extend their siphons higher than do non-toxic ones, in so doing possibly increasing their access to richer food-bearing regions of the boundary layer.  Sequestration of toxin, then, may provide double benefit: protection from siphon-nipping fishes and access to richer food resources.  Kvitek 1991 Mar Biol 111: 369.

NOTE  the difference between the heights of siphon extension is only 3mm, which begs the question as to whether such a small distance could be biologically significant to a clam in terms of increasing its access to more phytoplankton-rich water layers.  No-one knows the answer to this because it hasn’t been measured.  However, it may help to know that non-toxic clams extend their siphons 3mm above the substrate surface, while toxic ones extend theirs 6mm above the substrate surface…in other words, twice the distance

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How can this be explained?  How can the clams or their siphons “know” that it is safe to extend further out of the burrows? Study the answers provided, then CLICK HERE for explanations.

A chemical-recognition feedback system in the clam has evolved that sends a signal to the clam to extend its siphons further when toxins are present. 

The toxin is a neuro-inhibitor and relaxes the siphons so that they float our further. 

Toxic siphons are bothered less by potential predators and, hence, extend more. 

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Birds

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Research study 1
  photograph of glaucous-winged gull Larus glaucescensGlaucous-winged gulls Larus glaucescens prey upon a variety of intertidal organisms including, when they can get them, butter clams Saxidomus gigantea.  One often sees gulls fluttering upwards from a beach and dropping prey to crack them open.  Butter clams are too deep for most gulls to attack but, when supplied with a ready pile by scientists at Friday Harbor Laboratories, Washington, this is what the birds do.  Most (82%) clam-drops are made from the bird's standing height and only 18% from flying height, which seems to be a less-than-useful strategy.  Flying drops are made from 0.5-15m in height, averaging 4-7m.  From this height, on a hard substratum, it takes 1-2 drops to break an average-sized Saxidomus.  The height is probably a compromise between ensuring reasonable probability of breakage, and not incurring too great a risk of losing the prey to other gulls scavenging below.  Barash et al. 1975 The Wilson Bulletin 87: 60.
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Research study 2
 

Photograph of several seagulls flying aboutThere is evidence in the foregoing study that “clam-slamming” efficacy of gulls Larus glaucescens increases with age.  Young-of-the-year gulls tend to make most of their drops (78%) onto soft substrata instead of onto rocks, fumble their prey more during flying, and make more extra-low drops (<1m) and more extra-high drops (>10m) than do adults.  The results of these extra-high drops are mostly scavenged by birds on the ground.  By adulthood, the gulls are making most of their drops (81%) onto rocks and are much more adept at handling the clams during flight.  Barash et al. 1975 The Wilson Bulletin 87: 60.

 

Mixed species of seagulls, including some juveniles

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Research study 2a
 

graph comparing susceptibility of clams Saxidomus nuttallii to being dropped from different heights by Western gullsIn Bodega Bay, California 100% of adult Western Gulls Larus occidentalis drop clams to break them, while only 55% of immature gulls do so.  Flight distance and theft by other birds are important determinants of drop location.  Adults drop heavy clams Saxidomus nuttalli from lower heights (5-10m) than they drop light clams (15-20m) but, when dropped from the same height, the heavy ones tend to break less easily than the light ones (see graph).  Why not drop the heavy ones from a greater height?  The explanation may be in thievery by other gulls, which may occur more often during the longer time taken for retrieval.  Loss of a small clam to thieving conspecifics is less costly than loss of a large one.  The author agrees with other researchers (Research Studies 1 & 2 above) that clam-dropping is a learned behaviour, although in the present study the gulls do not learn to drop all their prey onto the hard surface of a nearby parking lot – only the heaviest clams receive this treatment.  Maron 1982 The Auk 99: 565.

NOTE  for some reason the author has eye-fitted 2 sigmoidal-shaped curves to the data but, judging by the variability shown in the data (no statistical tests are done on the 2 populations of data), a single linear relationship would probably be more appropriate

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Research study 3
 

photograph of broken shells of littleneck clams Protothaca staminea eaten by seagulls
In areas of northern Washington glaucous-winged gulls Larus glaucescens forage on a variety of clams including cockles Clinocardium nuttallii and littlenecks Protothaca staminea, and willingly eat catches of clam-diggers including butter clams Saxidomus gigantea and horse clams Tresus spp.  Kvitek 1991 The Auk 108: 381.

 

 

 

Bird-smashed litteneck clams Protothaca staminea 0.7X

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Research study 4
 

Studies on possible chemical defense of butter clams Saxidomus gigantea by their incorporation of Paralytic Shellfish Poison in their siphons in Tatoosh and Whidby Islands, Washington show that both chick and adult seagulls quickly learn to reject PSP-contaminated clams.  If contaminated flesh is swallowed by chicks, it is regurgitated within 5min and the chick avoids other toxic clams but readily eats non-toxic ones.  If an adult gull eats a toxic clam, it invariably regurgitates with no evident symptoms, and then discards the toxic siphons; however, if an adult eats a non-toxic clam, the siphons are never discarded. Kvitek 1991 The Auk 108: 381.

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Research study 5
 

In San Pablo Bay near San Francisco, California different species of scaups Aythya spp. feed on clams Potamocorbula amurensis and Macoma balthicaPoulton et al. 2002 The Condor 104: 518.

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Research study 6
 

photograph of northwestern crow Corvus caurinus courtesy For The Birds websiteHow much decision-making actually goes on in a crow when selecting a particular shellfish to eat?  This is investigated by researchers in Victoria, British Columbia by offering northwestern crows Corvus caurinus different sizes of Manila clams Venerupis philippinarum and whelks Nucella lamellosa in paired tests.  In some tests the clams are smaller than the whelks, in others they are about equal size, and in others, larger.  Both prey types are handled in the same way, by flying up and dropping them to crack the shell.  Results show that despite the clams being always the more energetically profitable options (profitability values for clams range from about 300-2000J/sec, as compared with about 200-270J/sec for whelks), the crows consistently select the larger (heavier) of the 2 choices.  The authors conclude that when feeding on hard-shelled prey requiring the same or similar handling techniques, the crows automatically choose the heavier items.  The crows know that heavier items require fewer drops to break open and energetic return is greatest.  What they appear not to be able to assess is that per unit mass, clams provide the better return because of their more favourable shell to flesh ratio.  The crows appear not to be learning from their post-ingestive experiences.  O’Brien et al. 2005 Ethology 111: 77. Photo courtesy FOR THE BIRDS.

NOTE  handling is divided into 3 components of flying, standing, and walking while consuming a prey item, with each activity being converted into Joules of energy from published values, and summed to determine total energetic cost.   This value is subtracted from edible flesh yield in Joules, also determined from the published literature, to give net energy gain from a particular prey item.  This last value, divided by the total time in sec that a crow spends handling that prey, gives a value for profitability for a particular prey item in J/sec

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