Predators
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Research study 1
 

Vertebrates, such as sea otters, minks, birds, black bears, raccoons, various shore birds, and fishes, are the principal causes of mortality in adult crabs.  Some of these are considered in Research Studies below. Invertebrate predators of adult crabs are most commonly other crabs and octopuses, and these are dealt with in Research Studies on the specific predator-type involved (e.g., octopuses will be found under topics of feeding/foods in their own section in the ODYSSEY.  Human "predators" have probably been gathering and eating crabs shortly after settling near west-coast beaches.  A study of household middens in a late-Holocene village in Netarts Bay on the northern Oregon coast shows that Dungeness crabs Cancer magister were a common dietary item.  From the generally small sizes and young ages of remains found, the authors surmise that they were likely gathered in shallow subtidal areas, perhaps along with collections of cockles.  Losey et al. 2004 J Archaeological Sci 31: 1603.

DEFENSES include a hard exoskeleton, biting claws, limb autotomy, hiding away and, for hermit crabs, occupation of sphotograph of spider crab being eaten by a sea anemone Anthopleura elegantissimanail shells. 

NOTE  except possibly for Dungeness crabs Cancer magister, disease as a cause of mortality in crabs is not well studied.  Reports of “mass mortalities” of Dungeness crabs Cancer magister from disease are often misidentification of normal springtime moultings.  The first report of mass mortality of this species is a large stranding of dead crabs at Grayland, Washington in April, 1979.  The authors could not identify its cause.  Stevens & Armstrong 1981 Fish Bull 79: 349.

NOTE  the authors add that during the year of the study, 1971, ten metric tonnes of Dungeness crabs were landed in commercial fisheries in the Bay


This spider crab, perhaps Pugettia sp., caught by a great green anemone
Anthopleura xanthogrammica, may be an example of opportunism, or
perhaps the crab is already dead or just a moult 2X

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Research study 2
 

Heavy-shelled invertebrates such as molluscs abundantly fossilise, but relatively lightly calcified invertebrates such as crabs only rarely do so.  Why this should be is investigated for the shore crab Hemigrapsus oregonensis by a researcher at Friday Harbor Laboratories, Washington.  photograph of moult of shore crab Hemigrapsus oregonensisComparison of H. oregonensis remains with various molluscan remains at different depths in a tidepool at False Bay shows the former to be heavily altered (bioerosion, dissolution, and encrustation), and the latter to be virtually unaltered.  As for crabs, carapace and chelipeds are more commonly represented than leg remains.  In general, crab remains (5%) are much less common than molluscan/barnacle remains (95%). The author estimates the average survival time of remains of H. oregonensis in the Bay at just a few weeks.  It appears, then, that the scarcity of fossil crabs in comparison with more heavily calcified taxa owes to the fact that they do not preserve as well.  Stempien 2005 Palaios 20: 400.

NOTE  crustacean cuticle is primarily composed of chitin and proteins (over 90%) and is only lightly calcified.  Although strong in life, after death the chitin rapidly hydrolyses and is consumed by various fungi and bacteria


This cast-off moult ofHemigrapsus oregonensis
has little chance of taphonomic preservation,
that is, of becoming a fossil 2X

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Fishes

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Research study 1
 

At least 12 species of fishes in kelp forests in Pacific Grove, California are known to eat spider crabs (5 species). Hines 1982 Ecol Monogr 52: 179.

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Research study 2
 

photograph of pinnortherid crab Pinnixa sp.A brief note in a fisheries journal describes young-of-the-year copper rockfish Sebastes caurinus in Humboldt Bay, California preying on crabs Pinnixa faba and, to a lesser extent, on P. littoralis.  As the crabs are parasitic in the mantle cavities of horse clams Tresus capax and other bivalves such as cockles, sea mussels, and varnish clams, the young fishes must capture the crabs as they venture to the openings of the siphons or to the edges of the mantle cavity, depending on which host species it is and the depth to which it buries.  This topic is worthy of further study. Prince 1975 Trans Am Fish Soc 104 (3): 539.

 

Female Pinnixa sp. crawling among the ctenidial
filaments of a sea mussel Mytilus californianus 3X

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Research study 3
 

Sculpins have wider relative mouth-gapes than other fish predators in estuaries and consequently consume a larger relative size of prey.  Studies in Grays Harbor, Washington show that the commonest size of Dungeness crabs Cancer magister consumed by sculpins Leptocottus armatus is 6-11mm carapace width. This represents about half the mean mouth gape of 482 fishes sampled (a fish of mean length 118mm has a mouth gape of about 19mm; see graph).  Perhaps some of the discrepancy is made up by the extra dimension of the chelipeds - usually extended defensively by the crab.  Based on these ratios, a size refuge for Cancer in Grays Harbor against predation by sculpins would be about 15mm; in other words, at a small juvenile size. Armstrong et al. 1995 Fish Bull 93: 456. Photo courtesy Oregon Dept Fish & Wildlife - Mar Res Progr, Newport, Oregon.

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Sea otters

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Research study 1
 

In the Santa Cruz region of California, crabs rank high on the list of preferred foods of sea otters Enhydra lutris nereis.  This could be for reasons of relatively large size, accessibility, tastiness, or nutritional content.  Another possibility is that crabs, especially large Cancer spp. (e.g., Dungeness and red rock crabs), are the prey type that most easily fulfils the energy needs of an adult sea otter. 

Let’s see how this would work.  Let’s say you’ve gone out and observed sea otters hunting and eating various prey.  An otter dives down, brings up a prey, and eats it at the surface.  It repeats this enough times to meet its daily energy expenditure of 28,000 Kj.  Later, you determine the amount of energy taken in by an otter for each type of prey item. The results, shown in the Table on the Right, indicate that Cancer crabs (representing several species), were they to be the sole prey caught and eaten, provide the best energy gain for time expended. Abalones are actually the hardest of the prey listed for a sea otter to catch. For example, while other prey types, such as Cancer crabs, yield one item per dive, it takes an average of 6 dives to produce a single abalone. Data from Ostfeld 1982 Oecologia 53: 170 adapted from Costa 1978 PhD Dissertation, U Calif, Santa Cruz.

NOTE because diets of sea otters are often heavily biased to a single prey species (up to 85% of a diet according to some reports), some scientists believe that their diet is not nutrient-limited but, rather, energy-limited.  Even though we know little about the nutrient requirements of sea otters and therefore little about how much of their requirements could be met from an extra 15% diversity of prey items in a diet, it is a fair assumption that energy intake is going to be an important factor in prey selection by a sea otter

NOTE  Kj = abbrev. for 1000 joules.  Formerly, energy requirements of animals were expressed in the more familiar term Calorie, equivalent to 1 Kcal.  However, because calories are a measure of heat energy, it is more appropriate to use joules, which are a measure of work energy.  1 Cal is equivalent to 4.187 Kj

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Research study 2
 

In Prince William Sound, Alaska sea otters Enhydra lutris prey on several large invertebrates including Dungeness crabs Cancer magister, horse crabs Telmessus cheiragonus, clams (several species), echiuroids Echiurus echiurus, and mussels Mytilus trossulus.  The authors monitor feeding activities of the otters using visual tallies combined with radiotelemetry data to determine, among other things, the mean rates of energy consumption on different prey types.  By use of telemetry data the researchers are able to monitor feeding activities of the otters over 24-h periods, rather than restricting observations to daylight as in most other similar studies. The data, shown here for 3 localities (see table on Right), show that crabs are the most energetically profitable food type, even though clams represent the most numerically dominant prey item in the otters' diets (clams: 75%, crabs: 3-13%, echiuroids: 5%, and mussels: 6%). In areas where otters have been present for a long time (>30yr) Dungeness crabs are virtually absent.  Garshelis et al. 1986 J Wildl Managem 50: 637.

NOTE  “profitability” is assessed by dividing the estimated energy content of food items obtained on a dive by the time taken to capture and consume it.  Clams consumed by the otters are small; hence, providing relatively less energy per unit effort

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Birds

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Research study 0
 

Researchers at the Point Reyes Bird Observatory, California provide data on summer diets of long-billed curlews Numenius americanus in Bolinas Lagoon north of San Francisco.  From 205 observations on feeding behaviour and examination of 30 regurgitated pellets, the authors photograph of long-billed curlew Numenius americanusdetermine that during autumn the birds are eating mainly crustaceans, with shore crabs Hemigrapsus oregonensis being most common (55-97% of observations), along with lesser numbers of mud and ghost shrimps.  The crabs are caught mainly by probing the burrows with the extra-long beak or, less often, with the beak just immersed, by sensing vibrations from the crab moving in its burrow and then catching the crab as it emerges.  The former method is used about twice as often as the latter, with up to 10 attempts being made per minute both methods combined.  Stenzel et al. 1976 The Wilson Bulletin, 88 (2): 314. Photograph courtesy Tom Grey, Palo Alto, California tgreybirds.

NOTE  the researchers also provide data for willets Catoptrophorus semipalmatus but, as their prey is much more diverse than that for curlews, it is not reported here

Long-billed curlew Numenius americanus
capturing a mole crab Emerita analoga

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photograph of a curlew Numenius americanus taken from a video

CLICK HERE to see a video of a long-billed curlew Numenius americanus drinking from a puddle, perhaps after eating a crustacean.

NOTE  the video replays automatically

 
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Research study 1
 

A group of California researchers report that summer diets of long-billed curlews Numenius americanus in the Elk River Estuary, California (Humboldt Bay) comprise mainly shore crabs Hemigrapsus oregonensis (about 30% by number).  Interestingly, out of 4805 prey captures in total over 3 seasons, the authors report 80 (2%) attempts of kleptoparasitism (food-stealing) mostly by gulls (Larus spp.).  Fifty-seven of these attempts are successful, but the authors do not comment on whether these numbers are of expected magnitudes.  A majority of the successful stealings are of worms, perhaps because they are larger than other prey types.   Leeman et al. 2001 The Wilson Bulletin 113 (2): 194. 

NOTE  other prey include bivalves (26%), worms (15%), fish (10%), and shrimp (4

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Research study 1.1
 

Although probably a common occurrence in nature, published accounts of bird predation on crabs are rare.  However, researchers from the University of Montana provide an account of ravens Corvus corax preying on sand crabs Emerita analoga on Netarts Spit, Oregon in mid-August.  Three ravens are photograph of raven Corvus corax courtesy My Audio Schoolseen to extract 4 crabs from the upper margin of the swash zone in 5 attempts during a 5-min period.  All prey are gravid females, and the egg clusters appear to be the focus of the ravens’ depredations.  Whether the ravens know to look for gravid mole crabs on that particular beach during late-summer breeding aggregations, and the means by which they located the buried prey are, of course, not known.  The authors note that theirs if the first description of the method used by ravens to hunt live crabs of any species.  Hendricks & Henricks 2011 Wilson J Ornithol 123 (2): 409. Photograph of raven courtesy My Audio School; photographs below of Emerita analoga courtesy the authors.

NOTE  a paper published on predators of sand crabs E. analoga in Peru notes that algal-fouled (mainly Enteromorpha spp.) individuals are preferentially preyed upon by several species of gullsOn one study beach near Ancón Bay, densities of Emerita in the lower swash zone vary seasonally from about 800-2000 . m-2, with about 1% of these being fouled.  The fouling makes the host visually obvious to the birds (see photos), most notably, Larus modestus, and this species alone can account for up to 35% mortality of the fouled crabs per day.  In comparison, the same gull species eats only about 0.1% of unfouled crabs per day.  No similar information on fouling-induced mortality is known for North American sand-crab populations and this might be something to look into. Hidalgo et al. 2010 Helgol Mar Res 2010 64: 367


Common raven Corvus corax

 
photograph of gravid Emerita analoga courtesy Paul Hendricks


Gravid Emerita
analoga
as eaten by ravens in Oregon

photographs of algal-fouled sand crabs Emerita analoga, courtesy Hidalgo et al. 2010 Helgol Mar Res 64: 367.

Enteromorpha-
fouled E. analoga as
eaten by gulls in Peru.
Left: crab fouled at
anterior end; Right:
similar crab buried

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Research study 2
 

graph showing relationship between parasite infestation in sand crabs and shorebird prevalenceThe research to be described doesn’t directly deal with predation by birds, but is it possible that the extent of parasitic infestation in sand crabs Emerita analoga might relate to the population density of certain shorebird and gull species that  are the definitive hosts of these parasites?  Investigation of birds and Emerita at 8 sites between Bodega Bay and San Diego, Caifornia shows, indeed, a strong positive association between bird abundance and prevalence of parasitic infection for trematodes and acanthocephalans (see sample graph for trematodes), but not for nematodes and trypanorhynchs, which use elasmobranches as their definitive hosts.  Further research on this topic may be justified.  Smith 2007 J Parasitol 93 (2): 265.

NOTE  the parasites in question are a trematode Spelotrema nicolli, an acanthocephalan Polymorphus kenti, a nematode Proletpus sp., and an unidentified trypanorhynch tapeworm

NOTE  most common are Western and California gulls (56% of the total bird population), and sanderlings, marbled gotwits, and willets (44%)

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Bears

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Research study 1
 

photograph of black-bear cub Ursus americanus on a beach at Tofino, British ColumbiaBlack bears Ursus americanus are commonly seen on the shores around the Bamfield Marine Sciences Centre, British Columbia. They sometimes engage themselves in turning rocks and eating the crabs beneath. On one occasion, from the vantage point of a boat, we came upon a cub of perhaps 80kg feeding in this way. The cub would raise a rock with one paw, then shovel crabs into its mouth with the other. After the cub moved away, we investigated the feeding area and found the crabs to be mostly Hemigrapsus spp. and Petrolisthes cinctipes. The last rock to be lifted by the cub with a single paw took 3 researchers with all their strength to move it back into place. Photograph courtesy Tony Costa eTravelPhotos.

 

 

Black-bear cub Ursus americanus perhaps
looking for porcelain crabs to eat

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