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 snail 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

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.

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.

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

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

Black 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