Defenses
 

Defenses are considered in this section, while PREDATORS are dealt with in another section. Defense by claws may be such an obvious defense that no west-coast studies have been done specifically on it; however, information on MECHANICS OF CRUSHING is extensive and is dealt with in its own section.





Another possible defense in crabs that no-one seems to
have worked on is the regurgitation of gut juices in response
to stress, as shown here for a shore crab Hemigrapsus
nudus.
To humans, the juices have an unpleasant taste 0.5X

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  Hide away
  Defenses of crabs include hide away, considered here, and LIMB AUTOTOMY and CAMOUFLAGE, considered elsewhere.
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Research study 1
 

photograph of hermit crab agurus granosimanus
Hermit crabs in San Juan Islands, Washington generally find shells in short supply and therefore commonly occupy shells that are smaller than preferred.  This may have lethal consequences when faced with a predator.  For example, laboratory tests with a predatory crab Cancer gracilis and 2 specimens of Pagurus granosimanus, identical save for size of Nucella emarginata shell occupied, show that the crab will select the smaller-shelled Pagurus in 15 of 16 trials.  The results strongly suggest that large shell size confers a selective advantage on the occupying crab.  Vance 1972 Ecology 53: 1075.

NOTE  because this species lives subtidally, its distribution does not normally overlap that of P. granosimanus.  However, when presented with the hermit crab, C. gracilis readily eats it

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Research study 2
 

photograph of hermit crab Pagurus hirsutiusculus courtesy Ron Long, SFU, Burnaby, B.C.In the Coyote Point Park area of San Francisco Bay, California, hermit crabs Pagurus hirsutiusculus occupy snail shells in the following proportion (230 crabs examined):

200 in mud snails Ilyanassa obsoleta
26   in oyster drill Urosalpinx cinerea
1     in channeled whelk Buscotypus canaliculatus
3     in winkle Littorina scutulata
1     in the dactylus of a shed chela of Cancer sp.

The author notes that almost all shells are species introduced into the San Francisco Bay region from the Atlantic coast in the early 1920s. The author also notes that these species were rare in the area during 1970-76.  Clearly, the hermit crab has been able to expand its range and degree of protection by exploiting non-indigenous snail shells.  Wicksten 1977 Veliger 19: 445. Photo courtesy Ron Long, SFU, Burnaby, B.C.

NOTE  the shell being used by the crab in the photograph is Nucella ostrina

NOTE  as to what shells P. hirsutiusculus used prior to this, the author draws reference to an early publication which refers to them using whelks Nucella lamellosa and Nassarius mendicus, the latter an inhabitant of both hard and soft substrata: Schmitt 1921 Univ Cal Publ Zool 23: 1

 
Research study 3
 

photograph of Dungeness crab Cancer magister buried
Intentional burial by a Cancer magister at a shallow depth for protection works well, as it can easily extricate itself.  However, accidental burial, such as by sand movement in storms, may be lethal if the crab becomes too deeply buried.  Studies at the West Vancouver Laboratory, DFO, British Columbia show that 100% of crabs (12-29cm carapace width) experimentally buried in sand to 10cm depth or less can reach the surface in 24h.  However, if buried to 20cm then 0% of crabs reach the surface in 24h.  Chang & Levings 1978 Estuar Coast Mar Sci 7: 409.

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  photograph of Dungeness crab Cancer magister with encrusting barnacles on its carapace courtesy Dave Cowles, Walla Walla University, Washington You now know enough to explain the accompanying photograph. It is a mature Dungeness crab Cancer magister with a growth of barnacles on the anterior part of its carapace. These are clues to 2 aspects of the crab's behaviour that should become obvious with a bit of thinking. CLICK HERE to confirm your ideas. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.
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Research study 4
 

photograph of juvenile king crabs Paralithodes camtschaticus in association with seastars Evasterias troschelii courtesy C. Braxton Dew, Nat Mar Fish Serv, Alaska
In areas around Kodiak, Alaska juvenile king crabs Paralithodes camtschaticus may be found in association with seastars Evasterias troschelii. The crabs, especially when small, nestle within the spaces between the arms, a behaviour that may provide them with protection. Dew 1990 Can J Fish Aquat Sci 47: 1944. Photo courtesy C. Braxton Dew, Nat Mar Fish Serv, Alaska.

 

 

 

 

 

 

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Research study 5
 

diagram of locomotory movements of limbs of a mole crab showing direction and type of movementphotograph of mole crab Emerita analoga half-buried in sandWhile foraging, Pacific mole or sand crabs Emerita analoga bury in the sand for protection between waves, and follow the wave-swash area up and down the shore as it moves with the tide.  The crabs hunt for food as a wave sweeps up the shore, then quickly bury in the sand when the wave recedes.  When buried, the antennae protrude from the sand, sensing for upstream food material.  A study in Victoria, British Columbia on limb movements and coordination done on specimens collected in Monterey Bay, California, shows that legs 2 and 3 do most of the digging.  These are coupled with a counterclockwise motion, and leg pairs alternate on either side of the body.  Meanwhile, leg 4 digs in a clockwise and smaller rotation, and at twice the frequency of the other limbs, an action that may liquify the sand and enable the shovelling movements of the other leg pairs to force the rear of the animal down into the sand.  If leg 4 were to move in the same direction as legs 2 and 3, presumably the animal would be propelled directly backwards.  Legs 2 and 3 cycle at a rate of 3-4 sec-1, while leg 4 cycles at 3-8 sec-1.   The authors propose that digging may have originated as a modified form of walking.  Faulkes & Paul 1997 J Exp Biol 200: 793 and Faulkes & Paul 1997 J Comp Physiol A 180: 161; see also Paul 1981 J Exp Biol 94: 159.

NOTE  the authors include 2 other species in their study, Lepidopa californica and Blepharipoda occidentalis.  However, since these are found almost exclusively in southern California, the results are not included here.  Emerita analoga usually occurs south of Oregon, but may temporarily recruit to beaches as far north as Vancouver Island, British Columbia

 
Research study 6
 

graph showing time to bury by different species of Californian mole crabs in different types of sediments
Of the 3 species of mole crabs referred to in Research Study 5 above, Emerita analoga is more widely distributed and more abundant.  Part of its success may owe to better burrowing and swimming abilities.  Laboratory studies in Santa Barbara, California show that E. analoga burrows equally or more quickly into sediments than the other 2 species, and does so over a broader range of sediment sizes (see accompanying graph).  The results indicate that Emerita is a substratum generalist, while the other species are substratum specific.  In field tests in wave swash, Emerita burrows at about the same speed as Lepidopa californica (1-14sec vs. 1-4sec, respectively) and both species are much quicker than Blepharipoda occidentalis (4-36sec). Through more strongly developed musculature and more efficient use of uropods and thoracic limbs, Emerita is a better swimmer than Blepharipoda, and possibly also better than Lipidopa.  The combined features of substratum generality, good swimming, and quick burrowing would be expected to lead to less exposure of Emerita to disorientation in wave swashes and thus to less risk of stranding, exposure to impact damage in the surf, and exposure to predators.  Dugan et al. 2000 J Exp Mar Biol Ecol 255: 229.

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photograph of a crab Cancer oregonensis hiding in a coralline alga-encrusted barnacle shell, taken from a video

CLICK HERE to see a video of Cancer oregonensis hiding away in a coralline alga-encrusted barnacle test.

NOTE  the video replays automatically

 
Research study 7
 


Many crabs including both Dungeness Cancer magister (shown in the photo) and red rock C. productus bury themselves in soft substrata during inactivity and/or low tide periods.  The two species differ slightly in how they bury.  The former species uses legs and chelae to excavate the sand, while the latter uses its legs to push itself into the sediment and its chelae for a final shove.  Complete burial is achieved in about 0.5min for C. magister and 1min for C. productus.  Some individuals remain buried for over 50h. While buried the crabs maintain a respiratory water flow through the gill chambers, driven by the beating of the gill bailers.  Water flows in over the legs and exits via apertures on either side of the mouth.  These shallow-burying crabs may create a channel between their claws and body, allowing water to flow into the openings at the bases of the legs.  Periodically, the bailers reverse their beat in order to push sediments back out these same openings.  Burial is a means to evade predators such as daytime hunting seals and larger fishes, and perhaps make good locations from which to ambush prey.  Equally important, burial may be a means to conserve energy. McGaw 2004 J Exper Mar Biol Ecol 3303: 47; McGaw 2005 Scientia Marina 69: 375.

NOTE  also known as scaphognathites (lit. “hollowed-out jaw” G.) referring either to its hollowed, membranous shape or to its location in a kind of a  tunnel connecting the gill chamber to the outside of the body on either side of the mouth

NOTE  when buried the crabs decrease blood flow to their muscles, may cease ventilating the branchial chambers for up to 30min, and exhibit periods of cardiac arrest.  A good hemolymph flow to the digestive organs is maintained, however, suggesting that burial may also be a time for digestion and absorption.  McGaw 2004 J Exper Mar Biol Ecol 3303: 47.

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Research study 8
 

histograms showing hiding times of hermit crabs Pagurus granosimanus after various chemical and visual stimulationsHermit crabs have good colour vision and good chemosensory abilities, the latter mediated mainly through sensory aesthetes on the antennules.  It therefore seems likely that hermit crabs can perceive chemical emanations from a predator, but is it able to differentiate these odours from those of non-predators?  And what is its response – to stay and hide, or to run away?  Finally, what if a view1 of the predator is included?  These ideas are tested at the Bamfield Marine Sciences Centre, British Columbia with the intertidal hermit crab Pagurus2 granosimanus and a known predator, the red-rock crab Cancer productus, measuring hiding time in seconds of the hermit crab in response to various combinations of chemical and visual stimuli.  The chemical stimulus is the effluent water from the predator Cancer, while the visual stimulus is a video of the predator. Responses in the presence of effluent water and/or video of an herbivorous kelp crab Pugettia productus are also included as a kind of control. A test begins with exposure of a hermit crab to one or both types of stimuli and then the crab's shell is given a sharp rap. The crab withdraws into its shell and the researchers then measure its hiding time. On the assumption that Pagurus would naturally run from a predatory crab, a shorter3 hiding time is interpreted by the experimenters as a significant escape response.  Results of laboratory tests provide some interesting data. 

First, and as expected, effluent water from the herbivorous Pugettia has no significant effect on hiding times (5th bar from Left).  A combination of video with Pugettia effluent, however, significantly lowers hiding time (6th bar from Left), and this depressing effect of video on hiding-time is seen in 3 other video treatments, whether the image projected is a neutral view of rocks and seaweed (2nd bar from Left), or whether images of crabs are included (3rd and 4th bars from Left).  Finally, exposure to effluent-water from Cancer produtus significantly lowers hiding time of Pagurus (by 41%; 7th bar from Left), but with no significant additional effect of video (8th bar from Left).  Thus, the hermit crabs can distinguish between chemical emanations from predatory and non-predatory crabs, and adjust their behaviour accordingly. With the caveats noted below in mind, the approach used in the study is fresh and interesting, and should generate considerable interest.  Rosen et al. 2009 Anim Behav 78: 691.

NOTE1  the researchers use a video image for this but, given that the resulting image is 2-dimensional, one wonders why a real crab, nearby, but "chemically" isolated, would not have been a better choice

NOTE2  as a means of standardising the kind of shells inhabited, only P. granosimanus inhabiting shells of black-turban snails Chlorostoma funebralis are used 

NOTE3  given that the shell is a place of sanctuary for the hermit crab, another interpretation is that a longer hiding time would be the important thing.  The authors briefly discuss the issue of expectation of shorter or longer hiding times, and opt for the former, arguing that C. productus is a durophagous-type predator, in other words, an eater of hard-shelled prey, and is relatively slow-moving. Thus, it would be more advantageous for a hermit crab to run away than to stay put, with the latter's risk of being caught and having the protective shell chipped away. The running away part of the response, at least, has been noted in studies of other hermit crabs.  But has the hermit evolved such a subtle array of responses?  The authors actually cite a previous study that finds that P. ganosimanus prefers to inhabit large snail shells into which it can withdraw completely, a fact that should be germane to such a discussion

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Research study 9
 

A study at the Hatfield Marine Science Center, Oregon in which newly settled juvenile red king-crabs Paralithodes camtschaticus are exposed to predation by 13cm-sized Pacific halibut Hippoglossus stenolepis in different complexities of experimental habitats shows, not surprisingly, that survival increases with complexity of habitat.  Lowest survival (zero) is in a habitat of plain sand, where attack frequency and capture success photograph of juvenile king crab Paralithodes californiensis courtesy Phillip Colla, by the predatory halibut is greatest.  With increasing complexity of habitat, prey and predator encounter one another less frequently, and attack rates decline significantly. Perhaps a more important finding is that the juvenile crabs are able to detect the presence of predators quickly and adjust their behaviour accordingly to seek protection in the densest microhabitat patches.  The study has relevance to fisheries scientists wishing to maximise survival of seed stock in P. camtschaticus rehabilitation programmes. Stoner 2009 J Exp Mar Biol Ecol 382: 54. Photograph courtesy Phillip Colla, Carlsbad, California oceanlight.com.

NOTE  the juveniles are spiny, red in colour, and about 2mm in carapace length

NOTE  8 different habitats of increasing complexity are created by adding various amounts and kinds of physical structures on top of sand substratum in several 1m-diameter circular tanks.  The structures consist of bundles of yarn to mimic filamentous seaweeds, empty mussel shells, smooth stones, and pea gravel

The juvenile king crab featured here is considerable
older and larger than the ones used in the study, and
is a different species (P. californiensis), but features
of colour and relative spinyness will be similar

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Research study 10
 

photograph of sand crab Emerita analoga courtesy Lovell & Libby Langstroth,CaliforniaAfter scurrying about in the wave-swash to find good feeding locations, sand crabs Emerita analoga re-burrow quickly to avoid predators and to minimize displacement (see photograph of buried crabs in Research Study 5 above).  One obvious factor influencing speed of burial is coarseness of the sand, but less obvious ones are relate to presence of egg masses and degree of parasitism.  Effects of these factors are assessed by researchers from California Polytechnic State University, San Luis Obispo for E. analoga at 4 local sand beaches. Only females are used in the study, as males are apparently much less common.  Results show that crabs bury slower in coarser sand.  Crabs with heavier loads of parasites burrow more slowly, a situation that makes them potentially more vulnerable to predation by marine birds.  Interestingly, some of these birds, especially surf scoters and gulls, are definitive hosts of the parasites.  Ovigerous females have a greater load of parasites than non-ovigerous ones, but burrow more quickly.  Finally, burrowing rates significantly decrease with repeated testing.  Kolluru et al. 2011 Behavioural Processes 88: 184. Photograph courtesy Lovell & Libby Langstroth, California.

NOTE  the main parasite is an acanthocephalan worm Profilicollis altmani, also known as thorny-headed worms because of the presence of an eversible proboscis, armed with spines, that is used to pierce and hold the gut wall of the definitive vertebrate host

Gravid female sand crab Emerita analoga. The pointy
white structure over the eggs is the abdomen flap 1.2X

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