Defenses
  Defense by use of claws may be such an obvious defense that no west-coast studies have been done on it; however, information on MECHANICS OF CRUSHING is extensive and can be found in its own section.
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  Limb autotomy
  Defenses of crabs include limb autotomy, considered here, and HIDE AWAY and CAMOUFLAGE, considered elsewhere.
 
Research study 1
 

diagram of crab walking leg showing the location of fracture plane for autotomyMost or all crabs are able to autotomise their limbs when under duress.  This occurs at a special fracture-plane located at the base of the limb, shown in the drawing on the Left for a walking leg of the shore crab Pachygrapsus crassipes.  A special “autotomiser” muscle is attached at the proximal side of the fracture plane on the ischium segment of the limb. 

diagram showing how protuberances on either side of the fracture lock together during autotomy of the limb to break it apartWhen stimulated, the muscle contracts and raises the entire limb until a special protuberance on the ischium locks into place with a similar protuberance on the coxa (highlighted in yellow).  With no place to go, further contraction of the muscle splits the ischium at the fracture plane and the limb drops away. This takes about 2sec.  Instantly, special fibers mat together at the wound site to form a protective diaphragm, with a central foramen for the main artery of the leg.  Special flaps located on the inner surface of the diaphragm immediately seal off this opening through hemocoelic pressure.  A few days later epithelium begins to grow over the outer surface of the diaphragm. In Pachygrapsus crassipes an autotomised limb will regenerate to full size in 3 moults.  Of several thousand individuals examined by the author, 30% are noted to have one or more regenerating appendages.  In comparison, similar surveys of subtidal cancroid crabs Cancer magister and C. productus disclose relatively few individuals with regenerating limbs, perhaps reflecting the hazards of intertidal life over that in subtidal areas.  Hiatt 1948 Pac Sci 21: 135.photograph showing autotomised limbe in the shore crab Hemigrapsus nudus

NOTE  lit. “self cut” G.

NOTE  lit. “blood space” G.  The hemocoel is filled with hemolymph. Although hemolymph has all the functions of blood, it is not technically blood because it generally lacks the cellular component of blood, and it circulates around the body in an “open” system, i.e., not contained within vessels.  Marine invertebrates with “closed” circulatory systems, like those of vertebrates, are few in number and include ribbon worms, polychaetes, and cephalopods



Shore crab Hemigrapsus nudus showing limb autotomy. The severed portion of the
limb is still partly attached. Note the diaphragm over the wound site, which is
completely sealed within a few seconds after autotomy 4X. A study on autotomy
of walking legs in shore crabs Hemigrapsus oregonensis collected in Puget Sound,
Washington shows that autotomy of one limb seems to facilitate autotomy of a
second leg, but not of subsequent ones.  Easton 1972 Mar Behav Physiol 1: 209.

 
Research study 2
 

map showing sampling station for study of limb damaga in Alaska king crabs Paralithodes camtschaticaphotograph showing autotomy membrane after limb loss in Alaska king crab Paralithodes camtschaticusLimb loss in crabs is greater in males than in females and juveniles, and is more prevalent in posterior limbs than in anterior ones.  These observations come from a study on limb loss and their regeneration in king crabs Paralithodes camtschaticus (an anomuran) and tanner crabs Chionoecetes bairdi (a brachyuran) in the Bering Sea, Alaska (see map). collections of several hundred crabs of each species shows that 15% of adult and 29% of juvenile Paralithodes are missing at least one limb.  Overall incidence of limb loss in tanner crabs is 39%.  The location of severence in all animals is at the autotomy plane at the base of each appendage. After autotomy,  a transverse membrane occludes the stump and prevents table of limb loss in 226 Alaska king crabs Paralithodes camtschaticusloss of hemolymph.  Within a short time the table showing comparative limb loss in tanner crabs Chionoecetes bairdimembrane darkens and forms into leathery scar tissue (see photo on Right).  Unlike the normal exoskeleton of all but very old animals, this scar tissue provides a good surface for settlement of barnacles and hydroids. 

Analysis of frequency distributions of limb-regenerate lengths in juveniles suggests that it takes at least 4 moults to restore limb symmetry. The frequency of limb loss in 226 Paralithodes with respect to limb position shows that hind limbs are more susceptible to loss (see Table on Left), but this is not seen in 229 Chionoecetes (see Table on Right). The author does note a tendency for more right limbs to be missing in Chionoecetes than left limbs, but this is likely to be neither statistically nor biologically significant (see Table on Right). Edwards 1972 Acta Zoologica 53: 105.

NOTE  the author implies that the slight bias in right- vs. left-hand limb loss in Chionoecetes bairdi signifies a tendency for the species to move with its right-side forwards, thus incurring disproportionate losses to predators in these leading limbs.  There is no basis for this assumption, as it is not known whether C. bairdi does, in fact, lead with its right side.  In any case, it could just as convincingly be argued that it is the left side that leads, and it is the right-side limbs trailing behind that are more vulnerable to attack by predators coming up from behind 

 
Research study 3
 

crab photo showing limb morphology in Dungeness crab Cancer magistergraph showing the extent of propodus length regenerated after limb loss in Dungeness crabs Cancer magister in relation to postmoult carapace width
An investigation into regeneration time of red-rock crabs Cancer productus following limb autotomy at the Bamfield Marine Sciences Centre, British Columbia reveals that at least 3 moults are required to regenerate a full-length cheliped.  Note in the grap on the Right that small crabs regenerate proportionately more of a missing claw during the first moult than do large crabs. 

Crushing strength in a regenerating claw, not surprisingly, is significantly less than in a normal (uninjured) claw even after 2 moults (see histogram below Left).

histograms showing mechanical advantages of regenerating and normal chelae in Dungeness crabs Cancer magisterInterestingly, during regeneration of an injured claw, the uninjured claw increases in mechanical histogram of crushing force in regenerating and normal chelae of Dungeness crabs Cancer magisteradvantage (favouring greater theoretical crushing ability), but not significantly greater than that of a normal (intact) crab (see histogram lower Right). Despite this compensatory response, however, a regenerating limb puts its owner at significant disadvantage during foraging because crushing strengths of both regenerating and normal claws in an injured crab are significantly lower than in comparably sized intact crabs suggestive of regenerative costs (see histogram lower Left).  There is, however, no significant decrease in body size over a moult cycle signifying increased energetic costs for crabs regenerating a single claw, but there is for a crab regenerating both claws.  Field data on claw loss in Cancer productus in Barkley Sound indicate that at any given time about 20% of small crabs and 40% of large crabs are injured in some way, mostly missing or regenerating a single claw. Thus, much of the population experiences prolonged foraging handicap owing to injury.  Brock & Smith 1998 Biol Bull 194: 53.

 
Research study 4
 

photograph of porcelain crab Petrolisthes cinctipesphotograph of autotomy site on a porcelain crab Petrolisthes In view of the uniqueness of limb autotomy in crabs it is surprising that not more has been done on the subject in common west-coast species.  A study in northern California using two Petrolisthes species helps to redress this.  The authors test the extent of autotomy with 4 potential sympatric predatory crabs, Cancer antennarius, Hemigrapsus nudus, Pachygrapsus crassipes, and Lophopanopeus bellus and find that all but the last will attack and eat porcelain crabs.  In total, attacks elicit 34% limb autotomy, but this rises to 67% if only attacks in which the prey is held by its cheliped are considered. 

cartoons demonstrating how limb loss in porcelain crabs Petrolisthes cinctipes may function in defenseLimb autotomy is highly effective as an escape mechanism in Petrolisthes.  Of 59 that autotomise, 58 escape successfully, usually aided by the predator taking time to eat the shed cheliped (see cartoon on Right). The authors note that reliance on autotomy as a primary escape mechanism may be particularly common in suspension-feeding crabs that do not use their chelipeds for feeding.  Wasson et al. 2002 Behav Ecol 13: 481.

NOTE  the study actually includes P. cinctipes, P. manimaculis, and P. eriomerus, but  only are first 2 are used in experiments.  Petrolisthes manimaculis inhabits shore areas from southern California north to at least Baker and Indian Beaches in northern California

NOTE  the authors show a significant negative correlation between numbers of purple shore crabs H. nudus and Petrolisthes spp. under rocks, suggesting a predator/prey interaction (the other crab species are too rare for any correlation to be made)

 
Research study 5
 

photograph of a lithodid crab Oedignathus inermis using its larger pincer-like right claw to bite,  while its smaller spatulate (but ridged) left claw holds on
In some crabs that autotomise their claws (e.g., Petrolisthes, Pachycheles), the freed claw may clamp down and continue to bite for some time, enabling the previous owner to crawl away from danger. One feature proposed, but never rigorously tested, to distinguish the species Petrolisthes cinctipes and P. eriomerus is that following autotomy the former species' claw pops open harmlessly, while the latter species' continues to bite down.

 

 

 

An Oedignathus inermis uses its larger pincer-like right claw
to bite, while its smaller spatulate (but ridged) left claw holds
on. It is not known whether this species autotomises its claws

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