Feeding & foods
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Crabs employ 6 pairs of appendages to catch, crush, manipulate, and chew food.  First in order of use are the large claws that catch, crush, and tear apart prey.  From the claws the food bits are passed to 3 pairs of outer mouth appendages, the maxillipeds.  These render and sort the photograph of some of the mouth appendages of Cancer magister. The outer parts of the 3rd maxillipeds have been reflected to the side and what is visible are portions of the maxillipeds, maxillae, and mandiblesbits, and then pass them to 2 pairs of inner appendages, the maxillae.  From the maxillae the food is moved to a single pair of mandibles where, after a final maceration, it is swallowed.  The swallowed food enters the foregut, a spacious chamber divided into an anterior cardiac chamber and a smaller posterior pyloric chamber.  The cardiac chamber contains an auxiliary grinding device, the gastric mill (bearing calcified ossicles).  Its primary function is mastication of the food, but some enzymatic breakdown starts here. A sieve device at the junction of cardiac and pyloric regions prevents any but the smallest particles passing into the midgut.  Ducts lead from the midgut into the large paired branches of the hepatopancreas where the food is further digested and then absorbed. The hindgut, or intestine, is a simple, thin-walled tube running the length of the abdomen and terminating in an anus.

NOTE  maxilliped lit. “jaw foot” L.; maxilla lit. “jaw, jawbone” L.; mandible lit. “jaw” L.

Some of the mouth appendages of Cancer magister. The outer parts of the 3rd maxillipeds have been reflected to the side and what is visible are portions of the maxillipeds, maxillae, and mandibles 1.7X

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  Carnivory
  Decapod crustaceans have a variety of feeding modes including carnivory, considered in this section, and HERBIVORY, SUSPENSION-FEEDING, and OMNIVORY/SCAVENGING considered in other sections.  This carnivory section first deals with mechanics of crushing, then leads to sections on feeding in the well-studied species CANCER MAGISTER and CANCER PRODUCTUS.
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photograph of a hermit crab Pagurus armatus, taken from a video, feeding on a cracked live mussel. The main appendages visible in the video are the chelae, which are tearing bits from the mussel and passing them to the 3rd maxillipeds. These, in turn, pass the bits to the 2nd and 3rd maxillipeds, and 1st and 2nd maxillae (not visible), and thence to the mandibles for final rendering and swallowing

CLICK HERE to see a video of a hermit crab Pagurus armatus feeding on a cracked live mussel. The main appendages visible in the video are the chelae, which are tearing bits from the mussel and passing them to the 3rd maxillipeds. These, in turn, pass the bits to the 2nd and 3rd maxillipeds, and 1st and 2nd maxillae (not visible), and thence to the mandibles for final rendering and swallowing.

NOTE  the video replays automatically

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Mechanics of crushing

 

photgraph of the left chela of a Dungeness crab Cancer magisterCarnivory in crabs commonly involves the crushing of hard-bodied molluscan prey, such as bivalves, with the chelae.  Most of the force applied is at the tips of the claws and, just as shell fatigue comes into play for a clam being crushed, so claw fatigue comes into play for the predator. If a tip of the claw is broken off, either on the moveable dactylus or stationary propus, normal functioning of the claw may be severely compromised and behaviours other than simple crushing may be affected.

NOTE  the terms chela (G. “claw”), cheliped (G. “claw foot”), and claw are used interchangeably in the ODYSSEY

 
Research study 1
 

histograms showing number of females and males of Dungeness crabs Cancer magister with worn or broken chelaeIn a collection of 800 crabs Cancer magister from Tofino, British Columbia in May (prior to mating season), about 50% of females show some tooth wear on the claws and about 12% show breakage (see left histogram, the X-axis scale indicates 0-2= increasing levels of wear, and 3=breakage).  Among mated males (from the previous mating season), 60% have worn claws and 22% have broken claws.  Unmated males are mostly undamaged.

Laboratory tests in which claw teeth of 5 adult male specimens are filed down to wear-level 2 reveal that handling times when attacking prey clams Protothaca staminea are significantly increased by about 50% over undamaged control crabs. For example, an intact adult crab takes about 120sec to break a 3-cm clam, while an experimentally created "wear-level 2" crab takes 175sec to break a same-sized clam.  Although not mentioned by the authors (perhaps as being too obvious), this indicates that pressure points from the dentition are important to the crushing process in addition to absolute force. Crabs wih broken claws consistently fail to crack even the smallest size of clam offered (1.5cm shell length), yet all crabs readily consume tissue from experimentally opened clams showing that they are, indeed, hungry. Juanes & Hartwick 1990 Ecology 71: 744.

NOTE the data shown here are originally divided into right and left claws by the authors but, as the two sets do not differ significantly, they are combined into one value here

NOTE  a numerical index of claw wear and breakage is used: 0 = no tooth wear; 1 = slight tooth wear, 2 = extreme tooth wear (>half of tooth volume missing), and 3 = a portion  of claw is broken

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Decreased crushing ability is just one effect of a broken or lost claw for either sex of crabs, but there may be other sex-specific effects. Consider the ideas presented below, then CLICK HERE to see explanations.

Moult interval is shortened. 

Growth increment is less. 

Fighting ability is lessened. 

The male's ability to hold a female during copulation is impeded. 

An injured male is less attractive to a female. 

Feeding is completely curtailed. 

 
Research study 2
 

drawings of claws of several west-coast crabs showing their morphology
histograms showing shell-breaking times for several crabs feeding on the winkle Littorina sitkana
Can the ability of crabs to crush shells of snails, clams, and other shellfish be correlated with functional morphology of their claws, such as shape, dentition, and mechanical advantage?  Comparison of 4 common intertidally-foraging crabs Hemigrapsus nudus, Lophopanopeus bellus, Cancer oregonensis, and C. productus at Friday Harbor Laboratories, Washington show that a male H. nudus (a generalist feeder whose diet includes macro- and microalgae as well as small snails and other shellfish) has finely serrated surfaces on the occluding surfaces of the claw, claw tips that abut rather than overlap (for good scraping and scooping), a mechanical advantage of  0.3, and a crushing force of 5 Newtons (for a 9-g male). In comparison, males (5-12g) of the specialist feeders L. bellus, C. oregonensis, and C. productus (whose diets include hard-shelled prey such as barnacles, snails, mussels, clams, and other shellfish) have stout, blunt molar teeth on the occluding surfaces of the claws, claw tips that overlap in shearing crossbill-like fashion (for good cracking and peeling back the shell), mechanical advantages of around 0.4, and crushing forces of 12-25 Newtons (for 9-g males).

When offered 3 size classes of Littorina sitkana prey in lab feedings (4, 6, and 8mm shell lengths), the generalist-feeding Hemigrapsus nudus can readily schematic indicating how mechanical advantage is calculated in a crab clawand quickly break only the smallest size class of prey (4mm shell length), but is incapable of breaking snails of the largest size class (see histogram upper Right).  In comparison, the specialist-feeding crabs Lophopanopeus bellus, Cancer oregonensis, and C. productus have no trouble breaking and eating all sizes of Littorina prey. Behrens Yamada & Boulding 1998 J Exper Mar Biol Ecol 220: 191.

NOTE mechanical advantage (MA) is a measure of the ratio of input to output force in a lever system.  Where force data are difficult to obtain, it can also be expressed as the ratio of input lever length to output lever length, as is done in this study. The output lever length is measured as the distance from the fulcrum to the tip of the dactylus (see photo on Right). The input lever length is the distance from the fulcrum to the insertion point of the flexor apodeme. In the photo this is hidden within the carapace. More detailed information on MAs in crabs is given in Research Study 3 below)

NOTE  measured by the extent of distortion of a stiff wire bent into a Ω-shape with strain gauge attached and calibrated with known weights.  Most crabs readily “bite” down on objects inserted within their claws and tests of several individuals will provide a reasonable estimate of force exerted.  As force is technically an acceleration, it is expressed in units of 1 kg mass accelerating at 1 meter per sec = 1 Newton

 
Research study 3
 

schemat showing the arrangement of levers, fulcrum, and apodemes in the claw of a crab
Here is a more complete explanation of lever systems and mechanical advantages in a crab, looking first at the arrangement in a chela and then in a walking leg. As noted in Research Study 2 above, the dactylus of a crab’s claw rotates on a fulcrum. The output lever is represented by the length of the dactulus. There are actually 2 input levers, one associated with flexing, or closing, the claw; the other, with extending, or opening, the claw. At the tip of each input lever is attached a flat sheet of calcified exoskeleton known as an apodeme. In turn, flexor and extensor muscles are attached along the entire length of each apodeme, contraction of which closes and opens the claw, respectively. The flexor muscles are pinnate, that is, arranged in blocks at an angle of about 30o or so (depending upon species) to the longitudinal axis of the apodeme. This arrangement allows relatively more muscle mass to be packed into the space contained within the claw. In contrast, the extensor muscles are parallel, with their fibers arranged parallel to the long axis of the apodeme, and less relative muscle mass can be packed into the space available.

An understanding of how a lever system works in a crab’s claw is made somewhat more difficult by the fact that each lever is angled, but the principle is the same as in a straight-armed lever system.  Note that because of the differing input lever lengths, the mechanical advantage (MA) of the flexor or closing system will be larger than that of the extensor or opening system (values of around 0.4 and 0.1, respectively, would be reasonable estimates for the MAs here). Now, add to the lever systems the different sets of muscles: a more massive pinnate set for the flexor system and a smaller parallel set for the extensor system. Just like a dog's jaw, the ratio of MAs in a crab chela makes it easy to hold closed, but difficult to hold open. As noted in the above Research Study, MAs differ depending upon function: higher MAs yield greater crushing force but slower relative movements, and lower MAs yield weaker forces but faster relative movements. Lever systems with low MAs are ones structured for speed; ones with high MAs are structured for force.

The lever systems in the joints of the walking legs tend to have more balanced MAs, reflecting the more equal flexing and extending functions. They do differ, however, especially in crustaceans like crabs that walk or pull themselves in a sideways gait, as compared with crustaceans that walk in a straight line like prawns.

MAs of a crab claw tell only part of the story of its evolution.  As noted, mass of muscles and orientation of their fibers are features to consider, and also whether the fibers are “quick twitch” or “slow twitch”.

Most species of intertidal-foraging crabs on this coast have monomorphic claws (lit. “one form” G, referring to right and left claws with the same shape), and mechanical advantages of the lever systems on each side are theoretically equal (e.g., see photos of Hemigrapsus nudus, Lophopanopeus bellus, Cancer magister, C. productus, and C. oregonensis).  In comparison, a species with dimorphic claws is Oedignathus inermis (last photo in second row). One expects that MA values in this species would be about 0.3 for the larger claw and about 0.2 for the smaller claw.

 
photograph of female shore crab Hemigrapsus nudus photograph of crab Lophopanopeus bellus photograph of male Dungeness crab Cancer magister
 
photograph of red rock crab Cancer productus courtesy Iain McGaw, U Nevada, Nevada photograph of male crab Cancer oregonensis photograph of crab Oedignathus inermis
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Research study 4
 

drawing showing loading procedure on a crab claw to measure breaking strengthAnother consideration about mechanics of crushing relates to safety factor1.  In other words, how closely do normal maximal loads approach breaking strength in crab claws?  This question is addressed in a study at the Bamfield Marine Sciences Centre, British Columbia using 6 species of predatory Cancer crabs.  The researchers use a force gauge with rings into which the claw tips can be inserted.  Biting is measured in both claws sequentially, with the rings adjusted at the start to 60% of maximum gape for each species.  After biting force is measured the breaking strengths of the same2 claws are measured by clamping the manus securely then adding weights to the tip of the claw as shown in the drawing on the Left. Breakage sites are variable (but generally in the location shown), and fracture angles are also variable.  Results show that the breaking strength increases in all species towards the tip of the pollex, or ventral fixed finger, of the claw, and this correlates with increasing thickness of the cuticle. The black colour on the claw tips generally signifies a thicker and stronger cuticle. The stronger cuticle structure towards the claw tip ensures that the force at which the pollex breaks remains roughly constant along its length. The thicker cuticle may also act to reduce abrasion in that well-used portion of the claw.  Interestingly, relative breaking strength decreases with claw size in all species.  Safety factors3 of the claw vary among species and range from 2-7.  Within a species they increase with increasing claw size owing to an allometric increase in cuticle thickness and proportionally lower maximum biting forces in larger claws.  The question as to why larger crabs should have proportionally lower biting forces is discussed by the researchers, but with no clear answer.  A similar pattern is seen in other crustaceans such as stone crabs and lobsters.  The higher safety factors in larger crabs is of adaptive value because stress cracks and wear have more time to accumulate between longer intermoult periods and larger crabs tend to use their claws increasingly more for aggression.  Palmer et al. 1999 Biol Bull 196: 281.

NOTE1  more on safety factors in claws of Cancer spp. can be found at LEARN ABOUT CRABS & RELATIVES: MOULTING & GROWTH

NOTE2  the authors remark on the statistical usefulness in their study of being able to measure both biting and breaking forces on the same claw

NOTE3 safety factor is calculated as breaking force (N)/maximum biting force (N). For example a value of 4 means that the breaking force is 4 times greater than the maximum biting force. This is the safety factor

 
Research study 5
 

In a paper entitled in part, “Why are crabs so strong?”, a research scientist working at the Bamfield Marine Sciences Centre, British Columbia measures maximum crushing forces in the claws of 6 species1 of Cancer crabs and compares the magnitude of these forces with sarcomere lengths of the muscles involved.  Crushing forces are measured using a strain-gauge device similar to that used in other studies.  Sarcomere lengths are obtained from fixed and stained portions of the closer muscles.  Results show that stresses generated by the claw closing muscles scale isometrically with resting sarcomers lengths, as predicted by the sliding filament model2 of muscle contration.  On a comparative basis, a crab claw3 exerts greater force during crushing than any force exerted in any other animal activity. Note that the data scale isometrically (the slope, b = 1.13, of the linear relationship does not differ significantly from an expected slope of 1).  Taylor 2000 Proc Roy Soc Lond B 267: 1475.

NOTE1  the species are C. antennarius, C. branneri, C. gracilis, C. magister, C. oregonensis, and C. productus

NOTE2  this predicts that force is proportional to the number of myosin-actin cross-bridge sites that can form within each half-sarcomere

NOTE3  the record crushing force is currently held by the stone crab Menippe mercenaria whose claw closer muscles can exert forces of up to 2000 KN . m-2

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Research study 6
 

drawing of claw of Cancer crab showing muscle blocks, apodemes, and fulcrum locationgraph showing negative relationship between crushing force exerted by a claw of a crab Cancer productus and claw sizeA comparison at the Bamfield Marine Sciences Centre, British Columbia of biting performance in 6 sympatric west-coast Cancer species confirms a pattern reported for other decapod crustaceans. The pattern is that while biting force increases with increasing claw size, the actual force per unit area exerted by the closing muscles declines allometrically. Thus, larger claws are proportionately weaker than smaller ones. The drawing on the Left illustrates some features of a claw, most notably the bundling of muscle fibres into discrete blocks.

A negative allometry is exhibited by 4 of the 6 species (sample results are shown for Cancer productus in the graph on the Right). Of the 2 species exhibiting no significant relationships, Cancer magister differs in having . Maximum biting force of all the crabs is exhibited by C. productus, at 194N for a carapace width of 13cm. Interestingly, within a species, 3 traits that affect maximum biting force actually increase in magnitude with increasing claw size.  These are relative claw size, mechanical advantage, and sarcomere length of the closer muscle. In contrast, the area of the flexor or closer apodeme and the angle of pinnation of the closer muscle-fibres vary isometrically with claw size. These features suggest a selection for greater biting forces in larger crabs, so there must be some as yet unidentified constraint that impairs muscle performance in larger claws. The author finds that claw height is a reliable predictor of maximum biting force among most Cancer species but not, apparently, among other decapod genera.  Taylor 2001 Evolution 55: 550.

NOTE Cancer antennarius, C. branneri, C. gracilis, C. magister, C. oregonensis, and C. productus

NOTE  the 2 exceptions are C. branneri and C. magister, neither of which exhibits a significant relationship. The author actually discusses the significance of a positive allometric relationship shown in C. magister, possibly associated with the fact that of the 6 species, it is the only one that has “cutter”-type claws, rather than the “crusher”-type possessed by the other species. However, because the slope of the relationship is non-significant, there is no relationship at all. Slope values for the 4 significant relationships range from -0.62 to -0.95

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