Foods & feeding
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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 part of carnivory deals with the red rock crab Cancer productus, while topics of MECHANICS OF CRUSHING, carnivory in Metacarcinus magister, and SPECIFIC DYNAMIC ACTION are dealt with in other sections.
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Cancer productus

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Research study 1
 

photograph of red rock crab Cancer productus
photograph of crab Canceroregonensis courtesy Ron Long, SFU, BurnabyIn Puget Sound, Washington Cancer productus preys on barnacles and smaller crabs, and scavenges dead fish and various invertebrates. The related Cancer oregonensis preys on polychaetes, barnacles, small crabs, amphipods, and shrimps.  Knudsen 1964 Pac Sci 18: 3. Photo of C. oregonensis courtesy Ron Long, SFU, Burnaby.

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Research study 2
 

map of area near Bamfield, British Columbia showing movements of sonically tagged red rock crabs Cancer productus
With good powers of locomotion red rock-crabs Cancer productus commonly roam about looking for food.  A study done at the Bamfield Marine Sciences Centre, British Columbia uses an acoustic transmitter attached to a crab’s carapace to provide a record of the path of an adult crab over a 23-d period. Total distance moved is about 410m. The map shows distances moved during most of the days travelled by the crab  Boulding & Hay 1984 Can J Fish Aquat Sci 41: 521.

NOTE 2 crabs with transmitters are used in the study, but only one record is shown here.

 

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Research study 3
 

photograph of littleneck clam Protothaca staminea
Red rock crabs Cancer productus are carnivores and scavengers.  Bivalves are favoured among live prey, especially mussels and clams.  However, while small specimens of littleneck clams Protothaca staminea are crushed easily, larger specimens require another technique.  Studies at the Bamfield Marine Sciences Centre, British Columbia show that a crab will spend 3d repeatedly squeezing the same spot on the edge of the shell of its prey.  After 200 or so loadings at 1-2 pulses per minute, tiny micro-cracks begin to bridge across adjoining crystals of the shell and eventually the crab can chip bits of shell from the edge to gain access to the flesh of the crab.  The authors note that because shell failure may occur at a weaker loading than the previous ones (say, at 70%), the strategy allows a crab to increase the maximum size of bivalve available for it to eat. Boulding & LaBarbera 1986 Biol Bull 171: 538.

 

Littleneck clam Protothaca staminea 1.5X

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Research study 4
 

histograms showing numbers of male and female red rock crabs Cancer productus foraging during nighttime vs. daytime
Other studies on red rock crabs Cancer productus at Bamfield, British Columbia, show that individuals forage through the intertidal zone during high tides at all times of day.  However, at night there are more foragers, and they are predominantly female (64%, lower histogram).  Fewer crabs forage during the day, and these are predominantly males (86%) and of a larger size than night-foraging males. Why the predominance of females foraging at night? As adults, females are smaller than males and they, along with the contingent of sub-adult males, may be at less risk during the night from visual predators.  Why don’t the large males forage at night as well?  Perhaps they are too full from their daytime feeding.  Analyses of gut contents show that the crabs are eating primarily mussels.  On the strength of their observations the authors suggest that these and other species of crabs may have important effects (hitherto not well studied) on the intertidal community.  Robles et al. 1989 J Nat Hist 23: 1041.

NOTE diet composition in this area is mussels Mytilus trossulus (71%), barnacles Balanus glandula (17%), and other items (12%)

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Research study 5
 

schematic of gut system of red rock crab Cancer productusgraph showing % digestion of food in different areas of the gut of a red rock crab Cancer productus
Progress of food through the gut of a crab can be monitored by use of radio-opaque meals and a fluoroscope.  Studies at Bamfield, British Columbia on red rock-crabs Cancer productus show that the foregut is emptied by rhythmic contractions by 30h, the midgut by 36h, and the hindgut mostly by 48h (at 11-13oC, see graph upper Right. 

If the crab is air-exposed  for 24h after feeding, movement of food from the foregut is delayed (see graph lower Right). On re-immersion, rate of contractions in the foregut takes 3-5h to return to normal. The author notes, however, that about 65% of all crabs on re-immersion regurgitate their stomach contents.  This may be a reflexive defensive response to stress or, as the author favours, a result of overall metabolic depression during air-exposure and a “reticence” to add a metabolic digestive burden to an already stressed system.  McGaw 2007 Mar Freshw Behav Physiol 40: 117.

NOTE  food is 65% homogenised fish muscle, 10% electrolytic iron powder, and 25% gelatin (setting agent), cut into cubes.  The crabs readily eat this food

NOTE  red rock-crabs often prowl the air-water interface for food and, on a receding tide, could become exposed to air for short periods.  Exposure to air for 24h in the natural habitat is unlikely, but the author notes that even longer emersions are common during commercial shipment.  The author presents results for 3-h and 6-h emersion experiments, but effects are less pronounced than the 24-h experiment and are not included here

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Research study 6
 

schematic showing food web for crabs Cancer productus, snails, and oystersIn certain areas of Washington the native red-rock crab Cancer productus tops a mini-food web that includes 2 invasive1 oyster drills Urosalpinx cinerea and Ocinebrina inornata, and 2 oysters, the native Olympic oyster Ostrea lurida and the introduced Japanese oyster Crassostrea gigas.  The crab eats all 4 species, while the drills tend to specialise each on a different species of oyster.  The crabs, then, have both direct effects on oysters, by eating them, and indirect effects on oysters by eating their snail predators (see schematic).  To determine the net balance of these effects, researchers at Shannon Point Marine Center, Anacortes compare consumption rates of crabs on each species of oyster alone, with consumption rates of crabs on oysters in the presence of an alternate prey of the crabs, namely, the oyster drills.  In the field, the drills are eating the oysters, but at the same time are being eaten by the crabs.  If this last were to happen at a faster rate than the drills can consume the oysters, it would produce a net savings of oysters.  Now, in preference tests with both types of prey the crabs are found to prefer oysters2 over drills by a factor of 6 (in single-prey feeding tests, they eat oysters and drills at about the same rate).  The drills, in turn, eat oysters at a rate of about 0.3 per day.  When a crab has a choice of prey, then, the balance sheet shows a direct negative effect on oysters of  minus 6 per day, and an indirect positive effect on oysters of plus 0.63 per day.  Overall, however, the net effect of crabs in the system is still strongly negative by a total of 5.4 oysters per day.  The interest in the study lies not so much in the effects of invasive oyster drills on oyster populations, but in the potential for mitigating effects of native crabs, especially were they to have exhibited a stronger preference for the drills than the oysters.  The study provides a novel look at trophic effects of invading predators, especially important in a system where one of the primary prey species, Ostrea lurida, is itself threatened, yet of potential commercial importance.  Grason & Miner 2012 PLoS ONE 7 (12): e51322. Photographs courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC and Smithsonian Marine Station, Fort Pierce, Florida.

NOTE1  Urosalpinx cinerea was introduced from the east coast on shipments of eastern oysters Crassostrea virginica and Ocinebrina inornata likely from Japan on shipments of Pacific oysters Crassostrea gigas, both species arriving sometime in the 1920s

NOTE2  juvenile oysters of both species are used in these tests because these are the preferred size of prey of the drills

NOTE3  this positive effect is calculated as follows: for every 6 oysters consumed a crab eats 2 drills.  If not eaten, each drill would consume 0.3 oysters.  So, by removing 2 drills a crab saves the oyster population 0.6 individuals per day

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