Habitat & community ecology

The section on HABITAT & COMMUNITY ECOLOGY includes a selection of topics such as intraspecific competition, considered here, and COMMUNITY INTERACTIONS, GENE FLOW, INTERSPECIFIC COMPETITION, and HERMIT-CRAB COMPETITION, considered in other sections.

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Intraspecific competition

  Competition in crabs, as in other animals, may be exploitative, where use of a common resource by one organism denies its use to another organism, or interference, where one organism actively prevents or interferes with another organism's use of a common resource. It can occur interspecifically or intraspecifically, and may involve food, mates, space, or shelter.  Most attention in the scientific literature has been paid to sympatric, congeneric pairs of species competing for space, and to hermit crabs fighting over shelter in the form of shells.
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Research study 1

photograph of juvenile crab Cancer magister
The early instars of crabs are aggressive to one another and may even practise cannibalism.  A study on the first 6 instars of Metacarcinus magister cultured from gravid females collected in San Francisco Bay shows an ontogeny of behaviour, from less aggression during Instars 1-2 (similar to the level exhibited by adults), to increasing aggression commencing from Instars 3-6.  The author notes that this ontogenetic difference allows the early instars to co-exist in dense packs in bays, while the later instars exhibit much greater inter-individual spacing and more cannibalism.  Jacoby 1983 Zeitschrift für Tierpsychologief 63: 1. 


Metacarcinus magister juvenile of unknown instar

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Research study 2

graph showing number of juvenile crabs Petrolisthes cinctipes surviving in treatments with different numbers of adult crabs of different species and in the presence or absence of a sculpin fishPreferential settlement of megalopa larvae of porcelain crabs to adults creates high-density aggregations.  What function is being served by clustering in this way, especially since clustering may lead to greater intraspecific competition for food?  Laboratory experiments with adult and juvenile Petrolisthes kept with tidepool sculpins Oligocottus maculosus (which eat the juveniles) test the idea that juveniles may gain protection by hiding under and between adults.

Results show that survival is significantly better in the presence of adults than with no adults, and significantly better in the presence of conspecific adults (P. cinctipes) than with P. eriomerus adults (see accompanying graph). Results of a control experiment show 100% survival of juveniles with 5 P. eriomerus adults and no sculpin, indicating that the disappearance in the other experiments owes only to feeding by the fish predator (extreme Right data point). Other experiments provide no direct evidence that adults actively protect the juveniles.  However, it is normal for adult porcelain crabs to push away fish and other crabs with their chelae, but to ignore juvenile porcelain crabs and also other small crustaceans such as amphipods.  The author remarks that the behaviour seems analagous to spine-canopy protection in sea urchins and to protection of larval sand dollars settling into beds of adults (the bioturbative activities of the adults exclude a predatory tenaid that feeds on newly settle sand dollars).  Jensen 1991 J Exper Mar Biol Ecol 153: 49.

NOTE  experiments consist of a single sculpin held in a bucket containing 5 adult crabs (either P. cinctipes or P. eriomerus) and 10 juvenile P. cinctipes crabs (≈2.5mm carapace width)

NOTE  to read about spine-canopy protection in sea urchins go to LEARN ABOUT SEA URCHINS: PREDATORS & DEFENSES: HIDING/SHELTERING/COVERING

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Research study 3

photograph of porcelain crab Petrolisthes cinctipesgraph showing growth rates of porcelain crabs Petrolisthes cinctipes at different densitiesGregarious settlement in Petrolisthes cinctipes has its benefits, but costs are incurred in terms of increased intraspecific competition.  Field studies using caged crabs in Sonoma County, California and related laboratory studies (not reported here) show that rates of feeding and growth decline with increasing density, with the effect being more severe for smaller individuals (see graph).

Note in the graph that the regression intercept corresponds to growth rate in the absence of competition (0 density), and the regression slope to the competitive effect on growth of each additional crab within an enclosure.  In terms of carapage width, this translates to a 5mm crab growing to 14mm in one year without competitors, but only to 11mm with 59 competitors (the equivalent of high densities in the field).  In terms of fecundity, this same lone crab would bear nearly 1100 eggs, while its crowded counterpart would bear only 500.  Recruits to the population therefore potentially suffer most from competition because of their smaller sizes, but the author notes that there may be a trade-off if the highly populated sites they settle in are ones with higher food availability. The experiments do not allow the type of competition to be identified, but the author makes a case for both exploitative and interference types, with the latter being slightly stronger.  Interference competition would explain some cheliped loss observed, but the author has too few data to be able to correlate greater limb loss with higher densities.   Donahue 2004 Mar Ecol Progr Ser 267: 219. 

NOTE  these include settlement in a guaranteed high-quality habitat (reduced risk of predation and desiccation, and good food suppy) and ready access to mates

NOTE  cages are 25cm-diameter PVC pipes, 10cm in length, with mesh ends and side windows.  Replicate cages house densities of 5, 10, 30, and 60 crabs . cage-1 The crabs are kept for 70d in the field.  Crabs that lose a cheliped (85 out of a total of 667) are considered separately in the analyses.  Interestingly, crabs that lose a cheliped appear to grow significantly faster than intact crabs, but comparable growth data for carapace width are contrary and, overall, it is hard to interpret the effects of cheliped loss in this study

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Research study 4

photographs of puncture wounds on the claws of porcelain crab Petrolisthes cinctipes courtesy Rypien & Palmer 2007 J Crust Biol 27: 59 graph showing proportion of individuals of porcelain crabs Petrolisthes cinctipes with punctured claws in wave-exposed and wave-protected areasPorcelain crabs Petrolisthes spp. are naturally aggressive, and use their claws in defense and intraspecific attack.  An examination of claws in P. cinctipes reveals distinctive puncture wounds characteristic of attack and wounding by conspecifics (see photo on Left). About 40% of field-collected crabs in the Bamfield area of British Columbia have puncture wounds.  A recent study at the Bamfield Marine Sciences Centre shows that density and size both play roles in injury frequency in the species.  First, crabs in wave-exposed areas tend to live in higher densities and attain smaller sizes than ones in wave-protected areas (12% smaller).

Second, crabs sustain significantly more injuries when densities are diagram of puncture wounds in the claws of Petrolisthes cinctipes in different-sized individualshigher, and there is a trend for smaller crabs to sustain more wounding
than larger crabs in the field (see histogram above Right).  

The concentration of wounds on propus and dactylus areas (see drawings on Right), the parts of the claw nearest an opponent during shoving interactions, strongly suggests that the claws mediate intraspecific interactions that can frequently escalate to injury.  But what are the crabs fighting over?  The authors note that injury frequency does not differ between the sexes, so sexual agonism, either within or between sexes, can probably be ruled out.  More likely, as suggested by the authors, is an interference-type competition for space required to spread out the maxilliped filtering baskets during suspension feeding.  Rypien & Palmer 2007 J Crust Biol 27: 59.

NOTE  densities in some areas reach 4000 . m-2

NOTE  throughout their paper the authors cite carapace widths
of 80-90mm for their specimens, which would be extraordinary,
indeed, as the species rarely exceeds 30mm in size

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