… the phenomenon which may be called "carcinization" … consists essentially in a reduction of the abdomen of a macrurous crustacean, together with a depression and broadening of its
cephalothorax, so that the animal assumes the general habit of body of a crab
Keiler et al., 2017 defines a carcinised
morphology as follows:[4]
"The
carapace is flatter than it is broad and possesses lateral margins."
"The
sternites are fused into a wide sternal
plastron which possesses a distinct emargination on its posterior margin."
"The
pleon is flattened and strongly bent, in dorsal view completely hiding the
tergites of the fourth pleonal segment, and partially or completely covers the plastron."
An important and visually evident marker of difference between true crabs and carcinised Anomura is the amount of leg pairs. While Brachyura (true) crabs have four pairs of legs used for locomotion, Anomura possess one much smaller set and therefore three sets of walking legs.
Examples
Carcinisation is believed to have occurred independently in at least five groups of
decapod crustaceans:[4]
Infraorder Brachyura (true
crabs)[10] First appearance: Early Jurassic
The extinct probable crustacean order
Cyclida are also noted to "strikingly resemble crabs," and probably had a similar ecology.[11][12]
King crabs
The example of king crabs (family Lithodidae) evolving from hermit crabs has been particularly well studied, and evidence in their biology supports this theory. For example, most hermit crabs are
asymmetrical, and fit well into spiral
snailshells; the
abdomens of king crabs, even though they do not use snail shells for shelter, are also asymmetrical.[13][14][15][16]
Hypercarcinisation
An exceptional form of carcinisation, termed "hypercarcinisation", is seen in the porcelain crab Allopetrolisthes spinifrons. In addition to the shortened body form, A. spinifrons also shows similar
sexual dimorphism to that seen in true crabs, where males have a shorter
pleon than females.[17]
Selective pressures and benefits
Independently arising from multiple ancestral crustacean taxa, the crab-like traits exhibited vary between individual species and taxa. However, all crabs and carcinised organisms are
decapods. Correlations between the folding of the pleon tail and widening of the cephalothorax across disparate decapod species suggest similar evolutionary pressures. Some occurrences of carcinisation are derived from convergent but distinct developmental pathways, while others may be instances of
homologous parallelism[15] from shared ancestral body plans.
Most carcinised organisms are descended from the infraorder Anomura, which includes hermit crabs.[18] Many carcinised Anomura evolved from ancestors with morphologically intermediate forms reminiscent of modern squat lobsters,[19] not including the King Crab which is hypothesized by researchers to be descended directly from a variety of Pagurid hermit crab. There may be various advantages to adopting brachyuraform (true crab-like) traits.
The adoption of a crab-like body structure can convey a number of selective advantages for crustacean species. Carcinisation is associated with a lowered center of gravity, allowing these creatures to invest in sideways walking.[20] This evasive adaptation is particularly useful in an ocean environment with forward-moving predators. The pleon is held tightly under the animal’s
cephalothorax with reduced musculature, which protects the pleon’s organs from attack.[20] The smaller and more balanced frame facilitates concealment within rocks and
coral. The folding of the pleon below the carapace reduces the crustacean’s exposed surface area, and associated hardening of the pleonal
cuticle are all thought to benefit the fitness of this body type.
Evolutionary tradeoffs
The
caridoid escape reaction is an innate danger response in crustaceans such as lobsters and crayfish, which contracts abdominal
flexions and sends the crustacean flying backward in the water.[21] Brachyura and species which have undergone carcinization have strongly bent and immobile tails, which prevent them from utilizing this evasion strategy. The necessary muscles are no longer developed enough in these species to facilitate the necessary tail flipping. Crabs and false crabs are best suited to escape by ground pursuit in comparison to the quick aquatic escape provided by the caridoid escape reaction.
Porcelain crabs’ closest relatives are
squat lobsters, taxa which occupy a morphological middle ground, described by Keiler et. al. as “half-carcinized” due to their partially flexed pleons and carapaces that remain longer than they are wide. Many species do not become fully carcinised but only undergo the crab-like adaptations that are contextually beneficial, to varying degrees.
Coconut crabs (Birgus latro)
While most incidences of carcinization are in aquatic Anomura populations, it has evolved in the planet’s largest land-dwelling invertebrate,
Coconut crabs. A number of true crab-like features, such as a wide carapace, and a low abdomen with strong supporting legs, allow these crustaceans to wield muscular
claws and manipulate their terrestrial environments with greater ease.[22] The lack of an extended pleon greatly benefits their mobility. In this case, brachyuraform traits accommodate comfortable terrestrial locomotion and are far more pronounced in maturity, after the
larval and post-larval stages which remain obligatorily aquatic.[23] The repeated emergence of carcinised morphological structures suggests selective pressures in various Anomura niches and habitus often favor carcinization, though this may fluctuate and is sometimes reversed by the opposite process of decarcinisation.[24]
Decarcinisation
Some crab-shaped species have evolved away from the crab form in a process known as decarcinisation. Decarcinisation, or the loss of the crab-like body, has occurred multiple times in both
Brachyura and
Anomura.[25][26] However, there are varying degrees of carcinisation and decarcinisation. Thus, not all species can necessarily be classified as "carcinised" or "decarcinised." Some examples include the
coconut crab as well as other hermit crabs that have lost or reduced their outer casing, often referred to as "domiciles." While they retain their crab-like phenotype, their reduction in or lack of domicile necessitates a "semi-carcinised" label.
^Jonas Keiler; Stefan Richter; Christian S. Wirkner (2014). "Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (Crustacea: Decapoda: Anomala): an insight into carcinization". Journal of Morphology. 276 (1): 1–21.
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10.1002/jmor.20311.
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^
abLing Ming Tsang; Tin-Yam Chan; Shane T. Ahyong; Ka Hou Chu (2011). "Hermit to king, or hermit to all: multiple transitions to crab-like forms from hermit crab ancestors". Systematic Biology. 60 (5): 616–629.
doi:
10.1093/sysbio/syr063.
PMID21835822.