"Handedness"

By far the preponderance of west-coast crab species have symmetrical or monomorphic claws, but what factors in evolution have led to this?  A study at the Bamfield Marine Sciences Centre, British Columbia on Cancer productus tests the effects of juvenile diet on claw morphology using 4 experimental treatments.  The treatments consist of 2 diets (“hard”: intact mussels, and “soft”: opened mussels of similar size) and 2 claw states (“free”: both claws free, and “immobilised”: one claw glued¹ shut).  The 4 treatment groups are maintained² on their diets, following which the volume of manus (the main part of the claw excluding the dactylus) of each crab is measured and crushing strength assessed.  Results show that crabs on the “hard” diet develop relatively larger (by 10-13%, and stronger, claws than those on the nutritionally equivalent “soft” diet after 1-2 and 2 moults, respectively.  Crabs with one claw immobilised develop asymmetrically sized claws, with the free claw presumably compensating by larger size for its constrained counterpart.  The authors note that such short-term adaptive responses to environmental stimuli, if heritable, could lead to long-term evolutionary changes in claw size and, if combined with behavioral biases³ to one side (“handedness”), possibly to claw dimorphism.  Smith & Palmer 1994 Science 264: 710. Photograph courtesy Iain McGaw, U Nevada.

NOTE¹ one claw, either right or left, is selected at random from each individual of a population of mixed male and female juveniles, and glued shut with cyanoacrylate adhesive within 48h after the first moult in captivity.  After each subsequent moult for the duration of the experiment the same claw is re-glued shut within 48h

NOTE²  it is not clear in the report how long the study lasted, except that it is over the duration of about 2 moult cycles

NOTE³  a possible research study, if it hasn’t been done already, might be to compare the extent to which monomorphic and dimorphic crabs differ in behaviours pertaining to “handedness”

Juvenile red rock-crab Cancer productus 1.5X

Lithodid crabs generally and box crabs Lopholithodes foraminatus specifically have asymmetrical claws, with the right claw in both sexes being larger than the left (see photo on Left).  Note in the photograph that not only do the claws differ in size, but their morphologies (and presumably functions) also differ. The condition appears early in development and is seen in the glaucothoe, 1st juvenile instar, and of course later stages.  During culture of this species in the laboratory, a researcher at University of Victoria, British Columbia observes a high incidence of reversed asymmetry, that is, a condition of left-handedness, and asks the question “is the handedness heritable or environmentally induced”.  Culture of eggs from wild-caught ovigerous females (3 normal females and one with reversed asymmetry) produces larvae with 20-30% incidence of reversed asymmetry, suggestive of a Mendelian inheritance ratio, but independent of maternity.  What about environmental effects?  No effect on reversal of asymmetry is seen following removal of the right cheliped in 4th-stage zoeae or glaucothoe stage, nor does rearing temperature (8-16^oC) affect the frequency of reversal.  The lack of evidence for either heritability or environmental induction of handedness is puzzling (based on published data for other species), and the author discusses the implications of the data with respect to the functional significance of asymmetries.  Duguid 2010 Evol & Development 12 (1): 74.

NOTE  in females, but not in males, there is also correspondence with the abdomen, which is asymmetrical (see photo on Right).  Reversed  asymmetry, as occurring in the laboratory cultures described here for both sexes, is only rarely seen in females in the field.  In such a case both the claws and abdomen become asymmetrical in a left-biased way

NOTE  the terms claw, chela, and cheliped are used synonymously here.  This is not meant to confuse; rather, it is done so beginning students will become familiar with the jargon