Of Cichlids and Snails: A Foray into Malacology

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Perissodus microlepis is a cichlid fish with an interesting specialization. They eat scales from other fish and come in two forms. “Left handed” forms attack the right side of fishes and “right handed” forms attack the left side. An interesting adaptation is that the jaws of these fishes are curved to the left or the right depending on which type they are:

The victims survive, though becoming wary of attackers from either side. If the population of left-handed scale eaters were to exceed that of right-handed scale eaters, however, the fish would become more wary of attacks from the right side. As a result, the right-handed scale eaters would have an advantage, and their population would increase. These forces ensure that the relative populations of left- and righthanded fish remain roughly equal.

This has usually been taken to be an example of frequency dependent natural selection. The same process operates in crabs and snails…


A recent article on BBC News has the story.
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The above is a picture of a Flame Box Crab which preys on whelks and cone shells. The picture below shows right and left handed shells from two different species.
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Sinistral and dextral prey: a) Sinistrofulgur adversarium, b) its right-handed counterpart Busycon carica; c) Conus adversarius, d) Conus cf. largillierti.
Researchers studied fossil shells dating to 2.5-1.5 MYA and found:

Ten out of the 11 pairs showed higher rates of scars on dextral shells, suggesting that crabs are attacking them in preference to their left-handed counterparts.

What does it mean:

Typically a crab grasps a shell with the pointed end away from its body.
With a dextral shell, this means the opening is on the right, and the special “tooth” on its claw can break in. But with a sinistral shell, either the opening is on the left, or it has to grasp the pointed end towards its body.
Both solutions are apparently too much trouble; the crab simply moves on.
“They picked them up, and they just dropped them,” said Dr Dietl.
“If we left them for a long period of time they would probably figure it out; but in nature these left-handed shells are really rare.”

There is a wrinkle though:

Presumably, if left-handed marine snails became more common, crabs would eventually evolve apparatus or techniques for eating them, and their advantage would disappear.
But that cannot explain why in some populations they persist only in extremely low proportions, about 1%, or why in others they have gone extinct; other factors must be at play.
Sinistral snails apparently find it much harder to find a mate, and so may be doomed to remain rare or die out completely, whether or not they evade can-opening crabs.

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One Response

  1. What interesting adaptations! Thanks for explaining it.

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