Mind-boggling Mangrove Mystery

Life on the beautiful Western Pacific island of Yap, one of the Federated States of Micronesia, brings fringe benefits in the form of a plentitude of fresh seafood. Specifically, among this bounty, succulent spiny lobsters and sweet mangrove crab (Scylla serrata), sometimes also referred to as the mud crab.

While picking my teeth in the aftermath of a delicious meal (a large female man­grove crab that our friend Fillmed caught for us earlier the same day) together with my wife, I took a closer look at some of the leftovers. As I have been interested in genetics and evolution for quite some time, pon­dering deep mysteries such as “explosive radiation,” “punctuated equilibrium,” and “converg­ent evolution,” what I saw truly boggled my mind… (Note: steaming the crab turned its color into a bright red—its life­time color is a shiny deep dark blue-green)

Mangrove crab cheliped (actual size ~10 cm)

Note the “hammers and anvil” crusher “molars,” and especially the faked “tooth separations” and different color “gum line”! The first larger “incisor” in the upper row, with its partner in the lower row, looks very much like my own vestigial canines! There are even lines along the “gum line” indicating where the teeth “grows” (which they—of course—do not: these false teeth are all part of a single chitinuous structure, recreated (and growing) with each molting the crab goes through)!

These forms have been brought forth by some six hundred million years of natural selection, working on the results of random mutations... But somehow, these digital (undoctored original) images seem to be saying some­­thing different: Why would natural selection favor “false teeth” separations and gum lining, in an already fierce looking claw?

And how could a multitude of mindless germ line DNA mutations and gene replication errors eventually produce exact copies of mammal molars, cutting teeth, and canines (that pointed "tooth" looks very much like a “vestigial canine”), when simple serrations and solid “molars” (without faked gum lines and tooth separations) would have done the job just as well, and maybe better? I mean, what are the odds?

Why would evolution "go through the trouble" of producing such an uncanny likeness to real, growing, mammal teeth? By what mechanism would random evolution even “know” what mammal teeth look like? And why would that likeness even include false tooth separations and false gum lines?

I can see the utility of the "gripping" and "crushing" constructs, but what is the added survival value of these fake teeth (and of their fake separation lines)—when much simpler structures would have done the job (of providing intimidating, crushing, and gripping tools) just as well? Why would natural selection favor crab individuals equipped with fake mammalian teeth?

Here is the same mudcrab pincer, viewed from the opposite side:

Mangrove crab cheliped (left view)

To me, these “fake teeth” represent an awesome and beautiful mystery! What could possibly be the explanation? What is the link—that surely seems to exist—between the evolution of mangrove crab chelipeds and land predator/vegetable chewer jaws, two current and incredibly similar results out of two dramatically different evolutionary pathways?

Could it be that something else is at play here? Could it be that what drives evolution—in addition to random mutations and germ line DNA replication errors—is an altogether different mechanism? Could evolution somehow use “templates,” that can be adapted to whatever the environ­ment happens to be where Life casts its nets and establishes footholds? And if so, how would these templates be stored and accessed? (And, please note that I do not consider interference from a "good omnipotent designer" to be a viable option!)

Many questions, no answers... Does anybody have a clue, or even an opinion?

Having scoured the Internet and libraries for clues for quite some time now—searching left, right, and center using all keywords I could come up with: I’m rather surprised to find that there seems to be a total absence of references to these attri­butes of the mangrove crab pincers on the web!

If you know anything at all about this, that might help me better understand what’s going on, please drop me a line!

Henry Norman
Email: henry.ko.norman at gmail.com