The Darwin Diaries #3: The Shellfishness of Nature

“Land-shells and fresh-water crustaceans are born having their proper forms, while the marine members of the same two great classes pass through considerable and often great changes during their development.” -Charles Darwin, On the Origin of the Species

Sally Lightfoot Crabs: After throwing up over the side of the boat during our transport from Santa Cruz to Floreana I was having a miserable morning in the Galapagos. Chunks of the scrambled eggs I had for breakfast were still caught in my hair and windbreaker as I ran from the boat for dry land. As I hung on to the nearest port pillar and pronounced my love for the stable and safe ground under my feet I saw a blur of red out of the corner of my eye. I walked closer to the edge of the dock and saw hundreds of large red crabs surrounded by small black rocks, until those rocks began to move and I realized they were crustaceans too. I abandoned my luggage and forgot my nausea as my curiosity compelled me to get closer, the black and red individuals were indeed the same species (which I recognized as Grapsus grapsus, or Sally Lightfoot Crabs), but they were all at different developmental stages.

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My view from the dock on Floreana Island

Upon further exploration while staying on the island for three days I found that each developmental group seemed to have it’s own specific phenotype which ranged from pure black (the youngest of the crabs) to the bright red of adults. The smallest (and thus the youngest) individuals were the hardest to see as they blended into the ebony lava rocks effortlessly. As they got a bit larger they seemed to produce speckles of green and blue, but still maintained black as their predominant color. At some point in development the black begins to become more crimson in hue, the green and blue phenotype was retained, and yellow begins to cover their faces. When full fledged adults their final form looked very different, their dark crimson brightened to a scarlet and the blue and yellow spots fully transformed into a yellow head and body, but their blue bellies still remained. Much like Darwin pointed out in the above quote these marine crabs go through a drastic morphological change between molting periods in their short life spans.

Sally

While sitting on a pier attempting to eat ice cream on the equator (something that results in a sticky mess) a classmate and I lamented on this drastic change in phenotype. ‘The red ones are so easy to spot from far away, why is that evolutionary beneficial?’ she asked, I looked up and realized she was precisely right. Even I, a visually challenged human (a species already visually challenged when compared to birds or other mammals) could clearly see hundreds of red adults from meters away just asking to be predated on. Then an idea struck me. I, as an embarrassingly over-enthusiatic science nerd, had read all about John Endler’s guppy experiments in which (long story short) he proved that the conflict between sexual selection and natural selection can create multiple phenotypes appropriate for the particular life stage of an organism.

To apply this finding to the Sally Lightfoot Crabs this would mean that the first priority of younger crabs is to survive natural selection in order to eventually reproduce. Having a dark color that allows one to fade into it’s environment ensures that they will have a better chance at surviving to adulthood and not be branded an evolutionary dead end. However, the adults have survived natural selection and must now compete against sexual selection. The best way to do this is to be more colorful than your fellow crabs. Color usually reflects health in animal systems (this has been studied in birds extensively) and thus females are more inclined to choose those with flashier colors (and thus better genes). Though the adult crab’s second priority is to survive natural selection and reproduce again next mating season, the need to beat out competitors is greater and thus saturated pigment is produced.

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Here lays Bob. He may have fallen to natural selection, but he had a lot of bitches.

Fiddler Crabs:

While walking through a mangrove on Isabella a classmate shouted ‘Fiddler crabs!’ I hurried over, very eager to see these queer creatures with assymetrical claws, but when I arrived I saw nothing. ‘Where?’ I asked, someone pointed for me… I still didn’t see it, I squinted my eyes attempting to follow their gaze looking for crabs of a comparable size to Sally Lightfoots when I saw a small claw open and close. ‘That’s them?!’ I exclaimed, feeling a bit guilty for insulting their size in front of them. I’m sorry, but when you hear about crabs with a mega claw you expect it to be attached to a bigger body. I began to examine each individual’s ‘handedness’: 3 out of 4 of the crabs on this particular rock were ‘left handed’, meaning their large claw was on their left. On another rock I found a group of fiddlers that were primarily right handed.

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As you can probably tell from the pictures the ‘mega claw’ is pretty useless, in fact it’s sometimes a hindrance when the crab attempts to be mobile. The smaller claw has more utility as the crabs use it to pick up their food. So, what’s the point of having this comically large claw? Sex of course. Males will often use their claws while fighting one another (similarly to how Ovis canadensis use their horns), however, as you may expect left handed males can only fight other left handed males (and vice versa). Interestingly enough, if a male loses their large claw before or during molting they’ll actually gain a large claw on the opposite sagittal plane. I had to wonder what the evolutionary advantage was of this, why would switching the claws be beneficial when an organism had adjusted to using the other side? My hypothesis is that it may be a survival technique. If you’re a male crab and you suck so much as a left handed fighter that you lost your hand, perhaps you may make a better right handed fighter. Despite my theory there is no evidence that one side is more powerful or more frequent than the other.

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Photo taken by Roxanne Astrid Van Hove

Males also use their large claw to draw the attention of females, they often will communicate with potential partners through a sequence of waves and gestures. These motions are where they got their name (Fiddler) as their large claw appears similar to a fiddle that they are playing. Fiddler crabs are also masters of what biologists call ‘dishonest signaling’ as they will intimidate other males with the mere size of their claw despite it’s relative weakness. Thus, males with smaller claws may (and usually do) have stronger claws, but are unwilling to fight a male with a larger (but weaker) claw. This presents an evolutionary conundrum as those individuals with the biggest claws are not necessarily the strongest with the best genes. So, unlike the Sally Lightfoot crabs, phenotype does not directly correlate with health.

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Hey ladies, don’t mind my inferior genes.

Mantis Shrimp, Penaeid Prawn, Caridean Shrimp, and Hermit Crabs

Yes! We found a mantis shrimp! It punched me a little, but because it was a baby it didn’t cause me to crack into a million pieces. For more information about the awesomeness of the mantis shrimp check out this.

On a side note, if anyone can help me identify the species of the small marine crab (pictured below) let me know!

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