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Dr. Shapiro's Puzzle of the Day

Today's Puzzle

Thursday, October 1
Can you identify these crabs? (This version of the puzzle has real crab facts. There is also a version with fake crab facts!)

This "crab" is not really a crab at all, nor is it even a crustacean. One of the oldest extant animal species, it had already been shuffling around sandy bottoms for 200 million years before dinosaurs came on the scene. Its blue blood is a critical ingredient in vaccines.
I suppose you're wondering about that giant claw. Per Wikipedia,
[REDACTED] crabs communicate by a sequence of waves and gestures; males have an oversized claw or chela, used in clashes of ritualised combat of courtship over a female and [to] signal their intentions between conspecifics. The movement of the smaller claw from ground to mouth during feeding explains the crabs' common name; it looks as if the animal were playing the larger claw like [REDACTED].
This crab collects objects (and even other living organisms) from its environment and sticks them to its carapace, for camouflage or to deter predators—some have been known to cover themselves in stinging anemones! They will use what's available, natural or otherwise; the specimen in the photo has bedazzled itself with pompoms.
Speaking of using what's available, this crustacean scavenges for a shell instead of making its own. As they outgrow the shells they occupy, they have to trade up. This passage from Wikipedia is too good not to share:
When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for up to 8 hours. As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab. As soon as a crab arrives that is the right size for the vacant shell and claims it, leaving its old shell vacant, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size.
Google this crab and you'll find more pictures of it on a plate with lemon than living in its natural habitat. Considered a delicacy, it's big business for California's fishing industry, but its season has been shortened or canceled in recent years due to the presence of a toxin called domoic acid.

   


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Yesterday's Puzzle + Solution

I was recently taking a walk at the edge of town when I saw a home with a banner out front that said

 SLOW! NO WAKE! 

A minute later, I saw another one! As it happens, there was something else unusual about these homes (and it's not that they were occupied by cavemen trying to ensure uninterrupted naps for their babies). What was unusual about them?

Solution They were houseboats. (Hence the subtle "edge of town" clue—I was walking on the waterfront between Oakland and Alameda.) A wake is the area of water churned up by a fast-moving boat; the houseboat owners didn't want to get rocked hard or splashed.

Congratulations to yesterday's solvers Charlie, Anna J., Jacob C., Nico, Yana, Leo S., Newton, Mr. Gregg, and Dr. Yetman. Thanks to everybody who made a guess!

About This Site

Though he now teaches mathematics, Dr. (né Mr.) Shapiro's first job in a K–12 school was as a lunch monitor in Davis, CA. It was there that he originated the Puzzle of the Day, even rewarding correct answers with tickets in denominations like "15 points" (though without a clear idea of how he'd ultimately redeem these). Dr. Shapiro's favorite puzzle from this pre-professional era was "Tell me the location of the beehive on this campus."

Ten years later, Dr. Shapiro revived Puzzle of the Day at Proof School, writing each day's puzzle on a name tag. After 600 puzzles or so, he was just starting to feel normal about students reading his chest all the time when campus closed and the puzzle, like the rest of our lives, moved online. New puzzles are posted daily on school days.

Want to catch up on old PotDs? There's an archive currently containing puzzles from March to June 2020.