Folk etymology and quick changes.

Folk etymologies occur most often when languages first come in contact and need words for each other's objects and actions. It is something like sparrow-grass for asparagus (though that might more properly be considered an eggcorn). An unfamiliar word changes in pronunciation to match with a word in English of similar sound. People then think that the word must come from"sparrow grass," because the grass part seems obvious and there must be some old story behind the sparrow part.

Crayfish from ME crevice, small freshwater crustaceans, which is from  13C Old French. That is in turn from Frankish krebvitja, related to kreb, the Germanic ancestor of crab. But it was not a "cray-fish." It just sounded like "fish" must have something to do with it, being right there in the stream and all. The "veess" sound morphed into "fish" by association. The pronunciation "crawfish" was uncommon in England but took off in the American South, likely tied to an unknown regional preference in the British Isles.

The first reference is in 1612, " I have caught with mine angle pike, carpe, eele,..creafish, and the torope or little turtle." Alexander Whitaker, in a letter from Jamestown.  Angle meant hook, so you can see how it is related to the other type of angle, meaning a bend. It was already becoming a synonym for rod and reel. The art of angling to catch a fish also involves the angle at which one casts the bait or lure in front of the fish. 

If you were alert to that "torope" in the sentence you may have connected it to the modern word terrapin, meaning a sea-turtle. Tortoise is used for land versions of the shelled reptile, and turtle became a more general word for either in North America. But we are not done with the oddities. Turtle originally meant a bird, as in turtle-dove. To turtle meant to speak fondly of a sweetheart, but later took on the meaning of fishing for turtles or of being turned over, unable to flip back or get up.

I was sure there must be some version of turtling that referred to hunkering down or retreating into a protected place, but I didn't find one. My OED is a few decades old, though, and it may have started coming into play. If I were to write "After Miss Bascomb humiliated him in front of the ladies, Algernon turtled into the background for the remainder of the evening," People would know what I meant.