The crab puzzle seems to be a Fibonacci sequence based math puzzle. Your goal is to select shells that will add up to the correct sum as shown by the blue crabs.


The puzzle is a three part puzzle, the first two seems to be randomly generated.


The shells on the right side of the screen are a table, showing the value of each shell. The sand dollar (round shell with a star in the center) is valued at 1. The next shell is valued at 2, then 3, then 4 and finally 5.




The shells placed in a spiral have different values, valued at (starting from the center of the spiral and continuing counter-clockwise) 1,1,2,3,5,8.




Add together the shell values shown by the blue crabs, then select shells that will add to the same sum. Note that you cannot select the same shell twice, except for sand dollars (1), of which there are two.




In other words, if the crabs are showing 3, 4, 5, add those together (12), then select shells that will add to 12. (8, 3, 1).
The Red Crabs show how many numbers you've selected. Once you run out of red crabs, you submit your answer.




The final shell puzzle is always the same, though I'm not quite sure the reasoning behind it. It's always two fives and then a final unmarked shell. For whatever reason, the answer to this one is 8, 5, 3, for a sum of 16. It's likely that this final shell is a 6 for this reason, but I cannot confirm.