why do crabs have more chromosomes than humans?

Answer 1

A lot of DNA can also be so-called "junk" DNA. This means that it does not code for anything. Introns are an example; others are SINES and LINES - short/long interspered elements. These are basically self-replicating portions of DNA that cut and paste (or just copy and paste) into new sections of DNA. This would increase the amount of DNA while not actually increasing the number of proteins made.
As stated above, number of cs is not equal to complexity. Neither is number of genes or amount of nucleic acid bases in the cs related to complexity. In fact, one of the largest genomes is found in a species of yeast.
Something during the evolution of crabs made it more efficient for them to have more cs than humans. Maybe they have a less reliable replication mechanism, so that they'd want smaller cs because their polymerse couldn't handle long cs. Maybe their method of wrapping DNA wasn't so hot, so they had to divvy up the genes more. (Humans and other species, like yeast, use histones to wrap un DNA and keep it organized.)

Answer 2

The number of chromosomes is not a measure of the complexity of an organism, as crab chromosomes may be very small, with very little genes in them.
That said, some simpler organism can have a huge amount of DNA to code their complete behavior, while humans may not need as much, since instinct is replaced by learning and parental care.

Answer 3

The number of chromosomes does not relate to the complexity of a species. I bet there are species of ferns with more chromosomes than crabs.

Answer 4

They're probably higher up on the evolutionary scale... We should bow to them and worship them, for surely that is the next step of evolution for us! Apes, humans, crabs...