[chris (OP)]

Why do crabs move sideways?

How did they evolve to do this in the first place?

[jeffreyH]

Maybe they are trying to sneak off the beach unnoticed.

[Colin2B]

Perhaps turn it around and ask what the advantage is of moving in the direction of the eyes.
Fast moving predators and prey have a big advantage in being able to see where they are running. Crabs don’t have many fast moving predetors and as scavengers they don’t chase prey, but the leg arrangement lets them get into very low crevices for protection. Interestingly, if threatened they can push themselves up against the top of the crevice and jam themselves in, a good reason not to try and pull one out if you are snorkelling - your hand can get trapped between crab and crevice roof,  not good.

[guest45734]

Why do crabs move sideways?

How did they evolve to do this in the first place?

Most crabs can like humans walk in the direction there legs are hinged for crabs they walk sideways normally. (But like humans who can walk sideways crabs can also can walk forwards). A crab’s legs are attached to the side of their body, and whereas our knees bend forwards allowing us to walk forwards, a crab’s joints bend outwards.

Maybe crabs find having more legs more attractive than two legs

It is thought that crabs’ bodies evolved this way because having their legs on their side gives them longer, flatter bodies, which makes it easier for them to hide in the sand. Crabs also scavenge for food, and because of this they do not need to run fast to chase their prey or hunt for food — the reason other animals have evolved with forward-facing, speedy legs.

There are however some species of crab that can walk forwards including the soldier crab. It’s able to do so because its joints allow for anterior-posterior leg movement.

[chris (OP)]

some species of crab that can walk forwards

Yes, indeed - i think horseshoe crabs walk "normally" don't they? But then, are they really crabs? I can't remember...

[guest45734]

some species of crab that can walk forwards

Yes, indeed - i think horseshoe crabs walk "normally" don't they? But then, are they really crabs? I can't remember...

Yes they are. I never new crabs could be so interesting and varied, I have tried to avoid them all my life

https://en.wikipedia.org/wiki/Crab

[Colin2B]

are they really crabs? I can't remember...

Something in your mind remembered correctly. Although they are arthopods they are a totally separate subphylum from the crabs we see on the beach. Thought to have branched off a long time ago from prehistoric sea scorpions.

[alancalverd]

'cos if they moved forwards, they'd be lobsters.