[Don_1 (OP)]

I'm not talking of their anatomical constraints, but more the evolutionary reasons.

Crabs have 360^o binocular vision, which is great for spotting predators and prey and picking bits of food with their pincers, but when walking sideways, the binocular effect must be impaired. So why did crabs evolve to walk sideways? Is there some benefit from this strange means of locomotion?

[maffsolo]

What did the crab look like before it evolved?

[Don_1 (OP)]

They probably descended from the Horseshoe Crab and before that the Trilobites. Neither walk sideways. So where, when and why over the past 600 million years did they come to walk sideways?

[maffsolo]

They probably descended from the Horseshoe Crab and before that the Trilobites. Neither walk sideways. So where, when and why over the past 600 million years did they come to walk sideways?

Beats me, can this question even be diputed with facts??

How could they evolve?
One of the most intriguing facts about crustaceans, as far as creation scientists are concerned, is not that they can climb trees, open coconuts, or hide behind a dot on this page. No, the most intriguing thing to creationists is that evolutionists have no idea how crustaceans could have evolved. Evolutionists don't even know how crustaceans are related to one another within their evolutionary framework.

http://www.creationtips.com/CrabMystery.html

Also;

http://www.cals.ncsu.edu/course/ent425/text02/arthropods.html

[Geezer]

Is it because they don't know whether they're coming or going?

[Don_1 (OP)]

[maffsolo]

Is it because they don't know whether they're coming or going?

Well on that note, It is best to have thenm on your piano then to place them on you organ

[RD]

They can move forward but because of their structure it may be more efficient to move sideways,
they can certainly shift quickly sideways, (see

0:37 - 0:44 ).

Animals change their gait depending on the speed they're trying to travel at, (e.g. horse: walk < trot < canter < gallop).
For the some crabs the top-speed gait is sideways.
 

[QuantumClue]

I'd agree with having something to do with their structure, and for the fact it may be more efficient to get to places quicker.

[SteveFish]

I have seen them at the pub. They always drink too much and when they head for the door, BAM! they go sideways into the wall. If you try to help them they get aggressive and those claws look nasty. It's a sneaky thing. They look like they are walking past and before you know it they are in your face because they went sideways. I don't like them, so I just throw them into the pot and eat their legs. Ha!

[keirastone]

they might be leading in going fast enough to reach first at the point, but they have turny legs ist's wy they walk like sideways.

[The supposed link an external picture has been removed as it flags up as spam. If you want post pictures then you can use the Insert-Image icon in the editor. Thanks, Mod]

[Dasyatis]

Crabs have 360^o binocular vision, which is great for spotting predators and prey and picking bits of food with their pincers, but when walking sideways, the binocular effect must be impaired. So why did crabs evolve to walk sideways? Is there some benefit from this strange means of locomotion?

I'm not sure what makes you think that moving sideways impairs their vision. They still retain the same field of vision, no matter what direction they are moving. And crabs don't always move sideways, but lateral movement does seem to provide the most efficient, and quickest way to travel in this group.

As for the evolution of this particular behavior, we may never know for sure, unless someone goes back in time and witnesses the divergence of crabs (Brachyurans) and hermit crabs (Anomurans), which previously diverged from and lobsters (Reptantians). Both hermit crabs and lobsters walk primarily forward. The evolution of the curling of the telson (what we sometimes call the crab's apron) was likely the first thing to happen, which then lead to the evolution of the behavior of walking sideways to compensate for the loss of balance that was previously provided by the telson.

In other words, when crabs first hit the scene, some may have missed the memo that without that junk in their trunk, moving forwards and backwards would be a little tricky. Others picked up the behavior quickly, and ran off with all the crab chicas to spread the good genes. That is a very loose interpretation of why crabs move primarily sideways, from an evolutionary perspective.