Nexu Crab are native to the Southern beaches of Ga-Wahi. Some do travel South to the lower areas of Mata-Nui, why they do is currently unknown.
They dwell is sea caverns far from shore and often live together in large groups for protection.
They are considered a delicacy to Ga-Matoran, but due to their protective nature they are hard to catch.
Since they often live together in large numbers, any attempt to capture one often upsets the rest of the herd and they will attack on mass using their sharp claws.
Many Ga-Matoran gather in large numbers at sunset to watch the Nexu Crabs cross the beach for food. Their golden shells glisten in the sunlight and create beautiful colours.
Naturally they are curious and shy creatures but are often savage and wild when provoked or threatened. They are very protective of their homes and family members.
Their natural predator is the Kohuru Bird. These birds are known to attack when Nexu Crab’s are out feasting on the beaches of Ga-Wahi. The Nexu Crab’s claws prove ineffective against Kohuru Birds due to their steel like feathers.
Fourth Wall Commentary:
I often mention in The Bionicle Inspiration Series, over on my YouTube how it's totally possible to make a MOC you can be proud of with limited parts. After recording some episodes on 'Low Parts Count' I was inspired to build some small Rahi that don't use too many parts. I decided on building a fun crab. I'm more then happy to make some more fun/small Rahi in the future, and likely will but, that said I currently have two other Bionicle MOC's, three new Star Wars Dioramas, a Marvel Diorama, A Boat, and a large Avengers Diorama that I still gotta post... so you know, you'll probably see them before you see these other Rahi... that said I may well build them soon and take some cool shots of them out around some trees/the beach etc. See what I can do :D
... You'll more likely see those shots on Instagram or Flickr though ;)