Splitting bta
 

Why do bta's split?
Thanks

many do it when they're stressed as a survival instinct.

They're a colonial species (i.e., they make colonies), so it's just another form of reproduction.

There seems to be some evidence that they also split due to stress. I.e., injury or other. Two halfs might stand a better chance of "making it" than one whole.

Is it the only way they reproduce?

No, they spawn as well. There are distinct males and females, during a spawning event they will release clouds of gametes (sperm and eggs), fertilization occurs in the water column. The larva are free swimming for several days then settle out as little tiny anemones on the substrata.

There is some work afoot on learning what triggers for spawning, and what it takes to rear the larvae into adulthood. Check this out:
http://www.sflorg.com/earthnews/en072406_01.html
http://www.abc.net.au/science/news/stories/s1000860.htm

They are both shrunk, I presume it's normal?

Yeah, sometimes they look a little rough for a day or two after a split.