Once I had determined what my mystery fruit was, I was ready to give it the "taste test." From it's rather prickly appearance, the first obstacle was getting it open. (click photos to enlarge)
Fortunately, a Brazilian friend who's well familiar with the ata told me that it actually comes apart easily in segments if you put your fingers gently into a fissure between segments, and slowly pull the fruit apart. So I gave it a try, and it opened easily.
As you can see from the photos below, the interior of the ata consists of a number of segments closely packed together. Each segment consists of a capsule of smooth and soft whitish flesh surrounding a shiny, dark-brown oval seed. The size of each capsule is no more than 1 inch long.
I found the best way to eat the ata was to pop a segment into my mouth, and then the seed is easily removed from the flesh and discarded. (Incidentally, the interior of the seed is said to be poisonous, so it's best not to bite on them). The flesh is incredibly sweet, with very low acidity, and flavors of pear, apple, and tutti-frutti. The texture is creamy, with a slight hint of the pear's grit. It's immediately evident why one of the English names for this fruit is custard-apple! There is not a strong aroma to the fruit.
Anyone who has eaten related tropical fruits like cherimoya, jackfruit, or even durian will be able to note both the visual similarity in the the fruits' structures, and the similar flavor profiles.
The ata is very high in caloric content, and I found that half an at provided a complete dessert. A whole one would be overfilling, I think.