Turns out this is an Oroblanco (called a Sweetie in Japan and Israel, and now apparently here). A cross between an acidless pomelo and a white grapefruit, created by University of California researchers, it’s apparently becoming something of delicacy in the Japanese market. The idea being that it is (surprise) sweeter than an ordinary grapefruit, and less bitter.
A single-serving review: Interesting, but a tad pricy. Ours was sweet, but not as sweet as a good ruby grapefruit. A decently complex taste, with a sour finish. Which I like. This particular one had very thick pulp walls, giving it a bit more of a crunch than a normal grapefruit. Definitely more intereresting to look at than an ordinary grapefruit, but at 99 cents apiece, I’m not sure they warrant the extra spendiness — unless you’re looking for a striking color for the table, in which case they’re an excellent pick indeed .