Dragonfruit, Tuna, Prickly Pear: Cactus flowers and fruit are simply not standard fare. The flower is so thoroughly surrounded by stem that even the outer layer of the bud (and then the floral tube) is covered with leaves. Yes, leaves. The ovary, then, is tucked down inside a spine-covered stem. When mature, people consume Cactus fruit as Tuna, or Dragon Fruit, or even Cactus Berry. And, they do resemble berries; the bulk of the flesh being seed-laden pulp, sometime white, other times beautifully red or light purple.
At one time, most cactus fruit were the Tuna harvested from Opuntias, but Dragon Fruit (from clambering Hylocereus plants) have become sufficiently popular and available as now to be generally available in markets.
It is no trick to pinpoint close relatives of cacti, because this plant family is in a branch that constitutes a major evolutionary group of plants which share many characteristics not seen in other plant groups. Outside the Cactaceae, those related plant families would be represented in Markets by Purslane, Spinach, Beets, Amaranth, and Quinoa (Chenopodium).
Carpologists find there are a lot of differences between Cactus fruit and what we observe in other plants. The presence of spine clusters reminds us the outer layer of the fruiting structures is stem tissue, covered by green, almost foliar leaves that make the exterior of the floral tube a fleshy involucre. This fruit parallels the pome structure of Apples, but is unique and absolutely diagnostic for the family. Dr. Spjut uses the term Acrosarcum to indicate the suite of characteristics. The problem is that he uses this same word for fruit of other plants that are incredibly different. We designate the fruit of Oaks as Acorns, and the fruit of Beans as Legumes, because the structure is distinctive and cleanly associated with those taxonomic groups. I propose we use the word Tuna as a collective term for the Cactus fruit type.