Riverfruit

Riverfruit was the name given to a form of plant life native to the planet Alzarius.

Riverfruit resembled Earth watermelon. It appeared to be part of the staple diet of the Alzarians who apparently farmed it as a crop. They considered riverfruit theft a relatively serious offence.

Before and during Mistfall, Alzarian spider eggs appeared within riverfruit. These would rapidly develop until the fruit split open and released the adult spider. (DW: Full Circle)

Given this development and the accepted link between the Alzarian spider, the Marshman, and the Alzarian humanoids, it is reasonable to assume riverfruit also contains the same unusual cellular characteristics as the descendant organisms.