It seems that this sea slug, which feeds on filamentous algae (Vaucheria sp.), intracellularly sequesters algal chloroplasts. Simple enough, right? No. The algae are secondary photosynthesizers, ancient heterotrophs whose ancestors acquired that ability from other algae. Their chloroplasts only encode a small fraction of genes required for their function, while the rest are ordinarily encoded by the nuclear genome. So, how does the sea slug maintain functioning chloroplasts without the supporting cast of nuclear genes? Horizontal gene transfer.
The authors find that at least one gene (psbO) is encoded by the sea slug's own nuclear genome. It is nearly identical to the algal gene. This strongly argues for the horizontal transfer of photosynthetic ability from prey to predator. Amazing!