The clam Calyptogena magnifica, which lives on deep-sea vents, depends on a bacterium to supply it with nutrients; the bacterium is transmitted through the clam’s eggsLast year we published a paper on the complete genome sequence of this symbiont (which I wrote about here when I was clearly in a whiny kind of mood). And Judson picks up on a part of the story on the clam that is rarely discussed - the symbionts are transmitted vertically from parent to offspring. Vertical transmission seems to be linked to multiple properties of the symbionts (see my discussion of this regarding the glassy winged sharpshooter symbionts here).
Judson's post is really worth checking out for the symbiosis fans out there. She does a good job of highlighting diversity and evolution of mutualisms in a relatively short post.
See my video of a dissection of a baby Calyptogena: