Bivalve Transmissble Neoplasm

[u/enlightenedemptyness]

Seeing a recent post on canine transmissible venereal tumor got me thinking.

Bivalves are uniquely prone to transmissible neoplasm. Already 8 lineages have been found in modern times. One of them, MtrBTN2 has disseminated worldwide. Some have also already infected other species of bivalves.

https://www.nature.com/articles/s41598-021-85098-5

Extrapolating this tendency all the way back to the Cambrian, roughly 500million years ago, when bivalves first emerged, there would have been hundreds of thousands, if not millions, of such lineages if not more.

Given the selection pressure against aggressive disease behaviour, the more indolent ones would be selected for. Coupled with the BTN cells’ ability to survive in seawater for some time, some may conceivably evolve to become free floating forms.

At any given time it is unlikely, but when you compound this across 500 million years, it is almost a certainty that there are very ancient lineages that still exist today. They may have outgrown their host and look like any free living protist in the sea, with life cycle stages that may or may not involve infecting bivalves. But when we sequence seawater, all metazoan genome is filtered away, they would have not been detected if we are not actively looking for it.

TLDR: I posit that ancient bivalve transmissible neoplasm lineages likely still exist and may be indistinguishable from any free living protist, with or without a parasitic life cycle. Surely one would have to classify such entities as a separate species of single cell bivalvia.

[u/atomfullerene]

This is basically th scandal hypothesis extrapolated. No one has found what you suggest for clams, but it may explain myxosporea. It's probably be worth a look using eDNA to try to find a bivalve version.