Since I just spent a little while tracking this information down I figured it would be useful for people to know.
The UNIO mailing list, devoted to the discussion of unionid mussels, seems to no longer have a home page that describes how to subscribe or unsubscribe. One can, however, go through the Listserv software at the Florida Institute of Technology.
List homepage: https://lists.fit.edu/sympa/info/unio
To subscribe: Send an email to firstname.lastname@example.org with no subject and this line only in the body of the message:
subscribe UNIO your-email-here
To unsubscribe, I suppose you would do the same, substituting “unsubscribe” for “subscribe.”