This marks the final appearances of Donna and Aldon Decker in the show.
Ryn's response, when she explained that she sensed that Helen was "one of us" is her referencing the time when she had discovered the scaly marks on Helen's left leg in "On the Road."
Helen and Ben are revealed be distant cousins, as Helen is the last living descendant of the mermaid/human hybrid daughter of Charles Pownall (Ben's paternal great-great-great-great grandfather) and his mermaid lover; being one-eighth mermaid through her great-grandmother. However, Helen lacks the agonizing aquatic ability to transform into her mermaid form, having skipped her generation.
Donna perishes from her wound and is laid to rest in peace, with fellow mermaids who were slaughtered while in human form centuries ago.