I think this may be the green form of irish moss, a seaweed that can also be red. Growing just below the surface of a rockpool in Scotland.
No interactions present.
in which case the dark swellings on the frond are probably the cystocarps, the female reproductive structures.
It's a shame you have to wait for the few Cranks (those who look at seaweeds). But I am certain you are right, and so it Jo.
I have agreed though.
See if you can tag all your Seaweed posts as Marine Alga, that way, if ever Admin decide to separate these from plants and even freshwater algae, they will be able to find them all! Hooray..
Lat/Lng: 56.0595, -2.7149
OS grid ref: NT555854