Interesting thread this.
In the Mediterranean we have four species OF Janthina which enter from
the adjacent Atlantic probably also with the Sargassum some of which
finds its way here.Some of them also end up beached in the Eastern
Mediterranean (Israel, Cyprus).
Janthina species are rare here and I have personally never found a
single specimen of Janthina and I spent a lot of time at sea. However,
my colleague Charles Cachia records an invasion of quantities of these
species, beached at Mellieha Bay, on the North East coast of the island.
This was during a persistent NE storm during the winter of 1978/79.
I have met at sea and seen large beach invasions of Velella, as in
winter/spring 1994,but there were no Janthina around.
At the near Atlantic there are two or three Nudibranchs which are found
on the Sargassum. I am not 100% sure, but two of these are Scyllaea
pelagica (L) and Fiona pinnata(Eschscholz).
The Dolphin fish, Coryphaena hippurus, follows the Sargassum annually
from the Gulf of Mexico into the Mediterranean during late spring to lay
its egg masses, together with the Blue fin tuna Thunnus thynnus.
Constantine.