I have observed a similar occurance at a locality  in Nova Scotia, where
the presumably carniverous species Nucella lapillus L. often lives on
Zostera ("eelgrass"), along with several  herbivores such as Littorina
obtusa L., L. saxatilis Olivi, and an elongate form of  Tectura
testudinalis (Muller).  Presumably, it knows a free lunch when it sees
one!  Might Mr. Herbert's Eupleura also be part of a community of
organisms living on the seagrass, such as juvenile barnacles, which
often occur on Zostera as well?
                                                                                        Ross Mayhew