It is not a "weed" - it is animals!
Episcophagus is spot on except for the maybe.1st it'll be obvious to a marine biologist that these are "goose barnacles" and which one can be found using the net's glaucus, (for the list) and species-identification (for illustration & description).
http://www.glaucus.org.uk/Barnacles.html gives at the TOP the overall classification of the part of Cirripedia containing barnacles, the Order Thoracica, of which the goose barnacles are in Suborder Lepadomorpha . at the BOTTOM is the list of UK species,with the 13 'goose barnacles' at the top.
(NB the barnacle classification seems to be 'in flux' so order Thoracica has become a superorder & Pedunculata's disappeared etc.)
Of the 13, 5 arent in species-identification, Alepas parasitica is parasitic on medusae,Anelasma squalicola deep[ly attaches to spine of the deep-water velvet-bellied shark, Scalpellum kempi I found no information, mitella pollicipes is in google images as is scalpellum velutinum, but as "arcoscalpellum michelottianum".
For the rest use http://species-identification.org , & in the search box enter "conchoderma aurita" & clicking 'next species' cycles through the remaining 8 + 1 extra, Scalpellum stroemi. These illustrations show the 5 Lepas sp's the only contenders. They all show 2 main parts, the stalk or "peduncle" & the other bit "capitulum".For all but 1 of the 5 the peduncle is given (in either description or size) as less than or about equal to the peduncle.But for Lepas anatifera the peduncle can be "sometimes several times longer than the capitulum" QED. (If the peduncle was only as long as the capitulum it could possibly be Lepas Hilli).
My Apologies, it was meant as an abbreviation for 'I would guess', & I don't know you but assume you're actually pretty sure as you gave the same (correct) answer (again a perfect photofit) 3 months ago, "Anyone know what these are".It surprised me that with good photofits & (as I believe) the right answers these stayed "unsolved" for months. But I hope my answer (given the authority of Glaucus & species.org as I'm no expert) holds.If not please disabuse me.
English is not my first language and I thought that I perhaps had expressed some uncertainty about my identification.
Martin & Davis in An Updated Classification of the Recent Crustacea (2001 - can be found at http://atiniui.nhm.org/pdfs/3839/3839.pdf) uses:
Subphylum Crustacea Brünnich, 1772
Class Maxillopoda Dahl, 1956
Subclass Thecostraca Gruvel, 1905
Infraclass Cirripedia Burmeister, 1834
Superorder Thoracica Darwin, 1854
Order Pedunculata Lamarck, 1818
Suborder Lepadomorpha Pilsbry, 1916
Family Lepadidae Darwin, 1852
On page 23 Martin & Davis says:
Although few new extant families have been suggested since 1982, there have been significant rearrangements of the cirripedes (or attempts to rearrange them) by workers using morphological and molecular data. Perhaps the most comprehensive is the cladistic study by Glenner et al. (1995), who concluded that many currently recognized groups appear to be paraphyletic, including the groups that appear in our classification under the headings ‘‘Lepadomorpha’’ and ‘‘Pedunculata.’’ However, Glenner et al. (1995) also noted that ‘‘we have far to go before a new taxonomy can emerge’’ and suggested the continued use of such commonly used terms as ‘‘lepadomorphs’’ or ‘‘pedunculates’’ as long as workers understand that these are groupings more of convenience than of common descent. We are not in agreement with this philosophy and would prefer to recognize taxa that reflect common descent, but in this group, it is apparent that we are not yet at the point where we know which clades are valid.
For the most part, we have followed the classification of the Thoracica given by Newman (1996). Thus, we are recognizing the order Pedunculata (an old name that was previously thought to lack validity but that Newman (1996) feels is a natural assemblage and thus has resurrected) as containing four suborders.[...]
My Enckell (Kräftdjur, 1980, 685 pages, ISBN 91-85330-27-2), which I use for identifying crustaceans, uses only subclass Cirripedia, order Thoracica and family Lepadidae and says "The often used partition in Lepadomorpha, Balanomorpha and Verrucomorpha has not been followed here."
But I agree with Martin & Davis and find Pedunculata useful for collecting all goose barnacles (which we call "långhalsar" - longnecks - in Swedish) in one taxon and separating them from the sessile barnacles (which we call "havstulpaner" - sea tulips), especially if Scalpellidae is included in Scalpellomorpha and not in Lepadomorpha.
Tolweb.org uses Pedunculata (http://tolweb.org/Cirripedia/8127).
And note that I use "perhaps", not "maybe".