These are like small stones, approx 2-3cm long, rounded and bulbous at the base and tapering to a small vent hole at the top. Appear to have only this one entry/exit/breathing hole... Found them washed up amongst shells on a sandy beach at the low tide mark. No signs of any animal inside them... but are they shells, or larvae of some sort - or maybe a by-product of the construction taking place offshore?
On further inspection there are actually TWO vent holes at the top, albeint very close together, and merging into one at the opening. I guess that one will be entry and one will be exit. The "shells" appear to be made of small stones, shell fragments or shingle, cemented together in a curved form, with no openings except for the single vent hol containing the two tubes. I havent yet chopped one open, but maybe that will have to be the next stage. Anyone got any thoughts yet?
My thanks to Dick Hornby for the following observation:-
.......they are protective casings produced by a benthic marine invertebrate....
Polychaete tubeworms make a variety of hard calcareous tubes, and in many cases by cementing together small stones, but these don't really look like the work of polychaetes. I think it is more likely to be the work of sipunculans (trumpet or peanut worms). One pointer in this direction is the form of the holes...... .... very reminiscent of the holes made in the base of solitary corals, such as Heteropsammia cochlea, by a symbiotic trumpet worm called Aspidosiphon sp. (family Aspidosiphonidae). The trumpet worm moves a lot and thus prevents the coral from settling down and becoming smothered.
Some species of Asidosiphon occupy old mollusc shells. Perhaps this one does that but then adds more stone and shell material in order to strengthen the protective casing.
I have been continuing my research into this odd Beachfind, and I am now coming to the conclusion that it could perhaps be Cucurbitula cymbium. This is listed as #1259 in the Bosch book, but with a photo that doesn’t really fit too well with the specimens that I have found. However, when I stumbled across the image at http://www.conchology.be/images/Label/330000web/332841.jpg - I think that I may have found a match….
I'm sorry to see you've been all alone on this quest.
Well done for your pursuit, and also for making the ID - I think you can mark that post as 'correct'!
It is a new one on me, so thanks.
For other interested readers, note that the image link above is from this page
A Remarkable & unexpected result, stm, & you should mark your answer as correct; amazing that the inhabitants are not only molluscs but bivalves. In google images there's a photo showing the rather crude valves of C.c. from the Natural Hist museum Rotterdam (nmr-pics.nl )
Information on the species is hard to find; there is an academic abstract on them at http://www.academia.edu/3642939/Cucurbitula_cymbium_Spengler_1783_Bivalvia_Gastrochaenidae_sometimes_an_accidental_shell-collector with photos, but unfortunately in Dutch
But it's also in chap.5 of 'The Silent Sentinels - the Demise of Tropical Coral Reefs' at http://biophysics.sbg.ac.at/reefs/ch5.htm the chapter starts with bacteria & (many screens down) reaches mollusca, bivalves & Gastrochaenidae =boring bivalves,which are "able to dissolve calcareous matter as well as to line their dwellings, particularly the siphonal parts" ...& "The small G.(Cucurbitula) cymbium settles on shell debris and, after penetration, builds ‘iglu’-tubes to encase itself."