Thanks to Kay I now know it is Trapa natans.
It seems to be a pest in the US, here in Western Europe it is rare.

http://usuarios.arsystel.com/p.m/c/c...stana_agua.jpg

http://gallery.cs.umb.edu/gallery/Trapa-Natans

http://www.invasiveplants.net/invasi...rChestnut.html

http://www.aquariana.be/artikels/planten/trapa.htm












"Theo Griep" schreef in bericht
...

http://home.planet.nl/~griep/seed.jpg


These StarWars-like seeds (I'm not sure these are seeds anyway...) where
floating in a lake in France.
As hard as stone, 4 very sharp points each with small hooks on it.

Anyone knows if it is a seed, if so from what plant, and why it has this
strange shape ('how is it supposed to work?') ?


Theo