The mind boggles at what this might be for!!
Or at the size of the animal it's intended to stop: caltrop.
Hello Dave,
This is an object which I don't know the name - not even in Portuguese - that is used to hold the tides and stop the advance of the sea water to go too far away ashore.
Look here at Etermar - a firm in Setubal - at the symbol itself and to the slide show at the right. That will give you an idea of what it is used for.