Thornmound

The thornmound replaced its ancestor on the southern Darwin beaches. They now grow their shells in a stack of lumps, which increases surface area for photosynthesizing. They have spread to all the southern edge of Darwin, as their black pigment endangers them with overheating.

Individuals live for 6 years. For the first year of their life, they live a plankton-like existence, transiting between the beach surf (where they swim life jellyfish) and sand (where they crawl like starfish). Afterwards they become terrestrial, crawling about on higher ground for the next year. When this period of movement ends and a suitable position is located (i.e. one where they do not move for 1 day), they secure their ground with root-like extrusions and remain sedentary, taking this time to grow their mounds. Individuals release spores in the 3rd and 6th years