They are always found growing on our floating docks. Halichondria has a symbiotic green algae in its tissues. It produces a large amount of spongin and is excellent for reaggregation studies. Halichondria is also good for demonstrating commensalism: multitudes of small pink anemones, polychaetes, and porcelain crabs live in the canal system. In the aquarium it starts growing rapidly into a thin layer, incorporating gravel, rocks, and shells into the colony. Size: 1 cm. and larger. It is the original source of Okadaic acid, an algae toxin.