Dictyosphaeria cavernosa

Dictyosphaeria cavernosa, a green alga belonging to the class Ulvophyceae and the family Siphonocladaceae, is one of just three species in that family. Its common name is green bubble seaweed. This alga is multicellular, with a surface covered in bumps or bubbles, and is hollow—unlike its lookalike sister species, D. versluysii, which is solid. When young, it has a green-yellow color and can reach about 12 centimeters across.

Found worldwide, especially in Southeast Asia, it typically grows on rocks in shallow reef areas. Around Oahu, it appears in Kaneohe Bay, where sewage discharge played a major role in its expansion. The alga often hides in rock crevices near tidal pools and can initially be mistaken for a green rock. Its spread speeds up when nutrients are abundant, and it has several periods each year of heavy growth.

Between the 1960s and 1970s, increased sewage release into the ocean supplied the nutrients that fueled this alga’s growth. By blanketing coral reefs in a thick layer, it smothers and kills the reef along with other organisms. Scientists have studied the physical, biological, and chemical conditions of the reefs to find ways to curb the alga’s spread. Cutting off its nutrient source led to a decline in growth by 1977.