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Close up of Dictyosphaeria cavernosa. Note the hollow chambers inside the bubbles.

Dictyosphaeria cavernosa, or "green bubble algae" is an opportunistic green alga which is invasive in overfished, high nutrient reef communities. The large "bubbles" efficiently trap sediments, nutrients and infaunal inside the chambers of the plant.

Dictyosphaeria cavernosa growing over Porites compressa at
Kaneohe Bay, Oahu.

Hawai‘i:  Northwest Hawaiian Islands, O‘ahu, Maui, Moloka‘i and Lana‘i.

Mechanism of Introduction:  Indigenous to Hawai‘i.

Worldwide:  Eastern Atlantic, Caribbean, Red Sea, Arabian Sea, Indian and Pacific Oceans.

Dictyosphaeria cavernosa is a native alga that has shown invasive tendencies in reef communities experiencing nutrient enrichment and overfishing. The thallus’ sheet-like morphology enables it to cover large areas of reef and create large hollow chambers where organisms and gases are trapped. Like other invasives, D. cavernosa efficiently captures available nutrients, resulting in high growth rates. The morphology of this particular green alga, however, is especially efficient: nutrients trapped in sediments are released to the water column and trapped in the chambers of D. cavernosa where they are held for uptake. This special "bubble" morphology has proven successful in areas of long term nutrient loading where the sediments have become a reservoir for additional nutrients.

The solid sheets of D. cavernosa can be devastating to coral reefs. Since the 1960’s, and possibly earlier, D. cavernosa has overgrown and displaced corals on reef slopes and outer reef flats in Kane‘ohe Bay, O‘ahu, Hawai‘i. The alga uses coral and limestone outcrops as an anchorage and proceeds to overgrow them, killing the corals by smothering. In a study of the standing crop of frondose algae at Waikiki as much as 300gm/m2 wet weight of the biomass was attributed to this species.

Dictyosphaeria species grow by segregative cell division, producing daughter segments that are initiated inside parent segments but grow outwards in the form of a bubble. The species also reproduces sexually by freeing flagellate reproductive cells through pores in the walls of the vegetative cells of the thallus. In conditions of high nutrient loading and overfishing, Dictyosphaeria species are fecund and successful competitors.