Montipora dilatata

Lore & Background

Montipora dilatata exhibits highly variable morphology, with colonies forming any combination of encrustations, plates, knobs, and branches. It is a glabro-favoleate type characterized by a very smooth surface. Colonies are usually purple or brown and can reach 3 feet (1 m) in diameter. The species occurs only in the Hawaiian archipelago, specifically in Kaneohe Bay, Oahu, and in the Northwestern Hawaiian Islands at Midway Atoll, Pearl and Hermes Atoll, Lisianski Island, Laysan Island, Maro Reef, and French Frigate Shoals.

Ecologically, this species requires calm water in sub-tidal environments and is restricted to shallow, low-water motion environments such as lagoons and bays. It is easily broken into fragments by storms or bioerosion, and these fragments readily grow into new colonies. The species is very rare in Kaneohe Bay, though it used to be more abundant and occurred in large patches on some reef flats.

Threats to M. dilatata include vulnerability to coral bleaching due to high temperatures (it was the first species to bleach during the 1996 event in Kaneohe Bay), fresh water kills and exposure at extreme low tide, habitat degradation from sedimentation, pollution, alien alga species (Gracilaria salicornia, Kappaphycus and Eucheuma spp. algae) and invasive green alga (Dictyosphaeria cavernosa) in Kaneohe Bay, limited distribution, and damage by anchors, fish pots, swimmers, and divers. Two similar species—M. turgescens at Kure Atoll and M. cf. dilatata (recorded at one site out of 30 sites surveyed at Maro Reef in 2000-2002)—require genetic analysis to confirm their separation into distinct species.

Reader's Guide

Montipora dilatata is significant as a rare, endemic Hawaiian coral with a very restricted distribution. Its decline is documented: only three colonies were found in a 2000 survey, and it was the first species to bleach during the 1996 event in Kaneohe Bay, indicating high sensitivity to thermal stress. The species faces multiple threats including bleaching, freshwater kills, habitat degradation from sedimentation and pollution, competition from alien and invasive algae, and physical damage from human activities. Its limited distribution makes it especially vulnerable. The species is listed as a U.S. National Marine Fisheries Service Species of Concern, meaning there are concerns about its status and threats but insufficient information to list it under the U.S. Endangered Species Act. Coral collection is not allowed in state waters of Hawaii without a research permit. The presence of similar species (M. turgescens and M. cf. dilatata) that may require genetic analysis underscores taxonomic uncertainties that could affect conservation priorities. The species' ability to regenerate from fragments offers some resilience, but overall its rarity and the array of threats highlight its precarious status.

Did You Know?

• Only three colonies of Montipora dilatata were found in a 2000 survey.
• The species is easily broken into fragments by storms or bioerosion, and fragments readily grow into new colonies.