Hydnophora rigida

Lore & Background

Hydnophora rigida was first described by James Dwight Dana in 1846. It is commonly found in shallow reef environments around Australia, the Philippines, Fiji, Indonesia, Samoa, Solomon Islands, the Great Barrier Reef, North West Cape, and the Coral Sea, as well as on the east coasts of Africa and around Madagascar. The species tends to inhabit lagoons and protected reef slopes.

The coral has thin branches that are cream or green without encrusting bases. Its colonies are bushy with small conical mounts called monticules, which form where the corallite walls of adjacent polyps fuse together. The polyps surrounding the base of monticules can appear brown and may contrast slightly. Tentacles can emerge at night.

Reproduction involves spawning after the full moon of November. H. rigida is a simultaneously hermaphrodite coral that develops eggs earlier than the testis. There are strong indications that many colonies carry out multiple gametogenic cycles each year. In the Houtman Abrolhos Islands, colonies contained ripe eggs in March, suggesting participation in a mass spawning event in western Australia. The population was found to be fertile from November to February, indicating a spawning season that may spread over several months.

Reader's Guide

Hydnophora rigida is significant as one of the most common species of Hydnophora, first described by James Dwight Dana in 1846. Its distribution spans shallow reef environments across the Indo-Pacific, including Australia, the Philippines, Fiji, Indonesia, Samoa, Solomon Islands, the Great Barrier Reef, North West Cape, the Coral Sea, and the east coasts of Africa and Madagascar. The species is notable for its natural green, brown, or cream coloration and its ability to exhibit fluorescent green and cyano-red emission. Its reproductive biology is distinctive: it is a simultaneously hermaphrodite broadcaster spawner that releases eggs rather than planulae larvae, with relatively small eggs about 220 μm in size. The coral spawns after the full moon of November, and evidence suggests multiple gametogenic cycles per year, with a possible biannual cycle including a March spawning event. The presence of ripe eggs before the full moon and ripe testes around the full moon of November, along with fewer fertile colonies in December, supports this pattern. The species' ability to spawn over several months each year distinguishes it from corals that spawn only once annually.

Did You Know?

• The coral can exhibit fluorescent green and cyano-red emission.
• It spawns after the full moon of November and may have multiple spawning cycles per year.
• The eggs are relatively small, about 220 μm, and the coral is a broadcaster spawner.