Sulphur Leather Coral

Rhytisma fulvum, commonly known as the sulphur leather coral, is a species of colonial soft coral classified under the family Alcyoniidae. This species is indigenous to the shallow reefs of the 🌊 Red Sea and spans across the Indo-Pacific region. It was first described by the Swedish naturalist Peter Forsskål in 1775.

Rhytisma fulvum is a zooxanthellate species, exhibiting two distinct color morphs: yellowish-brown and grey. Despite the color variation, there is no taxonomic differentiation between these forms. The species is characterized by its encrusting nature, forming extensive sheets over the substrate that may intertwine. The polyps are diminutive, densely packed in rows, and each is elevated on a cone-shaped peduncle.

The geographical range of Rhytisma fulvum extends from the 🌊 Red Sea, through Zanzibar and 🇲🇬 Madagascar, to 🇮🇩 Indonesia, Papua 🇵🇬 New Guinea, and the Great Barrier Reef in 🇦🇺 Australia. This species thrives in reef environments at depths ranging from 3 to 40 meters (10 to 131 feet). As a pioneering species, Rhytisma fulvum is often among the first to colonize areas where corals have died or reefs have suffered damage.

Rhytisma fulvum is a dioecious species, with colonies being either male or female. In the 🌊 Red Sea, the reproductive season spans from late June to early August. By this period, the oocytes have been maturing for approximately ten months and the sperm sacs for seven months. Male colonies release sperm into the water column during a spawning event synchronized with the lunar phase, with shallow-water colonies spawning earlier in the year compared to those in deeper waters. Fertilization occurs inside the polyps' cavities in female colonies, followed by brooding of the embryos on the colony's surface, entangled in a sticky mucus. After approximately six days, planula larvae complete their development, are liberated, and soon settle on the seabed. Each polyp in a female colony produces roughly twenty eggs, a relatively modest quantity, which is offset by the brooding process that enhances the larvae's survival rate by minimizing predation risks.

In the 🌊 Red Sea, Rhytisma fulvum often falls prey to the nudibranch Marionia levis. This seaslug's appearance closely mimics the coral's surface, with its gills resembling the feeding polyps, providing effective camouflage. The presence of the nudibranch is typically revealed when the coral is disturbed, causing the polyps to retract and exposing the seaslug. Studies indicate that approximately 25% of Rhytisma fulvum corals examined were found to harbor Marionia levis.

Rhytisma fulvum plays a significant role in the ecological dynamics of reef environments in the Indo-Pacific region. Its adaptive reproductive strategies and interactions with marine organisms, such as the nudibranch Marionia levis, underline its importance in the coral reef ecosystem. Further research into the species' biology and ecology can provide deeper insights into the resilience and conservation of reef habitats.