Madracis auretenra

Madracis auretenra, called the yellow finger coral or yellow pencil coral, is a colonial stony coral belonging to the family Pocilloporidae. It is fairly common and lives in the Caribbean Sea and western Atlantic Ocean. This species was not originally recognized; it was separated from Madracis mirabilis due to differences in form and the depths where it grows.

Taxonomy

The species was formally described in 2007 by Locke, Weil, and Coates. They distinguished it from Madracis mirabilis, a deep-water coral, by its thin, branching, tree-like shape and its depth range of 1 to 60 meters. Other unique traits include bare skeletal areas at the base lacking living tissue, a fairly smooth surface between the polyps, a clear line of spines between the corallites, and the absence of secondary septa in the tightly packed corallites. The authors noted that many earlier studies on Madracis mirabilis might need to recheck which species they were actually studying.

Description

Colonies of M. auretenra form hemispherical clumps that can reach over a meter across. They are made of densely packed, cylindrical branches with blunt, finger-like tips. In fore-reef areas the branches are slender, but in back-reef and lagoon habitats they are thicker and the clumps are larger.

Unlike most stony corals, the living tissue covering the skeleton dies back on the lower parts of the branches as the coral grows, leaving bare skeleton, so only the branch tips stay covered. The corallites measure 1.1 to 1.6 millimeters across and have at least ten septa. The coral is bright yellow.

Ecology

This coral is zooxanthellate, hosting symbiotic single-celled protists in its tissues that provide products from photosynthesis and use some of the coral’s waste. To supplement this, the polyps extend their tentacles to catch zooplankton, mainly feeding on larvae of crustaceans, polychaete worms, and arrow worms. M. auretenra is a hermaphrodite, with each colony containing both male and female gonads. The release of gametes into the water is tied to the moon’s phase and other factors.

After fertilization, the planula larvae drift as plankton before settling on the seabed and turning into polyps. In some cases, the coral holds the gametes on its mesenteries and briefly pseudo-broods the larvae before releasing them. The coral also reproduces easily by fragmentation, a form of asexual reproduction.