Yellow Pencil Coral

Madracis auretenra forms hemispherical clumps that can be a metre or more across. Each colony is formed of densely packed, cylindrical branches with blunt, finger-like tips. In fore-reef habitats the branches are slender but in back-reef and lagoon habitats they are more robust and the clumps are larger. The hard skeletal material of which the colony is built is in most coral species covered by a thin layer of living tissue, the coenosarc. M. auretenra is unusual in this respect because, as the coral grows, the coenosarc progressively dies back on the lower parts of the branches leaving the skeleton bare, and only the tips of the branches are covered with living tissue. The corallites are from 1.1 to 1.6 mm (0.04 to 0.06 in) in diameter and have at least ten septa. This coral is bright yellow.

Madracis auretenra is a zooxanthellate coral, housing symbiotic single-celled protists within its tissues. These provide the products of photosynthesis to the coral and use some of the coral's waste products. To supplement this food supply, the coral polyps spread their tentacles to catch zooplankton, feeding mostly on the larvae of crustaceans, polychaete worms and arrow worms.

M. auretenra is a hermaphrodite; individual colonies contain both male and female gonads. Liberation of gametes into the sea is linked to the phase of the moon and other factors. After fertilisation, the planula larvae form part of the plankton and eventually settle on the seabed and undergo metamorphosis into polyps. In some instances, M. auretenra has been observed to retain the gametes on its mesenteries and pseudo-brood the larvae briefly before liberating them into the sea.