Settlement of a colonial ascidian on an artificial reef in the Gulf of Mexico

Abstract

An obsolete oil production platform, located in 58 m water depths in High Island A- 532, 12 nautical miles west of Stetson Bank, was converted into an artificial reef in 1997. The upper 27 m of the structure was mechanically cut off and placed on the sea floor, next to the undisturbed lower section of the rig. During “pre-construction” biological monitoring, divers observed a thin white encrusting ascidian colonizing most of the structure below the water line. Divers confirmed that between 1998 and 2000 this ascidian colonized the majority of both portions of the structure from 27 m to below 42 meter depths. Divers have recently observed this same species colonizing other platforms, within 12 nautical miles of this reef site.

Samples of this ascidian were collected and compared to other species found in the Gulf of Mexico (GOM) and the Pacific. The High Island A-532 Reef’s species was identified by ascidian expert, Dr. Gretchen Lambert, as Didemnum perlucidum, in the family Didemnidae. This ascidian forms a thin tunic layer over the colony, less than 1 mm thick, and appears white due to the aster-like spicules in the tunic. This species of ascidian can colonize structures through sexual reproduction of gametes and settlement of larvae or through asexual budding of itself. It appears to settle or grow over other encrusting organisms such as sponges, corals, bryozoans, hydroids, and mollusks.

Monitoring stations on the structure have been set up at this reef site to determine how quickly Didemnum perlucidum colonizes and what other factors may influence its growth and/or reproduction. First-quarter observations show that this ascidian can completely re-colonize 100 cm surfaces that were cleared of all attached organisms. This monitoring study is still in progress, andenvironmental as well as seasonal factors are being considered.