Mesophotic Coral Ecosystems

Lionfish on the Pulley Ridge mesophotic reef in the Gulf of Mexico

6.7.3. The Green Alga Avrainvillea sp. Avrainvillea sp., an invasive green alga (Figure 6.18) known for its propensity to displace native seagrass populations in the intertidal zone (Peyton 2009), was first reported in Hawai‘i in 1981 in 13 m of water off Kahe Point, O‘ahu (Brostoff 1989), and is now found offshore of O‘ahu at depths to 90 m (Peyton 2009, Spalding 2012). Originally identified as Avrainvillea amadelpha , recent research suggests that this identification is incorrect and work is now underway to confirm the invasive alga’s identity (A. Sherwood pers. com.). The Avrainvillea sp. spread from its first collection site at Kahe Point toMaunalua Bay (50 kmdistance) within about six years (Brostoff 1989). Presently, the distribution of this species in Hawai‘i extends along most of O‘ahu’s southern and western shores, and has been documented in a one-time collection from Kaua‘i (Smith et al. 2002). The mechanism(s) of its rapid propagation is not known. The species’ high abundance at mesophotic depths introduces the possibility that it may have originated in mesophotic waters and then moved into shallower waters (Spalding 2012). Some species of Avrainvillea form dense mats on either hard or soft substrata. These mats sequester fine sediments under their holdfast structures (Littler et al. 2005), creating anoxic mounds of soft sediment in otherwise hard-substratum- dominated environments (H. Spalding pers. obs.). Within its natural range, members of the genus Avrainvillea have been found at mesophotic depths in Mauritius ( A. amadelpha ) at 90 m (Gepp and Gepp 1911) and on a Bahamian seamount ( A. levis ) at 125 m (Littler and Littler 1992). Despite its high abundance around west and south O‘ahu, Avrainvillea sp. has not yet been found in the nearby ‘Au‘au Channel (located between the islands of Lāna‘i and Maui), which contains extensive MCEs, and its competitive impact on dominant native mesophotic macroalgae, such as Halimeda kanaloana meadows in this region is unknown. Lionfish are now common throughout the Western Atlantic Ocean, theGulf of Mexico andCaribbean.The first sightings in the Gulf of Mexico occurred in 2010, with lionfish populations having since increased dramatically on many shallow and mesophotic reefs. During submersible dives in 2010, only a few lionfish were documented along the West Florida Shelf MCEs in the Gulf of Mexico (Reed and Rogers 2011) and none were reported in the same region in 2003 (by a United States Geological Survey study). However, by 2013, a total of 703 lionfish were observed during remotely operated vehicle surveys at Pulley Ridge, a MCE (60– 80 m depth) on the West Florida Shelf (Reed et al. 2014; Figure 6.17). Of the 33 1–km 2 blocks surveyed at Pulley Ridge, 72 per cent contained lionfish, most of which were associated with active red grouper, Epinephelus morio, burrows. Burrows (5–15 m in diameter) are excavated by red grouper and provide habitat and act as oases for many small reef fish and lionfish. From multibeam sonar maps of Pulley Ridge, over 136,000 red grouper burrows are visible in the region, providing the potential for tens of thousands of lionfish.

Figure 6.17. The invasive lionfish has been found in increasing numbers at red grouper burrows at Pulley Ridge at 70 m (photo John Reed, NOAA Coral EcosystemConnectivity Expedition 2014).

The high abundance and ability of Avrainvillea sp. to grow on a diversity of substrata types increases the likelihood of its transport between the islands by potential transfer on construction materials for underwater development, such as pipelines, or by attachment to boat anchors. Removal efforts for mesophotic populations of Avrainvillea sp. would be costly and difficult, highlighting the need to limit the spread of this highly adaptable and invasive algal species.

Figure6.18. Avrainviella sp., an invasive green alga at 50m, is found offshore of O‘ahu’s southern and western shores from shallow to mesophotic depths. The diver is measuring the sediment depth under the algal mat. This species has the potential to transform a diverse, coral-dominated area into dense mats of macroalgae over anoxic mounds of sediment (photo Heather Spalding).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 81

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