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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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.
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.
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.
Lionfish on the Pulley Ridge mesophotic reef in the Gulf of Mexico
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).
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).