EPA's Vessel General Permit and Small Vessel General

established coral assemblages, and chemical cues (Lewis 1974, Birkeland 1977, Goreau et al.
1981, Rogers et al. 1984, Baggett and Bright 1985, Harriott 1985, Hughes and Jackson 1985,
Sammarco 1985, Morse et al. 1988, Fisk and Harriott 1990, Richmond and Hunter 1990).
Growth rates are relatively rapid, expressed as the linear extension of branches, ranging from
1.57 to 4.33 inches annually, and have enabled elkhorn coral to construct significant reefs in
several locations throughout the Caribbean (Vaughan 1915, Jaap 1974, Adey 1978). Branching
species, such as acroporid corals, grow differentially in response to light such that coral polyp
growth maximizes exposure to available light (Kaniewska et al. 2009). Growth can also occur
from fragmentation and dispersal (Tunnicliffe 1981, Bak and Criens 1982). A broken branch
may be carried by waves and currents to another location and, if favorable, branches grow into a
new colony. Rapid growth and fragment dispersal facilitate a competitive advantage for elkhorn
coral relative to other coral and benthic species (Shinn 1976, Neigel and Avise 1983, Jaap et al.
1989).
Elkhorn coral require relatively clear water and depend almost entirely upon symbiotic
photosynthesizers (zoozanthelle) for nourishment (Porter 1976, Lewis 1977, Jaap et al. 1989,
Mieog et al. 2009) and is much more susceptible to increases in water turbidity than are some
other corals. Different strains of symbiotic zoozanthelle (Symbiodinium spp.) can confer
different thermal and light tolerances to acroporiids (Abrego et al. 2009, Ainsworth and Hoegh-
Guldberg 2009, Abrego et al. 2010). The type of Symbiodinium spp. may change during
ontogeny or remain the same, depending upon acroporiid species, and may be the same as parent
colonies or not (Baird et al. 2007, Gómez-Cabrera et al. 2008, Abrego et al. 2009).
Habitat
Colonies of elkhorn coral often grow in dense stands and form interlocking framework known as
thickets in fringing and barrier reefs, ranging in depth from 3.3 to 49 feet (Jaap 1984, Dustan
1985, Dustan and Halas 1987, Tomascik and Sander 1987, Wheaton and Jaap 1988). However,
optimal depth range is considered to be 3.3 to 16.4 feet in depth, with possible exposure at low
tide (Goreau and Wells 1967). Colonies generally do not form thickets below 16.4 feet, with
maximum water depths of framework construction ranging from 10 to 39.4 feet (Lighty et al.
1982). Elkhorn coral thrive in shallow reef zones where wave energy is a significant factor. In
areas with strong wave energy conditions only isolated colonies occur, while denser thickets may
develop in intermediate wave energy conditions (Geister 1977). The preferred habitat of elkhorn
coral is the seaward face of a reef (Shinn 1963, Cairns 1982, Rogers et al. 1982).
Status and trends
Elkhorn coral was listed as threatened under the ESA on May 9, 2006 (71 FR 26852). Elkhorn
coral underwent precipitous declines in the early 1980s throughout its range and this decline has
continued. Although quantitative data on historical distribution and abundance are scarce, best
available data indicate declines in abundance (coverage and colony numbers) by greater than
97%. Recovery from a bleaching event in 2005 is expected to take 10-12 years; this is after a
previous event in 1997. In all, roughly one-third of the Acropora palamata genotypes have been
lost as a result of these events (Miller and Williamson 2010).
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