Papers and Publications: Interdisciplinary Journal of
Undergraduate Research
Volume 3
Article 14
8-26-2014
Coral preference of the polychaete Spirobranchus
giganteus in the Belize Barrier Reef
Ethan Evans
Sewanee: The University of the South,
Callie Oldfield
Sewanee: The University of the South,
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Recommended Citation
Evans, Ethan and Oldfield, Callie (2014) "Coral preference of the polychaete Spirobranchus giganteus in the Belize Barrier Reef,"
Papers and Publications: Interdisciplinary Journal of Undergraduate Research: Vol. 3, Article 14.
Available at: http://digitalcommons.northgeorgia.edu/papersandpubs/vol3/iss1/14
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Evans and Oldfield: Coral preference of Spirobranchus giganteus in the Belize Barrier Reef
INTRODUCTION
On coral reefs, there are numerous symbiotic relationships among reef organisms
(Goldberg 2013). Many of these symbioses are commensalisms, in which one participant (the
commensal) benefits while the other participant (the host) is unaffected. However, there are
some instances of reef commensalisms in which the host inadvertently benefits (Glynn and
Nochs 2011). For example, xanthid crabs that carry coral fragments as protection can potentially
help these fragments become established in different areas (Lamberts and Garth1977).
Spirobranchus giganteus (the Christmas tree worm) is a polychaete that commonly burrows in
corals in what is likely a commensalistic relationship (Gibbs 1969). The polychaete may offer
benefits to the coral host, such as greater water circulation from filter-feeding and nutrients from
its waste (Ben-Tzvi et al. 2006). Further, coral colonies in the immediate vicinity of the
polychaete may be protected from coral-feeding fish, facilitating coral colony regrowth after
heavy predation (DeVantier et al. 1986). This polychaete-coral relationship occurs in only certain
species of coral, because of the settlement preferences of the polychaete larvae (Marsden 1987).
The Christmas tree worm reproduces by mass spawning. Larvae are not sessile like adults
and swim until they find a suitable coral colony for settlement (Qian 1999). In a laboratory
preference study, Marsden (1987) demonstrated larval settlement preference on the coral species
Acropora prolifera. Populations of the genus Acropora have been severely reduced in the Belize
Barrier Reef over the last 30 years because of extreme weather and disease (Aronson et al. 2002).
A field study of coral preference performed in Barbados (Conlin 1988) determined that the coral
genus Diplora was the most preferred by S. giganteus. Hunte et al. (1990) demonstrated that S.
giganteus individuals thrive and grow larger on Diplora when compared to other coral
substrates.
Goldberg (2013) hypothesized that S. giganteus host preference may vary within large
geographical areas. In addition, coral preference by S. giganteus could result from preference of
larvae for a particular coral substrate color. Little is known about the abundance of S. giganteus
color morphs, but investigations on color morph spatial patterns in other genera of polychaetes
have been published. Crisp and Ekaratne (1984) noted that the distribution of Pomatoceros
triqueter color morphs was correlated with latitudinal gradients, but found no spatial pattern in P.
lamarckii. Rankin (1946) determined that the distribution of color morphs in Clymenella
torquata was related to environmental conditions, with flesh-colored individuals associated with
sandy substrates and green individuals associated with muddy areas.
The purpose of this study was two-fold. We investigated (1) whether S. giganteus prefers
a specific coral host in the Belize Barrier Reef (the second largest barrier reef system in the
world), thus testing Goldberg’s (2013) hypothesis that geographic variations exist in host
preference behavior of this polychaete. The Conlin (1988) study in Barbados was used for
comparison. In addition, we investigated (2) whether S. giganteus color morphs are associated
with coral preference.
METHODS
Data was collected from two patch reefs: Whale Shoals (16° 46’ 39.39”N 88° 06’
53.41”W) and Long Reef (16° 46’ 43.78”N, 88° 04’ 44.09”W), in May 2013 along the Belize
Barrier Reef south of Carrie Bow Cay. To assess coral composition, six 10 m length transects
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Papers and Publications: Interdisciplinary Journal of Undergraduate Research, Vol. 3 [2014], Art. 14
were placed parallel and at regular intervals within a 100 by 100 m study area at each site. We
identified all mounding corals to the level of genus including Diplora, Porites, and Montastraea.
Five 0.5 by 0.5 m quadrats were placed at 1.5 m intervals along each transect. In each quadrat, an
estimate was made of the percent composition of each genus. To determine S. giganteus settling
patterns, an investigator searched the study area in each site by moving at a constant pace along
parallel transects. We searched for S. giganteus individuals for the duration of 40 min at each
site. For each individual, we recorded the color of the stalk of the worm’s branchial crown and
coral substrate on which it was found. Color was classified as red, white, or yellow (Fig. 1).
To calculate the expected number of S. giganteus per coral species, the total number of
worms found for each site was multiplied by the percent cover of each coral species. Chi-squared
tests were used to test for patterns of substrate preference and for color morph association with
coral genera.
RESULTS
In both sites, S. giganteus preferred Diplora to the genera Porites and Montastraea (Fig.
2). At Whale Shoals, despite making up a small percentage of the overall coral cover (7.7%),
Diplora hosted a significantly higher (χ2 = 150.29, df = 2, p < 0.0001) percentage of S.
giganteus. Porites, even though it was the most abundant coral (80.5% of coral cover), hosted a
significantly lower percentage (χ2 = 12.12, df = 2, p = 0.0023) of S. giganteus. The number of S.
giganteus found on Montastraea was insufficient for proper analysis, despite having a similar
percent coral cover (11.9%) to Diplora. The S. giganteus individuals exhibited similar
preferences in Long Reef; the majority of the coral composition was Porites (77.9%), but fewer
than predicted S. giganteus were observed on the coral (χ2 = 9.75, df = 2, p = 0.0076). Diplora
made up slightly more of the coral composition at Long Reef (18.2%), and more worms than
expected were observed there (χ2 = 50.01, df = 2, p < 0.0001).
In both sites, red was the dominant color morph, regardless of coral substrate (Fig. 3).
The two study sites showed different trends in S. giganteus color morph in relation to coral genus
distributions. In Whale Shoals, there was no significant color morph association with coral
genus. In Long Reef, there was a significant positive color association between Diplora and red
color morphs (χ2 = 10.01, df = 2, p = 0.0067) (Fig. 1), and a significant negative association
between Diplora and yellow color morphs (χ2 = 9.33, df = 2, p = 0.0094).
Figure 1. Red
color morph S.
giganteus on
Diplora coral
substrate
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Evans and Oldfield: Coral preference of Spirobranchus giganteus in the Belize Barrier Reef
A.
B.
Figure 2. Percent coral composition and percent S. giganteus individuals found on coral
substrate at Whale Shoals (A) and Long Reef (B)
A. Significantly fewer S. giganteus were found on Porites and significantly more S. giganteus on
Diplora at Whale Shoals. The number of S. giganteus found on Montastraea was insufficient for
proper analysis. B. The S. giganteus individuals exhibited similar preferences in Long Reef;
Porites made up the majority of the coral composition, but more worms were found on Diplora.
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Papers and Publications: Interdisciplinary Journal of Undergraduate Research, Vol. 3 [2014], Art. 14
A.
B.
Figure 3. Percentage of color morphs per coral genus at Whale Shoals and Long Reef
A. Data from Whale Shoals shows a random distribution of color morphs on Porites, but nonrandom trends on Diplora for yellow and red color morphs. B. Long Reef had a slightly different
distribution, with significant negative association with yellow and positive association with red
morphs. The white color morph displayed no significant association in either site. We did not
observe S. giganteus on Montastrea coral on Long Reef.
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Evans and Oldfield: Coral preference of Spirobranchus giganteus in the Belize Barrier Reef
DISCUSSION
S. giganteus were most frequently found on corals in the genus Diplora. This settlement
preference for Diplora in the Belize Barrier Reef matches a similar pattern found by Conlin
(1988) in Barbados and therefore does not support the Goldberg (2013) hypothesis that there is
likely to be geographical variation in substrate preference. Diplora made up a small percentage
of the overall coral composition, yet settlement rates were significantly higher than on Porites or
Monastraea, so it is evident that S. giganteus were not randomly distributed among corals. The
non-random distribution may be a result of larval preference or differential mortality (Conlin
1988). Data regarding settlement on Montastraea was inconclusive due to the rarity of the coral
at the two study sites. The limited S. giganteus settlement could be related to the depth at which
the coral is found within the study site (Conlin 1988).
Although there were site-specific associations between worm color and coral substrate,
there were no significant overall trends between the two sites. The random distribution of color
morphs may reflect density-independent pressure from predation and habitat constraints.
Previous studies in the Red Sea have shown that a wide variety and even distribution of different
color morphs prevents predators from forming an association of S. giganteus with a certain color
morph (Grassle 1973). Further, the Indo-Pacific region hosts a variety of S. giganteus colors not
found in the Caribbean, such as blue, brown, and purple. The cause of these regional effects is
unclear. Even in our small-scale study, the color morphs varied greatly between the two sites.
Environmental stress and other abiotic factors, such as depth and temperature, may also
contribute to varying color morph distribution (Etter 1988). Whether these factors relate to S.
giganteus settlement preferences is an area of future research.
While our data and that of Conlin (1988) suggests S. giganteus preference is strongly
linked with Diplora in the Caribbean basin, studies on the Great Barrier Reef (Marsden 1988)
and in Taiwan (Dai and Yang 1995) demonstrated that S. giganteus is not solely dependent on
one species of coral for survival. Many factors influence S. giganteus larval preference, including
depth, prevalence of light, and percent coral cover (Marsden 1984). It is likely that the larval
preference towards Diplora is due to the expansive size and surface area of Diplora colonies, as
well as the relative depth of Diplora in the Belize Barrier Reef (Conlin 1988).
In reef ecosystems, mutualistic and commensalistic relationships among organisms are
common (Goldberg 2013). This interdependence between species makes them vulnerable to the
loss of either partner (Kohl et al. 2004). Corals are especially vulnerable to environmental threats
such as acidification, bleaching, extreme weather, eutrophication, or disease. Each coral species
has a different susceptibility to these threats (Burke and Maidens 2004). Diplora coral have been
shown to be less susceptible to coral bleaching in the Caribbean as compared to species in other
coral genera (van Hooidonk et al. 2012). Our results show a strong preference by S. giganteus
towards Diplora as a coral substrate. If this commensalism is a result of the resilience of Diplora
to environmental pressures, S. giganteus may be showing a preference towards coral types that
are less susceptible to bleaching. However, if this preference is obligatory, a future decline in
Diplora could cause a decline in S. giganteus abundance. Our study was located in the Caribbean
region, which contains some of the most endangered reef systems in the world (Burke and
Maidens 2004). In the two coral reefs we studied, we found a lower density of polychaete worms
(fewer than five per square meter) than seen in previous studies of other systems (Conlin 1988).
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Papers and Publications: Interdisciplinary Journal of Undergraduate Research, Vol. 3 [2014], Art. 14
Whether S. giganteus abundance is linked to coral decline is a question that is worthy of future
study in the Caribbean region.
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