Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Skeletal fusion of probable clonal fragments in Acropora
Luke D. NOTHDURFT 1 & Gregory E. WEBB 2
1 Biogeosciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia;
l.nothdurft@qut.edu.au
2 School of Earth Sciences, University of Queensland, Brisbane, St. Lucia, QLD 4072, Australia;
g.webb@uq.edu.au
Modern reef-building corals inhabit high energy environments. Waves and currents are capable of
causing physical damage to coralla, such as fragmentation, as well as generating and mobilizing large
sediment clasts. Corals have developed various defence mechanisms to adapt to sedimentation events (e.g.,
HUBBARD & POCOCK 1972). Scleractinia can reject sediment to a certain degree by means of morphological
adaptations and by directed behaviour of the soft parts. Some studies have documented coral communities
in gravel-cobble dominated environments (e.g., BRAGA et al. 1990; PERRY & SMITHERS 2009), but the effects
of such large sediment on corals is poorly understood. Relatively little research has been conducted on the
incorporation of foreign material into scleractinian coral skeletons, and incorporation of gravel and pebble
sized sediment has only rarely been documented. Regardless, corals in reef environments are commonly
impacted by large debris, and that debris is in some cases incorporated into the growing colony (e.g.,
NOTHDURFT et al. 2011). Most of the gravel produced in clean reef environments consists of broken skeletal
material, such as coral branches. Here we report on two different outcomes when a colony received pebblesized
coral debris during a sedimentation event.
Pebble sized Acropora branch sections were observed enclosed within the skeletons of subfossil Acropora
coral skeletons collected from a beach at Bargara, Australia. They are encased within the skeleton in the
same way as other – non-carbonate clasts (Fig. 1A). In all cases, the cavities are primarily lined by obvious
clypeotheca. These coral branches are interpreted to have been dead when deposited on the living coral
surface becausethey are moderately adraded. In other cases, clearly broken and redeposited Acropora
fragments were found to have been incorporated onto existing colonies (Fig. 1B) on the reef flat on Heron
Reef, an enclosed lagoonal platform reef, approximately 70 km from the mainland coast of Australia in the
southern Great Barrier Reef. In this case, the branches rested at odd angles on the larger colony and were
then fused into place by coral growth both from the underlying colony and the branch itself. Samples were
collected for microstructural analysis on polished and etched sections with scanning electron microscopy
(SEM) and x-ray computer tomography.
The fused coral branch has come to rest in horizontal orientation amongst surrounding branches in the
underlying corallum that are predominantly upright (Fig. 1B). The axial corallites of the horizontal clast
and the upright branches that it is fused to are perpendicular to each other. Either end of the broken branch
is completely covered with growing corallites of similar architecture to those in the rest of the corallum. At
the depressed junction between the fused branches closely-spaced smaller corallites occur with no lips.
Underlying and overlying corallites directed toward the junction of the branch and corallum do not appear
to change direction and must terminate, but the exact nature of the process is not clear. Corallites over the
top of the fused branch appear to originate from the branch and from the underlying colony with no
discernable juncture. There appears to be a large amount of thickening deposits in the broken branch (Fig.
1C) where synapticulae have completely filled areas between septa, costae and ceonosteum. Importantly,
the broken end of the clast branch was completely covered by subsequent corallite growth.
We interpret the fused branches as most likely representing sections of the same genotype (i.e., genetic
clones) as they were so readily incorporated into the underlying colony. However, no genetic analyses have
so far been carried out to test that hypothesis. It is possible that they might represent a different genotype
within the same species, but it seems more likely that the branches came from clones. Fragmentation of
coral colonies is an important mode of reproduction for certain corals (HIGHSMITH 1982) and asexual
reproduction and fragmentation is an important process of replication/propogation of colonies in high
energy reef settings. This process has been documented particularly in branching colonies of the genus
Acropora. The degree to which this process occurs is largely dependent on the morphology of the colony
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