Aretz et al_2011.pdf - ORBi - Université de Liège

_________________________________________________________________________________________________________
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
_________________________________________________________________________________________________________
Regularity in budding mode and resultant growth morphology of
azooxanthellate colonial scleractinian Cyathelia axillaris
Asuka SENTOKU & Yoichi EZAKI
Department of Geosciences, Faculty of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan; sentoku@sci.osaka-cu.ac.jp; ezaki@sci.osaka-cu.ac.jp
Azooxanthellate corals obtain nutrients by the capture of plankton and suspended organic particles
without symbionts. Their colonial growth is therefore not influenced by light. This study examines
regularity in budding in the branching (bushy), azooxanthellate scleractinian oculinid species Cyathelia
axillaris, with the aim of better understanding developmental constraints on the formation of colonies.
A sympodial form of Cyathelia axillaris develops its bushy morphology by repeated dichotomous
branching here and there with no corallites clearly axial. Two buds often originate simultaneously on
opposite sides, at individual distal ends of corallites. The parent corallites are in some instances immersed
in coenosteum occurring between two branching corallites. Their aragonite skeletons are hard and finegrained.
The largest colonies known attain about 20 cm in height, and consist of approximately 150
individuals. Corallites are circular in outline when small, and become elliptical, or medially constricted if
located at the proximal parts of branching corallites. The greater part of its calice forms according to two
directive septa. The calicular long diameter is approximately 1.0 cm, whereas the calicular short diameter is
about 0.6 cm. In order to decipher regularity in budding processes, the following features are noted in
particular.
(1) The offsets (lateral corallites) always occur in the vicinity of two parental second-order septa, not in
the sectors of the two first-order directive septa; (2) the two directive septa of lateral corallites are oriented
perpendicular to the directive septa of the immediate parental corallites; (3) the lateral corallites grow
more-or-less diagonally upwards; and (4) these regularities remain valid from parental to derived lateral
corallites throughout colony growth. Apparently, complex bushy corals are thus formed according to
certain rules, irrespective of generation of individual corallites.
It is highly probable that the presence of these strict developmental constraints on asexual reproduction
greatly affects colonial growth of the Scleractinia, both extant and extinct. In addition, these regularities in
budding revealed by azooxanthellate scleractinians, as one of the representative colony-bearing metazoan
groups, provide us with fundamental data towards the understanding of how colonies are constructed.
157