Regular budding modes in a zooxanthellate dendrophylliid Turbinaria peltata (Scleractinia) revealed by X-ray CT imaging and three-dimensional reconstruction.

The zooxanthellate dendrophylliid coral, Turbinaria peltata (Scleractinia), exhibit various growth forms that increase the photoreception area through the development of coenosteum skeletons. Because it is difficult to make detailed observations of the internal structures, we visualized inner skeletal structures using nondestructive microfocus X-ray computed tomography (CT) imaging. After removal of the coenosteum skeletons from the X-ray CT images, three-dimensional 3D-models were reconstructed for individual corallites. Regular budding was observed from the 3D-model and cross-sectional images as follows: 1) lateral corallites occurred only near the two primary septa on one side, apart from a directive primary septum with distinct polarity; 2) the budding occurred upward at acute angles; and 3) these regular structures and polarity were repeated throughout growth with every generation. Even in zooxanthellate dendrophylliids, the same budding modes as observed in azooxanthellate equivalents control the colonial growth. These characteristics provide clues for understanding the mechanisms that regulate the shapes of modular marine organisms.

Internal skeletal analysis of the colonial azooxanthellate scleractinian Dendrophyllia cribrosa using microfocus X-ray CT images: underlying basis for its rigid and highly adaptive colony structure.

Dendrophyllid Scleractinia exhibit a variety of colonial morphologies, formed under the strict constraints on (1) budding sites, (2) orientations of the directive septa of offsets, (3) inclination of budding direction, and (4) those constraints in every generation. Dendrophyllia cribrosa exhibits a sympodial dendroid form, characteristically large coralla, and occasional fusions of adjacent branches within the same colony. Adjacent corallites are bound and supported by coenosteum skeleton. This study examined the inner skeletal structures at the junctions of fused branches using a non-destructive microfocus X-ray computed tomography (CT) imaging approach, and considered the reasons for the large colonial sizes and their adaptive significance. Three-dimensional reconstructions of two-dimensional X-ray CT images reveal that individual corallites are not directly connected in fused parts. Additionally, no completely buried individuals were found within fused skeleton. When adjacent branches approach one another, constituent corallites change their growth directions to avoid collisions between the branches. The adjacent branches fuse without a reduction in the number of constituent corallites, leading to the establishment of reticular and rigid colonial structures. In addition, a nearly even distribution of individuals on the colony surface facilitates efficient intake of nutrients. Thus, the growth of large D. cribrosa colonies involves avoidance of collision between constituent individuals, the reinforcement of colonial structure, and efficient uptake of nutrients. These observations provide insights on the dynamics of interrelationships between colony-making mechanisms and the adaptive strategies required under habitat conditions such as specific current activities.