Asuka Sentoku

Regularity in budding mode and resultant growth morphology of the azooxanthellate colonial scleractinian Tubastraea coccinea

Scleractinia exhibit a variety of growth forms, whether zooxanthellate or azooxanthellate, according to factors that control asexual reproduction and ensuing coral growth. The azooxanthellate branching scleractinian Dendrophylliaarbuscula shows regular modes of budding in terms of the locations of budding sites, the orientations of directive septa, and the inclination angle of budding throughout colonial growth. This study reports that such regularities are also found in the apparently different growth form of the massive dendrophylliid Tubastraeacoccinea, which shows the following growth features: (1) the offsets (lateral corallites) always occur near four primary septa, except the two directive primary septa, meaning that the lateral corallites do not appear in the sectors of the two directive septa; (2) the two directive septa in lateral corallites tend to be oriented subperpendicular to the growth direction of the parental corallites; (3) the lateral corallites grow approximately diagonally upwards; and (4) these regularities are seen in the axial and derived lateral corallites among all generations during colony growth. Large differences in growth form are found between the branching D.arbuscula and massive T.coccinea, irrespective of the presence of specific regularities. It is likely that subtle modifications of certain parameters (e.g., budding interval, branch length, corallite size, and inclination angle of lateral corallites) have a strong effect on the overall growth morphology. A precise understanding of such regularities, which occur regardless of generation or taxonomic position, would contribute to understanding the “shape-controlling mechanism” of corals, which are an archetypal modular organism.

Regularity in Budding Mode and Resultant Growth Morphology of the Azooxanthellate Colonial Scleractinian Cyathelia axillaris: Effective and Adaptive Ways of Utilizing Habitat Resources

Abstract. Azooxanthellate coral species obtain nutrients by themselves, retaining their colonial growth forms independently of other species. This study examines the regularity of budding in the bushy, azooxanthellate scleractinian oculinid Cyathelia axillaris in order to identify the developmental constraints on colony formation and the ecological significance of colonial forms. The sympodial form of C. axillaris develops its bushy morphology by repeated dichotomous branching without clear axial corallites. The regularities in budding process are as follows: (1) in most cases, two buds originate simultaneously on opposite sides of medially constricted corallites near two parental second-order septa at individual distal ends; (2) the two directive septa of lateral corallites are oriented almost 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 growth. Thus, apparently complex, threedimensional colonies with numerous offsets are formed according to certain rules, irrespective of the generation of individual corallites. The strict developmental constraints and the subtle modifications on asexual reproduction greatly affect the colonial growth that is unique to C. axillaris, for which the regularities in budding are effective and adaptive ways of utilizing limited resources (i.e., growth spaces and nutrients).

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. J. Morphol. 276:1100–1108, 2015.

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.

Burrowing hard corals occurring on the sea floor since 80 million years ago.

We describe a previously unknown niche for hard corals in the small, bowl-shaped, solitary scleractinian, Deltocyathoides orientalis (Family Turbinoliidae), on soft-bottom substrates. Observational experiments were used to clarify how the sea floor niche is exploited by turbinoliids. Deltocyathoides orientalis is adapted to an infaunal mode of life and exhibits behaviours associated with automobility that include burrowing into sediments, vertical movement through sediments to escape burial, and recovery of an upright position after being overturned. These behaviours were achieved through repeated expansion and contraction of their peripheral soft tissues, which constitute a unique muscle-membrane system. Histological analysis showed that these muscle arrangements were associated with deeply incised inter-costal spaces characteristic of turbinoliid corals. The oldest known turbinoliid, Bothrophoria ornata, which occurred in the Cretaceous (Campanian), also possessed a small, conical skeleton with highly developed costae. An infaunal mode of life became available to turbinoliids due to the acquisition of automobility through the muscle-membrane system at least 80 million years ago. The newly discovered active burrowing strategies described herein provide new insights into the use of an unattached mode of life by corals inhabiting soft-bottom substrates throughout the Phanerozoic.

Intrinsic Constraints on Sympodial Growth Morphologies of Azooxanthellate Scleractinian Coral Dendrophyllia

Background Asexual increase occurs in virtually all colonial organisms. However, little is known about the intrinsic mechanisms that control asexual reproduction and the resultant morphologies of colonies. Scleractinian corals, both symbiotic (zoaxanthellate) and non-symbiotic (azooxanthellate) corals are known to form elaborate colonies. To better understand the growth mechanisms that control species-specific type of colony in azooxanthellate dendrophyllid scleractinian corals, we have studied details of the budding pattern in the sympodial colonies of Dendrophyllia boschmai and Dendrophyllia cribrosa. Principal Findings Budding exhibits the following regularities: (1) the two directive septa of offset corallites are oriented almost perpendicular to the growth direction of parent corallites; (2) offsets generally occur in either of the lateral primary septa that occur on one side of a corallite; the individuals thus show a definite polarity with respect to the directive septa, and only when branching dichotomously offsets occur in both primary septa; (3) the lateral corallites grow more-or-less diagonally upwards; and (4) the regularities and polarities are maintained throughout growth. Given these regularities, D. boschmai grows in a zigzag fashion by alternately budding on the right and left sites. In contrast, D. cribrosa grows helically by budding at a particular site. Conclusions/Significance The strict constraints on budding regularities and shifts in budding sites observed in the sympodial growth forms of corals greatly affect resulting morphologies in azooxanthellate coral colonies. A precise understanding of these intrinsic constraints leads to a fundamental comprehension of colony-forming mechanisms in modular organisms.

Morphological Variability in Azooxanthellate Scleractinian Dendrophylliids Governed by Regular Modes of Asexual Reproduction: A Computer Simulation Approach

Abstract. Morphogenetic mechanisms in modern Scleractinia provide insights into the habitats and growth forms of corals living in different environmental conditions. Scleractinian morphogenesis has been studied primarily in relation to extrinsic (e.g. environmental) factors, irrespective of specific regularities in budding and their impacts on growth forms. Morphogenesis in colonial dendrophylliids can be modelled by taking these developmental traits into account, so as to understand how intrinsic developmental factors affect morphologies resulting from colonial growth. We used a simple, voxel-based program to simulate the morphogenesis of dendrophylliid corals. Three parameters (budding orientation, inclination of budding, and interval of budding) were adopted to simulate a range of colonial growth forms, regardless of whether the forms exist in nature or not. The regularity of budding was fully taken into account, and various colonial morphologies were gained, even under strict regularity. The regulation of two of the parameters (inclination of budding and budding interval) gives rise to two distinct types of morphologies (branching and massive forms) that are represented by the extant taxa Dendrophyllia arbuscula and Tubastraea coccinea, respectively. This suggests that the growth forms of individual dendrophylliid colonies vary according to habitat conditions, and that they can be simulated by modifying only a few parameters, even given rigid developmental constraints on budding.