# The structure of scleractinian coral skeleton analyzed by neutron diffraction and neutron computed tomography

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Jul 30, 2020

Concerning the hard tissue of coral skeleton, QTA evidenced the orientation of identically oriented mineral associations, in our case aragonite fibers/bundles with respect to a physical or geometrical coordinate system31. Following this model, we were able to analyze the preferred orientations of the principal crystallographic planes of aragonite. This was accomplished within the coordinate system of coral, whose Z axis coincides with the coral vertical axis.

The crystallographic network of aragonite, as reflected by the PF (001), (010) and (100) appeared well-pronounced and symmetrical (Fig. 3). Accordingly, the (001) PFs exhibits two weak reciprocal perpendicular and non-coincident maxima (Fig. 3a). At the same time, the (100) and (010) PFs display two belt like bands perpendicular to the Pfs equator along with a well defined central single maximum perpendicularly to them (Fig. 3b,c).

The degree of crystallographic preferred orientations can be characterized by the texture index, J, which represents a bulk measure for the strength of texture:

$${J = frac{{1}}{{{8}pi^{{2}} }}sumlimits_{i} {left[ {fleft( {g_{i} } right)} right]^{{2}} g_{i} } }$$

(1)

where g are the orientations in the G space.

For a random texture, J is equal to 1.0. In our case, the texture index J, calculated using WIMV algorithm29 implemented in the program package BEARTEX30, was found to be equal to 1.33; this signifies a weak, less organized, but not a random texture. Consequently, aragonite monocrystals form bundles representing the main constituents of the coral skeleton. Also J is a single parameter, in contrast to the ODF, which is a functions of many parameters.

Following this model, texture of aragonite bundles can be easier described using the fibers as individual components whose orientation is distributed along some well defined directions31,32,33,34.

According to the texture analyzing method utilized in this study, there are two types of components, i.e. “peak” and “fiber” whose orientation is parallel and respectively normal to corallites growth axis34. The orientation of peak component is described by three Euler angles (α, β, γ) as well as by the half-width parameter b which characterizes the distribution of preferred orientation. “Fiber” component is defined by two unit vectors with parameters θy, φy and θh, φh and by the fiber distance ωf. The first vector is the fiber direction (“skeleton line”) in the sample coordinates, and the second one is the fiber direction in the crystal coordinates (Table 1).

For a better understanding of the spatial distribution of aragonite bundles which form coral skeleton, we have fitted the experimental PFs reproduced in Fig. 3 following the Barnes and Luogh11 model. Consequently, we have used a linear combination of three texture components: aragonite bundles oriented along the individual corallites axes, in our case Z axis, and two aragonite bundles, normal to the previous ones.

While the PF (Fig. 6a,b) correspond to the vertical growth axis of the coral, the PF reproduced in Fig. 6c correspond to the normal to grow axis fibrils which assure the coral skeleton rigidity. The superposition of these three components is reproduced in Fig. 6d. To simulate the natural distribution of aragonite bundles, we have included a certain degree of fluctuation of the Euler angles (α, β, γ) characterized by a b parameter of about 30°.

This model does not take into account any randomly oriented fibers. Regardless this approximation, PF illustrated in Fig. 6d appear to be in god correlation with experimental PF reproduced in Fig. 3.

By comparing the experimental PF reproduced in Fig. 3 with the calculated ones illustrated in Fig. 6d, it appears that PF presents the same features. At the same time, PF appear more distorted in the case of experimental ones, most probable due to some imperfection of coral skeleton.

This observation proved that aragonite fibrils, which compose the coral skeleton, are not randomly oriented but follow a well established pattern as Barnes and Luogh11 suggested.

As for the coral skeleton structure, a better interpretation of the information furnished by the NCT can be done by comparing the optic (Fig. 4a) and the corresponding 3D volume rendering tomographic model (Fig. 4b). On the photographic image, we observed only the lacunar structure of coral colony skeleton with a multitude of small empty spaces allowing different polyps to communicate. Due to the high resolution of the photographic image, we could observe at the superior extremity of photography (Fig. 4a) the fragments of septa. In contrast, on the tomographic data (Fig. 4b), the internal structure of skeleton offers better clarity. Here, due to dependence of neuron beam attenuation on wall thickness, the distribution of individual coral cups (calices) is more acuratelly illustrated.

Due to the fact that hydrogen has the greatest linear attenuation coefficient (LAC) for neutrons, in order to evidence the presence of organic matter rich in hydrogen, we have selected the voxels with top 10% highest value and represented them in shades of orange (Figs. 4b, 5a).

More details on skeleton morphology are provided in Fig. 5b,c. The reciprocal distribution of calices is well represented on tomographic slice (Fig. 5b). Although the spatial resolution does not allow visualizing internal septa, this image shows with clarity the walls (theca) of individual cups (calices) as well as the way they are interlocked.

The same image helped us to estimate NCT spatial resolution at about 0.15 mm, enough to represent the most important skeleton features. Their presence is evidenced by an alternation of lighter and darker bands (Fig. 6c,d), similar to those previously as seen on thin section radiography of the Porites sp.11. Such features, in our opinion, could be associated with polyp annual growth, as reported in35.

The most intuitive model of the D. pallida colony is a beehive like structure which starts from an individual polyp, growths radially by adding consecutive generation which gives the colony an almost hemispheric shape, and whose surface is made of living polyps (Figs. 4a,b, 5a, 7).

Although ND and NCT are based on the same physical principles as X-ray ones, the lack of electric charge and the presence of rest mass, make neutrons quite different from the X-ray as far as, the interaction mechanism is concerned. In the case of D. pallida, ND not only to confirm the aragonite as the exclusive mineral component of exoskeleton, but also shows the spatial non-randomly distribution of aragonitic fibrils that compose the skeleton. In contrast to X-ray CT, the NCT 3D representation of coral skeleton shows the spatial distribution of minute amounts of organic matter.