# Study on the Development and Growth of Coral Larvae
## 4 Discussion  
The skeleton of a coral polyp consists of the columella, septa, theca, costa, and coenostea. The costa and coenostea are part of the skeleton of the exterior of the coral polyp. This study demonstrated two skeleton growth mechanisms: the skeleton-over-polyp mechanism and the polyp-over-skeleton mechanism. The _G. fascicularis_ and _M. elephantotus_ polyps exhibited the skeleton-over-polyp mechanism. Their primary polyps developed laminar skeletons from the bottom sections. The skeletons surrounded the surface of the polyps and formed the epitheca. With the growth of the laminar skeletons, they connected to the epitheca and formed septa. The epitheca covered the young polyps as they grew outward, forming the theca. The _P. verrucosa_ and _S. caliendrum_ polyps exhibited the polyp-over-skeleton mechanism; both the types of corals appeared similar to branches. Their primary polyps developed laminar skeletons from the bottom sections. Next, horizontal laminar skeletons were formed near the center of the polyps, forming the theca. Then, spindle-shaped skeletons were formed on the laminar skeleton. _P. verrucosa_ had compact cylindrical skeletons. The epitheca grew along the surface of the polyps and were smooth, and graduated stripes were observed. The septa lined the interior of the epitheca and protruded from the epitheca after the polyps developed tentacles. _S. caliendrum_ had a radial appearance. Its long laminar skeletons were arranged along the basal plate, and the skeletons closer to the center were taller. New long skeletons were formed around the center, connecting to the laminar skeletons, and forming the theca. This formation process caused the theca to become a polygon. The formation of the young skeletons suggested that the coenostea of the skeletons of _G. fascicularis_ and _M. elephantotus_ grew slowly through continuous stacking and compaction, whereas the skeletons of _P. verrucosa_ and _S. caliendrum_ were connected by the thin and stripe-shaped skeleton structures. The spindle-shaped skeletons under the coenosarc of _P. verrucosa_ formed stripe-shaped skeletons that were connected together. Therefore, the coenostea of _S. caliendrum_ may have formed new laminar skeletons from the spindle-shaped skeletons in the same manner. The skeletons formed through this mechanism were fragile but did not undergo the slow process of stacking, enabling the structure to respond to the rapid growth of the polyps. No study has examined the live skeletons of growing corals. Similar studies include [16] that observed similar skeleton structures. However, in this study, the development of the polyps was affected by the size of the corals and was not correlated with the species. In addition, [16] did not observe live specimens and thus could not determine the effects of the development of the polyps on the formation of skeletons. Other studies on coral skeletons have mainly investigated their components and microstructures [24; 25]. This study observed changes in the mouths, tentacles, and coenosarcs of the adult coral polyps. The polyps grew through mitosis. We speculated that the presence of Symbiodiniaceae species might have affected the development of the young _G. fascicularis_ and _M. elephantotus_ polyps. The eggs and larvae of these two corals were not infected by Symbiodiniaceae species, and they could only engage in symbiosis with the Symbiodiniaceae species after their larvae had settled and the mouths and coelenternos of their primary polyps had fully developed. The symbiosis between the corals and Symbiodiniaceae species was affected by several factors and caused the infection of Symbiodiniaceae to be probabilistic. We observed that the young corals infected by Symbiodiniaceae species developed polyps and tentacles; however, the growth of the young corals not containing Symbio