Coral (Prabal) bottom dwelling, sessile, marine invertebrates that belong to the class Anthozoa of the phylum Coelenterata. Anthozoans with a stony or calcareous skeleton generally secrete the skeletal substance around the epidermis, so that the structure, which is a true exoskeleton, conforms to the external surface of the lower part of the polyp. Each polyp thus builds around and under itself a cup or disk of some sort. This structure is known as a theca or corallite. Colonial anthozoans bind adjacent corallites together into a variety of structures by secreting additional skeletal material between individual polyps.
Corals of St Martin's Island (These Corals died after 1945)
A complete skeleton, whether the single theca of a solitary anthozoa or the united theca of a colony, is designated as a corallum. The shape, wall structure and internal architecture of corallite, and in some cases of coralla, also are of great importance to the palaeontologists studying fossil corals. Corals are abundant in the fossil record in all periods later than the Cambrian (570 to 505 million years old). At certain times corals were so abundant that the ancient seas, like certain oceanic areas of today, are referred to as 'coral seas'. Ancient corals built banks and reefs that now lie as great structureless masses in the midst of stratified limestones and shales. In recent years these ancient reefs, which are also known as bioherms, have attracted considerable interest as possible reservoirs in which petroleum could be accumulated.
Corals flourish best in waters with temperatures of about 22'C and common reef building corals cannot endure temperatures below 18'C. As a consequence, most modern coral reefs are limited to continental and island shores in tropical and subtropical waters between 28' north and south latitudes. The largest known coral reef, the Great Barrier Reef off the east coast of Queensland, Australia, extends for a distance of more than 1930 km and varies from 16 to 145 km in width. In Bangladesh st martin's island is the only island where coral was formed. Here the oldest coral is dated (C14 dating) as 33,238 years old. Though favourable conditions prevail today for the growth of coral colonies, neotectonic activity on the island is causing greater damage to them.
Corals of St Martin's Island are mainly scattered among the boulders on the beach and on the land in the interior of the island. Some isolated blocks are also found on the sandy beach. The majority of the coral boulders are small in size. However, some massive blocks of corals are noted at some places. A total of 21 species of fossil corals have been identified from the island. They are: Porites labota, P. lutea, P. solida, Platygyrn sp, Goniopora stokesi, G. columna, Favia favus, Favites chinesis, F. helicora, Goniastrea retiformes, G. aspera, G. pendulus, Cyphastrea seralila, Cyphastrea sp, Favites sp, Galaxea astrea, Acropora aculeus, A. glaucarudis and Montipora informis. However, many of them do not represent the real reef forming species.
Similarly, 39 species of living corals have also been identified. Among them the species of Porifies, Favites, Goniopora, Cyphastrea and Goniastrea are most abundant. The corals are represented by 7 families. A number of soft corals and Zoanthids have also been recorded. But these living corals will soon become dead as a result of neotectonic activity on the island (the island is now uplifting approximately at a rate of 19 mm/year).
The local people collect corals from mid-October to April, during which period the sea is calm, water is clear and the tide level is also favourable for collection. Corals are normally collected during the full moon and new moon periods. Mainly four kinds of corals are collected. They are locally called pataphul (Acropora sp), gacch phul (Acrora sp), Shaibal (Favia sp) and mog (Goniastrea sp). Among them Acropora is the most sought after. Corals are mostly used as decorative pieces and also for making jewellery. [Sifatul Quader Chowdhury]