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YM COLUMN

March 10, 2010

Is coral a plant or an animal?

When you are talking with Okinawan children, coral is often brought up for subject of conversation. Coral has a foundation made of calcareous skeleton at its bottom, upon which sits a main body called “polyp” or “coral insect”. Coral’s digestive organ is only mouth right in the middle of polyp and stomach connected with mouth through tube. Medical checkup of coral is very easy as compared to men’s, for they have only two digestive organs. The mouth double functions as anus from where spawns are laid, so it’s embarrassing what is a proper way to call it.

Outer mouth is extended fancy digital tentacles by which they capture zooplankton in the sea to eat them, which is to say coral is no doubt an animal. In a precise sense, it is classified to “Cnidarian” that has venomous stinger on the body or tentacles. Coral belongs to the group of jellyfish and sea anemone.

What makes conversation with children lively is how coral grows. There are two ways of procreation. One is division of polyp to procreate new cells around the body i.e. literally “nonsexual reproduction” to self-create new polyp. The other is “sexual reproduction” to make it fertilized with spawns released from other corals.

Although it’s simply said “spawns”, the fact is that it is a kind of spherical capsule in which are mingled numerous spawns and sperms. The fertilized spawns become larva called “planula” floating in the sea to eventually settle down in the ocean bed to grow into coral. By “nonsexual reproduction” coral may not be able to expand its distribution beyond one’s territory, but “sexual reproduction” may produce gigantic coral reef depending upon how far and wide released spawns expand of their distribution. Please click URL below and you will see beautiful images of great spawn laying of coral to color the sea of Ishigaki Island in pink all over.

http://www.churaumi.net/yonehara/coral/07coralspawn.html

What we must not forget is that just zooplankton is not sufficient enough for nutrition of coral, and so coral lives in a symbiotic relationship with the plant called “zooxanthella” that helps to nourish coral by photonic synthesis through chemical reaction of carbon dioxide from coral and sunlight. The reason why coral can only survive in clean seawater is because co-habitant zooxanthella needs sufficient sunlight to live.

Temperature rise in seawater due to abnormal climate or too drastic change in environments causes to extinguish zooxanthella from polyp and eventually extinguishing polyp itself only to leave skeleton of limestone behind. This is a phenomenon of so-called “coral bleaching”. Crown-of-thorns starfish also causes significant damage to the coral, which is becoming a grave social problem.

Okinawan children are so sensitive to such social problems. I’m writing this column by remembering a little girl named Angera who came from Tuvalu to join “Space Summit for Children” held in Hokkaido in springtime last year. At the Summit meeting she said, “My country is going under water in another ten years or so” and asked Japanese children, “When this happens, may I come to Japan again to stay?” Never before than now, we mankind have confronted really grave problem of global environments to think about in a serious manner.

I welcome your opinions on this column to the following E-mail address.
matogawa@planetary.or.jp

(Translated by The Planetary Society of Japan)

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