Ecowatch
Coral gardens
RINA MUKHERJI
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Two projects, one in Israel and another in Japan, look at ways of cultivating corals in laboratories and transplanting them to degraded reefs to rejuvenate the marine ecosystem.
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A thriving colony: A coral reef before (left) and after transplantation at Eilat; A year-and-a-half old cultured Aprocora corals at Okinawa (below).
Global warming and acidification has affected sea levels and the general health of oceans the world over. Coral reefs and marine species have been a major casualty of these phenomena.
High temperatures have caused the bleaching of corals and thereby affected the many marine species reefs are home to. This is particularly of concern since 25 per cent of all marine species live and grow on coral reefs. Serving as natural wave barriers, healthy coral reefs protect coastal communities, and provide them with an important source of protein. Being more biologically diverse than rainforests, coral reefs provide the raw material for many life-saving medical treatments such as the life-saving HIV drug AZT.
There have been several attempts to restore degraded coral reefs the world over. These have meant transplanting whole coral colonies taken from healthy localities into denuded areas. However, low survival rates of the transplants and the stress caused to donor colonies have proved this to be ineffective.
New techniques
Prof. Baruch Rinkevich of the Israel Oceanographic and Limnological Research, Haifa and Yael Haroszowski of the University of Quebec have hence adopted the “gardening coral reefs concept. This method consists of two steps: 1) Minute coral fragments are cultured and generated in an in situ nursery; 2) These colonies are then transplanted, when adult, on the degraded reef.
The transplantation of whole colonies instead of coral fragments substantially increases the latter’s ability to adjust to the new environment. It enables the generation of thousands of new colonies, thus allowing restoration of large zones in a short period of time.
The coral reefs of Eilat, Israel, are the world’s northernmost coral reefs. Although classified as among the richest in terms of biodiversity, the reefs have been in decline over the last four decades. In May 2007, 400 coral colonies were prepared at the nursery for transplantation. Once cleaned, the coral trays were placed in large recipients filled with seawater and transferred by boat from the coral nursery to the restoration site. The colonies were then transplanted onto two knolls by divers. Each colony was marked with a plastic tag in order to follow the corals’ acclimation and survival during the following months.
The scientists found that the transplanted colonies of Stylophora and Favia were, surprisingly, attacked by local fish. Those attacks led to tissue damage, loss of branches, and in extreme cases to the detachment of the colony from the substrate. However, the attacked coral colonies staged a surprisingly fast recovery. This was ample evidence of the good physiological condition of nursery-grown colonies which permitted them to cope with biological disturbances encountered on the natural reef.
Besides, the transplants were quickly colonised by invertebrates like Trapezia crabs, Spirobranchus worms and Alpheus shrimps in the following months. The spatial and ecological niches created by the presence of the reef-building corals were filled by obligatory coral invertebrates. A month later, the researchers also found that large numbers of larvae were being released by coral colonies at the site in the course of sexual reproduction.
Different approach
Half way across the globe, Dr. Makoto Omori and his team of researchers have been working at the Akajima Marine Science laboratory (AMSL) at Okinawa in Japan to restore coral reefs by culturing and transplantation by sexual propagation. To them goes the credit of having developed the world’s first technique of culturing and growing Acropora corals from eggs to juvenile colonies. Dr. Omori and his team fertilized the gametes, cultured the planula larvae, settled the larvae onto tiles, and cultured juvenile corals in cages.
Mass spawning of Acropora corals occurs around the full moon on early summer nights. Bundles of eggs and sperm are released together at night. Fertilization between different colonies takes place immediately below the sea surface soon after. Countless fertilized and unfertilized eggs aggregate and drift in the form of a slick the morning after spawning. The eggs and embryos are then collected from for mass coral culture. Alternatively, the bundles are collected by net underwater and fertilization immediately induced in the laboratory after spawning.
The embryos are then bred in large water tanks to grow into planula larvae. Five to six days after fertilization, the planulae swim down to the bottom to look for suitable places to settle. Searching for places to attach, planulae follow special chemical signals emitted by certain bacteria and crustose coralline algae on substratum. The larvae are induced to settle onto ceramic tiles that are placed on the seabed more than one month beforehand to grow algae and bacterial films on their surfaces. On the tile, each planula metamorphoses into a polyp (juvenile coral). These polyps multiply by producing clones to form a colony. The juvenile corals are then cultured in cages held by floating frames and suspended at 1.5 to 3 m depth into the sea. Though many planulae settle on the upper surface of a tile in the sea, sediments or algae overgrowth can easily discourage coral development. Tiles were hence vertically placed in the cage.
At this stage, the algae-eating juvenile top-shell snail, Trochus niloticus is introduced at the rate of 100 snails per cage Although only one to two percent of juvenile colonies generally survive, the yielding of a maximum of 10 colonies out of 700 polyps on a tile proved sufficient in this case. The corals are monitored periodically and the cages cleaned by brushing
Successful transplantation
Nearly 18 months after spawning, the Acropora tenius in the cages grow to an average diameter of six cm. In December 2006, about 2,000 colonies were transplanted onto nearby degraded bommies and fixed in place using pegs and underwater glue .Six months later, in June 2007, 89 per cent of the colonies were found to be alive and measured an average diameter of 9.1 cm across. By June 2008, these 3-year-old colonies had grown to 15-20cm in diameter.
According to Dr. Omori, accounting for the cost of cultivating one-year-old juvenile corals in open water in terms of maintenance personnel, equipment and consumables, with 10 colonies growing on each tile, about 3200 juvenile colonies could be
produced “at a per colony cost of
US $ 7-8.”
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