Phuket scuba diving: andaman's amazing anemones
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Phuket Scuba Diving

 Andaman's Amazing Anemones

The seas around Phuket hold a wealth of marine life including a diverse range of anemone species. These flowerlike animals come in many different colours. Anemones are found from the edge of the beach at low tide to depths of less than 20m.

They do not live deeper than this because they contain' Zooxanthellae'. These are microscopic organisms similar to algae. They photosynthesize and provide the anemone with most of its nutritional requirements.

Zooxanthellae also produce much of the colour that anemones show. If an anemone is kept in very low light conditions, the Zooxanthellae die or evacuate in search of a better home,

just as happens with corals when the water temperature rises to high. The result is a white anemone (or coral) that will slowly starve to death even if fed. Small of fishes or prawns will be accepted by the tentacles and be slowly moved towards the central mouth, where they are ingested and after a couple of days excreted though the same opening.

Flesh alone is not enough to sustain anemones. They need the vitamins manufactured by the Zooxanthellae. All corals need these. Anemones can be thought of as a coral polyp without a calcium carbonate exoskeleton. The structure is fundamentally the same in all Coelenterates

The anemones we see at the beach, exposed at low tide, are mainly brown but amongst the coral many small species can be found in a range of colours. Some are quite remarkable with colourings such as apple green with purple streaks, measuring up to 4" (or 10cm) across when fully expanded. Anemones expand and contract by pumping water in or out of themselves. As they do so larval Zooxanthellae are sucked in or blown out along with various body wastes.

Sea anemones reproduce by'budding'. Small anemones form below the disk on the side of the column. When large enough they detach and float away to fine a new place to live. If its not to their liking they will detach, float off and try again.

Anemones have been kept alive at public aquariums for over 30 years. Estimates of their life span are up to 70 years or more. Some Pacific specimens have been found with a diameter of over a metre.

Nothing seems to eat them; they are all stings and slime. They inside is just mush. However the islanders of Koh Samui do eat one species. By wading knee deep at low tide, they collect a very beautiful species that lives just offshore. This species comes in a variety of colours, bright yellow or green being the most common. The anemones are dried in the sun before being added to various recipes. Fortunately, conservation laws have been introduced and this practice is fading away. In Thailand at least, anemones are not at risk, as they cannot be exported for the aquarium trade anymore.

Anemones catch their prey by using nematocysts. These stinging tentacles work by having a small barbed arrowhead attached to a thread that is coiled like a spring. When pushed against, these sting cells release their arrows and capture the intruder, if small enough. Larger animals break free. The stings from species around Phuket are not to bad but Caribbean species can be quite dangerous. Different people have varying degrees of sensitivity to the stings. This writer can handle local anemones and suffer only minor skin irritation for a few days. Some people have huge purple marks and broken skin for up to 2 weeks afterwards. Experimentation is not advised.

The best-known anemones in the vicinity of Phuket are the large purple species; Heteractis magnifica 25 - 30 cm in diameter. These are home to the common anemone fish or clown fish, Amphiprion ocellaris. The Thais call them cartoon fish for obvious reasons.

These beautiful small fish are only about 7cm long and lay their eggs on the side (column) of the anemones and guard them. Small groups of clownfish of every size'hover' above their host. If danger threatens or a diver approaches too near, they dive in amongst the tentacles for protection. They have an extra thick layer of mucus to protect them from the stings and are resistant to the toxins used in the stings. However, the anemones are also adapted to the fish and recognizing their mucus, tries not to sting them. If the mucus is wiped of by a scientist, as in an experiment, the released fish is killed by the anemones as soon as it seeks protection in its former home.

The purple green-fingered anemones used by the common clownfish are mainly found in coral reefs or, as at Hin Muang, they can be the dominant species and actually fight off corals. Other species of anemone fish live in different types of anemone. The Seabee clowns with their black and white stripes can grow to 5inches and live on top of sand anemones such as Heteractis aurora or Stoichactis gigas. Only their surface disk, 8 - 16 inches in diameter, is kept above the sand. These anemones bury their column and can contract rapidly by ejecting water from within there body cavity and pull their shrunken disk down below the surface when disturbed e.g. when a shark decides to lie on that particular spot!

A group of Ampiphrion sebae the yellow-tailed clown fish or Ampiphrion clarkii, hover over their chosen home. When threatened, their leader, the brood female, dives headfirst into the mouth of the anemone. This causes it to rapidly contract and fold its disk, the smaller clowns dive into the folds to wait for the danger to pass. Though the disks are mainly brown or grey these buried parts are a bright orange-red colour. The columns, hidden in the sand, go down 6 - 10 inches to anchor the anemone by its basal disk, which grips onto pieces of buried coral.

Phuket has three other species of clown fish; the skunk clown which is pinkish with a white dorsal line and two species of tomato clown. Each has its own niche and they are seldom found together. Anemones are also home to some species pf commensal shrimp. Commensal describes a mutually advantageous relationship. These small shrimps such as Periclimenes brevicarpalis are largely transparent with a few patches of white and purple. They provide a service to the anemone by keeping it clean and receive protection in return. It is not known quite how they avoid being stung and eaten by their host?

Anemones are also hosts to an amazing species of porcelain crab, Neopetrolisthes maculatus. At only about 12 mm across the carapace, or shell, these crabs have two legs modified to be sweep nets and they sit on top of the disk'sweeping' the water as it flows past. Fish don't try to catch them, as the anemone would sting their face. Where the anemone's advantage lies in this relationship is hard to see. Possibly they lull small fishes into a false sense of complacency that ends with the fish becoming the anemones dinner!

- September Issue, 2002


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