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An Anemone Fish shelters among the stinging tentacles of the Anemone.
Photo: Genevieve Johnson

June 20, 2001
Anemone Fishes
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Log Transcript

Today the crew took some time off from the ongoing maintenance work aboard the Odyssey to explore the coral reefs in Madang harbor. This area contains an incredible biological richness, where the view is equally fascinating above and below the surface of the sea. For most families of tropical marine organisms, there are more species represented in the Indo-Pacific region than any other area on earth.

A fish that was quick to capture our attention today was the aggressive and highly territorial anemone fish. We encountered several different species, all appearing equally determined to drive us away from their domain with swift charges and retreats. The size of the apparent intruders, appeared to be of little consequence to this determined fish.

Anemone fishes, also known as clown fishes, consist of numerous similar species varying in the number and position of bands on the body, and in body color. They live in close association with anemones, with each species having a preferred host, and although it is possible to see a suitable anemone without a fish, an anemone fish is never seen without an anemone.

Occasionally, different species of fish will cohabitate within the same anemone. Large anemones are often seen hosting a monogamous pair of adults and several small juveniles. One of the most fascinating aspects of the lives of these fishes is that all individuals mature as males, with all females being sex-reversed males. In other words, in the absence of a female, the largest male will turn into a female, while the largest juvenile will rapidly mature as a male. The adult pair then stunts the growth of the remaining juveniles living within the same anemone.

The species in the photo is nestling down amongst the stinging tentacles of its host, yet it is perfectly safe. The fish acquires immunity from the lethal toxins excreted by the anemone, by slowly covering itself with a layer of mucus. Initially the fish will be stung, but as it builds up a sufficient layer, any tentacles encountered 'assume' they are touching other tentacles and withhold their deadly sting. If the fish is separated from its host for a day or so however, it loses its immunity and needs to rebuild its coating of anemone mucus.

The advantages of living within this safe haven of tentacles are obvious for the anemone fish, yet it is still unclear as to how the anemone benefits from the presence of this brightly colored little fish. Perhaps it lures predators close enough for the stinging tentacles to kill. The anemone is an animal closely related to corals and jellyfish. It has a sac-like body, its tentacles are armed with stinging capsules, which act as small harpoons that are jabbed into prey. Injecting a paralyzing toxin, the captured prey is slowly moved toward the mouth and consumed.

This is Genevieve Johnson speaking to you from the Odyssey in Papua New Guinea.

Log by Genevieve Johnson

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