Not everything I come across underwater is pleasant. On an evening dive at Batu Angus in Lembeh, my dive guide and I ran across what I can only describe as a full-blown invasion of crown-of-thorns starfish (Acanthaster planci).
As the sun set and we started to surface, I spotted first one, then two, then five, then dozens, then hundreds and hundreds of these toxic devourers of coral. I've certainly heard about outbreaks like this, but it's the first time I've seen so many with my own eyes.
According to this Wikipedia entry, crown-of-thorns dine alone. I can say definitively that this is not always the case. They hunt in packs too. Many had congregated together into large groups, making their way across lettuce corals, staghorns and other reef structures. Wherever they passed, only the white calcium-based skeletons of corals remained.
When I came up from the dive, I was torn between two conflicting desires: On the one hand, ecosystems should be left alone in most circumstances, to sort themselves out and reach equilibrium. On the other hand, I could see that the deadly predators were consuming the reef, and I wanted to remove as many as possible.
After I slept on it, I decided to go back and take another look the next day. We went prepared with hooks and crates, just in case. That evening, the crown-of-thorns were again out by the hundreds, and there was visibly much more damage than the day before.
We pulled up bucket loads, literally. We were careful not to leave any pieces, or allow anything from inside the starfish to spread, as the starfish can regenerate.
We dried the ones we brought up in the sun, then burned them to ensure no one risked injury from accidentally stepping on their toxic spines.
I have no idea what's causing the outbreak, how long it's been going on or how long it will continue. From researching a bit on the net, it seems like we actually don't know very much about outbreaks like this. Anecdotal evidence from other areas suggest that actively picking them off the reef might be an exercise in futility, though no one's really sure.
In any case, I'm sure they're still there, out in their legions, marauding in slow motion across the reef. If you're interested in reading more about these animals, try this link. And if you visit Lembeh and dive Batu Angus in the evening, I'd be very interested in hearing if the hordes are still there. They were present from 10 metres up to 2 metres, with the largest packs in the 3-5 metre range.