Coral and Parasites

Often when I take pictures of critters on the reef, I am trying to show how beautiful the residents are.

Beautiful coral polyps off of Koh Rong

However, coral reefs around the world, including those around Koh Rong are up against a lot.  Polyps on the reef are being smothered by sediment, which prevents them from feeding through photosynthesis with their symbiotic zooxanthellae and prevents them from feeding with their tentacles. 

 I tried to dust the coral head above off, so that the polyps would be free of the clinging silt.  Underneath I found bleached coral that is likely to be dead soon.  The coral could recover, but conditions would have to improve.  Yesterday I did I dive at about 13 meters over vast expanses of silt.  Normally there would be many critters (e.g., urchins, sea cucumbers, and seahorses) crawling across the bottom, but we did not see anything like that.  We saw lines left by trawlers.  These boats are not supposed to trawl in areas that are less than 20meters deep, but clearly they had been doing so and fairly recently.  The sea floor had been cleared of much of its life and in the process silt had been stirred up.  The silt greatly reduces the visibility in the water and covers everything.  Even this scorpionfish.

Scorpionfish sitting on long-dead coral that is covered in algae.

Reefs already have plenty of other problems to deal with: rising sea temperatures, anchor damage, dynamite fishing, coral harvesting, harvesting of algae-eating fish and diseases.  I have seen a number of diseases in the Caribbean, but I was exposed to a new one here, a parasite.  The pink on the coral below is part of a trematode (flatworm) life cycle.  

I'm pretty fascinated by parasites and their complicated life cycles.  Most trematodes require a vertebrate and a mollusc to complete their life.  This one includes a snail, a coral polyp and a butterflyfish.  It's always hard to know where to begin in a cycle, but we could start with the fish.  The butterflyfish deficates out  eggs that hatch and infect a snail, grow in the snail and gives rise to a swimming stage, which burrows into the coral polyps.  The worm is then encysted in the polyp and, apparently, the polyps are easier for a butterfly fish to eat.  Butterflyfish prey on polyps and thus consume the encysted worms.  The worm matures and produces more eggs.  Then it all starts over. 

Trematode infection on hard coral

There are other parasites going around on the island.  Many volunteers, especially on the other island, walk around barefoot in the sand.  A fine mixture of feces is present in the sand and in the fecal/sand mixture are worms.  I'm not sure if they are threadworms or hookworms, I've only seen the feet of people that get them.  You see, the worm burrows into their foot (some evidence suggests that they are stimulated by certain chemicals present on feet) and travels in their blood to their lungs, where they are coughed up and swallowed.  Finally, the worms mature in the intestines and begin pumping out eggs that go with the feces into the soil.  Then volunteers or other people feel the sand between their toes and start the cycle all over.  Pretty cool, eh?