ocean's predators, a visiting scientist said.

Corals not only sting and paralyse other sea life before pushing them into their mouths, they attack and kill other corals who compete with them for space, Dr. Ken Sebens of the University of Maryland said in a recent lecture at the Bermuda Biological Station for Research. "Most corals really are pretty good predators,'' Dr. Sebens said.

Corals -- the sea invertebrates that produce stony skeletons behind their living polyps -- have stinging cells on the end of their tentacles, and barbs that stick into the soft zooplankton they eat, Dr. Sebens said. Zooplankton are small living animals that drift about in the water.

For plankton with tougher exteriors, some corals have a type of glue on the end of their tentacles, he said. The tentacles also have sensors on their ends that can detect amino acids. That was how they distinguished their prey from other floating objects, he said. As well as pushing zooplankton into their mouths, corals draw chemicals from seawater that contain the nutrients they need. The chemicals are used to produce bacteria that is sometimes ingested directly, and sometimes released to be ingested by other corals. And corals benefit from energy produced by photosynthesis in algae that grow on them.

Dr. Sebens said that in shallow water, photosynthesis can produce up to twice as much energy as a coral needs each day. But photosynthesis requires sunlight, and that is a problem in deeper water.

Zooplankton are mainly abundant at night and disappear at dawn, he said. Their sizes can range from half a millimetre to several centimetres, and there is little correlation between the size of polyps and the size of the prey they catch, he said.

Some corals have special long polyps, whose tentacles are used to sting and kill other corals and take over their space, Dr. Sebens said.

Much of Dr. Sebens' work has centred on the effects of various seawater flow rates on the success of various corals in capturing prey. Corals with long polyps did not do well in water with high flow rates, because their polyps tended to get flattened and stop working, he said.

Flow rates in the shallow parts of coral reefs were about optimal for prey catching by most corals, he said. Knowledge of the effect of flow rates on corals was important for reef conservation, he said.