Fight club

A bryozoan colony.
Image: Gary Cranitch.


21 November 2010
Competition is fierce on coral reefs, and it's often a fight to survive, according to Museum Victoria honorary associate and bryozoan expert Phil Bock.
Phil is collecting bryozoans – tiny invertebrate animals that grow in colonies on dead corals or the underside of rocks – on the CReefs field trip to Heron Island this month.
"The key resource for any of the animals and the plants which encrust on rocks and coral is space," Phil explains.
"They start in one small spot and then spread. Plants grow; bryozoans spread by budding, a process of cloning in which an individual animal repeats itself over and over, expanding exponentially into a larger colony. Usually, the plant or the bryozoan colony will come up against a neighbour which is also trying to spread," he says.
The first rule governing this competition in ecology: the law of competitive exclusion, which states that two species competing for the same resources cannot live together if other factors are constant. Even the slightest advantage of one species will force the other species to adapt or die.
Naturally, then, organisms cultivate various forms of advantage in order to win out over competitors.
"The competitions don't all work the same way. Sometimes if you have three competing organisms, you can have A overgrowing B, and then B overgrows C, and then C overgrows A. The actual reasons why some are more successful than others are often hard to work out," Phil explains.
"Different organisms have different strategies by which they deter or kill off the competing neighbour. Some plants and animals, such as sponges, produce rather nasty chemicals to deter other things from overgrowing.
"Bryozoans gain some advantage because they can grow faster than many plants and other colonial animals. They can also dodge competitors: if they come up against a barrier in one direction, they can produce buds to grow in another part of the colony," he says.
Several groups have evolved a strategy of growing upwards like a bush or a tree into the water. These have to compromise between increasing area for filtering food particles, and reducing the possibility of damage by strong waves or currents.
The specimens Phil collects on CReefs expeditions may enable scientists to further investigate bryozoans' competitive strategies.