Eternal life


Scanning electron micrograph of part of a bryozoan colony showing male and female brooding zooids and feeding autozooids.
Image: Phil Bock.

 

4 September 2010
 
The tiny invertebrate organisms being studied by Phil Bock can, theoretically, live forever.
 
Phil, a Museum Victoria honorary associate, is collecting bryozoans on the CReefs field trip to Lizard Island this month.
 
Bryozoans, also known as moss animals or lace corals, are tiny creatures, typically about 0.5 millimetres across. Individual bryozoans, called zooids, cannot survive independently, but instead grow in colonies on dead corals or the underside of rocks in coral reefs.
 
They do this by cloning.
 
"A larva released from an existing colony will settle down on a rock and almost instantly start budding. It does this by a process of cloning, repeating itself over and over. Then some of the cloned zooids will bud into more clones, and so on," Phil explains.
 
"Each colony is made up of thousands or even millions of these zooids," he says.
 
Colony members are genetically identical and co-operate for the good of the colony as a whole. Different zooids within a colony will have different tasks, "a bit like an ant colony," says Phil. The key players are the autozooids, which are responsible for feeding the colony, but various types of specialist heterozooids are responsible for defence, cleaning, breeding and structure.
 
Phil describes the structure of each zooid as "a kind of box with organs in it" – an exoskeleton and body wall protecting the gut, nervous system and feeding apparatus such as the mouth and tentacles, or in heterozoids, specialised organs such as the reproductive system.
 
In addition to asexual reproduction through cloning, bryozoans reproduce sexually, with cross-fertilisation occurring between colonies.
 
 
"Some species have separate male zooids and female zooids, but in many colonies, each reproductive zooid has both male and female organs. Colonies release sperm into the water, which floats to other colonies to fertilise the eggs held in the brood chamber of reproductive zooids there," Phil explains.
 
"Fertilised eggs are released into the water, and each one tries to float to a new place to settle down, begin budding, and begin a new colony," he says.
 
Although little is known yet about the life spans of zooids, it does seem that each dies after fulfilling its task.
 
"It can do some feeding or do some breeding, and then it dies back, leaving behind a little dark spot in the colony. But then, another zooid can bud again – so the same hole is occupied by a new set of tentacles, but it is all the same clone," Phil says.
 
"This cycle of death and rebirth can be as short as a few weeks. Some colonies die back after only a few months," he says.
 
"There are others, though, that, as far as we know, live for tens of years on the undersides of great shelves of coral. Parts of the earlier forms of zooids will die, be broken off or wash away, while the colony is still growing at the edges. A colony that lives for a decade may have completely renewed itself hundreds of times over – they appear to have an almost infinite life."