by
Jenny Edwards
In another example of misleading common names, we have tube worms living on our rocks that are called “Sydney Coral”. They are not confined to Sydney and they are not even related to corals. Their scientific name is Galeolaria caespitosa.
The worms are segmented, like earthworms, but each of the body segments has bristles (chaetae) and they belong to a large group called polychaetes. Their relatives come in all shapes and sizes and there are some in just about every marine habitat.
The family that Galeolaria belongs to is the Serpulids. They live in limy tubes and have their head ends modified with feathery gills that are used to trap tiny food particles and extract oxygen from the water. The delicate gills are pushed out of the tube when the tide is in and can be instantly withdrawn if danger threatens.
The unlucky worm that has all or part of its exposed head eaten can regenerate the lost bits from the tissue that is left.
Galeolaria’s limy tube is made of calcium carbonate, the same stuff shells and coral use. It is cemented to the rocks towards the low tide mark and in some locations with broken surf the tubes can form very thick tangled encrustations.
Attached to the worm’s head is a sculptured calcium carbonate plug with 9 to 11 movable spines. The plug forms a door to the tube when the animal withdraws. Since the door (operculum) is only 2 to 3mm wide you will need a strong magnifying glass to really appreciate its beauty. The operculum helps protect the worm from predators and prevents it drying out when exposed at low tide.
Galeolaria worms are either male or female. This statement may seem obvious but in the case of marine animals without backbones “it ain’t necessarily so”. They time the release of their eggs or sperm to coincide and fertilization takes place in the sea. The fertilized egg develops first into a larva which looks a bit like a microscopic football with a tuft of hairs (cilia) at one end and a band of them around the middle. Gradually segments bud off at one end and the animal starts to resemble a tiny worm. If it survives the hazards of life in the plankton the baby worm settles on a suitable hard surface and secretes its tube.
All marine animals that secrete calcium carbonate are indirectly threatened by humans’ insatiable appetite for energy. Some of the rapidly growing amount of carbon dioxide in the atmosphere is being absorbed by the oceans turning them more acidic. It is not just corals that will suffer from having their calcium carbonate dissolved.