Split personalities
by Jenny Edwards
Some of our marine animals without backbones really do split their personalities and bodies too. They actually divide themselves into two parts and each goes its separate way to form a complete “new” individual.
Many sea anemones can reproduce in this way. Their cylindrical bodies pinch in from opposite sides, usually top to bottom. Viewed from above the circular surface would look more like a figure 8. Then the pinched part gets thinner and thinner until it splits apart making two new anemones.
Flatworms are another group where individuals regularly split into two. The common Oyster Wafer (Notoplana australis) is a thin, flat, semi-transparent animal that slides into its prey when the oyster opens its shell to feed. It is common in estuaries but can also be found in sheltered locations below low tide level on the open shore.
You might think that only uncomplicated animals could do this trick, but sea-stars are more comples animals that regularly split themselves in two. The exponent of this feat that you are most likely to see in our area is the Spiny Sea-star (Allostichaster polyplax). The Spiny Sea-star occurs in and below the low tide region, usually sheltering under rocks, in crevices or inside dead shells.
This small, long-armed sea-star usually has eight arms but can have between six and nine. However, several of them are nearly always much shorter than the others. The animal has split itself in two and each part is growing new arms to replace those lost. It does not need to split in half. You may even find one large arm with the rest small ones regrowing.
The notorious Crown-of-Thorns Starfish that eats the corals of the Great Barrier Reef can also regrow from bits. If you attacked one with a machete and threw the bits back you would only end up with more starfish than before.