High Density Housing on Coral Reefs

We often see hatchings of minute baby domino damselfish (Descyllus Trimaculatus) precariously and optimistically sharing the protection of a very small (Heteractis Magnifica) anemone. The dominos develop an immune relationship with the anemone where their own mucous coating inhibits the firing of the anemones’ stinging cells and they are protected from its poison. It’s the most ideal symbiotic relationship, as the anemone feeds on anything that comes its way from the dominoes, and it protects its tenants. However, there was no way a small anemone could provide enough cover once they were fully grown, and we fully expected most of them to succumb to food chain predation.

Surprisingly, a year later not only did the dominos grow, but the anemone itself expanded to keep up with the growing baby dominos. A unique characteristic of the (Heteractis Magnifica) anemone is that it is capable of practicing asexual reproduction, and each new animal is an exact biological replica of the parent. This cloning enabled the dominos to swim between host anemones in prefect safety from hungry predators and without being poisoned.

Each anemone can grow to up to 60 cm in diameter, and they can form quite large carpets over time. They occur in places where there is a reasonable flow of current, and at depths of between 8 and 40 metres. Their long greyish green tentacles have a yellowish tip, and in some parts of Indonesia they have spread over areas of several square kilometres.

Each anemone has a disc which attaches to the substrate of the reef and is almost impossible to dislodge. They are capable of drawing their tentacles into this disc which can become quite large and fleshy over time. I have seen huge fleshy basal discs on these anemones in Mauritius. When a cold current passes through they enclose their tentacles, forming large reddish balls, only releasing them once the cold current passes. The inhabitants seem to be quite happy to enjoy a cosy spell in their cocooned accommodation.

The Mauritius (Heteractis Magnifica) anemones also play host to porcelain crabs and the exquisite Mauritain Clown (Amphiprion Mauritiensis) anemone fish. If you get a chance to see past the tentacles to the flesh below you may be lucky enough to spot clownfish eggs, the size of a pearl and pinkish in colour.

If its environment becomes hostile under certain adverse conditions, the anemone can release its tenacious hold on the substrate and allow itself to be carried by current to a more hospitable habitat. Moving house? Why not simply curl up inside and move with it?