Friday, October 29, 2021

Floating blue life

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Blue animals on the beach
Photo: LHI Signal

In September, strong winds often drive tens of thousands of blue sea creatures ashore, and the beaches are littered with their dead bodies. The surface of the open ocean supports large populations of these organisms that live their lives afloat, sometimes forming huge rafts kilometres long, carried by the currents and blown along by the wind.

Blue bottle
Blue bottle. Photo: LHI Signal

The most common is the Bluebottle, Physalia physalis easily recognised by the prominent blue, crested, air-filled float about five cm long, with blue, stringy tentacles trailing underneath. Each Bluebottle is actually a colony of polyps that function together as one animal. Some of the individuals are modified to make up the float chamber; others comprise the tentacles that dangle into the water, armed with tiny stinging nematocysts that are used to capture prey. Captured prey is taken up to the feeding cells underneath the float chamber.

By te wind sailor
By the wind Sailor. Photo: LHI Signal

Other animals washed ashore are related to the Bluebottle, and have similar colour but different shapes. One is the By-the-wind Sailor, Velella velella. It consists of a small disc-shaped body about 3 cm across, on which sits a triangular sail membrane. The tentacles hanging below the disc are only 1 to 2 mm long and although they feed in the same manner as Bluebottles, their stinging cells are not as potent or harmful to humans.

The third member of this group is Porpita porpita, the body being a small blue disc 2 cm in diameter, with a fringe of knobbed feeding tentacles around the rim.

The great number of these animals living close together would normally be an attractive food source to other animals, but the stinging cells are a major deterrent. Certain animals, however, have evolved the ability to neutralise the nematocysts, and live by hunting in this floating world of the Bluebottles and their relatives.

Blue snail shells
Violet snail. Photo: LHI Signal

The most conspicuous predator is the Violet Snail Janthina janthina, which is about the same size as the common garden snail. The light purple coloured shell is thin and delicate. These snails float just below the surface of the water, suspended by a raft of bubbles trapped by the foot. They prey upon the Bluebottles and By-the-wind Sailors without being harmed.

There are also three species of nudibranch sea slugs inhabiting the floating colonies that also eat the Blue bottles and By-the-wind-sailors.

The Lord Howe Island Signal 30 September 2021

This article appeared in The Lord Howe Island Signal, 30 September 2021.

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