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Unwelcome Blow-ins

By Em Allen on June 20, 2012 in

Photo: Jason Pisani

Dealing with blow-ins at the beach during summer is a fact of life if you live on the Eastern Beaches. However, one blow-in that is arguably the hardest to deal with, and one that can easily turn a pleasant day at the beach into a painful ordeal, is the infamous ‘bluebottle’.

Bluebottles, or Portugese man-of-war as they are sometimes called, usually live in open waters off the coast where they drift around at the mercy of the currents and the winds with the aid of their gas-filled floats. Unfortunately for both us and the bluebottles, prolonged periods of north-easterly winds force bluebottles into coastal regions where they wash into the surf zone and, inevitably, come into contact with humans. Bluebottles dry out and eventually die after being washed onto the beach, but their stingers can remain potent for some weeks.

Although it looks like it, the bluebottle is not actually a single animal but rather a colony of four kinds of highly modified individuals (zooids) working together. The float is one individual, which holds the colony together and aids in movement. The tentacles are another individual responsible for capturing and killing prey. Two other individuals within the colony carry out digestion and reproduction.

The stinging cells, or nematocysts, are located in the single long tentacle that hangs from the bluebottle float. This tentacle can reach up to ten metres in length and contains millions of nematocysts. When activated, each nematocyst fires a tiny dart into any prey it captures and numerous toxins are released to immobilise and kill it. The tentacles then contract and the prey is hauled up close to the bladder, where feeding zooids attach themselves and begin digesting it.

While bluebottle toxins do not kill humans, they inflict a painful sting that can last for more than an hour. It is estimated that bluebottles are responsible for tens of thousands of stings to beach-goers each year.

But bluebottles are not the only floating blue-coloured life forms to be blown onto our beaches. Nor is their sting the most painful.

The blue sea slug, Glaucus atlanticus, is a nudibranch with spectacular blue and silver colouration, and it eats bluebottles for breakfast. Literally. Nudibranchs are a type of mollusc, similar in appearance to snails but without a shell and much more colourful. With the aid of a gas-filled sac in its stomach, the blue sea slug floats upside down on the surface of the ocean where it feasts on bluebottles.

Blue sea slugs eat bluebottles whole, tentacle and all. Amazingly, not only are they immune to the sting of the bluebottle, they store and concentrate the stinging cells into specialised sacs, which they then use on their own prey and as a defence against predators. Because the stinging cells are more concentrated than in the bluebottle, coming into contact with a blue sea slug will produce a much greater sting.

Just like bluebottles, blue sea slugs are also at the mercy of the wind and can be blown onto our beaches, although they are not very common and are rarely seen.

And while you may consider bluebottles and blue sea slugs a nuisance, remember that they are not particularly fond of washing up onto the beach (or you) either.