Sunday, September 21, 2008

The blue layer - Blue bottle, By-the-wind sailor, Violet shell

I went to the South Beach of Fremantle this morning hoping to snorkel. But the water conditions were poor, so I beachcombed instead.

I found many blue bottles (a.k.a. Portuguese Man O' War, Physalia species) and violet shells (Janthina janthina) on the shore. They must have been blown to shore during the recent storm. And I saw a by-the-wind sailor (Velella velella) too, for the first time.

These animals are part of "the Blue Layer", which is basically a huge group of floating animals that drift across the ocean, and that they are camouflaged by having dark blue colors on the upper body surfaces (so the sea birds won't see them easily) and whitish colors on the underside (so that the fishes won't see them easily).

By-the-wind sailor (Velella velella)
This animal is similar to the Porpita (Blue button), which is also a "member" of the Blue Layer. Unlike Porpita, Velella has a sail and has shorter tentacles at the margin of the blue disc. Nevertheless, both species occur as a colony, not as a single animal.

Blue bottle (Physalia species)
Like Porpita and Velella, the blue bottle is also a colony of organisms. They have trailing tentacles that can sting and paralyze small fishes. They have a single coiled tentacle that can be meters in length. They are said to be capable of delivering painful stings even when they have been washed ashore.

Now, the violet shell (Janthina species) - which unlike the last two species - is a single organism. And it is actually a predator of Velella and Physalia! This animal is always born as a male and would later change into a female. It makes the bubble raft by secreting some kind of mucus (waterproof, of course) that can be used to trap air and keep the animal afloat.

The MESA website has an explanation about the Blue Layer.


Mosura said...

I remember well the time I was stung by blue bottles. I probably deserved it for all the times I popped them on the beach as a kid.

I've seen Violet shells before but never with the raft still attached like that. What a great find!

Tsun-Thai Chai said...

I was too scared of getting stung when I saw them on the shore. So I have never popped any. I suppose if I try to pop them at a safe distance from the tentacles, I should be alright. Is it really that painful?

Snail said...

Beachcombing is such a delight. That was a good haul, Chai, especially the violet snails. (Of course!)

Tsun-Thai Chai said...

Hi Snail,

I knew you would say that - about the violet snails :)