Thursday, April 19, 2007

The Amazing Blanket Octopus!

Dear Constant Readers,

I was going to write a blog on cryptic giant octopuses, but as it got increasingly bloated, I stumbled on something that I knew I had to cover. My time is rather limited between a block of testing and finals, so another one of those 10 hour time-killers is sorta out of the picture. Don't worry, who knows what monstrosities I'll cook up over summer, muhahaha. Cryptozoology and monsters will have to wait, but this animal is just as interesting, if not more so, because it is quite real.

Octopuses (not octopi or octopodes) are some of the most fascinating creatures on the planet, well, to me of course. They are highly derived cephalopod molluscs like the squids of my past posts; except having eight-arms, a fused head and mantle, and a very reduced or absent shell. Other stereotypical characteristics (benthic habitat, rapid color changes, lack of fins and cirri, et cetera) apply to the family Octopodidae which is by far the most familiar to the public. However, the group has a surprising amount of diversity including totally pelagic (free-swimming) forms, a blind form, jello-like forms, forms with fins, a form with a "shell", and so forth.

The taxa that I'm going to talk about belong Argonautoida superfamily which have specialized to be free-swimming. These of course include the bizarre "shelled" Argonauts, and possibly the world's largest species of octopus, both of which I'll have to talk about later. They are also united by the strange feature of having one of the male's arms detach during mating. But I think one can argue that there is one oddball in this already peculiar group.

Tremoctopus is a genus consisting of four species and resting in its own family. For octopuses they are quite large, getting up to 2 meters (6.5 feet) long and weighing around 10 kg (22 lbs). However, this is only for females as the males only reach 2.4 cm with a weight of 0.25 grams! This makes the females up to 40,000 times larger than their male counterparts, nearly two orders of magnitude. This is apparently the most extreme sexual dimorphism seen in an animal that could be considered "large", see this paper for more. The males appear to have large eyes for finding the females, and after their arm detached during mating it is believed to be fatal.

Also remarkable about these species is the defensive behavior of the males and immature females. They have been known to take the stinging tentacles from Portugeuse Man O' War siphonophores and use them as a defensive (and offensive?) weapons while protecting themselves with unknown mechanisms. A combination of this and the pelagic habitat may have caused the incredible sexual dimorphism, as other animals (sea slugs, hermit and boxer crabs) who use this bizarre defence do not exhibit the dimorophic trait. Another trait due to the pelagic habitat is the presence of a "swim bladder", a trait present only in a few other Argonautoids and a convergence upon fishes.

Odd thought these traits may be, what is really impressive (and the reason for this post) is the general appearance of this animal. The name is a reference to very heavy webbing between the four dorsal arms (the same used to hold tentacles earlier in life), which makes it look distinctive from all other octopuses. I hope I can use these images, because I'm definitely not going to be able to match this, especially as far as coloration is concerned. People seeing me work on this could not even identify the images as coming from an octopus. Enjoy:

There is an excellent picture at the Tree of Life webpage, that was taken by Arthur Silva and appeared in the April 2007 BBC Wildlife magazine, although this does unfortunately appear to be copyright violation. So I guess I'm morally obligated not to post it here or link to it. Sigh, it really was a very nice picture. Now with summer on the way I'll be sure to create lots of nice public domain images of an unfortunate lesser quality.

There were also nice photographs taken of Tremoctopus violaceus by Cassandra L. Cox off the coast of Florida in 2005. They can be found here since there seem to be difficulties uploading at the moment.

Finally, an actual image! Taken from this page which has other fantastic pictures. This specimen is identified as Tremoctopus gracilis and was taken by Marcello Conticelli off Ponza, Italy. Let's see how the copyright holds up on this one...

For the sake of posterity I am a bit worried about putting up a YouTube video. When other octopuses swim, they put their arms in a similar wing-like posture, but this is something else entirely...

I hope I can see you all sometime before the month is out. More cephalopods? Something different? Who knows.

Oh yes, and Happy Birthday Caitlin!


1 comment:

Caitling said...

Believe it or not, that was worth reading just to get to the very last sentence.

It is my birthday and I expect presents from everyone!