12 August 2007

Do isopods grow on trees?

Isopods are crustaceans, like crabs and lobsters. And like crabs and lobsters, isopods originated in the oceans and then evolved to become terrestrial. In fact, they have become so successful on terra firma that, according to Brusca, there are more terrestrial isopod species than there are marine species. It seems that they are now on their way up the tree trunks. If they face a strong enough selective pressure, who knows, maybe one day they will even evolve wings to conquer the skies.


During my recent trip to Antwerp, Belgium, I encountered these isopods on wet tree trunks late in the afternoon after a rainy day in a wooded park. I was actually looking for slugs; instead, I found isopods. According to the notes I took, they were on both smooth- and rough-barked trees. The highest ones I saw were more than 2 meters above the ground. As you can see in the pictures, the tree barks were covered with green algae (cyanobacteria) and probably also with fungi. I suspect the isopods were partaking of the algae and the fungi.

So, if they were feeding on the trees, then the answer to the question posed in the title of this post is Yes, isopods grow on trees.

I don't know anything about isopod taxonomy, but based on the patterns and colors visible on their dorsal surfaces (the technical term for that part of their bodies is pereon), there were at least 3 species.

This one:

This one:

And this one:

Of course, they may all be the same species and the observed pattern and color differences may be due to age, male-female dimorphism or polymorphism, etc.

Isopods were featured on this blog before in this post and in this post.


Tristram Brelstaff said...

Last summer I came across one of these in a curled-up leaf on a tree about 2 metres off the ground. At the time it occurred to me that if it started to get too dry, or got attacked, then it could have quickly got back to cover in the leaf litter by simply walking off the edge of the leaf.

I seem to remember that the little bumps on the segments are the characteristic of one of the isopod species (at least here in the UK) but I can't remember the name.

Roger B. said...

I often find dead isopods (woodlice) in our attic. It's not particularly damp up there, but I suspect they may be feeding on the dead leaves that accumulate in the guttering.

umit said...

first and second are the same species. The last one is different, suggested by the antennae. The coloration (not the pattern)is almost zero importance as in slugs. I dont know the IDs either, although much too simplier to know it than here, as there are some species there still we dont get in this part of the world. But i suppose both species occur in Tristram's backyard also.

They are numerous on land as it serves as an enlarged morph of littoral to them. As you see land coverage and richness correlate.

Ayatto said...

Dear Aydin I Said a Haiku for one of your photographs.
read it here

Kevin Z said...

Having studied marine isopods, I agree with Umit that coloration, even ornamental design, is nearly useless. For marine isopods, patterns on the body may reflect trace minerals it accumulates into its exoskeleton which will be moulted at some point. See my post on crabs moulting to rid themselves on excess lead.

I think #3 is different too, based on the dorsolateral margin of each segment. It is more hooked back slightly, while the other 2 are more straight.


Isopod expert Helmut Schmalfuss has e-mailed the following comments regarding the identifications of these isopods:

"the isopods on your photographs no. 2 and 3 belong to the same species, they are different color morphs of Porcellio scaber, which is the most common terrestrial isopod in western Europe. The animal on your first picture seems to be a different species, but the picture does not allow any identification. In rainy weather Porcellio scaber [is] often found on tree trunks."