Sunday 10 July 2011

From Beach to Desert: Journey of the Woodlouse

Image: Biopix
The Coconut Crab may be the biggest piece of exoskeleton on land today, but when it comes to crustaceans getting out of the sea and stretching half a dozen legs on the beach and beyond it's the woodlice you have to look to. No other crustacean has had the terrestrial success of these isopods. Today, woodlice can be found across the world, quietly going about their business without a hint of the pomposity they could easily be entitled to.

Here, we will take a little look at how the humble, little woodlouse might have come across this impressive achievement.

Image: Filip Nuyttens
Baltic Isopod
There are well over 10, 000 species of isopod; parasitic ones, deep sea ones, giant ones (of course!) and many others. Woodlice might have evolved from something like the Baltic Isopod. These can be found in coastal waters across Europe, eating algae, seaweed and much else. They keep to the very bottom of shallow waters, scurrying around and swimming amongst the underwater foliage. Sometimes they get washed up in seaweed or found in rockpools when the tide goes out.

One strong advantage that all isopods have is the possession of a brood pouch. Several other crustaceans have this too, but crabs, lobsters and true shrimp don't. It means that isopods lay relatively few eggs but can carry and protect them until they hatch. They also skip the larval stage that most other crustaceans go through. These will prove a tremendous help to the woodlouse in their journey to the centre of the continent.

Image: Malcolm Storey
Sea Slater
The next step would be the Sea Slater. The Common Sea Slater really is very common; they can be found from Norway all the way down to the Mediterranean, as well as a small part of north eastern USA.

They are essentially terrestrial but still breathe using gills, so they need lots and lots of moisture. Their realm is under and around rocks in the splash zone, where sea water sprays over them but they're not totally immersed.

Sea Slaters emerge in the evening to feast on algae, sea weed and all sorts of plant and animal detritus. Filling their little tummies is very important for keeping beaches clean and healthy. Having said that, at some 3 cm in length Common Sea Slaters are actually bigger than any other woodlouse, so I guess their tummies aren't that small after all.

Image: Biopix
And finally, we reach your friendly, neighbourhood woodlouse. I have always found these critters curiously charismatic, slowly walking along with their antennae waving and caressing the ground ahead. They have a whole host of other common names like pill bug, butcher boy and roly poly, indicating that these inconspicuous little crustaceans are nevertheless very noticeable. It seems like everyone's talking about them!

Being crustaceans, they need damp environments to live in such as leaf litter or rotting wood. They are active in the cool of night, when they can eat that self same wood and other soft foods. This can sometimes include plant saplings, but it tends to be rotting plant matter. They're omnivorous though, and some even find meat preferable. Sometimes a woodlouse, soft after a recent moult, can find himself getting unwanted attention from his own neighbours. Cannibalism is rare among woodlice, but it can happen. The bigger threat comes from their own environment...

The cuticle of a woodlice isn't waterproof like that of insects, so water can evaporate or be absorbed straight through their exoskeleton. They are so adapted to life on land that too much water will actually drown them, and it's their  escape from this fate that leads them to climb up walls and sometimes enter people's home. They are totally harmless, though. And utterly charming, if you ask me!

Woodlice have lungs much like their old gills, kept on their underside toward the tail end. Some have none, some have 2 pairs, others have 5. Basically speaking, the more lungs they have the drier the environment they can tolerate. It's all about keeping a tiny, damp atmosphere about their person for them to breathe in.

Image: BioImages
Here's a picture of the underside of a female woodlouse, head at the top, tail at the bottom. You can see her brood pouch, full of eggs and taking up more than half the length of her body. Beneath that are her greyish white gills.

Image via Wikipedia
Desert Woodlouse 
I must admit, this astonished me! It turns out that there are several woodlice that have managed to make their living in the desert! This is amazing for a crustacean. It's amazing for most things really, but a crustacean? Wow!

One called Hemilepistus reaumuri lives in and around the deserts of North Africa and the Middle East. This is the driest habitat any crustacean has conquered, and they had to adapt to achieve it.

One little detail is that it keeps its body much higher up than other woodlice who tend to really hug the ground. It's tempting to think they are keeping themselves away from the heat of the desert sands. There may be something in that, but I don't really know.

The real thing is that they look after their children. A solitary woodlouse digs a burrow for herself, eating damp sand to gain moisture along the way. They have no particular body modifications to help, no hugely powerful legs or shovels sticking out of their toes, so it's a slow process.

Image via Wikipedia
Eventually a woodlouse of the opposite sex will join in and they'll continue excavation work together, teamwork that will hopefully be the start of a respectful, monogamous relationship of equals who value each other's contributions, thoughts and opinions.

Unless a bigger male comes along and chases the resident one away. It can happen.

In any case, the female bears around 80 live young who will be cared for by the parents. Digging that burrow was so difficult and time consuming that only one parent goes out foraging at a time, the other staying to defend against invaders. The young also release a pheromone that their parents learn to recognise.

It only takes 2 or 3 weeks for the young to be old enough to forage for food on their own, but they still return home for a nice dinner from their parents. It will be several months before the little'uns can totally fend for themselves and go out to build their own burrows, and that will be after the whole family enjoys the winter months quietly huddled in their underground abode. Snug as a pillbug in a burrow they dug!

In a really desirable location, location, location there can be some 20 of these burrows per square metre. And to think just a few days ago I didn't know there were any!


TexWisGirl said...

well, i like the good parenting part of their nature. :) but golly, i have never heard the term woodlouse! UGH! i'll stick to roly poly or pillbug! sounds much more pleasant!

Joseph JG said...

Really? I had no idea 'woodlouse' was so localised. I can appreciate how you wouldn't like the word LOUSE in there! I've gotten used to it so I immediately think of the sweet, little character.

Still, plenty more names to choose from!

Meg said...

ah, the roly-poly; one of the few bugs that don't make me shudder. They're just so darn cute when they roll up into a ball like a teeny-tiny, multi-legged armadillo.

Joseph JG said...

I know, and it's amazing what crustaceans are allowed to get away with! They have it so easy compared to insects and arachnids.