I keep finding new-to-me and fascinating forms of marine life on our walks up and down our stretch of Red Sea coastline. Two life forms in particular have caught my attention: cuttlefish and chitons.
I first encountered the chitons when we visited a snorkeling and diving camp on the coast – one of those places that’s fenced off to the low-tide mark to discourage anyone from gathering sea life there. The tide was going out, and I was picking my way along the rocks and emerging tidal pools when I spied a creature clinging to a wet rock face that looked almost exactly like a sowbug.
You’re probably more familiar with the term “pillbug” or even “wood louse” than “sowbug.” When I was growing up, we called them sowbugs, and we loved the way they rolled up; we held them in our little-girl palms and watched them rolling around. My mother used to say she had to empty my little sister’s pockets of them before doing the wash.
I had never seen anything like these sowbug-looking creatures, and I wondered if the two animals were related. Diving down the rabbit hole of the internet, I discovered that the sowbug, or armadillidium vulgare, is an isopod. Isopoda is an order of crustaceans that includes both land-dwelling creatures (like sowbugs) and water-dwelling animals. The water-dwelling isopods live in both sea and fresh water. (If you want to read a wonderful article about pillbugs and their evolutionary origin, go to https://www.pbs.org/newshour/science/pill-bugs-emerged-sea-conquer-earth)
I was convinced that the creatures clinging to the rocks along the shore were isopods until I searched for images to match my own photos. As I looked and read, I figured out that the creatures I was looking at are chitons.
Chitons and isopods are actually quite similar in one important way. Both have a plated shell (in the case of isopods, it’s an exoskeleton) and can roll up to protect themselves. But there the similarity ends. Chitons are actually molluscs, the largest phylum of marine animals. Where isopods have legs, chitons have a fleshy foot that clings to rock faces – kind of like a snail. In fact, that foot is most of its body below the shell.
Whatever the differences, they all look quite prehistoric to me. In fact, the earliest fossils of chitons go back 400 million years, Wikipedia says; for wood lice, it’s about 100 million years. Ancient enough!
The other creature that’s captured my attention is the cuttlefish. I have always loved that word – as a little girl, it sounded to me like a cuddly fish – but I didn’t know what they looked like. Some days ago I spotted a purplish fleshy-looking creature on the shore, recently dead – its body seemed intact. It had legs like an octopus, but much shorter in proportion to the longish body. I wondered if it were a squid, but couldn’t quite figure it out from the images I found on the internet.
Then Alan showed me something he’d found: a cuttlefish bone. He’d shown these to me before, on the beach at Finike in Turkey. Alan was familiar with cuttlefish bones from his childhood summers on Mersea Island in England.
The cuttlefish bones here are prettier than those we saw in Finike: a soft peach color, smooth and ribbed on one side, with a needle-like protrusion at one end. They’re actually quite stylish, reminding me of a tatting shuttle.
Cuttlefish bones have been used by people for a lot of different purposes, but the most common use nowadays is to be placed in bird cages for the birds to peck on. Cuttlefish bone is made of aragonite, which turns out to be a form of calcium carbonate that’s very common in marine life. For birds, it’s apparently quite nutritious.
I wondered what function this bone could actually have. To me, it doesn’t really look like a bone at all. As it turns out, the cuttlefish bone isn’t part of a skeletal structure: It’s a porous internal shell with chambers, and it allows the cuttlefish to control its degree of buoyancy in the water by changing the ratio of gas to water in the chambers. This structure is apparently unique to cuttlefish, and is the main distinction between these animals and squid. (Both, though, are in the cephalopoda class of molluscs, distinct from the polyplacophora class that chitons belong to.)
All this was so clever and fascinating, I had to read more about cuttlefish. They are, like the chitons, an ancient creature; fossils of cuttlefish bones go back about 70 million years. Not as old as chitons or isopods, but anyway, old. And their cleverness manifests in ways other than buoyancy control. They can communicate by changing the color of light reflecting off their skin, changing the texture of the skin, and by the movements of their bodies and their eight arms and two tentacles. They even sleep: REM (rapid eye movement) has been observed in resting cuttlefish.
The Wikipedia article on cuttlefish says they’re considered one of the most intelligent form of invertebrates. The more I read about them, the more complex, fascinating and remarkable they seem. When I read about their mating behavior, it’s like reading about another culture. So there’s another rabbit hole for me to dive down – though perhaps in this case I should call it a tube worm hole. I’ve seen quite a few of those recently, too.