Tuesday, October 03, 2006

Remember ... walking in the sand.



My sister (the same one from the Katrina video) took this picture on her recent vacation in Georgia. I thought it was so cool; I just knew that I had to blog about it. Unfortunately, I know next to nothing about starfish--but that's never stopped me before.

This is actually a really good picture. The composition and the lighting are perfect. And you can make out the starfish's tracks quite well. Looking at the picture, the ocean is off to the upper left. So the starfish started to briefly head off away from the water, then he banged-u and headed on home. Shortly after the picture was taken, my sister picked him up and gently tossed him into the ocean (which was quite a ways away and still receding).

The next thing that pops out at you about the picture is that the starfish left exactly three whole body prints in the sand. Was he making sand angels? I'm not sure what he was "thinking," but these were surely spots where he stopped and began to sink in due to the soft, wet sand. These couldn't have been caused by waves crashing in around him, as these would've erased the record of his past travels. By that same token, the first whole body print must represent where our friend was resting when the tide first receded. Also, the possiblity that he didn't slow down but hit some unusually soft patches of sand seems unlikely to me. There's nothing in the sand around those three spots to indicate that such soft spots are there. These were almost certainly pit-stops.

The initial impression is the shallowest of the three, and the second print--which lies so close to the first as to overlap it--is the deepest. My guess is that shortly after the tide began to recede, our starfish friend moved to a more "comfortable" position in the sand and stayed there a while longer. Then he began his trek.

The third impression is the most interesting to me. This represents his turn back towards the ocean. It would be too easy to anthropomorphise our spineless, pentapodal friend and say that this was where he realized that he was heading in the wrong direction and stopped to get his bearings straight. He pulled the beach map out of his glovebox, all the while his wife was nagging him about how "I told you that wasn't the ocean exit!" In truth, I don't know a thing about how starfish navigate. I did some looking on the internets (including wikipedia) and came up empty. I even asked my sister what she thought the starfish was thinking. Her unhelpful reply was "Damn! I was right there with him and I totally forgot to ask him." That leaves me no choice but to stick with my lost traveler metaphor.

The other interesting thing about that third impression is the difference in the track marks leading to it and away from it. In the tracks leading away from the third impression, you can clearly see where the hind appendages were dragged through the sand. The tracks to the impression seem more evenly dispersed. It's like the difference between tracks left by a sled (with rails) and a toboggan. My extemporaneous expertise tells me there are two possible explanations for this. Perhaps he was in a bigger rush after changing direction (this fits in nicely with my lost traveler metaphor). Or maybe it was just the difference between moving with or against the grain (in the sand, the "grain" would be created by the receding tide).

All this made me curious as to how starfish actually do move. Here's a good explanation from Jonathan Dale's website.

The underside of the starfish is covered with hundreds of tube feet, which it uses for walking around, for attaching tightly to rocks, and for holding on to prey. To move, each tube foot swings like a leg, lifting up and swinging forward, then planting itself on the ground and pushing back. At the tip of each tube foot (in most species) is a suction cup. These aren't used when walking on level ground, but can be used when walking up sheer surfaces.







Here's a nice close-up photo of the tube feet from Wikipedia:




Here's a cool time-lapse video of some starfish moving around an aquarium from Jan Ellenberg's site.











And finally, take a look at this "creepy" YouTube video of a brittle star walking along the bottom of a tank. It actually moves its limbs like an octopus. Unfortunately, it has three few limbs to get PZ all excited.



My sister tells me that she has entered the photograph in a "nature photo" contest on the internets. I sure hope she wins!

UPDATE: The picture is now up on National Geographic! Hurray!!!

5 comments:

coturnix said...

At what times does the star move and what times rest? Is it day-night difference or low/high tide difference?

JohnnieCanuck said...

The top photo is showing movement over a period of maybe an hour. The last wave was a big one (boat wake?) that wiped the sand clean over the image area. This must be a very shallow beach where the horizontal tidal range is large.

The sea stars around here often are exposed by low tides, but they tuck themselves in rocky crevices and tidal pools to wait it out. I've never seen them moving across the sand.

The big glaucous-winged gulls here would have made a meal of any sea star that unfortunate. Unless your sympathies are with the victim, it is quite humourous to observe the process. You'll see them perched somewhere with mouth and throat extended and two or three arms still on the outside.

Google image search for - gull starfish.

The Science Pundit said...

From what I've been read so far, it seems the star's primary means of navigation is Chemosensory Orientation. This would tend to suggest that my sister's star's pit-stops may have been to dig out a mollusk that was hiding in the sand.

As for the day/night preference, I couldn't find much on echinoderm circadian rhythms. Though I found this one study from the Baltic that says that stars (Asterias rubens) are diurnal, but tend to feed more during the day. I also found several references to photosensitivity around the tube feet. This might explain jc's observation that they tuck themselves into rocky crevices.

As for the shallow beach, I've never been to Georgia's barrier islands, but my sister did say that the receding tide was "pretty far away" from where she saw the star.

I've also never seen a picture of them moving accross the sand. That's why I decided to blog about it and also why I told my sister to enter that picture in every nature photo contest she can find.

fireweaver said...

wow, congrats to your sister. the pic is beautiful, she should do well in the photo contest.

MH said...

Beautiful picture! Of course he would have been going slightly uphill as he moved away from the sea and thus downhill as he finally turned towards the sea, making it easier going...