99. Spider Rider

A brief episode, occurring on a tiny scale.

The spider-like animal in the middle of the photo is a blue-knee sea spider. Sea spiders are not exactly spiders, but they are probably moderately close, in evolutionary terms, to land spiders – arachnids. This species is common and the photo above is a pretty poor one, but there’s something to see if one looks closely. Right on top of the spider, as if riding in a saddle, is another animal, wearing (or comprising?) a slender grey tube with three tentacles out front.

Here’s a photo from 20 seconds before.

The tube-with-tentacles is now below the sea spider, apparently climbing aboard.

Here’s a closer view of the first photo, showing the rider:

The sea spider moved forward and reached out over a chasm, from the sponge it started on to a tunicate that is not quite visible in the upper right of the first photo.

The tube-with-tentacles was still in the saddle.

But after the sea spider settled on the tunicate, it began tangling its body up, and – in a way only semi-clear in the photos – seems to bring one leg under the body of the tube.

I’ve marked the leg (red) and tube (yellow) here….

And then, begone!

The rodeo ride was over. This was almost exactly a minute after the second, climb-aboard photo above.

The climb on, the period in the saddle, and the bronco-style eviction all occurred while I was watching, and as the scale was so tiny, I had no idea. I’m guessing, but I’d say the sea spider was perhaps 3 cm with its legs stretched out. The rider was hence quite tiny.

Who was the rider? We have a grey tube, something like three visible tentacles, and that’s it. It’s like a tiny version of a giant Paleozoic cephalopod, Cameroceras. I thought it might be some sort of tube worm, but Marty Hing, who seems to know every obscure and tiny marine creature personally, tells me it is almost certainly a tube-building amphipod – hence an arthropod like the sea spider, and a relative of the skeleton shrimp I wrote about here a few years ago.

Tube-building amphipods have a small literature, as far as I can see – I marvel at the dedication of people who patiently unravel the secrets of such animals. I read that to build their tubes, “amphipods use special secretions that are typically called ‘amphipod silk,'” though I learn this from a paper about “masts” rather than tubes. Amphipod masts are thin and large towers – relative to the size of the animal – that the animals use to raise themselves higher into the water column for collecting food. Some masts are used by more than one animal, and: “Additionally, masts can be used by amphipods to cultivate diatoms.”

The best website for tube-building amphipods I’ve found is this one – “Real Monstrosities” – and the Japanese video included is suitably spooky.

The episode occurred at Fly Point in Nelson Bay, during my long-awaited first trip back, earlier this month.

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