Findings on the beach, and a puzzle

A while back now I went out on a low tide even though the actual low was after sunset. I figured that it was low enough that I’d have plenty of time to poke around as the tide was receding. And given that there were promising clouds in the sky, I took my good camera along just in case the sunset proved to be photo-worthy. Having had enough of crowds in the intertidal at Natural Bridges the previous day, I decided to venture up to Pistachio Beach, which isn’t as heavily visited.

I ended up spending only 45 minutes in the intertidal, all the while watching the sun sink lower in the sky. It was already too dark to take many photos in the tidepools, but there were some interesting things on the beach.

The majority of shells that wash up on any beach are going to be molluscs, usually either gastropods or bivalves. I’ve often seen living red abalone (Haliotis rufescens) hidden in nooks and crannies at this site, so it’s not surprising to find their shells on the sand. Usually, though, the shells are a little beat up. This one was intact, with a lovely layer of nacre inside.

This butterfly-shaped object is one of the shell plates of Cryptochiton stelleri, also known as the gumboot chiton. Cryptochiton is the largest of all chiton species; the largest one I’ve ever seen is the length of my forearm from elbow to fingertip. Like all chitons, C. stelleri has a row of eight shell plates running down the dorsal side of the body. Unlike other chitons, however, in Cryptochiton the plates are covered by a layer of tissue called the girdle and not visible from the outside. If you run your finger down the back you can feel the plates under the girdle. I never thought about it before now, but it seems that the name Cryptochiton refers to the hidden chiton-ness of the animal.

Anyway, Cryptochiton lives mostly in the subtidal, although you can occasionally see them in the very low intertidal. As subtidal creatures they have neither the ability nor the need to cling tightly to rocks, as their intertidal cousins do. This means that when big swells come through at low tide, they can get dislodged and wash ashore. I know from personal experience that the tissue of Cryptochiton is really tough. Once a pal and I were trudging back after working on a low tide and came across several dead Crytochiton scattered over the beach. We decided to do an impromptu dissection and try to salvage the plates, hacking away with her pocket knife. The smell was horrendous, and after several minutes we made practically zero progress, so we gave up. I’ve seen gulls pecking at dead Cryptochiton, too, and they didn’t seem to have any success either. However, their bodies do eventually disintegrate, or something manages to eat them, and their naked plates can often be found on beaches.

One of the coolest pattern I’ve ever seen in the intertidal was this:

I’ve never seen anything like this before. It’s hard to tell from the photo, but these two rock faces converge into the crevice, sort of like the adjacent pages in an open book. This side of the rock surface faces away from the ocean and will never be subject to the main force of pounding waves. The barnacle in the middle is attached pretty much in the deepest part of the crevice, and is surrounded by mussels, which are then surrounded by limpets.

Now, all of these animals recruited to this location after spending some period of time, from a few days to a few weeks, in the plankton. The barnacle certainly can’t move once it has settled and metamorphosed. Newly settled mussels have a limited ability to scoot around a bit but are generally stationary once they’ve extruded their byssal threads and fastened them to something hard. The limpets, on the other hand, are quite mobile. The barnacle and mussels gave up their ability to move around after they became benthic, but limpets can and do locomote quite a bit–in fact, they have to, in order to feed. So in a sense, these limpets “chose” to aggregate together long after settlement.

What are the ecological implications of this pattern?

Well, for one thing, that barnacle is a genetic dead end. I’ve written before about the bizarre sex lives of barnacles. This one lone barnacle, far from any others of its species, is not able to reproduce. It has nobody to copulate with. It is possible that other barnacles will recruit to the mussels (Pollicipes is often associated with Mytilus), but until then there will be no sexy times for this individual.

Another ecological consequence concerns the limpets. If these are owl limpets (Lottia gigantea), then some of them will grow up to be the big females that maintain farms on the rocks where they manage and harvest the crop of algal film that grows. These big females are territorial, and will bump or scrape off any creature found to be trespassing on their farms. Clearly, none of the limpets in the photo above are demonstrating any type of territorial behavior! So they are either some other species of Lottia, or are younger individuals of L. gigantea that haven’t yet made the change from male to female.

In any case, I do think the pattern is very interesting, even though I don’t understand it. Or maybe because I don’t understand it. I’m always intrigued by something that I can’t explain, which is a good thing because it means I don’t get bored very often. If anyone reading this has an explanation for this pattern, let me know about it!