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For animals that do essentially nothing when you see them where they live, chitons have a lot of charm. They are the kind of animal that, once you develop the search image for them, you start seeing everywhere. It helps that they are easily recognized as being chitons because of their eight dorsal shell plates—nothing else looks like them. Depending on species, those shell plates can be smooth or sculpted, and pigmented or not. Patterns of sculpting and pigmentation (or lack thereof) are diagnostic features used to distinguish different species. Some species are reliably consistent in appearance and look the same wherever you happen to see them. Other species show a lot of phenotypic variation, often even at a single site.

One of my favorite chitons is Mopalia muscosa, the mossy chiton. It's one of the easiest of our chitons to identify, because its girdle (the layer of tough tissue in which the shell plates are embedded) is densely covered by long, curved spines. They're called spines, but they're quite soft and flexible. Your basic Mopalia muscosa looks like this:

Mossy chiton with bare shell plates, in the rocky intertidal
Mossy chiton (Mopalia muscosa) at Pigeon Point
2016-04-24
© Allison J. Gong

Mopalia muscosa is one of the species whose appearance is quite variable. Many of them wear algae, usually reds but occasionally greens or browns, on their shell plates. Not all species of chiton do this. I've often wondered why some chiton species wear algae and others do not. This individual is probably fairly old, judging by the worn condition of the shell plates. The plates show signs of erosion, but are not decorated. There are some small pieces of coralline algae amongst the spines of the girdle, though, which I always associate with age. Smaller, and presumably younger, M. muscosa tend not to have algae on the girdle even if they are wearing some on the shell plates.

The degree of shell decoration in M. muscosa varies from none, as above, to heavy encrustation. This individual below has been colonized by only a small bit of coralline algae and perhaps some brown diatom-ish film on the edges of the shell plates:

Mossy chiton (Mopalia muscosa) at Pistachio Beach
2021-02-09
© Allison J. Gong

This next one has only a small bit of coralline alga, but sports a jaunty sprig of something quite a bit larger.

Mossy chiton (Mopalia muscosa) at Asilomar
2019-07-04
© Allison J. Gong

This season's fashionable chiton will go all out with the coralline algae, wearing both encrusting and upright branching forms. Look at this:

Mossy chiton (Mopalia muscosa) at Pigeon Point
2017-06-28
© Allison J. Gong

and this:

Mossy chiton (Mopalia muscosa) at Pigeon Point
2018-01-01
© Allison J. Gong

Sometimes the chitons wear the larger leafy red algae, in addition to or in place of the coralline algae. I always think that these individuals must be very old, by chiton standards.

Mossy chiton (Mopalia muscosa) at Pigeon Point
2020-11-14
© Allison J. Gong

And sometimes the chitons are so covered with algae that they blend in perfectly with the surrounding environment.

Mossy chiton (Mopalia muscosa) at Pistachio Beach
2021-04-06
© Allison J. Gong

These chitons can get very heavily fouled by algae. Is there any benefit to the chiton, to carry around a load of red algae? And if wearing algae is for some reason advantageous, is there a way for a chiton to attract algae to settle on their shell plates? Well, let's think about that. Chitons' main predators would be sea stars, crabs, and birds. Sea stars do not locate prey visually, so camouflage would not be very helpful in avoiding them. Birds such as oystercatchers and surfbirds certainly do pry up chitons and limpets, and blending in with the background just might help a chiton go unnoticed by an avian hunter.

Regarding the matter of how the algae end up living on chitons' bodies, I want to start with the question of how prevalent algal fouling is on Mopalia muscosa, and the extent of fouling on the chitons that are wearing algae. A little research study might be a fun way to spend my time in the intertidal. Pigeon Point is a lovely site on a foggy summer morning, and many of the most heavily decorated M. muscosa in my photo library are from there. Yes, I can foresee several visits up the coast over the next few months. Laissez les bons temps rouler!

This morning I went to Pigeon Point to poke around and do some collecting. It's a favorite site of mine, as it's exposed and dynamic, with the diversity you'd expect. Of the sea stars, the most common by far are the six-armed stars in the genus Leptasterias. They are small (less than 8 cm in diameter, often smaller than 1 cm), somewhat drably colored, and sometimes on the underside of rocks, all of which means that they are not always conspicuous. But once you get the right search image, you see them everywhere.

Six arms, see?

Sea stars are well known for their ability to regenerate lost arms. It is not uncommon to see a star that looks healthy in every way except that one of its arms is shorter than the others. This must happen in Leptasterias, too. Searching through my library of pictures of Leptasterias, I did find a couple of examples of regeneration.

When these stars finish regrowing those arms, they will have the typical number of arms for the genus, which is six.

Today I saw something that I'd never seen before. It was a Leptasterias that was regenerating arms. Only this was weird. It had three full-size arms and was growing four!

Sea star with 3 arms and regenerating 4 arms
Leptasterias star regenerating lost arms at Pigeon Point
2021-04-03
© Allison J. Gong

When (if) this star survives, it will eventually have seven arms. And that's strange. I asked my friend Chris Mah, who is the sea star systematist at the Smithsonian, if it was common for Leptasterias to do this. He said he'd never seen it, either. So it is indeed a rare phenomenon.

Now, there are stars in the genus Linckia that actually reproduce by deliberately leaving behind an arm, which then goes on to regenerate the rest of the body. While they do so they look like comets:

"comet" star. One arm is regenerating the remaining arms and central disc.
Linckia multiflora comet in the Maldives
Ahmed Abdul Rahman and Frédéric Ducarme for MDC Seamarc Maldives (CC BY-SA 4.0)

My regenerating Leptasterias isn't quite a comet, but it is doing something equally strange and wonderful. I really wished I could bring it to the lab and keep an eye on it over the next several months. However, Leptasterias are on the no-take list, I think because their populations are so patchy. It is extremely unlikely that I will ever see that same individual again, so we will never know what happens to it. Unless, of course, I happen to come across a 7-armed Leptasterias at Pigeon Point sometime in the future. If you see it, take a photo and let me know!

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Way back in 2015 I wrote about some Ulva that spawned in a bowl at the lab, and delved into the mysteries of reproduction in the green algae. This morning I was out at Franklin Point and saw this:

Spawning algae at Franklin Point
2021-04-01
© Allison J. Gong

I had seen the sea lettuces (Ulva spp.) spawning in these high pools at Franklin Point before, and usually cursed the murkiness of the water. But today the water was dead calm, with the tide low enough that there were no waves to slosh into the pools. The result was a gorgeous marbled swirl in the water. The patterns were stunning.

Yellow streams of algal spawn in a shallow tidepool
Spawning algae at Franklin Point
2021-04-01
© Allison J. Gong
Yellow streams of algal spawn in a shallow tidepool
Spawning algae at Franklin Point
2021-04-01
© Allison J. Gong
Yellow streams of algal spawn in a shallow tidepool
Spawning algae at Franklin Point
2021-04-01
© Allison J. Gong

What these photos show is the Ulva releasing either spores or gametes. Without microscopic examination it's impossible for me to know whether these tiny cells are spores or gametes. What I can say is that the spawn is released from the distal ends of the thallus, making the body of the alga look ragged.

Sea lettuce in a tidepool. Some blades are clear.
Sea lettuce (Ulva sp.) at the edge of a tidepool at Franklin Point
2021-04-01
© Allison J. Gong

The parts of the thallus that have already spawned are now clear. The tissue itself will soon disintegrate, leaving behind only the healthy green parts, which should be able to regrow.

All of these photos were taken in pools where the spawning itself had either completely or mostly stopped. Obviously when the tide comes back all of this yellow spooge will get mixed up. It's only when the water is perfectly still that these streams would form. It was hard stepping around the pools to take the photos, as the last thing I wanted to do was stomp my big booted foot into a pool and disrupt the beautiful patterns. Fortunately the sun angle was a little cooperative this morning, and I was able to find a pool where active spawning was happening.

What appears to be an act of destruction—the alga's brilliant green thallus being reduced to yellow streaks that drift away with the tide—is really an act of procreation. This is terminal reproduction, literally the last thing an organism does before it dies. Salmon do this, as do annual plants. The sheer amount of algal spawn in these tidepools is astounding. Imagine the number of 2-micron cells needed to color the water to this degree. But if reproducing is the last thing you're going to do in your life, you might as well go all in on your way out, right?

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