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3

Yesterday I had the great fortune to visit a new intertidal site. It can be accessed only by crossing private property. The property owner is my next-door neighbor, and he said I can visit any time. As I said, lucky me! The site is a little north of Pigeon Point, and at first glance the terrain is not very different from Pigeon. But I could tell that it a site that is rarely, if ever, visited by humans. It just had that look of being mostly undisturbed. Yesterday's marine layer was low, making for dark skies and pretty lousy light for picture-taking, so I had to try something new.

This site has a lot of lovely pools and channels to explore, and at this time of year the water is very clear, which does make for good picture-taking. Halosaccion glandiforme, one of the charismatic red algae, is more abundant here than at other sites, and in the pools it grows quite a bit taller than it does on the rocks.

Here's what it looks like on the tops of the rocks. This is a cluster of young thalli. The tallest of these "bladders" is about 4 cm tall. Note that they are about 2/3 full of water, with a large air space at the top.

Many olive-green spherical and ovoid bladders, attached to rock.
Young Halosaccion glandiforme thalli along the San Mateo County coast
2022-08-14
© Allison J. Gong

The really cool thing is what happened when I stuck the camera in the water and took a shot. I got something like this:

Two elongate olive-green bladders, filled about 2/3 with water, submerged in a tidepool
Halosaccion glandiforme and other algae submerged in a tidepool
2022-08-14
© Allison J. Gong

I got a little carried away. But don't things look interesting from the turban snail's perspective?

Olive-green towers rising from a carpet of pink algae. A black snail is nestled between a trio of the towers.
Halosaccion glandiforme and a black turban snail (Tegula funebralis) in a tidepool
2022-08-14
© Allison J. Gong

I'm kind of enraptured by these towers of algae.

Olive-green towers rising from a carpet of pink algae.
Halosaccion glandiforme in a tidepool
2022-08-14
© Allison J. Gong

But the best part of these experiments was the reflections on the surface of the water. Check it out.

Olive-green towers rising from a carpet of pink algae.
Halosaccion glandiforme in a tidepool
2022-08-14
© Allison J. Gong

And this is the money shot! I just love how this turned out.

Olive-green towers rising from a carpet of pink algae in the bottom half of the image. The same tower are reflected in the top half of the image.
Halosaccion glandiforme in a tidepool
2022-08-14
© Allison J. Gong

This was a super fun morning. I'm looking forward to visiting this site again, when the light is better. When the daylight low tides return in a few months they will be in the afternoon. I anticipate some fantastic light shows in these pools and channels. I'll be teaching most afternoons by then, but will get out as often as I can.

One of the many delightful animals in the rocky intertidal is the vermetid snail, Thylacodes squamigerus. Unlike their more typical gastropod relations, the vermetids don't live in a shell, per se. Instead, they live in a calcareous tube, which forms a loose coil draped over the surface of a rock. The tubes can be up to about 12 mm in diameter, and, if straightened out, about 15 cm long. In some locations, Thylacodes can be very abundant. In a recent visit to Point Pinos in Pacific Grove, I saw many of them in the low intertidal. I occasionally see them on the northern end of Monterey Bay and points farther north, but at nowhere near the abundance I see in Pacific Grove. At a larger scale, iNaturalist shows observations of T. squamigerus from northern British Columbia down to southern Mexico.

Three coiled white tubes and one spherical snail on a rock amid greenish seaweed
Trio of vermetid snails (Thylacodes squamigerus) with their more conventional cousin, the black turban snail (Tegula funebralis)
2022-06-03
© Allison J. Gong
Loosely coiled whitish tube on a rock
Thylacodes squamigerus
2022-06-03
© Allison J. Gong

Most snails are either grazers (e.g., abalones, limpets, turban snails) or predators (e.g., whelks, conchs, cone snails). Thylacodes is a bit of an outlier with regards to feeding as well as housing, for it is a suspension feeder. Being entirely sessile, it cannot go out and forage. And unlike its doppelganger, the tubeworms Serpula columbiana and S. vermicularis, Thylacodes does not create a water current to catch food on ciliated tentacles. Instead, it spins threads of sticky mucus that thrash around in the current and capture suspended detritus. When the tide is out the snail hunkers down in its tube, same as any worm. It cannot feed unless it is immersed. Where the worms live in the low intertidal on exposed rocky coasts, the water is moving constantly, and it requires relatively little energy for Thylacodes to feed the way it does. As a bonus, even the calories expended in producing the mucus are recouped, as the snail ingests the mucus strands as well as the food particles they capture.

When the tide came back, I got to watch Thylacodes in action. At Point Pinos there are some areas that form lovely tidepools, deep enough for animals to react to the return of the water and clear enough to make photography and videography possible. So standing knee-deep in a pool I stuck the camera underwater and hoped for the best. And I got lucky—you can see the mucus threads!

See here:

Thylacodes squamigerus
2022-06-03
© Allison J. Gong

and here:

Thylacodes squamigerus
2022-06-03
© Allison J. Gong

And not only that, but I captured some video footage. I use a point-and-shoot for these underwater shots, and usually don't know what or whether I've shot anything good until I download images and video at home. Color me happy to have seen these clips!

Despite the unusual aspects of its biology, Thylacodes is indeed a snail. It has a conventional snail's radula, and uses it the way, say, an owl limpet (Lottia gigantea) uses hers to scrape algae off rocks at Natural Bridges. Only instead of scraping the radula against rocks, Thylacodes uses its radula to reel in the detritus-laden mucus threads. That's what's going on in the second video clip above.

So there you have it, another of my favorite animals. Thylacodes is one of those animals that doesn't look like much when you see it just sitting there. But we get to see it only during the tiny fraction of its life that it spends emersed. As with most inhabitants of the rocky intertidal, much of Thylacodes' life occurs out of sight for human eyes. This makes the occasional sighting of Thylacodes under water especially enlightening. And delightful!

1

Big waves breaking on beach, with cliffs on the right side

One of the things that I've been doing with my Ecology class since almost the very beginning is LiMPETS monitoring in the rocky intertidal. Usually we have a classroom training session before meeting in the field to do the actual work. This year we are teaching the class in a hybrid mode, with lecture material being delivered remotely, so we don't have class meetings except for our field trips. The LiMPETS coordinator for the Monterey Bay region, Hannah, and I arranged to meet at our sampling site, where she would do a training session on the beach before we herded everyone out into the intertidal. It truly was a great plan! But the weather intervened and a spring storm blew through, bringing in a big swell. There was a high surf warning for our area the day of our scheduled LiMPETS work. Hannah and I conferred via email and decided that we'd still give it a shot, and at least the students would have an opportunity to learn about the LiMPETS program and practice with the datasheets and gear.

I arrived early to see how the surf was looking, and it was impressive. The waves were regularly covering our sampling location with whitewash, even as the tide was going out. When my co-instructor arrived and I showed him where the transect would lie, it was an easy decision to make to cancel the monitoring. But we would still be able to do the practice stuff, so we convened with Hannah on the bluff and she went into teacher mode.

College students standing in a circle, listening to instructor
Hannah (right) explaining the LiMPETS program
2022-04-22
© Allison J. Gong

We didn't bother with the transect, but had groups of students work through some quadrats out on the intertidal bench, which you can just see in the background of the photo above. Hannah kept everyone out of the danger zone and we stressed the importance of having one member of each group keep an eye on the ocean at all times. We stayed mostly in the high zone, venturing down into the upper mid zone only when the tide was at its lowest. Even then, the big swells would surge up the channels and splash up onto the benches. Nobody got swept off, though, or even more than a teensy bit damp.

Most of the students left after what little work we had for them to do, and that gave me the freedom to poke around on my own and take pictures. I hadn't had a chance to do this in a long time, and intended to make the most of a decent low tide that was almost wiped out by huge swell.

So here we go!

First up, the high-intertidal seaweeds:

Olive-green seaweed on rock, with mussels surrounding
Silvetia compressa
2022-04-22
© Allison J. Gong

And here's a typical high intertidal community at Davenport Landing. Inhabitants include:

  • Several large clumps of rockweed (Silvetia compressa and Fucus distichus)
  • Several smaller bunches of tufty reds (Endocladia muricata)
  • Mussels (Mytilus californianus)
  • Many blotches of "tar spot alga" which is the encrusting tetrasporophyte phase of Mastocarpus papillatus
Clumps of olive-green seaweeds, dark red seaweeds, and mussels on rock
High intertidal community at Davenport Landing
2022-04-22
© Allison J. Gong

The water was pretty murky, so not great for underwater photography. Some of the shots turned out pretty well, though. The soft pale purple structures that you see in the photo below are papullae, used for gas exchange. You can see these only when the star is immersed.

Clumps of pale purple transparent tubes interspersed with white blotches
Aboral surface of the ochre star Pisaster ochraceus, showing papullae and spines
2022-04-22
© Allison J. Gong

The anemones were, as always, happy to be photographed. In this shot, the anemone was being photobombed by a turban snail.

Large green sea anemone and small purple snail in a tidepool
Green anemone (Anthopleura xanthogrammica) and black turban snail (Tegula funebralis)
2022-04-22
© Allison J. Gong

Here's another typical intertidal assemblage:

Clump of sandy tubes with mussels, barnacles, and greenish-purple seaweed
Sandcastle worm (Phragmatopoma californica), iridescent alga (Mazzaella flaccida), gooseneck barnacles (Pollicipes polymerus), and mussels (Mytilus californianus)
2022-04-22
© Allison J. Gong
Gooseneck barnacles (Pollicipes polymerus)
2022-04-22
© Allison J. Gong

A couple of students stayed after the rest of the class had left. They were happy to see the nice fat ochre stars, and so many of them in one small area.

It's always good to see so many big ochre stars. For this species, in the intertidal areas that I visit, sea star wasting syndrome (SSWS) no longer seems to be a problem. Fingers crossed! We'll have to see what unfolds in the next months and years.

On this winter solstice, as we anticipate the return of light, I thought I'd write about a different kind of light.

Merriam-Webster defines fluorescence as "luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate reradiation usually at a different wavelength and that ceases almost at once when the incident radiation stops". It is a type of luminescence that occurs in both biological and non-biological objects. For example, mushrooms and scorpions are notably fluorescent, as are several minerals. Technically, to qualify as "fluorescent" an object can absorb any wavelength of radiation and re-radiate any other, although the re-radiated wavelength is usually longer than the absorbed wavelength.

We humans, with our three (and occasionally four) color photoreceptor types, can see only the tiny fraction of the electromagnetic spectrum that we call visible light. The visible light range (approximately 400-700nm) is bounded by UV on the short end and infrared on the long end. Other organisms have very different light perception capabilities. We know, for example, that insects can see in UV and pit vipers can see in infrared. And as for mantis shrimps, which have as many as 12 types of photoreceptors, we don't yet understand how they see the world, but you can bet it's nothing like the way we do. For practical purposes, fluorescence is most easily seen when an object absorbs UV light and re-radiates light of a longer wavelength that falls into the visible light range.

When you shine a UV light on one of these fluorescent objects, you see an apparent color change from whatever it looked like under visible light. This color change is most striking in the dark, because the fluorescent object will appear to glow. The same thing happens in daylight, but is obviously more difficult to see.

Here, let me show you. A few weeks ago I went to Natural Bridges to photograph the anemones, first under normal daylight conditions and then under UV light. I have a pretty wimpy UV flashlight, it turns out, but you can still see the fluorescence.

Here's Anemone #1, under daylight:

Sea anemone in daylight
Sunburst anemone #1 (Anthopleura sola) at Natural Bridges
2021-12-07
© Allison J. Gong

And here's Anemone #1 under UV light:

Sea anemone under UV light
Sunburst anemone #1 (Anthopleura sola) at Natural Bridges, under weak UV light
2021-12-07
© Allison J. Gong

Striking difference, isn't it?

This is Anemone #2. It was getting dark by then, but this photo was also taken without flash and I did not increase exposure of the image.

Sea anemone
Sunburst anemone #2 (Anthopleura sola) at Natural Bridges
2021-12-07
© Allison J. Gong

And, under UV light:

Sea anemone under UV light
Sunburst anemone #2 (Anthopleura sola) at Natural Bridges, under weak UV light
2021-12-07
© Allison J. Gong

Here's what's going on. Pigment molecules in the anemones' tissues are absorbing the UV radiation and re-radiating light in the visible range. It's easier to see the fluorescence in Anemone #2 because it was much darker when I took that set of photos. Fluorescence still occurs during the day, but we can't see it as well in the daylight. This is why our local bowling alley does their Atomic Bowling at night! They can dim the overhead lights, crank up the black lights, and let the tunes roll.

Incidentally, if you've ever wondered why so-called black lights are purple, there's a reason for it. A true black light emits only UV light. UV light is invisible to us, hence the term "black", as in pure darkness. UV lights that ordinary folks like us can buy are tinged purple so that we can see it. The purple isn't UV, of course, but seeing the purple light keeps people from looking into the beam and frying their retinas from the actual UV radiation.

Sea anemones, of course, do not celebrate the solstice, but they do perceive it. They, and just about every other living thing, can sense the cyclical changes in day length as the year progresses. After tonight the days will start getting longer as we move through winter and towards spring. Personally, I cannot wait until we get the early morning low tides in the spring.

In the meantime, happy solstice, everyone!

4

For some reason, many of the sunburst anemones (Anthopleura sola) in a certain area at Davenport Landing were geared up for a fight. I don't know what was going on before I got there yesterday morning, but something got these flowers all riled up. We think of them as being placid animals, but that's only because they operate at different time scales than we are used to. A paradox about cnidarians is that they don't do anything quickly except fire off their stinging cells; that, however, they do with the fastest known cellular mechanism in the animal kingdom. Go figure.

Pale green sea anemone with slender feeding tentacles surrounding the oral disc. Below the ring of feeding tentacles there is a ring of thick club-shaped tentacles used for fighting.
Sunburst anemone (Anthopleura sola) with inflated acrorhagi
2021-06-27
© Allison J. Gong

What looks like an anemone wearing a tutu is actually an anemone ready to fight. The normal filiform feeding tentacles are easily recognized. But those club-shaped white tentacles below the ring of feeding tentacles are called acrorhagi. They are all about fighting. The tips are loaded with potent cnidocytes that usually aren't used to catch food. They are used to fight off other anemones, and possibly predators.

Here's another shot of the same animal, which shows how the feeding tentacles and acrorhagi are arranged in concentric rings:

Pale green sea anemone with slender feeding tentacles surrounding the oral disc. Below the ring of feeding tentacles there is a ring of thick club-shaped tentacles used for fighting.
Sunburst anemone (Anthopleura sola) with inflated acrorhagi
2021-06-27
© Allison J. Gong

So who would this anemone be fighting? This individual was the only one of its kind in the pool where it lives. I don't know why its acrorhagi are inflated. I suppose they could be used to fend off a would-be predator, but I didn't see any other animal in the pool that seemed a likely candidate.

But look at this duo:

Two pale green sea anemones with slender feeding tentacles surrounding the oral disc.The anemone on the right has inflated fighting tentacles. The animal on the left has fewer inflated fighting tentacles.
Sunburst anemones (Anthopleura sola) with inflated acrorhagi
2021-06-27
© Allison J. Gong

Now, clearly there is (or had been) something going on between these individuals. They both have their acrorhagi inflated. I've been looking at this photo for a while and can't decide which is the aggressor. At first I assumed that the anemone on the right had initiated an attack on the other. But now I wonder if that is a defensive posture rather than an offensive one. That animal does seem to be more bent out of shape than the one on the left.

I've seen anemone fights before, and I've also seen anemones living side by side, tentacles touching, in apparently perfect amity. It's very clear that they can coexist peacefully. Why, then, do they sometimes choose to fight? It's important to point out that Anthopleura sola is an aclonal species. Unlike its congener A. elegantissima, whose primary mode of growth is cloning, each A. sola represents a unique genotype. With these anemones, whether or not two individuals fight is not determined by relatedness.

In a different pool these two anemones are sharing the carcass of a rock crab.

Sunburst anemones (Anthopleura sola)
2021-06-27
© Allison J. Gong

Maybe that third anemone at the top had also taken part in the feast, but at this point it seemed to be minding its own business. Given the demonstrated aggression of some A. sola, it would be interesting to know whether or not this trio ever fight amongst themselves. When we 'ooh' and 'aah' over them in the tidepools they look like passive flowers, and we forget that they are active predators. But we humans have access to the anemones' home for only a few hours every month, and I have no doubt that they get up to all sorts of shenanigans when we're not looking.

3

This morning I went to Natural Bridges. The tide this morning was the lowest of the season, but early enough that for the most part I had the intertidal to myself for a couple of hours. I always like those mornings best.

I did meet a docent out there, and we chatted for a few minutes. Towards the end of the excursion, when the tide had turned and I realized I had to get to the marine lab for the usual Friday feeding chores, she pointed out something that didn't make sense to her. She described it as two anemones side-by-side, but one was really stretched out down towards the water. She wondered what could be going on, as the other anemone looked normal.

Two large sea anemones at the edge of a tidepool. The anemone on the left is stretched down to more than twice the length of the anemone on the right.
Sunburst anemone (Anthopleura sola) and giant green anemones (Anthopleura xanthogrammica)
2021-06-25
© Allison J. Gong

Looks strange, doesn't it? What this anemone is doing, I think, is disgorging the remains of its most recent meal. If you look at the oral end, which is indeed stretched down towards the sandy bottom of the pool, you can see two things sticking out. The whitish blob is the internal part of the anemone's pharynx. It is not at all uncommon for anemones to sort of prolapse the pharynx, especially after a big meal. Remember, anemones have a two way gut with a single opening for both food ingestion and waste expulsion. The other thing sticking out of the mouth is a clump of mussel shells thickly coated with slime.

Here's a close-up of what's going on at the mouth of this anemone:

Sunburst anemone (Anthopleura sola) disgorging mussels
2021-06-25
© Allison J. Gong

It's hard to tell whether or not the mussels have been opened and digested by the anemone. It looks like at least some of the acorn barnacles attached to the mussel might still be alive, although smothered in slime. Nor can we see how many mussels are still inside the anemone's gut. In any case, the anemone is getting rid of this part of the mussel clump. However, this isn't a phenomenon that can really be watched, unless you can watch in time-lapse. The docent asked, "Doesn't it use peristalsis, or something like that?" The answer is that no, anemones don't use peristalsis. They don't have the type of muscles that can contract in that way. The anemone still has to somehow expel wastes and undigestible matter from its gut, through that single opening that we call a mouth but functions as both mouth and anus.

Our human gut, of course, uses peristalsis to move food along from esophagus to rectum. And while for the most part we don't like to think about how that works, we have all experienced what happens when things don't go as planned. I doubt that anybody gets through life without vomiting, so it is probably safe to say we all know that it is a violent way to thoroughly expel food, toxins, and other noxious items from the stomach. Anemones, however, have no peristalsis and cannot vomit. How, then, does an anemone void its gut of something larger than the typical digestive waste?

This particular anemone is ideally situated to let gravity do the work. Hanging down like this and relaxing the simple sphincter muscle around the base of the tentacles will allow the mussel clump to eventually fall out. Without peristalsis to speed things along, it will probably take a while. Would it be finished by the time the tide comes back? I couldn't stick around to watch, so I can't say. But it was a very cool thing to see, even though it happens about as fast as paint drying.

1

As we speed towards the summer solstice the days continue to get longer. The early morning low tides are much easier to get up for, as the sky is lightening by 05:30. Even so, when traveling an hour to get to the site, it's nice when the low is later than that. This past Saturday the low wasn't until 08:00. My parents were in Monterey for the weekend, so I decided it would be a good day to work the tide at the southern end of Monterey Bay, and then visit my parents. The Monterey Peninsula has some of the most spectacular tidepooling terrain in the region, and if I lived closer you can bet I'd know those sites better. Not that there is anything at all wrong with the sites on my end of the Bay and up the coast. But sometimes it's good to get out of one's comfort zone and explore the less well known.

Rocks and tidepools
Rocky intertidal at Asilomar State Beach
2021-05-15
© Allison J. Gong

So explore we did. It was cold and windy. The tide wasn't all that low and the swell was up, so we didn't get beyond the mid-tidal zone. My hip boots have deteriorated to the point that I have pinprick leaks at the seam where the boot part meets the leg part. Usually the tiny leaks don't bother me, but when the water is cold I definitely feel the trickles. What all this means is that I didn't get down into the low zone, which is fine. Biodiversity is highest in the mid zone anyway. The mediocrity of the low tide meant that I had to keep an eye out for sneaker swells, so less heads-down poking around and more scanning from above and then zooming in on individual items of interest.

One thing we noticed right away is that groups of Tegula funebralis, the black turban snail, were clumped together above the waterline of the high pools.

I'm trying to decide whether or not this is noteworthy. The pattern did catch my eye, but that might be only because it's unusual (although not particularly interesting). It was a cold and drizzly morning, so the snails didn't have to worry about desiccation. Was the clumping together benefiting the snails in any significant way? Hard to say.

The T. funebralis were also clumping together in the water! Here's a large clump of Tegula shells in a pool.

Clump of black turban snails in a tidepool
Black turban snails (Tegula funebralis) and one hermit crab (Pagurus samuelis)
2021-05-15
© Allison J. Gong

Almost all of these are snails, but can you see the one that is a hermit crab?

Poor Tegula funebralis. It is so common that it is invisible and vastly underappreciated. I find them quite charming, though. There's something about a grazing snail's slow way of life that is very soothing. Not that you might not fall asleep waiting for them to do something interesting, but it is good to slow down to the pace of nature. Anyway, Tegula is one of my favorite animals, precisely because it is so unassuming and ignored. One of delightful things about Tegula funebralis is when it plays host to Crepidula adunca. I've written about the biology of C. adunca before and don't want to rehash that here. I just wanted to show off my favorite photo of this trip to Asilomar:

Black turban snail with two attached slipper snails
Black turban snail (Tegula funebralis) wearing two slipper snails (Crepidula adunca)
2021-05-15
© Allison J. Gong

I don't know why I like this photo so much. It certainly isn't the best shot I've ever taken. There isn't any vibrant color at all. The subjects are the same color as the background. But it works for me.

When it comes to a snail's pace, you can't find anything slower than Thylacodes. That's because Thylacodes squamigerus is the snail that lives in a calcareous tube. Much like a barnacle, or the serpulid worms that have similar tubes, Thylacodes makes one decision about where to live and lives there for the rest of its life. I see Thylacodes at places like Pigeon Point up north, but they are much more abundant on the Monterey Peninsula.

Tube snail (Thylacodes squamigerus)
2021-05-15
© Allison J. Gong

And the snail winners in the Most Likely to be Overlooked have got to be the littorines. These little snails (most of which are smaller than 15 mm) live in the highest intertidal, where they get splashed by the ocean just often enough to keep their gill sufficiently moist. They are never entirely submerged, but they do tend to gather in cracks, even the tiniest of which will hold water longer than a flat rock surface.

Littorines (Littorina keenae) in the splash zone
2021-05-15
© Allison J. Gong

If you look closely at the photo above, you might see pairs of mating snails. Given where they live, high up in the intertidal where they are rarely covered by water, broadcast spawning isn't a viable option for the littorines. They have to copulate. There are, I think, eight copulating pairs in this group of ~30 snails.

Copulating pairs of Littorina keenae
2021-05-15
© Allison J. Gong

Because Littorina's habitat makes broadcast spawning an unfeasible option, the snails must lay eggs. But the splash zone isn't a very friendly place for the eggs of marine animals. The littorines lay eggs in gelatinous masses in crevices or depressions where water will remain. After a week or so of development, the egg mass dissolves as it gets splashed, and veliger larvae emerge. They recruit back to the intertidal after spending some period of time in the plankton.

When all is said and done it's difficult to make the claim that snails live exciting lives. Nonetheless, they are interesting animals. The diversity of morphology and lifestyle we see in the intertidal snails makes them eminently worthy of study and appreciation. I like to think that, as biologists once again "discover" the usefulness of natural history, students will be encouraged to fill in some of the gaps in our understanding of these and other abundant animals.

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!

It has taken me months to gather all the photos and videos I needed for this post. I could blame it on the stress of teaching online for the first time, the COVID-19 pandemic itself, or residual malaise from the dumpster fire that was 2020. But really, it's the animal's fault.

In this case the animal is the orange cup coral, Balanophyllia elegans. I've written about this beast before, and lately I've been paying more attention to the corals that we have in the lab. In many ways it is the typical anthozoan—its life cycle consists of only a polyp stage (i.e., no medusa), it is benthic, and its body is vaguely anemone-like. Like the reef-building corals of the tropics, Balanophyllia is a scleractinian coral. This means that it secretes a calcareous base, or exoskeleton, upon which sits the living tissue of the polyp. But unlike the reef-building corals of the tropics, Balanophyllia is solitary, which means that it does not clone or form colonies. Nor does it contain photosynthetic zooxanthellae in its cells, as the reef-builders do. This means that Balanophyllia is a strict carnivore: unable to rely on photosynthetic symbionts to do the hard work of fixing carbon, Balanophyllia has to catch its own prey.

Balanophyllia has separate sexes and reproduces sexually. Males release sperm that are ingested by the female, and she then broods larvae for several months. What is strange about this is that, when you consider the anatomy of Balanophyllia (or any cnidarian polyp, for that matter) the question that comes to mind is, "Where are the larvae brooded?" So let's take a look at the basic cnidarian body plan.

Adult cup coral (Balanophyllia elegans)
© Allison J. Gong

The professor who taught my undergraduate course in invertebrate zoology described a cnidarian's body as a baggie inside a baggie, with a layer of jelly between the two baggies. The inner and outer baggies represent the tissue layers that developmental biologists refer to as endoderm and ectoderm, respectively. The jelly between the two tissue layers is called mesoglea. For the sake of this discussion, it's the endoderm that interests us. In a cnidarian, the endoderm is also called gastrodermis, because it is, literally, the skin of the digestive system. The cnidarian gut is a cavity that opens to the outside via a single opening. We could call that opening either a mouth or an anus, since both food and wastes pass through it, but for politeness' sake we call it a mouth. The gut itself does double duty as both the digestive system and the circulatory system, so its formal name is gastrovascular cavity (GVC).

It's easy to imagine the GVC as being essentially a tubular vase, but it's more complex than that. The GVC extends into each of the tentacles, so the tentacles are actually hollow. The main cavity of the GVC, in the stalk of the polyp, is partitioned by thin layers of endoderm. The partitions are called septa, and serve to increase the surface area of the endoderm for digestion. Think of a large office building, divided into small cubicles by movable partial walls—there's much more total wall space for hanging things such as calendars than there would be in the big room with only four walls. In anthozoans, gonad tissue is associated with the septa.

Now back to how Balanophyllia broods its larvae. As we've seen before, Balanophyllia's planula larva is an orange-reddish wormlike blob about 2 mm long, which is ciliated all over. It doesn't feed, but survives on energy reserves supplied by the mother in the egg; it may also be able to take up dissolved organic material directly from the seawater. After brooding larvae for some period of time in their GVC, Balanophyllia females release larvae. Or rather, the larvae ooze out of their mother's mouth and crawl around to settle and metamorphose nearby.

The planulation season, when larvae show up amongst the corals we keep in the lab, begins in the fall and runs through winter into the spring. Sometimes the larvae metamorphose right away—one day there are no larvae and the next day there are baby corals. Other larvae squirm around for days or weeks, not getting any smaller but not metamorphosing, either. This past season (Fall 2020-Spring 2021) we saw both extremes: planulae that settled and metamorphosed right away, and others that I collected weeks ago and are still worming around.

Orange cup coral (Balanophyllia elegans) with a planula larva
2020-12-12
© Allison J. Gong

I peered into a bunch of corals, hoping to see what the larvae are doing inside the GVC, with no luck. Then I decided to try some time-lapse video under the dissecting scope, and had a bit of success with that. In the video below you'll see about two hours of action compressed into about 1 minute. Watch for a small red ball squirming around inside the GVC. And remember that the GVC extends into the tentacles, so the larva can wiggle its way in there, too.

All of which makes me wonder if the pathway from mother's GVC to the scary world outside travels in only one direction, or if the larvae can retreat back into the safety of the mom's digestive system. How strange is it, that the safer location might be inside the mother's gut!

Eventually, whether it be a matter of hours or weeks, the planula larva settles (i.e., sticks to a surface and stops crawling) and metamorphoses (i.e., makes the anatomical and physiological changes into the juvenile body). There doesn't seem to be a clear connection between surface characteristics and whether or not larvae will settle there. You might think that they'd choose to settle nearby or maybe actually on the parent, but that's not always the case. At some point, however, the larva will have to either settle and metamorphose, or die. When they metamorphose into juveniles, they reveal other aspects of their nature as scleractinian corals.

From what I've seen, the larva begins the settlement process by sticking to a chosen spot and becoming a more circular (i.e., less elongated) blob. On the other hand, I've seen perfectly round red blobs that look for all the world as though they might be pushing out tentacles, change their minds and go vermiform again. But once they stick permanently and begin to metamorphose by forming the polyp's body, they have to continue.

Newly settled orange cup coral (Balanophyllia elegans)
2021-01-29
© Allison J. Gong

The juvenile in the photo above has not yet pushed out any tentacles, but it does show signs of its scleractinian ancestry. The Scleractinia (stony corals) and Actiniaria (sea anemones) are both members of a group called the Hexacorallia. The Hexacorallia and Octocorallia are in turn grouped together as members of the class Anthozoa. As you might infer from the name 'Hexacorallia', animals in this group of a body symmetry based on the number six. And you can see in the photo above that the juvenile coral's body is divided into 12 wedges. When the juvenile begins growing tentacles and a more polyp-like body, the hexamerous symmetry becomes even more evident:

Young orange cup coral (Balanophyllia elegans)
2021-02-22
© Allison J. Gong

The young coral begins by forming six primary tentacles, which establishes the visible hexamerous symmetry. Then it grows the six shorter secondary tentacles. The bumps on the tentacles are cnidocyte batteries, clusters of stinging cells, indicating that the animal can already capture and kill prey. Good thing, too, because if it hasn't already then it soon will deplete the energy stores its mother partitioned into its egg.

By this stage in its development the coral will remain where it is for the rest of its life. As it grows it will deposit CaCO3 and grow taller, but the living tissue will always be restricted to a layer sitting on top of the calcareous base. Once the base grows more than a few millimeters tall, there is no living tissue in contact with the surface. Thus there is no mechanism for the animal to move to another location, or even to re-attach itself if it gets broken off its surface. There are many animals that are likewise unable to move once their larvae have attached and metamorphosed. The decision of where to put down roots becomes quite stark for them, as a bad one can result in a very short life indeed. The selective pressures that enforce a good decision are quite clear, and are important factors in the distribution patterns we observe in the intertidal.

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