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.
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:
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:
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.
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.
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.
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:
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.
Sometimes things just work out, through no fault of my own. In terms of good minus tides occurring in daylight hours, this weekend's tides are the best we will have all season. Today (Saturday 29 May) is the third of five intertidal excursions I have planned. This morning I went up to Pistachio Beach to collect some things for the Seymour Center. I always feel a teensy bit apprehensive agreeing to collect for anybody but myself, because it is quite likely that I will get skunked and not be able to bring back what is needed. So usually I just agree to keep my eyes open for things that are on the wish list and make no promises.
The current wish list for the Seymour Center includes fishes. I've already brought them some sculpins and a clingfish, but small pricklebacks are also welcome. Pistachio is a popular place for people who fish for large pricklebacks. Apparently they (the pricklebacks) put up a good fight and make tasty eating. The usual way of fishing for them is poke-poling. I am not entirely sure how that works, but it involves a long pole and baited hooks. I think the idea is to lure a prickleback out from its hiding place at low tide, when it is sort of stranded away from open water. Adults get up to 70-80 cm long, and are as big around as my forearm.
Unlike the fishermen, I was fishing for young pricklebacks, hoping to find some that were about the length of my hand. Possessing the ideal set of characteristics for avoiding capture—a long eel-like body, small head, slimy coating, and the ability to augur really quickly into even the tiniest crack amongst the cobbles—these small fish led me on a merry chase for quite a while. However, the advantages that I have over even a wily prickleback are an enlarged cerebral cortex, opposable thumbs, and the dexterity to use both a dip net and a zip-loc baggie. When all was said and done I had two appropriately sized pricklebacks in my baggie, and two others had gotten away from me. Oh, and I did also bag another clingfish!
Having had that bit of success and not wanting to press my luck, I started poking around just for the hell of it, without any clear objective in mind. As I've said before, what we gain from a super low tide like this (-1.6 ft) is not only access to more real estate in the low intertidal, but more time to spend there before the tide returns. I took lots of photos, which I will present in chronological order. These will give you an idea of what it was like out there this morning.
Even the hike across the beach yielded something nice—this small stand of Postelsia palmaeformis, the sea palm. These poor junior kelps will be taking a beating with these spring tides rushing up and down. That's the price they pay for living out there on those exposed rocky points.
The leather star Dermasterias imbricata isn't one of the most common stars in the intertidal around here. It was one of the species that was hit pretty hard by the most recent outbreak of Sea Star Wasting Syndrome. We see one every so often, but they are nowhere as abundant as the ochre stars or bat stars.
Pistachio Beach isn't the best place for large anemones, but of course there are some. This is one of the few big Anthopleura anemones that I saw today. There are many of the small cloning anemones, A. elegantissima, in the high intertidal, as well as the moonglow anemones, A. artemisia, in the mid and low sandy areas.
I was so pleased to see my favorite red alga doing really well in the low zone! It is so pretty.
And at the same time I accidentally discovered a pretty big rock crab, which was tucked under a rock. For its species, this one was pretty calm and didn't come at me with big claws up. It could be that this crab is a male, and is clasping a female beneath him. I didn't check.
One of the things I found while turning over rocks to look for fish is this purple urchin:
And a bit later, a nice healthy group of Dictyoneurum californicum. As these thalli age, they will develop longitudinal splits at the base of the blades. Right now they are young and crispy.
And who can resist such an exuberantly decorated limpet? Certainly not I! Reminds me of the fancy hats that ladies used to wear for Easter. Or Beach Blanket Babylon.
Chitons, the overlooked molluscs that reach peak abundance and diversity in the intertidal, can be very common along the coast. Species composition varies from site to site, though. Here at Pistachio Beach, the two species of Tonicella are very common. I found several of them on the undersides of rocks. This one is T. lokii.
After two hours of catching fish and looking around, I was getting cold. Time to head back up and out. That took an additional half-hour or so, because I kept getting distracted by the algae. For example, look at how beautiful this Fucus is. And note the swollen tips, which mean this thallus is getting sexy. 'Tis the season, after all.
One of the other rockweeds, Pelvetiopsis limitata, was also very thick and abundant.
The rockweeds share the high intertidal with a few species of red algae. The most common reds in this zone are the two (or however many there are) species of Mastocarpus, and Endocladia muricata.
I always want to stop and look around in the high zone on my way down. Because when I walk past sights like this, it's hard not to stay and study more closely. Then I remember that I can take as much time as I want in the high zone on the way out. This morning I took lots of photos of these reds and rockweeds.
How many different types of seaweed can you see?
So there you have it, my morning summarized in about a dozen photos. I hope your Saturday was as enjoyable as mine was!
The rocky intertidal is coming into its full summer glory right now. The early morning low tides have been spectacular in May, and they'll get better for the remaining few days of the month. This morning I went out to Franklin Point to poke around. Low tide was -1.8 feet (yippee!) at 06:13. And for once the swell was also down, so the ocean seemed very far away from the mid-tidal zone. See?
One thing that's nice about Franklin Point is that despite its exposure, especially on the north side of the point, all those boulders provide a lot of protection from the incoming waves. It's amazing how they serve to dissipate the water's energy. Of course, that doesn't prevent the inevitable rise of water in the pools, but at least when it arrives it just floods boots instead of knocking down a distracted marine biologist.
Here's a 20-second video I shot from the same spot.
Just as in any terrestrial habitat, summer is when the photosynthetic organisms come to dominate the rocky intertidal. Even a cursory glance shows that every surface is covered with algae and/or surfgrass. So why not showcase some of these organisms when they look their best?
In terms of biomass, Egregia is by far the most abundant alga along our intertidal coast. Individual fronds can be 5+ meters long, and several fronds arise from each holdfast. Higher up in the mid tidal zone the Egregia was forming curtains hanging down along vertical faces.
But Egregia does know how to share the spotlight. Here it is posing with a couple of other low tidal denizens:
That's Egregia on the left, of course. One of the laminarian kelps, Laminaria setchellii, is taking center stage in this shot. When it lives in the subtidal Laminaria setchellii is an understory kelp; it gets to about 1.5 meters tall and can form dense stands. In this species each holdfast gives rise to a single stipe that in turn opens into a wide blade that is deeply divided, as you can see. The surfgrass Phyllospadix torreyi is on the right. There is a lot of surfgrass in the rocky intertidal these days. It's pretty treacherous stuff, too. It's very slippery and likes to cover pools that are deeper than you'd expect. I've learned the hard way that it cannot be trusted at all.
My favorite seaweeds are always the reds. And my favorite of the reds is Erythrophyllum delesserioides, looking so lush and pretty this time of year. It is a low intertidal species, and can be locally abundant. Some years it seems to get beat up and look ratty, but this year it looks great. Here it is, surrounding a couple of Laminaria setchellii.
Here's a grouping of Erythrophyllum and some other reds. I can see two species of Mazzaella, and of course there are Egregia and Phyllospadix mingled together on the right. So pretty!
When the tide is as low as it was this morning, a marine biologist has a lot of time to explore. I had just about exhausted the batteries in both my camera and my phone and was getting uncomfortably cold when I decided to head in. On the way back I stopped to take a look at the rockweeds, which live in the high intertidal. Franklin Point isn't a hotspot for rockweed abundance or diversity, but I did see this nice thallus of Fucus.
Fucus is the seaweed with the bifurcated branch tips. The tips are starting to swell up, which means this thallus is getting ready to spawn. Of all the algae, rockweeds are unusual in that they have what phycologists call an "animal-like" life cycle. They don't have sporophytes or gametophytes. They just have bodies, or thalli. Some thalli are female and some are male. Instead of releasing multiple kinds of spores and whatnot, they release eggs and sperm. The resulting zygote develops as you would expect, only instead of forming a young animal it grows into a baby seaweed.
I do love that olive green color of the rockweeds, which belong to the phylum of brown algae (Ochrophyta). Notice that there's a bit of similarly colored sheetlike seaweed right below the Fucus. That seaweed has the same color, but is in the red algae (Phylum Rhodophyta). Once again, we are reminded that the algae cannot be reliably sorted into phyla based solely on color. Mother Nature can be very tricksy!
So there you have it, my trip report for this morning's excursion to Franklin Point. The tides are excellent for the next several days, and I will be out there for most of them. This is my favorite time of the year.
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.
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.
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. aduncabefore and don't want to rehash that here. I just wanted to show off my favorite photo of this trip to Asilomar:
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.
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.
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.
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.
A few weeks ago I went out to Franklin Point and saw that the sea lettuces (Ulva sp.) were spawning in the high pools. I revisited the site today, with a lower tide to work with, and spent a considerable amount of time looking for and photographing the staurozoans. I did find some, too! But they are not the focus of this post.
As the tide came back in, I spent more time working my way through the higher pools. At Franklin Point there are very few places where the water is still. Even in the high regions the intertidal terrain is more surge channels than pools. But if you go high enough up the beach there are some quiet areas where the water, if it moves, does so very slowly. It is in these areas where the algal spawn forms those beautiful patterns that I photographed at the beginning of the month. Today there was much less algal spawn accumulating in the calm areas. It was also windy (and cold) this morning, so the patterns were not as crisp as they had been in early April. Still pretty, though!
On my way back up the beach I saw something that looked like an iceberg viewed from the air.
This is an accumulation of foam being pushed ashore. I didn't have any way to collect a sample to bring back to the lab for closer observation, but foams like this are usually due to algal particulates. Surface agitation whips up the organic matter, which act as surfactants and produce tiny bubbles. I'd be willing to bet that the Ulva spawn is at least partly responsible for this foam.
I watched the foam for several minutes, and was rewarded for my vigilance. I found an area where the highest reach of the incoming tide was gently washing back and forth.
I found the slow swirling to be rather mesmerizing. Maybe that was due to the early morning, the brisk sea air, or hunger pangs. But when I saw this I thought to myself, "I've seen that somewhere before." You might be able to guess where.
To validate my intuition, when I got home I looked up some images and found that I was sort of right after all.
Okay, so maybe the resemblance isn't as strong as all that. But I can still imagine the streams in van Gogh's painting swirling and flowing the way the algal foam does. What do you think?
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:
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:
This next one has only a small bit of coralline alga, but sports a jaunty sprig of something quite a bit larger.
This season's fashionable chiton will go all out with the coralline algae, wearing both encrusting and upright branching forms. Look at this:
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.
And sometimes the chitons are so covered with algae that they blend in perfectly with the surrounding environment.
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!
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:
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!
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.
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.
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?
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.
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.
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.
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:
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.