Of course, sea anemones don't have faces. They do have mouths, though, and since a mouth is usually part of a face, you can sort of imagine what I'm getting at. The sunburst anemone, Anthopleura sola, is one of my favorite intertidal animals to photograph. Of the four species of Anthopleura that we have on our coast, A. sola is the most variable, which is why it keeps catching my eye.
This afternoon I met the members of the Cabrillo College Natural History Club for the low tide at Natural Bridges. Here are some of the A. sola anemones we saw.
Such an amazingly photogenic animal, isn't it?
This past Fall semester the NHC went tidepooling at Pigeon Point. Today we were at Natural Bridges, and later in the spring we are going to Asilomar. I didn't intend it, but this school year the club is getting a look at three very different intertidal sites.
It has been a while since I've spent any time in the intertidal. There isn't really any reason for this, other than a reluctance to venture out in the afternoon wind and have to fight encroaching darkness. There's also the fact that I much prefer the morning low tides, which we'll have in the spring. However, this past weekend we had some spectacular afternoon lows, and although I was working on Friday and couldn't spare the time to venture out, I went out on Saturday and Sunday.
Saturday was a special day, because I had guests with me. A woman named Marla, who reads this blog, contacted me back in the fall. She said she wanted to do something special for her husband's birthday, and asked if I'd be willing to take them to the intertidal. It turns out that Andrew's birthday was around this past weekend, and he had family coming out from Chicago to celebrate. They picked the perfect weekend, because the low tides we had were some of the lowest of the year. So on Saturday I met up with Marla, Andrew (her husband), and Betsy (Andrew's sister) and we all traipsed out to Natural Bridges.
This was our destination for the afternoon:
Taking civilians into the intertidal can be tricky, because they often come with expectations that don't get met. Like expecting to see an octopus, for example. I explain that the octopuses are there, but are better at hiding from us than we are at finding them, but that never feels very satisfactory. This trio, however, were fun to show around. The tide was beautifully low and we had fantastic luck with the weather. It had rained in the morning, but the afternoon was clear and sunny. I congratulated Marla on remembering to pay the weather bill. And the passing stormlet didn't come with a big swell, so the ocean was pretty flat. We were able to spend some quality time in the mid-tidal zone, with occasional forays into the low intertidal.
The typical Natural Bridges fauna--owl limpets, mussels, chitons, anemones, etc.--were all present and accounted for. Of course, there isn't much algal stuff going on in mid-January.
Given the time of year (mid-January) and the time of day (late afternoon), the sun was coming in at a low angle. This was tricky for photographing, both in and out of water. However, sometimes good things happen, as in this photo below:
That's a big kelp crab (Pugettia producta) nestled among four sunburst anemones (Anthopleura sola). Kelp crabs are pretty placid creatures, for crabs, and usually take cover when approached. But this one remained in plain sight, holding so still that I thought it was dead. Even when I hovered directly over it and blocked the sun, it didn't move at all. Then it occurred to me that maybe he was having the sexy times with a lady friend. So I very carefully reached down and gave him a tap on the carapace. He flinched a little, so I knew he wasn't dead, but made no move to get away. And I caught a glimpse of a more golden leg underneath him.
Crabs live their entire lives encased in a rigid exoskeleton, and can mate only during a short window of opportunity after a female molts. Early in the breeding season, a female crab uses pheromones to attract nearby males. When a suitable male approaches, she may let him grab her in a sort of crabby hug. That's what this male kelp crab is doing to his mate. They may remain in this embrace for several days, waiting until the female molts and her new exoskeleton is soft. At that point the male will use specialized appendages to insert packets of sperm into the female's gonopores. The two will then go their separate ways.
We didn't disturb these crabs, and let them go on doing their thing. By now the sun was going down, so we headed back up and were rewarded with a glorious sunset.
When we stop to marvel at the wonders of the natural world, we usually forget about all the life that is going on that we don't get to see. But there is a lot happening in places we forget to look. For example, any soil is an entire ecosystem, containing a variety of small and tiny animals, bacteria, and fungi. In fact, if a fungus didn't send up a fruiting body (a.k.a. mushroom) every once in a while, most observers wouldn't realize it was there at all. We humans tend to behave as though something unseen is something that doesn't exist, and I admit to the very same thinking with regards to my own kitchen: anything stored way up in cupboards I can't reach, may as well not be there at all.
But there are places where we can witness the life occurring below our feet, and floating docks in marinas and harbors are some of the best. Of course, the trick is to "get your face down where your feet are", a piece of advice about how to observe life in tidepools that applies just as well to investigating the dock biota. Once you get used to the idea of lying on the docks, which can be more or less disgusting depending on time of year and number of birds hanging around, a whole new world literally blossoms before your eyes.
Some of the flower-looking things are indeed anthozoans ('flower animal') such as this plumose anemone:
and this sunburst anemone:
Other animals look like dahlias would look if they were made of feathers. Maybe that doesn't make sense. But see what I mean?
This is Eudistylia polymorpha, the so-called feather duster worm. These worms live in tough, membranous tubes attached to something hard. They extend their pinnate tentacles for feeding and are exquisitely sensitive to both light and mechanical stimuli. There are tiny ocelli (simple, light-sensing eyes) on the tentacles, and even casting a shadow over the worm causes it to pull in its tentacles very quickly. This behavior resembles an old-fashioned feather duster, hence the common name. These were pretty big individuals, with tentacular crowns measuring about 5 cm in diamter. Orange seems to be the most common color at the Santa Cruz harbor.
One of the students pointed down at something that he said looked like calamari rings just below the surface. Ooh, that sounds intriguing!
And he was right! Don't they look like calamari rings? But they aren't. These are the egg ribbons of a nudibranch. They appeared to have been deposited fairly recently, so I went off on a hunt for the likely parents. And a short distance away I caught the nudibranchs engaging in the behavior that results in these egg masses. Ahem. I don't know if the term 'orgy' applies when there are three individuals involved, but that's what we saw.
To give you some idea of how these animals are oriented, that flower-like apparatus is the branchial (gill) plume, which is located about 2/3 of the way down the animal's dorsum. The anterior end bears a pair of sensory organs called rhinophores; they look kind of like rabbit ears. You can see them best in the animal on the left.
When you see more than one nudibranch in such immediate proximity it's pretty safe to assume that they were mating or will soon be mating. Nudibranchs, like all opisthobranch molluscs, are simultaneous hermaphrodites, meaning that each can mate as both a male and a female. The benefit of such an arrangement is that any conspecific individual encountered is a potential mate. The animals pair up and copulate. I'm not sure if the copulations are reciprocal (i.e., the individuals exchange sperm) or not (i.e., one slug acts as male and transfers sperm to the other, which acts as female). In either case, the slugs separate after mating and lay egg masses on pretty much whatever surface is convenient. Each nudibranch species lays eggs of a particular morphology in a particular pattern. Some, such as P. atra, lay eggs in ribbons; others produce egg masses that look like strings of miniature sausages.
This is the first time I've seen big Polycera like these. The slugs were about 4 cm long. They eat a bryozoan called Bugula, and there is a lot of Bugula growing at the harbor these days. Maybe that's why there were so many Polycera yesterday. Nudibranchs are the rock stars of the invertebrate world--they are flamboyantly and exuberantly colored, have lots of sex, and die young. They can be very abundant, but tend to be patchy. Quite often an egg mass is the only sign that nudibranchs have been present.
The next time you happen to be at a marina poke your head over the edge and take a look at the stuff living on the dock. Even if you don't know what things are, you should see different textures and colors. With any luck, you'll be pleasantly surprised at the variety of life you find under your feet.
In my experience, the most difficult organisms to photograph in the wild are staurozoans. Even birds in flight are easier. The problem with staurozoans is where they live. I never see them in calm, still pools, where taking pictures would be easy. Instead, they seem to like surge channels where the water constantly sloshes back and forth, and even in the few seconds between a wave coming in and receding they never really stop moving. Their bodies are extremely soft and squishy, so the slightest current causes them to flutter and make blurry photos. When they are emersed their bodies don't really look like anything except a soggy booger, so they aren't recognizable as staurozoans unless they are underwater. And when underwater they don't hold still, and so on and so forth.
Still, finding them is always a treat, even if I can't capture photographic proof. They really are extremely gorgeous creatures.
They are also enigmatic creatures. Much of staurozoan biology, including their evolutionary relationships, remains poorly understood. Until recently the staurozoans were considered a subgroup of the Scyphozoa, the taxon that includes the large medusae such as moon jellies (Aurelia spp.) and sea nettles (Chrysaora spp.). However, using data from more extensive morphological and molecular studies, most taxonomists now agree that the Staurozoa should be elevated to a level equivalent to the Scyphozoa. In other words, the staurozoan lineage probably evolved alongside, but separate from, the scyphozoan lineage.
Whatever their evolutionary history and relationships, what we know about staurozoans is very limited. They are considered to be stalked jellies (hence their previously assumed close affinity to the scyphozoans) that do not have a separate polyp stage. Their bodies consist of an adhesive peduncle, or stalk, that attaches to algae or surfgrasses, and a calyx or goblet-shaped portion surrounded by eight tapering arms. Each of the eight arms is topped with a puffball of stinging tentaches which are uses to catch food and presumably to defend the animal against predators. The mouth is located in the center of the calyx, usually lifted up on a short stalk called a manubrium. The animal feeds by capturing prey on the tentacles and flexing the arm so the food is brought to the mouth. Staurozoans are not permanently attached and can sort of 'walk' with a somersault-like motion, flipping end-over-end.
Haliclystus 'sanjuanensis' at Franklin Point grows to a length and diameter of ~3 cm, although most of the ones that I see are smaller than that. The most common color is this reddish brown, but I've also seen them in a gorgeous bottle green that makes them much easier to see against the background of their habitat. I usually see them attached to pieces of red algae, but I'm not sure they actually prefer red algae to either green or brown algae. I don't think I've ever seen one attached to a rock.
Last week I had one of those moments in the intertidal when I felt something stuck on my finger and I couldn't get rid of it. That happens frequently, with small bits of algae getting caught on everything; usually I just flick my hand and they go flying off. But this thing wouldn't leave. I finally stuck my hand in the water to rinse it off, and saw that I had been glommed onto by a small staurozoan!
See how the animal stuck to me with its tentacles, while its peduncle is still attached to a piece of Ulva?
As I mentioned, not much is known about these strange animals. They possess the stinging cells to prove their inclusion within the Cnidaria, but are aberrant medusae which stick to algae instead of swimming around in the water column. Their life cycle is more or less cnidarian-like, but their planula is non-ciliated. Their ecological relationships haven't really been studied at all.
Which is why this photograph is so informative. It's not a great picture, by any means, but it shows a glimpse of how staurozoans interact with other species.
This is a picture of two animals, a staurozoan (H. 'sanjuanensis') and a nudibranch (Hermissenda opalescens). Both of these animals are predators. Hermissenda is well known for its affinity for general cnidarian prey, from which it steals the stinging cells to defend its own body (a behavior known as kleptocnidae). But the staurozoan should be quite capable of defending itself. So, who is doing the eating, and who is being eaten?
Given the dastardly nature of Hermissenda, I'd bet on it as the eater. Those damned nudibranchs have to spoil everything! The staurozoan will probably sustain damage, perhaps losing a tuft of tentacles, but should be able to regrow the lost parts. And the sting of the staurozoan may keep the nudibranch from eating as much as it would like. That's the thing. We just don't know.
I'll definitely be keeping an eye out for the staurozoans at Franklin Point the rest of this tide season. I may even bring a few back to the lab for closer inspection; my collecting permit allows me to do so. I could then photograph them under controlled conditions and hopefully get some better pictures. I find these animals very intriguing, being both so clearly cnidarian-like and simultaneously so inscrutable. I always did like a good mystery story!
When low tides occur at or before dawn, a marine biologist working the intertidal is hungry for lunch at the time that most people are getting up for breakfast. And there's nothing like spending a few morning hours in the intertidal to work up an appetite. At least that's how it is for me. Afternoon low tides don't seem to have the same effect on me, for reasons I can't explain. A hearty breakfast after a good low tide is a fantastic way to start the day.
Sea anemones are members of the Anthozoa (Gk: 'antho' = 'flower' and 'zoa' = 'animal'). These 'flower animals' are the largest cnidarian polyps and are found throughout the world's oceans. They are benthic and sedentary but technically not sessile, as they can and do walk around, and some can even detach entirely and swim away from predators. The anthozoans lack the sexual medusa stage of the typical cnidarian life cycle, so the polyps eventually grow up and have sex. In addition to the sea anemones, the Anthozoa also includes the corals, sea pens, and gorgonians.
With their radial symmetry and rings of petal-like tentacles, the sea anemones do indeed resemble flowers. You've seen many of my anemone photos already. Here's one more to drive home the message.
Sea anemones are cnidarians, and cnidarians are carnivores. Most of the time anemones in the genus Anthopleura feed on tiny critters that blunder into their stinging tentacles, although the occasional specimen will luck into a much more substantial meal. I've watched hermit crabs crawl right across the tentacles of a large anemone (Anthopleura xanthogrammica), and while the anemones did react by retracting the tentacles, the crabs easily escaped their grasp.
Of course, not all potential prey items are so fortunate. Sometimes even big crabs get captured and eaten, like this poor kelp crab (Pugettia producta):
There's no way to know exactly how this situation came to be. Was the crab already injured or weakened when the anemone grabbed it? Or was the anemone able to attack and subdue a healthy crab? I've always assumed that the exoskeleton of a crab this size would be too thick for the rather wimpy nematocysts of an Anthopleura anemone to penetrate, but maybe I'm wrong. A newly molted crab would be vulnerable, of course; however, they tend to stay hidden until the new exoskeleton has hardened, and the crab in the above photo doesn't appear to have molted recently.
Even big, aggressive crabs can fall prey to the flower animals in the tidepools. I'd really like to have been there to watch how this anemone captured a rock crab!
And crabs aren't the only large animals to be eaten by sea anemones. Surprisingly, mussels often either fall or get washed into anemones, which can close around them. Once a mussel has been engulfed by an anemone, the two play a waiting game. Here's what I imagine goes on inside the mussel: The bivalve clamps its shells shut, hoping to be spit back out eventually; meanwhile, the anemone begins trying to digest the mussel from the outside; sooner or later the mussel will have to open its shells in order to breathe, and at that point the anemone's digestive juices seep inside and do their work on the mussel's soft tissues. When the digestive process is finished, the anemone spits out the perfectly cleaned mussel shells.
In the photo above, the anemone is working on a clump of several mussels. I can't see that any of these mussels have been compromised, but the pale orange stringy stuff looks like mussel innards and slime. It could be that several mussels are still engulfed within the anemone. There is always a chance that an anemone will give up on a mussel that remains tenaciously closed, and spit it out covered with slime but otherwise unharmed. I assume that hungry anemones are less likely to give up their meals than ones that have recently fed.
So how, exactly, does an anemone eat a mussel, or a crab? The answer lies within the anemone's body. Technically, the gut of an animal is outside its body, right? Don't believe me? Let's think it through. An animal with a one-way gut can be modeled as a tube within a tube, and by that reasoning the surface of a gut is contiguous with the outer surface of the body. Our gut is elaborated by pouches and sacs of various sizes and functions, but is essentially a long, convoluted tube with a mouth on one end and an anus on the other. Sea anemones, as all cnidarians, have a two-way gut called a coelenteron or gastrovascular cavity (GVC), with a single opening serving as both mouth and anus. Anemones, being the largest cnidarian polyps, have the most anatomically complex gut systems in the phylum.
Imagine a straight-sided vase with a drawstring top. The volume of the vase that you'd fill with water and flowers represents the volume of the anemone's gut. Anemones can close off the opening to their digestive system by tightening sphincter muscles that surround the mouth; these muscles are analogous to the drawstring closure of our hypothetical vase. Now imagine that the inner wall of the vase is elaborated into sheets of curtain-like tissue that extend towards the center of the cavity. These sheets of tissue are called mesenteries. They are loaded with various types of cnidocytes that immobilize prey and begin the process of digestion. The mesenteries greatly increase the surface area of tissue that can be used for digestion. The mesenteries are also flexible and can wrap around ingested prey to speed things up.
This anemone (below) that was eating both a mussel and a piece of kelp:
Those frilly ruffles are the mesenteries. You can see how greatly they'd increase the surface area of the gut for digestion. They are also very soft, almost flimsy. Here's a close-up shot:
Maybe I'm especially suggestible, but seeing these animals working on their own meals makes me hungry, too. After crawling around the tidepools for a few hours I'm always ready for a second breakfast or brunch of my own.
This weekend I was supposed to take a photographer and his assistant into the field to hunt for staurozoans. I mean a real photographer, one who has worked for National Geographic. He also wrote the book One Cubic Foot. You may have heard of the guy. His name is David Liittschwager. Anyway, his assistant contacted me back in March, saying that he was working on something jellyfish-related for Nat Geo and hoped to include staurozoans in the story, and did I know anything about them? As in, maybe know where to find them? It just so happens that I do indeed know where to find staurozoans, at least sometimes, and we made a date to go hunting on a low tide. Then early in May the assistant contacted me to let me know that David's schedule had changed and he couldn't meet me today, and she hoped they'd be able to work with me in the future, and so on.
None of which means that I wouldn't go look for them anyways. I'd made the plans, the tide would still be fantastic, and so I went. And besides, these are staurozoans we're talking about! I will go out of my way to look for them as often as I can. Not only that, but I hadn't been to Franklin Point at all in 2018 and that certainly needed to be remedied.
The sand has definitely returned. The beach is a lot less steep than it was in the winter, and some of the rocks are completely covered again. This meant that the channels where staurozoans would likely be found are shallower and easier to search. But you still have to know where to look.
See that large pool? That's where the staurozoans live. They like areas where the water constantly moves back and forth, which makes them difficult to photograph in situ. And given that the big ones are about 2 cm in diameter and most of them are the same color as the algae they're attached to, they're a challenge to find in the first place. I looked for a long time and was about to give up on my search image when I found a single small staurozoan, about 10 mm in diameter, quite by accident. It was a golden-brown color, quite happily living in a surge channel. I took several very lousy pictures of it before coming up with the bright idea of moving it up the beach a bit to an area where the water wasn't moving quite as much. I sloshed up a few steps and found a likely spot, then placed my staurozoan where the water was deep enough for me to submerge the camera and take pictures.
Cute little thing, isn't it? I had my head down taking pictures of this animal, congratulating myself on having found it. When I looked around me I saw that I had inadvertently discovered a whole neighborhood of staurozoans. They were all around me! And some of them were quite large, a little over 2 cm in diameter. All of a sudden I couldn't not see them.
I know I've seen staurozoans in the same bottle green color as the Ulva, but this time I saw only brown ones. As you can see even the animals attached to Ulva were brown. Staurozoans seem to be solitary creatures. They are not permanently attached but do not aggregate and are not clonal. Most of the ones I found were as singles, although I did find a few loose clusters of 3-4 animals that just happened to be gathered in the same general vicinity.
Not much is known about the biology of Haliclystus, or any of the staurozoans. I collected some one time many years ago, and brought them back to the lab for closer observation. They seemed to eat Artemia nauplii very readily, and I did get to observe some interesting behaviors, but they all died within a week or so. Given that I can find them only in certain places at Franklin Point, they must be picky about their living conditions. Obviously I can't provide what they need at the marine lab. The surging water movement, for example, is something that I can't easily replicate. I need to think about that. The mid-June low tides look extremely promising, and my collecting permit does allow me to collect staurozoans at Franklin Point. Maybe I'll be able to rig up something that better approximates their natural living conditions in the lab.
For several centuries now, Earth's only natural satellite has been associated with odd or unusual behavior. Lunatics were people we would describe today as mentally ill, who behaved in ways that couldn't be predicted and might be dangerous. The erratic behaviors were attributed to the vague condition of lunacy. These words are derived from the Latin luna, which means 'moon'. The cycles of the moon have long been thought to influence human behavior as well; hence such legends as the werewolf.
We do know that the moon indeed has a very strong influence on aspects of many organisms, primarily through the tides. For example, reproduction in many marine animals is timed to coincide with a particular point in the tidal cycle. Grunion (Leuresthes tenuis, small, silver, finger-shaped fishes) run themselves up onto California beaches at night to spawn following the full and new moon high tides in the early summer months. Corals in the Great Barrier Reef spawn together in the handful of nights after the full moon in November. Animals such as these, which reproduce via broadcast spawning, are the ones most likely to benefit from synchronized spawning; after all, there is no point in spawning if you're the only one doing it. Invertebrates don't have watches or calendars; they keep time by sensing the natural cycles of sun and moon. The moon's strong effect on the tides is a signal that all marine creatures can sense and use to coordinate spawning, increasing the probability of successful fertilization for all.
Last night, Wednesday 6 September 2017, the moon was full. Yesterday at the lab, I noticed that the large Anthopleura sola anemones living in the corner of my table had spawned.
That diffuse grayish stuff in the right-hand side of the photo is a pile of sperm. I looked at a sample under the microscope, just to be sure. By this time they had been sitting at the bottom of the table for several hours and most of them were dead. But they were definitely sperm:
Whenever I see something unusual like this my first impulse is to see if it's happening anywhere else at the lab. So I started poking around. The aquarists at the Seymour Center told me that some of their big anemones had spawned in the past couple of days; however, since they clean and vacuum the tanks every day all evidence was long gone.
Fortunately there are several A. sola anemones in other labs that aren't cleaned as regularly as the public viewing areas. One of the animals in the lab next door to where I have my table had also spawned. . .
. . . and this one is a female! What looks like a pile of fine dust is actually a pile of eggs.
And the eggs are really cool. See those spines? They are called cytospines and apparently deter predation. Other species in the genus Anthopleura (A. elegantissima and A. xanthogrammica) are known to have spiny eggs, so it appears that this is a shared feature. Now, if only I could get my hands on eggs of the fourth congeneric species--A. artemisia, the moonglow anemone--that occurs in our area, I'd know for certain, at least for California species. I examined the eggs under higher magnification, but due to their opacity I couldn't tell if the had been fertilized. Most appeared to be solid single undivided cells; they could, however, be multicellular embryos.
All told, of the anemones that had obviously spawned, 1 was female and 4 were male. I sucked up some of the eggs and put them in a beaker of filtered seawater. I doubt that anything will happen, but I may be in for a pleasant surprise when I check on them tomorrow.
Last week I went up to Davenport to do some collecting in the intertidal. The tide was low enough to allow access to a particular area with two pools where I have had luck in the past finding hydroids and other cool stuff. These pools are great because they are shallow and surrounded by flat-ish rocks, so I can lie down on my stomach and really get close to where the action is. At this time of year the algae and surfgrasses are starting to regrow; the surface of the pools was covered by leaves of Phyllospadix torreyi, the narrow-leafed surfgrass.
Parting the curtain of Phyllospadix leaves to gaze into the first pool I was pleasantly surprised to find this. What does it look like to you?
There are actually two very different organisms acting as main subjects in this photo. The pink stuff is a coralline alga, a type of red alga that secretes CaCO3 in its cell walls. Coralline algae come in two different forms: one is a crust that grows over surfaces and the other, like this, grows upright and branching. Because they sequester CaCO3, corallines are likely to be affected by the projected increase of the ocean's acidity due to the continued burning of fossil fuels. Ocean acidification is one of the sexy issues in science these days, and although it is very interesting and pertinent to today's world it is not the topic for this post. Suffice it to say that changes in ocean chemistry are making it more difficult for any organisms to precipitate CaCO3 out of seawater to build things like shells or calcified cell walls.
It's the tannish featherlike stuff in the photo that I was particularly interested in. At first glance the tan thing looks like a clump of a very fine, fernlike plant. It is, however, an animal. To be more specific, it is a type of colonial cnidarian called a hydroid. I love hydroids for their hidden beauty, not always visible to the naked eye, and the fact that at first glance they so closely resemble plants. In fact, many hydroid colonies grow in ways very similar to those of plants, which has often made me think that in some cases the differences between plants and animals aren't as great as you might assume. But that's a matter for a separate essay.
I collected this piece of hydroid and brought it back to the lab. The next day I took some photos. To give you an idea of how big the colony is, the finger bowl is about 12 cm in diameter and the longest of these fronds is about 3 cm long.
And here's a closer view through the dissecting scope.
Each of the fronds has a structure that we describe as pinnate, or featherlike--consisting of a central rachis with smaller branches on each side. This level of complexity can be seen with the naked eye. Zooming in under the scope brings into view more of the intricacy of this body plan:
At this level of magnification you can see the anatomical details that cause us to describe this animal's structure as modular. In this context the term 'modular' refers to a body that is constructed of potentially independent units. A colony like this is built of several different types of modules called zooids, some of which are familiarly referred to as polyps. Each zooid has a specific job and is specialized for that job; for example, gastrozooids are the feeders, while gonozooids take care of the sexual reproduction of the colony. In this colony of Aglaophenia each of these side branches consists of several stacked gastrozooids, which you can see as the very small polyps bearing typical cnidarian feeding tentacles. Aglaophenia is a thecate hydroid; this means that each gastrozooid sits inside a tiny cup, called a theca, into which it can withdraw for protection. Those larger structures with pinkish blobs inside are called gonangia. A gonangium is a modified gonozooid, found in only thecate hydroid colonies, that contains either medusa buds or other reproductive structures called gonophores.
Pretty complicated, isn't it? Who would expect such a small animal to have this much anatomical complexity?
In the second pool I found an entirely different type of hydroid. At first glance this one looks more animal-like than Aglaophenia does, although it is still a strange kind of animal. This is Sarsia, one of the athecate hydroids whose gastrozooids do not have a protective theca. It might be easier to think of these and other athecate hydroids (such as Ectopleura, which I wrote about here and here) as naked, with the polyps not having anywhere to hide.
Each of these polyps is about 1 cm tall. The mouth is located on the very end of the stalk. The tentacles, not quite conforming to the general rule of cnidarian polyp morphology, do not form a ring around the mouth. Instead, they are scattered over the end of the stalk.
Here's a closer view:
In the hydroid version of Sarsia, the reproductive gonozooids are reduced to small buds that contain medusae. You can see a few round pink blobs in the lower right of the colony above; those are the medusa buds. The medusae are fairly common in the local plankton, indicating that the hydroid stage is likewise abundant. Here's a picture of a Sarsia medusa that I found in a plankton tow in May 2015.
The medusa of Sarsia is about 1 mm in diameter and has four tentacles, which usually get retracted when the animal is dragged into a plankton net. Sometimes, if the medusa isn't too beat up, it will relax and start swimming. I recorded some swimming behavior in a little medusa that I put into a small drop of water on a depression slide. It refused to let its tentacles down but you might be able to distinguish four tentacle bulbs.
There's a lot more that I could say about hydroids and other cnidarians. They really are among the most intriguing animals I've had the pleasure to observe, both in the field and in the lab. I've always been fascinated by their biphasic life cycle, with its implications for the animals' evolutionary past and ecological present. Perhaps I'll write about that some time, too.
A long time ago in a galaxy called the Milky Way, a great adventure took place. We don't know exactly when it happened, but it must have been very shortly after the evolution of the first cells. Some small prokaryotic cell walled itself off from its surroundings. Then it learned how to replicate itself and as cells continued to divide they began interacting with clones of themselves. Sooner or later, however, our clone of cells encountered cells from a different genetic lineage. These foreign cells were "other" and were recognized as such because they had a different set of markers on their outer covering. Perhaps there was an antagonistic interaction between the two clones of cells. In any case, this ability to distinguish between "self" and "non-self" was a crucial step in the evolution of life on Planet Earth.
The entire immune system in vertebrates is based on self/non-self recognition. It is why, for example, transplanted organs can be rejected by their new host--the host's immune system detects the transplanted tissue as "non-self" and attacks it. As a result, patients who receive donor organs usually take immune-suppressing drugs for some period of time after the transplant.
The vertebrate immune system is quite complex and very interesting. It has two main components: (1) cell-mediated immunity, in which the major players are T cells; and (2) humoral (i.e. blood-based) immunity, which is the part of the immune system that produces antibodies to a pathogen when you get a vaccination. However, even animals much less structurally complex than vertebrates have some ability to recognize self from non-self.
Sponges, for example, exist as aggregations of cells rather than bodies with discrete tissues and organs. Most zoologists, myself included, consider sponges to be among the most ancient animal forms. They have different types of cells, many of which retain the ability to move around the body and change from one type to another; this totipotency is a feature that sponge cells share with the stem cells of vertebrates. There are sponges that you can push through a mesh and disarticulate into individual cells, and then watch as the cells re-aggregate into an intact, functioning body. As if that weren't cool enough, if you take two different sponges and mush them into a common slurry, the cells from the distinct lineages re-aggregate with cells to which they are genetically identical. So even animals as primitive as sponges have some degree of self/non-self recognition.
If you're lucky, you can see self/non-self recognition and aggression in the intertidal. Here in northern California we have four species of sea anemones in the genus Anthopleura:
Anthopleura xanthogrammica, the giant green anemone
Anthopleura sola, the sunburst anemone
Anthopleura elegantissima, the cloning anemone
Anthopleura artemisia, the moonglow anemone (and my favorite)
Of these species, only A. elegantissima clones. It does so by binary fission, which means that the animals rip themselves in half.
It looks painful, doesn't it? As the two halves of the animal walk in opposite directions they pull apart until the tissue joining them stretches and eventually rips. Then each half heals the wound and carries on as if nothing had happened. Each anemone is now a physiologically and ecologically independent animal, and can go on to divide itself. And so on ad infinitum. The logical consequence of all this replication is a clone of genetically identical anemones spreading over a rocky surface. And that's exactly what you get:
Okay, it's hard to tell that these are sea anemones, but this is what they look like when the tide goes out and leaves them emersed. They pull in their tentacles, close off the oral disc, and cover themselves with sand grains. They look like sand but feel squishy and will squirt water if you step on them. In this photo, each anemone is probably 4-5 cm in diameter.
There are three patches of anemones in the photo above, separated by narrow strips of real estate where there are no anemones. Each patch is a clone, essentially a single genotype divided amongst many individual bodies. The anemones in each clone pack tightly together because they are all "self." However, they recognize the anemones of an adjacent patch as "non-self" and they won't tolerate the intrusion of neighbors onto their territory. Those strips of unoccupied (by anemones) rock are demilitarized zones. When the rock is submerged the anemones along the edges of the clones reach out their tentacles and sting their non-self neighbors. This mutual aggression maintains the DMZ and nobody gets to live there.
Because A. elegantissima lives relatively high in the intertidal the clonal patches are usually emersed when I go out to the tidepools. Its congener, A. sola, lives lower in the intertidal and is more often immersed at low tide. Anthopleura sola is larger than A. elegantissima and is aclonal, meaning that it does not divide. Anthopleura sola also displays quite dramatically what happens when anemones fight.
These two anemones, each about 12 cm in diameter, were living side-by-side in a tidepool. You can see that each animal has two kinds of tentacles: (1) the normal filiform feeding tentacles surrounding the oral disc; and (2) thicker, whitish club-shaped tentacles below the ring of feeding tentacles. These club-shaped tentacles are called acrorhagi, and are used only for fighting. The acrorhagi and the feeding tentacles may contain different types of stinging cells, reflecting their different functions. All tentacles are definitely not the same.
These animals, which represent different genotypes, are non-self to each other, so they fight. They inflate their acrorhagi, move their feeding tentacles out of the way, and reach across to sting each other. See how some of the acrorhagi on the animal on the right don't have nice smooth tips? Those tips have been lost during battle with the animal on the left; the tips are torn off and remain behind to continue stinging the offender even after the tentacle itself has been withdrawn.
Here's another picture of the same two anemones, taken from a different angle:
The goal of these fights is not to kill, but to drive the other away so that each anemone has its own space. Eventually one of them will retreat, and a more peaceful coexistence will be established. Fights like these have been going on for over half a billion years. Eat your heart out, George Lucas.
This morning I went out on the first morning low tide of the season. I was so excited to have the morning lows back that I got to the site early and had to wait for the sun to come up. Awesome thing #1 about early morning low tides: Having the intertidal to myself.
The purpose for the trip was to collect some algae for a talk I'm preparing; I'll be speaking to the docents at Natural Bridges State Beach at their monthly meeting this coming Wednesday. They invited me to talk to them about algae. I already have a lecture on algae prepared, but last year I set the bar pretty high with this particular audience and want do something a little different. So I'll talk to them for a bit, show them some of my pressings, and invite them to press a couple of specimens. This morning I collected a few pieces of algae and took a bunch of pictures.
The Anthopleura anemones continue to fascinate me. At Davenport Landing there's an area where the rock has eroded and forms a sort of channel. In this channel at low tide the water comes about up to my knees. The rock in the channel remains clear of algae but sometimes contains sand. Scattered over the bottom of this channel are several A. artemisia anemones, which can burrow into the sand when it is present. I've photographed these animals many times, as they are magnificently photogenic and in deep enough water that I can just stick my camera below the surface and click away.
This morning the first anemone I looked at in this channel had some orange gunk on its oral surface. At first I thought it had latched onto a piece of bleached algae, but then noticed that others had the same thing. My second thought was, "Ooh, eggs!" If I were at the lab I'd have sucked up some of the gunk and examined it under the microscope.
Usually when animals spawn the gametes are quickly dispersed by water currents. But this channel is high enough that at low tide it doesn't exchange water with the ocean so there are no currents except those generated by the wind. Awesome thing #2 about early morning low tides: No wind. Once I used the camera as a sort of underwater microscope I could see the granular texture of the orange gunk, which told me that these were, indeed, eggs. Cool! Because I was on a hunt for algae I didn't spend a lot of time censusing these anemones, but I figured that statistically speaking they couldn't all be females. And sure enough, after a very short search I found some males.
So today I learned that April is when the A. artemisia anemones have sex. Makes sense, as spring is the time of year when many organisms (algae and invertebrates) in the intertidal reproduce. Reproduce sexually, that is.
Some animals reproduce clonally as well as sexually, and while sexual reproduction tends to be seasonal, clonal reproduction doesn't seem to be. Along the coast of central/northern California we have four species of anemones in the genus Anthopleura:
A. artemisia, the moonglow anemone (see above)
A. elegantissima, the aggregating anemone
A. sola, the sunburst anemone
A. xanthogrammica, the giant green anemone
Of these four species, only A. elegantissima clones readily. It does this by ripping its body in half in a process called binary fission. The two halves of the animal pull away from each other and the tissue between them gets stretched thinner and thinner until it rips. Then each former-half heals the wound and gets on with life, completely independent of the other. It sounds rather awful but is a very effective way to form clones of genetically identical units that can monopolize large areas in the intertidal.
It'll probably take this anemone another day or two to completely tear itself into two pieces. Anemones can continue to clone like this, with each individual splitting into a pair of individuals, for a long time. Eventually this process can form large clones. More about the ecology of these clones in a separate post some time.