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.
If, like me, you are fortunate enough to live near the coast in Northern California, you get to visit the tidepools. And when you do, you may notice something that looks like a pile of sand in the mid tidal zone below the mussel beds. When you venture down and touch the sand, you'll find that it's hard--hard enough to walk on, if you step very carefully, but also somewhat brittle.
It might look something like this:
Meet Phragmatopoma californica, the sandcastle worm. Hard to believe that these mounds, which can be the size of a small dining room table, are constructed by little worms, isn't it? Phragmatopoma is one of the many marine segmented worms grouped together as the Polychaeta. We have lots of polychaetes on our coast, ranging in size from greater-than-hand-length nereids and glycerids that can take a bite out of you and draw blood, to tiny worms small enough to swim in the layer of water between sand grains. In fact, the majority of our worm fauna on the coast consists of polychaetes.
Polychaetes make up a large and very diverse taxon, comprising some 80 or so families. Polychaete taxonomists might argue against it, but to make things simpler, we can divide them into two subclasses, the Errantia and the Sedentaria. As the name implies, the Errantia comprises the worms that are errant, or free-crawling. That said, most of them don't actually crawl around in plain sight; they tend to burrow in sediment, shell debris, or gravel, or wiggle their way through various benthic faunal communities. Some of them make temporary shelters by wrapping themselves in pieces of algae sewn shut with mucus threads. The Sedentaria, on the other hand, are pretty much all, well, sedentary. They live in more or less permanent tubes made of various materials, and generally can't live outside of them.
Phragmatopoma is very much a sedentary worm. It lives in a tube that it builds out of sand grains. Yes, this little worm is a mason!
What you see in these mounds is an aggregation of hundreds of individual worms. The mounds do not form by accident or chance. Phragmatopoma has a planktonic larval stage that floats around on ocean currents for some weeks before returning to the shore. The larva is attracted to areas already colonized by members of their species, which it detects by sniffing out the chemical signal of the glue used to create the tubes (more on that below). This phenomenon is called gregarious settlement. If you consider the challenge of being a tiny creature searching the entire coastline for a place to settle and live forever, one big clue as to the suitability of a given location is the presence of conspecific adults. After all, if your parents' generation grew up there, chances are it's a good spot for you to grow up, too.
Each of those holes in the big sandy mound is the entrance to a worm's tube. Tubes might be as long as 15 cm, but the worm itself is much smaller: a whopping big one would be 4 cm long, and most are in the 2-3 cm size range. From this pair of observations I infer that the worms can and do move up and down the tube. They have to move to the open end of the tube to feed, and can withdraw towards the closed end to avoid predators, or seek protection from desiccation.
Phragmatopoma's tube is not a haphazardly constructed object. It is the worm's home for the entirety of its post-larval life, and is constructed to shield its builder/occupant from the mechanical bashing that occurs twice daily as the tide floods and ebbs. As such, it must be strong and able to maintain its structural integrity. Let's take a closer look at an isolated tube under the dissecting scope:
The tube itself is made of debris--sand grains, bits of shell, the occasional tiny sea urchin spine--that the worm gathers from its environment. Glandular regions at the worm's anterior region secrete around the body a cylinder of sticky cement that is chemically similar to both spider silk and the byssal threads that mussels use to attach themselves to rocks in the intertidal. The inside of the tube is lined with a chitin-like material. The worm uses tentacles on its head region to collect and sort the 'stones' and glues them to the outside of the lining. There is some degree of selection involved; in the photo above you can see that all of the sand grains are more or less the same size, with none standing out as being conspicuously smaller or larger than the others. Growing worms that are actively building their tubes may be geographically restricted at least partly by the availability sand grains of the right size; if the sand is too fine or too coarse, the worm's either can't or don't live there.
Life inside a tube
Living in a tube may provide significant protection from wave bashing and predators, but does present some challenges as well. One thing that comes to mind is the matter of personal hygiene: What happens to the worm's poop? As we know, the worm lives inside the tube but it not attached to it, and can crawl up and down within it. To understand how it does, we have to review some basics of polychaete anatomy.
The word 'polychaete' comes from Greek ('many bristles') and refers to the fact that these segmented worms have chaetae, or bristles, along the left and right sides of the body. In some worms the segments, including chaetae, are pretty much the same from the anterior end of the body to the posterior end. In others, the segments and chaetae are differentiated from one body region to another. In the case of Phragmatopoma, all you can see sticking out of a tube is the head region, consisting of the slender feeding tentacles and a large disc-shaped structure called an operculum, which made of fused cephalic chaetae and serves as a door to close off the tube when the worm withdraws. Behind the head is a collar region, a series of three adjacent segments that have very stiff chaetae that can be pushed out against the lining of the tube to anchor the body in place. The rest of the body behind the collar is the trunk, which bears smaller chaetae on each segment. The entire epidermis is ciliated, which keeps water flowing around the body.
But what about the poop? As in most vermiform animals, Phragmatopoma's anus is at the posterior end of the body, which is oriented towards the closed end of the tube. How, then, does it defecate without fouling its home? The answer is both simple and ingenious. Phragmatopoma has a long rectum, which is curved to run anteriorly back towards the head. The anus, located at the terminal end of the rectum, discharges fecal pellets about halfway up the length of the body. The ciliary currents of the epidermis then flush the fecal pellets the rest of the way up the tube and out the top.
The skinny cylindrical things in the photo below are Phragmatopoma's fecal pellets!
Gas exchange is another challenge for animals that live within tubes. Aquatic animals exchange respiratory gases with the water that surrounds them, which is easy for animals that live where the water is constantly moving over their bodies. But for tube-dwellers, gas exchange is much more difficult. Phragmatopoma has paired gills on each segment of the trunk region of the body, which greatly increase the surface area for gas exchange. Any gas exchange surface is useless unless it connects with the circulatory system, so blood vessels flow into and out of each gill. Dissolved oxygen diffuses from the water into the blood, and is then circulated throughout the body. A certain amount of gas exchange probably occurs across the surface of the tentacles, too. To make things easier, the ciliated epidermis of the body keeps that small amount of water inside the tube moving, minimizing stagnation. When the worm's head is extended out for feeding, the tube is flushed with clean water. When the worm is withdrawn into the tube at low tide, its only oxygen supply is in the water contained in the tube with it. Like most of its intertidal neighbors, Phragmatopoma hunkers down and waits for the tide to return, when it can feed and breathe more easily.
And speaking of feeding, I should mention that Phragmatopoma is a filter feeder. Those purple tentacles are ciliated and create a water current that brings small suspended particles towards the mouth located at the base of the tentacles. In the video below the operculum is the darker object to the left; it represents the dorsal side of the worm's body. The long, filiform tentacles are the feeding tentacles.
As you may imagine, living in a tube also affects the way that Phragmatopoma reproduces. The worms never leave their tubes, so copulation isn't an option for them. Despite their occurrence in large groups they are not clonal, and reproduce only sexually. Both sexes of Phragmatopoma spawn gametes into the water, where fertilization and larval development take place. Living in dense aggregations and spawning at the same time as everyone else maximizes the chance that egg and sperm of the same species will find each other. Many marine invertebrates throughout the oceans, from corals to sea urchins, spawn synchronously. After all, it does an individual no good to throw gametes out into the world if it is the only one of its type around--all of the metabolic energy that went into producing and maintaining the gametes would be entirely wasted.
Clearly, the advantages of living in a tube outweigh the costs and inconveniences. Phragmatopoma has evolved physiological, anatomical, and behavioral adaptations to deal with life in the intertidal. One of those adaptations is the tube, which solves one set of problems but creates others which also need to be solved if the animal is to survive. Evolution comes up with solutions like this all the time. Every trait has metabolic and/or fitness costs, and an organism's biology is based on this type of evolutionary compromise. Life in the intertidal is a tough game. It is probable that none of the various biological processes that keep Phragmatopoma alive work function quite as well as they could, if they were isolated systems. But inside the bodies of these little worms, everything works just well enough for them to be one of the more conspicuous inhabitants of the intertidal.
Today is the first day of the week of low tides dedicated to Snapshot Cal Coast, a statewide citizen science project headed in my area by the California Academy of Sciences. This week groups and individuals will be making photographing the organisms they see in the ocean or along the coast, and uploading observations to iNaturalist. Participants will include both scientists and non-scientists, making the week-long event one of the biggest citizen science projects that I regularly take part in. Next Monday I'll be taking a group of Seymour Center volunteers and staff up to Davenport to conduct a Bioblitz. The other days I'll be out on my own, or with 1 or 2 people.
This morning the low tide was very early (-1.3 feet at 05:09), so I stayed close to home and went to Natural Bridges. The tide was low but the swell was big and I wasn't able to get down to the low spots I could normally reach with this kind of tide. However, this meant that I could spend more time in the low-mid-intertidal, where there is a lot of biodiversity to document.
Today I want to write about polychaete worms. These are the segmented marine worms in the Phylum Annelida, which also includes earthworms and leeches.
Worm #1: Phragmatopoma californica
One of the most conspicuous inhabitants of this zone is the tube-dwelling polychaete worm, Phragmatopoma californica. This worm has a couple of common names: honeycomb worm, which refers to the mounds of tubes they build; and sandcastle worm, for the fact that the tubes are built of cemented sand grains. In effect, these worms are tiny masons!
Each of the tubes is inhabited by a single worm. Mounds form because competent Phragmatopoma larvae, looking for a place to settle out and live permanently, are attracted to the tubes of existing adults. This phenomenon is called gregarious settlement. Once settled and metamorphosed, juvenile worms build their tubes by selecting sand grains and cementing them together around a lining of chitin-like material. How they do it, underwater, nobody knows. And these tubes are tough! The worm inside is skinny, and a humongous one would be all of 4 cm long, but it takes a lot of force to pry apart those sand grains. The openings to the tubes are 5-10 mm in diameter. Each worm can close off its tube with a circular-ish disc of stiff, fused chaetae called an operculum; this protects the worm from both predators and desiccation.
When the tide is out the worms withdraw into their tubes and clap the operculum down. They wait for the water to return. Phragmatopoma is a filter feeder; like most of the tube-dwelling polychaetes these worms use a crown of ciliated tentacles to create water currents that draw food particles to the mouth. When the tide is in the worms pull down the operculum and extend their feeding tentacles into the water. In the field, this is the most you can see of the worm's body.
Worm #2: Serpula columbiana
Many polychaetes live in tubes, and tubes can be made of a variety of materials. Phragmatopoma californica builds tubes out of sand grains. Another worm that I saw today, Serpula columbiana, builds tubes out of CaCO3 precipitated from seawater. Like other animals that build calcareous skeletons, S. columbiana may in the future have difficulty precipitating CaCO3 in an increasingly acidic ocean. Tubes of Serpula worms are white when new and soon become fouled with algal growth, and tend to wander over the substrate. The best photo I could take this morning is a little blurry but you can see the general morphology of the tubes.
These worms are incredibly shy, and react to any perceived threat by pulling into their tubes. Their tentacles have tiny eyespots that can detect changes in light, so passing a hand over them can cause them to withdraw. Fortunately I was able to sneak up on one lazy worm in a pool, and grab a shot of its 'head' region. Worms that live in tubes are poorly cephalized, with none of the structures that we generally associate with a head. Serpula columbiana's 'head' looks like this:
The tentacles of S. columbiana are morphologically complex compared to those of Phragmatopoma. Serpula's tentacles are pinnate, or feather-shaped, and in cross-section look like a V. Cilia on the side branches of the tentacle create the feeding current, and food particles are transported by other cilia down the trough of the V to the mouth.
See that long, trumpet-shaped structure? That's the worm's operculum!
Worm #3: Unidentified cirratulid
Unlike Serpula and Phragmatopoma, worms of the Family Cirratulidae don't live in tubes. Instead, they live with most of the body hidden in crevices, and extend tentacles to feed.
As you can imagine, it is extremely difficult to identify a worm when all you can see of it are its tentacles; with the rest of the body hidden in a crevice, there are no visible characteristics to use to distinguish species. Cirratulids use their tentacles to feed, but in a way that is entirely unlike how Phragmatopoma and Serpula use theirs. Instead of feeding on particles suspended in the water, cirratulids are deposit feeders. They sweep their tentacles across the surface and collect organic deposits. Sticky mucus on the tentacles picks up organic matter, and cilia on the tentacles sweep the organic matter to the worm's mouth.
Don't believe me? Watch this!
It doesn't matter if the surrounding substrate is sand or rock. The cirratulid's sticky tentacles are very effective at gathering organic muck.
Worm #4: Flabesymbios commensalis
This worm remains an enigma. There doesn't seem to be much known about its biology. I have seen them twice, both times on the body of purple urchins (Strongylocentrotus purpuratus), and although the genus name has changed twice since the first time, I'm pretty sure it's the same worm. As the species epithet commensalis implies, this worm is a commensal on sea urchins. This means that it neither harms nor benefits its echinoderm host. Similar to the worm I've seen on bat stars, F. commensalis presumably cruises over the urchin's body and feeds on detritus or scraps of kelp that the urchin grabs.
When I took the photo in a tide pool this morning I didn't see the worm. It wasn't until I downloaded the pictures from the camera onto my computer that I saw it. See how well it blends in with the urchin's color?
Here's a tighter crop of that photo:
For many polychaete worms, another animal's body seems to be the ideal habitat. And for some reason, echinoderms are likely hosts for such commensal worms. I've written about the bat star worms, here is the urchin worm, and there's also a scale worm that I've seen crawling around on the body of a sea cucumber. What is it about echinoderms that makes them habitat for worms? Or is this type of commensalism also common, but less observed, between polychaetes and other non-echinoderm invertebrates? I don't know the answer to either of those questions, but am very intrigued.
Five days ago I collected the phoronid worms that I wrote about earlier this week, and today I'm really glad I did. I noticed when I first looked at them under the scope that several of them were brooding eggs among the tentacles of the lophophore. My attempts to photograph this phenomenon were not entirely successful, but see that clump of white stuff in the center of the lophophore? Those are eggs! Oh, and in case you're wondering what that tannish brown tube is, it's a fecal pellet. Everyone poops, even worms!
Based on species records where I found these adult worms, I think they are Phoronis ijimai, which I originally learned as Phoronis vancouverensis. The location fits and the lophophore is the right shape. Besides, there are only two genera and fewer than 15 described species of phoronids worldwide.
Two days after I first collected the worms, I was watching them feed when I noticed some tiny approximately spherical white ciliated blobs swimming around. Closer examination under the compound scope showed them to be the phoronids' larvae--actinotrochs! Actinotrochs have been my favorite marine invertebrate larvae--and that's saying quite a lot, given my overall infatuation with such life forms--since I first encountered them in a course in comparative invertebrate embryology at the Friday Harbor Labs when I was in graduate school.
The above is a mostly top-down view on an actinotroch, which measured about 70 µm long. They swim incredibly fast, and trying to photograph them was an exercise in futility. They are small enough to swim freely in a drop of water on a depression slide, so I tried observing them in a big drop of water under a coverslip on a flat glass slide. At first they were a bit squashed, but as soon as I gave them enough water to wiggle themselves back into shape they took off swimming out of view.
Here's the same photo, with parts of the body labelled:
The hood indicates the anterior end of the larva and the telotroch is the band of cilia around the posterior end. The hood hangs down in front of the mouth and is very flexible. At this stage the larva possesses four tentacles, which are ciliated and will get longer as the larva grows. These are not the same as the tentacles of the adult worm's lophophore, which will be formed from a different structure when the larva undergoes metamorphosis.
As usual, a photograph doesn't give a very satisfactory impression of the larva's three-dimensional structure. There's a lot going on in this little body! The entire surface is ciliated, and this actinotroch's gut is full of phytoplankton cells. You can see a lot more in the video, although this larva is also a little squished.
I've been offering a cocktail of Dunaliella tertiolecta and Isochrysis galbana to the adult phoronids, and these are the green and golden cells churning around in the larva's gut. However, good eaten is not necessarily food digested, and the poops that I saw the larvae excrete looked a lot like the food cells themselves. Today I collected more larvae from the parents' bowl and offered them a few drops of Rhodomonas sp., a cryptonomad with red cells. This is the food that we fed actinotrochs in my class at Friday Harbor. We didn't have enough time then to observe their long-term success or failure, but I did note that they appeared to eat the red cells.
I don't know if phoronids reproduce year-round. It would be a simple task to run down and collect a few every month or so and see if any worms are brooding. Now that I know where they are, it would also be a good idea to keep an eye on the size of the patch. Some species of phoronid can clone themselves, although I don't know if P. ijimai is one of them. In any case, even allowing for the possibility of clonal division, an increase in the size of the adult population would be at least partially due to recruitment of new individuals. If recruitment happens throughout the year, it follows logically that sexual reproduction is likewise a year-round activity. Doesn't that sound like a nifty little project?
Besides, it's never a bad idea to spend time at the harbor!
If I asked you to draw a worm and designate the front and back ends, you'd most likely come up with something that looks like this:
And you would be entirely correct. A worm, or any creature described as 'vermiform' for that matter, has an elongated, wormlike body. Some worms have actual heads with eyes and sensory tentacles, but many don't. The great many polychaete worms that live in tubes don't have much of a head at all: usually all you can see sticking out of the tube is a crown of tentacles used for feeding. Although even the use of the word 'crown' more than suggests the presence of a head, doesn't it? After all, where else does one wear a crown?
Most worms, including the worm that we imagined above, are bilaterally symmetrical, with bodies elongated along the Anterior-Posterior axis. This means the head is at the anterior end and the rear is the posterior end. For animals that don't have a prominent head, the Anterior can also be defined by the direction of locomotion. Worms crawl with their bellies against the ground, which sets up a second axis of symmetry, the Dorsal-Ventral axis. The third axis of symmetry is the Left-Right axis. These axes should sound familiar, because they apply to our own bodies, as well of those of all other vertebrates and many invertebrates. Because of our upright stance we actually walk with our ventral surface forward, which is a little confusing, but if you don't trust me you can see for yourself by crawling around on hands and knees for a while.
Now back to our worms, hypothetical and otherwise. Consider a worm that is elongated not along its Anterior-Posterior axis, but along its Dorsal-Ventral axis. It sounds strange, but such worms do exist. They are called phoronid worms, and are classified within their own phylum, the Phoronida. They all live in tubes, and the few times I've seen them they have been in pretty dense aggregations. As with most tube-dwelling worms the only part of the body that you can usually see is the crown of feeding tentacles, which in these animals (as well as in the Bryozoa and Brachiopoda) is called a lophophore.
The other day I was at the harbor looking for slugs with my friend Brenna, and spotted these pale tentacles swaying in the current.
These are the lophophores of an aggregation of phoronids! I'd never seen them at the harbor before, so I was pretty excited about it. They were on the side of a floating walkway, down almost beyond the reach of my outstretched arm. The current caused the lophophores to sway continuously and I was barely able to snap some blurry photos without falling in (I couldn't really see what I was doing and just hoped for the best) when I accidentally caught this one shot. I wanted to have at least one clear-ish shot to submit to iNaturalist. I did manage to scrape off some bits of stuff that I hoped contained intact phoronids, so I could observe them under the dissecting scope at the lab.
And these are some lovely little worms!
The tubes that these phoronids inhabit are more like burrows of slime to which the surrounding sediments adhere. The tube itself isn't anything particularly interesting, but the bodies of the worms are beautifully transparent. One of the coolest things you can see in a living phoronid is its circulatory system. They have red blood that, like ours, contains hemoglobin, so it's easy to see the vessels that run along the length of the worm (which is the Dorsal-Ventral axis, remember) and the two blood rings around the base of the lophophore. If you get the lighting right you can even see the vessels that extend into each tentacle of the lophophore.
I was disappointed to see that none of the video clips I took really do justice to these worms. They are so pretty when I look at them through the microscope, and I wish I could capture their beauty. You may at least be able to see blood moving through the larger vessels of the body in this short video.
Seems I need to upgrade my photomicroscopy set-up. Anybody have a few thousand bucks they want to donate to the cause?
I'm keeping the phoronids for as long as I can, although I don't know what to feed them. I had time to take just a quick look at them this morning, and they look fine. Just for kicks I offered them a little phytoplankton to see what they'd do with it and couldn't see if they were reacting at all. Still, they are filter feeders, and if I can adjust the lighting and get a good view of those ciliated tentacles I should be able to see if they are creating a water current that is bringing food to the mouth. Friday is the next day I have time to spend with these animals that I don't get to see very often. Maybe then I'll have something else to report.
If I ask my invertebrate zoology students to name three characteristics of the Phylum Annelida, they would dutifully include segmentation and chaetae (bristles) in the list. And they would be correct. Annelids, for the most part, are segmented and many of them have chaetae. But in biology there are many exceptions for every rule we teach, and it's these exceptions that make a deeper study of biology so rewarding.
A couple of weeks ago I did some collecting in the intertidal at Pigeon Point. It was a very accommodating low tide, and I had a lot of time to poke around and explore. I found an area that had several decently sized rocks that I could turn over, and had fun seeing what lives on the side away from the light. Some of the animals on the underside of rocks are the common ones you see everywhere--turban snails, limpets, Leptasterias stars, and the like. Some, however, prefer a life of darkness and actively move away from the sun when their rock is turned over. And others happen to live in the sand under the rock and might not care one way or the other about the light.
Peanut worms, scientifically known as sipunculans, are delightful small worms that in my opinion are vastly underappreciated. This is understandable, as they are usually hidden in sand or rubble and aren't exactly conspicuous even when uncovered. Phascolosoma agassizii is our local sipunculan. Like all sipunculans it is unsegmented, and it has no chaetae. Peanut worms used to be elevated to their own phylum, the Phylum Sipuncula; however, molecular evidence shows that they are indeed annelids despite their apparent loss of key features such as body segmentation and chaetae.
They do look vaguely peanut-ish, don't they? They're small, maybe 6 cm all stretched out, which you hardly ever see. Phascolosoma agassizii is a grayish pink color, with irregular black stripes that usually don't form complete hoops around the body. Peanut worms are sedentary, living with most of the body buried in sand, rubble, shell debris, kelp holdfasts, etc. One of the weird things about them is that the mouth in located on the distal end of a long tube called the introvert. Most of the time the introvert is stuffed inside the main body region, or trunk. It is eversible and unrolls from the inside out, sort of like when you remove a long sock by pulling the top edge down over your leg and off your foot. The mouth on the end of the introvert is surrounded by short sticky tentacles, and the introvert dabs around to pick up organic deposits from the surfaces. Mucus and cilia on the tentacles convey the yummy organic gunk to the mouth, and a pharynx pushes food through to a long esophagus that runs the length of the introvert and leads to the long coiled intestine in the trunk.
Watch these peanut worms extending and retracting their introverts. Cute, aren't they?
I brought three peanut worms back to the lab with me, where they are happily living in my sand tank. Their housemates are ~15 sand crabs (Emerita analoga) and a clump of tube-dwelling polychaetes (Phragmatopoma californica). I never see them unless I dig them up from the sand, which leads me to believe that they do most of their feeding at night. Either that or they actually do actively shy away from the light.
Despite not sharing much in the way of apparent morphological similarity with more typical annelids, sipunculans are indeed annelid-like in other ways. Many of their internal structures are like those of annelids, and at least their early development (cleavage pattern and differentiation of tissue layers) follows the annelidan pathway. The species that have indirect development have a trochophore larva, typical of the marine annelids, that in some cases morphs into a second larval stage called a pelagosphera.
Sipunculans are the poster child for Animals That Are Not What They Seem. But they are interesting in their own way, and I always have a "yay!" moment when I find them in the field. It's really hard not to make sound effects as they're rolling their introverts in and out. You should try it yourself some time.
Animal associations can be strange and fascinating things. We're used to thinking about inter-specific relationships that are either demonstrably good or bad. Bees and flowering plants--good. Mosquitos on their vertebrate hosts--bad. In many cases the 'goodness' or 'badness' of these associations is pretty clear. However, there are cases of intimate relationships between animals of different species that cannot be easily categorized as good or bad.
Take, for example, the barnacles on the skin of gray and humpback whales. From the barnacles' perspective the skin of a whale isn't a bad place to live: as the whale swims through the water the barnacle is continually flushed by clean water, which should make feeding easier. But is the whale affected in any way by its barnacle passengers? I suppose they might increase the drag coefficient a little bit and make swimming marginally less efficient, and maybe they itch, although it's hard to imagine that the whale would really care much one way or the other.
A week ago I went to the intertidal up at Pigeon Point. It's a great spot for certain animals, especially the small six-rayed stars of the genus Leptasterias. These stars rarely get larger than 8 cm in diameter and always have six arms. I've been told by a friend who just happens to be a sea star taxonomist at the Smithsonian, that making species identifications in the field is very difficult for this genus, so I've stopped trying. I do know that some of the Leptasterias stars have slender rays and others have thicker rays.
The most common large star at Pigeon Point is the bat star, Patiria miniata. These stars get about as big as my outstretched hand, and come in a variety of colors. Last week I didn't see very many Patiria, but all of them were reddish orange, like this one:
Unless they're so abundant as to be annoying, I like picking up bat stars and looking at their underside. That's because sometimes they have these little dark squiggles in their ambulacral groove:
That little squiggle is a polychaete worm, Oxydromus pugettensis. It is one of many polychaete worms that forms a symbiotic relationship with another animal species. Some symbiotic polychaetes live in the tubes of other worms, or within the shells of bivalves, for example. Oxydromus crawls around inside the ambulacral groove of Patiria, where it feeds on scraps of leftover food from the star's meals. The worms don't like light, and as soon as I picked up this star and flipped it over the worm started burrowing down between the star's tube feet to get back to the dark. The next day I found another star with a worm and was able to take a picture of it before it disappeared.
Oxydromus pugettensis is clearly segmented, evidence of its annelidan roots. It doesn't look very different from many other free-crawling polychaetes. A member of the family Hesionidae, it lives in fine silty sediments in the intertidal as well as in the ambulacral grooves of sea stars. According to one source, it is the most common intertidal member of its family along the California and Oregon coast. For reasons as yet undetermined, P. miniata seems to be the favored host, although I have also seen the worms in the ambulacral grooves of the leather star Dermasterias imbricata.
Over two days at Pigeon Point last week I examined a total of five bat stars, and all of them had worms. One of the stars had three worms! It's possible that more worms were hiding deep within the ambulacral grooves, too. I always wonder how, in this type of association, the partners manage to find each other. How does one "lucky" star end up with three worms? Do the worms every migrate from one star to another? Does the star do anything to attract the worms? In what way(s) would the star benefit from having a few worms in its ambulacral regions? It does seem that the worms don't stick around very long once a star is brought into the lab--I don't know if they die or just leave on their own--but since they also live in the sand maybe they do actively migrate between stars. There hasn't been much work done on these worms in recent decades, probably because of the overall decline in natural history studies. However, I'll keep this worm in mind for my Marine Invertebrate Zoology students this fall, when one of them asks me for help coming up with an idea for his or her independent research project.
Northern California is currently being pummeled by a meteorological phenomenon called an atmospheric river. The storms produced by these "rivers" tend to be warm and can be very wet, such as the Pineapple Express storms that carry atmospheric moisture from Hawai'i to California. The weather station on the roof of our house has recorded 4.26 inches of rain over the past three days, and more will come in the next few days. In addition to the rain, the atmospheric river has brought very strong winds, gusting to 40+ mph. Combined with the saturated ground, winds like this can uproot trees and utility poles. So far we haven't lost power, but are prepared with candles, firewood, and extra water. . . just in case.
I am not a meteorologist, and this is not a blog about weather. I mention all the rain because it brought out the worms. Earthworms, to be more precise.
Earthworms are oligochaete worms in the phylum Annelida, which also contains the polychaetes (marine segmented worms) and hirudineans (leeches). The body plan for annelids is based on segmentation, or metamerism. Let me explain what that means.
Imagine a round water balloon. Now imagine two sets of rubber bands encircling the balloon along perpendicular axes. You'd have something like this:
where the green and red lines indicate different muscle types. Remember that we're discussing a three-dimensional object here. We'll call the red lines circular muscles, and the green lines longitudinal muscles. Now picture in your mind what happens when the circular muscles contract; how does this change the shape of the water balloon? What happens when the longitudinal muscles contract?
An annelid's body consists of many fluid-filled segments, each with its own set of circular and longitudinal muscles. The segments are arranged along the anterior-posterior axis, with the head being located at the anterior end.
Adjacent segments are separated by a layer of tissue called a septum (anatomically speaking, a septum is any tissue that divides a cavity into two or more smaller spaces; think of the septum that divides your nasal cavity into left and right nostrils). An incidental amount of fluid may escape from one segment into the next, but for the most part they function as separate water balloons. Water isn't compressible but is deformable, so contracting muscles around one part of the water balloon simply displaces that water to another part, and the balloon's shape changes. Because each segment in our worm has its own complement of body wall musculature, its shape can be modified independently from that of its neighbors.
Rather than draw up another pedagogical worm, I'll show you a real one. As I mentioned earlier all the recent rains have brought the earthworms out from their burrows. I was out and about myself this afternoon, and took pictures. This is the anterior (front) end of a worm. Not much of a head, is there? Earthworms are poorly cephalized, which makes sense when you consider that they live underground: an animal that spends almost all of its time in complete darkness has no need for eyes, and having sensory organs hanging off the body would impede its burrowing activities.
That pale pink apparently unsegmented bit of worm is the clitellum, a glandular structure used in reproduction. Another feature you can see is the difference in size among all the segments. Some of them are much wider than others. These are the localized deformations. The anterior-most segments are the widest; which type of muscle is contracted in this part of the worm? Which muscles are contracted in the segments immediately in front of the clitellum?
The annelid body plan originally evolved to facilitate burrowing through soft substrates. The fluid in each segment provides a stiffness against which the body wall muscles can contract, and the separation of adjacent segments allows the aforementioned localized deformations. An earthworm burrows by making its front end long and pointy (by contracting the circular muscles), jabbing it into the soil, swelling those anteriormost segments (by relaxing circular muscles and contracting longitudinal muscles), and pulling the rest of the body along. Next time you have a live earthworm at your disposal, watch how it moves either on top of or through the ground.
As you may imagine, while an earthworm's body volume remains constant, its shape varies greatly. This has consequences for internal anatomy as well. For example, an earthworm's gut is essentially a straight tube within a tube; it doesn't have distinct compartments or side chambers as ours does. But it can't really be straight, can it? If the overall body shape of an individual worm can change as much as we see in a burrowing earthworm, it follows that the internal morphology must be equally plastic. This means that the blood vessels and major nerve cord remain functional whether the worm is stretched out or scrunched up. Kinda hard to imagine that in the body of any vertebrates.