Skip to content

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!

Lophophore of a phoronid worm (Phonoris ijimai)
18 Septenber 2017
© Allison J. Gong

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.

2-day-old actinotroch larva of Phoronis ijimai
22 September 2017
© Allison J. Gong

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:

2-day-old actinotroch larva of Phoronis ijimai
22 September 2017
© Allison J. Gong

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!

1

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?

Polychaete worms, Phragmatopoma californica, sticking their 'heads' out of their tubes at Natural Bridges
26 May 2016
© Allison J. Gong

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.

Phoronids at the Santa Cruz Yacht Harbor
18 September 2017
© Allison J. Gong

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.

Single phoronid worm extending its lophophore
18 September 2017

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.

Peanut worms (Phascolosoma agassizii) at Pigeon Point
30 April 2017
© Allison J. Gong

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.

Peanut worms (Phascolosoma agassizii) at Pigeon Point
30 April 2017
© Allison J. Gong

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.

Two stars of the genus Leptasterias, at Pigeon Point
9 May 2016
© Allison J. Gong

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:

Bat star (Patiria miniata) at Pigeon Point
30 April 2017
© Allison J. Gong

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:

Patiria miniata with commensal worm, at Pigeon Point
30 April 2017
© Allison J. Gong

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.

Commensal worm (Oxydromus pugettensis) in the ambulacral groove of Patiria miniata, at Pigeon Point
1 May 2017
© Allison J. Gong

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.

Data from our weather station in Santa Cruz, CA.
8 January 2017

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.

Earthworm on wet pavement.
8 January 2017
© Allison J. Gong

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?

Earthworm on wet pavement.
8 January 2017
© Allison J. Gong

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.

Does the title of this post make any sense now?

%d bloggers like this: