Category: Marine invertebrates

17 days old

Posted on by Allison J. Gong

What a difference a week makes! The Pisaster larvae have grown and developed quite a bit since I looked at them a week ago. Here they are as little space ships again.

Early brachiolaria larvae of Pisaster ochraceus, age 17 days. 19 June 2015. © Allison J. Gong
Early brachiolaria larvae of Pisaster ochraceus, age 17 days
19 June 2015
© Allison J. Gong

Since they are getting so big, Scott and I decided to redistribute the larvae from four jars into six. This will give them room to grow and ensure that they aren’t overcrowded. To do this we first concentrated them all into a single beaker, then divided the entire population into two jars, then subdivided each jar into three jars, for a total of six. See all the larvae in the beaker?

Brachiolaria larvae of Pisaster ochraceus, aged 17 days. 19 June 2015. © Allison J. Gong
Brachiolaria larvae of Pisaster ochraceus, aged 17 days
19 June 2015
© Allison J. Gong

The largest larvae are ~1200 µm long, getting big enough to fill up the field of view under the lowest magnification of the compound microscope. The most noticeable difference from last time, aside from the overall increase in size, is that the ciliated band is becoming more lobed. These lobes will eventually be elaborated into the long arms of the mature brachiolaria larva (‘brach-‘ is Greek for ‘arm’). See below:

Brachiolaria larva of Pisaster ochraceus, age 17 days. 19 June 2015. © Allison J. Gong
Brachiolaria larva of Pisaster ochraceus, age 17 days
19 June 2015
© Allison J. Gong

The other rather obvious development is that the left and right coeloms from the previous observation a week ago have fused together in the anterior (top of the picture) and posterior (bottom of the picture) region of the body.

From here on out the larvae won’t get too much bigger; if I remember correctly they’ll grow until they’re about 1500 µm long. Their brachiolar arms will get really long and pretty, though, greatly increasing the length of the ciliated band. Eventually their juvenile rudiments will form . . . but that’s a post for another day. More on that when it happens.

Growing fast

Posted on by Allison J. Gong

Today my Pisaster ochraceus larvae are 10 days old. Although they seemed to be developing slowly, compared to the urchins that I’m more used to, in the past several days they have changed quite a bit. They’ve also been growing quickly, which makes me think that they’re off to a strong start. Of course, there’s still a lot of time for things to go wrong, as they have another couple of months in the plankton. However, at this point in time I feel optimistic about their chances.

In the dish under the dissecting scope they swim around like bizarre space ships. All the bits of detritus in the water add to the effect. The only thing missing is the sound effects.

The magnification of my dissecting scope goes up to 40X. To see any details of anatomy I have to use the compound microscope, through which I can see this, under 100X magnification:

10-day-old bipinnaria larva of Pisaster ochraceus, 12 June 2015. © Allison J. Gong
Ventral view of 10-day-old bipinnaria larva of Pisaster ochraceus
12 June 2015
© Allison J. Gong

Aside from the dramatic increase in overall size (almost 1 mm long now!), the larva’s body has gotten a lot more complicated. For one thing, the animal’s marginal ciliated band, which propels the larva through the water, has started becoming more elongate and elaborate. In this view the larva is lying on its back, and I have focused on the plane of its ventral surface. The left and right coeloms are in the plane of the dorsal surface, and thus are not really in focus. You should still be able to see how long they have gotten, though. Eventually they will fuse anteriorly to form a single cavity. The stomach of the larva has a nice green-golden color due to the food it has been eating. It makes perfect sense, as we are feeding them a cocktail of green algae and a diatom-like golden alga.

The larvae are very flexible and can be quite animated when they’re swimming around. They bend, scrunch up, and swallow food cells. They have already gotten so big that they take up much of the field of view under the microscope, even at the lowest magnification. Watch some larval gymnastics:

Part of the reason that I wanted to spawn Pisaster and raise the larvae this summer is that I want to put together a series of pen-and-ink drawings of the developmental stages. I did the same for the bat star Patiria miniata several years ago, but the Pisaster larvae will have longer and more elaborate arms when they mature; capturing these in drawings will be a challenge for me. I also hope to include the juveniles in this set of drawings. With that goal in mind, I’ve been sketching the larvae every few days, just to get some practice under my hand and remind myself what it feels like to draw. I’ve missed it!

10-day-old bipinnaria larva of Pisaster ochraceus, drawn from life. 12 June 2015. © Allison J. Gong
10-day-old bipinnaria larva of Pisaster ochraceus, drawn from life
12 June 2015
© Allison J. Gong

For whatever reason, I really like how this sketch turned out. It’s not pretty, but it does truly represent what I saw under the microscope. I’m going to have to work on depicting three-dimensional structures on a two-dimensional page, which will take some practice. Fortunately I have several weeks to brush up on my skills!

Feeling blue?

Posted on by Allison J. Gong

This spring and summer the local beaches have at times been covered by what appear to be small, desiccated, blue or white potato chips. They would typically be seen in windrows at and just below the high-tide line, or blown into piles. The most recently washed up ones are a dark blue-violet color, while the ones that have been on the beach for more than a day or two are faded to white.

Windrows of Velella velella (by-the-wind sailor) washed up on the beach at Point Piños, 9 May 2015. © Allison J. Gong
Windrows of fresh Velella velella (by-the-wind sailor) and algal detritus washed up on the beach at Point Piños
9 May 2015
© Allison J. Gong
Desiccated Velella velella on the beach at Franklin Point, 22 April 2015. © Allison J. Gong
Desiccated Velella velella on the beach at Franklin Point
22 April 2015
© Allison J. Gong

These animals are Velella velella, commonly called by-the-wind sailors. Taxonomically they are in the Class Hydrozoa of the Phylum Cnidaria. Other members of this class are the colonial hydroids and siphonophores (such as the Portuguese man-o’-war, Physalia) as well as the freshwater hydras that you may have played around with in high school. Technically speaking, Velella isn’t a jellyfish. Actually, if we want to get uber-technical about it, there’s no such thing as a jellyfish at all; or if there is, it’s a vertebrate (i.e., some kind of actual fish) rather than a cnidarian. Most of the gelatinous creatures that people generally refer to as “jellyfish” are in fact the medusae of cnidarians.

That said, Velella is a special kind of hydrozoan. Its body consists of an oblong disc, 3-10 cm long, with tentacles and such hanging down and a sail sticking up. The little sail catches the wind that propels the animal:

Single Velella velella washed up on beach at Franklin Point, 22 April 2015. © Allison J. Gong
Single Velella velella washed up on beach at Franklin Point
22 April 2015
© Allison J. Gong

How do so many of these animals end up on the beach? The answer is that they float on the surface of the ocean and are at the mercy of the winds, hence their common name. This is an extremely specialized habitat called the neuston. Organisms living here have to be adapted to both aerial and marine factors. In fact, the blue pigment in these animals is thought to act as a sunscreen, reflecting the blue (and probably UV) wavelengths and protecting the underlying cells. We all know that UV radiation damages DNA, right? That’s why we wear sun protection. Other cnidarian inhabitants of the neuston are things like Physalia and Porpita porpita (blue buttons), which are also blue in color. A former boss of mine used to say that for every hydroid there’s a nudibranch that lives on it, eats it, and looks just like it. Porpita isn’t exactly a hydroid, but it does have a predatory nudibranch, Glaucus atlanticus, which is (of course) blue-purple! Glaucus eats Velella, too.

Porpita porpita (left) and its predator, the nudibranch Glaucus atlanticus. Diameter of P. porpita approx. 2 cm.
Porpita porpita (left) and its predator, the nudibranch Glaucus atlanticus. Diameter of P. porpita approx. 2 cm.

The Monterey Bay Aquarium Research Institute (MBARI) has, of course, one of the best video explanations of what Velella is all about. I certainly can’t do any better, so you should watch this:

By the way, MBARI’s YouTube channel is like marine biology and oceanography porn. Just sayin’. If you have some time to kill on the Internet, you could certainly do worse than to spend it there!

Slow and steady (I think)

Posted on by Allison J. Gong

Today the Pisaster larvae that Scott and I are following are a week old. Happy birthday, little dudes! Yesterday we did the twice-weekly water change and looked at them. They’re getting big fast since we started feeding them on Saturday when their mouths finally broke through. At this stage they are sort of jellybean-shaped and extremely flexible–they don’t have the calcified skeletal rods that sea urchin larvae have so they bend and flex quite a lot. They are also beautifully transparent, which allows us to see their guts in fine detail. We can even watch them swallow food cells!

Front view of Pisaster ochraceus bipinnaria larva, age 7 days, 8 June 2015. © Allison J. Gong
Front (ventral) view of Pisaster ochraceus bipinnaria larva, age 7 days
8 June 2015
© Allison J. Gong

In profile view you can see that the larvae are shaped sort of like fat C’s. Here’s a side view of a different individual:

Right side view of bipinnaria larva of Pisaster ochraceus, age 7 days. © Allison J. Gong
Right side view of Pisaster ochraceus bipinnaria larva, age 7 days.
© Allison J. Gong

In the short term (over the next couple of weeks or so) the larvae will continue to get longer. Their guts won’t change much, but their coelomic systems will develop and become more complex. I’ll try to capture that in photos and drawings to share with you.

A glass half full

Posted on by Allison J. Gong

It’s becoming quite clear that I don’t have to worry about having too many sea star larvae to deal with. While the embryos from my F1 x M1 (Purple x Purple) cross had hatched this morning, nothing from the F2 x M1 (Orange x Purple) cross looked promising. I’m about ready to write off these guys and dump them all down the drain, but will give them until tomorrow to pull themselves together and do something that doesn’t look all wonky.

In the meantime, it’s really fun looking at the good embryos from the F1 x M1 mating. They hatched out of their fertilization envelopes and have become elongated, sort of like stubby Tylenol caplets. This elongation defines a functional anterior-posterior axis, and the animal swims with its anterior end forward.

Gastrulating embryo of Pisaster ochraceus, 4 June 2015. © Allison J. Gong
Gastrulating embryo of Pisaster ochraceus
4 June 2015
© Allison J. Gong

Gastrulation is the process of forming the first larval gut, or archenteron. Remember how yesterday the embryo was a hollow ball of cells called a blastula? In these echinoderms gastrulation is simply an invagination into the blastula. Imagine poking your finger into an inflated balloon:  The balloon is the blastula and your finger forms an invagination, or channel, through it. In embryos, gastrulation begins at a site on the blastula called the blastopore; this is where you’d stick your finger into the balloon in our analogy.

Most animal guts have two openings, a mouth and an anus. You understand what happens at each of those openings. The archenteron is a gut, one of whose openings is the blastopore. The fate of said blastopore is to be either the mouth end or the anus end of the archenteron. In echinoderms, the major invertebrate phylum that makes up a larger grouping of animals called the deuterostomes, the blastopore becomes the anus, with the mouth breaking through as the process of gastrulation finishes. And lest you think that possessing an anus before a mouth is somehow less evolved than the reverse would be, you might be interested in knowing that we humans are also deuterostomes. That’s right, each of you reading this blog, as well as the one who writes it, built an anus first and a mouth second.

These sea star embryos swim really fast! I had to squash them under a cover slip to snap some halfway decent pictures, and even then it wasn’t easy to slow them down or chase them around on the slide. You can get a feel for how fast they can move in this short video clip:

The archenteron appears to wobble because it doesn’t go straight through from the blastopore to the apex of the embryo. The mouth will break through along one of the sides, resulting in a curved gut. I suspect that when I look at the embryos tomorrow they will have graduated to the status of larvae, with complete guts. Then I get to start feeding them and watching them grow.

Questions and answers

Posted on by Allison J. Gong

I’ve been fielding questions about my recent sea star spawning work from people I’ve shared this blog with, which is a lot of fun! To streamline things and make the info available to anybody who might be following, I decided to put together a very brief FAQ-like post to address the most recent questions.

Question:  Can you watch the eggs divide in real time?

In a time-lapse sense you can watch cleavage divisions occur, but not in real time. What I can do is set up a slide on the microscope and leave it there for a while. The gradually warming temperature speeds up development to the point that I can sort of see the division in real time. Of course, the danger is that the embryo will cook on the slide. I generally figure that once I’ve pipetted some embryos onto a slide and dropped a cover slip on top of them, they’re goners (it’s not really possible to remove the cover slip without damaging the cells underneath it) so I feel marginally less bad about sacrificing a few to the gods of observation.

Questions:  I’m fairly certain that the stars can go back to the sea, but are you able to keep their eggs with them, too? How difficult is that transport?

Actually, my scientific collecting permit specifically states that I’m not allowed to return animals to the wild. If I needed to, I could apply for additional permits but it has never been necessary for the work I do. Surplus eggs and larvae, therefore, are discharged into the seawater outflow at the lab and do return to the ocean but the parents remain in my care.

Question:  Are orange and purple stars usually able to cross with each other?

As far as anyone has been able to determine, the color of stars has zero effect on whether two individuals’ gametes are able to do the nasty together. The sea stars that I’m working with–Pisaster ochraceus, the ochre star–are broadcast spawners, meaning that each individual spews his/her gametes into the water, where fertilization and development occur. The stars are also synchronous spawners, meaning that if one individual in an area begins spawning other stars in the immediate vicinity will also spawn. After all, it does take two to tango, and to spawn while nobody else does is a tremendous waste of energy.

So yes, a purple star and an orange star should be able to mate without any problems… at least not any problems due to the parents’ colors.

Question:  If so, what color do they end up being, statstically?

This is a very interesting question. Two of my colleagues are going to spawn Patiria miniata (bat stars) next week to address this. Their plans are to cross a Blue female with an Orange male, an Orange female with a Blue male, and both pure-color matings. They did a preliminary version of this experiment a couple of years ago but didn’t end up with enough juveniles at a size that color could be ascertained; thus they couldn’t calculate any statistically meaningful color ratios.

Questions:  Do you suppose that the wasting disease could be now in the genetic makeup? Any thoughts (unofficial of course) about this?

My thought is sort of the opposite, actually. The animals that we brought in from the field are all survivors of SSWS; if anything, I’d expect them to be resistant to whatever causes the plague, and to (hopefully) pass on this resistance to their offspring. Of course, there’s no way of knowing if and how exposure to SSWS affects the quality of the gametes. It’s quite possible that these survivors are less fit after the SSWS outbreak than they were before.

Question:  Purple Male with Purple Female developed well and purple Male with Orange female didn’t…some sort of incompatibility?

Well, given what I saw today the Orange (female) x Purple (male) cross almost certainly did not work. Fertilization occurred, but almost none of the embryos had any indication of normal development. Since we know the Purple male was able to mate successfully with the Purple female, we can infer that his sperm were fine. It could be that there was something going on with the Orange female’s eggs; there were a lot of them, but maybe their quality just wasn’t very good. Or perhaps we somehow mistreated and wrecked them the other day.

Any other questions? Use the Comments section to ask them, and I’ll address them in a future post.

Strangeness abounds

Posted on by Allison J. Gong

Wow, they weren’t kidding about “early developmental asynchrony” in sea stars! This morning I looked at the embryos that I had started almost 24 hours earlier, and noticed two things right off the bat:

Thing #1:  Within the F1 x M1 (Purple female x Purple male) mating , developmental rates among full siblings were all over the map. Some embryos had progressed to the blastula stage, which is essentially a hollow ball of ciliated cells, while others were still in the early cleavage stages and rather a lot hadn’t divided at all. In fact, with 24 hours of hindsight I can see that several of these eggs had not even been fertilized.

Embryos of Pisaster ochraceus, age 24 hrs. 3 June 2015. © Allison J. Gong
Embryos of Pisaster ochraceus, age 24 hrs
3 June 2015
© Allison J. Gong

My first reaction upon looking into the microscope and seeing all these assorted blobs was, “Oh, crap.” But then I looked more closely at some of the embryos and realized that they had become blastulae!

Here’s a picture of a blastula. This embryo is freely swimming inside its fertilization envelope, although it doesn’t have a lot of space (remember that narrow perivitelline space from yesterday? that’s all the elbow room it has). The hollow space in the center of the embryo is the blastocoel ‘sprout cavity.’ Given that the embryo hasn’t grown (or even hatched) yet, it’s still ~165 µm in diameter, the size of the original egg.

Blastula of Pisaster ochraceus, 3 June 2015. © Allison J. Gong
Blastula of Pisaster ochraceus
3 June 2015
© Allison J. Gong

The stage that precedes the blastula (a hollow ball of cells) is called a morula (a solid ball of cells). The embryo that is partially visible in the bottom of the above photo may be a morula. Imagine the following sequence of events: (1) an egg is fertilized by a sperm, forming a zygote; (2) the zygote undergoes a number of cleavage divisions, with the cells becoming more numerous and smaller in size; (3) at some stage a solid ball of small cells, the morula, is formed; (4) as cell division continues, the cells migrate toward the outside of the sphere, forming a cavity (the blastocoel) in the middle.

The blastula is a ciliated stage, and in this video clip you can see the cilia moving. I shot this video at only 100X magnification to capture as much depth of field as possible, and suggest viewing at full-screen. This should enable you to see the three-dimensional structure of the embryo, and that it is indeed a sphere.

Thing #2:  The F2 x M1 mating (Orange female X Purple male) isn’t doing well at all. I looked at several slides and didn’t see any embryos that were developing normally. They had all been fertilized, as I could see the fertilization envelope surrounding each egg, but most had not even divided. The ones that had divided were all strange and just plain wrong. Here, see for yourself:

24-hr embryos of Pisaster ochraceus, 3 June 2015. © Allison J. Gong
24-hr embryos of Pisaster ochraceus
3 June 2015
© Allison J. Gong

Many of the eggs are blurry because they’re below the focal plane of the microscope. But see how many of them are undeveloped? And how, in the ones that have started dividing, the cells are disorganized and of different sizes? Typical echinoderm cleavage, as I see in echinoids (our local urchins and sand dollars) and in my other crossing of these ochre stars, results in a blastula made up of cells that are all approximately the same size. Most of these embryos, on the other hand, appear to consist of one large cell and a bunch of tiny ones.

I assume that these abnormal-looking-to-me embryos will not hatch, although I could be pleasantly surprised tomorrow. I don’t yet have much of an intuition about these Pisaster ochraceus embryos, so this is a huge learning experience for me. I do expect to see hatching in the F1 x M1 cross tomorrow. Fingers crossed!

From zero to cleavage in. . . nine hours

Posted on by Allison J. Gong

A recent college graduate and fellow marine lab denizen (Scott) and I are collaborating on a project to quantify growth rates in juvenile Pisaster orchraceus stars. This is one of the intertidal species whose populations in the field and in the lab were decimated by the most recent outbreak of sea star wasting syndrome (SSWS). We are interested in seeing how quickly the stars grow once they metamorphose and recruit to the benthos, and hope that the information will help researchers guesstimate the age of the little stars that are now being seen in the field. This would in turn tell us whether the little stars are survivors of SSWS or post-plague recruits. I keep seeing people refer to them as “babies,” but they could very well be several years old. We just don’t know, hence this study.

Large, healthy specimen of Pisaster ochraceus at Davenport Landing. 20 May 2015. © Allison J. Gong
Large, healthy specimen of Pisaster ochraceus at Davenport Landing
20 May 2015
© Allison J. Gong

But before we get to measure juvenile growth we have to get through larval development, which is perfectly fine by me because I’m always up for observing marine invertebrate larvae. Two weeks ago Scott and I ventured into the field in search of prospective parents. We brought back eight individuals from two different sites, making sure to leave many more in place than we took away. It was actually rather gratifying to see how many hand-sized-or-larger P. ochraceus there were. This morning we met at 07:30 to shoot up the stars with magic juice and then wait for them to spawn.

We have injected the stars (Pisaster ochraceus) and are waiting for them to spawn. 2 June 2015 © Allison J. Gong
We have injected the stars (Pisaster ochraceus) and are waiting for them to spawn
2 June 2015
© Allison J. Gong

It has been a while since I tried to induce spawning in Pisaster, and I had forgotten how much longer everything takes compared to the urchins. For one thing, the magic juice itself isn’t the same stuff that we use on the urchins, and works by an entirely different mechanism. The stars’ response to the magic juice takes 1.5-2 hours, whereas if the urchins aren’t doing anything 30 minutes after getting shot up they either need another injection or simply don’t have gametes to share.

However, despite my misgivings the animals spawned. Two large females gave us enormous quantities of eggs, and three more donated trivial amounts that we didn’t end up using.

This purple individual is the one we designated Female 1. See the huge piles of salmon-pink eggs?

Large purple female Pisaster ochraceus, spawning. 2 June 2015 © Allison J. Gong
Large purple female Pisaster ochraceus, spawning
2 June 2015
© Allison J. Gong

and

Large orange female Pisaster ochraceus, spawning. 2 June 2015 © Allison J. Gong
Female 2, a large Pisaster ochraceus, spawning
2 June 2015
© Allison J. Gong

Although we had to wait for a male to spawn, we finally did get some sperm and fertilized the eggs at about 12:30. Another thing I had forgotten was that Pisaster eggs, when shed, are lumpy and strange. I was used to the urchin eggs, which are usually almost all beautifully spherical and small. The stars’ eggs are about twice as big, at ~160 µm in diameter. The lumpiness doesn’t seem to hamper the fertilization process, as you can see below.

Fertilized eggs of Pisaster ochraceus, 2 June 2015 © Allison J. Gong
Fertilized eggs of Pisaster ochraceus
2 June 2015
© Allison J. Gong

In this photo you can see the fertilization envelope surrounding most of the eggs. In stars the perivitelline space (the space between the egg surface and the fertilization envelope) is very narrow, which makes it difficult to see the envelope; in urchins the space is much larger, and as a result the envelope quite conspicuous. The rising of the fertilization envelope off the surface of the egg is referred to as the slow block to polyspermy, a mechanical barrier that keeps multiple sperms from penetrating any individual egg. There’s also a fast block to polyspermy, but it happens on a molecular level milliseconds after a sperm makes contact with the egg surface; you can’t see it happen in real time.

Cleavage in stars proceeds much more slowly than it does in urchins, too. In embryological terms, “cleavage” refers to the first several divisions of the zygote, during which the cell number increases as the cell size decreases. This inverse relationship between cell size and number logically has to occur because the embryo can’t get any larger until it has a mouth and begins to feed, which won’t happen for at least a couple of days. It took our zygotes about four hours to undergo the first cleavage division.

2-cell embryo of Pisaster ochraceus, 2 June 2015 © Allison J. Gong
2-cell embryo of Pisaster ochraceus
2 June 2015
© Allison J. Gong

I left the slide on the microscope to warm up and speed development a bit, and about 45 minutes later was rewarded with this mishmash of embryos at different stages. Nine hours after we started this whole process, there were 2-cell, 4-cell, and 8-cell embryos, as well as eggs that had not divided yet.

Embryos of Pisaster ochraceus, about four hours post-fertilization. 2 June 2015 © Allison J. Gong
Embryos of Pisaster ochraceus, about four hours post-fertilization
2 June 2015
© Allison J. Gong

This asynchrony in early development is another way that stars differ from urchins, and it takes some getting used to. I expect that development will become more synchronized as the embryos continue to cleave, and that hatching will occur for all of them at about the same time, probably before Thursday. At least it won’t take another 9-hour day to see how far they’ve come.

Sea urchins have no manners

Posted on by Allison J. Gong

On Monday of this week (today is Thursday) I was transferring my baby urchins into clean bowls as I always do on Mondays, and for some crazy reason decided that I needed to measure all 300+ of them. I don’t remember how the details of how this decision came about, but it probably went something like this:

  • Me #1:  You know, we should probably measure these guys. We do want to see how fast they’re growing, after all.
  • Me #2:  Are you kidding? Do you know how long it’s going to take to measure 300 urchins under the microscope? We don’t have that kind of time today!
  • Me #1:  Oh, come on, don’t be so lazy. How long can it take, really? Let’s do it for science!
  • Me #2:  These things always take twice as long as you think they will.
  • Me #1:  It’s not as though you have anything better to do this afternoon. I mean, aside from writing a final exam and grading all those research papers you assigned.

Three-and-a-half hours later, Me #2 was soundly kicking Me #1 in the butt and we were all tired. But the urchins got measured and now I have some baseline data so I can track further growth. And, no, I don’t have the urchins separated into individual containers so I won’t be following individual growth, but will be able to calculate average growth rates across the cohort.

Having to look at each urchin long enough to get it lined up with the ocular micrometer in the dissecting scope gave me a chance to observe how their colors are developing. In the field, urchins of this species (Strongylocentrotus purpuratus) in this size range (mm-3 cm) are usually greenish in color; when these individuals are brought into the lab they turn purple as they continue to grow. I seem to recall that my last batch of lab-grown urchins (in Spring 2012) didn’t go through that green phase as juveniles, at least not as vibrantly as what we see in the field. So while I was holding down the current batch of urchins to measure them, I noted their color.

Some of them have a definite green tinge at the base of the spines, which then fades to a mauve-y purple towards the tips. The green coloration is most evident on the younger spines:

Strongylocentrotus purpuratus juvenile, age 118 days. This individual has a test diameter of 2.7 mm. 18 May 2015. © Allison J. Gong
Strongylocentrotus purpuratus juvenile, age 118 days. This individual has a test diameter of 2.7 mm
18 May 2015
© Allison J. Gong

In addition to giving the urchins something more substantial than scum to eat, having them on coralline rocks gives me a chance to see some of the other critters that live on the rocks. This particular rock is inhabited by a number of spirorbid polychaete worms that build tiny circular tubes made of calcium carbonate, as well as assorted small barnacles cemented to the rock and other crustaceans crawling around.

This is a close-up shot of one of the spirorbid worms. The tube is entirely covered by pink coralline alga, but the worm’s orange tentacular crown and trumpet-shaped operculum (used to close the tube when the worm withdraws) are extended as the worm filter-feeds:

Spirorbid polychaete worm on coralline rock, 18 May 2015. © Allison J. Gong
Spirorbid polychaete worm on coralline rock
18 May 2015
© Allison J. Gong

Another photogenic animal that I happened to find was a very small chiton. By the time I found it after measuring all the urchins I didn’t have the brain energy to try and key it out; if I can find it again once I’ve finished grading final exams I’ll give it a shot. It is extremely cute, with its bright blue spots, and was very slowly creeping around on the rock when one of the urchins barged in and ran right over it:

The chiton is probably about 4 mm long, just a bit longer than the urchin’s test diameter. To the urchin, walking over a chiton isn’t much different from walking over a rock; and while the chiton probably doesn’t like being walked on it isn’t significantly affected by the incident unless the urchin starts gnawing on it. Chitons are the masters of just hunkering down and waiting for things to get better, whether that means the tide coming back or an uncouth urchin moving along and minding its own business.

When is a tube worm not a tube worm?

Posted on by Allison J. Gong

Answer:  When it’s a snail! Yes, there are snails that secrete and live in white calcareous tubes that look very similar to those of serpulid polychaete worms. Here, see for yourself:

Serpula columbiana, a serpulid polychaete worm, at Point Piños, 9 May 2015. © Allison J. Gong
Serpula columbiana, a serpulid polychaete worm, at Point Pinos
9 May 2015
© Allison J. Gong

The worms secrete calcareous tubes that snake over whatever surface they’re attached to. When the worm is relaxed, it extends its delicate pinnate feeding tentacles and uses them to capture small particles to eat; they are what we call suspension feeders.

Serpula columbiana polychaete worms, Seymour Marine Discovery Center, 11 May 2015. © Allison J. Gong
Serpula columbiana polychaete worms, Seymour Marine Discovery Center
11 May 2015
© Allison J. Gong

But there are gastropods that secrete calcareous tubes, too. They are the vermetid snails, the local species of which is Thylacodes squamigerus. This is one of my favorite animals in the low intertidal, probably because it is so delightfully un-snail-like.

There are three individuals of T. squamigerus in this photo:

The vermetid snail Serpulorbis squamigerus at Point Piños, 9 May 2015. © Allison J. Gong
The vermetid snail Thylacodes squamigerus at Point Pinos
9 May 2015
© Allison J. Gong
Serpulorbis squamigerus at Point Piños, 9 May 2015. © Allison J. Gong
Thylacodes squamigerus at Point Pinos
9 May 2015
© Allison J. Gong

Thylacodes is also a suspension feeder, but it gathers food in a very different way. When submerged, it spins out some sticky mucus threads that catch suspended particles, then reels in the threads and eats them.

So how would you tell these animals apart if you see them? Here’s a hint:  Look at the tubes themselves.

I invite you to use the comments section to tell me how you’d distinguish between Serpula and Thylacodes.