Category: Marine invertebrates

My oldest baby urchins have been actual sea urchins for eight days now. Their total age, counting from the time they were zygotes, is 58 days. When an animal undergoes a life history event as drastic as this metamorphosis, it can be tricky deciding how to determine its age. Do you count from when egg and sperm formed the new zygote, or from when the juvenile (and eventually adult) body form was achieved? For the sake of this discussion I’m going to count from the date of fertilization, simply because I know exactly when that date was and it’s the same for all of these larvae, larveniles, and juveniles. This just makes sense to me.

So, at the grand old age of 58 days, which is five days post-metamorphosis for the oldest individuals, the baby urchins have grown a lot more tube feet, spines, and pedicellariae. However, they haven’t gotten any bigger. This is because they aren’t eating yet. I’ll explain why in a bit. The individual in the picture below measures about 490 µm in test diameter–that’s the opaque part in the center of the animal. The spines make the apparent size much larger.

In this short video clip you can see how many more tube feet this animal has, compared to the original five it started with. The movements are now much more coordinated, too, and these animals can walk with what appears to be purposeful direction. You can also see the texturing of the spines and the little pincher-like pedicellariae.

To see the surface details of the animal when it’s this opaque, I needed to use a different kind of lighting. Instead of using the transmitted light that shines through the object on the stage of the microscope, giving a brightfield view, I used my fiber optic light to create a darkfield effect that shows the surface details of the animal. Then I shot another video clip with this epi-illumination and focused up and down on the oral surface to see what was going on there. Fortunately the baby urchin isn’t yet able to right itself very quickly, and it stayed oral-side-up for as long as I needed to take the photos and video.

What this video clip of the urchin’s oral surface shows very clearly is that the animal doesn’t have a mouth yet. The pinkish star-shaped structure in the center is actually the negative space between the five triangle-shaped white teeth which all point to the middle. Soon, I expect in the next handful of days or so, that thin membrane covering the mouth will rupture, and the teeth will be exposed for the first time to the outside environment. At that point the urchin will begin feeding.

You may well be wondering, How the heck are they living if they haven’t eaten in over a week? They’re babies, after all, and don’t babies have to eat all the time? Well, yes, they are babies. But before they were baby urchins they were larvae, and as larvae were kept well fed by yours truly for their entire larval life. Part of becoming competent as a larva is sequestering enough energy stores to power the process of metamorphosis and keep the juvenile going until it has a mouth and can feed itself. Remember, this new animal has to do everything–locomote, eat, avoid predators–with body parts that it didn’t have when it was a larva. Building whole new body parts and learning how to use them takes time. So these newly metamorphosed juveniles have about 10-12 days to fast until their mouths break through and they can begin eating. Any individual that didn’t store enough energy to make it through the fast, will die.

I’ll check on them again tomorrow (day 59) and see if it’s time to transfer the juveniles to their food source, which will be algal scuzz that I’ve been cultivating on glass slides for a few weeks. They’ll grow quickly once they’re eating. I hope I have enough scuzz to keep up with them!

Imagine spending your entire life up in the water column as a creature of the plankton. You use cilia to swim but are more or less blown about by the currents, never (hopefully!) encountering a hard surface, and feeding on phytoplankton and other particulate matter suspended in the water. Then, several weeks into your life’s adventure, you fall out of the plankton, dismantle your body while simultaneously building a new one, and about a day later have to begin walking using anatomical structures that you didn’t have 24 hours earlier. Not only that, but the food that you’ve been eating your entire life is no longer available to you, for you no longer possess the apparatus that can capture it. And, finally, your body symmetry makes a wholescale change from bilateral to pentaradial–just think of what that means in terms of how your body is oriented and moves through three-dimensional space. That’s what metamorphosis is like for sea urchins and many other echinoderms.

The objects of my complete and utter obsession for the past month and a half have started metamorphosing from small larvae into tiny urchins. When I did my daily check yesterday I had two that had completed metamorphosis since the previous day. One of them still had a bit of puffiness on the aboral surface, which I think may be the very last remnants of the larval body. This little guy has only its first five tube feet, from the juvenile rudiment of the competent larva.

Its companion in metamorphosis was a bit farther along in terms of development; while it still had only the first five tube feet, it has more spines:

But just having feet doesn’t mean you automatically know how to walk with them, and it’s no easier for these guys than it is for humans. It’s probably more difficult, actually, because the urchins have to coordinate movement of five appendages simultaneously. They typically pick up one or two tube feet from the same side of the body and wave them around until one of them randomly sticks to something. Then they remain stretched out until the tube feet on the opposite side of the body let go. Well, you can watch for yourself; this is the same individual that is in the top photo above:

Being a bit farther along in the developmental process means having more spines, but not necessarily any more coordination. I watched the second urchin for several minutes, and while it repeatedly detached and re-attached tube feet, it didn’t actually go anywhere. Here’s a short clip:

It’s amazing how quickly they learn, though. When I go to the lab to look at them tomorrow, they’ll be running around as though they’ve been ambulatory their entire lives. Which, in a peculiar sense, depending on when you start counting, maybe they have.

After much teasing and titillation, my urchin larvae have finally gotten down to the serious business of metamorphosis. It seems that I had jumped the gun on proclaiming them competent about a week ago, or maybe they were indeed competent and just needed to wait for some exogenous cue to commit to leaving the plankton for good. In any case, I’ve spent much of the last five days or so watching and photographing the larvae to document the progress of metamorphosis as it occurs. While I was unable to follow any individual larva through the entire process of metamorphosis, I did manage to put together a series of photographs that document the sequence of events.

To recap: A competent larva is anatomically and physiologically prepared to undergo metamorphosis. This batch of larvae reached competence at the age of about 45 days. The larva below is very dense and opaque in the main body. It can still swim, but has become “sticky” and tends to sit on the bottom of the dish.

Sometimes the first tube feet emerge from the larva while it is still planktonic. Other times the larva falls to the benthos and lands on its (usually) left side, where the rudiment is located.

This larva is lying on its right side, so the tube feet are sticking straight up out of the plane of view. You can clearly see two of them, though.

Just for kicks, here’s the same larva, photographed with dark-field lighting. This kind of light illuminates the surface of the object being viewed, which is very helpful when the subject is opaque, making it possible to see four tube feet in this picture.

As the tube feet are emerging from the juvenile rudiment, the larval body contracts and gets denser. The arms shrink and the internal skeletal rods that supported them are discarded. At this stage the larval juvenile (larvenile? juvenal?) begins to crawl around on the bottom. The ciliated band that used to propel it through the water and create the feeding current may still be beating, but eventually will stop, as the larvenile will no longer need it. This is usually the time that I see the first spines waving around; it’s interesting to note that tube feet, which originate from the inside of the animal, come first, then are followed by spines. Then again, the spines are part of the animal’s endoskeleton, so maybe it’s not so noteworthy after all.

So they’re getting close to becoming real urchins! Next up: Learning to walk.

Did you notice that I invented a new word? I’m going to start using it regularly.

In the parlance of invertebrate zoologists, competence is the state of development when a larva has all of the structures and energy reserves it needs to undergo metamorphosis into the juvenile form. In the case of my sea urchins, this means that they have four complete pairs of arms, each with its own skeletal rod, and a fully formed juvenile rudiment, which contains the first five tube feet of the water vascular system. A continuous ciliated band runs up and down all eight arms and provides the water current used both for swimming and feeding. The larva will have been eating well and its gut will be full of food. It will have lost the transparency it had when it was younger and will appear to be more solid-looking in the central area.

The first batch of larvae that I began culturing this season are now 42 days old. Some of these are competent, or very nearly so. Last week I isolated about a dozen of these big guys into a small dish, making it easier for me to observe them closely every day. Today they looked decidedly opaque and dumpy, and although some of them were still swimming others were heavy and tended to rest on the bottom of the dish.

Here’s a photo that I took yesterday:

General orientation: This is a ventral view. The animal swims with its arms forward, which defines the anterior portion. Thus the bottom of the cup-shaped body is the posterior. This larva measures ~900 microns along the anterior-posterior axis. Plutei have bilateral symmetry that goes all to hell during metamorphosis, from which the urchin crawls away with typical echinoderm pentaradial symmetry. This wholescale change in body organization is one of the truly amazing things about metamorphosis in these animals. It boggles my mind every time I think about it.

You can see that this pluteus has eight arms. The oblong reddish structure in the middle is the stomach, which has taken on the color of the food the animal has been eating. The strange mixed-up looking structure adjacent to the stomach on the animal’s left side is the juvenile rudiment. Focusing up and down through the rudiment shows that it contains five tube feet. After metamorphosis, the juvenile urchin will use those first five tube feet to walk around as a benthic creature, having spent all of its life up to this point as a member of the plankton.

Today I captured about 20 seconds of a larva feeding. This individual is a day older than the one in the photo above and has more of that opacity that I associate with competence.

This is a dorsal view; if you imagine that you’re looking at the animal’s back, you see that the rudiment is indeed on its left side. The larva’s ciliated band is moving a lot of water, and the little specks that you can see flying around are food cells. There wasn’t enough water in this drop for the pluteus to do any actual swimming, but at this point it’s pretty heavy and would tend to sink to the bottom.

Some time in the next several days these guys are going to start metamorphosing. I will be examining them every day; keep your fingers crossed that I catch them in the act!

On another glorious afternoon low tide the other day, with the help of a former student I collected six purple urchins, Strongylocentrotus purpuratus. Given that we’re in about the middle of this species’ spawning season, I reasoned that collecting six gave me a decent chance of ending up with at least one male and one female that hadn’t spawned yet.

Yesterday, after the urchins had been in the lab for somewhat less than a whole day, I shot them up and waited. Three females began spawning almost immediately (yes!) and one male started a few minutes later. When all was said and done I ended up with four females and two males. It turns out that the largest individual, with a test diameter of almost 10 cm, was a male but didn’t spawn very much at all. I infer from this that he had already spawned in the field before I collected him.

At the current ambient sea water temperature of 14°C, hatching begins around 24 hours post-fertilization. Early this afternoon I checked on the beakers and they had indeed begun hatching. Sea urchins hatch at the blastula stage of development, when they are essentially a ciliated hollow ball of cells. The cilia allow the larvae to swim, but at this size they are at the mercy of even the weakest current. Thus, for the most part they act as particles, getting carried wherever the current takes them.

As the embryos hatch, they swim up to the top of the beaker, then move down towards the bottom. I call this “streaming.” At this point in our artificial culturing system the embryos are living in still water without any current, so this behavior is due primarily to their ability to swim. There is probably some interesting physics involved, but I’m not enough of a physicist to figure out what’s going on at that level. But whatever it is, it’s a really cool behavior to watch:

Rather mesmerizing, isn’t it? Each of those tiny orange dots is an individual embryo. Once the embryos hit the water column I pour them off into larger jars and begin stirring them. Right now they’re small enough to swim on their own, but once they start feeding and growing they get heavier and would sink to the bottom without some current to keep them suspended. The contraption we use to stir jars of larvae is a manifold of paddles connected to a motor that moves the paddles back and forth, creating the right amount of current to keep the larvae from settling on the bottom without getting beat up by the turbulence.

Here’s the paddle table in action. It’s a noisy SOB.

For now the embryos just hang out in the jars and get stirred. Their first gut, the archenteron, will be visible tomorrow and the larvae will be able to eat on Friday. Stay tuned!