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My baby urchins have become scum-eating machines! They are 88 days old now and I am beginning to wonder if I can generate scum fast enough to keep up with them. I did a head count this morning and have three bowls, each of which holds a population of ~100 urchins, and a bowl that contains another 33. The first three bowls are going through food very quickly, and I change their scum slide every 2-3 days. And, since eating results in pooping I change the water every day.

Hungry urchins looking for food:

Juvenile sea urchins (Strongylocentrotus purpuratus), 18 April 2015 © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus), 18 April 2015
© Allison J. Gong

After they eat through the food on the upper surface of the slide, the urchins migrate to the lower side and begin munching there. Once most of the food is gone they go on the prowl, and I'll find them on the sides of the bowl looking for something to eat. In the photo, can you tell which urchins are on the underside of the slide? Most of them are, actually. They're the ones where you see a darkish ring around the center; that ring is the peristomial membrane that surrounds the mouth. That's what "peristomial" means, by the way, but you didn't need me to tell you that, did you?

Changing the slide involves using a paintbrush to pick up each urchin and drop it into the new bowl. It's rather tedious but is also the most convenient time to count them. And they do seem happy every time they find themselves in a new bowl with plenty to eat.

Baby urchins with lots of food to eat now:

Juvenile sea urchins (Strongylocentrotus purpuratus), 18 April 2015 © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus), 18 April 2015
© Allison J. Gong

Using the clue I gave you up the page, can you find the single urchin on the underside of the slide?

Having pentaradial symmetry means that the urchins don't have forward-backward or left-right axes, and they can and do move in any direction on the horizontal plane. They do, however, have a strong oral-aboral axis, and they definitely have a preference for how their bodies should be oriented with respect to gravity. The normal position is to have the mouth (oral surface) facing downward, with the opposite side (aboral surface) facing up. And for this species, at least, even in the field you don't see them sticking upside-down on overhanging surfaces, unless they have a vertical surface to hang onto as well. These little guys can hang onto the underside of the slide because they're not very heavy yet. Once they get bigger, it'll be a lot more difficult for them.

Setting an urchin down on its aboral surface, with its mouth facing up, will keep it from crawling away very quickly, but sooner or later it will right itself and take off. Even my little babies don't like to be flipped upside-down. This guy was pretty stubborn at first and spent a minute waving its tube feet at me while I looked at it through the microscope, but then took another minute or so to get down to the business of turning over. Don't worry, I cut out the boring first minute so this clip shows only the action sequence.

Quite clearly, urchins don't care about forward-backward or left-right, but they do care about up-down. Like most animals that live in essentially two dimensions, adult urchins prioritize knowing the orientation of one's body with respect to gravity. But remember those bilateral larvae? They swim in any direction in their pelagic, three-dimensional world, although the body always moves through the water in a particular orientation (arm tips first). It seems this is another aspect of metamorphosis that gets overlooked: the transition from a bilateral body that swims in both the horizontal and vertical planes (three body axes, weak response to gravity), to a body with pentaradial symmetry that walks only in the horizontal plane (one body axis, strong response to gravity). Hmm. I'm going to have to think about that for a bit.

Most of the animals that we are familiar with (think of any pets you've ever had) have bilateral symmetry: they have a head end and a tail end, a left and a right, and a top and a bottom. In scientific terms that translates to the anterior-posterior, left-right, and dorsal-ventral axes. Also, most bilateral animals are elongated on the anterior-posterior axis and have some sort of cephalization going on in the anterior end of the body; in other words they have a head, or at least a concentration of neural tissue and sensory structures in the part of the body that encounters the environment first.

Even your basic worm meets all these criteria. Here's a video clip of Nereis sp., an intertidal polychaete worm. The body is conspicuously segmented, as this animal is a somewhat distant relative of earthworms. The body symmetry is clearly bilateral, and you can see that it has an anterior end, which in this case is defined by both the direction of locomotion and the presence of a head:

As "normal" as bilateral symmetry may seem, there are many animals that have a completely different type of symmetry. The cnidarians, for example, are the largest group of animals with radial symmetry. This means that instead of being elongated along an anterior-posterior axis, these animals' bodies are either columnar or umbrella-shaped. In either case, when you look down on them you see a circular shape:

Anthopleura sola, photographed at Natural Bridges State Beach © Allison J. Gong
Anthopleura sola at Natural Bridges State Beach
© Allison J. Gong

An animal with this sort of body plan obviously has no head--no eyes, nose, or concentration of either neural or sensory structures. Being a sea anemone, it lives attached to the sea floor and doesn't walk around much, so there's also no locomotory clue as to a possible anterior end, either. Rather than have most of its neural apparatus located in a particular region, its nervous system is diffusely scattered over the entire body. This animal has the advantage of meeting its environment from all sides and across all of its external surface. It can't be snuck up on, because it has no front or back.

Let's now return to the echinoderm pentaradial symmetry. As you might imagine, the five-way symmetry of echinoderms has strong implications both for other aspects of the animal's anatomy and the way that it interacts with its environment.

Take the example of a sea star:

Dermasterias imbricata at Pigeon Point, 18 January 2015. © Allison J. Gong
Dermasterias imbricata at Pigeon Point, 18 January 2015.
© Allison J. Gong

Echinoderms are structurally more complex than cnidarians, with distinct internal organs. The central disc contains most of the organs, but there are extensions of both the gut and the gonads in each of the five arms. Although, like the cnidarians, the echinoderms don't have a centralized nervous system, they do have very simple eyes that can detect light and dark. And guess where, in an animal with a form of radial symmetry, the eyes are located? Hint:  Think about how the animal encounters its environment. Yes, the eyes are in the tips of the arms, along with chemosensory receptors. Makes sense, doesn't it?

Pentaradial symmetry also affects how an animal locomotes. Since they have no front or back, sea stars and sea urchins can walk in any direction. They can also change the direction of locomotion easily, without needing to turn around the way we would.

There's a natural human tendency to regard creatures like us as somehow better than those different from us. I try to teach my students that complex is not always better (think of the pervasive damage done to a person who has suffered a major brain or spinal cord injury); that there are multiple types of complexity (morphological, behavioral, reproductive, and life cycle); and that the best way to understand an animal is to put yourself in its "shoes" and try to imagine what its life is like, with its anatomy, physiology, and lifestyle. It can be difficult to shed our human-centric biases, but we have to put them aside at least temporarily if we truly want to make sense of what's going on in the world around us.

2

Finally! At long last I have evidence that my juvenile urchins have mouths and are feeding. A week ago I put a batch of seven teensy urchins onto a scuzzy glass slide and have been watching them daily ever since. And yesterday, just as I was beginning to worry that they'd never be able to eat, I saw that some of them had eaten little tracks through the scuzz on the slide.

Here's an example:

Juvenile urchin (Strongylocentrotus purpuratus), age 73 days, 3 April 2015. © Allison J. Gong
Juvenile urchin (Strongylocentrotus purpuratus), age 73 days, 3 April 2015.
© Allison J. Gong

The little urchin still has a test diameter of about 0.5 mm, so it hasn't really started growing yet. However, see the squiggly dark paths? Those are areas of the slide that have been eaten clean. The scuzz is algal in origin, giving the slide an overall brownish-green color, so the scuzz-free parts of the slide are clear--or dark, actually, given that I took this photograph against a black background--having been munched clean by the urchin's teeth. And the other bit of evidence that I saw? Poop! Yes, there were fecal pellets on the slide, which proves that the little urchin has a complete functional gut.

And those small round golden objects you see on the slide? Those are big centric diatoms of the genus Coscinodiscus. They are the only local diatoms that I know of that are big enough to be seen with the naked eye.

Lastly, because I just can't seem to stop myself, here's a video of the little urchin:

I love the sculpturing of the spines. And do you see that three-pronged structure at about 9:00 on the urchin? That's a pedicellaria. On adults of the genus Strongylocentrotus there are four types of jawed pedicellariae, three of which, in my experience, are easy to distinguish on a living specimen. But in this young an animal I can't yet tell how many types of pedicellariae it has. I suppose that the formation of pedicellariae might be the next event for me to follow as these urchins continue to grow and develop.

Anyone who went to graduate school in the sciences remembers what oral exams are like. I remember not having any fun at all in mine, and by the time I was dismissed I wasn't sure what my own name was. Fortunately, that is all ancient history and now I get to spend my time performing a different kind of oral examination on other creatures.

My oldest urchins are now 17 days post-metamorphosis and I've been watching them to see when their mouths break through. It seems to me that 17 days is a long time, but the time is near. Besides, the animal is always right. In the urchin that I examined closely the five teeth of Aristotle's lantern are very close to breaking through the thin membrane covering the mouth opening. The teeth are also much more active than they were a week earlier, as you can see in this short video clip:

I also checked out another tiny urchin and noticed that this individual has startlingly red buccal tube feet:

Juvenile sea urchin (Strongylocentrotus purpuratus), age 66 days, 27 March 2015. © Allison J. Gong
Juvenile sea urchin (Strongylocentrotus purpuratus), age 66 days. 27 March 2015.
© Allison J. Gong

Sea urchins have five pairs of large tube feet on the oral surface, surrounding the mouth. As with all tube feet, the buccal tube feet are part of the animal's water vascular system and are situated in the ambulacral region of the test; they are used to manipulate and grab food. In adults of this species, the buccal tube feet are much larger and more robust than the other tube feet. In this little guy the tube feet are noticeably red, but I can't yet tell if they're bigger than the others.

And just for kicks I took another video:

Yesterday I transferred seven urchins onto a glass slide that I've had basking in the sun in an outdoor tank to develop a thin film of algae. As the urchins' mouths become functional they should be able to start munching on the scuzz on the slide. So far they seem happy to be crawling around on the slide but this morning I didn't see any signs that they'd actually eaten anything.

Juvenile sea urchins (Strongylocentrotus purpuratus), age 67 days. 28 March 2015 © Allison J. Gong
Juvenile sea urchins (Strongylocentrotus purpuratus), age 67 days. 28 March 2015.
© Allison J. Gong

The waiting continues....

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.

Juvenile urchin (Strongylocentrotus purpuratus), age 55 days. Five days post-metamorphosis, 11 March 2015. ©Allison J. Gong
Juvenile urchin (Strongylocentrotus purpuratus), age 55 days. Five days post-metamorphosis, 16 March 2015.
©Allison J. Gong

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!

3

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.

Newly metamorphosed juvenile urchin (Strongylocentrotus purpuratus), 11 March 2015. Age = 50 days. ©Allison J. Gong
Newly metamorphosed juvenile urchin (Strongylocentrotus purpuratus), age 50 days, 11 March 2015. 
© Allison J. Gong

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:

Juvenile sea urchin (Strongylocentrotus purpuratus), 11 March 2015. ©Allison J. Gong
Juvenile sea urchin (Strongylocentrotus purpuratus), age 50 days, 11 March 2015.
© Allison J. Gong

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.

7

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.

Competent pluteus larva of Strongylocentrotus purpuratus, 6 March 2015. ©Allison J. Gong
Competent pluteus larva of Strongylocentrotus purpuratus, age 45 days, 6 March 2015.
© Allison J. Gong

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.

Metamorphosing larva of Strongylocentrotus purpuratus, 8 March 2015. Age = 47 days. ©Allison J. Gong
Metamorphosing larva of Strongylocentrotus purpuratus, age 47 days, 8 March 2015.
© Allison J. Gong

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.

Metamorphosing larva of Strongylocentrotus purpuratus, photographed with dark-field lighting, 8 March 2015.  ©Allison J. Gong
Metamorphosing larva of Strongylocentrotus purpuratus, photographed with dark-field lighting, age 47 days. 8 March 2015.
© Allison J. Gong

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.

Metamorphosing larva of Strongylocentrotus purpuratus, showing spines and tube feet, 9 March 2015. Age = 48 days. ©Allison J. Gong
Metamorphosing larva of Strongylocentrotus purpuratus, showing spines and tube feet, age 48 days. 9 March 2015.
© Allison J. Gong

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.

3

Anybody who has visited one of the sandy beaches in California has probably seen kids running around digging up mole crabs (Emerita analoga). These crabs live in the swash zone at around the depth where the waves would be breaking over your ankles, moving up and down with the tide. They are bizarre little creatures, burrowing backwards into the sand with just their eyestalks and first antennae reaching up into the water.

Although it's called a mole crab, Emerita's external anatomy isn't very similar to that of other crabs. For one thing, it doesn't have claws. In fact, its legs are quite unlike the legs that you'd see in a typical crab. Check out Emerita's appendages:

External anatomy of Emerita analoga
External anatomy of Emerita analoga

The crab's head faces to the left in this diagram. The real surprise that these little crabs hide is the nature of the second antennae. Usually the crab keeps these long, delicate antennae protected under its outer (third) pair of maxillipeds. This is why you don't see them when you dig up a mole crab.

You do see them when the crabs are feeding. As a wave washes over the crab, it extends the second antennae and pivots them them around on ball-and-socket joints. The feathery antennae catch particles in the water, then are drawn underneath the maxillipeds so the food can be slurped off and eaten.

Here's a top-down view of two Emerita feeding. The purple-grayish thing in the field of view is a sand dollar (Dendraster excentricus).

This side view gives a better angle of what's going on:

I find these little crabs quite captivating. I love how they rise up when I put food into their tank.  Watching them feed always makes me smile.

3

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:

41-day-old pluteus larva of Strongylocentrotus purpuratus, 2 March 2015. ©Allison J. Gong
41-day-old pluteus larva of Strongylocentrotus purpuratus, 2 March 2015.
©Allison J. Gong

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!

 

 

Until recently I hadn't closely observed what it looks like when a leather star (Dermasterias imbricata) succumbs to wasting syndrome. When I had the outbreak of plague in my table almost 18 months ago now, my only leather star was fine one day and decomposing the next, so I didn't get to see what actually happened as it was dying.

(Un)fortunately, one of the leather stars at the marine lab started wasting a bit more than two weeks ago, and this time I was able to catch it at the beginning. This animal wasn't in my care so I didn't check on it as frequently as I would if it had been living in one of my tables, but one of the aquarists pointed it out to me when it began getting sick.

The first symptom was a lesion on the aboral surface. I say "lesion" but it's more of an open wound.

Dermasterias imbricata with aboral lesion, 2 February 2015. ©Allison J. Gong
Dermasterias imbricata with aboral lesion, 2 February 2015.
© Allison J. Gong

You can see that the animal's insides are exposed to the external environment. In the photo above the whitish milky-looking stuff is gonad (I'm pretty sure this animal was a male) and the beige ribbon bits are pyloric caeca, essentially branches of the stomach that extend into the arms. What typically happens along with the development of lesions like this is an overall deflating of the star as the water vascular system and other coelomic systems become increasingly compromised, and the tendency for the animal to start tearing off its arms.

Which results in this, a week later:

Wasting Dermasterias imbricata, autotomizing its arm, 9 February 2015. ©Allison J. Gong
Wasting Dermasterias imbricata, autotomizing its arm, 9 February 2015.
© Allison J. Gong

This poor animal had torn its arm off, and continued to live for a while. I find it fascinating that the lack of a centralized nervous system means that this animal literally didn't know it was dead. It was finally declared officially dead two days later. Compared to how quickly wasting syndrome kills the forcipulates that I've seen (Pisaster, Pycnopodia, and Orthasterias), the leather stars take a long time to die--several days from start to finish, opposed to a matter of hours as I saw with my stars. The leathers didn't seem to be hit as hard by the first wave of the disease outbreak, either. Is Dermasterias somehow able to fight off the infection a bit longer? It would be interesting to know, wouldn't it?

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