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As a long-time student of invertebrate zoology I have for most of my life appreciated the immense variety and ingenuity of animal body plans. And one of the things I've always found the most intriguing is the pentaradial symmetry of echinoderms. I remember thinking, the first time I encountered a live echinoderm (probably a star at the beach, when I was in elementary school), "Wow. Five arms. That's weird." And now, all these years later, knowing a bit more than I did then, I still find it weird.

Pentaradial symmetry doesn't occur in any animal group except the echinoderms, and even they begin life as bilateral larvae. Remember these guys?

31-day-old pluteus larva of Strongylocentrotus purpuratus, 20 February 2015. © Allison J. Gong
31-day-old pluteus larva of Strongylocentrotus purpuratus, 20 February 2015.
© Allison J. Gong

There isn't a more perfect example of bilateral symmetry out there. Although, even at this stage there are developments within the body that are beginning to interrupt the bilateral-ness of the animal. This is a picture of the animal lying on its dorsal surface, so you are looking down on its ventral surface. See how, to the (animal's) left of the stomach there is a darkish squiggle running mostly horizontally between the stomach and the skeletal rod of that arm, that you don't see on the right side? That squiggle indicates where the juvenile rudiment, which contains the first five tube feet of the water vascular system, will form.

As we've seen already, the rudiment grows to the point that it occupies most of the internal space of the pluteus larva. When the larva settles it lands on its left side, where the tube feet erupt during metamorphosis. The end result is (hopefully!) a little urchin walking around on tube feet that it didn't have the day before. Well, I guess technically it had them, but there weren't useful yet. And the body symmetry will have changed from the bilateral larval form to the pentaradial juvenile.

When looking at a live sea urchin it can be difficult making sense of all the stuff that's going on. A sea urchin is a very active animal, with spines and tube feet waving all over the place. It looks like total chaos at first, but examination of a naked sea urchin test (the endoskeleton made up of interlocking calcareous ossicles) lends a lot of insight into the body plan of this animal.

Here's a cleaned intact urchin test:

Cleaned test of adult sea urchin (Strongylocentrotus purpuratus), 13 April 2015. © Allison J. Gong
Cleaned test of adult sea urchin (Strongylocentrotus purpuratus), 13 April 2015.
© Allison J. Gong

Now the pentaradial symmetry of this body plan becomes apparent. You can see that there are five regions of doubled rows of plates that have little holes in them. The holes are where the tube feet protrude to the outside, and the plates that bear them represent the animal's ambulacrum, or ambulacral region. The structures of the water vascular system run up along the inside surface of the test in the five ambulacra. The ambulacral regions are separated from each other by five intermabulacral regions, which do not have holes for tube feet because there are no tube feet here. The bumps on the test are called tubercles, and are where the spines attach. The tubercles fit into the base of the spines like a ball-and-socket joint, similar to our shoulder, that allows the spines to rotate 360˚. You can see this for yourself the next time you have a live urchin available: touch one of the spines and observe how the animal reacts.

There is interesting stuff going on at the apex of the urchin, too. The five large-ish holes, one at the point of each interambulacral area, are the gonopores. When I shoot up urchins to make them spawn, the gametes are released from these holes. The arrangement of the gonopores in the interambulacral regions makes sense, once you remember that on the inside of the test the ambulacral areas are where the water vascular system structures (including tube feet) are located. The only space available for the gonads is in the interambulacral areas. I know, it's confusing. And people think invertebrates are simple. Ha!

That may be enough to digest about urchin symmetry for now. I'll have more on this soon, including the implications of pentaradial symmetry. Stay tuned!

On Easter Sunday we got a call about a big swarm of bees in our neighborhood. The woman who called has a couple of hives in her backyard, one of which had swarmed three weeks earlier. We caught that swarm and installed it into our Green hive at our house. This time it was her other hive that swarmed, and the swarm was HUGE. The bees went to her neighbor's yard and gathered in a tree about eight feet off the ground. It was very considerate of them to end up in such an accessible spot!

The swarm consisted of two lobes, each of which was about 1/2 meter deep and 1/3 meter wide. It probably contained 20,000 bees. See?

Large bi-lobed swarm of bees, 5 April 2015.
Large bi-lobed swarm of bees, 5 April 2015.

Each of those lobes easily contains as many bees as were in the packages that we bought when we got our very first bees four years ago. It was too big a swarm to simply shake into a box, so we tried lifting a hive body box with frames under the bottom of the lower lobe, hoping that the bees would find the smell of wax enticing and go into the box on their own. They proved to be not quite that cooperative, and we had to brush and scoop them into the box. But they did go willingly once we got started.

We lifted a box of frames under the swarm to entice the bees inside, 5 April 2015.
We lifted a box of frames under the swarm to entice the bees inside, 5 April 2015.

Then we brushed and scooped.

We brushed bees off the lobe into the box, 5 April 2015.

Eventually we got most of the bees from the lower lobe into the box. Then we shook the branch with the upper lobe to drop them into the gathering of their sisters. This put a lot of bees in the air, and we maneuvered the lid onto the hive and backed away to let everything settle down. As we were leaving we saw that some of the bees were lifting their abdomens into the air and fanning their wings. This behavior disperses a pheromone that is secreted from the Nasonov gland in the bees' abdomen, a sort of "this is home" type of thing. They generally won't do this unless they have a queen, so somewhere in that mishmash of 20,000 or so bees there's a queen.

Later in the evening, when it had cooled down quite a bit, we brought the hive home and placed it on our yard, between the Green and Purple hives. Now we have a Blue hive at our house!

The three hives in Apiary #1. The new swarm is in the Blue hive. 9 April 2015. © Allison J. Gong
The three hives in Apiary #1. The new swarm is in the Blue hive. 9 April 2015.
© Allison J. Gong

We can infer a bit about the origin of this swarm and the hive it came from. The woman whose hive threw it has two hives in her yard. One hive threw a swarm three weeks ago. On that same day, the beekeeper inspected both of her hives and found lots of queen cells in the hive that threw the swarm (I'll call this Hive 1, just for the sake of convenience) and none in the second hive (Hive 2). Then this humongous swarm emerged on Easter Sunday. That afternoon the beekeeper inspected her hives again and found several queen cells in Hive 2. So in the three weeks that passed between swarms from this beeyard, the bees in Hive 2 decided that they were going to swarm. They produced a bunch of queen cells, and probably just as the first new queen was about to emerge and announce her presence with authority about half the colony dragged away the old queen and landed in the neighbor's tree.

Typically, the first swarm that a colony throws in the biggest. One-third to one-half of the bees can leave, and they do drag along the old queen. One of the queen's daughters will have to emerge from her queen cell, take her mating flights and be successfully inseminated, and return to take over the egg-laying duties of the original hive. In the meantime, the swarming bees (with the old queen in tow) gather in a temporary resting spot somewhere and send out scouts to search for suitable place to set up a permanent residence. The scouts return to the swarm and try to persuade their sisters that they've found the perfect spot. The decision-making process can take just a few hours, or stretch out and last for days. A beekeeper on the lookout for swarms generally has to act quickly once a swarm has been spotted, because there's no way to know how long the bees will hang out and be catchable.

All told, this is the fourth swarm we've caught so far this season. We have populated all of our hives and used up most of our equipment. We will have to do a honey harvest in the next month or so, because a couple of the hives are getting pretty tall. In the short term, at least, there will be lots of honey for anybody who wants it.

 

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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.

1

Everybody knows that climate change is a hot--pun intended!--topic in both science and politics these days. Here along the northern California coast it seems that sea surface temperature (SST) has been elevated for at least a year now. I remember a time, not too many years ago, when I would put my hands into my seawater table and they'd go numb after several minutes. This told me that the water temperature was in the 11-12 ºC range. But that hasn't happened for a while, and recently I'd put my hands in the water and it didn't even really feel cold. My trusty not-fancy thermometer has been telling me that the temps have been hovering at around 14ºC.

The other day it occurred to me that I have a 20-year record of water temperatures from my seawater table, which is a pretty fair proxy for SST in the area. The numbers may not jive exactly with SST data produced by oceanographic instruments, but the trends should be very similar. If you click on the figure you'll be able to see a larger version of it.

Temperature in my seawater table at Long Marine Lab, July 1994-March 2015. © Allison J. Gong
Temperature in my seawater table at Long Marine Lab, July 1994-March 2015.
© Allison J. Gong

There are a couple of notable trends in these data. I was pleased to see a strong signal for the 1997-1998 El Niño event, visible as a prolonged period of elevated temperatures in the fall and winter. This was followed by a La Niña in 1998-1999, when temperatures were lower than average for a few months. Aside from those events, SST fluctuates between about 16º in the summer-fall and 11-12º in the winter-spring.

One more thing. Take a look at the far right end of the graph. Notice what appears to be a cooling trend so far in the spring of 2015?

Here are the data from March and the first three days of April:

Temperature in my seawater table at Long Marine Lab, March-early April 2015. © Allison J. Gong
Temperature in my seawater table at Long Marine Lab, March-early April 2015.
© Allison J. Gong

So there's definitely a cooling trend in the past few days. The interesting question is:  Why is this happening now, when it hasn't happened for about two years?

The answer, in a nutshell, is the wind. For the past week or so, we've had screaming afternoon winds at the marine lab, coming from the northwest. Northwest winds blowing down the coast drive the process of coastal upwelling, which results in cold water rising to the surface; it usually takes 3-4 days of sustained winds to start upwelling. This upwelled water, in addition to being cold, also contains a lot of nutrients, which are used as fertilizers by the primary producers of the marine ecosystem, the phytoplankton. Most of the phytoplankton are photosynthetic unicellular algae (NOT plants) that harvest the energy from sunlight and use it to fix carbon dioxide into organic molecules. The fixed carbon in turn feeds grazers such as copepods, which are then eaten by small predators, which are eaten by larger predators and so on up the food chain.

What this all means is that we may, for the first spring in two years, be getting some productive upwelling. I don't think I'm the only marine biologist in the area who is looking forward to seeing whether this apparent upwelling continues. If it does, then we should see the biota respond accordingly. Mind you, a four-day streak does not indicate a long-term return to typical spring upwelling conditions, and it may be merely a blip in the warmer conditions that are the new normal for us, but it is a stronger signal than we've seen in a few years. In any case, I will be keeping an eye on both the water temperature and the critters living in it.

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When I was in graduate school I found myself drawn to the "old-fashioned" skills of classical zoology:  observation of and experiments with living animals. I had, and still have, very little interest in the new-fangled high-tech methods of studying animals, and part of me strongly resents having to homogenize an animal to know what its name is. I leave that sort of biology to the systematists because, after all, the animal doesn't care what name we give it, and the names themselves are merely a way for us humans to communicate amongst ourselves (although in the best of all possible worlds the taxonomy reflects the evolutionary history of the group in question, which is itself a Very Useful Thing). I am much more interested in what the animal actually does as it goes about living its life.

A priest I know has said repeatedly that observation is a passive activity, and I've told him that he wouldn't think so if he actually knew how to do it properly. Careful observation takes a tremendous amount of mental focus, and like all other skills gets easier the more one does it. But I fervently disagree that observation is at all passive. If it is, then you're not doing it right. There are tricks of the trade that facilitate observation, of course, such as sketching and annotation, and I have all of my students do at least some drawing. My upper-division students keep a lab notebook that consists of drawings and notes that document their lab activities during the semester. Sometimes they grumble about having to draw and most of them worry about their self-perceived lack of artistic skill, but they do come around and realize that drawing forces them to really pay attention to what they observe. And no, I do not allow them to substitute photographs for drawings.

In graduate school I was fortunate enough to fall under the tutelage of one Todd Newberry, who directly and indirectly shaped how I think about animals and biology. He also taught me a specific type of scientific illustration, which he himself had learned from a pair of biologists in Paris. I used these techniques to draw the life history stages of my dissertation study organism, the moon jelly Aurelia sp. My favorite of this group of drawings is the juvenile medusa, or ephyra:

Pen-and-ink drawing of ephyra of Aurelia sp. Scale bar indicates 1 mm. © Allison J. Gong
Pen-and-ink drawing of ephyra of Aurelia sp., oral view. Scale bar indicates 1 mm.
© 2015 Allison J. Gong

Several years ago now, I put together a handout of sea star larval development for my invertebrate zoology students. These pen-and-ink drawings were done using the same techniques, but I left out the stippling as the larval anatomy became more complex. Part of the beauty of drawing as a means of documenting my observations is that I can select what to include which obviously reflects what I feel is important. Sometimes the decision about what to omit helps me focus on the structures that really matter, which of course depends on the purpose for any particular drawing.
Patiria development

 

So there are some of my drawings. One of my goals for the summer is to put together a suite of similar drawings of my sea urchin larvae, to complement the sea star set. It will give me an excuse to clean out my technical pens--they've been sadly neglected for years, and I hope they can be revived--and spend time with my photographs. I also have plans for some pencil drawings on black paper. I'm looking forward to tapping into the artistic side of my brain again for a while!

3

This afternoon we inspected our Purple hive to check on how the queen is doing and see if they need more space for either brood or honey. For the past few weeks I've been able to smell that they're making some very tasty honey (it smells like buttered popcorn) and we want to make sure that they have plenty of room to continue storing and curing nectar. I hope we'll be able to harvest some of that popcorn honey later this spring.

These bees are very calm and sweet. I love how they look up from between the frames.

We look down on the bees and they look up at us. 29 March 2015. © Allison J. Gong
We look down on the bees and they look up at us. 29 March 2015.
© Allison J. Gong

This hive isn't too crowded but they are busy bringing in nectar. The queen is doing her job, and although the brood might be a little spotty for us to be entirely convinced that all is well. It could just be that she's back-filling cells from which young bees had emerged, and those cells might happen to be not in a contiguous patch. We did find the queen, and she's a big fat one. We were able to catch her in a little cage and put a little blue dot on her thorax. Her daughter, on the outside of the cage, could smell her mother and was very reluctant to leave.

Here's the queen with a blue dot on her thorax. 29 March 2015. © Allison J. Gong
Here's the queen with a blue dot on her thorax. 29 March 2015.
© Allison J. Gong

The queen needs to be released back into the hive pretty quickly, as she depends on her worker daughters to be fed and kept warm. While it's always tempting to do something dramatic like release her in the front of the hive and watch her walk into the bottom, there's always a risk of her flying away instead of cooperating with the beekeeper. Or she could get snarfed by a bird. So we released her into the top of the hive and watched her crawl down between the frames. You can watch here:

See how the workers respond to the presence of the queen? They know that if she's there then all is well with the hive, and they quickly rush to surround and attend her. And now we'll be able to spot this queen more easily when we inspect the hive because of her blue dot, and if for some reason the workers decided that they need to supercede their mother, we'll be able to recognize the new queen because she won't be wearing a blue dot.

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!

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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.

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