Hitting the wall

We humans use the term “hitting the wall” when we find ourselves in situations in which progress is elusive despite extreme effort. For endurance athletes or anyone doing any serious physical training it can mean not being able to break one’s personal best time for a race, or not being able to continue getting measurably stronger. For me, it felt as though much of graduate school involved hitting various hard walls and coming up with a headache. Maybe it’s like that for everyone, but from the perspective inside my own head it sure did seem that I was struggling harder than most.

Sometimes the wall is literal rather than figurative. And for small animals, a surface that we might be able to break through without any effort at all (or without even perceiving it as a surface) can be an impenetrable barrier. The biggest of my baby sea urchins has a test diameter of ~2700 µm now; including spines it probably measures a bit bigger than 3 mm across. While tiny urchins can use the surface tension at the air-water interface to crawl, this big guy is too heavy now to stick to the underside and would fall off of it.

However, I thought this urchin might be able to use the surface tension of a water bubble to grab onto and right itself. A hypothesis like this requires empirical data, so I picked up the little urchin and plopped it, oral side up (so, upside-down), in a bubble of water on a depression slide. As I expected, the urchin crawled over to the edge of the bubble and I could see its tube feet attaching to the underside of the surface tension. Watch here:

I watched continuously for about a minute, and the urchin never did figure out how to turn itself over. I think there may be two reasons for this:

  1. The water bubble at the edge of the depression in the slide was very shallow, probably not deep enough to cover the whole animal for the few seconds that it would be positioned on its edge. If, to the animal, the surface tension proved to be impenetrable, then a comparable situation would be for me to pin you, lying on your side sandwiched between a solid wall in front of you and a hard board against your back, then telling you to roll over. You wouldn’t be able to do it, either.
  2. The surface tension of the bubble may simply not have been firm enough for the urchin to grab it and pull. Urchins use their tube feet to pull against hard objects, and adults can actually generate enough leverage to push bricks around. Obviously, it’s easier to pull (or push) hard against a solid surface (say, a rock or the side of a glass bowl) than a malleable one such as the inner surface of a bubble.

Now, I’m not by any means an expert in biomechanics, but it seems pretty clear to me that the surface tension is either too hard or too soft to be used by an urchin this size to right itself. Smaller urchins would just crawl on the underside of the surface tension until they reached the side or bottom of the container, and larger urchins would push right through it to reach for whatever was on the other side. I may need to do some more experiments with these urchins and bubbles of various sizes.

Just for fun I took another video of the same animal, this time situated upright. It was much happier this way.

See?  It has pedicellariae in addition to spines and tube feet. It’s also getting easier to distinguish the ambulacral and interambulacral areas. These urchins are already starting to develop some purple coloration. Typically they go through a greenish stage before turning purple; maybe that will come later. We’ll have to see.

Posted in Marine biology, Marine invertebrates | 1 Comment

Eating, pooping, and turning over

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.

Posted in Marine biology, Marine invertebrates | Leave a comment

The good father

In case you were wondering, here’s what our red-tailed hawk nest looks like from our deck:

Red-tailed hawk nest in eucalyptus tree, 16 April 2015. © Allison J. Gong

Red-tailed hawk nest in eucalyptus tree, 16 April 2015.
© Allison J. Gong

See that little red circle? That’s the nest. Without the spotting scope, even with binoculars it’s hard to find.

WITH the spotting scope, we can spy on the nest from our deck. And using a nifty gadget that clips an iPhone to the lens of the scope, we can take photos and video. This video shows the dad feeding the bigger of the two chicks. I can’t see what the prey is, but I hope to god the hawks are eating a lot of gophers.

In hawks, as is typical for raptors, the female is larger than the male. But when there’s only one bird on the nest it’s difficult to tell if it’s the bigger one or the smaller one. In general, the dad has longer looking legs, while the female looks a bit bulkier and heavier. We know this parent is the dad because he was seen flying in with food. The mom hopped out of the nest for a bit of respite while her mate took over the feeding duties. I think that as the chicks get bigger they’ll need more food, and both parents will have to spend time away from the nest foraging.

Posted in Birds | 2 Comments

Symmetries II

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, photographed 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.

 

 

Posted in Marine invertebrates | Leave a comment

Feeding babies

For the past several weeks we have been watching a pair of nesting red-tailed hawks across the canyon. They built a nest in a eucalyptus tree, then the female began incubating a clutch of eggs. The male would bring her food and spell for short stints on the nest, while we spied on them through the spotting scope. The phrase “eyes like a hawk” is very a propos, I found. Every time I trained the scope on her she looked right back at me through the other end. It was a little unnerving.

We first noticed chicks in the nest about two weeks ago, I think. We could see a parent eating and feeding something (presumably babies) in the nest but couldn’t see exactly what was going on. Several days ago now, the babies got big enough for us to see over the edge of the nest. They were floppy fluffy white blobs.

Today I finally got some pictures of the babies. There are two chicks in the nest, and one looks quite a bit bigger than the other. I took some photos through the scope but never managed to get even a half-way decent shot of both chicks at the same time. Here’s the best that I was able to capture today:

Red-tailed hawk chick in nest, 13 April 2015. © Allison J. Gong

Red-tailed hawk (Buteo jamaicensis) chick in nest, 13 April 2015.
© Allison J. Gong

I also got lucky enough to see the female return to the nest. I think she had been perched in a tree on our side of the canyon while her mate was flying above, screaming loudly. There must have been a mid-air prey exchange that I missed.

Female red-tailed hawk (Buteo jamaicensis) returning to nest with prey, 13 April 2015. © Allison J. Gong

Female red-tailed hawk (Buteo jamaicensis) returning to nest with prey, 13 April 2015.
© Allison J. Gong

The female then proceeded to tear apart whatever prey item it was, and feed it to the chicks.

If you can stomach the somewhat shaky video, I did catch about a minute-and-a-half of the feeding.

From this angle I couldn’t see if both chicks were getting fed, but they are both growing. So far these hawks are good parents!

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Symmetries I

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, is a gonopore. 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!

Posted in Marine biology, Marine invertebrates | 1 Comment

Catching a mondo swarm

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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