Category: Marine invertebrates

Today was a big day for me. I got to graduate some of my baby urchins from glass slides onto coralline rocks. They were growing very quickly on the slides, chowing down on scum faster than I can grow it, so now it’s time for the biggest ones to really put their Aristotle’s lanterns to the test and chew up some rocks.

Coralline algae are red algae that have calcified cell walls, giving them a crunchy texture. They come in two morphs–erect branching forms and as encrusting sheets–and are pink in color. The corallines that I’m using for urchin food are growing as sheets on rocks. In the field it is not uncommon to see little urchins on coralline rocks, and their teeth are more than capable of grinding up the calcified algae.

So today I used my trusty frayed paintbrush to scoop up a total of ~90 urchins from their slides and dropped them onto rocks. I should have taken a picture of this valuable tool of mine, so you can see just how low-tech (and cheap!) my type of marine biology is.

The largest urchin on this rock has a test diameter of ~2800 µm. Almost 3 mm now!

Here’s a closer view of three of the urchins in the photo above:

It didn’t take long for the little urchins to start crawling around on their new substrate. I think they’ll be happy with this more natural surface to explore and food to eat.

In the meantime, the remaining babies will stay in their jars or on their slides, eating scum. I will continue graduating urchins to rocks as they get too big for slides, feeling more nostalgic each time.

Just think, only 97 days ago these urchins were zygotes! It’s not often that you can say that you’ve known an organism for its entire life, from the moment of fertilization. I am grateful for the privilege of having the opportunity to undertake such an intimate study of these animals’ lives. Although I try at least once every year, this is my first successful urchin spawning since 2012. Those animals, by the way, are what I call my most perfect urchins because, well, they just are. I had originally thought I could use them for dissection, but after caring for them as larvae and the three years since they’ve metamorphosed, I just can’t bring myself to sacrifice them. They are simply perfect.

I don’t think I could ever get tired of this.

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.

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:

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:

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.