Answer: When it’s a snail! Yes, there are snails that secrete and live in white calcareous tubes that look very similar to those of serpulid polychaete worms. Here, see for yourself:
The worms secrete calcareous tubes that snake over whatever surface they’re attached to. When the worm is relaxed, it extends its delicate pinnate feeding tentacles and uses them to capture small particles to eat; they are what we call suspension feeders.
But there are gastropods that secrete calcareous tubes, too. They are the vermetid snails, the local species of which is Thylacodes squamigerus. This is one of my favorite animals in the low intertidal, probably because it is so delightfully un-snail-like.
There are three individuals of T. squamigerus in this photo:
Thylacodes is also a suspension feeder, but it gathers food in a very different way. When submerged, it spins out some sticky mucus threads that catch suspended particles, then reels in the threads and eats them.
So how would you tell these animals apart if you see them? Here’s a hint: Look at the tubes themselves.
I invite you to use the comments section to tell me how you’d distinguish between Serpula and Thylacodes.
This morning I took a small group of Seymour Center volunteers on a tidepooling trip to Point Piños (see red arrow in the photo below). Point Piños is a very interesting site. It marks the boundary between Monterey Bay to the right (east) of the point and the mighty Pacific Ocean to the left (west).
Map of Monterey Bay. Red arrow indicates Point Piños.
As is my usual habit, we began our exploration on the Pacific side of the point. Almost immediately, Victoria found an octopus! And a couple of meters away, she found another one!
As we approach the summer solstice, the algae and seagrasses are at their most lush. Point Piños is a fantastic site for algal diversity; every time I come here I want to take some back with me so I can study it at the lab. Alas, collecting at Point Piños is not allowed even for someone (like me) who holds a valid scientific collecting permit.
And yes, that log-like object towards the upper-left corner is a harbor seal (Phoca vitulina). A handful of seals were hauled out on the rocks.
However, I was much more interested in the invertebrates. I wasn’t looking for anything specific, but in the back of my mind I was keeping track of certain nudibranchs and looking for small stars.
We did see many Patiria miniata (bat stars) in the 1-2 cm size range. Most of them were a bright orange-red color, but some were beige, yellow, or blotchy. There was one large (bigger than my outstretched hand) Pisaster ochraceus that was intensely orange. And Point Piños is always a good spot to see many of the six-armed stars in the genus Leptasterias.
In terms of nudibranchs there were many Doriopsilla albopunctata, a yellow dorid with tiny white spots. We saw quite a few of them crawling around on the emersed surf grass, as well as in pools. And of course Okenia rosacea (Hopkins’ rose) was there, although not in the huge numbers I was expecting.
In the low zone I saw a few thalli of the intertidal form of Macrocystis pyrifera, the giant kelp that forms the forests that the California coast is famous for. I’d seen this intertidal form named Macrocystis integrifolia, but it appears that now the two forms (intertidal and subtidal) are both considered to be M. pyrifera. To my eye, the intertidal form differs morphologically by having rounder pneumatocysts (floats) and a holdfast that is less dense than the subtidal form.
Hermit crabs are diverse and abundant at Point Piños. Here’s an example of Pagurus samuelis, the blue-banded hermit crab; even when you can’t see the blue bands on the legs, the bright red antennae are a major clue to this crab’s identity.
When we climbed over the point to the Monterey Bay side, I found two of these little gastropod molluscs, which I didn’t recognize. They are about 1 cm long, with a brown lumpy mantle that can covers the shell, which is pinkish in color. After putting it out on Facebook that I needed help with the ID, a bunch of friends and friends of friends chimed in (thanks John, Rebecca, Barry, and David!) and I was able to determine that these little guys are Hespererato vitellina:
On our way back up the beach we noticed long windrows of Velella velella, the by-the-wind sailors, washed up. While most of them were faded and desiccated, there were enough freshly dead ones that were still blue, which may have washed up on the previous high tide.
All in all, a very satisfactory morning. I saw things I expected to see, some things I didn’t quite expect but wasn’t surprised to see, and some things I’d never seen before. That Hespererato vitellina was completely new to me, which is always exciting.
Next up: What kinds of things live in white calcareous tubes?
The red-tailed hawk chicks across the canyon from us continue to practice their flapping, preparing to take their eventual first flights. We frequently see one of the chicks standing up in the nest, flapping away and whacking its sibling in the head. They’re too big now for both to be flapping at the same time.
The parents are being kept busy bringing food to their hungry offspring. One or the other is often perched on the top of a pine tree within sight of the nest occupants, usually being pestered mercilessly by a marauding crow, while the other is out hunting. The grown-ups are also, I think, trying to entice the kids out of the nest, by hanging out where the they can be seen and showing the kids how it’s done. I expect that the young ones will fledge in the next couple of weeks. We may not see the actual fledging flights, but I’m certain we’ll hear about them.
See how it’s covered in water? I took this picture at about 13:00, probably right at high tide. And of course when I was out there this morning at 06:00, it was low tide. It wasn’t the greatest of low tides but it allowed me to see what I needed to see and have a front-row seat watching the early morning surfers going up and down on the big swell that’s blowing in.
Obviously, visits to the intertidal need to be timed with the tide cycle. At this time of the year we get our lowest spring tides in the morning every two weeks or so, which is great for me because I am a creature of the morning. I can get up hours before the sun rises, but don’t ask me to do anything that requires any intense brain activity after about 21:30.
Low tide this morning was at 05:29, when it was still almost full dark. There was plenty of light to see by the time I got out to the rocks. The tide wasn’t very low and the swell was big, a combination that makes for some pretty spectacular wave watching. Here’s a view towards the marine lab from my intertidal bench; look at all that frothy water!
What a difference seven hours can make! See that tiny black dot in the ocean? That’s a surfer. While I was out there none of the three surfers I was watching did any actual surfing.
I was able to watch the octopus for a couple of minutes. Its mantle was about 3 cm tall, and I’d guess that all spread out the animal was perhaps a bit larger than the palm of my hand. When I got up to move around to the other side of the pool for a different camera angle, the octopus oozed underneath the mussels and just disappeared.
Before it vanished I was able to catch it in the act of breathing.
Although it looks like a head, given the position of the animal’s eyes, the part of the animal that’s pulsating is the mantle. The visceral mass and gills are contained in the space enclosed by the mantle; not surprisingly, this space is called the mantle cavity. The octopus flushes water in and out of the mantle cavity to irrigate its gills. When it wants to swim it closes off the opening to the mantle and forces water out through a funnel which can be rotated 360° so it can jet off in any direction. But this time the octopus didn’t use jet propulsion. It just oozed away.
Our red-tailed hawk chicks are growing bigger every day, and trading fluff for feathers as well. Their bodies are almost completely feathered by now, which makes their heads look small and strange, as though the heads are developing more slowly than the rest of the body. Given that the head is where the brain is located, maybe it actually is growing at a different rate from the body.
For quite long stretches of time now, both parents are away from the nest. Usually the chicks are just lazing around, napping below the level of the nest rim so that we can’t see them. But occasionally they stand up and look around. Already they’ve got that “eyes like a hawk” thing going, and they’ll stare back at us through the spotting scope. And we can tell when the parents are approaching with food before we can see them, because the chicks make a holy hell of a racket. From what I’ve observed, they’ve been eating a lot of rodents lately. Good hawks! Eat all the gophers!
Sometimes the chicks get up and stretch. They need to build strength in their growing flight muscles, so they stretch up and flap their wings a bit. They’re pretty long-legged and gangly now. They look sort of like bald eagles, but that’s only because they don’t have feathers on their heads yet.
Watch this:
Having never kept close eyes on baby red-tailed hawks before, I can’t guess how long it’ll be until these chicks fledge. My experience watching peregrine falcons fledge at the marine lab tells me that, for those raptors at least, fledging doesn’t occur until the head is more completely feathered. If that also holds for red-taileds, then these guys have a bit of feather-growing to do. Besides, the more time they spend stretching and flapping, the better shape their muscles will be in for when they take that eventual first journey into the air.
I’ve been told what to expect when these guys get close to fledging, and what to do if one of them ends up on the ground. I’ll keep you posted!
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.
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.
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:
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.
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.
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.
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.
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.
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:
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.
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.
