For several weeks now I've been raising another batch of bat star (Patiria miniata) larvae, from a fortuitous spawning that occurred in early January. Since this is rather old hat by now I'm not diligently taking photos or drawing the larvae as often as I would have years ago when this kind of undertaking was new to me. But I still change the water twice a week and look at them on Fridays, and I still have the set-up that attaches my old phone to the microscope so I can take pictures of them.
Last Friday it occurred to me that: (A) my gizmo holds the camera steady over the microscope, so I can take pictures at multiple focal planes within objects under the scope; and (B) I have software that will stitch those many snapshots into a single image. Neat!
So I made this:
This larval stage is called a bipinnaria or a brachiolaria. From top (anterior end) to bottom (posterior end) the larva is about 1 mm long. It swims with the anterior end in front. In some sea stars the bipinnaria grows long arms, at which point we call it a brachiolaria ('brachio' = 'arm' in Greek). Bat stars don't grow long arms, so the distinction between bipinnaria and brachiolaria is much fuzzier.
I took 11 photos of this larva, each one focused on a different horizontal plane, and did a focus merge in my photo processing software. Crossed my fingers as the software did its magic, and then peeked at the result. It worked! When looking through the microscope I have to focus up and down through the body to get an idea of its three-dimensional structure. But if the animal holds still long enough, I can do the focus merge thing and get images like this one.
And that slight halo that you see around the exterior surfaces of the larva? That is not an artifact of the photo taking or processing. That halo is due to the cilia that cover the body. There is a ciliated band, which you can see as the dark gold ribbon that snakes along the lobes of the body, and the other body surfaces are ciliated as well. The ciliated band is what the larva uses to swim through the water. Each photo freezes the ciliary action at the moment it was shot, but stitching several photos together causes the cilia to blur into that pale halo.
Intact shells are a limited resource in the rocky intertidal. Snails, of course, build and live in their shells for the duration of their lives. A snail's body is attached to its shell, so until it dies it is the sole proprietor of the shell. Once the snail dies, though, its shell goes on the market to whoever manages to claim it. Empty shells tend not to remain on the market for long.
Hermit crabs also live inside snail shells. They are the ones that compete for empty shells when they do become available. Here in California, at least, the hermit crabs can't kill snails for their shells; they have to wait for a snail to die. And once a shell comes on the market, it will have a taker even if it's not the ideal size for the crab. It's not at all uncommon to see hermit crabs that can fit only their abdomen into the shell, leaving the head and legs exposed and vulnerable. On the other end of the spectrum, many hermit crabs are so small that they can pull into the shell and not be seen by an inquisitive tidepool visitor. Anybody taking a snail shell home as a souvenir—where such takes are allowed, of course—must be certain that there is no tiny hermit crab hiding deep in the depths.
From a hermit crab's perspective, the best shell is one that is big enough to retreat into but light enough to be carried around. Snail shells come in a variety of shapes and corresponding internal volumes. Turban snails, with their roughly spherical shape, have a large interior space and are coveted by larger hermit crabs. For example, the grainy hand hermit crab (Pagurus granosimanus) seems to really like both black and brown turban snail shells.
Original inhabitant and builder of the shell:
And opportunistic second inhabitant of the same type of shell:
Other snails are not even remotely spherical. Olivella biplicata, for example, is shaped like the pit of an olive. Unlike Tegula, of which both intertidal species are found in rocky areas, O. biplicata burrows in sand. Note the shape and habitat of this olive snail:
These olive snails have a smaller internal volume, and thus tend to house smaller hermit crabs. Young individuals of P. granosimanus can be found in olive snail shells, but they quickly outgrow the cramped quarters and need to find a larger home. Smaller hermits such as Pagurus hirsutiusculus, though, are often found in olive shells.
Any hermit crab that finds itself robbed of its snail shell has a short life expectancy. The front end of the hermit resembles the front end of any crab, with the familiar armored legs, claws, eyestalks, and antennae. But the abdomen is soft and unarmored, covered by only a thin cuticle. The abdomen is coiled to follow the coiling of the snail shell, which allows the crab's body to curl around the columella, the central axis around which the shell spirals. In this way the crab can hang onto its snail shell and resist a tug by a would-be predator. A strong enough tug, though, will rip the crab's front end (head + thorax) away from its abdomen. So if you ever find yourself with a hermit crab in hand, do not be tempted to remove it from its shell by yanking it out!
The next time you encounter gastropod shells in the tidepools and want to know whether the inhabitant is a snail or a hermit crab, watch to see how it moves. Hermit crabs scuttle, as crabs do, while snails glide along very slowly. You would also notice a difference as you pick up the shell: snails stick to the rock with their foot, which you will feel as a suction. Hermit crabs don't stick at all, so if the shell comes away easily it likely houses a crab instead of a snail. See? Easy peasy lemon squeezy!
Sometimes even a well-known site can present a surprise. Here's an example. Yesterday I went up to Davenport to scope things out and see how the algae were doing. This is the time of year that they start growing back after the winter senescence. I also took my nature journal along, hoping to find a spot to sit and draw for a while.
The first thing I noticed was the amount of sand on the beach. Strong winter storms usually carve sand off the beaches, making them steeper. And during the calmer months of summer the beaches are flatter and less steep. Yesterday the beach was very thick and flat. It makes trudging across the sand in hip boots much easier!
The accumulation of sand meant that I could walk around the first point. Unless the tide is extremely low, such as we see around the solstices, the water is too deep for that. But yesterday I walked around it, and it wasn't until I got to the other side that it occurred to me that: (1) hey, I walked around the point; and (2) I could do that only because there was so much sand. See, a thick beach with a lot of sand makes a mediocre low tide feel lower because the water isn't as deep as it would be if the beach were thinner. When the tide isn't low enough for me to walk around the point, I have to clamber down a cliff. The cliff height varies depending on how much sand has built up, obviously, but is about head height for me. Getting down usually involves scooting on my butt and hoping my feet land on something that isn't slippery. As with most climbing, up is easier and less scary than down.
It's hard to imagine the amount of sand there was yesterday. Look at this picture.
See how the rocks in the foreground end? Usually that's the edge of the cliff. Yesterday I could have just taken a tiny step off the top of the cliff onto sand. That's over 1.5 meters of sand in that one spot! If the couple in the background were visiting this area for the first time, they'd have no idea of the conditions that made it so easy for them to get out onto the reef.
There was a lot of sand in the channels between rocks, too.
Normally those channels are deeper. You can see that some anemones were able to reach to the surface of the sand, but many more are buried, along with any other critters and algae unfortunate enough to be attached to the lower vertical surfaces. And while some of them will either suffocate or be scoured off as the sand washes away, many will survive and be ready to get on with life.
The second surprise of the day was a bright orange object. What I could see of it was about as big as my thumb, and at first I thought it was a nudibranch. Then when I crept closer for a better look, what popped into my head was "snailfish". Which was an odd thing, because I'd never seen a snailfish before. But something about the creature's posture looked somehow familiar.
Fortunately I had the presence of mind to take photos before trying to draw this little fish, because this is all I had time to get:
When I spooked the critter it took off really fast, confirming that it was no nudibranch. It was, indeed, a snailfish! It came to rest in a small hole in a rock, from where it looked out at me.
The snailfishes are a very poorly studied group. As a group they are related to the sculpins. There are snailfishes throughout the northern temperate and polar regions, from the intertidal to the deep sea. iNaturalist shows 43 observations of L. florae, eight of which are in California. Before yesterday, none had been recorded at Davenport Landing.
So there you have it, a snailfish! We don't know much about any of the snailfish species, even the intertidal ones. They apparently have pelvic fins modified to from a sucker, similar to the clingfishes, but I didn't have a chance to examine this specimen closely enough to confirm that. I don't know why they are called snailfishes, either. They're not snail-shaped at all.
Now, about that thing up there where I said "snailfish" came to mind even though I'd never seen one before. That happens quite a bit—a name will jump into my head before I've had a chance to think about it. Sometimes I'm wrong, but often I'm right. I know I hadn't seen a live snailfish before, but obviously I'd seen photos of them or I wouldn't have been able to recognize this orange creature as being one. It's fascinating how the brain forms search images, isn't it?