Here's another photo, taken from farther away to give you a bigger picture of the scale of things.
Believe it or not, the maker of these trails is the little black turban snail, Tegula funebralis. They are one of my favorite animals in the intertidal, for a number of reasons:
I always root for the underdog and the under-appreciated, and these snails are so numerous in the intertidal that they are practically invisible. People literally do not see them. I know, because I ask.
They are very useful creatures to keep as lab pets. I throw a few of them into each of my seawater tables, except for the table that contains a resident free-ranging sea star, and they do a fantastic job keeping algal growth to a tolerable minimum. They're my little marine lawnmowers!
They come in very handy when I'm teaching invertebrate zoology. Students study them live to observe behavior, and the snails are not shy. They are very tolerant of being picked up and gently prodded, and soon emerge from their shells and carry on their little snail lives. Students also dissect them in lab to learn about gastropod anatomy.
So yes, these tracks in the sand are made by T. funebralis in the high intertidal. In areas where a layer of sand accumulates either at the bottom of a pool or on a flat exposed rock, it is not uncommon to see a turban snail pushing sand out of the way as it crawls along, like a miniature snow plow.
Tegula funebralis and its congeners are called turban snails because their shells are shaped like turbans. Given their small size (a big T. funebralis would have a shell height of 2.5-3 cm), pushing sand around must be a tiresome chore. They do it because they have no choice. Most grazing gastropods, such as turban snails and limpets, can feed only when they are crawling. There may very well be a nice yummy layer of algal scum on the surface of this rock, but the snail has to push the sand out of the way before it can feed on it.
Here's another photo, taken at the snail's level.
This snail is pushing through a wall of sand as tall as itself! I don't know about you, but I sure as heck couldn't do that. Props to these little snails!
There are certain creatures that, for whatever reason, give me the creeps. I imagine everyone has them. Some people have arachnophobia, I have caterpillarphobia. While fear of some animals makes a certain amount of evolutionary sense--spiders and snakes, for example, can have deadly bites--my own personal phobia can be traced back to a traumatic childhood event involving an older cousin and a slew of very large tomato hornworms. Even typing the words decades later makes me want to rub my hands on my jeans.
But enough about caterpillars. This Halloween I want to share something that isn't nearly as disgusting, but can still creep me out sometimes. Commonly called skeleton shrimps, caprellid amphipods are a type of small crustacean very common in certain marine habitats. They are bizarre creatures, but a close look reveals their crustacean nature. For example, they possess the jointed appendages and compound eyes that only arthropods have.
Around here the easiest place to find caprellids is at the harbor, where they can be extremely abundant. The last time I went to the harbor to collect hydroids for my class, the caprellids were swarming all over everything. When I brought things back to the lab I had to spend an hour or so picking the caprellids off the hydroids. I don't think they eat the 'droids, but they gallop around and keep messing up the field of view, making observation difficult. They're essentially just a PITA to deal with, and everything is easier after they've been removed.
Caprellids are amphipods, members of a group of crustaceans called the Peracarida (I'll come back to the significance of the name in a bit). They have the requisite two pairs of antennae that crustaceans have, and seven pairs of thoracic appendages of varying morphology. Some of these thoracic legs are claws or hooked feet that like to grab onto things. A caprellid removed from whatever it's attached to and placed by itself in a bowl of seawater thrashes around spastically. Only when it finds something to grab does it calm down. Even then, they attach with their posterior appendages and wave around the front half of the body in what I call the caprellid dance: they extend up and forward, and sort of jerk front to back or side to side. It isn't pretty.
A bunch of caprellids removed from their substrate and dumped into a bowl together will use each other as something to grab. This forms the sort of writhing mass that makes my skin crawl. I was nice enough to give them a piece of bryozoan colony to hang onto, but even so they ended up glomming together.
Now, back to the thing about caprellids being peracarids. The name Peracarida means "pouch shrimp" and refers to a ventral structure called a marsupium, in which females brood their young. Males don't have a marsupium, so adult caprellids are sexually dimorphic. When carrying young, a female caprellid looks like she's pregnant. See that caprellid in the top photo? She's a brooding female. That's all fine, until her marsupium itself starts writhing. This ups the creepiness factor again. Here's that same brooding female, in live action:
Crustaceans obviously don't get pregnant the way that mammals do, but many of them spend considerable energy caring for their young. Well, females do, at least. A female caprellid doesn't just carry her babies around inside a pouch on her belly. Although she isn't nourishing them from her own body in the way of mammals (each of the youngsters in the marsupium is living off energy stores provisioned in its egg), the mother does aerate the developing young by opening and closing the flaps to the marsupium. This flushes away any metabolic wastes and keeps the juveniles surrounded by clean water. As the young caprellids get bigger, they begin to crawl around inside the pouch, and eventually leave it. They don't depart from their mother right away, though; rather they cling to her back for a while, doing the caprellid dance in place as she galumphs along herself.
Until the juveniles strike out on their own they form a small writhing mass on top of a female who can herself be part of a larger writhing mass. And the sight through the microscope of all these long skinny bodies jerking around spasmodically can indeed be very creepy. Fortunately not as creepy as caterpillars, or I wouldn't be able to teach my class or go docking with my friend Brenna. And it's a good thing caprellids are small, 'cause if they were any bigger. . . just, no.
Although the world's oceans cover approximately 70% of the Earth's surface, most humans interact with only the narrow strip that runs up onto the land. This bit of real estate experiences terrestrial conditions on a once- or twice-daily basis. None of these abiotic factors, including drying air, the heat of the sun, and UV radiation, greatly affects any but the uppermost few meters of the ocean's surface so most marine organisms don't need to worry about them. Despite the apparent paradox of where they live, intertidal organisms are also entirely marine--they cannot survive prolonged exposure to in air or freshwater. So how do they manage to live here?
Some organisms have a physiological tolerance for difficult conditions. These tidepool copepods and periwinkle snails, for example, are able to survive in the highest pools in the splash zone, where salinity can be either very high (due to evaporation) or very low (due to rain or freshwater runoff), dissolved oxygen is often depleted due to high temperature, and temperature itself can be quite warm. Sculpins and other tidepool fishes cope with low oxygen levels by gulping air and/or retreating to deep corners of their home pools.
Of course, animals that can locomote have the option of moving to a more favorable location. Other creatures, living permanently attached to their chosen site, aren't quite so lucky. Let's take barnacles as an example.
Barnacles have two planktonic larval stages: the nauplius and the cyprid. The nauplius is the first larval stage and hatches out of the egg with three pairs of appendages. It can be distinguished from the nauplius of other crustaceans by the presence of two lateral "horns" on the anterior edge of the carapace. The nauplius's job is to feed and accumulate energy reserves. It swims around in the plankton for several days or perhaps a couple of weeks, getting blown about by the currents and feeding on phytoplankton.
After sufficient time feeding in the plankton, a barnacle nauplius metamorphoses into the second larval stage, the cyprid. A cyprid is a bivalved creature, with the body enclosed between a pair of transparent shells. It has more appendages than the nauplius, and these are more differentiated. If the nauplius has done its job well, then the cyprid also contains a number of oil droplets under its shell. These droplets are of crucial importance, because the cyprid itself does not feed. For as long as it remains in the plankton it survives on the calories stored in those droplets. The cyprid's job is to return to the shore and find a suitable place on which to settle. Somehow, a creature about 1 mm long, being tossed about by waves crashing onto rocks, has to find a place to live and then stick to it.
Returning to the topic of the challenges that marine organisms face when they live under terrestrial conditions, let's see how these barnacles manage. Along the northern California coast we have a handful of barnacle species living in the intertidal. In the higher mid-tidal regions at some sites, small acorn barnacles of the genera Balanus and Chthamalus may be the most abundant animals.
However, nowhere is a particular pattern of barnacle distribution more evident than at Natural Bridges. Here, the barnacles in the high-mid intertidal are small, and concentrated in little fissures and cracks in the rock.
I think most of these small (~5 mm) barnacles are Balanus glandula:
And here's a closer look:
If all of the rock surfaces were equally suitable habitat, the barnacles would be distributed more randomly over the entire area. Instead, they are clearly segregated to the cracks in the rock. Each of these barnacles metamorphosed from a cyprid into a juvenile exactly where it is currently located. The cyprid may be able to move around to fine-tune its final location, but once the decision has been made that X marks the spot and the cyprid has glued its anterior to the rock, the commitment is real and lifelong. The barnacle will live its entire life in that spot and eventually die there. It is quite probable that cyprids landed in those empty areas on the rock, but they didn't survive to adulthood.
How did this distribution of adult barnacles come to be?
There is one very important biological reason for barnacles to live in close groups, and that is reproduction. They are obligate copulators, which I touched on in this post, and as such need to live in close proximity to potential mates. But today I'm thinking more about abiotic factors. In a habitat like the mid-mid rocky intertidal, desiccation is a real and daily threat. Even a minute crack or shallow depression will hold water a bit longer than an exposed flat surface, giving the creatures living there a tiny advantage in the struggle for survival. No doubt cyprid larvae can and do settle on those empty areas of the rock. However, they likely die from desiccation when the tide recedes, leaving only the cyprids that landed in one of the low areas to survive and metamorphose successfully. There are other factors as well, such as the presence of adult individuals, that make a location preferable for a home-hunting cyprid. In addition to facilitating copulation, hanging out in a cluster slows down the rate of water evaporation, giving another teensy edge to animals living at the upper limit of their thermal tolerance.
Lower in the intertidal, where terrestrial conditions are mitigated by more time immersed, barnacles and other organisms do indeed live on flat rock spaces. But at the high-mid tide level and above, macroscopic life exists mostly in areas that hang onto water the longest. Pools are refuges, of course, but so are the tiniest cracks that most of us overlook. Next time you venture into the intertidal, take time on your way down to stop and salute the barnacles for their tenacity.
Five days ago I collected the phoronid worms that I wrote about earlier this week, and today I'm really glad I did. I noticed when I first looked at them under the scope that several of them were brooding eggs among the tentacles of the lophophore. My attempts to photograph this phenomenon were not entirely successful, but see that clump of white stuff in the center of the lophophore? Those are eggs! Oh, and in case you're wondering what that tannish brown tube is, it's a fecal pellet. Everyone poops, even worms!
Based on species records where I found these adult worms, I think they are Phoronis ijimai, which I originally learned as Phoronis vancouverensis. The location fits and the lophophore is the right shape. Besides, there are only two genera and fewer than 15 described species of phoronids worldwide.
Two days after I first collected the worms, I was watching them feed when I noticed some tiny approximately spherical white ciliated blobs swimming around. Closer examination under the compound scope showed them to be the phoronids' larvae--actinotrochs! Actinotrochs have been my favorite marine invertebrate larvae--and that's saying quite a lot, given my overall infatuation with such life forms--since I first encountered them in a course in comparative invertebrate embryology at the Friday Harbor Labs when I was in graduate school.
The above is a mostly top-down view on an actinotroch, which measured about 70 µm long. They swim incredibly fast, and trying to photograph them was an exercise in futility. They are small enough to swim freely in a drop of water on a depression slide, so I tried observing them in a big drop of water under a coverslip on a flat glass slide. At first they were a bit squashed, but as soon as I gave them enough water to wiggle themselves back into shape they took off swimming out of view.
Here's the same photo, with parts of the body labelled:
The hood indicates the anterior end of the larva and the telotroch is the band of cilia around the posterior end. The hood hangs down in front of the mouth and is very flexible. At this stage the larva possesses four tentacles, which are ciliated and will get longer as the larva grows. These are not the same as the tentacles of the adult worm's lophophore, which will be formed from a different structure when the larva undergoes metamorphosis.
As usual, a photograph doesn't give a very satisfactory impression of the larva's three-dimensional structure. There's a lot going on in this little body! The entire surface is ciliated, and this actinotroch's gut is full of phytoplankton cells. You can see a lot more in the video, although this larva is also a little squished.
I've been offering a cocktail of Dunaliella tertiolecta and Isochrysis galbana to the adult phoronids, and these are the green and golden cells churning around in the larva's gut. However, good eaten is not necessarily food digested, and the poops that I saw the larvae excrete looked a lot like the food cells themselves. Today I collected more larvae from the parents' bowl and offered them a few drops of Rhodomonas sp., a cryptonomad with red cells. This is the food that we fed actinotrochs in my class at Friday Harbor. We didn't have enough time then to observe their long-term success or failure, but I did note that they appeared to eat the red cells.
I don't know if phoronids reproduce year-round. It would be a simple task to run down and collect a few every month or so and see if any worms are brooding. Now that I know where they are, it would also be a good idea to keep an eye on the size of the patch. Some species of phoronid can clone themselves, although I don't know if P. ijimai is one of them. In any case, even allowing for the possibility of clonal division, an increase in the size of the adult population would be at least partially due to recruitment of new individuals. If recruitment happens throughout the year, it follows logically that sexual reproduction is likewise a year-round activity. Doesn't that sound like a nifty little project?
Besides, it's never a bad idea to spend time at the harbor!
If I asked you to draw a worm and designate the front and back ends, you'd most likely come up with something that looks like this:
And you would be entirely correct. A worm, or any creature described as 'vermiform' for that matter, has an elongated, wormlike body. Some worms have actual heads with eyes and sensory tentacles, but many don't. The great many polychaete worms that live in tubes don't have much of a head at all: usually all you can see sticking out of the tube is a crown of tentacles used for feeding. Although even the use of the word 'crown' more than suggests the presence of a head, doesn't it? After all, where else does one wear a crown?
Most worms, including the worm that we imagined above, are bilaterally symmetrical, with bodies elongated along the Anterior-Posterior axis. This means the head is at the anterior end and the rear is the posterior end. For animals that don't have a prominent head, the Anterior can also be defined by the direction of locomotion. Worms crawl with their bellies against the ground, which sets up a second axis of symmetry, the Dorsal-Ventral axis. The third axis of symmetry is the Left-Right axis. These axes should sound familiar, because they apply to our own bodies, as well of those of all other vertebrates and many invertebrates. Because of our upright stance we actually walk with our ventral surface forward, which is a little confusing, but if you don't trust me you can see for yourself by crawling around on hands and knees for a while.
Now back to our worms, hypothetical and otherwise. Consider a worm that is elongated not along its Anterior-Posterior axis, but along its Dorsal-Ventral axis. It sounds strange, but such worms do exist. They are called phoronid worms, and are classified within their own phylum, the Phoronida. They all live in tubes, and the few times I've seen them they have been in pretty dense aggregations. As with most tube-dwelling worms the only part of the body that you can usually see is the crown of feeding tentacles, which in these animals (as well as in the Bryozoa and Brachiopoda) is called a lophophore.
The other day I was at the harbor looking for slugs with my friend Brenna, and spotted these pale tentacles swaying in the current.
These are the lophophores of an aggregation of phoronids! I'd never seen them at the harbor before, so I was pretty excited about it. They were on the side of a floating walkway, down almost beyond the reach of my outstretched arm. The current caused the lophophores to sway continuously and I was barely able to snap some blurry photos without falling in (I couldn't really see what I was doing and just hoped for the best) when I accidentally caught this one shot. I wanted to have at least one clear-ish shot to submit to iNaturalist. I did manage to scrape off some bits of stuff that I hoped contained intact phoronids, so I could observe them under the dissecting scope at the lab.
And these are some lovely little worms!
The tubes that these phoronids inhabit are more like burrows of slime to which the surrounding sediments adhere. The tube itself isn't anything particularly interesting, but the bodies of the worms are beautifully transparent. One of the coolest things you can see in a living phoronid is its circulatory system. They have red blood that, like ours, contains hemoglobin, so it's easy to see the vessels that run along the length of the worm (which is the Dorsal-Ventral axis, remember) and the two blood rings around the base of the lophophore. If you get the lighting right you can even see the vessels that extend into each tentacle of the lophophore.
I was disappointed to see that none of the video clips I took really do justice to these worms. They are so pretty when I look at them through the microscope, and I wish I could capture their beauty. You may at least be able to see blood moving through the larger vessels of the body in this short video.
Seems I need to upgrade my photomicroscopy set-up. Anybody have a few thousand bucks they want to donate to the cause?
I'm keeping the phoronids for as long as I can, although I don't know what to feed them. I had time to take just a quick look at them this morning, and they look fine. Just for kicks I offered them a little phytoplankton to see what they'd do with it and couldn't see if they were reacting at all. Still, they are filter feeders, and if I can adjust the lighting and get a good view of those ciliated tentacles I should be able to see if they are creating a water current that is bringing food to the mouth. Friday is the next day I have time to spend with these animals that I don't get to see very often. Maybe then I'll have something else to report.