Be honest now. When you think of clams, what comes to mind? If you're like most people, visions of clams steamed in white wine, garlic, and butter might dance in your head. Or perhaps clams in cioppino or a hearty chowder would be your go-to. In any case, I doubt that clams, as actual living creatures, occupy much of your brain. Because let's face it, at first glance even living clams aren't the most energetic and charismatic animals. Most of the really cool things that they do, like suck water through their shells for filter feeding and gas exchange, they do while buried in the mud.
When you think about it, though, just the fact that clams live in the sand or mud while depending on water that may be quite far from them is rather amazing. All animals require oxygen, and for marine animals that oxygen comes from seawater. Animals that move freely through the water have access to a ready supply of oxygen. But clams live more or less fixed lives encased in sediment, and water can be quite far from their bodies. How, then, do they pull water into their shells and across their gills? They use siphons, which can reach up to the surface of the sediment into the water column.
A clam has two siphons--one pulls clean water into the shells and the other expels water from the shells. This arrangement allows for one-way flow across the gills, which serve double duty as both feeding and gas exchange organs. The siphons themselves are somewhat muscular and can open and close, but it's the ciliary action of the gills that create the actual water current. In a living clam the only visible body parts are the siphons, which in some species (e.g., geoducks) are so large that they cannot be entirely withdrawn into the shells.
Of the two siphons in the picture above, can you tell which is the incurrent and which is the excurrent? What do you think is the functional significance of that network of white structures that cover the opening of one of the siphons?
Not only do clams live buried in sediment, but some of them can actually bore into rocks. These boring clams, the pholads, have shells that are morphologically and functionally different from the typical clams you've encountered in cioppino. They are elongated on the anterior-posterior axis and the anterior ends are heavily sculpted and fortified to grind into rock. Of course, they can do this only in areas where the rock is soft--you don't see pholads burrowed into granite, for example.
Fortunately for the pholads, much of the rock in the Santa Cruz area is a soft mudstone, easily eroded and burrowed into. I've seen pholads at intertidal sites from Capitola to Davenport. Both dead pholads and live pholads can be seen, but it takes a careful eye to spot the live ones. Of course, all you'd ever see of a living pholad is the siphons. When the animal dies, though, the shells are left behind. As the mudstone continues to erode the shells can be exposed, just like fossils. And as a matter of fact, the mudstone formations around here are known for their fossil contents. I think, but am not certain, that these empty shells in holes belong to Parapholus californica.
How does a clam burrow into even soft rock? A description of burrowing activity of Parapholus californica can be read here. As you can imagine, it's a slow and continuous process. Fortunately, these clams don't have much else going on and can take their time. In some ways, their lifestyle sounds pretty ideal: hang out in a snug burrow where predators can't get at your soft body and extend your siphons out to bring in clean water for food and oxygen. Sure, when it comes to reproduction the only option available is free-spawning and hoping for the best, but that has proven to be a successful strategy for countless generations of your kind. Aside from the cost of making gametes, it's a pretty low-energy way to produce offspring. Maybe the old saying "happy as a clam" isn't that far off the truth.
Professor Emeritus John Pearse has been monitoring intertidal areas in the Monterey Bay region since the early 1970s. Here on the north end of Monterey Bay, he set up two research sites: Opal Cliffs in 1972 and Soquel Point in 1970. These sites are separated by about 975 meters (3200 feet) as the gull flies. My understanding is that the original motivation for studying these sites was to compare the biota at Soquel Point, which had a sewage outfall at the time, with that at Opal Cliffs, which did not. The sewer discharge was relocated in 1976, and the project has now morphed into a study of long-term recovery at the two sites. In the decades since, John has led students, former students, and community members to conduct Critter Counts at these sites during one of the mid-year low tides. Soquel Point is visited on the first day, and Opal Cliffs is visited the following day. When John founded the LiMPETS rocky intertidal monitoring program for teachers and students in the 1990s, the Soquel Point and Opal Cliffs locations were incorporated into the LiMPETS regime.
I have participated in the annual Critter Counts off and on through the years--around here, one takes any chance one gets to venture into the intertidal with John Pearse! I usually have my own plans for this series of low tides, but try to make at least one of the Critter Count mornings. This year (2019) the first 16 days of June have been designated the official time frame for Snapshot Cal Coast, giving marine biologists and marine aficionados an excuse to go to the ocean and make observations for iNaturalist. I had set myself the goal of submitting observations for every day of Snapshot Cal Coast, knowing that every day this week would be devoted to morning low tides. That's the easy part. Next week, when we lose the minus tides, I'll do other things, like look at plankton or photograph seabirds. My plans for this week included a trip to Franklin Point on Wednesday and doing the Critter Count at Opal Cliffs on Thursday. John asked me if I could also do the Wednesday Critter Count. As I alluded above, I'm not going to say "No" to an invitation like that! So I didn't make it out to Franklin Point to document the staurozoans for Snapshot Cal Coast, but that's okay. Some plans are meant to be changed.
Day 1- Soquel Point
Both the Soquel Point and Opal Cliffs sites are flat benches with little vertical topography. The benches are separated by channels that retain water as the tide recedes. The Soquel Point site has deeper channels that make the benches more like islands than connected platforms.
The benches are pretty easy to get around on, as long as you remember that surfgrass (Phyllospadix spp.) is treacherous stuff. The long leaves are slippery and tend to cover pitfalls like unexpected deepish holes. The difficulty at this site is that it takes very little rise in the tide for water in the channels to get deep. You can be working along for a while, then get up to leave and realize that you're surrounded by water. Keeping that caveat in mind, we worked fast.
For the Critter Count we keep tabs on only a subset of the organisms in the intertidal. The quadrat defines our sample; we put it down at randomly determined coordinates within a permanent study area. Some animals, such as anemones, turban snails, and hermit crabs, are counted individually. For other organisms (surfgrass, algae, Phragmatopoma) we count how many of the 25 small squares they appear in. Some quadrats are pretty easy and take little time; others, such as ones that are placed over channels or pools, are more difficult and take much longer.
Because of the rising tide I didn't have a lot of time to look around and take photos of the critters we were counting. Linda and I were worried about finishing our quadrats before the channels got deep enough to flood our boots. But here are two of the things that caught my eye:
Day 2 - Opal Cliffs
The next day we met a half hour later and a few blocks down the road. The Opal Cliffs site is a popular spot with surfers: If you've ever heard of the surf spot Pleasure Point or seen the movie Chasing Mavericks, you know about this location. As far as the intertidal goes, it's an easy site to study. The channels aren't as deep as those at Soquel Point so we could work at a more leisurely pace. As the rest of the group hauled up all the gear and left to get on with their day, I stayed behind to take pictures for my iNaturalist observations. The sky was overcast, making for good picture-taking conditions. I'll just add a gallery of photos to share with you.
There is one critter that deserve more attention here, because I'd never seen one in the intertidal before. Two of the guys finished their quadrats early and started flipping over rocks to look for an octopus. To my knowledge they didn't find any octopuses, but they did find a bizarre fish. At first it didn't look like much:
Hannah, the LiMPETS coordinator for Monterey and Santa Cruz Counties, recognized the fish right away and grabbed it by the body. She held it up so we could see the ventral surface.
This is a plainfin midshipman. These are nearshore fish found in the Eastern Pacific from Alaska to southern Baja. Clearly, I need to spend more time flipping over big rocks! The midshipman is a noctural fish, resting in the sand during the day and venturing out to feed at night. Like many nocturnal animals, it is bioluminescent--those white dots on the fish's belly in the photo above are photophores. Midshipmen are heavily decorated with photophores all over the body. This bioluminescence is used both for predator avoidance and mate choice.
The lives of plainfin midshipmen and human beings intersect in the wee hours of the morning. During breeding season these fish sing or grunt. They breed in intertidal areas, where females lay eggs in nests that are subsequently guarded by males. Both sexes make noise, but it's the breeding males that are the noisiest. They grunt and growl at each other when fighting for territory, but hum when courting females. Females typically grunt only when in conflict with others. People who live in houseboats on the water in Sausalito have reported strange sounds emanating from the water beneath them, only to learn that what they hear are the love and fight songs of fish!
I've always been a fan of the intertidal fishes. They seem to have a lot of personality. Plus, any aquatic animal that lives where the water could dry up once or twice a day deserves my admiration. Of course, all of the invertebrates also fall into this category, which may explain why I find them so fascinating.
After we admired the midshipman's photophores and impressive teeth, we put it back in the sand and replaced the rock on top of it. It was probably happy to get back to snoozing away the next few hours before the tide returned. I don't know how I never realized the midshipmen were in the intertidal. I think I just assumed that they were in deeper water. Now that I know where to find them, I will spend more time flipping over rocks. And who knows, maybe I'll even find an octopus!
If, like me, you are fortunate enough to live near the coast in Northern California, you get to visit the tidepools. And when you do, you may notice something that looks like a pile of sand in the mid tidal zone below the mussel beds. When you venture down and touch the sand, you'll find that it's hard--hard enough to walk on, if you step very carefully, but also somewhat brittle.
It might look something like this:
Meet Phragmatopoma californica, the sandcastle worm. Hard to believe that these mounds, which can be the size of a small dining room table, are constructed by little worms, isn't it? Phragmatopoma is one of the many marine segmented worms grouped together as the Polychaeta. We have lots of polychaetes on our coast, ranging in size from greater-than-hand-length nereids and glycerids that can take a bite out of you and draw blood, to tiny worms small enough to swim in the layer of water between sand grains. In fact, the majority of our worm fauna on the coast consists of polychaetes.
Polychaetes make up a large and very diverse taxon, comprising some 80 or so families. Polychaete taxonomists might argue against it, but to make things simpler, we can divide them into two subclasses, the Errantia and the Sedentaria. As the name implies, the Errantia comprises the worms that are errant, or free-crawling. That said, most of them don't actually crawl around in plain sight; they tend to burrow in sediment, shell debris, or gravel, or wiggle their way through various benthic faunal communities. Some of them make temporary shelters by wrapping themselves in pieces of algae sewn shut with mucus threads. The Sedentaria, on the other hand, are pretty much all, well, sedentary. They live in more or less permanent tubes made of various materials, and generally can't live outside of them.
Phragmatopoma is very much a sedentary worm. It lives in a tube that it builds out of sand grains. Yes, this little worm is a mason!
What you see in these mounds is an aggregation of hundreds of individual worms. The mounds do not form by accident or chance. Phragmatopoma has a planktonic larval stage that floats around on ocean currents for some weeks before returning to the shore. The larva is attracted to areas already colonized by members of their species, which it detects by sniffing out the chemical signal of the glue used to create the tubes (more on that below). This phenomenon is called gregarious settlement. If you consider the challenge of being a tiny creature searching the entire coastline for a place to settle and live forever, one big clue as to the suitability of a given location is the presence of conspecific adults. After all, if your parents' generation grew up there, chances are it's a good spot for you to grow up, too.
Each of those holes in the big sandy mound is the entrance to a worm's tube. Tubes might be as long as 15 cm, but the worm itself is much smaller: a whopping big one would be 4 cm long, and most are in the 2-3 cm size range. From this pair of observations I infer that the worms can and do move up and down the tube. They have to move to the open end of the tube to feed, and can withdraw towards the closed end to avoid predators, or seek protection from desiccation.
Phragmatopoma's tube is not a haphazardly constructed object. It is the worm's home for the entirety of its post-larval life, and is constructed to shield its builder/occupant from the mechanical bashing that occurs twice daily as the tide floods and ebbs. As such, it must be strong and able to maintain its structural integrity. Let's take a closer look at an isolated tube under the dissecting scope:
The tube itself is made of debris--sand grains, bits of shell, the occasional tiny sea urchin spine--that the worm gathers from its environment. Glandular regions at the worm's anterior region secrete around the body a cylinder of sticky cement that is chemically similar to both spider silk and the byssal threads that mussels use to attach themselves to rocks in the intertidal. The inside of the tube is lined with a chitin-like material. The worm uses tentacles on its head region to collect and sort the 'stones' and glues them to the outside of the lining. There is some degree of selection involved; in the photo above you can see that all of the sand grains are more or less the same size, with none standing out as being conspicuously smaller or larger than the others. Growing worms that are actively building their tubes may be geographically restricted at least partly by the availability sand grains of the right size; if the sand is too fine or too coarse, the worm's either can't or don't live there.
Life inside a tube
Living in a tube may provide significant protection from wave bashing and predators, but does present some challenges as well. One thing that comes to mind is the matter of personal hygiene: What happens to the worm's poop? As we know, the worm lives inside the tube but it not attached to it, and can crawl up and down within it. To understand how it does, we have to review some basics of polychaete anatomy.
The word 'polychaete' comes from Greek ('many bristles') and refers to the fact that these segmented worms have chaetae, or bristles, along the left and right sides of the body. In some worms the segments, including chaetae, are pretty much the same from the anterior end of the body to the posterior end. In others, the segments and chaetae are differentiated from one body region to another. In the case of Phragmatopoma, all you can see sticking out of a tube is the head region, consisting of the slender feeding tentacles and a large disc-shaped structure called an operculum, which made of fused cephalic chaetae and serves as a door to close off the tube when the worm withdraws. Behind the head is a collar region, a series of three adjacent segments that have very stiff chaetae that can be pushed out against the lining of the tube to anchor the body in place. The rest of the body behind the collar is the trunk, which bears smaller chaetae on each segment. The entire epidermis is ciliated, which keeps water flowing around the body.
But what about the poop? As in most vermiform animals, Phragmatopoma's anus is at the posterior end of the body, which is oriented towards the closed end of the tube. How, then, does it defecate without fouling its home? The answer is both simple and ingenious. Phragmatopoma has a long rectum, which is curved to run anteriorly back towards the head. The anus, located at the terminal end of the rectum, discharges fecal pellets about halfway up the length of the body. The ciliary currents of the epidermis then flush the fecal pellets the rest of the way up the tube and out the top.
The skinny cylindrical things in the photo below are Phragmatopoma's fecal pellets!
Gas exchange is another challenge for animals that live within tubes. Aquatic animals exchange respiratory gases with the water that surrounds them, which is easy for animals that live where the water is constantly moving over their bodies. But for tube-dwellers, gas exchange is much more difficult. Phragmatopoma has paired gills on each segment of the trunk region of the body, which greatly increase the surface area for gas exchange. Any gas exchange surface is useless unless it connects with the circulatory system, so blood vessels flow into and out of each gill. Dissolved oxygen diffuses from the water into the blood, and is then circulated throughout the body. A certain amount of gas exchange probably occurs across the surface of the tentacles, too. To make things easier, the ciliated epidermis of the body keeps that small amount of water inside the tube moving, minimizing stagnation. When the worm's head is extended out for feeding, the tube is flushed with clean water. When the worm is withdrawn into the tube at low tide, its only oxygen supply is in the water contained in the tube with it. Like most of its intertidal neighbors, Phragmatopoma hunkers down and waits for the tide to return, when it can feed and breathe more easily.
And speaking of feeding, I should mention that Phragmatopoma is a filter feeder. Those purple tentacles are ciliated and create a water current that brings small suspended particles towards the mouth located at the base of the tentacles. In the video below the operculum is the darker object to the left; it represents the dorsal side of the worm's body. The long, filiform tentacles are the feeding tentacles.
As you may imagine, living in a tube also affects the way that Phragmatopoma reproduces. The worms never leave their tubes, so copulation isn't an option for them. Despite their occurrence in large groups they are not clonal, and reproduce only sexually. Both sexes of Phragmatopoma spawn gametes into the water, where fertilization and larval development take place. Living in dense aggregations and spawning at the same time as everyone else maximizes the chance that egg and sperm of the same species will find each other. Many marine invertebrates throughout the oceans, from corals to sea urchins, spawn synchronously. After all, it does an individual no good to throw gametes out into the world if it is the only one of its type around--all of the metabolic energy that went into producing and maintaining the gametes would be entirely wasted.
Clearly, the advantages of living in a tube outweigh the costs and inconveniences. Phragmatopoma has evolved physiological, anatomical, and behavioral adaptations to deal with life in the intertidal. One of those adaptations is the tube, which solves one set of problems but creates others which also need to be solved if the animal is to survive. Evolution comes up with solutions like this all the time. Every trait has metabolic and/or fitness costs, and an organism's biology is based on this type of evolutionary compromise. Life in the intertidal is a tough game. It is probable that none of the various biological processes that keep Phragmatopoma alive work function quite as well as they could, if they were isolated systems. But inside the bodies of these little worms, everything works just well enough for them to be one of the more conspicuous inhabitants of the intertidal.
In my experience, the most difficult organisms to photograph in the wild are staurozoans. Even birds in flight are easier. The problem with staurozoans is where they live. I never see them in calm, still pools, where taking pictures would be easy. Instead, they seem to like surge channels where the water constantly sloshes back and forth, and even in the few seconds between a wave coming in and receding they never really stop moving. Their bodies are extremely soft and squishy, so the slightest current causes them to flutter and make blurry photos. When they are emersed their bodies don't really look like anything except a soggy booger, so they aren't recognizable as staurozoans unless they are underwater. And when underwater they don't hold still, and so on and so forth.
Still, finding them is always a treat, even if I can't capture photographic proof. They really are extremely gorgeous creatures.
They are also enigmatic creatures. Much of staurozoan biology, including their evolutionary relationships, remains poorly understood. Until recently the staurozoans were considered a subgroup of the Scyphozoa, the taxon that includes the large medusae such as moon jellies (Aurelia spp.) and sea nettles (Chrysaora spp.). However, using data from more extensive morphological and molecular studies, most taxonomists now agree that the Staurozoa should be elevated to a level equivalent to the Scyphozoa. In other words, the staurozoan lineage probably evolved alongside, but separate from, the scyphozoan lineage.
Whatever their evolutionary history and relationships, what we know about staurozoans is very limited. They are considered to be stalked jellies (hence their previously assumed close affinity to the scyphozoans) that do not have a separate polyp stage. Their bodies consist of an adhesive peduncle, or stalk, that attaches to algae or surfgrasses, and a calyx or goblet-shaped portion surrounded by eight tapering arms. Each of the eight arms is topped with a puffball of stinging tentaches which are uses to catch food and presumably to defend the animal against predators. The mouth is located in the center of the calyx, usually lifted up on a short stalk called a manubrium. The animal feeds by capturing prey on the tentacles and flexing the arm so the food is brought to the mouth. Staurozoans are not permanently attached and can sort of 'walk' with a somersault-like motion, flipping end-over-end.
Haliclystus 'sanjuanensis' at Franklin Point grows to a length and diameter of ~3 cm, although most of the ones that I see are smaller than that. The most common color is this reddish brown, but I've also seen them in a gorgeous bottle green that makes them much easier to see against the background of their habitat. I usually see them attached to pieces of red algae, but I'm not sure they actually prefer red algae to either green or brown algae. I don't think I've ever seen one attached to a rock.
Last week I had one of those moments in the intertidal when I felt something stuck on my finger and I couldn't get rid of it. That happens frequently, with small bits of algae getting caught on everything; usually I just flick my hand and they go flying off. But this thing wouldn't leave. I finally stuck my hand in the water to rinse it off, and saw that I had been glommed onto by a small staurozoan!
See how the animal stuck to me with its tentacles, while its peduncle is still attached to a piece of Ulva?
As I mentioned, not much is known about these strange animals. They possess the stinging cells to prove their inclusion within the Cnidaria, but are aberrant medusae which stick to algae instead of swimming around in the water column. Their life cycle is more or less cnidarian-like, but their planula is non-ciliated. Their ecological relationships haven't really been studied at all.
Which is why this photograph is so informative. It's not a great picture, by any means, but it shows a glimpse of how staurozoans interact with other species.
This is a picture of two animals, a staurozoan (H. 'sanjuanensis') and a nudibranch (Hermissenda opalescens). Both of these animals are predators. Hermissenda is well known for its affinity for general cnidarian prey, from which it steals the stinging cells to defend its own body (a behavior known as kleptocnidae). But the staurozoan should be quite capable of defending itself. So, who is doing the eating, and who is being eaten?
Given the dastardly nature of Hermissenda, I'd bet on it as the eater. Those damned nudibranchs have to spoil everything! The staurozoan will probably sustain damage, perhaps losing a tuft of tentacles, but should be able to regrow the lost parts. And the sting of the staurozoan may keep the nudibranch from eating as much as it would like. That's the thing. We just don't know.
I'll definitely be keeping an eye out for the staurozoans at Franklin Point the rest of this tide season. I may even bring a few back to the lab for closer inspection; my collecting permit allows me to do so. I could then photograph them under controlled conditions and hopefully get some better pictures. I find these animals very intriguing, being both so clearly cnidarian-like and simultaneously so inscrutable. I always did like a good mystery story!
All semester I've been taking my Ecology students out in the field every Friday. We've visited rivers, forests, natural reserves, endemic habitats, and fish hatcheries--none of which fall into my area of expertise. This year I have several students interested in various aspects of food production, natural/holistic health practices (which sometimes conflict with actual science!), mycology, as well as some who haven't yet decided in which direction to take their academic endeavors. Until very recently I haven't been able to share with my students much of what I really know, which is marine biology. I did have them learn the organisms that live on docks at the harbor, but that was to study the process of ecological succession rather than natural history.
Yesterday, finally, I took the class into my real field, the rocky intertidal. This year it happened that the best Friday to do our annual LiMPETS monitoring was at the end of the semester. We welcomed the new regional LiMPETS coordinator, Hannah, to our classroom on Thursday for some training. Students learned about the history of the LiMPETS program, some natural history of the rocky intertidal in California, and got to practice some organism IDs with photo quadrats of actual intertidal areas.
The real fun, of course, occurs in the field where the organisms live. So we went here:
We didn't have a very good student turnout, unfortunately, but the ones who did show up were diligent workers and we got everything finished that Hannah needed. Most of the time was spent sampling along the permanent vertical transect line. This line is sampled at 3-meter increments along a line that runs from the high intertidal into the low. The same quadrats are sampled every time, and the data collected are used to determine how specific sites change over time. The most difficult part of the monitoring is finding the eye bolts that mark where the transects begin!
I admit, I was a little bummed at the low turnout and late arrival of my students. But the intertidal is the intertidal, and it didn't take long for me to adjust my attitude. I worked up a handful of quadrats with Hannah, then let the students do the bulk of the heavy lifting. This was their field trip, after all. So I wandered around a bit, remaining within hearing distance in case I was needed. I needed to find some stuff!
I just want to show some of the animals and algae in the intertidal yesterday. I didn't realize how much I missed this basic natural history stuff until I got to spend some time simply looking at things.
Such rich life to see! One of the students was astounded when she learned that we could visit sites like this only a few days each month. "At dinnertime today the spot where you're standing will be under several feet of water!" I told her. Mind blown.
Looking more closely, there were, as usual, interesting zonation patterns to observe. One was the restriction of large brown algae to the vertical faces of rocky outcroppings.
In the mid-intertidal, mussels (Mytilus californianus) rule the roost. They are often (but not always) accompanied by gooseneck barnacles (Pollicipes polymerus). The barnacles, for reasons discussed in this earlier post, always live in clumps and are most abundant in the lower half of the mid-intertidal mussel beds.
During the training session on Thursday, Hannah told the students that Pollicipes is easily identifiable because the barnacles look like dragon toes. I think I can sort of see that. They are scaly and strange enough to be dragon toes.
The algae are taking off now, and the site is starting to look very lush.
Even algae start as babies! These balloon-shaped things are young Halosaccion glandiforme thalli, surrounded by other red algae. The large blades belong to Mazzaella flaccida, which makes up a large portion of algal biomass in the mid-intertidal zone.
The tidepools at Davenport Landing are good places to see fish, if you have the patience to sit still for a while and watch. This woolly sculpin (Clinocottus analis) posed nicely in the perfect pool for photography--deep enough to submerge the camera, with clear, still water.
And I was finally able to take a good underwater shot of a turban snail carrying some slipper shells. I've already written about the story of this gastropod trio in case you need a refresher. I'm still waiting to see a taller stack of slipper shells some day.
It was impossible not to feel satisfied after spending some time looking at these creatures. My attitude was mercifully adjusted, and we all departed feeling that we'd done a good morning's work. Our small group of students was able to collect a full set of data for Hannah. That ended up being a very important accomplishment, as Hannah doesn't have any other groups monitoring at Davenport this spring. This means that our data will probably be the only data collected this year at this site. I'm glad the tide and weather conditions allowed us to stay out there as long as we did.
For a number of reasons--a lingering injury to my bum knee, scheduling difficulties, and ongoing postconcussion syndrome--I missed the autumn return of the minus tides. At this time of year the lowest tides are in the afternoon, and at the end of the day I just didn't have the energy to deal with field work. It took until today, the winter solstice, for me to find my way back to the intertidal. An additional motivating factor was a request from both the Seymour Center and Seacliff State Beach for critters to populate their displays. So off I went!
Over the past day or so a storm system blew through the area. It didn't drop any rain on us in Santa Cruz, but earlier this week the National Weather service issued a small craft advisory and suggested that people stay off the beach, due to a combination of big swell and high tides. Usually when I go collecting at Davenport I go to the reef on the north end of the beach, which has more varied vertical topography and a similar, but generally richer, biota than the gently sloping benches to the south. However, the big swell had washed away a lot of the sand, leaving the beach steeper than it would be in the summer, and even the -1.0 ft tide didn't make the reef safely available to someone not clad in a wet suit.
So I trudged across the beach and went to the south instead. It gave me an excuse to poke at the stuff that had been washed up onto the beach and look for nice pieces of algae to take to the Seymour Center. Algal pickings are rather slim in the winter, but I did find several decent small clumps that will do nicely in the touch table. One noteworthy find was a dead gumboot chiton (Cryptochiton stelleri). There were four such corpses washed up on the beach, in varying states of decay and stench.
Cryptochitonstelleri is the largest of the chitons, routinely growing to length of 20 cm. It's a hefty beast, too. The chitons as a group have their greatest diversity in the intertidal, but Cryptochiton is a subtidal creature. Unlike the intertidal residents, Cryptochiton's sticking power is pretty weak. Living below the worst of the pounding of the waves, it generally doesn't have to cling tightly to rocks. However, because it doesn't stick very well, Cryptochiton often gets dislodged by strong surge, especially during spring tides. Then they get tumbled by the waves and wash up dead on the beach. I don't think I've ever seen a live Cryptochiton washed up.
The reef to the south of the beach consists of flat benches that slope down to the ocean. There are some channels and a few pools, but otherwise there is no real topography. Of course, for creatures living in the intertidal, there is topography--the nooks and crannies, as well as vertical faces, provide a variety of microhabitats.
Mopalia lignosa is one of the intertidal chitons that I'm always delighted to find because it's not as common as some of the others, and it's a beautiful animal. The species epithet lignosa means 'wood' and refers to the patterning on the dorsal shell plates.
As usual, there were spectacular anemones to be seen. And I saw something new! Anthopleura sola, the sunburst anemone, is one of the large aclonal anemones that is very common. At Natural Bridges there is a brilliant fluorescent A. sola in a pool on one of the benches I visit. I've been keeping an eye on this animal for a couple of years now, just to reassure myself that it's still there and doing well. The animal is hardly hidden, but it feels like a little insiders' secret that not everybody knows about.
For the first time, I saw fluorescent A. sola at Davenport Landing. Three of them, in fact! And boy, were they all bright!
The third fluorescent anemone was closed up. There were just enough partial tentacles visible to see that it is indeed a fluorescent specimen.
Now, I don't spend as much time at the south end of the beach as I do on the north side, but until today I had never noticed these fluorescent animals. Could I have missed them all this time? It's kind of hard to miss a neon green animal the size of a cereal bowl! At any rate, now that I know they exist and hopefully remember where they are, I'll be able to keep an eye on them, too.
When I teach sponge biology to students of invertebrate zoology, I spend a lot of time describing them as phenomenal filter feeders, and suspect that most other professors do the same. There really are no animals that come close to possessing sponges' ability to remove very small particles from the water. Sponges have this ability despite the fact that their bodies are extraordinarily simple. I can draw pictures on the board to diagram the variety of sponge body types, but I've always wanted to show students how these bodies actually work.
Thing is, from the outside sponges just aren't that interesting. Some grow into large, conspicuous tube or vase shapes, but most occur as crusts of varying thickness and color. For example:
Not much to write home about, is it?
But as with most things invertebrate, sponges are more complex than they appear to be at first glance. And of course their complexity can be best appreciated when you observe sponges under the microscope. That's what I've been doing over the past few weeks: making wet mounts of living sponge and looking at them under the compound microscope. I'm still figuring out the best way to take photos through the scope, and trying to find the magic combination of lighting, magnification, and depth of field to obtain the clearest images.
Let's take a step back and review some basic sponge fundamentals. Sponges are animals in the phylum Porifera. Their bodies are characterized by a lack of true tissues; in other words, a sponge's body consists of various types of cells that do not form permanent connections. The different types of cells have different functions, but most of the cells retain the characteristic of totipotency, the ability to differentiate into another cell type as needed.
The sponge cells that do the filtering are called choanocytes. They form the lining of the sponge's body cavity. Choanocytes consist of a cell body and a collar region of microvilli that form a ring. From the center of the ring protrudes a single flagellum, whose undulations travel from base to tip. The choanocytes are arranged so that the flagella face into the body cavity, and their collective beating draws water through the body. The flagella also capture food particles, which are phagocytosed by the cell.
In its simplest tubular form, a sponge can be visualized as a miniature vase, with a single body cavity called a spongocoel ('sponge cavity') which is lined with choanocytes. Water enters the sponge through many microscopic pores on the outer skin of the body, is filtered by the choanocytes, and exits through a single opening called the osculum. This system works, but the efficiency of filtering is limited by the surface area of the choanocyte layer lining the spongocoel, and very few sponges have this body type.
Now if you imagine making invaginations into the choanocyte layer and continue the choanocytes into the channels you create, you could increase the filtering surface area of a sponge without having to increase its overall body size. Continue this maneuver to its logical end and you'd end up with something that resembles a cluster of grapes. The skin of the grapes would represent the layer of choanocytes, all oriented so that their flagella face the hollow interior of the grape, which would correspond to what we call a choanocyte chamber. This type of body plan has a vastly expanded surface area to volume ratio compared to the tubular form, and these sponges achieve the largest sizes. Incidentally, natural selection has used this exact same strategy to maximize the respiratory exchange surface area of your lungs: gas exchange occurs in the alveoli, which are tiny thin-walled sacs where oxygen diffuses into and carbon dioxide diffuses out of capillaries. The total respiratory surface area of your lungs is about 70 m2--i.e., roughly equivalent to one side of a standard tennis court, without the doubles lanes--all tucked neatly into the volume of your thoracic cavity.
The canals leading into and out of each choanocyte chamber are smaller than the chamber itself, and this arrangement takes advantage of some fundamental fluid dynamics: a given volume of water flows faster through a tube with a narrow diameter and slower through a tube with a wider diameter. Water travels relatively fast through the narrow canals on either end of a choanocyte chamber and slows down significantly within the chamber proper. This gives the choanocytes time to capture all of the food particles in the water stream, and speeds the water to the outside of the body once it has been filtered.
Now we can get back to the animals themselves. Their external appearance may not look like much, but sponges are very interesting when viewed under a microscope. I've been taking samples and squashing them under coverslips for a close look.
Here's a view under darkfield lighting:
The clear-ish objects that look like the back roads of a map are spicules. They provide a bit of skeletal support for the sponge's body and help to deter predators--who would want to bite a mouthful of glass splinters?
When I switched to higher magnification and phase-contrast lighting I could see hollow spherical structures that vaguely resembled blackberries. I felt a thrill of excitement to realize that these were probably choanocyte chambers, and I was looking at the choanocytes themselves!
Here's another view at the same magnification, which shows more clearly the cells of the chamber:
The chambers themselves closely resemble the blastula stage of early animal embryology. Like a blastula, a choanocyte chamber is a hollow ball of cells; unlike a blastula, which has a ciliated outer surface, a choanocyte chamber consists of flagellated cells with the flagella oriented towards the inner hollow space. At a bit less than 40 µm in diameter, the chambers are about half the size of my sea urchin blastulae.
Remember how I said that the structure of the choanocyte chambers is similar to that of our alveoli? You may not be able to visualize the alveoli in your lungs, but this photo shows how the chambers resemble a cluster of grapes.
Because it's impossible to see the three-dimensional structure of the chambers from the single plane of focus you get with a photograph, I shot some video while focusing up and down through the sample on the slide.
This semester I am teaching a lab for a General Biology course for non-majors. I polled my students on the first day of lab, and their academic plans are quite varied: several want to major in psychology (always a popular major), some want to go into business, a few said they hope to go into politics or public policy, and some haven't yet selected a field of study. I think only one or two are even considering a STEM field. Which is all just to say that I have a group of students whose academic goals don't have much in common except to study something other than science. Several of them are the first in their families to go to college, which is very exciting for them and for me.
Most of the activities we do in this class are lab studies. Last week, for example, the students extracted DNA from a strawberry (100% success rate for my class, thank you very much) and then used puzzles and 3-dimensional models to understand the structure of DNA. We do have a couple of field trips scheduled, though, which are the days that students really look forward to. Outside the classroom is where most of the fun stuff happens.
Today I took my class to the beach! We were there to do some monitoring for LiMPETS (Long term Monitoring Program and Experiential Training for Students). For the past few years I've taken my Ecology students out to the intertidal to do the rocky intertidal monitoring. The General Bio students don't have the background needed for the intertidal monitoring and I don't have the classroom time to train them, so we take them to do sand crab monitoring instead. This is a simpler activity for the students, although the clean-up on my end is a lot more intensive even though I get them to help me.
Emerita analoga is a small anomuran crab, more closely related to hermit and porcelain crabs than to the more typical brachyuran crabs such as kelp and rock crabs. It lives in the swash zone on sandy beaches and migrates up and down the beach with the tide. Its ovoid body is perfectly shaped to burrow into the sand, which this crab does with much alacrity. The crabs use their big thoracic legs to push sand forward and burrow backwards into the sand until they are entirely covered. They feed on outgoing waves, sticking out their long second antennae (which can be almost as long as the entire body) and swivel them around to capture suspended particles.
We went out to Seacliff State Beach to count, measure, and sex sand crabs. The protocol is to lay out a 50 m transect along the beach, roughly parallel to the shore where the sand remains wet but isn't constantly covered by waves. Students draw random numbers to determine their position along the horizontal transect and venture out into the ocean, measuring the distance between the transect and the point where they are getting wet to the knees. Then they divide that distance by 9 to yield a total of 10 evenly spaced sampling points along a line running perpendicular to the transect.
The corer is a PVC tube with a handle. It is submerged into the sand to a specified depth and collects a plug of sand that is dumped into a mesh bag. Sand is rinsed out of the bag and the crabs remain behind. Students then have to measure and sex each of the crabs.
Each crab is classified as either a recruit (carapace length ≤9 mm) or a juvenile/adult (carapace length >9 mm). Students get to use calipers to measure carapace length, which they enjoy. Adult crabs are sexed, and females are examined for the presence of eggs.
A sand crab's sex is determined by the presence or absence of pleopods, abdominal appendages that females use to hold onto eggs. If a female is gravid, the eggs are visible as either bright orange or dull tannish masses tucked underneath the telson (see below):
The pointed structure in the photo above is the telson. You can see the tan eggs beneath the telson. They look like they would fall off, but they adhere together in a sticky mass until they are ready to be released. Adult females have pleopods whether or not they are gravid, making it easy to sex the crabs even when they are not reproductive.
Most of the larger crabs today were gravid females and could be sexed with a quick glance at the ventral surface. Sexing the smaller individuals requires a lot more effort. The crab's telson has to be gently pulled back to expose the abdomen, which isn't easy because the crab doesn't like having its parts messed with. In fact, one of the ways to determine whether or not a crab playing dead is really dead is to pry up its telson--a dead crab will let you without making a fuss, while a live one will start thrashing about.
It was a good day to spend time at the beach. The weather got better as we worked and the water wasn't very cold. The students had a good time splashing around in the waves, and they all fell in love with the crabs. There were a few sad moments when crabs got chopped in half by the edge of the corer, but the vast majority were released back to the ocean unharmed. From a teaching perspective, I was happy to give the students an opportunity to do some outdoor learning. After all, the world is our biggest and best classroom. Most students learn best when they get to actually 'do' science, and even though most of this group will not go on to complete a science major, they hopefully have a better appreciation of what it is like to collect real data as citizen scientists.
When low tides occur at or before dawn, a marine biologist working the intertidal is hungry for lunch at the time that most people are getting up for breakfast. And there's nothing like spending a few morning hours in the intertidal to work up an appetite. At least that's how it is for me. Afternoon low tides don't seem to have the same effect on me, for reasons I can't explain. A hearty breakfast after a good low tide is a fantastic way to start the day.
Sea anemones are members of the Anthozoa (Gk: 'antho' = 'flower' and 'zoa' = 'animal'). These 'flower animals' are the largest cnidarian polyps and are found throughout the world's oceans. They are benthic and sedentary but technically not sessile, as they can and do walk around, and some can even detach entirely and swim away from predators. The anthozoans lack the sexual medusa stage of the typical cnidarian life cycle, so the polyps eventually grow up and have sex. In addition to the sea anemones, the Anthozoa also includes the corals, sea pens, and gorgonians.
With their radial symmetry and rings of petal-like tentacles, the sea anemones do indeed resemble flowers. You've seen many of my anemone photos already. Here's one more to drive home the message.
Sea anemones are cnidarians, and cnidarians are carnivores. Most of the time anemones in the genus Anthopleura feed on tiny critters that blunder into their stinging tentacles, although the occasional specimen will luck into a much more substantial meal. I've watched hermit crabs crawl right across the tentacles of a large anemone (Anthopleura xanthogrammica), and while the anemones did react by retracting the tentacles, the crabs easily escaped their grasp.
Of course, not all potential prey items are so fortunate. Sometimes even big crabs get captured and eaten, like this poor kelp crab (Pugettia producta):
There's no way to know exactly how this situation came to be. Was the crab already injured or weakened when the anemone grabbed it? Or was the anemone able to attack and subdue a healthy crab? I've always assumed that the exoskeleton of a crab this size would be too thick for the rather wimpy nematocysts of an Anthopleura anemone to penetrate, but maybe I'm wrong. A newly molted crab would be vulnerable, of course; however, they tend to stay hidden until the new exoskeleton has hardened, and the crab in the above photo doesn't appear to have molted recently.
Even big, aggressive crabs can fall prey to the flower animals in the tidepools. I'd really like to have been there to watch how this anemone captured a rock crab!
And crabs aren't the only large animals to be eaten by sea anemones. Surprisingly, mussels often either fall or get washed into anemones, which can close around them. Once a mussel has been engulfed by an anemone, the two play a waiting game. Here's what I imagine goes on inside the mussel: The bivalve clamps its shells shut, hoping to be spit back out eventually; meanwhile, the anemone begins trying to digest the mussel from the outside; sooner or later the mussel will have to open its shells in order to breathe, and at that point the anemone's digestive juices seep inside and do their work on the mussel's soft tissues. When the digestive process is finished, the anemone spits out the perfectly cleaned mussel shells.
In the photo above, the anemone is working on a clump of several mussels. I can't see that any of these mussels have been compromised, but the pale orange stringy stuff looks like mussel innards and slime. It could be that several mussels are still engulfed within the anemone. There is always a chance that an anemone will give up on a mussel that remains tenaciously closed, and spit it out covered with slime but otherwise unharmed. I assume that hungry anemones are less likely to give up their meals than ones that have recently fed.
So how, exactly, does an anemone eat a mussel, or a crab? The answer lies within the anemone's body. Technically, the gut of an animal is outside its body, right? Don't believe me? Let's think it through. An animal with a one-way gut can be modeled as a tube within a tube, and by that reasoning the surface of a gut is contiguous with the outer surface of the body. Our gut is elaborated by pouches and sacs of various sizes and functions, but is essentially a long, convoluted tube with a mouth on one end and an anus on the other. Sea anemones, as all cnidarians, have a two-way gut called a coelenteron or gastrovascular cavity (GVC), with a single opening serving as both mouth and anus. Anemones, being the largest cnidarian polyps, have the most anatomically complex gut systems in the phylum.
Imagine a straight-sided vase with a drawstring top. The volume of the vase that you'd fill with water and flowers represents the volume of the anemone's gut. Anemones can close off the opening to their digestive system by tightening sphincter muscles that surround the mouth; these muscles are analogous to the drawstring closure of our hypothetical vase. Now imagine that the inner wall of the vase is elaborated into sheets of curtain-like tissue that extend towards the center of the cavity. These sheets of tissue are called mesenteries. They are loaded with various types of cnidocytes that immobilize prey and begin the process of digestion. The mesenteries greatly increase the surface area of tissue that can be used for digestion. The mesenteries are also flexible and can wrap around ingested prey to speed things up.
This anemone (below) that was eating both a mussel and a piece of kelp:
Those frilly ruffles are the mesenteries. You can see how greatly they'd increase the surface area of the gut for digestion. They are also very soft, almost flimsy. Here's a close-up shot:
Maybe I'm especially suggestible, but seeing these animals working on their own meals makes me hungry, too. After crawling around the tidepools for a few hours I'm always ready for a second breakfast or brunch of my own.