It has been almost three and a half years since I first documented seastar wasting syndrome (SSWS) in the lab. Since then many stars have died, in the field and in the lab, and more recently some species seem to be making a comeback in the intertidal. This circumstantial evidence may not be reason enough to conclude that the epidemic is over, but I think there is reason to be hopeful. Any disease outbreak eventually runs its course, and despite its death toll there are always at least some survivors. And I have an individual star that was very sick but seems to be recovering.
In September of 2015 one of my bat stars (Patiria miniata) developed the first tell-tale lesion of SSWS on its aboral surface. At the time the lesion was small (less than 10 mm in diameter) and superficial. Knowing that SSWS starts with minor symptoms and rapidly progresses to something horrific within a day or so, I wanted to keep an eye on this star. It held the same morbid fascination as a car accident or any other impending catastrophe.
By November 2015 the main lesion hadn’t grown much but a few others had developed. The star still wasn’t acting sick and was eating every once in a while, although it occasionally ignored the food that I offered.
So far, so good. I was thinking that the star doesn’t look too much worse, so maybe it wouldn’t keep getting sicker. I checked on it regularly, offered food a few times a week, and left it alone.
4 May 2016
Several months later I noticed that the first lesion had gotten much deeper. The outer dermal layers had been completely compromised, exposing the animal’s internal organs (gonad and digestive caecum) to the external environment. This was bad, very bad. Even in stars, internal organs are supposed to be internal, except when stars extrude their stomachs to feed.
This was the point in time when things started going south. The star lost the ability to maintain its internal turgor pressure and became lethargic and floppy. It stopped eating, or even responding to food. It spent most of its time in a corner of the seawater table where it lives, although a few times I saw it wrapped around one of the hoses that feeds the table. However, its body never started disintegrating the way I’d seen with other SSWS victims.
19 January 2017
Fast-forward another several months. About a month ago the sick bat star began perking up a bit when I placed food near the tip of one of the arms. A week later it actually wrapped its arm around the food, and I assume ate it. It has since been eating about once a week, after fasting for at least eight months. I began to think it would recover.
Today I had some time to photograph the star again, and it really appears to be doing much better!
The lesions are apparently healing over; at any rate, the internal organs are no longer exposed to the outside. The body margin between the arms has a few small divots, but they look superficial. Lately the star has been more active, too, cruising around the table instead of hunkering down in a corner. I’m going to keep feeding it to see if it continues to improve.
One of the most remarkable things about many animals with a decentralized nervous system, such as echinoderms and cnidarians, is their ability to regenerate lost parts and repair damage to their bodies. This bat star is a prime example. It has been sick for almost a year and a half now, and for at least half that time it hasn’t eaten. Yet it somehow had the metabolic reserves to heal a major wound to its body wall. That’s some astounding resilience there. I am very impressed, and you should be, too.
Although at this stage it’s a close race. Two and a half weeks ago I spawned sea urchins in the lab, setting up several purple urchin crosses with the hope of re-doing the feeding experiment that I lost this past summer when I was on the DL (that’s Disabled List, for those of you who don’t speak baseball). I was also fortunate enough to set up a hybrid cross, fertilizing purple urchin (Strongylocentrotus purpuratus, or “Purp”) eggs with red urchin (Mesocentrotus franciscanus, or “Red”) sperm. I would have done the reciprocal hybrid cross (red eggs by purp sperm) as well if I’d gotten any of female red urchins to spawn. However it wasn’t really spawning season for the reds, and I consider myself lucky to have persuaded that one male to release some sperm for me.
This is the first time that I’ve tried to raise the hybrid larvae, although I know it can be done because my colleagues Betsy and John did it many years ago, before I came to the marine lab. All of my larvae are the exact same age and are being raised side-by-side, so I can make direct comparisons between the Purp by Purp crosses and the Purp by Red hybrids. Incidentally, when speaking or writing about a hybrid cross the convention I’ve adopted is to reference the female parent first, so when I say Purp by Red I mean a Purple eggs fertilized by Red sperm. A Red by Purp hybrid would logically result from red urchin eggs fertilized by purple urchin sperm.
My experience raising sea urchin larvae is that things almost always go well for the first 48 hours or so; most (but not all) of the fertilized eggs develop into embryos and undergo the crucial processes of gastrulation and hatching. In some cultures the hatching rate is close to 100%. After that there’s a window of 3-4 days when cultures can crash for no apparent reason, although food availability or quality may be a factor. If the larvae make it past their first week of post-hatching life they generally cruise along until the next danger period which occurs at about 24 days. I change the water in the culture jars and observe the larvae under the microscope twice a week.
Today the larvae are 18 days old. It’s a little early for that second mortality period, but some of the Purp by Purp cultures never really took off. The larvae don’t seem to be growing or developing as quickly as I’m used to. Perhaps this has to do with lower water temperatures, especially after the prolonged period of high temps in 2014-2015. In any case, two of the four Purp by Purp crosses are doing well and the other two are just hanging in there.
There are two things I can see with the naked eye that give me a heads-up when cultures are crashing: the first sign is an accumulation of debris at the bottom of the jar and the second is an absence of larvae in the water column. The debris can be due to excess food, a build-up of fecal matter (not usually the case, as I’m pretty good at doing the water changes on time), the disintegration of larval bodies, or some combination thereof. If the water column is clear then the culture has already crashed and everybody is dead.
Today one of my jars had crashed. The water column was very clear and there was a lot of fluff at the bottom of the jar. I’d been wondering if I could figure out what the fluff was made of, so I sucked up a bit in a pipet and examined it under the microscope. I thought I’d see dead algal cells or pieces that look like defecated algal cells. This is what I saw:
Silly me. I had forgotten that the corpses of pluteus larvae would disintegrate pretty quickly, leaving behind only the skeletal rods. The rods get all tangled together and trap the organic stuff, which is probably a mixture of uneaten and defecated algal cells and the soft tissues of the larval bodies. This explains the clear water column in the jar.
While the Purp by Purp larvae have had mixed success so far, the Purp by Red hybrids have been doing well. From the outset they appeared to be more robust than the Purps, and even though the fertilization rate was only about 50% the post-hatching mortality seems low. The hybrid larvae are also larger than the Purps, and are developing more quickly. In the two photos below the scale bar indicates 100 µm.
The hybrid larva is about 10% larger than the Purp larva. Other than that they look similar, but to me the hybrid larva seems farther along the developmental process: its arms are proportionally longer and have a more mature look (although I don’t have any way to describe that to a naive observer). There’s something about the gestalt of the animal that makes me think it’s more robust than the Purp individual.
We’ll see how the pure Purps and the hybrids do from here on. I actually have the Purp larvae divided up into different feeding treatments, which I may discuss in a future blog post. In the meantime I’m trying to baby the hybrid larvae as much as possible, to maximize their probability of successful metamorphosis in six weeks or so.
Sea urchins have long been among my favorite animals. From a purely aesthetic perspective I love them for their spiky exterior that hides a soft squishy interior. I also admire their uncanny and exasperating knack for getting into trouble despite the absence of a brain or centralized nervous system. Have you ever been outsmarted by an animal without a brain? I have. It’s rather humbling.
Red sea urchins (Mesocentrotus franciscanus) and purple sea urchins (Strongylocentrotus purpuratus) share a common geographic range along the northeastern Pacific but generally live in different habitats. S. purpuratus is the common urchin in tidepools, while reds are almost always subtidal (although I have seen them in the intertidal on very low minus tides). The two species’ habitats do overlap a bit, as the purple urchin can live in subtidal kelp forests alongside the reds. There is a commercial fishery for the gonads of red urchins, which are prized as uni by sushi aficionados. I’ve tried uni once, and it tasted exactly the way I imagined the gonads of a sea urchin would taste. Not a fan. I’d much rather make a different use of urchin gonads.
The other week I collected some urchins from the field, hoping that they’d have nice full gonads. Gametogenesis in many marine invertebrates, including sea urchins, is governed at least partly by annual light cycles. Provided they have sufficient food, purple urchins have ripe gonads and spawn in the winter, from December through March. Reds spawn in the spring, from March through June. In my experience the best time to induce spawning of purps in the lab is December or January, when the urchins have developed gonads but likely haven’t spawned yet. There is no way of knowing the sex of any given urchin or the condition of its gonads, so this exercise is somewhat of a crap shoot even with the best of planning.
Today I shot up my eight field-collected purps, hoping to get at least one male and one female out of the deal. I got lucky with the timing, as one of the smallest urchins was a female and began spewing out eggs. This little female gave a lot of eggs! She was followed by three males and two more females. So out of my eight purps I ended up with three of each sex, and a spawning rate of 75% ain’t bad.
I set up some mating crosses and fertilized all of the eggs. I divided the little female’s eggs into two batches and fertilized them with the sperm of two different males (M1 and M2). Each of the other females’ eggs was fertilized by M1, who gave huge amounts of sperm. When I checked on the eggs about two hours post-fertilization most of them had gone through the first cleavage division and seemed to be developing normally and on schedule.
Just for the hell of it I decided to shoot up some of the red urchins we have in the lab. I didn’t really think they’d spawn, as it’s not the season for them to be gravid. Red urchins are large, heavy animals with long and sharp spines and they are much more difficult to handle. Four of the five that I shot up did nothing, as expected. It took a long time, but just as I was about to give up on them the biggest red began dribbling out a couple thin streams of sperm. I examined the sperm under the microscope and they were very active and healthy. Fortunately I hadn’t returned the purps to their tanks, and two of the female were still putting out some eggs. I rinsed the purp eggs into a clean beaker, pipetted up some of the red sperm, and added it to the eggs.
Sea urchin eggs are covered by a thick jelly coat. In the video you can see many of the red urchin sperm embedded in the jelly coat of the egg. Despite the frantic activity of the sperm, fertilization (as evidenced by the rising of the fertilization envelope off the surface of the egg) took much longer than it does when eggs and sperm come from the same species.
Look at that beautiful zygote! Fertilization success in this hybrid cross was low, only about 50%. The eggs that did get fertilized went through the first cleavage division after about two hours later, which is right on time.
It remains to be seen whether or not the few hybrid embryos I have continue to develop. I have a colleague who has hybridized red and purple urchins successfully in the past, and has raised the offspring to adulthood. I don’t have any expectations of great success with this little experiment, but it would be very informative to raise known hybrid urchins. I’ve seen animals in the field that look like hybrids and there’s no reason to assume that hybridization between these two free-spawning species never occurs. The adults can be found living side-by-side subtidally, and there’s enough overlap in their reproductive seasons that some individuals of each species could very well spawn at the same time. On the other hand, hybridization that can be forced in the lab doesn’t necessarily occur in the field. I dumped a lot of red urchin sperm on those purple urchin eggs, and such high sperm concentration may overcome any mechanisms of reproductive isolation that exist under real-life conditions.
About three weeks ago I collected some mussels from the intertidal, to use both in the lab and in the classroom. A mussel can itself be an entire habitat for many other organisms. Many of the mussels I brought into the lab this last time were heavily encrusted with barnacles and anemones. I wanted to look more closely at one of the anemones so I took the mussel to the microscope. And, as often happens when I look at stuff under the microscope, I got totally distracted by things other than what I intended to study.
But for the record, this is the anemone that started the whole chain of events:
Below the anemone there’s a thick mat of small acorn barnacles (Balanus glandula) and a couple of leaf barnacles (Pollicipes polymerus). They were all alive when I brought the mussel into the lab, and over the weeks a few of them have died. But many of them are still kicking, both figuratively and literally.
Barnacles are most strange animals. Believe it or not, they are crustacean arthropods, somewhat closely related to crabs and lobsters. They live encased within a shelter of calcareous plates, which they can close seal up against predators and desiccation. I’ve never figured out why they are called “acorn barnacles,” as they don’t look anything like acorns to me, but in Balanus and such the base of the shelter is glued directly to a rock or some other hard surface. Leaf barnacles are shaped very differently, and have a fleshy stalk between the shelter that houses the main body of the animal and the rock surface.
To picture what’s going on with a barnacle, imagine a shrimp lying on its back, then curl it up and stick the whole thing inside some calcareous plates. The thoracic appendages would be facing up. In barnacles the thoracic appendages are modified to be clawlike feeding structures called cirri. Barnacles are filter-feeders, collecting particles from the water by maneuvering the cirri in a sort of grasping fashion. So in a nutshell, or more precisely a test, a barnacle lies on its back and kicks its legs out to catch food.
Here’s what B. glandula looks like when feeding. Note the clearly jointed cirri, with fine hairs that help catch particles. The cirri can be controlled independently, as you can see when they flick towards the center, and the entire apparatus can be rotated quite a bit.
Same deal with Pollicipes.
So that’s the feeding part. A little strange, but not as interesting as the barnacles’ sex lives. Let’s start with some background about sexual function. And get your mind out of the gutter; this is real science stuff! Most of the animals that you’re familiar with are described as dioecious (Gk: “two houses”). This means that female and male sexual functions are segregated; in other words, there are male bodies and female bodies. Other animals are described as monoecious (Gk: “one house”), so that a single body has both female and male sexual function. Monoecious animals could also be described as hermaphroditic. Some monoecious animals have male and female function in a single body at the same time; we call these simultaneous hermaphrodites. If a body first functions as one sex and then either acquires or switches to the other sex, we say the animal is a sequential hermaphrodite. Many fishes, including the California sheephead and the anemone fishes of coral reefs, are sequential hermaphrodites. Make sense?
Barnacles are simultaneous hermaphrodites. If you dissect an adult barnacle you will find mature ovaries and testes. This means that every barnacle can be both a mother and a father. The logical assumption is that monoecious animals should just fertilize their eggs with their own sperm. . . however, this generally isn’t the case. The whole point of sexual reproduction is to combine the genomes of two individuals, and self-fertilization obviously doesn’t accomplish this. So even though there are many hermaphroditic animals, very few of them are self-fertile.
One other weird thing about barnacles, and crustaceans in general, is their sperm. Arthropods have non-flagellated sperm, which means they don’t swim (although some of them have amoeboid sperm that can ooze around a little bit). Many marine animals reproduce by broadcast spawning; that is, by throwing their gametes into the water, where fertilization takes place. Fertilization is facilitated by the sperms’ ability to swim towards conspecific eggs.
Barnacles, with their non-swimming sperm, generally cannot rely on broadcast spawning to get sperm to egg. They must copulate. How do you suppose they do this? The same way that other animals (e.g., Homo sapiens) copulate, by using a penis or some other structure to transfer sperm from one individual to the body of another. In barnacles the penis’s technical name is intromittent organ. The penis is inserted into the test of a neighboring barnacle and sperm is delivered. The receiving barnacle uses the sperm to fertilize its eggs. Unlike the cirri, the penis is unjointed and flexible, the better to seek out and slip into potential mates. You can see the intromittent organ unrolled and poking around.
Now think about the ramifications of these constraints. Barnacles live their entire post-larval lives permanently cemented to a rock. They also have non-motile sperm so sperm transfer can occur only by copulation. If the key to reproductive success is to mate with as many other individuals as possible, what do you suppose natural selection has done? That’s right: barnacle anatomists, including the great Charles Darwin himself, have noticed that barnacles have incredibly long penises. In fact, compared to overall body size, barnacles have the longest penises in the animal kingdom, up to 15 times the length of the body! That’s what you call bragging rights. Not all barnacle species are so amply endowed, however. The same leaf barnacle that I observed today (P. polymerus) has recently been reported to be a spermcaster; their penises are shorter than body length, and they release sperm that are captured by their downstream neighbors.
My friend Peter Macht is the aquarium curator at the Seymour Marine Discovery Center. He is responsible for all of the live (i.e., wet) exhibits and has a team of student and volunteer aquarists who help him care for the animals in the hall and behind the scenes. Peter and I go way back together, to years before the Seymour Center opened in 2000. Back then the only public space at Long Marine Lab was called the Shed Aquarium because it was, literally, in a wooden shed. I do miss the marine lab the way it was then, when I knew everybody who was there and it was a quieter and more peaceful place to work. However, we’ve come a long way, baby, and the Seymour Center is in just about every way imaginable, a huge improvement over the Shed Aquarium.
For one thing, there are two large exhibits in the Seymour Center, each of which would occupy about half the volume of the old Shed Aquarium. One of these tanks, the Sandy Seafloor tank, has housed many different animals over the years: surf perches, sand dabs, sharks, rays, señoritas, and various invertebrates. My personal favorite continues to be the burrowing sea star Astropecten, although she hasn’t been on exhibit for several years now. The current inhabitants are a close second favorite, even though when they first arrived I didn’t expect them to be nearly as fascinating as I’ve found them.
Pleuroncodes planipes is a little red crab commonly called the pelagic crab or the tuna crab. For once the common names reveal something about the biology of the animal–these crabs spend their lives in the water column over the continental shelf, at least as youngsters, and are one of the favored food items of tunas. They are usually found in the waters of southern California and Mexico, but during the El Niño event of 2015 they washed onto the beaches around Monterey Bay in humongous numbers; they also did so during the ENSO event of 1982-1983.
Although they resemble crayfish, Pleuroncodes is a crab. They are anomuran crabs more closely related to hermit crabs and porcelain crabs than to “regular” brachyuran crabs such as shore crabs and rock crabs. The way you tell the difference between anomuran and brachyuran crabs is to count the number of thoracic walking legs, keeping in mind that the claws are included as walking legs: anomurans have four pairs while brachyurans have five pairs. You can see in the picture of the lateral view that this crab has three pairs of stick-like legs and one pair of chelipeds (claws).
Being arthropods, red crabs molt periodically. Peter has been collecting data on frequency of molts for individual crabs since the spring of 2016. Doing so requires isolating crabs in separate containers, to keep track of which crab molts when and also to prevent the crabs from ripping apart a freshly molted compadre, which they do with great enthusiasm. It is not unusual to see one or more of the inhabitants of the Sandy Seafloor tank missing a leg.
Here’s one of Peter’s tables containing crabs in baskets:
It’s just as well that these guys have extraordinary regenerative capabilities, as they are eager to rip each other’s legs off. With most crabs that I’ve observed in the lab limb regeneration is a gradual process, with the new leg growing a bit with each successive molt. Chelipeds, even with their increased size and complexity, seem to regrow faster than the other walking legs, likely reflecting their importance to the animal’s lifestyle.
Peter told me last week that he’d seen one of his isolated crabs regenerate an entire cheliped with a single molt, going from nothing to an almost-full-size functional limb essentially overnight. This seemed very unlikely to me, but Peter said he’d seen the before (the empty molt) and after (the actual crab) together in the same container. Unfortunately the crabs end up demolishing and eating their molts within a couple of days, so the evidence doesn’t stick around very long.
Sometimes, though, you get lucky. When I was at the lab yesterday morning Peter told me that he’d seen another of his crabs molt, and that it had grown a missing cheliped since the previous day. And this time he could show me the proof. Voilà!
Note that the molt has only one cheliped, the left, while the crab itself has two. How cool is that? The crab’s right cheliped is a bit smaller than the left, as might be expected of a regenerating limb, but it’s definitely intact and functional. It was pretty exciting to see evidence of wholescale limb regrowth taking place in such a short period of time, which must be incredibly energetically expensive. On the other hand, chelipeds are extremely important for defense, and there is obvious selective pressure to regrow them as quickly as possible should a crab be unfortunate enough to lose one.
Peter gave me permission to examine the molt more closely, so I took it back to the lab where the lighting is better. And surprise! The right cheliped apparently didn’t grow from nothing overnight. If you look really hard at the photo above, you can just barely see a ghostly transparent sheath where the missing arm would be. Hmm. This was not at all what I expected. Did I really see that?
It turns out that, yes, that is exactly what I saw.
See that translucent tiny limb up front? That’s a little cheliped! And it had been there at least six months, as this crab’s last recorded molt was in April. Why hadn’t anyone seen it before? I think because this limb was so small that the crab kept it tucked underneath the carapace, where it wouldn’t be seen from the dorsal (top) side.
In the course of one morning I got taken for quite a roller coaster ride. Peter reminded me that he’d seen a crab apparently regrow a missing appendage in a single molt cycle . . . and had just found a crab whose molt showed exactly that . . . and then that molt ended up including a claw after all. What fun!
Now, why is that little claw so transparent? An arthropod’s exoskeleton is made of a material called chitin, with varying degrees of calcification depending on species. The large marine crustaceans (e.g., crabs and lobsters) have heavily calcified exoskeletons, while insects have much more lightweight, less-calcified exoskeletons. As a crab prepares to molt, one of the things its body does is resorb some of the minerals that it had deposited in the soon-to-be-discarded exoskeleton, so they can be re-used in the new one. If you find a discarded molt on the beach, pick it up and note how little it weighs; you’d be surprised at how flimsy it is.
Here’s my hypothesis. I think that this little cheliped, because it was newly regenerated before this most recent molt, was only lightly calcified. The crab may have used it, but it wouldn’t have been much use for defense. Then, the next time the crab molted the claw was shed along with the rest of the exoskeleton, and the limb was significantly larger. This crab now possesses a complete pair of chelipeds again. After examining the molt I returned it to the crab, which has probably torn it to pieces and eaten already. It’s a way for the animal to recover some of the nutrients it allocated into building the exoskeleton in the first place.
Seeing as today is the third anniversary of the first blog post I wrote about sea star wasting syndrome (SSWS), I thought it would be appropriate to take inventory of my remaining stars and see how they’re doing. Right now I have custody of ~10 bat stars (Patiria miniata), 7 ochre stars (Pisaster ochraceus–collected last year for the juvenile survival experiment I did with Scott), and 1 Mediaster aequalis. For whatever reason the M. aequalis hasn’t been affected by SSWS so I’m going to disregard it for now. Of the 10 or so bat stars, four live in one of my seawater tables, roaming free-range in quite a large volume of water. The other half-dozen or so live in a tank in a different building. The Pisasters live in 1s and 2s in tanks distributed in two rooms in the same lab.
After the initial horror and shock of the spectacular onset of SSWS, in which we watched stars rip themselves into pieces right before our eyes, what we’ve seen has followed the standard epidemiology pattern. Any time a novel pathogen enters a population, the individuals that have no immunity or resistance are the first to die. The disease spreads rapidly through the population, wiping out all of these weaker individuals. However, not everyone dies. Even during the Black Death of the 14th century, the very fact that 1/3-1/2 of the human population died of bubonic plague means that 1/2-2/3 survived. Those survivors presumably had some degree of resistance to the disease.
At the same time three years ago that all of my forcipulate stars died, divers were noticing similar phenomena happening subtidally. It didn’t take long for us to realize that Something Big was going on, which was eventually dubbed SSWS. Fast-forward three years and now I’m seeing healthy, hand-sized P. ochraceus in the intertidal again. These individuals are certainly survivors from the SSWS outbreak; they were likely small juveniles during the plague, and were able to come out of hiding and expand into open niches after so many of the adults died. Whether or not natural populations will recover completely remains to be seen, but as of right now things look promising.
About a year ago, having gone two years without showing any signs of being sick, one of my bat stars developed lesions on its aboral surface. It’s the red star in the middle of that blog post. This star is one of the four that live in my shallow table. It has now been sick for a year. See how it has changed since then:
The lesions have all gotten worse–the largest is about 2 cm long now–and the body margin has some ripples that it didn’t have before, but the star is still alive. For a while it wasn’t eating, as far as I could tell, but two days ago I watched it eat a piece of fish. Perhaps the return of cooler water is helping this animal survive.
One of its tablemates, however, hasn’t been so lucky. I first noticed apparent SSWS damage in a second star several months ago. Today was the first chance I had to look closely at it.
The most noticeable injury to this star is that big interradial divot. It looks like someone took a bite out of the body at that spot. The margins of the wound are white and fluffy, similar in appearance to the lesions caused by SSWS.
For years now this star has had an abnormal spot on its aboral surface. I’ve been calling it a bubble, for lack of a better word. The bubble may be an over-inflated papulla (skin gill) and it didn’t seem to be causing any problems for the star. I’d touch it and it would deflate, then re-inflate almost immediately. When I touched it today, it shrank back a little but didn’t really deflate.
If you look really closely at the above photo, you can make out clusters of small, clear, clublike projections. These are papullae, extensions of the internal body lining that project through the skeletal ossicles to the outside and act as gas exchange surfaces. The bubble is many times larger than the normal papullae. Because it has been there for so long, years before the divot in the interradial margin, I don’t think the bubble is due to SSWS. I don’t even know if it’s a wound, or merely an overinflated papulla. The largest star in this table has also had a bubble for many years, but no lesions or wounds indicating SSWS or other disease.
So. Three years after the outbreak of SSWS I still have stars that are sick. They’ve been sick for a long time and aren’t getting worse very quickly, from which I conclude they may eventually recover. At the very least they must have some resistance to the SSWS pathogen because they’ve managed to survive so far. One more thing. Way back in 2013 when all of the forcipulates were tearing themselves into pieces and melting into piles of goo, these bat stars were among them, scavenging on the dead and decaying tissue. For a while I feared that eating contaminated tissue might cause the disease, but that doesn’t seem to be the case, as these two didn’t get sick until two years after the initial exposure.
I hope these two stars make it. Cooler water temperatures should help. When they’re really sick they stop eating (they haven’t eaten much in the past year) but if they’re going to eat now I’ll keep feeding them. Fingers crossed!
A long time ago in a galaxy called the Milky Way, a great adventure took place. We don’t know exactly when it happened, but it must have been very shortly after the evolution of the first cells. Some small prokaryotic cell walled itself off from its surroundings. Then it learned how to replicate itself and as cells continued to divide they began interacting with clones of themselves. Sooner or later, however, our clone of cells encountered cells from a different genetic lineage. These foreign cells were “other” and were recognized as such because they had a different set of markers on their outer covering. Perhaps there was an antagonistic interaction between the two clones of cells. In any case, this ability to distinguish between “self” and “non-self” was a crucial step in the evolution of life on Planet Earth.
The entire immune system in vertebrates is based on self/non-self recognition. It is why, for example, transplanted organs can be rejected by their new host–the host’s immune system detects the transplanted tissue as “non-self” and attacks it. As a result, patients who receive donor organs usually take immune-suppressing drugs for some period of time after the transplant.
The vertebrate immune system is quite complex and very interesting. It has two main components: (1) cell-mediated immunity, in which the major players are T cells; and (2) humoral (i.e. blood-based) immunity, which is the part of the immune system that produces antibodies to a pathogen when you get a vaccination. However, even animals much less structurally complex than vertebrates have some ability to recognize self from non-self.
Sponges, for example, exist as aggregations of cells rather than bodies with discrete tissues and organs. Most zoologists, myself included, consider sponges to be among the most ancient animal forms. They have different types of cells, many of which retain the ability to move around the body and change from one type to another; this totipotency is a feature that sponge cells share with the stem cells of vertebrates. There are sponges that you can push through a mesh and disarticulate into individual cells, and then watch as the cells re-aggregate into an intact, functioning body. As if that weren’t cool enough, if you take two different sponges and mush them into a common slurry, the cells from the distinct lineages re-aggregate with cells to which they are genetically identical. So even animals as primitive as sponges have some degree of self/non-self recognition.
If you’re lucky, you can see self/non-self recognition and aggression in the intertidal. Here in northern California we have four species of sea anemones in the genus Anthopleura:
Anthopleura xanthogrammica, the giant green anemone
Anthopleura sola, the sunburst anemone
Anthopleura elegantissima, the cloning anemone
Anthopleura artemisia, the moonglow anemone (and my favorite)
Of these species, only A. elegantissima clones. It does so by binary fission, which means that the animals rip themselves in half.
It looks painful, doesn’t it? As the two halves of the animal walk in opposite directions they pull apart until the tissue joining them stretches and eventually rips. Then each half heals the wound and carries on as if nothing had happened. Each anemone is now a physiologically and ecologically independent animal, and can go on to divide itself. And so on ad infinitum. The logical consequence of all this replication is a clone of genetically identical anemones spreading over a rocky surface. And that’s exactly what you get:
Okay, it’s hard to tell that these are sea anemones, but this is what they look like when the tide goes out and leaves them emersed. They pull in their tentacles, close off the oral disc, and cover themselves with sand grains. They look like sand but feel squishy and will squirt water if you step on them. In this photo, each anemone is probably 4-5 cm in diameter.
There are three patches of anemones in the photo above, separated by narrow strips of real estate where there are no anemones. Each patch is a clone, essentially a single genotype divided amongst many individual bodies. The anemones in each clone pack tightly together because they are all “self.” However, they recognize the anemones of an adjacent patch as “non-self” and they won’t tolerate the intrusion of neighbors onto their territory. Those strips of unoccupied (by anemones) rock are demilitarized zones. When the rock is submerged the anemones along the edges of the clones reach out their tentacles and sting their non-self neighbors. This mutual aggression maintains the DMZ and nobody gets to live there.
Because A. elegantissima lives relatively high in the intertidal the clonal patches are usually emersed when I go out to the tidepools. Its congener, A. sola, lives lower in the intertidal and is more often immersed at low tide. Anthopleura sola is larger than A. elegantissima and is aclonal, meaning that it does not divide. Anthopleura sola also displays quite dramatically what happens when anemones fight.
These two anemones, each about 12 cm in diameter, were living side-by-side in a tidepool. You can see that each animal has two kinds of tentacles: (1) the normal filiform feeding tentacles surrounding the oral disc; and (2) thicker, whitish club-shaped tentacles below the ring of feeding tentacles. These club-shaped tentacles are called acrorhagi, and are used only for fighting. The acrorhagi and the feeding tentacles may contain different types of stinging cells, reflecting their different functions. All tentacles are definitely not the same.
These animals, which represent different genotypes, are non-self to each other, so they fight. They inflate their acrorhagi, move their feeding tentacles out of the way, and reach across to sting each other. See how some of the acrorhagi on the animal on the right don’t have nice smooth tips? Those tips have been lost during battle with the animal on the left; the tips are torn off and remain behind to continue stinging the offender even after the tentacle itself has been withdrawn.
Here’s another picture of the same two anemones, taken from a different angle:
The goal of these fights is not to kill, but to drive the other away so that each anemone has its own space. Eventually one of them will retreat, and a more peaceful coexistence will be established. Fights like these have been going on for over half a billion years. Eat your heart out, George Lucas.
Well, we can’t—at least, not very well. I suppose we can eat it in small amounts, but sand itself is one of the most nutrient-poor substances imaginable. Sand is, after all, ground up bits of rock. It would provide certain minerals, depending on the type of rock, but none of the essential macronutrients—carbohydrates, proteins, and lipids—that animals need to survive.
When I was a kid I thought that sand dollars were called sand dollars because I’d find their broken tests on sandy beaches. I knew they lived in sand, hence the name. As I started studying marine invertebrates in college I learned that sand dollars don’t just live in the sand; they also eat sand. In addition to organic matter, usually in the form of detritus, sand dollars eat sand to create ballast. This makes them heavy and keeps them from being picked up and carried away by waves. It is also why, if you come across an intact sand dollars test and break it open, sand will fall out of it.
I have a batch of recently settled Dendraster excentricus, the common sand dollar in northern California. They began metamorphosing only 30 days post-fertilization. As the larvae settled and transformed into tiny sand dollars, I decided to try to figure out what to feed them. These animals aren’t grown commercially and there doesn’t seem to be a definitive answer on how to raise them. One of the suggestions I got was “Well, we know they eat sand, so feed them sand.”
Which is what I did. The first time I just sprinkled a bit of sand in the dish with the juvenile sand dollars. Then I looked under the microscope to see that the sand grains were about 10 times the size of the animals. Oops. But the sand dollars didn’t look unhappy so I let them be. I decided that they also needed something organic to eat so I ground up a small piece of Ulva and dropped some of the resulting slurry on them.
The second time I offered sand to the sand dollars I ground it up in a mortar and pestle that I scrounged from the lab next door. Let me tell you, grinding sand makes a sound that is every bit as horrible as you imagine. At least it produced smaller particles that the sand dollars might be able to eat. I continued to offer Ulva mush in addition to the fine sand. If they end up eating either sand or Ulva, I can provide that pretty easily. The question is, how do I know whether or not they’re eating?
How many sand dollars can you find in the above photo? They are exactly the same color as the sand. I don’t have real proof that these little guys are eating sand; even their poops would look like the sand. The animals do tend to clear the space in their immediate vicinity, but I think that might be due to the action of the tube feet and spines rather than consumption of either sand or Ulva. In this video clip you will see that the sand dollars are very active, even though all the motion doesn’t seem directed the way it does in urchins at this stage.
They do a lot of waving around, but don’t actually walk. They do, however, seem to like being tilted up a bit, similar to the way adult sand dollars position themselves when in calm water:
By Chan siuman at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23041434
I do have circumstantial evidence that my sand dollars are eating something. The first ones metamorphosed at 30 days post-fertilization. Today is day 51 post-fertilization, which means some of the animals have been post-larvae almost as long as they were larvae. I know it takes about a week for newly metamorphosed sea urchins to form their new guts and begin feeding, and I assume it’s the same for sand dollars. In fact, because these sand dollars raced through larval development so quickly I expected their juvenile mouths to break through quickly as well. If this were the case, then these animals should have had complete and functional guts for almost two weeks now. The fact that they’re not dead or dying makes me think that they have to be eating.
Call it a hunch, call it intuition, call it wishful thinking. I’m not sure how they’re doing it, but I think they’re fine. Next week I hope I can find time to measure them.
I’ve already written several times about seastar wasting syndrome (SSWS) and you’ve probably seen your share of photos of wasted, melting, self-mutilating stars. However, you may also be wondering about the current state of affairs regarding SSWS, and whether or not sea star populations have recovered at all since the outbreak began three years ago now. The question “How does SSWS affect the stars?” can be addressed on two different levels: the level of an individual star, and the level of the population of stars. In this post I discuss the first aspect, and in a subsequent post I’ll share my observations of sea star populations in the field.
Level 1: SSWS as it affects individual stars
I remember very vividly the feeling I had when I opened the door to the wet lab and glanced into my table to see this:
And after that it only got worse, until (almost) every star was dead. It was interesting to watch how the disease manifests in different species of stars, though. The forcipulates–genera Pisaster (ochre stars), Pycnopodia (the huge sunflower star), Orthasterias (rainbow star)–succumbed quickly and violently. These were the animals that ripped their own arms off, often without showing any prior signs of distress, and then melted away.
On the other hand, other species seemed to be more resistant to SSWS. At least, they didn’t succumb right away. Perhaps the disease (if it is indeed a disease) progresses more slowly in some groups of species compared to others. These stars, including the bat stars (Patiria miniata) and leather stars (Dermasterias imbricata), didn’t rip their arms off. The only leather star in my care died about a week after the forcipulates bit the dust, and the bat stars seemed fine for months. And when these species got sick they showed different symptoms.
Instead of self-mutilation, the leather and bat stars developed lesions on their skin. The lesions could be very deep, exposing the animal’s internal organs (guts and gonads) to the external environment.
The white objects inside the yellow circle are the star’s skeletal ossicles, which have fallen away because the tissue holding them in place has been severely eroded. I haven’t seen a leather star survive longer than a week once the lesions appear. Bat stars, on the other hand, can and do live for months with lesions. For example, this star of mine first developed lesions back in September 2015:
The lesions were small and superficial, and for a long time the animal didn’t actually seem sick. It wandered around its table, remained sticky, and even ate. Now, seven months later, the star is still hanging in there. I took this photo of it yesterday:
The lesion is bigger and deeper and now the innards are exposed. The star is also a little deflated, which might be a bad sign. From what I’ve observed, once an animal can no longer maintain its internal turgor pressure, it probably can’t recover. However, this one isn’t totally deflated yet, so I still have hope for it. Heck, this animal has been sick for over half a year now and hasn’t died yet. It obviously has some ability to resist the illness, or perhaps it’s just dying very slowly.
Just for kicks I zoomed in on the lesion under the dissecting scope, and it actually looks sort of cool. It isn’t every day that you can see the internal structures of an animal without cutting it open.
Sea stars don’t have a lot of space in the central disc of the body, so they keep their gonads and guts in their arms. Each arm contains a pair of pyloric caeca (extensions of the gut) and a pair of gonads. In the photo above, the whitish ribbons are the pyloric caeca and the tan bits are gonad. Just for kicks I snipped off a piece of the gonad and looked at it under the compound scope. And lo and behold, it’s a girl!
Those large round-ish blobs are oocytes in varying stages of maturity. I’m a little surprised to see any developing oocytes at all, given that this poor star has been sick for so long. Maybe this is a good sign. The internal fluid of the animal’s main body cavity is essentially seawater, so having the gonads and guts exposed to the outside might not be the direct avenue to infection that it would be for us. From what I can tell the tissue itself looks healthy: it doesn’t appear to be decomposing, the oocytes are full and more or less round, and there aren’t a lot of ciliates swarming all over it. So I think there’s hope for this animal, which has already survived so much, to pull through.
Another bat star that I’ve been keeping an eye on is a beautiful 8-armed star that was collected by Prof. John Pearse. Somehow I never managed to take a picture of this animal until it got sick about two weeks ago. One of the lab assistants noticed that it looked a little off on a Saturday, and two days later it had some nasty lesions.
Because this bat star went from zero symptoms to ulcerated lesions in two days, we didn’t think it would last much longer. The lab assistants isolated it in a tub filled with 0.2-µm filtered seawater and have been changing its water daily. Just as it didn’t take long for symptoms to appear, it didn’t take long for this individual to show signs of recovery. About five days after first being isolated the star was sticking to the side of its tub, indicating that its water vascular system was still functioning. A week after that, I looked at it again and saw that the lesions seemed to be healing!
The surface of the lesion appears to be more solid, as if the epidermis had been knitted back together. There’s still a bit of gonad exposed, though. Is this significant? At this point I’m not sure. The animal will remain in ICU, separated from all other echinoderms, until we are absolutely certain that it has recovered. And of course I may be jumping the gun to say that the animal is recovering at all. Only time will tell. It is, however, extremely refreshing even to think about SSWS without despair, for which I am grateful.
This morning I drove up the coast to Pigeon Point. It was cold and very windy, and I was grateful to have decided to wear all of my layers. I don’t remember any cold mornings from last year’s low tides, which made me think that perhaps we’re returning to a more normal non-El Niño weather pattern. The wind was screaming down the coast from the north, and if it keeps up we should get some upwelling in a few days. Fingers crossed!
Even the pelicans, which can fly through strong winter storms, were having a bit of trouble with the wind:
My favorite kelp grows in the intertidal, and it wasn’t having any difficulty at all with the strong surf. It’s not large and doesn’t form the magestic kelp forests that divers flock to, but it is very charming in its own way. The sea palm Postelsia palmaeformis is a small (1/3-1/2 meter tall) kelp that lives only on exposed rocks sticking out into the brunt of the waves. It requires the full force of the crashing waves, where other algae would get broken off. They have a thick flexible stipe that bends with the waves and then pops back up. Postelsia is a protected organism and I can’t collect it even with my scientific collecting permit, which is fine with me.
This is the kind of environment in which Postelsia thrives:
You can tell how windy it was by the sound of the wind and my inability to hold the camera steady. As the tide comes in the pounding from the waves will only get worse. These little algae are pretty damn impressive!
Pigeon Point has always been a good place to see the 6-armed stars of the genus Leptasterias. Unlike the five arms that most of the local asteroids have, Leptasterias has six. And unfortunately for us naturalists, the taxonomy of the genus is incompletely understood. All that is agreed upon is that there are several species in the genus. This is referred to as a species complex, acknowledging that the genus contains more than one species but that the species have yet to be definitively described.
As you can see, these stars vary quite a bit in terms of arm thickness and color pattern. Most of the time they are blotchy but the blotches can be pink, gray, orange, or cream-colored. Some of the stars have slender arms with very little taper, while others have thicker arms that taper strongly to the tips. For the time being, until the sea star systematists come to consensus about the species in this genus, I’ll refer to all of them as Leptasterias sp.
Most of the Leptasterias that I see in the field are in the size range of 1-4 cm in diameter, usually no longer than my thumb. Today I saw a big one, which would have been about the size of the palm of my hand.
The reason this star doesn’t look quite as big as that in the above photo is that it was eating when I disturbed it. The star was humped up over its breakfast!
The unfortunate breakfast item, the turban snail Tegula funebralis, was about 2 cm in diameter. It seems like a very large and well-protected prey item for a star this size, doesn’t it? And yet, there it is. The animal is always right, and Leptasterias certainly knows what it should be eating.
And lastly, because they were just so beautiful and I can’t help myself, I’m going to close with photos of anemones.
