Revert to the Ascidian

I've spent most of the last week running full-tilt at work. I had a higher-than-usual number of talks and meetings to attend, got a stack of papers to grade, and have spent this weekend with an experiment that has me run to the ground. /end grad student whining.

One of the talks I attended this week was about ascidian development, which reminded me that I have been meaning to write about these cool invertebrates (some of our closest relatives!). 

This is a group of Ciona intestinalis, the solitary vase tunicate.

Ascidians are also called tunicates or sea squirts. Their body is covered by a protective covering called a tunic, hence "tunicate," and as you can see in the photo above, they have two siphons to pump water through their bodies. If you squeeze them, water shoots out the excurrent siphon, hence "sea squirt." The ones pictured above are solitary animals, but they also live in colonies like this orange species below.

The colonial ascidian Botrylloides violaceus. Each of the holes is a zooid, and the whole gloopy thing is a colony.

 Ascidians are part of a group of organisms known collectively as "fouling organisms" because they live on pretty much any hard substrate they can find. Sometimes that is rocks, and sometimes it is the underside of docks or the bottoms of boats. They generally grow really quickly and can outcompete other species like barnacles. Many species are nasty invaders all over the world.

I am, of course, particularly interested in tunicate larvae. They look like this:

Tadpole larvae of B. violaceus. Photo from UNH.

 These larvae are called tadpole larvae for obvious reasons. They do not feed, and they only live in the water for a few hours to a day before metamorphosing and turning into a little colony of ascidians.

And these larvae bring me to the part where they are your closest invertebrate relatives. They have all of the traits that make chordates unique as a phylum. So, just like you did when you were an embryo, they have a notochord, a muscular postanal tail, a hollow dorsal nerve chord, and pharyngeal gill slits. Many people are interested in studying the larvae because of these similarities with the rest of the chordates (like us!). I, on the other hand, am interested in the larvae because they can only disperse over short distances before they metamorphose -- and that can affect their ecology and evolution in turn.

Walter Garstang, author of The Ballad of the Veliger, did not write about ascidian larvae. But I have found this gem, by Andrew Lang, and excerpted the part of it describing metamorphosis here. Go read the whole thing -- it's a lovely metaphor of life.

Th' Ascidian tadpole, young and gay,
Doth Life with one bright eye survey,
His consciousness has easy play.
He's sensitive to grief and pain,
Has tail, and spine, and bears a brain,
And everything that fits the state
Of creatures we call vertebrate.
But age comes on; with sudden shock
He sticks his head against a rock!
His tail drops off, his eye drops in,
His brain's absorbed into his skin;
He does not move, nor feel, nor know
The tidal water's ebb and flow,
But still abides, unstirred, alone,
A sucker sticking to a stone.

-Andrew Lang, from "Man and the Ascidian"

Time & Tide Wait for No Man

It's spring break in this neck of the woods, and that means that it is time for SCIENCE. "Spring break" in grad school really means "time to work without being interrupted for teaching responsibilities, seminars, lab meetings, etc.," and I've been trying to buckle down on a number of things.

Sadly, the weather here (like many places) has also failed to get the spring break memo. I spent yesterday out collecting snails for an experiment that I'm hoping to run this weekend, as soon as the baby snails are ready to metamorphose. Normally, when I go to my field site, it looks something like this:

Field site at "low tide." Photo credit A. Cahill.

 And here is what it looked like yesterday, from approximately the same vantage point. Same point in the tidal cycle (low tide). You can tell there's a whole lot more water in the picture below, and that made for an unpleasant day in the field.

Field site at "low tide." Photo credit A. Cahill

So what gives? Why is "low tide" so different on two different days? And what do I mean when I say the tides are "good" or "bad" for my fieldwork? It has to do with the phases of the moon.

Tides are caused by the pull of gravity of the sun and the moon on the ocean. Since the ocean is not fixed to the earth, it sloshes around based on these gravitational forces. The moon rotates around the earth on a 4-week cycle. Sometimes it is aligned with the sun and sometimes they are perpendicular to each other.

Spring and neap tides explained. Photo from Wikipedia.

 When the sun and the moon are in a line (the new moon & full moon phases in the diagram above), they pull together on the ocean and make high tides higher & low tides lower. These are the spring tides in the above diagram. For me, these are "good" tides, since the snails I study live really low on the shore. If the low tides aren't really low, I can't get out to the animals without getting really wet. In March around here, the idea of going swimming for snails is distinctly unpleasant.The top photo of my field site was taken during a spring low tide.

When the sun and the moon are at right angles (1st quarter & 3rd quarter moons, above), they exert their forces in opposing directions. Since the moon is much closer to the sun, it has the stronger effect on the tides, but without the added pull from the sun the water doesn't move as much. These are the neap tides, or for me, the "bad" tides.

Yesterday was a neap low tide. That was compounded by the wind, which was blowing water directly towards shore (see the waves in that second photo), pushing the water level up even higher. I couldn't get anywhere near where I usually need to go at the site without going swimming -- and yesterday it was about 40 degrees Fahrenheit and raining. Luckily, the wind and waves have moved many snails way high up on the shore, so I was able to collect without getting hypothermia.

Now if only my veligers would decide that they are ready to cooperate, I could really do some good work this week.

(Bonus points for anyone who read the post title and immediately pictured Eowyn pulling off her helmet and screaming "I am no man!" in Return of the King. I only wish the tides would work like that for me.)

The Veliger's a Lively Tar

More than anything, this is a post to promise that I'm not dead yet. Things appear to be ramping up for field and lab season here, and I suddenly have a lab full of baby snails. We've also just had a stream of prospective graduate students through the department, which led to a nice day of showing off my lab to a bunch of folks but also took a lot of my time.

So, instead of a general marine inverts post as promised, BABY SNAILS!

A Crepidula fornicata veliger, about 24 hours after hatching. It's about 300 microns long -- 1 micron = 1/1000 of a milimeter. Photo taken at 100X magnification. Photo credit: A. Cahill

I work with these guys every day and I absolutely love them. Marine larvae in general are beautiful, but I am currently partial to the baby snails. These are called veliger larvae. They have a coiled shell, like they will as an adult, and look remarkably similar to their adult form overall. The one difference is an organ called a velum. It's a large pair of lobes that have cilia around the edge (on the left side of the snail in the photo above). Those cilia beat and allow the snail to swim and to feed (they capture algae and move it to the snails' mouth). The shell of the larva is transparent, so in the picture above you can see the digestive tract as a dark brown squiggly line, and the stomach is the brown blob at the right end of the shell.

A lot of the questions that I answer in my lab have to do with these larvae. Adult snails don't move much, but these larvae have the ability to swim in the water for weeks and can travel hundreds of kilometers along the coast. It's therefore the larval stage that is critical for exchanging DNA and individuals among populations of snails. The larvae are also vulnerable because they are so small. Many researchers are concerned that they will be more impacted by human-caused disturbance (climate change, pollution, ocean acidification) than adults.

These veligers will be a recurrent feature on the blog, because they take up an inordinate amount of my life.

And just for fun, a larval poem by Walter Garstang. It is just one of many that he wrote about many different larval forms. We had our Invertebrate Zoology students interpret bits of it on their final exam last year, because it's full of technical details if you are familiar with gastropod development. If you're not, it's a cute little rhyming poem.

The Ballad of the Veliger (by Walter Garstang)

The Veliger’s a lively tar, the liveliest afloat,
A whirling wheel on either side propels his little boat;
But when the danger signal warns his bustling submarine,
He stops the engine, shuts the port, and drops below unseen

He’s witnessed several changes in pelagic motor-craft;
The first he sailed was just a tub, with a tiny cabin aft.
An Archi-mollusk fashioned it, according to his kind,
He’d always stowed his gills and things in a mantle-sac behind.

Young Archi-mollusks went to sea with nothing but a velum—
A sort of autocycling hoop, instead of pram—to wheel ‘em;
And, spinning round, they one by one acquired parental features,
A shell above, a foot below—the queerest little creatures.

But when by chance they brushed against their neighbours in the briny,
Coelenterates with stinging threads and Arthropods so spiny,
By one weak spot betrayed, alas, they fell an easy prey—
Their soft preoral lobes in front could not be tucked away!

Their feet, you see, amidships, next the cuddy-hole shaft,
Drew in at once, and left their heads exposed to every shaft.
So Archi-mollusks dwindled, and the race was sinking fast,
When by the merest accident salvation came at last.

A fleet of fry turned out one day, eventful in the sequel:
Whose left and right retractors on the two sides were unequal:
Their starboard halliards fixed astern alone supplied the head,
While those set aport were spread abeam and served the back instead.

Predaceous foes, still drifting by in numbers unabated,
Were baffled now by tactics which their dining plans frustrated.
Their prey upon alarm collapsed, but promptly turned about,
With the tender moral safe within and the horny foot without!

This manoeuvre (fide Lamark) speeded up with repetition,
Until the parts affected gained a rhythmical condition,
And torsion, needing now no more a stimulating stab,
Will take its predetermined course in a watchglass in the lab.

In this way, then, the Veliger, triumphantly askew,
Acquired his cabin for’ard, holding all his sailing crew—
A Trochophore in armour cased. with a foot to work the hatch,
And double screws to drive ahead with smartness and despatch.

         But when the first new Veligers came home again to shore,
         And settled down as Gastropods with mantle-sac afore,
        The Archi-mollusk sought a cleft his shame and grief to hide,
        Crunched horribly his horny teeth, gave up the ghost, and died.

Evolution, Ecology, and Baby Urchins

In my first post, I promised to unpack my research statement a little bit. What exactly do all these terms mean and why are they important? Today I'll focus on the first term: evolutionary ecology.

Like all compound nouns, this is really an adjective (evolutionary) combined with a noun (ecology).

Ecology, according to the dictionary definition, is the study of the interaction between organisms and their environment. How do animals and plants interact with each other, and how does the environment influence what they do?

Evolution is the study of how organisms change through time due to changes in their genes (usually changes in the sequence of their DNA).

So evolutionary ecology is the study of how evolution impacts the way organisms interact with their environment, or how interactions between organisms and environment can influence the course of evolution. Two sides of the same coin, and I find both perspectives fascinating.

Some of my favorite science stories come from the field of evolutionary ecology. Seriously, go read about how toxic newts and their garter snake predators are fighting to the death in an evolutionary arms race. Or what happens when you watch bacteria evolve for twenty-five years while changing their environment in controlled ways (that's a lot of bacterial generations!)? But there are plenty of cool questions and answers in marine evolutionary ecology as well, and that is what I spend a lot of time thinking about. Here is an example of evolutionary ecology thinking applied to a classic marine invertebrate problem.

<- This is a picture of a larval echinoderm (a heart urchin), as seen through a microscope. It will grow up to look like this, as seen not through a microscope. ->

Like any organism, a female echinoderm has a decision to make when she reproduces. (I use 'decision' here as if it were some sort of conscious choice, but since the female echinoderm doesn't have a brain, she is not making a 'decision' like you did when you wanted breakfast this morning. She just does her thing in response to cues from the environment and from her genes. Having seen my brother trying to make decisions before breakfast, he might actually be part echinoderm, but I digress.) Assuming that she has a certain amount of energy stores to make eggs, she can either use that energy to make many many tiny eggs, or a much smaller number of big, energy-rich eggs. Each choice comes with costs and benefits. Many tiny eggs make more offspring, but they may be more vulnerable out in the water. Larger eggs means fewer offspring, but because they got more energy from their mom, they are bigger and may have better chances of survival. So what is a female echinoderm to do?

I chose the example of an echinoderm here more-or-less at random. All marine invertebrate groups are faced with this 'choice.' Some species of echinoderm do one thing while other closely related species of echinoderms do the other. Some species of worms and sea slugs can do both (maybe more on that some other time). Understanding why these different strategies might evolve is the purview of evolutionary ecologists. If food is scarce, is it better for mom to make larger babies that don't need to feed themselves? and if there's lots of food for the babies to eat, is mom more likely to make tiny babies? what if the food supply changes through time in an unpredictable fashion?

My goal here is not to answer these questions (there are whole research programs devoted to them -- see the reference below for a thorough overview), but to give you a feel for the sort of questions that I study as an evolutionary ecologist of marine invertebrates. Next time I'll talk more about those invertebrates -- who are they, why should we care about them, and which ones are the absolute coolest?

Oh! and the answers to the questions about the Octopi Wall Street cartoon on the last entry. We have pictured, from left to right:
- a jellyfish (phylum Cnidaria)
- a grasshopper (phylum Arthropoda)
- a snail (phylum Mollusca)
- an octopus (phylum Mollusca)
- a sea star (phylum Echinodermata)
The sea star is our closest relative in the picture. More on that next time.

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Further reading:
The Evolutionary Ecology of Offspring Size in Marine Invertebrates (Marshall & Keough 2008, Advances in Marine Biology, 53: 1–60.)