Science journalists are regularly thrown into new topics, which we have to master quickly in order to meet deadlines. Because of this, there are times where we’re introduced to an idea, a field of study or even an animal within a limited context (particularly for generalist writers, like yours truly). This week, I became intimately acquainted with an animal that I thought I already knew: the tunicate.
A few days ago, we learned about tunicates–also called “sea squirts,” for their water-gun reaction when squeezed–in the context of animal modeling. Prior to this, I only knew of tunicates through a short feature I wrote on marine invasive species–plants or animals shifted to a new environment via artificial means. Invasives are bad news for local ecosystems that have not evolved to deal with them.
So, I learned of tunicates in this context: right here in Cape Cod, an invasive species called Didemnum vexillum is infringing on local eel grass beds, which are also a favorite spot for juvenile bay scallops. The fear is that the Didemnum may edge out the scallops, which are economically important here (regrettably, I just realized that the published piece did not include the species name, likely because the story originally appeared in print and the words were cut for space. It’s unfortunate because there are also native tunicates in the region, so leaving out the species name may lead to confusion. This, however, is a topic way outside the scope of this blog post…)
What I’ve learned since my article is, like sea urchins, tunicates are handy for cell and developmental research because you can watch the fertilization process under a microscope. Tunicates are even more commonly used in evolutionary biology, where they give insight to chordates, a phyla that includes vertebrates and therefore includes you and me. Reducing the products of specific genes in a tunicate, particularly if its genome has been sequenced, can show how those same changes might affect a vertebrate.
Tunicates are primitive chordates which mean, like us, they have a ventricle heart, a dorsal nerve cord and a notocord (in human embryos, the notocord eventually is surrounded by tissue that turns into vertebrae, after which the notocord disappears). If you don’t believe me, here is a tunicate’s heart beating under a microscope:
Yesterday, the Logan Science Biomed team attempted a first for the program: fertilizing up some tunicate babies. We used a species called Ciona intestinalis, a non-invasive Didemnum cousin whose genome has been sequenced. Here’s what we did:
First, we selected a tunicate (don’t look like much, do they?).
Then, we cut off its tunic (yes, they wear tunics, hence “tunicate”).
And then used a dissection microscope to find some sperm and eggs (tunicates are hermaphoditic; they have both gonad flavors).
Here’s the view through the microscope. The sperm is held in squiggly white sacs, and the eggs in a pink kidney-bean-shaped ovary. Some eggs have come out of the ovary. They are the white flecks.
Next, we pipetted as many eggs as we could…
…and transferred them to a clean dish filled with seawater.
Then, we checked the eggs under the microscope. They looked like a child’s drawing of a sun; the rays are actually cilia, or hair-like structures, which help protect the egg.
Next, we broke the sperm casing and pipetted as much as we could into an eppendorf tube, then mixed in some sodium hydroxide to get the boys swimming. We peeked at them under the scope, but they don’t photograph well so you’ll have to use your imagination (envision grains of black pepper swimming and you’re almost there).
We mixed the sperm and eggs from two separate tunicates together in a dish, because although they have all the equipment to go at it alone, they prefer to dance with a partner (likely to generate a more diverse and robust population). A few hours later, the fertilized eggs had blossomed into 16-cell embryos. Here is what they looked like under the microscope.
This morning when we checked in with the cells, we had tunicate larvae, which resemble tadpoles. In this photo, taken through through the eyepiece of a microscope, a larva’s tail is wrapped around its body. The black dot is an eye.
And here, another larva swims free. It will eventually metamorphosize into the sessile blob you saw in the first still photograph. Ah, the circle of life.
-Brooke Borel
