When you think of, like mussels and  clams and other bivalve animals,   you might think of something like this.

Sheltered and static, and perhaps sitting  on your plate at a fancy restaurant. But before the mussel got to  your plate, it led a life— and all things considered,  a surprisingly active one.

Like the animals themselves, the  word “bivalve” seems simple enough. These are mollusks made from a shell that is hinged in two parts, which are called valves. But the diversity of bivalves is vast,  encompassing an array of animals like clams,   mussels, scallops, and oysters that are  found in all sorts of watery habitats.

Some have even been found in  hydrothermal vents deep in the sea. Now luckily, because bivalves are so widespread,   we didn’t have to travel as far as a  deep sea hydrothermal vent to find one. These samples came to us from the Black Sea.

And when James, our master of microscopes,   first saw them in his samples, they  were just tiny black dots on seaweed. They were so small that he  thought they were sand grains… until he noticed that one of those  little black dots was moving! Putting those dots under  the microscope and finding   a tiny shelled creature was an exciting surprise.

But we here at Journey to the Microcosmos  are not experts in the ways of bivalves. So we decided to reach out  to someone who is an expert: J. Evan Ward, Professor and Head of the Department   of Marine Sciences at the  University of Connecticut.

And one of the things Evan told us after we showed  him our videos is that it is hard to tell exactly   what bivalve species we are looking at because  these are young bivalves, which are called spat. When bivalves reproduce,  they produce larvae,   which can then develop through different  life strategies depending on the species. Some will become pests to  fish, others just simply grow.

But eventually, they develop into spat. And Evan told us that most bivalve spat are pretty   active because they have a  very important goal in mind: they’re trying to find a place to settle down,   so they have to move around  to find a good home. So how long they remain a spat will depend  on the conditions of the water around them.

And at the size ours seem to be, Evan estimated  that if it were summer and they had plenty of   food, they would likely grow out of this stage  in just a few weeks and become juveniles. One of the ways that these bivalves  try to find a home is by using what   looks like a giant tongue sticking  out of their shells to crawl around. That tongue-like thing is actually called the   “foot,” which is used to find  a good place to settle down.

If these spat are tiny mussels,   they will produce strong fibers made from  protein that are called byssal threads. For those of you who have cooked  mussels, you might know byssal   threads as the beard-like thing that gets  removed before you actually cook them. While these spat are too young for us to name,   there are still some potential  clues to their identity.

In this clip, it seems like we might be looking   at a clam based on the shape of  the shell, which is more rounded. Meanwhile, here the oblong shape of some of   these shells suggests that we  might be looking at mussels. But even if we don’t know  exactly what these species are,   there are still fascinating details  that Evan pointed out to us.

Well, one of these details is unfortunate: most of these spat are dead, or dying, which is  why you can see a lot of ciliates inside of them. But if you look towards the spat that’s  a bit more left of center in the screen,   you can see that this one is still alive,  and that its shell is opening and closing. Along the edges of the shell are  little bundles that look like hair.

Those are the spat’s tentacles, and  their job is to act as a guard so that   if something intrusive brushes along it, the  bivalve knows to clamp the shell back down. And the ones at the top left and top  center here appear to be alive, too. The translucency of their shells means that we can  see movement of cilia along the gill filaments,   which are those lines that look almost  like a rib cage through the shell.

Most bivalves are filter feeders,  and the movement of those cilia   helps them capture food from the  water that passes over their gills. And it’s really cool that we can see these  things up close because it gives us a   little hint of just how complicated  the inner worlds of bivalves are. But the transparent shells of young bivalves  don't last forever, and as bivalves develop,   that inner world becomes harder to access.

Much of the early knowledge of  how bivalve organs worked came   from studies on young bivalves like  ours and their transparent shells,   or from studies on dissected bivalves or  bivalves that had holes drilled into them. And while those techniques  gave us important insights,   they also weren’t necessarily  able to give the full picture. A young bivalve’s organs may not have  developed enough to reveal how it works.

An isolated organ is missing important context  that might change the way it functions. And a bivalve that’s just undergone surgery might   not provide the best example  of how its organs perform. But what if there were a way to see into bivalves,   to peer into the secrets they  keep buried between their shells?

Well, that’s what we’re looking at here. These are the gills of the  blue mussel Mytilus edulis. As we watch, water is passing  over and through the gills,   and yellow particles are getting trapped  along the surface of the filaments.

As you can probably tell, this is not a  typical Journey to the Microcosmos video. This is a J. Evan Ward production, a  result of a technique that he and other   scientists developed in 1991 using  an endoscope to peer into bivalves.

Not all of the bivalves they tested this  technique on were immediately open to the idea. In their paper, they described one particular  clam species’ sensitivity to the endoscope,   though they were able to make some observations. But other bivalves seemed to be more amenable,   giving the scientists a whole  new view of the creatures.

When we talked to him, Evan said that part  of what made developing this technique so   cool was that he could see things working  in these creatures in a way no one had   seen before, and it was so much more  sophisticated than people had known. Videos like these helped scientists  understand how bivalves pick and select   particles to consume, and better  understand more of the complex   machinery in these sophisticated animals that  otherwise seemed like simple creatures. So much of the way we understand the world around   us comes in the choices we  make in how we observe it.

Whether it’s changing the way you shine  light from a microscope, or finding a way   to get a camera into a shelled creature,  there is always another way to see things— and always another way to be  surprised by the complexity of nature. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us. The folks on the screen right  now are our Patreon patrons.

Every single name, is a person in the world who  supports this channel, and we are so grateful   to them that they have allowed us to go on  this really remarkable and wonderful journey. Thanks, everybody for being a part of this. If you want to see more from our  Master of Microscopes, James Weiss,   you can check out Jam and Germs on Instagram.

And if you want to see more from  us, just click on the channel name. We have a ton of other videos.