Previous: Microbes in Slow Motion
Next: Looking for Answers in the Skull of a Zebrafish



View count:70,535
Last sync:2023-01-05 22:15
Thank you to GiveWell, an organization that makes it easy to find excellent charities to trust, for sponsoring today’s episode!

GiveWell is matching donations from first-time donors, dollar for dollar, up to $250. Click on my link below to donate and be sure to use the fundraiser code MICROCOSMOS at checkout to make sure your donation gets matched.

We here at Journey to the Microcosmos have been very excited for everyone who has ordered their very own Journey to the Microcosmos microscope and started their own explorations. And we love seeing the photos and videos you’ve shared with us on Twitter of all of the rotifers and nematodes that you’ve been finding already. Back before these microscopes were available to buy though, my trusty co-host Hank took a demo out for a spin and saw a lot of…this floating around in the water.

And as any of us would do if we had an expert in our contacts, Hank decided to turn to our master of microscopes James for help identifying these hairy lima beans. And James replied with the following warning: “ they are ostracods and they will eat everything in your sample and poop all the time, until there is nothing left in your sample but a bunch of ostracods swimming in their poop.” Ostracods are a crustacean, encompassing tens of thousands of species and whose bodies mostly range from around 0.2 to 2 millimeters in length. And thankfully, their size makes them large enough for James to filter out using a simple tea infuser, which is how he prevents these animals from turning his samples into a toilet.

But their size is not just a useful feature for filtering. It’s also a clue into a much more fundamental question about the trade-offs between sexual reproduction and survival. To explain what that means, we need to establish two things.

The first is that if you’re looking at a male ostracod, you are essentially looking at a crawling shelled penis. Do you remember how we said earlier that the ostracod usually range from around .2-2 mm in length? Well, for male ostracods, about a third of that length is made up of penis—or rather two hemipenes, which make up what ostracod scientists have called “the large, muscular sperm pump.” The two hemipenes in the males correspond to the two openings on the female ostracod, which hold several dozen eggs ready for fertilization.

And these eggs can have different fathers, so male ostracods compete with each other in part by having their large, muscular sperm pump actually pump out large sperm. And when I say “large,” I don’t just mean “large for such a tiny animal.” Ostracods produce some of the largest sperm in the animal kingdom...sometimes up to 10 millimeters in length, which is around 182 times longer than human sperm. The second thing that we need to establish is that ostracods are remarkably well-preserved.

Their bodies are protected by two shells connected at a hinge, which shelters their soft bodies in life and secures their remains in death. There are ostracod fossils dating back at least 450 million years. And in their many hundreds of millions of years, they have spread to so many waters and been so well-preserved that their fossils tell us stories of how the world’s climate has changed over that long period and how this little crustacean has adapted in response.

But in addition to their geological storytelling, ostracod fossils have a few other very notable distinctions. One distinction is that the oldest fossilized animal penis we have found belongs to an ostracod, one that existed around 425 million years ago. The species was named Colymbosathon ecplecticos, which translates to “astounding swimmer with a large penis.” Another distinction is that the oldest fossilized sperm belongs to—yes, you guessed it—the ostracod.

The remains were found in an Australian bat cave, where the phosphorus from many years worth of bat poop ensured that the soft insides of the ostracod would remain frozen in time. Scientists even found a female ostracod whose remains held post-coital sperm. And these simple facts about the ostracod may paint a somewhat absurd image of ancient crustacean genitalia.

But within that absurd image is also a profound question: is it worth it? Is it worth the exceptional amount of space and energy that the male ostracod devotes to reproduction? To ensuring that its sperm will outcompete all others?

The animal kingdom is full of examples of this seemingly simple question. Think of the peacock, whose ornate train is exquisitely crafted to attract the comparatively drab peahen. The ostracod’s equivalent is its size: the longer the male ostracod, the more it has invested in its reproductive needs.

And in general male ostracods are longer than their female counterparts. The accumulation of these kinds of differences is called sexual dimorphism. The higher the degree of dimorphism, the more different the male and female of a species are.

Some animals have high dimorphism, some have low. And so on both ends, the question remains: is it worth it? On the one hand, for species with high sexual dimorphism, the energy an individual might spend ensuring it stands out from other potential mates is energy that goes towards a biological legacy, one that hopefully aids the species overall.

But that is also energy that potentially comes at the cost of other essential elements of an animal’s survival or makes them vulnerable, leading eventually to their extinction. It can be difficult to untangle this balance with animals we see today, whose present day survival at the very least means their choices haven’t led to extinction…yet. But consider the ostracod.

This is a remarkably well-preserved animal with a long history of investment in sexual reproduction. And over that long history, many species of ostracod have gone extinct, their beginnings and ends documented in the fossil record. So in 2018, researchers studied 93 extinct ostracod species that lived in the Mississippi area around 66-84 million years ago.

They turned to the fossil record to quantify the differences in size and shape between male and female ostracods of each species, as well as to measure how long it took for each species to go extinct. They found that when males were larger than females—meaning that they invested more energy in sexual reproduction—those species tended to go extinct 10 times faster than species where the males were smaller. We’re talking about the ability to survive as a species for 1.6 million years versus 15.5 million years.

So when it comes to the original question “is it worth it?” this result seems like a resounding “no.” For the ostracod, the compulsion to invest in sexual reproduction at the individual level seems to come at a cost to the species overall. But this is one very specific context, with one very specific animal and its one very specific sexual dimorphism. The story of survival and reproduction is more than just any one animal, and the question of “is it worth it?” has only living answers told across past, present, and future.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. We’d also like to thank GiveWell for sponsoring this episode. GiveWell makes it easy to find excellent charities to trust.

They spend over 20,000 hours each year doing intensive research on charities and all of that research is available for anyone to view on their website. GiveWell emphasizes donating to charities that have evidence of improving people’s lives like the Against Malaria Foundation which provides nets for people to sleep under so they are not bitten by malaria-carrying mosquitoes. Or you could donate to their Maximum Impact Fund which directs donations to the charities that GiveWell has determined to be the most effective each quarter.

GiveWill is here to give you the confidence that your donations are being used effectively, and you have the option to do a one-time donation or set up recurring donations. Givewell is free to use and does not take fees from those donations and they are all tax deductible in the US. And right now, GiveWell is matching donations from first-time donors, dollar for dollar, up to $250.

Click on my link below to donate and be sure to use the fundraiser code MICROCOSMOS at checkout to make sure your donation gets matched. The folks you’re seeing on the screen right now. They are people who have supported us on Patreon.

Patreon is a place where you can go and give a little bit of money to the things that you really think should exist in the world, and all of these people really think that Journey to the Microcosmos should exist. And we agree with them and we’re so thankful to them. If you want to see more from our Master of Microscopes James Weiss, you can check out Jam & Germs on Instagram.

And if you want to see more from us, hey, there’s always a subscribe button somewhere nearby.