For our 6th door, I want to introduce you to a puzzling aspect of my favorite study organism, Calanus—a group of tiny marine zooplankton that play a crucial role in the North Atlantic and Arctic ecosystems.

These crustaceans, no larger than a grain of rice, sustain our oceans and fisheries by serving as a crucial food source for fish, seabirds, whales, and other marine life. My research focuses on understanding how well Calanus can withstand and adapt to climate change, as their fate will shape the future of marine ecosystems.

To determine how well equipped they may be to face future climate changes, I analyse their DNA, studying how their genetic code—passed down through generations—might hold clues to their resilience. But here’s where the story gets fascinating: the DNA of Calanus species- and indeed many marine zooplankton- is surprisingly vast and complex, presenting a significant scientific challenge. 

Tiny Zooplankton, Giant Genomes

Every living organism has a genome- a complete set of DNA containing all the instructions needed for life. For humans, this genome consists of about 3.5 billion base pairs of DNA—those famous A, C, T, and G letters arranged in complex patterns.

Now, compare this to Calanus finmarchicus, one of my primary study species. Its genome is more than twice the size of ours! And its Arctic relative, Calanus glacialis, has a genome four times larger than humans.

Let that sink in: a creature smaller than your fingernail carries a genome vastly larger than ours.

Why Do They Need Such Large Genomes?

Carrying and maintaining such a massive genome isn’t free—it demands energy. So why would Calanus invest in such a costly feature? 

We don’t know for sure, and their small body size presents a practical hurdle too: extracting enough DNA from a single specimen for full genome sequencing is challenging. I’ve developed a specific DNA preparation protocol to analyse parts of this genome, but getting the full picture remains out of reach- for now.

 One hypothesis is that their enormous genomes provide them with an evolutionary advantage, possibly tied to the extreme and changing environments they inhabit.

For example, a study on Calanus hyperboreus found that individuals living in the high Arctic had larger genomes (around 12 billion base pairs) than those in the milder Oslofjord (9 billion base pairs). 

This raises an exciting question: does this extra DNA contain useful information that helps these animals adapt to their environment, or is it just “genetic baggage”?

What’s Inside These Giant Genomes?

One possibility is that these genomes grew through duplication events during their evolutionary history. Such events might occur in response to drastic environmental changes, like ice ages, and could provide organisms with “extra” genetic material to “tinker with” for adaptation. These duplicate genes could serve as a larger toolbox for adaptation, offering multiple versions of a gene with slight variations. 

Another explanation comes from what was found in krill, another group of marine crustaceans with even larger genomes (up to 48 billion base pairs!). Researchers discovered that much of krill DNA is made up of transposable elements—or “jumping genes”. These are repetitive sequences that copy and insert themselves throughout the genome, drastically increasing its size over time while potentially introducing new genetic variations that could aid in adaptation.

Could the same be true for Calanus? That’s one of the mysteries I’m trying to solve.

Why It Matters

Understanding what makes up these enormous genomes and why they evolved this way could reveal how Calanus and other zooplankton respond to environmental stress. If we can link specific genetic traits to their ability to thrive in warming oceans, it could help us predict how marine ecosystems will cope with climate change.

Studying such a vast and complex genome is no small feat—especially in creatures as tiny as Calanus. But science thrives on curiosity and persistence, and I hope to uncover more of their secrets in the years to come.

So next time you think about the ocean, remember: even the smallest creatures can have the biggest surprises!