The world of marine fish is a strange one, to say the least. Not only do fish species vary dramatically in form, behavior and just about everything else, but the ocean they inhabit is a proverbial black box. What do I mean by this? Well, the ocean is both vast and largely still unexplored, introducing the distinct possibility that fishermen, scientists, or even regular people will pull out something random/ unknown/ thought to be dead for millions of years. And it’s happened before…more than once. I mean, we are talking about an ocean that covers 71% of our planet’s surface…that’s a casual 335 million square kilometers (or 129 million square miles if that’s your jam) or, when factoring in depth, 1.3 billion cubic kilometers (310 million cubic miles). All this ocean has an average depth of 3,682 meters (or 12,080 ft). So yeah, it’s not only expansive, but it’s also really, really deep, with many parts not super accessible to us humans, what with our constant, limiting need for air, and non-body crushing pressure levels. So maybe it’s less than surprising that according to NOAA’s recent estimates, more than 95% of this ocean remains unexplored. Let that sink in for a second. Ninety-five percent! We, as a species, have an intrinsic desire to explore the unknown, whether it’s Lewis and Clark trekking across the US, or astronauts bounding across the surface of the moon. Yet we have only explored 5% of the ocean!? No wonder we are always pulling out weird and unexpected stuff…
As a child my favorite book was Slimy, Creepy, Crawly Creatures by Joe Kaufman (which I reread frequently enough to concern my parents). The book describes all kinds of critters that I found absolutely riveting, ranging from frogs that raise their babies in their mouths to whale sharks topping 15 meters (50 feet) in length, but none fascinated me more than the coelacanth. What’s a coelacanth, you ask? Or maybe you didn’t ask, but I will tell you anyway. Spoiler alert, it’s an order (or group) of fish species. A super old order of fish species that looks kind of like this:
The story of the coelacanth starts with a brief lesson in fish phylogeny. You see, when osteichthyes, or bony fish were evolving way back when, a key split occurred in their evolutionary lineage. Namely, the sarcopterygians, (a.k.a. the lobe or fleshy finned fish) went one way, and the actinopterygians (the ray finned fish) went another. Now, in general the actinopterygians were more successful, mostly because they contain evolutionarily advanced teleosts, a group with almost 30,000 different species, which is more than all other vertebrate species combined, and make up 96% of all extant (still living) fish species. While teleosts are remarkably diverse, all of them possess “ray fins” that are simply webs of skin stretched over bony or horny spines for support. These fins do not contain independent muscles, and are controlled entirely by muscles within the fish body.
The sarcopterygians, on the other hand, went another route. Coelacanths, which fall within this category, once contained 80 different species, and entered the fossil record roughly 400 million years ago. These species developed musculature in their fins, allowing independent movement of these appendages, almost like…legs or something. Hmmmm…weird, right? Why would fish need muscular fins that can move independently kind of like legs? Well, turns out they don’t, which is why the teleosts with their non-muscular fins took off as a group, becoming dominant among modern fish species. The coelacanths, meanwhile, quietly evolved into amphibious tetrapods… you know, just the first vertebrates to transition from the ocean onto land, giving rise to every other terrestrial vertebrate species on earth. No big deal. Definitely not a defining moment in the evolution of life as we know it, or anything…
So as terrestrial species flourished and diversified, coelacanths became extinct along with the dinosaurs near the end of the Cretaceous period. Or so we thought. Fast forward about 66 million years (+10 years just to be safe) to 1938 when a South African museum curator Marjorie Courtenay Latimer is called over to a fishing boat to sift through their trawl catch in search of museum specimens, something she has done countless times before. As she sorts through the pile of fish, suddenly she sees it…something extraordinary! A live coelacanth! Turns out these guys were slightly less extinct than everyone thought, and still exist today. Apparently while dinosaurs and innumerous other species were evolving and dying off, two of the 80 coelacanth species were hanging out at the bottom of the ocean, virtually unchanged, for millions of years.
These two species, the West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis) are very elusive deep sea fish that live 700 meters (or 2,300 feet) below the surface of the ocean…not really a depth where humans would inadvertently cross paths with them. Since successfully confirming their existence on earth (that was step one), we have ascertained that coelacanths can grow to 2+ meters (6.5 feet), weigh up to 90 kilograms (200 pounds), and live for 60+ years. Unsurprisingly, no one knows much about their overall population size, other than it is more than zero, as we initially thought. So there you have it…reach into the black box of the ocean and pull out a living fossil, a fish that not only looks weird, but also represents a key link in the evolution of all terrestrial species, and fell off the radar for a not so brief period before we realized that these guys are more shy and deep-sea dwelling than actually extinct. I bet Ms. Marjorie Courtenay Latimer told the story about the day she stumbled across a coelacanth every single day for the rest of her life…give or take 66 million years.
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Catherine Caruso is a master’s student in the Berlinsky Lab at the University of New Hampshire. As you might have guessed from the post above, she gravitates towards weird fish, and is currently studying summer flounder, which collectively have a face only a mother could love. She is focusing on how temperature effects sex differentiation early in the developmental process.