Episode 136 – Seahorses

Listen to Episode 136 on PodBean, YouTube, iTunes, or find it somewhere else!

Not all fish are shaped, well, like fish. In this episode, we discuss the bizarre group of tube-mouthed, prehensile-tailed, mostly finless, bony-plated wonders: Seahorses

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Horses of the Sea?

Seahorses are very strange fish. They have an unusual vertical posture with an angled horse-like head; their bodies are covered in bony plates; their mouths are toothless tubes; they have no tail fins or pelvic fins, instead using mainly their dorsal fin for propulsion and using their tails to grasp onto their environment; and, very famously, seahorse males are the ones that brood and develop young. Weird.

A sampling of Syngnathidae.
Top left: Short-snouted seahorse (Hippocampus hippocampus). Image by Hans Hillewaert, CC BY-SA 4.0
Top right: Leafy sea dragon (Phycodurus eques). Image by James Rosindell, CC BY-SA 4.0
Bottom: Alligator pipefish (Syngnathoides biaculeatus). Image by TheMadBaron

And yet, seahorses are not alone in their strangeness. They belong to a group called Syngnathidae, which also includes pipefishes and sea dragons, which share many features with seahorses, including tubular mouths and reduced fins, and – believe it or not – all members of this group exhibit some degree of male pregnancy!

Top: CT scans of two seahorse skeletons: Hippocampus japapigu and Hippocampus pontohi. Image by Short et al 2018, CC BY-SA 4.0.
Bottom: Leafy seadragon skeleton. Photo by Nesnad, CC BY-SA 4.0

How Did this Happen?

Not much is known about the evolutionary history of seahorses and their relatives. Despite their bony bodies, they don’t often fossilize very well. The oldest syngnathid fossils date to the Eocene, around 50 million years ago, which lines up with some genetic estimates for the origins of the group around the time of the Paleocene-Eocene Thermal Maximum. The oldest fossils of true seahorses are Miocene, around 12 million years ago, but there’s plenty more for us to learn.

Plenty of research has been dedicated to the evolution of the most bizarre features of seahorses: their upright posture and male pregnancy. Fortunately, not only are there genetic clues as to how these features came to be, there are also lots of living seahorse-cousins that exhibit a spectrum of variations on these themes. Between their fellow syngnathids and their more distant cousins, modern species give us many examples of what earlier stages in seahorse evolution might have looked like.

Diagram of a pregnant male seahorse by Lin et al. 2016, CC BY-SA 4.0
Notice the position of the anal fin: that’s where a seahorse’s anus is and where its tail begins. You’ll hear us shocked by this in the episode!

Learn More

The oldest fossil seahorses (technical, paywall)

The evolutionary origins of Syngnathidae (technical, paywall)

The evolution and physiology of male pregnancy in syngnathid fishes (technical, paywall)

Recent research on the extreme end of seahorse male pregnancy: a placenta-like structure for nourishing developing embryos

Recently-discovered species (2015): ruby seadragons (and the technical paper)

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