The Complete Beginner’s Guide to Seahorse Evolution
When you look at a seahorse, it’s easy to wonder how such a bizarre creature could come to be. The seahorse’s behavior and appearance is so radically different from most other fish that one can’t help but ponder how they evolved into what we see today. With it’s unusual horse-like head, chameleon eyes, monkey tail, kangaroo pouch and insect-like armor; how did did it evolve to be so strange?
To understand that, we need to look at some of the seahorses relatives. One issue we face with discovering how seahorses evolved is the lack of fossils. There are a few fossils that show some early seahorses, but like most sea-dwelling creatures, it’s a very limited number. Fortunately for us, many of it’s living relatives give us a glimpse at the seahorse’s evolutionary path. While these relatives aren’t exactly what seahorses evolved from, they give a pretty clear picture of how changes over time go from subtle to extreme to become seahorses.
One of our readers pointed out in the comments that this is already out of date, and new molecular genetic data points to a very different origin of the seahorse family, and that sticklebacks are no longer considered to be part of the evolutionary tree to seahorses. I will be revising this shortly, once I’ve had time to digest the information. A quick cursory glance suggests that the beginning of the article is completely different, and somewhere before pipefish comes something like dragonnettes. Until I’ve digested and updated the article, you can see for yourself in the paper here. Thanks Joe!
First, we start out with the seahorse’s more normal but distant relatives. These are scorpion fish, a large group of ray finned fish. Some are elaborately ornate, like the lionfish.
Others are camouflaged to match their surroundings.
Many though, look just like normal fish.
The next interesting ancestor analog is the stickleback. Many sticklebacks look like a pretty normal fish by all accounts. But their is something new starting. Sticklebacks are starting to develop the armored skeleton for protection, and lacks scales. But it’s still overall very fish-like. The male protects the eggs in a bubble nest he creates. The Three Spine Stickleback Gasterosteus aculeatus pictured below still looks overall fish-like.
Then we come to the Fifteen Spine Stickleback Spinachia spinachia. Its mouth is elongated, its body stretched out; it’s starting to look less like your common fish. In sticklebacks, the parental care is done by the male. This also is not uncommon in fish, with many males taking on the role of primary caregiver.
Now we come to the middle of the journey. Here we have trumpetfish, Aulostomus spp. Still fishlike, still swimming like a fish, but the mouth of a seahorse is clearly evident. It’s an open water spawner, with no parental care. We don’t know where it diverged from it’s common ancestors or why it’s a broadcast spawner, but other traits, such as the elongated body, and suction like mouth are similar to seahorses.
It’s body is still fish like, and it swims like most common fish; it shares a similar mouth shape to seahorses, but less refined.
An ancestor similar to the cornetfish Fistularia spp. probably came next. Also known as flutemouths, these elongated fish still swims mid water, but has reduced fins and a very long snout. They are probably the largest of the fish we’ll be looking at, with some species growing up to 6′ (~180cm).
Next in line is the ghost pipefish, which grows only to a maximum of 6″ (~15cm). They are probably a branch off of a common ancestor that shared many of the traits seahorses do, but with some differences. These fish have started to move to caring for their eggs on their body, like most close seahorse relatives. However, it’s the female that carries the eggs, clutching them in her pelvic fins.
Flagtail pipefish are the next on this evolutionary ride. Care of the eggs is once again the realm of the males. Chances are it never left, but it’s not clear why some living relatives like the trumpetfish and ghost pipefish developed different reproductive strategies. It’s pretty clear this is the beginning of what we think of as the paternal care common in these fish. The male carries eggs laid by the female in an intricate dance along his belly. Dunckerocampus spp. carries the eggs on their bellies completely exposed, while Doryrhamphus spp. has a flap of skin that helps protect the eggs. Flagtail pipefish swim midwater much like the fish listed above.
From there we go to pipefish that carry the eggs at the base of their tails, some in partial pouches, later with pouches that almost entirely encase the eggs. Most still have a tail fin, but it is getting smaller. They slither close to surfaces, using their bodies as anchors. Many use their bodies and even their tails to help grip on to rocks, seagrass, or floating algae.
There are at least 200 species of pipefish, with a dizzying array of body types and behaviors. Some live in seagrass beds, others on coral reefs. Some are only 2″ (~5cm) long, but the biggest species grows to over 2′ (~60cm). The photo below shows a literal handful of different species found off the coast of North America.
Now we start to see an amazing transformation. Pygmy pipehorses are the next in this evolutionary march. These tiny fish are all 2.5″ (~65cm) or smaller in length. They hitch just like seahorses and lack a tail fin, and their body is starting to take the angular shape that seahorses have, but their heads are still overall in alignment with their long bodies. Interestingly, males prefer to keep their body vertically, but females perch more upright, similar to seahorses.
Pygmy pipehorses are loosely grouped as pipefish-like pygmy pipehorses and seahorse-like pygmy pipehorses because of how similar they are to one or the other. The first of which is the pipefish-like pygmy pipehorses. They do not have a tail fin, instead using their prehensile tails to grasp onto algae and wait for food to swim by. They are frequently misidentified as pipefish or missed all together because of their diminutive size.
The seahorse-like pygmy pipehorses could almost be mistaken for seahorses. One beautiful example is the Sydney Pygmy Pipehorse Idiotropiscis lumnitzeri. They look much closer to that of a true seahorse, and even have some of the head structures that seahorses have.
The head is distinct from the body, the male has a full brood pouch at the base of the tail. The head can bend, but is usually held in alignment with the body. They don’t chase down prey; instead waiting for it to drift past their holdfast.
And finally we get to seahorses, the strangest fish of them all. They’ve made the leap to standing upright most of the time, the bent head allowing for a longer reach to snap up prey. But like their distant ancestors, still relies on camouflage to hunt and gulps their prey whole; only this time through a straw.
I hope you enjoyed this look into seahorse evolution. As mentioned earlier, this is a rough map based on living relatives, not the exact ancestors of seahorses. Taxonomy, the study of how animals are related and categorized is always changing so we may find new information about these relationships as time goes on. But hopefully these examples will make it easier to understand how the seahorse became what it is today.