How the Seahorse Got its Shape (an evolutionary tall tail)

January 23, 2013 • 6:53 am

Attend and listen, oh my Best Beloved: I’m not going to even try to mimic Matthew Cobb’s famous use of Kiplingesque language to explain How the Beetle Got His Handles.  Instead, I’ll use plain English to talk a bit about seahorses.

The other day I put up a funny video in the “True Facts” series, “True facts about the seahorse“, and at least one reader wondered why these silly animals even exist. They can barely swim (yes, they’re fish), and although they’re cute, they hang onto vegetation with their tails all day, snapping at small animals like shrimp or fish larvae to get food.

And the males get pregnant—females insert eggs in the males’ pouches, and the pregnant males have to do all the gestation. (That leads to a shortage of available males, by the way, for there are fewer non-pregnant males than there are females with eggs to donate. Thus, unlike what happens in most animal species, female have to compete for males. And this reverse sexual selection explains why if only one sex in a seahorse species is brightly colored or ornamented, it’s the females.  For most animals it’s the reverse, since males are competing for females and evolve ways to call attention to themselves.)  For you ladies who have suffered the pains of human childbirth, you can perhaps take consolation in this video of a male seahorse in “labor,” then producing a gazillion young:

Anyway, I’m not going to explain the pregnancy difference; I’m sure evolutionists have concocted a story, but I don’t know it. Instead I want to talk about a recent theory for why seahorses have that crazy shape—like tiny horses without legs.

Seahorses evolved, in fact, from straight fish, something probably like their closest relatives, the pipefish (subfamily Syngnathinae).  You can see in the video below that, with their big eyes and long snouts, pipefish look like stretched-out seahorses:

Like seahorses, pipefish swim using only a single dorsal fin (a few species have other fins as well), which isn’t really a great way of swimming, but it does the job, especially since seahorses don’t swim very much.

We’re pretty sure that the ancestral seahorse was straight, and then evolution turned it into a horsey form, for molecular and morphologically-based phylogenies show that the “outgroup” for seahorses are pipefishes. That is, all seahorses have shared derived characters within a larger group that includes pipefish and then other fish:


So why give up your vocation as a straight-swimming pipefish to evolve a horselike morphology with a deep chest and then a prehensile tail to hang onto vegetation all day? That means that you no longer search for food (seahorses spend about 80% of their time attached to the substrate), but become a “sit-and-wait” predator that noms anything that swims by. When I was looking for other seahorse videos, I found one made by the journal Nature highlighting a 2011 paper on seahorses by Sam Van Aassenbergh, Gert Roos, and Lara Ferry (reference and link at bottom). It purports to explain the adaptive significant of the seahorse shape and behavior:

As Van Wassenbergh showed via both modeling and filming of seahorse feeding, by turning your body into a bent rather than straight shape, and developing that deep “chest,” seahorses are able to strike at prey at a farther distance from their eyes. As the paper notes:

Forward dynamic simulations of cranial rotation revealed two main effects of sharpening the angle between the head and the trunk in the first model (based on the morphology of the pipefish S. leptorhynchus). First, the velocity of the mouth decreased (Fig. 3a). Second, strike distance (defined as the distance between the starting position of the eye and the final position of the mouth) increased considerably (that is, + 28%) when gradually transforming the pipefish model into a more seahorse-like shape (Fig. 3b).

In other words, by aligning oneself so that the head is bent, and counterbalanced by a heavy “chest,” you can thrust up farther towards prey than you could if you were a straight-bodied and free-swimming pipefish.

A 28% increase in strike difference means, of course, that you can get more food, and that’s the supposed “selective advantage” of evolving that seahorse shape. The shape apparently evolved after the seahorse had already become sit-and-wait predators since some species of pipefish, the “pygmy pipehorses”, also attach themselves to the substrate but don’t have the bent body. Here’s one species, of pygmy pipefish: look at this camouflaged beauty!:

Anyway, in their abstract the authors claim that they’ve explained the adaptive advantage of turning from a pipefish into a seahorse. It’s feeding efficiency:

The results from the mathematical modelling were confirmed by kinematic data of prey-capturing syngnathids: compared with straight-bodied pipefish, all seahorse species studied consistently show an additional forward-reaching component in the path travelled by the mouth during their strikes at prey. This increased strike distance enlarges the volume of water they can probe for food, which is especially useful for tail-attached, sit-and-wait predators like seahorses. The biomechanics of prey capture thus provides a putative selective advantage that may explain the bending of the trunk into a horse-like shape.

But there’s one problem.  Seahorses can strike farther than pipefish, but they also strike more slowly:

In accordance with the pipefish model results, our second model, based on the seahorse Hippocampus reidi, showed reduced velocities of the mouth travelling towards the prey (that is, − 36%) compared with more elongate versions of this model (Fig. 3c), whereas the body shape facilitated striking at prey located a greater distance away (Fig. 3d). Consequently, our model highlights a trade-off between strike velocity (favoured by a head in-line with the trunk) and strike distance (favoured by sharper angles between head and trunk as observed in seahorses).

That’s a problem!  Seahorses can strike farther than pipefish, or something shaped like a pipefish, but they strike more slowly. And presumably speed of strike is important, too, because if you lunge too slowly, the prey will get away.  So, as the authors note, there’s a “trade-off” between strike distance and strike speed.

Given the trade-off, how do they know that turning into a seahorse will, on the whole, get you more food? They don’t!  They just assume it, and in this way the paper limns a clever but unproven evolutionary story, somewhat akin to the tales of bad evolutionary psychology.  In fact, in the conclusion, the authors seem pretty sure that it’s feeding efficiency that drove the evolution of the seahorse shape:

If the evolution of the seahorse body shape occurred as a result of natural selection, this anatomical shift should have resulted in an increase in fitness, as might be facilitated by an increase in strike distance during feeding.

Well, I’m pretty sure that natural selection was somehow involved in this change (but wait—we haven’t yet heard from Larry Moran!), but I’m not convinced that it’s selection due for feeding efficiency.  The authors haven’t shown, given that a bigger strike is also a slower strike, that feeding efficiency drove the evolution of that funny shape. It may in fact lower your food intake to have that shape, but that there’s some other advantage to the shape that we don’t understand. Remember that attachment to the substrate by a prehensile tail probably preceded the evolution of that shape, so there may be other reasons connected with reduced predation, inconspicuousness, and so on.

In the end, we don’t understand how the seahorse got its shape. We have one possible explanation, but it’s not very convincing.  So, sadly, I can’t answer the readers’ question, but we can still marvel at these silly but endearing animals.

It’s embarrassing to me that the Nature video above doesn’t mention the big problem with the study—the problem of slower strikes associated with the seahorse’s shape. Instead, the narrator blithely concludes, after describing the study, “And that’s how the seahorse got its curiously curved shape.”

Well, no, we don’t know that. It may be true, but some caveats were certainly in order. One would expect better science reporting from Nature!

I end with an exhibit from Buzzfeed’s “31 kids who are too clever for their own good” (some funny stuff there):

Picture 3

h/t: SGM


Van Wassenbergh, S., G. Roos, and L. Ferry. 2011. An adaptive explanation for the horse-like shape of seahorses. Nature Communications 2011/01/25/online.

25 thoughts on “How the Seahorse Got its Shape (an evolutionary tall tail)

  1. Perhaps strike-speed is not as important for different types of prey? Another thought: If you are glued to the ground (so to speak), the pipe-shape may not allow you to achieve the faster strike speed. If that is the case, then the horse-shape advantage would predominate.


    1. Strike speed may not be important. For very small prey organisms like planktonic crustaceans the viscosity of seawater hampers the ability of the planktonic prey to move through water. For them, it’s roughly analogous to humans swimming through honey or molasses. The larger size of the seahorse head enables it to move more efficiently through water than the plankton. The slow strike speed of the seahorse may still be faster than the prey’s ability to escape. See Reynolds Number.

      1. “The slow strike speed of the seahorse may still be faster than the prey’s ability to escape.”

        This is certainly true. Otherwise seahorses would be extinct!

  2. I would think the environment would have a big effect on what was beneficial. I second gbjames’ comment; if the slowest strike is faster than most or all prey could evade then it would be irrelevant. Perhaps it would be more important in an open environment compared to dense algae?
    Could camouflage have something to do with it? Could there be some select environments where pipefish are evolving slighter larger chests?

  3. Very amusing student response. Also a good argument for why women should buy a fish rather than get married. 😉

  4. Ever since reading the van Wassenbergh paper in Nature, I wondered and puzzled and hoped to learn more about the evolution of seahorses here.
    May I humbly nominate this post for the “Henry Albert Bivvens, A.B., Award for Infinite-Resource-and-Sagacity” ?

  5. That was great post. I must admit, when the old ‘male seahorses get pregnant’ thing comes up, I always thought it was bit of hyperbole – males have a pouch and the eggs (provisioned from the female) get bigger and hatch with no input from him, so all he is doing is carrying a bit of extra weight around (not much of problem for someone who doesn’t move 80% of the time), but that video really made me think – that parturition looked exhausting and even painful.

    1. As I remember it the male provides at least oxygen through blood vessel gas exchange. More effort invested than in a mere pouch, certainly.

  6. Cute animals!

    By the way, wouldn’t the conclusive test be to look for feeding efficiency, say as growth rate?

    And maybe I’m making this up, but I seem to remember someone waaay back claiming that the placenta would mean the animal would have to bulk up before pregnancy (re body form and feeding efficiency).

    It is funny how you can substitute “just-so story” for “seahorse”, “don’t-know response” for “barnacle”, and “evolution theory” for “aquarium” in the kid’s response. In fact, I suspect that is how Larry Moran views adaptationists.

  7. If there are pipe fish with similar lifestyle & prey, perhaps it is the anchoring which is most important. Perhaps they evolved where there were stronger currents? I am sure I read an article last year that said the most dynamic areas for marine evolution are margins/estuaries…? Ring a bell?

    PS The child clearly had no idea of the marvels of barnacle sex!

  8. Strike velocity also may trade-off with accuracy, so one cannot assume that velocity = better. Also, many trade-offs are easily compensated by concomitant changes in other traits that affect the system. If strike velocity is important, then the seahorse can have its cake and eat it too with modifications to muscle volume, muscle contractile properties, and muscle gearing. For example, the muscle that fish use to escape is high-geared, which enables large bending into the characteristic C-shape at the start of the evasion response. But as anyone who has ridden a bike knows, high-gearing trades-off with output force. Fish compensate for the low output force by having a large volume of muscle that powers this behavior. What I didn’t see in the paper was actual strike speeds in the seahorse relative to the pipefish. That is, is the seahorse using one of the above mechanisms to maintain high strike speed?

  9. Barnacles almost drove C. Darwin around the bend, & the close resemblance between the free-swimming larvae of barnacles to other sea creatures not closely related has led to some crackpot theories about cross-phylum mating events. Barnacles are not completely without interest.

    1. Camouflage seems to be the primary defense for most sea horses, though I believe I’ve heard that some species have armored bodies, like a box fish.

  10. Hey, has anyone checked the metabolic rates of sea horses compared to pipefish? Being a more sedentary animal that attaches itself to seaweed or other structures much of the time might have lowered the sea horse’s energy requirements enough compared to a free-swimming fish that it didn’t need to be as efficient at food collection.

  11. Do males & females change sex as many fish do depending on size? (just been listening to part 2 of this BBC Radio programme where they discuss this)
    That would put the whol;e male pouch thing into perspective ie do males become females at some point or females males? If so the stress of brooding (rather than pregnancy?) would be paid back by becoming an egg producer.

    1. Nope, seahorses and pipefish are not among the fish that do this, though they all have the males carrying the eggs. But fish have very flexible sex “roles” in general, with lots of species where the males provide parental care.

  12. Delightful and interesting post.

    There is an amazing seahorse exhibit at the Monterey Bay Aquarium. If ever a set of animals looked like they were not evolved but dreamed up by Disney, it was these. In real life, the leafy sea dragons look even less real than they do in pictures. Even their texture looks vaguely like plastic.

  13. Well, I am tempted to posit my own hypothesis. Although I am no evolutionary biologist of any measure, here goes.

    JC, you state that it is likely that the prehensile tail preceded the bent shape. Proceeding on that assumption, I draw attention to the Pygmy Pipe-fish video. In response to changes in direction of currents, the Pygmy is pushed about (or so it seems) in a manner that puts its mouth “downstream”. Food would likely also be caught in that current. However, the Pygmy’s mouth would no longer be in a position to feed on the oncoming prey.

    It appears, and I base this solely on the videos I have viewed here, that the Seahorse is more resistant to changes in current, or at the least, its mouth remains aligned with the current more efficiently, allowing it to benefit when currents drive prey toward it.
    Imagine the economy of energy when you wait for the food to come to you, rather than swimming about chasing it.

    How’d I do?

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