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):
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.