Sunday, August 31, 2014

Shape of the seahorse

yellow thorny seahorse
Standing up straight is relative
when you are a seahorse
photo: Peter Ryngaert, Project Seahorse
Guylian Seahorses of the World 2005
You may have been told to stand tall and sit up straight, but seahorses do the opposite: they maintain a weird curvy body shape to find food and avoid becoming some other fish's dinner.

Seahorses are master hunters, catching prey on over 90% of attacks. Who knew?

Seahorses are ambush predators, meaning they sit still and pretend to be vegetation until a suitable prey – in their case, tiny copepods and other shrimp-like crustaceans – swims by, and then they attack with a lightning-fast strike.

To be so effective, a sit-around-and-wait predator must be sneaky and stealthy, and seahorses excel at hiding in plain sight.

Although they are slow swimmers, according to marine biologist Brad Gemmell, at the University of Texas at Austin, seahorses "...tend to anchor themselves to surfaces like seagrass with their prehensile tails." (Prehensile tails, like those of some monkeys and possums, can grasp items, like branches or coral.)

So they are slow swimmers, and they prefer to attach themselves to a rock or grass. Their prey, the copepods, are super-sensitive to motion in the ocean and can swim away at a speed equivalent to a 6-foot human swimming 2,000 miles per hour.  How do the seahorses catch them?

Seahorses' weird shape, with its crazy spines and appendages, not only helps them hang onto and blend into the aquatic vegetation. It also helps camouflage their actual movements in the water: the horse shape of the seahorse's head minimizes vibration of water around it, rendering it nearly invisible to its little crustacean prey.

That narrow snout and forward-tilting head help the seahorse hunt by keeping it sonically "hidden" until it is too late for the prey, even for the speedy copepods that make up a bulk of the seahorse's diet.

Three seahorses living in the minimal vegetation structure of "muck" habitat in Indonesia - photos by this highly novice photographer on my first underwater photography attempt!  Note how they either hang on or bob (seahorse #3) and blend in with their surroundings (yes, some corals are very colorful!).  For professional photos of seahorse diversity, click here.

In addition, its bent neck and head act like a spring. While seahorses are latched onto some vegetation with their tails, their curved bodies and upright posture allow them to snap their heads forward to catch their prey faster than you can say "copepod", or even "shrimp". Sort of like a snake. Baam. The ability to snap their head forward also expands the area they have to probe for prey.

Not only that, but they can use that long tubular snout to poke into nooks and crannies in reefs or rocks to find little critters and then inhale them with vacuum power. Whoa.

Here's a great look - these guys at D-News are funny but have done their seahorse homework:

Natural selection

The wavy shape of sea horses might, in fact, have actually evolved to help them better capture prey, researchers now suggest. Experiments using high-speed video footage of feeding by both sea horses and their straight-bodied relatives, the pipefish, revealed that sea horses can strike farther than pipefish.

As you might guess by now, the ability to strike outward is especially useful for stealth predators like seahorses.  Any genetic variation that facilitated the forward strike probably helped its owner chow down a little more frequently and produce a few more baby seahorses (seafoals?), who, in turn, were more likely to be successful in catching food than their neighbors who lacked that ability.

You can chart the progress from "normal" fish to trumpetfish to pipefishes to seahorse (still a fish!) at this beginner's guide to seahorse evolution - or look below.

Chinese trumpetfish has seahorse-features
photo: Dfmalan, Wikimedia

pipefish really blend into their surroundings vegetation

ghost pipefish take camouflage to a whole new level (it's on the right)

This trajectory results in fish that are longer and thinner, with mouths at the end of long, thin snouts, smaller fins, and bony plates rather than scales, making them look ever weirder...

And here is the result!

Natural selection is amazing!  image: FreakingNews

OK, here is the real result, times two!  Natural selection IS amazing.

Their long, thin, angled heads are curved and ready to spring forward
to catch a copepod (or they are just acting shy)
photo: Bettina Balnis, Project Seahorse
Guylian Seahorses of the World 2010