FISH FILES: Seahorses: Inept and Really Good at What They Do

This thorny seahorse (Hippocampus hystrix), about six inches in length, was found hanging out (literally) in the southern Philippines.

This thorny seahorse (Hippocampus hystrix), about six inches in length, was found hanging out (literally) in the southern Philippines.

FINDING A SEAHORSE IS PRETTY MUCH ON EVERY DIVER’S WISH LIST. But if you’ve ever watched one after you found it, your reaction very likely was, “Now what?” Mostly, actually watching seahorses seems like watching paint dry. They seem to just sit there looking bored, attached to a gorgonian or something and wishing you would go away. And, they’re notoriously bad swimmers, among the worst in the ocean.

On the outside, they’re cute, unique and inept. On the inside, they’re busy little guys.

SEAHORSE BASICS

First of all, seahorses are widely famous for the fact that the males do the childbearing – perhaps the only animal species that does this. During mating, a female seahorse deposits her eggs in a brood pouch on the male’s abdomen, after which he fertilizes and nurtures them with nutrients and oxygen until they hatch, usually a period of several weeks. The result is fully formed teeny, tiny seahorses. Seahorses are mostly believed to be monogamous and mate for life, and sometimes the females hang around to support the pregnant fathers. In many species, each pair does a courtship dance each day, interlocking their tails.

MORE BASIC BASICS
This longsnout seahorse (Hippocampus reidi) does its thing at Bonaire in the southern Caribbean.

This longsnout seahorse (Hippocampus reidi) does its thing at Bonaire in the southern Caribbean.

Found in tropical waters worldwide, seahorses are represented by some 35 species in the genus Hippocampus (from the Greek for horse and sea monster), ranging in size from about a half-inch to 14 inches, with bodies protected by interlocking bony rings. In the Caribbean and tropical Atlantic, they’re represented by only a handful of species, significantly more  in the Pacific.  In many species, colors – including black, gray, yellow and orange – are quickly changeable, often helping them camouflage themselves among their hiding places.

They’re closely related to pipefishes and sea dragons essentially, they’re pipefishes that evolved to sit upright, with prehensile (grasping) tails that allow them to anchor themselves to holdfasts on the seafloor.

In transit -- Powered only by a small dorsal fin, this Pacific seahorse (Hippocampus ingens), photographed in the Galapagos, demonstrates seahorses laborious swimming technique.

In transit — Powered only by a small dorsal fin, this Pacific seahorse (Hippocampus ingens), photographed in the Galapagos, demonstrates seahorses laborious swimming technique.

Seahorses are well advised to do this because they are terrible swimmers – they swim positioned upright, propelled by the small dorsal fins on their backs. Their pectoral fins are used for steering; unlike other fishes, they have no fins on their tails. When they swim, their propelling dorsal fins may flutter 35 to 50 times a second (not clear who’s counting), and still their progress is slow and painful to watch. Seahorses tire easily and can die of exhaustion when tossed about in the undersea surges of a storm.

 TOOTHLESS — AND STOMACHLESS — WONDERS

But as planktivores – animals that eat microscopic copepods, brine shrimps and other tiny stuff that drifts by in the planktonic soup of the ocean currents – their stationary lifestyles serve them well. They do have challenges.  Seahorses are singular in having in having neither teeth nor stomachs. They’re extremely good at grabbing prey; they’ve very bad at digesting it, which means they have to eat pretty much constantly to stay alive and healthy.

 FAST. AND SNEAKY.

Apparently, catching things to eat is what seahorses do well. In a study published in Nature Communications in 2011, researchers at the University of Antwerp and Arizona State University used video and biomechanical analysis to conclude that the seahorse’s curved-body architecture increased their speed and distance for lunging at its crustacean-prey – giving them as much as a 20 percent strike-distance advantage compared to the pipefish from which they evolved.

For a study published in Nature Communications at the end of 2013, researchers at the University of Texas Marine Science institute used video to study the anatomy of dwarf seahorses (Hippocampus zosterae, endemic to the Bahamas) and found that they captured their intended prey 90 percent of the time. This was the best record of any fish tested. Normally, the prey in question – tiny copepods – can sense vibrations in the water caused by a predator’s movement and bolt away at extremely high speeds.

Except the horsey heads of seahorses, the researchers determined, are craftily shaped to produce minimal disturbances as they approach their targets, letting them get very close to their target before striking (like many fishes, seahorses feed by drawing the prey in with suction, but the effective range for such action is only about one millimeter. A strike takes less than one millisecond).

STILL, THEY’RE VULNERABLE.

A seahorse’s major goal in life may be to just mind his own business and produce little miniature seahorses, but unfortunately, they’ve vulnerable to a  number of outside factors, including pollution, loss of their habitat, and harvesting by humans. Some of this is for use  in aquariums, but a great deal is for use in Asian traditional medicines.

Principal Sources: “Morphology of seahorse head hydrodynamically aids in capture of evasive prey,” Nature Communications, November 26, 2013; “An adaptive explanation for the horse-like shape of seahorses,” Nature Communications, January 25, 2011;  Encyclopedia of Fishes, 1998; Eyewitness Fish, 2005; “Seahorse,” National Geographic.com.