Seahorses' Odd Head Shape Enables Them To Stealthily Catch Their Prey With 90 Percent Accuracy

By Ajit Jha on November 26, 2013 4:28 PM EST

Thorny Seahorse in seagrass
The unique head shape of the seahorse it what makes it such an effective hunter. (Photo: Shutterstock)

One of nature's mysteries is how a docile marine animal like a seahorse can catch their prey at an incredible accuracy of 90 percent, a feat rarely, if ever, demonstrated by even the fastest and stealthiest of predators. Scientists from the University of Texas at Austin now have the answer: their heads are perfectly shaped to sneak up and quickly snatch prey. The research study co-authored by Gemmell, Buskey and Sheng is published this week in Nature Communications.  

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"A seahorse is one of the slowest swimming fish that we know of, but it's able to capture prey that swim at incredible speeds for their size," said Brad Gemmell, research associate at the University of Texas Marine Science Institute, which is part of the College of Natural Sciences. Seahorses prey on on a type of marine animal called copepods, which extremely small crustaceans forming a critical component of the marine food web. They are also extremely fast: copepods jolt away at speeds of more than 500 body lengths per second. This speed is comparable to a six foot person swimming underwater at the speed of 2,000 mph. They are so good at escaping predators because they can detect waves produced in advance of an attack. 

It's amazing that seahorses other marine animals of the syngnathid family such as pipefish and seadragons are able to catch these lightning-fast little animals.  "Seahorses have the capability to overcome the sensory abilities of one of the most talented escape artists in the aquatic world - copepods" said Gemmell. "People often don't think of seahorses as amazing predators, but they really are."

In calm conditions, seahorses can catch pretty much any prey at least 90 percent of the time, which is extremely high, according to Gemmell, who found it intriguing enough to design the study. Gemmell and his colleague Ed Buskey, professor of marine science, zeroed in on Hippocampus zosterae, a dwarf seahorse native to the U.S and the Bahamas. They used high-speed digital 3-D holography techniques to observe the seahorses and the copepods in action. The high-speed digital 3-D techniques were developed by mechanical engineer Jian Sheng at Texas Tech University. The technique combines laser-fitted microscopes and high speed digital cameras to observe microscopic animals in a 3-D volume of liquid.

The technique revealed that the shape of seahorse's head is so designed that the least disturbance in water before its mouth takes place when the animal strikes its prey. The seahorse's nostrils have a "no wake zone" just above and in front. In addition, the seahorse positions its head in an angle against its prey that water fluid disturbance doesn't reach it.  This is the unique advantage that seahorses have over other small fish with blunter heads, such as the three-spine stickleback.  

The unique head shape of seahorses family probably evolved partly as a survival strategy, since they are dependent on catching their prey. In the long term, the individuals that went closer to prey without generating an escape response were more successful, according to Gemmell.  "It's like an arms race between predator and prey, and the seahorse has developed a good method for getting close enough so that their striking distance is very short," Gemmell said.

The feeding mechanism used by seahorses is known as pivotal feeding. They rapidly rotate their heads upwards while the prey is drawn in with suction for short distances. Seahorses have a 1 mm strike range, and it takes them less than 1 millisecond to make their strike, while copepods take 2 to 3 milliseconds to respond to predator movements. This is a huge speed to escape any other predator except the incredibly fast seahorse.   

Photo above courtesy of Shutterstock.

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