Seahorses are at once weird and wonderful, exotic and underwhelming and unique among bony fishes. Underneath their obvious horsey-head charm, seahorse anatomy is really, really different from other fishes.
THE MOST OBVIOUS THING ABOUT SEAHORSES IS THEIR BODY DESIGN – an upright torso connecting a horse-shaped head and a monkey-like tail. They’re bony fishes, but pretty much the only bony fish that swims upright. When they swim.
If searching for them carries an air of anticipation, finding them is like watching grass grow. When spotted, they’re likely to just be hanging out attached to a sea rod or something, ignoring us and wishing we would go away. It works. Eventually we do.
Looks aren’t everything. Far more than just fish with horse-shaped heads, they’re complex little guys.
SEAHORSES IN BRIEF:
- For fishes, they’re notoriously bad swimmers, among the worst in the ocean.
- Yet they’re super-predators of tiny shrimps and other planktonic delectables, and have few predators of their own.
- They may not swim well but their horse-shaped heads and square tails power body mechanics that contribute to their success in capturing prey.
- Their finless tails are prehensile, enabling them to grasp onto solid objects and anchor themselves.
- Unlike most bony fishes, they lack scales, sporting an armor of interlocking bony rings, covered by a thin layer of skin.
- Internally, seahorses lack teeth and a stomach and have to eat pretty much continually to live.
- They’re largely monogamous, with charming, elaborate daily rituals between mates.
- Beyond their general horsey shape, they may be best known for the fact that the male does the childbearing.
SEAHORSES IN THE WORLD
Found in oceans worldwide, seahorses are represented by some 44 species in the genus Hippocampus (from the Greek for horse and sea monster), ranging in size from the recently defined grain-of-rice-sized “Japan pig” pygmy seahorse (Hippocampus japapigu) found in coastal Japanese waters to the foot-long big bellied seahorse (Hippocampus abdominalis) found in southern Australia and New Zealand seas. Members of the Syngnathiadae Family, they’re related to pipefishes and seadragons.
Most of us probably associate seahorses with tropical seas because that’s where we see them. But a species like the lined seahorse (Hippocampus erectus) is found not just in the Caribbean but in the Atlantic as far north as Cape Cod. Two species are native to British and Irish waters as far north as the Shetland Islands. Three species are found in the Mediterranean. Obviously, the greatest number of species are found in the Indo-Pacific basin.
They typically inhabit shallow waters – reefs, ledges, mangrove forests and eel grass beds – but move into deeper waters during rougher winter months.
Despite their shapes, seahorses are bona fide bony fishes, equipped with bony skeletal structures, swim bladders and gills. Their shapes, of course, makes them unique, and they’re the only fishes who swim in an upright posture.
Bony skeletons they have, but more like exoskeletons. Their bodies are encased in a structure of interlocking plates covered by a fleshy skin. They lack the usual fishy scales. This bony structure, in effect, confers on them a suit of armor that accounts for their lack of predators – they’re hard to eat. They’re most vulnerable to being grabbed by the pincers of larger crabs.
Fortunately, they’re masters of camouflage. Not only do their colors range from red and orange to purple and white, they can change their colors to blend in with their surroundings.
HOW SEAHORSES SWIM
On the other hand, seahorse anatomy makes the little guys the slowest-swimming fishes in the ocean. Without tail fins, their propulsion is provided by a single, small dorsal fin that beats furiously – as much as 50 times per second. Their small pectoral fins don’t help much with speed, mainly serving to steer the little fish around. And, swimming upright, these horse-shaped fishes are hardly streamlined for rapid movement through the water.
Put all this together and they typically move at a heart-racing 5 ft./1.5 m. per hour. It is heart-racing. After a strenuous journey they can die of exhaustion. Still, they have one swimming trait nearly all other fishes lack. They can move upwards, downwards and backwards.
And the lack of swimming finesse may not matter. Like a lot of small fishes in the sea, they tend to spend their lives pretty much in one small patch of seabottom. Females forage around territories of about 1,100 sq. ft./100 sq. m. Perhaps because males are busy taking care of the babies, they have a particularly confined territory – 5½ square feet/.5 square meters.
A TALE OF TAILS
If their tails aren’t much use for swimming, another weirdness of seahorse anatomy is they’re prehensile – like monkeys’ tails that can be wrapped around tree branches and held tight. In seahorse world, this means they can anchor themselves firmly onto a sea rod or seagrass stalk or mangrove root and hang on in place.
And, despite their lack of swimming agility, if they want to travel they can lock on to loose seaweed or debris with their tails and hitchhike for long distances. Doing so may be more a matter of anchoring onto something firm than intending to shift territories.
Here’s where weird seahorse anatomy pays off: They’re really, really good at capturing prey – mostly small mysid shrimps, copepods, other invertebrates and occasionally, small fishes and fish larvae that drift by in the water or move along the seafloor. Anchoring themselves with their tails, their curved bodies and horse-shaped head-and-neck design maximize the speed and distance with which they can capture prey.
Normally, any prey in question can sense vibrations in the water caused by a predator’s movement and bolt away at extremely high speeds. But the horsey heads of seahorses, researchers have determined, are craftily shaped to produce minimal disturbances as they approach their targets, letting them get very close to their target before striking.
Like most other fishes, seahorses feed by drawing the prey in with suction. Depending on the species, the effective striking distance ranges from only about one millimeter to three centimeters. And they’re fast. A strike takes less than one millisecond.
They have a 90 percent success rate, among the highest for any fish (great white sharks achieve about 55 percent).
TOOTHLESS – AND STOMACHLESS – WONDERS
Seahorse hunting techniques depend on their habitat. In areas with little vegetative cover, they’ll sit and wait for prey to come to them – the tiny shrimps, copepods and other stuff that drift by in the planktonic soup of the ocean currents. In habitats with significant vegetative cover – seagrass patches and mangrove forests, for example – they’re likely to move around and hunt while swimming. The slender headshape of seahorse anatomy is useful here, too: It’s helpful for probing in nooks and crannies.
And they need that 90 percent kill rate. Another seahorse anatomy weirdness: they have neither teeth nor stomachs. They’re extremely good at catching prey; they’ve very bad at digesting it, which means they have to eat pretty much continuously to stay alive and healthy. Scientists estimate that a typical seahorse consumes as many as 3,000 prey each day.
DADDY DAY CARE
Seahorses are widely famous for the fact that the males do the childbearing – one of the few 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. The number varies, from as few as five to as many as a thousand. Volume is good; the rate of survival into adulthood is estimated to be less than one percent. They may be well-nourished and protected during pregnancy but once they’re born they’re on their own. Emerging from their daddy’s pouch headfirst, fully able to swim, they drift with the plankton until they find a territory suitable for settling down.
Male lined seahorses (Hippocampus erectus), a common species found in the Caribbean, develop their brood pouches at about five to seven months. They’re considered mature at just over two inches in length and can grow to about seven inches during lifespans that can last as long as four years.
MONOGAMOUS, MORE OR LESS
Seahorses often mate for life. Some are more serially monogamous, staying together only for extended mating seasons. Whichever, it’s strategy that fosters reproductive success during the course of a mating season.
In any event, male and female seahorse couples start each day with a greeting ritual, moving through complex, rhythmic twists and twirls, sometimes changing colors and interlocking their tails. For one thing, the rituals ensure that their reproductive cycles are synched.
These dances last for several minutes before they separate for the day to pursue their predation strategies, the female going off into her significantly larger hunting territory.
STILL, THEY’RE VULNERABLE
Seahorses’ major goal in life may be to just mind their own business, eat well and produce little miniature seahorses. They have few predators among reef denizens but they do have us humans. The nonprofit Seahorse Trust estimates that more then 1.5 million seahorses are taken from the oceans annually for the traditional Chinese medicine trade, one million for sale as dried curios and one million for the aquarium pet trade.
On its Red List, the International Union for the Conservation of Nature (IUCN) lists two species as endangered, 12 as vulnerable, one as near threatened and 17 as of least concern. But it also indicates that data is insufficient for 17 species.
PRINCIPAL SOURCES: Seahorse, National Geographic.com; 10 Things You Never Knew About Seahorses, Smithsonian Ocean; Seahorse Facts, The Seahorse Trust; Seahorses Are Super Killers!, Discover News; Beautiful Images Of Male Seahorse Giving Birth, Reproduction Live TV; Hippocampus erectus, Florida Museum of Natural History; Seahorse, Wikipedia.org; Why the seahorse tail is square, Science Magazine; IUCN Seahorse, Pipefish & Seadragon Specialist Group, International Union for the Conservation of Nature; Encyclopedia of Fishes, 1998; Eyewitness Fish, 2005; 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;