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I tackled seahorses last week but only got as far as how they reproduce. You can read that to get caught up and then we’ll see about some of the other behaviors and characteristics of this amazing animal.

There are thirty-five species of seahorses worldwide, but I’ll be using my local Lined Seahorse (Hippocampus erectus) as the main example. Although this group of fish are pretty consistent in their behavior and anatomy.

Seahorses are carnivores, feeding mainly on small crustaceans such as shrimp and amphipods. They are "lying-in-wait" predators, meaning that instead of actively hunting for prey, they stay in one place and wait for the prey to come to them.

Although they don’t have teeth, they can still feed on animals that are surprisingly large. The snout is long and tubular, terminating in a small opening at the end. This tube serves as a powerful vacuum. When a shrimp swims by the seahorse will snap open its jaws, sucking up water and, hopefully, the prey. If the food is too large to be swallowed in one gulp, the seahorse will repeatedly snap its jaws open. The power created by the snout is pretty impressive and will quickly kill and then dismember a shrimp that is more than five or six times the size of the mouth opening.

So the seahorse now has a mouthful of prey and water. But it doesn’t want to swallow the water, so it compresses the food against the roof of its mouth and expells the water out of its gills. Because the food has been crushed by the force of the vacuum, you’ll see two little puffs of smoke shooting out of the gill slits just before it swallows the food.

This fish feeds constantly in daylight hours and may consume as many as two hundred small shrimp per day.

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Seahorse feeding on mysid shrimp.

Seahorses are unique among fish in that they swim upright. The dorsal fin is found on a fish’s back. Many fish have two (a few types, like cod, have three).  Seahorses only have one. However this fin is not used for balance as it is in other species, but for locomotion. Because the fish is swimming upright this dorsal fin is more analogous to the tail fin of other fish. The dorsal fin moves back and forth very rapidly, pushing the seahorse slowly forward. If the animal needs more speed it will lunge its head forward and "dive" through the water propelled by the dorsal. This only works over very short distances and the speed still hardly rivals other types of fish.

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The caudal fin is found on the tail of a fish and is usually used to move the animal forward, especially when speed is required.  Seahorses are unique among fish in that they completely lack this fin.  Instead the tail is prehensile, like a monkey’s or opossum’s, and used to anchor the animal by wrapping it around weeds or coral.  The tail is very flexible and can curl and twist around even the thinnest objects.  Although some drawings of seahorses (including the logo of at least one seafood chain) show the tail curled backwards, the seahorse is actually unable to do this.  Yes, the tail can stretch almost to the back of the head (I had one seahorse that had a habit of doing this to scratch its neck) but it cannot curl backwards.

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This tail position is anatomically
impossible to achieve.

Ventral means "bottom", and the ventral fin is the one opposite the dorsal. Which means on the upright swimming seahorse it is facing forward. In most fish the ventral helps to balance the animal as it moves through the water. A seahorse’s ventral fin is tiny and mostly useless.

Pectoral fins are very important in swimming because they are used both to turn and to brake. These fins are found in pairs, one on each side of the head just behind the gill slits. They are often described as being "wing-like".  Seahorses have two well developed pectoral fins on the head which resemble strange "ears". These fins help the fish change direction and to stop moving.  They are also used, along with the dorsal fin, to move the fish forward.

Seahorses do not have pelvic fins, the paired fins found on the chest of most fish. Also, like the pufferfish we discussed a few weeks ago, the seahorse lacks ribs and depends on the armor covering to protect its internal organs (more on this below).

The upright way that this fish swims is what gives it that unusual horse-like look, sort of like the knight in a chess set. If the head didn’t bend as it does the fish would be looking straight up instead of forward.

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In New England seahorses are confined almost exclusively to shallow water eelgrass beds, where they can cling to the long, thin grass blades and remain hidden from their predators. Although to survive the winters they need to migrate (usually in pairs) to deeper water which may not have enough sunlight for this plant to grow. In this case the fish needs to get creative and will search out anything that they can wrap their tails around. It is not uncommon for lobster boats to pull up traps in the winter with seahorses still clinging to the wire mesh.

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Scales, not!
Seahorses are known as "armored fish" because of the hard outer shell covering and protecting their bodies. This shell is often mistaken for a series of overlapping scales. The skin is actually covered with a set of fused bony plates known as an integument. Although the integument helps to protect the seahorse, this is offset by restricting flexibility, and thus speed, of the animal.

One of the most intriguing features of the integument is the coronet. This is the series of spines and growths that each seahorse develops on the top of its head. This structure is unique to each individual and can be used like fingerprints to identify any given fish. Interestingly, and I really can’t explain this, the spines on the coronet shrink and eventually disappear when a wild-caught specimen is kept in captivity.

For years the function of the coronet was a mystery. It is now believed that this structure amplifies the clicking sound made by the jaw which may be how seahorses communicate with each other. By having a unique coronet each seahorse makes a sound that in turn is unique to itself. In this way these fish can establish territories and identify their mates.

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Seahorses in New England are rare, but I think their populations are safe since there is no real market for them and they are not fished for. The same can’t be said for other species throughout the world.  Tropical species are being wiped out and many species are close to extinction. Although not used for food, countless numbers are caught for the pet trade, dried out to be sold in gift shops as "curios" (I hate that word) or used in traditional Asian medicines.  A full 32 out of the 35 known species of seahorses are covered under the Convention on International Trade in Endangered Species (CITES).

China has exempted itself from the rules set by CITES, claiming that traditional medicinal practices benefit the human population. In that country alone over twenty million seahorses are imported each year, dried out and crushed into a powder to be used in herbal remedies (supposedly it cures impotency and high blood pressure). I know we are no environmental saints in America, but the damage to wildlife caused by the Chinese food and pharmaceutical industries is just stunning. (From what I understand every native species of turtle in that country has been nearly wiped out.)

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As far as the pet trade goes, seahorses are very demanding and difficult to keep in captivity and nearly all of those sold as pets will die in a very short time. Although fortunately today most of those sold in pet shops are more and more likely to have been bred in captivity.  These captive-bred animals not only take pressure off of wild populations, but are more likely to survive because they are more receptive to frozen foods and less likely to be carrying parasites and disease.

Fun Fact: The hippocampus, the part of our brain responsible for processing new memories, is called this because it is shaped like a seahorse. (In Greek hippo means "horse" and campus means "sea monster".)

Video Bonus: In the comments of the last MLS diary RainyDay links  to some videos that perfectly show what I was trying to describe last week. Check ’em out.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Dec 01, 2006 at 02:18 PM PST.


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