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It’s pretty much a given across the animal kingdom that females of the species are responsible for pregnancy and childbirth. One glaring exception to this rule is found in a family of fishes known as Syngnathidae, which include seahorses, pipefish and their close relatives. There are about 200 species found in this family, about 35 of which are true seahorses, and all of these are members of the genus Hippocampus.  Most seahorses are tropical, but there is one species found in New England, called the Lined Seahorse (Hippocampus erectus).

All members of this group have reversed roles, if not actually in producing the eggs, carrying and birthing the young that hatch from them.

Unlike many types of fish, seahorses are very easy to sex. Males have a brood pouch on their bellies. Females do not. Unlike the hard armored plates that cover and protect the rest of the body, this pouch is a soft, flexible tissue that can be clearly seen even when not expanded with the developing young. In the image below the one on the left is a male and the one on the right is a female.

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Most seahorses are monogamous, including the Lined Seahorse. Once a pair has mated they will stay together for the entire breeding season (spring and summer), and in some species it is believed that they pair for life (a short five to seven years). The pair bond is reinforced by a daily greeting ritual, usually in the morning, where the couple wrap tails and swim side by side through the eelgrass beds. This is often accompanied by an impressive color-changing behavior where the two change their skin tone from jet black to sheet white, and sometimes even shades of yellow or gold. During the day the male stays in its home territory of a mere few square yards while the female goes off to hunt, returning each afternoon before dark.

While they are year-round residents of Narragansett Bay, keeping to deeper water during the winter and shallow eelgrass beds in warmer weather, they are still uncommon and very hard to find. Being long familiar with this pair bonding behavior, if I catch one seahorse I will continue scouring the area for the other, knowing they will never be too far apart. The pair will then continue the relationship even in captivity. I had to place a pair in adjacent aquariums once and the two never left the sides of the glass except to eat, trying to keep the bond alive (I put them together as soon as I could since this was rather painful to watch).

In order to form this bond however, the male must first win the right to claim the female. He does this by challenging other males to duels as the female looks on. This duel involves slow-motion violence that, because this fish really has no weapon, results in one of the two males being humiliated rather than harmed.

The two methods of battle are head-butting and tail wrestling. The two combatants will line up face to face until their foreheads touch. Using the strength of the jaw snapping open they will try to tip each other over (a perfect hit will result in one of them sumersaulting slowly backwards). This is combined or alternated with an intertwining tail battle that is hard to describe, but think of two people thumb wresting. Eventually one will concede defeat and show this by turning black and lying down sideways on the sand. The winner swims off into the sunset with his new bride.

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Now they can mate and they may do this several times in a breeding season (in captivity, because the water temperature is pretty constant they will breed year round). Male seahorses follow the female around trying to get her to transfer her eggs. This is comical to watch as he continuously moves in front of her, bends his body and flares the opening of his pouch. She’ll ignore this advance over and over (she’s waiting until the eggs actually form inside her). Once the eggs mature she will approach the male, place her egg tube, known as an ovipositor, at the entrance of his pouch, and produce a stream of roughly a hundred eggs. The pouch closes and the male fertilizes them within his own body. Below is a fantastic shot I found of the mating process actually occuring (these are captive fish and the female is malnourished, as can be seen by the indentations on the abdomen):

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Now it’s all up to the male to carry the young to hatching. Seahorse eggs do not contain a yolk. As the eggs develop the skin that makes up the brood pouch thickens, swells and a system of veins weave its way through the egg mass. The inner lining of the pouch is filled with nutrients and this is what actually nourishes the eggs. As the eggs absorb this lining it thins to a point, just before birth, where it is nearly transparent, and a hundred pairs of tiny eyes can sometimes be seen right through the skin.

After three weeks of incubation the eggs hatch. You can tell when this is going to happen because about a day before birth the brood sac suddenly swells to an enormous size. Most of this swelling is due to fluid taken into the sac that will help the young be ejected from the pouch and into the surrounding water.

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Now it’s time for hatching. The brood pouch opening enlarges for the first time since mating occured and the tiny quarter-inch long babies begin to stream out, helped by the dad using muscle contractions as he contorts and bends his body to move the more reluctant young towards the opening. The hatchlings are fully formed and able to swim and fend for themselves.

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One potential problem the pair must overcome is the instinct to eat the newly hatched babies since they are the same shape and size as their favorite foods (fish fry, amphipods and tiny shrimp).  Especially since the young aren’t always eager to swim very far away because, like the adults, the long prehensile tail instinctively grasps onto any object it can hold onto. Even dad’s nose.

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As it happens, for roughly a day after he gives birth the feeding instinct of the parents is suppressed keeping them from accidentally mistaking one of their own young for potential food. This won’t protect it from other seahorses nearby, nor by any other fish that happen to wander in to the eelgrass beds that form this animal’s nursery grounds. There is no parental care once the young are born, and in fact the parents will eventually eat any young that remain close by for more than a day. And there is no truth to the belief that the young can return to the pouch for protection, which, amazingly, is a claim made by Bigelow and Schroeder, which is the bible of fish identification in the Northern Atlantic (although to be fair the text was last updated in 1953).

To make matters worse, the first action of a baby seahorse is a perilous trip to the water’s surface; a trip that will take it out of the protection of the weeds and into the water column. Most bony fish have a gas bladder that helps to control their bouyancy as they swim (a discussion of gas bladders can be found in this diary on toadfish). The problem with seahorses is they are born with a deflated bladder and must swim to the surface to take a tiny gulp of air in order to fill up this organ. Out of the 100 babies born only about two or three will actually survive to become adult seahorses.

As a side note, in monogamous species like the lined seahorse, if one dies the other will often not select a new mate for the remainder of the breeding season. It’s interesting that once it has chosen a new mate the number of young produced in each brood is reduced, sometimes by as much as 50%. I can usually tell when a pair is not an original couple because the male will generally only give birth to 50 or 60 babies. I’m not sure why this happens although it’s possible that the first bond an individual makes is the strongest, and this enhances reproductive productivity.

Finally, here’s a closup shot of a seahorse that is about a few minutes old.

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There is so much more to write about this fish, including its feeding and protective behaviors, why it has such an odd shape, the fact that it is perhaps the only fish that lacks a caudal fin, and conservation troubles many species face. But I’ll save that for another essay.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Nov 24, 2006 at 03:00 PM PST.

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