Male seahorses, rather than females, are the ones that become pregnant and give birth to live young. Females produce the eggs and transfer them to the male’s enclosed brood pouch where the male fertilizes the eggs, ensuring that his genes are passed to the next generation.
When seahorses mate, they perform a lengthy courtship dance with displays of vibrant colors in both sexes. During mating, the pair rise in the water, trying to align the female’s ovipositor with the male’s pouch opening. The female deposits dozens to thousands of eggs into the male’s brood pouch.
The embryos settle into the tissue of the male’s brood pouch, where they receive nutrition and oxygen from the placental fluid distributed through a network of capillaries. The adult male’s diet appears to vary according to the stages of embryonic development.
Can a Male Seahorse Really Become Pregnant?
As a biologist, I become extremely suspect when I hear about a male of any species becoming pregnant. Sounds too much like one of those magazine headlines in the grocery checkout line!
In the case of seahorses, however, the males really do become pregnant!
The females insert their eggs into the male’s brood pouch. The male releases sperm to fertilize the eggs as they enter, and the embryos become embedded into the tissue of the pouch. Once attached, they are supplied with nutrients and oxygen via the placental fluid (just like in a real female uterus!).
The pregnant male will incubate the embryos for about 24 days until they are born
Wild isn’t it!? Pregnant male seahorses are a real thing!
Do Seahorses Have a Courtship Dance or Display?
Seahorses may court for several days prior to mating.
Seahorse courtship strikes most human observers as very romantic! They are monogamous, with a lifetime commitment to just one partner. The pair will gather daily in the male’s territory and engage in a kind of dance during which they may circle one another or an object, change colors, or even hold tails.
The courtship behavior, according to scientists, synchronizes the animals’ movements and reproductive states so that the male can catch the eggs just as the female is about to lay them. They may also undergo color changes, swim side by side while holding tails or grasp a single strand of sea grass with their tails and engage in a coordinated wheeling motion known as a “predawn dance.”
Watch this pair of seahorses in their courtship dance prior to mating:
Seahorses go through four distinct stages of courtship, each of which is distinguished by distinct behavioral shifts and variations in the intensity of courtship behavior.
Phase 1: One or two days before physical copulation the first stage of courtship usually occurs in the early hours of the morning. The potential partners brighten in color, tremble, and exhibit quick side-to-side body vibrations during this phase. Both the male and female seahorses alternate between these displays.
Phases 2 through 4 occur sequentially on the day of copulation.
Phase 2 is marked by the female pointing, a behavior in which the female will raise her head to form an oblique angle with her body.
Phase 3: males will also begin the same pointing behavior in response to the female.
Phase 4. The pair eventually perform an eight-hour-long “true courtship dance” in which the male pumps water through the egg pouch, which enlarges, and gapes open to display an alluring emptiness.
This final phase of courtship consists of 5-8 rounds. The beginning of each round involves the male and female being anchored to the same plant. Usually, they are facing one another and are still brightly colored from the previous round. Following the facing behavior, the seahorses will rise a few inches in unison in the water column.
During the final round of courtship, the female will insert her ovipositor and transfer her eggs into the male’s brood pouch.
How do Seahorses Mate?
After repeatedly rising upward together in the water column the pair will mate in mid-water copulation, in which the female will transfer her eggs directly into the male’s brood pouch.
The female deposits dozens to thousands of eggs into the male’s brood pouch using her ovipositor. The female’s body shrinks while the male’s expands as she releases her long, sticky strings of eggs into the pouch. They finally succeed in transferring the eggs after numerous failed attempts, and then they split.
This protected form of fertilization reduces sperm competition among males, ensuring that the pregnant male is the one that gets to pass his genes to the next generation.
SEE my post on Sexual Selection to discover how having the right looks and moves is so critical in the life of animals. Being a good dancer can make the difference between attracting a mate … or not!
While the female swims off, the male descends and gently sways the eggs to settle them.
The male resumes feeding by sucking tiny creatures like small shrimp, plankton and fish larvae through his snout. The female swims off until the following morning when she rejoins the male for a daily greeting. The female will continue to make daily “morning greetings” during the entire period of gestation.
Larger females (with more, larger eggs) mate with larger males (with larger brood pouches), and pairs generally tend to be somewhat similar in size.
What Makes the Male Seahorse’s Brood Pouch so Special?
The brood pouch is a truly unique adaptation that makes it possible for male seahorses to become pregnant.
The family Syngnathidae, which includes seahorses and pipefishes, is the only group of vertebrates in which embryonic development takes place inside a special pouch in the males.
This pouch gives the embryo or offspring aeration, protection, osmoregulation, and nutrition after the females deposit eggs during mating.
Seahorse dads even appear to shield their offspring from disease by producing antibacterial and antifungal molecules to fend off pathogens.
How does Male Pregnancy Work in Seahorses?
The embryos become embedded within depressions of the interior lining of the brood pouch, which is similar to the placental function in mammals and acts like the mammalian uterus.
Following fertilization, the eggs are incorporated into the pouch wall and surrounded by spongy tissue. Prolactin, the hormone responsible for milk production in pregnant mammals, is provided to the eggs by the male.
The pouch offers an incubator with a controlled environment and oxygen. The male sea horses provide additional nutrients, such as energy-rich lipids and calcium to allow them to build their skeletal systems, by secreting them into the brood pouch that are then absorbed by the developing embryos. The egg yolk also provides nourishment to the developing embryo.
Additionally, they provide immune defense, osmoregulation, gas exchange, and waste transport.
Depending on the species and water temperature, male pregnancy lasts between 10 days and six weeks. During this time, salinity in the pouch changes to resemble the ocean more closely.
In the pouch, where the water’s salinity is controlled, the eggs then hatch, preparing the young for life in the sea.
How Do Male Seahorses Give Birth?
The male goes into labor at the end of the pregnancy, pumping and thrusting for hours to push his young out of the pouch.
Usually, he gives birth at night, and when his mate comes back in the morning, he is prepared for the next batch of eggs.
Watch a male seahorse giving birth to a litter of young:
For most species, the male seahorse releases 100–1000 young, but smaller species may only release 5 and larger species as many as 2,500. Young of most species of seahorse are tiny, measuring less than half an inch (8-10 mm).
Most fish species, including seahorses, do not care for their young after birth. Infants are vulnerable to predators and ocean currents that could wash them away from feeding areas or into temperatures that are too high for their fragile bodies.
Video of male releasing young out of pouch
Fewer than 0.5% of the young survive to adulthood.
Because so few infants reach adulthood, it is not surprising that litter sizes can be large.
Because of their protected gestation, these survival rates are quite high when compared to other fish, making the process worthwhile despite the high cost to the father. Most other fish abandon their eggs right away after fertilization.
From birth on, the young are totally independent, with the majority drifting around as plankton.
As soon as one litter is released the male is prepared to mate once more.
Why Do Seahorses Form Pair Bonds?
One male and one female typically form a lifelong pair bond in many species, though not in all. Project Seahorse made the initial discovery of this in the Hippocampus whitei from eastern Australia.
In seahorses, pair bonding improves reproductive success. If a male and a female spend a significant amount of time together (at least seven days) prior to mating, the male gives birth to significantly more young.
This is most likely because the male is somehow stimulated by the female’s presence to increase the size of his ovarian pouch or his production of hormones like prolactin, which are crucial for incubation.
A female who has been with a male during his pregnancy has already prepared eggs and is ready to mate again as soon as he has given birth. This reduces the time between matings in pair-bonded seahorses, so they can mate without a three-day courtship. Additionally, pairing reduces the likelihood that a female will have to drop her clutch out of necessity.
Early each the morning, the female will approach the male, and the two will then dance in greeting. They promenade for an average of six minutes while pirouetting around a shared holdfast as their bodies visibly brighten. The remainder of the day is spent with the animals apart.
The male may be cueing the female to mate on the day of delivery by performing this greeting dance throughout the entire course of the male’s pregnancy.
The male and female’s bond is so strong that even if an injury prevents mating or pregnancy, the female will still come see him. Additionally, it appears to be advantageous because paired seahorses mate again quickly and persistent pairs tend to have more offspring.
Because pair-bonded seahorses don’t need to look for partners, their courtships are shorter, and they can likely transfer eggs after fewer mating rises, the risk of predation should be decreased in these animals as well.
Why Do Seahorses have Sexual Role Reversal? Why do the Males Become Pregnant?
This is still a mystery to biologists. Given that females are responsible for getting pregnant in every other animal, we still don’t know why seahorse fathers conceive.
Some scientists hypothesize that sex role reversal speeds up reproduction, enabling more babies to be born in a shorter amount of time.
While the male is gestating, the female can begin preparing the next batch of eggs, making for a quicker mating cycle.
Seahorse mothers still contribute nutrient-rich egg yolks to support growing embryos, but after mating, they no longer have any parental responsibilities.
Male pregnancy drains the male’s energy. This raises the question of why there is even a sexual role reversal.
You would think that females would compete the most for a mate because males carry out all of the incubation work.
However, it is the males that demonstrate a greater level of aggression and prove to be much more competitive than females. Males engage in rough physical contact or lash out by flicking their snouts at one another’s heads.
Some males adopt a submissive posture when the competition becomes intense, by darkening and flattening against the ground. Submissive males can occasionally use this position as a cover to approach the dominant male and snap at him.
Typically, the largest and busiest males prevail in physical contests, and are the most successful in mating and produced offspring.
Only the males engage in head-snapping and tail-wrestling, claims Amanda Vincent of Project Seahorse. This discovery inspired additional research on energy costs.
- The energy contained in each egg was chemically analyzed to determine the direct contribution of the female. Utilizing oxygen consumption, the burden placed on the males was calculated.
- The male consumed almost 33 percent more oxygen by the time incubation was complete than before mating.
- Females expend twice as much energy producing eggs as males do during incubation.
- Males have the potential to have 17% more offspring than females during a breeding season, given an infinite number of ready and willing partners.
- Additionally, females take 1.2 times longer “time-outs” from the reproductive cycle than do males. This appears to be based more on mate preference than on physiology.
- The female demands a lengthy courtship as insurance against losing a clutch of eggs.
- The female must lay her eggs in a matter of hours or eject them into the water column.
- She physically pays a high price for making eggs because they make up about one-third of her body weight.
How Widespread is Male Pregnancy?
Seahorses are not the only ones that display this peculiar sex role reversal.
According to researchers at Texas A&M University who study the taxonomic family Syngnathidae, which includes seahorses, pipefishes, and sea dragons, male pregnancy occurs commonly in Syngnathidae species.
Are Seahorses Really Fish? More Interesting Facts!
Seahorses might not look like it, but they are, in fact, proper fish closely related to the common sticklebacks. They have gills for breathing and fins for swimming.
- These voracious ambush predators can keep an eye out for food in all directions thanks to their independently moving eyes.
- Once a suitable small crustacean is found, the seahorse quickly dispatches the prey with powerful suction from its long, tubular snout.
- Seahorses have no stomach. They need to eat almost constantly because food quickly passes straight through the digestive system. An adult seahorse will eat 30 to 50 times per day, while baby seahorses eat 3,000 pieces of food per day.
- Seahorses still manage to break down tough crustaceans despite lacking teeth or a stomach.
- Gills resemble bunches of grapes because they are grouped into lobes.
- The skin of seahorses is taut and stretched over a network of interlocking bony plates with spines and knobs at the junctions. Seahorses lack scales.
- Since the tail is prehensile and so powerful, seahorses are frequently impossible to pry free from a holdfast.
- The dorsal fin is used for propulsion, along with two tiny, ear-shaped pectoral fins that are used for stabilization and steering. The tiny anal fin doesn’t seem to do much.
- The Pacific seahorse, which is the largest, grows to a height of about 11 inches, compared to under an inch for an Australian species.
- Seahorses are found bobbing in sea grass meadows, mangrove stands, and coral reefs in confined areas along tropical and temperate coasts.
- Seahorses are masters of concealment, covered in murky brown-and-gray patterns that blend into backgrounds of sea grass.
- Females have a range that may be a hundred times larger than that of males, who tend to stick to small areas of about 10 square feet (1 square meter).
- Based on behavior observed in the lab, interlopers of either sex commonly attempt to interfere with courting pairs.
Can Seahorses Be Bred in Captivity? Breeding Tips
Seahorses raised in captivity are more expensive, but they have no negative effects on wild populations.
Captive breeding has gained popularity recently. Such seahorses are less likely to contract diseases and fare better in captivity. They don’t go through the stress of leaving the wild and they eat frozen crustaceans (mysidacea) that are easily obtained from aquarium stores.
Keep seahorses in an aquarium with calm tank mates and low flow. Fast, aggressive feeders will deprive them of food because they are slow feeders. Numerous varieties of shrimp and other bottom-feeding animals can coexist with seahorses. Gobies are also good tank companions.
Eels, tangs, triggerfish, squid, octopus, and sea anemones should all be avoided.
Seahorses must have high-quality water to survive in aquariums. Due to their fragility, they shouldn’t be introduced to a brand-new tank.
Although these fish may eventually acclimate to different water, the following water characteristics are advised:
- 23-28 °C (73-82 °F) water temperature.
- pH: 8.1-8.4
- Ammonia: 0.01 mg/l (0.01 ppm), may be tolerated for brief periods.
- Nitrite: 0.125 mg/l (0.125 ppm), may be tolerated for brief periods of time.
- S.G. = 1.021–1.024 at 73–75 °F (23–24 °C)
Watch for signs that your seahorse is under stress.
Seahorses with clamped fins, reduced appetites, erratic swimming, and lingering at the surface are signs of poor water quality.
NOTE: Seahorses use the length of the aquarium as well as its height by swimming up and down. As a result, the tanks should ideally be twice as deep as the adult seahorse’s length.
Are There Any “Freshwater Seahorses?”
Usually, the closely related pipefish are the creatures marketed as “freshwater seahorses.” A few species of pipefish inhabit the lower reaches of rivers.
These might resemble seahorses, but they are actually freshwater fishes found in estuarine environments.
References
Kvarnemo, C., Andersson, S. E., Elisson, J., Moore, G. I., & Jones, A. G. (2021). Home range use in the West Australian seahorse Hippocampus subelongatus is influenced by sex and partner’s home range but not by body size or paired status. Journal of Ethology, 39(2), 235-248.
Lin Q, Li G, Qin G, Lin J, Huang L, Sun H, Feng P. (2012) The dynamics of reproductive rate, offspring survivorship and growth in the lined seahorse, Hippocampus erectus Perry, 1810. Biol Open. 2012 Apr 15;1(4):391-6. doi: 10.1242/bio.2012398. Epub 2012 Feb 24. PMID: 23213429; PMCID: PMC3509459.
