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Honey, I’m pregnant!

Atualizado: 3 de jul. de 2023

By Raquel Moreira Saraiva and Yonara Garcia

Image (right): Flickr

Today we are going to talk about the Super Dad of the animal kingdom, the seahorse! This peculiar organism is considered a Super Dad for a good reason: the males become pregnant! That's it! Seahorses stand out in the animal kingdom because the males are responsible for all parental care after fertilization: they carry the pups during gestation, experience the "birth pangs," and finally give birth! Recent research also shows that seahorse daddies have even more similarities to human mommies than we thought! But before we talk about those peculiarities, let's get to know a little bit about seahorses in general.

Two "pregnant" seahorses. Image: Flickr

Seahorses are bony fishes (teleosts) belonging to the genus Hippocampus and the syngnathidae family (Syngnathidae). This family has the unique developmental characteristic of viviparity, where embryonic development occurs within the body (the same as humans), which in this case, is the paternal body. There are more than 50 species of seahorses distributed throughout the world in tropical and temperate regions. Of these, three species occur on the Brazilian coast: Hippocampus reidi, Hippocampus erectus, and Hippocampus patagonicus, present in the marine and estuarine environment.

Representatives of the three species of seahorses that occur in Brazil: Hippocampus reidi, Hippocampus erectus, and Hippocampus patagonicus, respectively.

Images:  Projeto Hippocampus

These fish move vertically through wave movements of their dorsal fins, which vibrate rapidly. This type of vertical locomotion slows them down to the point of being considered one of the slowest fish in the oceans. Seahorses are predators, with a diet based on plankton, crustaceans, and small animals that are sucked through their tubular snout. They are also skillful at camouflaging themselves: if they feel threatened, they can change color and develop skin projections that mimic algae or coral polyps. Additionally, they can become rigid and immobile, fixing themselves on algae and corals through their prehensile tail. But these disguises are not infallible: crabs, some carnivorous fish (e.g. tuna), penguins, sea birds, and even humans predate upon adult seahorses (to learn more about plankton, read our post O que você sabe sobre o plâncton?).

Seahorses mimic the environment when they feel threatened. Images: Flickr

Most seahorses are monogamous, so that both the male and the female of a formed pair repel other partners who try to interfere with the relationship. For mating, they perform a type of dance in which they synchronize their movements, turning around one another with interlaced tails. Male pregnancy has interesting implications for the classic sex roles in mating. In most species, males compete for access to females, so it is common to see the evolution of secondary sex characteristics * in males. According to researcher Adam Jones of the University of Texas, in the case of seahorses, females exhibit a competitive behavior that is typically characteristic of males. In addition, males appear "demanding" in relation to the choice of their partners, an attribute commonly observed in females.

Now let's get down to business: how can males in this group get pregnant? The male seahorse has a specialized brood pouch where the female places her oocytes (reproductive cells). When it is ready to mate, the male signals the female by filling the pouch with water. The female, in turn, swims and presses against it, placing her ovipositor into a dilated hole in the male's pouch. After the oocytes are transferred, the hole closes, and the male fertilizes them. Thus begins the development of the babies (called fry) inside the body of the male.

The gestation period of this group varies greatly, according to the species and the water temperature, and can occur between ten days to six weeks. In tropical regions, seahorses have a gestation period of around 12 days. They reproduce throughout the year, and from the first year of life, a couple is able to produce more than 1000 larvae per gestation.

Seahorse giving birth.

The challenges of pregnancy are the same for all animals, including ensuring the adequate supply of oxygen and nutrients to the embryos. Recent studies have shown that several animal taxa have overcome these challenges in a similar way. Seahorse embryos, like many other viviparous animals, acquire many nutrients from the vitellus of the mother’s egg, which is equivalent to the egg yolks of chickens. Researcher Dr. Camilla Whittington and colleagues at the School of Biological Sciences, University of Sydney, Australia, have shown in studies published in Molecular Biology and Evolution that additional nutrients, such as calcium and some lipids, are secreted by the fathers from the brood pouch and absorbed by the embryos. In addition, the dad’s pouch also maintains the complex challenges of gas exchange, excreta removal, and providing immunological protection to the young!

Pregnancy is accompanied by many morphophysiological adaptations, such as the remodeling of the brood pouch, transport of nutrients and residues, gas exchange, osmoregulation, and immunological protection of embryos. Another curiosity discovered by researchers is that the genetics related to these adaptations are very similar to the genetic expression of the internal reproduction of mammals, reptiles, and other fish. It is surprising that, even in animals with very distant evolutionary histories, the genetic tools for reproduction have developed remarkably similar to each other, even between viviparous aplacental (seahorses) and placental (mammalian) animals (Caspermeyer, 2015; Whittington et al., 2015).

Seahorse populations are declining worldwide. In addition to their limited locomotion capacity, the destruction of their habitat and incidental and targeted fisheries have threatened the lives of these fish. There is high demand for live specimens among ornamental fish enthusiests. Dehydrated, they are used as ingredients of homemade and industrialized drugs and as decoration, which leaves them even more vulnerable. The purchase of these fish, even alive, encourages their capture and trade, in addition to contributing to the ecological imbalance. Genetic, physiological, and ecological studies of these animals help not only to understand their biology and the evolutionary steps that led to the inversion in sexual behavior, but also contribute knowledge to the management of these species. The best option is to leave seahorses in their natural habitat, reduce exploitation, and take care of the environments in which they live, including coral reefs and mangroves. This way you can get to know these fish better while helping in their preservation.

Dehydrated seahorses being sold at a market in Hong Kong. In Asia they are much appreciated in cooking and as raw material for the manufacture of medicines. Image: Britt-Arnhild’s House in the Woods

*secondary characters: characteristics that develop during the sexual maturity of animals, but which, unlike the sexual organs, are not part of the reproductive system.


To learn more about the subject:

Projeto Hippocampus - Iniciativa do Laboratório de Aquicultura Marinha - LABAQUAC para educação ambiental e estudos de conservação de cavalos-marinhos.

Caspermeyer, J. Unraveling the Genetic Basis of Seahorse Male Pregnancy Mol Biol Evol (2015) 32 (12): 3278 first published online November 17, 2015 doi:10.1093/molbev/msv238

Jones, AG & Avise, JC. Mating Systems and Sexual Selection in Male-Pregnant Pipefishes and Seahorses: Insights from Microsatellite-Based Studies of Maternity J Hered, 2001.

Rosa IL, Oliveira TPR, Osório FM, Moraes LE, Castro ALC, Barros GML & Alves RRN. Fisheries and trade of seahorses in Brazil: historical perspective, current trends, and future directions. Biodivers Conserv, 2011.

Silveira, R. B. Dinâmica populacional do cavalo-marinho hippocampus reidi no manguezal de Maracaípe, Ipojuca, Pernambuco, Brasil. (2005).

Whittington CM, Griffith OW, Qi W, Thompson MB & Wilson AB. Seahorse brood pouch transcriptome reveals common genes associated with vertebrate pregnancy.Molecular Biology and Evolution, 2015.

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