Male seahorses are nature's Mr. Mom, Texas A&M researchers say

COLLEGE STATION – Male seahorses are nature’s real-life Mr. Moms – they take fathering to a whole new level: Pregnancy.

Although it is common for male fish to play the dominant parenting role, male pregnancy is a complex process unique to the fish family Syngnathidae, which includes pipefish, seahorses and sea dragons. Texas A&M University evolutionary biology researcher Adam Jones and colleagues in his lab are studying the effects of male pregnancy on sex roles and sexual selection of mates and are trying to understand how the novel body structures necessary for male pregnancy evolved. By doing this, the researchers hope to gain a better understanding of the evolutionary mechanisms responsible for changes in the structure of organisms over time.

“We are using seahorses and their relatives to address one of the most exciting areas of research in modern evolutionary biology: the origin of complex traits,” Jones said. “The brood pouch on male seahorses and pipefish where females deposit eggs during mating is a novel trait that has had a huge impact on the biology of the species because the ability for males to become pregnant has completely changed the mating dynamics.”

When seahorses mate, the female inserts her ovipositor into the male’s brood pouch (an external structure that grows on the body of the male) and deposits her unfertilized eggs into the pouch. The male then releases sperm into the pouch to fertilize the eggs. “It wouldn’t be that interesting if the brood pouch were just a flap of skin where the females put regular fish eggs and they developed in the bag instead of on the sea floor,” Jones said. “But the male pregnancy in some species of seahorses and pipefish is physiologically much more complex than that.”

After the female deposits her unfertilized eggs into the male, the outer shell of the eggs breaks down, and tissue from the male grows up around the eggs in the pouch. After fertilizing the eggs, the male closely controls the prenatal environment of the embryos in his pouch. The male keeps blood flowing around the embryos, controls the salt concentrations in the pouch, and provides oxygen and nutrition to the developing offspring through a placenta-like structure until he gives birth.

Male pregnancy has interesting implications for sex roles in mating, Jones explained, because in most species, males compete for access to females, so you usually see the evolution of secondary sex traits in males (for example, a peacock’s tail or antlers in deer). But in some species of pipefish, the sex roles are reversed because males become pregnant and there is limited brood pouch space. So females compete for access to available males, and thus secondary sex traits (such as brightly colored ornamentation) evolve in female pipefish instead of males.

“From a research standpoint, it’s interesting because there aren’t very many species in which there is a sex role reversal,” Jones said. “It provides a unique opportunity to study sexual selection in this reversed context.”