Male pregnancy in seahorses and pipefish: beyond the mammalian model

Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union. Despite strong similarities between viviparity in mammals and other vertebrate groups, researchers have historically been reluctant to use the term pregnancy for non-mammals in recognition of the highly developed form of viviparity in eutherians. Syngnathid fishes (seahorses and pipefishes) have a unique reproductive system, where the male incubates developing embryos in a specialized brooding structure in which they are aerated, osmoregulated, protected and likely provisioned during their development. Recent insights into physiological, morphological and genetic changes associated with syngnathid reproduction provide compelling evidence that male incubation in these species is a highly specialized form of reproduction akin to other forms of viviparity. Here, we review these recent advances, highlighting similarities and differences between seahorse and mammalian pregnancy. Understanding the changes associated with the parallel evolution of male pregnancy in the two major syngnathid lineages will help to identify key innovations that facilitated the development of this unique form of reproduction and, through comparison with other forms of live bearing, may allow the identification of a common set of characteristics shared by all viviparous organisms.