The evolution and physiology of male pregnancy in syngnathid fishes

Abstract

The seahorses, pipefishes and seadragons (Syngnathidae) are among the few vertebrates in which pregnant males incubate developing embryos. Syngnathids are popular in studies of sexual selection, sex‐role reversal, and reproductive trade‐offs, and are now emerging as valuable comparative models for the study of the biology and evolution of reproductive complexity. These fish offer the opportunity to examine the physiology, behavioural implications, and evolutionary origins of embryo incubation, independent of the female reproductive tract and female hormonal milieu. Such studies allow us to examine flexibility in regulatory systems, by determining whether the pathways underpinning female pregnancy are also co‐opted in incubating males, or whether novel pathways have evolved in response to the common challenges imposed by incubating developing embryos and releasing live young. The Syngnathidae are also ideal for studies of the evolution of reproductive complexity, because they exhibit multiple parallel origins of complex reproductive phenotypes. Here we assay the taxonomic distribution of syngnathid parity mode, examine the selective pressures that may have led to the emergence of male pregnancy, describe the biology of syngnathid reproduction, and highlight pressing areas for future research. Experimental tests of a range of hypotheses, including many generated with genomic tools, are required to inform overarching theories about the fitness implications of pregnancy and the evolution of male pregnancy. Such information will be widely applicable to our understanding of fundamental reproductive and evolutionary processes in animals.

Seadragon genome analysis provides insights into its phenotype and sex determination locus

“…Like other syngnathids, seadragons have specialized morphological traits such as a tube-like toothless mouth, the absence of pelvic fins and scales, a bony armor covering the body, and "male pregnancy." Males of all syngnathids incubate the fertilized eggs on a specialized incubation area, and in the modern lineages, even in enclosed brood pouches, making them prime examples for sex-role reversal (1)(2)(3). On the basis of the position of the brood pouch, syngnathids have been divided into tail-brooders (subfamily Syngnathinae represented by seahorses, some pipefishes and seadragons) and trunk-brooders (subfamily Nerophinae represented by Manado pipefish) (4,5).…”

A Multidisciplinary Experimental Study on the Effects of Breeders Diet on Newborn Seahorses (Hippocampus guttulatus)

“…Due to the uncommon nature of reproduction, syngnathids offer a unique opportunity to examine pathways of repercussion of breeder's dietary resources on breeding performance and the resulting newborn quality. Reproduction in seahorses has received increasing attention in recent years, mostly on the effect of environment (e.g., photoperiod regime and temperature) (Lin et al, 2006(Lin et al, , 2008bPlanas et al, 2010Planas et al, , 2013 zootechnical conditions (e.g., sex ratio, aquaria design, and seahorse size) (Woods, 2000;Dzyuba et al, 2006;Faleiro et al, 2008Faleiro et al, , 2016Planas et al, 2008) and biological / physiological features (Boisseau, 1967;Carcupino et al, 2002;Poortenaar et al, 2004;Stölting and Wilson, 2007;Scobell and MacKenzie, 2011;Zhang et al, 2019;Wittington and Friesen, 2020). Only a few studies have focused on the effects of feeding and dietary sources on breeding success and newborn features in seahorses under ex-situ conditions (Lin et al, 2007;Faleiro and Narciso, 2010;Binh and Serrano, 2012;Otero-Ferrer et al, 2012Palma et al, 2012Palma et al, , 2017Saavedra et al, 2015).…”

Diversity of Seahorse Species (Hippocampus spp.) in the International Aquarium Trade

“…and copepods), keeping of breeders, and maintenance of a nursery facility for juveniles as they grow into an appropriate size [43,44,51,181,183]. There is no larval breeding stage because fertilized eggs are incubated inside the male's pouch [43,[184][185][186][187]. Seahorse species in captivity usually become sexually active at ages of around 3 to 6 months.…”