Fishes care for their young in a variety of ways. While some fishes are ‘transfer brooders’, which forget about their young after laying their eggs, others are ‘pelvic brooders’ and carry their eggs for long periods of time, allowing the embryos to develop and grow while protected by their parent. Ricefish have evolved a peculiar sticky structure that emerges from the genital pore and holds their eggs on the outside of the body. This ‘plug’ keeps eggs safe until hatching then deteriorates so that the fish can prepare for its next spawn. However, we know little about the anatomical changes in the reproductive system of fishes that have allowed some to carry their eggs, in contrast to those that just lay their eggs, or how the plug evolved in the first place. To understand how the plug forms, Alina Schüller from Museum Koenig, Germany, and an international team of colleagues documented the anatomy of a Oryzias eversi, an egg-carrying ricefish during the spawning period. They aimed to categorize which cell types are involved in plug formation and how the anatomy of these fish changes across time.
The researchers collected O. eversi females at various points in the spawning process and examined their reproductive tract and egg bundles attached to the plug using a microscope. They also CT scanned the abdomens of the ricefish on two occasions: in the late stages of the spawning cycle and after the eggs had hatched and left the mother. They compared the anatomy of these egg-carrying species with a close relative, Oryzias latipes, which doesn't carry its eggs.
The team discovered that the plug-like structure is made up of filaments anchored within the tube that delivers the eggs from the ovary to the outside world and the reproductive anatomy of the two ricefish species didn't differ much between the egg carriers and those that simply lay their eggs. However, the entire spawning process is much slower for egg-carrying species. While the egg-laying species can develop new eggs within the ovaries while spawning, the egg-carrying species cannot simultaneously form new eggs while still carrying the previous clutch. The plug also prevents the ovary from becoming infected by closing the tube that leads from the ovary to the outside world. Interestingly, the researchers found giant cells with multiple nuclei within the plug used to anchor the eggs of their mother. Even more intriguingly, humans also produce these specialised cells, called macrophages, when we pick up a splinter, or some other foreign body, and our skin swells and reddens as part of an inflammatory response, suggesting that formation of the plug may have evolved from an immune response.
Learning how some mechanisms in the body, such as immune responses, can change in their function for new uses, such as new reproductive strategies, can help us to better understand the origins and intricacies of these mechanisms. This knowledge can help scientists to establish the relationships between various species and how diverse animals such as fishes are, not only in their anatomy, but also in the ways that they live and reproduce.
