Figure 1. The blood vessel arrangement in conjoined yolk sac fry. (Photo: Tom Hansen)

Paper of the Month: Twinning salmonids – a new source to knowledge about conjoined twins?

Conjoined twins are frequently observed in fish hatcheries. However, they normally die during the first feeding period and rarely achieve adulthood. In this study we investigated different types of conjoined salmonid twins at the yolk sac stage, and dissected a fully developed conjoined twin specimen obtained at Institute of Marine Research, Matre Research Station. This may contribute in filling in knowledge – gaps on this subject also in higher species.

Conjoined salmon twins may help us to elucidate the early development of conjoined twins. After hatch, the salmon fry is supported with nutrient from the yolk sac. The yolk sac is transparent (Fig. 1) enabling scientists to study the developing blood vessels in detail – making salmon an ideal model organism in which to study this topic.

The yolk sac conjoined twins showed a wide range of phenotypes, with twins that were laterally conjoined at different levels, or twins only connected to a common yolk sac by their abdomens. The twins form by a primary fission of the early cleaving blastoderm that results in the formation of two adjacent blastoderms that undergo secondary fusion during epiboly. The time between primary fission and secondary fusion affects the twinning phenotype. Early secondary fusion leads to twins fused from the head backwards, while later fusion leads to twins only fused by a common tail. If the two embryos do not fuse, they are only connected by a common yolk sac.  ‘Separate’ twins only fused by a common yolk sac undergoes secondary fusion after the yolk sac is absorbed.



Figure 2. Different twinning phenotypes, from nearly completely fused (a) to embryos only fused by a common yolk sac (e). (Photo: Tom Hansen)


For the fully grown conjoined twin specimen, one was normal and one was small and severely malformed (Fig. 3). Histology of the gills of the small twin showed that these were malfunction, not allowing for normal respiration. Also its mouth was blocked. Dissection showed that the normal twin supported the small twin with oxygen and nutrients through a joined cardiovascular system. Earlier studied, on other fish species, show that conjoined yolk sac twins that are only fused by a common yolk gets connected by their abdomens as the yolk sac is absorbed, resembling a phenotype similar to our fully grown specimen. In this study, among the yolk sac conjoined twins, some specimens with this type of phenotype – ‘separate’ embryos fused to a common yolk sac – had cardiovascular connections via a clearly joined vena vitellina hepatica. Since the yolk sac in salmonids is transparent and the embryo is relatively large, twinning in salmonids may be a useful model in which to study cardiovascular morphogenesis in conjoined twins. This may contribute in filling in knowledge -gaps on this subject also in higher species. All the phenotypes of conjoined twins defined in this study have similar human phenotypes.


Figure 3. ‘Separate’ twins that sheared a common yolk sac and fused as the yolk sac got absorbed. These twins survived first feeding, which is very rear. (Photo: Harald Kryvi)