aplacental

Aplacentals are a group of fish that lack a placenta. This means that the embryo develops within the mother but receives nourishment from the yolk sac of the egg. This contrasts with placental mammals, where a specialized organ called the placenta facilitates the transfer of nutrients and waste between the mother and the developing fetus.

In aplacentals, the eggs are typically oviparous, meaning they are laid and hatch outside the mother's body, or ovoviviparous, where the eggs hatch inside the mother and the young are then born live. The yolk sac provides all the necessary nutrients for embryonic development until hatching or birth.

Examples of aplacentals include most bony fishes, such as sharks, rays, and many teleosts. These groups have evolved diverse reproductive strategies, with varying degrees of parental care and dependence on yolk reserves. The absence of a true placenta is a defining characteristic that distinguishes them from the placental mammals and some other vertebrate groups. Understanding aplacentals helps in comprehending the evolutionary pathways of reproduction in vertebrates.