Bryozoan Placentas

Celleporella hyalina. Source.
Celleporella hyalina. Source.

Pictured above is Celleporella hyalina, a species of cheilostome bryozoan. Bryozoans are aquatic, filter-feeding, colonial organisms – each opening up there is the house of an individual bryozoan, and they all live together in this colony that can sprawl over any substrate, often playing a significant role as foulers. Each house is technically called a zooid, and they’re made of calcium carbonate.

I bring them up because I got an e-mail asking whether any animals other than mammals have a placenta. In terms of the actual organ, no, the mammalian placenta is unique. But organs that serve the same function as the placenta do exist in some other animals, such as some bryozoans.

In bryozoans, fertilised eggs are kept in a special, fortified chamber, the ovicell, within the mother’s zooid. You can consider this like a womb. The ovicell has a door, the ooecial vesicle, that closes the ovicell off once implantation occurs – this protects the eggs from seawater.

Ovicell longitudinal section. Source.
Ovicell longitudinal section. Source.

In some bryozoans, the ooecial vesicle becomes enlarged as a special structure forms from its mother-facing wall: the embryophore. The light microscope picture above shows a longitudinal section through an ovicell. em is the embryo, eph is the embryophore; not shown to the right is the maternal zooid.

The embryophore keeps growing larger as the embryos grow, and its function is to pass on nutrients from the maternal zooid to the embryos in the ovicell. Therefore, we term it a placental analogue.

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