MadSci Network: Genetics

Re: Would a 'mermaid' or 'merman' feasibly survive in the ocean?

Date: Fri Jan 12 13:31:26 2007
Posted By: Paul Szauter, Staff, Mouse Genome Informatics
Area of science: Genetics
ID: 1167617545.Ge

"Mermaidism" is sirenomelia, a rare and usually lethal malformation characterized by severe 
anomalies of the caudal part of the fetus. Besides the fusion of the lower limb into a single limb 
that gives the malformation its name, the other malformations include bilateral renal agenesis 
(failure of both kidneys to develop), absence of a urinary outflow tract, absence of genitalia, and 
imperforate anus.

Please see the OMIM entry for Urogenital Adysplasia (%191830):

Sirenomelia is associate with other inherited conditions besides the one listed above.

Sirenomelia is usually lethal, and affected individuals are far from helathy. In February 2005, a 
nine-month old girl in Peru made the news when she was scheduled for surgery to correct the 

Mermaids, as fantasy creatures, have wide appeal and appear in many different cultures across 
the ancient world. The sea, to many, symbolizes freedom, and it is also a source of abundant 
food. It is fun to think about humans adapted for life in the sea, living in a kind of natural state 
while the rest of us have to deal with the mess that we have made of the land.

So let's look at the changes to humans that you have speculated about:

1. Legs fused into a mermaid tail
2. Fins instead of feet
3. Presence of gills
4. Ability to live on an all-seafood diet

We know that the first change, fusion of the lower limbs, is possible, but it is most often 
associated with malformations in the kidneys and urogenital tract (see above).

Fins instead of feet are possible, although well-developed fins are not observed in humans. 
There are a number of marine mammals (whales, dolphins, walruses and seals) that have fins 
instead of feet, and flippers instead of forelimbs.

Gills do not occur in mammals. This is a radical change in development that is difficult to 
imagine. You suggest that it might occur in humans if they were "forced" to adapt, but whales 
and dolphins have evolved from the first fully recognizable whales living entirely in the ocean 
about 38 million years ago. You can even see the hind limb atavism in the skeleton of a sperm 
whale. The hind fin is an adapted tail, not a fusion of the hind legs. Whales have had 38 million 
years of living in the ocean, and they have not developed gills. There is some good information 
on the evolution of cetaceans here:

As for being able to live on an all-seafood diet, many different cultures around the world already 
come very close to doing so. A diet consisting only of foods available in the ocean could certainly 
be consistent with proper human nutrition.

Thank you for an interesting question!


Paul Szauter
Mouse Genome Informatics

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