The alleged increase in two-headed sharks

Why So Many Two-Headed Sharks?

Formerly the subject of horror movies and surreal art, real life two headed sharks have been appearing in the science media over the last few years.

A recent National Geographic article asks the question why have so many two headed sharks been reported in recent years? While there is not firm answer to this question, mutations perhaps caused by disease or toxins, and inbreeding are suggested as causes.

My own suggestion would be that the frequency of occurrence may not have changed at all. Recent publicity may have increased the chances of fishermen looking for, and reporting, two-headed finds. (As adult specimens have never been found, the location of specimens requires close examination of pregnant females or eggs.) This would explain why reports of this phenomena cluster together in time. One cluster around the late 1930s and one in the current era.

It should also be noted that while the media reports are closely clustered, the collection dates for the samples are spread over a wider timescale.

 Rather than being a purely new occurence, two headed shark reports can be found going back centuries. Vincenzo de Romita, for example, is said to have collected a specimen in Italy in 1876. He remarked that such a specimen was "a not common monstrosity", but this suggests it was rare and sought after by natural historians, rather than unheard of.

Examples:

Below is a list of the of the examples I can locate. If you know of more examples please let me know!

2016 (1)

• An embryo from an egg-laying species of shark, the Atlantic sawtail catshark (Galeus atlanticus)

2013 (1)

• Fetus of a bull shark (Carcharhinus leucas).

2008 (1)

• Small tail shark specimen (Carcharhinus porosus) from Columbia. Muñoz‐Osorio, L. A., Mejía‐Falla, P. A., & Navia, A. F. (2013). First record of a bicephalic embryo of smalltail shark Carcharhinus porosus. Journal of fish biology, 82(5), 1753-1757.

2000 (3)

• Two specimens of Rhizoprionodon porosus found in the water of Brazil.Zaera, D., & Johnsen, E. (2011). Foetal deformities in a smooth-hound shark, Mustelus mustelus, from an oil exploited area in Angola. Cybium, 35(3), 231-236.
• Two-headed blue shark (Prionace glauca).Ferreira, L. A., Ferreira, T. G. A., & Amorim, A. F. (2002). Embryo anomaly of blue shark, Prionace glauca (Linnaeus, 1758) Carcharhinidae, Carcharhiniformes. III Reuniao da Sociedade Brasileira para Estudo em Elasmobraˆnquios SBEEL. Caderno de resumos, 38-39.

1990 (2)

• An example of smalleye smooth-hound (Mustelus higmani) and  blue shark (Prionace glauca) from the Caribbean sea. Drinan, D. P., Galindo, H. M., Loher, T., & Hauser, L. (2016). Subtle genetic population structure in Pacific halibut Hippoglossus stenolepis. Journal of Fish Biology.

1981 (4)

• Four specimens of blue shark (Prionace glauca) found in Japan.
  Goto, M., Taniuchi, T., Kuga, N., & Iwata, M. (1981). Four dicephalous specimens of the blue shark, Prionace glauca, from Japan. Japanese Journal of Ichthyology (Japan).

1938 (1)

• A two-headed shark, species not noted, is found in Australia.

1934 (2)

• Two preserved examples of two-headed tope sharks (Galeorhinus galeus) were found in a private collection.  Delpiani, S. M., Deli Antoni, M. Y., Barbini, S. A., & Figueroa, D. E. (2011). First record of a dicephalic specimen of tope Galeorhinus galeus (Elasmobranchii: Triakidae). Journal of fish biology, 78(3), 941-944.

1876 (1)

• Specimen, species unknown, collected by  Romita

1642 (1)

• A two-headed shark reportedly from the river Nile. Possibly apocryphal? Gudger, E. W. (1929). An Unusually Large (53 Mm.) Two-headed Brook Trout, Salmo Fontinalis. American Museum of Natural History.