A few days ago, it occurred to me to browse the different streaming video platforms in search of a marine-themed film. In the end, I found one with a title and messages that caught my attention, “The two-headed shark attack”: one body, two heads and 6000 teeth (Fig.1). It was Saturday afternoon, and it was raining, so I took the chance.

The film starts with a sequence in which a two-headed great white shark attacks a group of wakeboarders. Minutes later, an oceanographic boat is damaged when it hits another shark. That was enough for me, and I stopped watching the film. But I wondered: has a two-headed shark ever been found?

The answer is yes, and what’s more, it seems to be an increasingly common occurrence.

The presence of two heads in the animal world is technically called bicephaly and refers to two twins fused side by side with two totally separate heads and a single body. In the case of sharks, it seems that it all started in September 2008, when a fisherman sent photographs of a blue shark embryo (Prionace glauca) with two heads to National Geographic (Fig.2).

Subsequently, in 2011, scientists published a study of two-headed blue shark embryos from females caught in Baja California, Mexico (Fig. 3).

This study does not explain the cause of this bicephaly, although it is speculated that it may be due to parasitic infections, tumors, poor feeding, or genetic abnormalities. However, it is noted that developmental deformities in sharks are usually cephalic, whereas postnatal abnormalities involve mostly spinal deformities.

Later, in 3021, bicephaly was described for the first time in the bull shark (Carcharhinus leucas). The study, carried out by Wagner et al. (2003) indicated that external examination, radiography, and magnetic resonance imaging revealed a case of bicephaly in which the axial skeleton and internal organs had divided into parallel systems before the pectoral girdle, giving rise to two well-developed heads (Fig. 4).

It was not until 2016 that Sans-Coma, et al. described a case of bicephaly in the Atlantic sawtail catshark (Galeus atlanticus). In this case, the two-headed specimen was detected among 797 embryos destined for cardiovascular studies. Each head had two eyes, a brain, a notochord and five gill slits on each side of the head. The two heads were fused behind the gills. Notably, although it had two esophagi, two stomachs and two livers, it had only one intestine (Fig. 5).

The authors of this study suggested that the most plausible cause of bicephaly could be a genetic disorder.

Two-headed sharks seem to continue to appear. There is no clear explanation as to what causes this curious phenomenon. There is talk of infections, chemical agents, radiation, or metabolic problems. The most interesting theory is that overfishing is diminishing the gene pool of some species, and this would lead to inbreeding that would cause genetic abnormalities resulting in this bicephaly.

Whatever the cause, the film industry continues in its usual line of entertaining us on rainy Saturday afternoons. We can now find new stories starring 3-, 5- and 6-headed sharks (Fig. 6).

Where is the limit to the number of heads on a shark? It seems that for Hollywood this question has no answer.