Two-headed sharks… fact or fiction?

Figure 1: Movie poster.

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).

Figure 2: Two-headed blue shark from a pregnant female captured in the Indian Ocean (Source: National Geographic). Figure 3: Two-headed blue shark from the Gulf of California (Punta Arena), Mexico (Galvá-Magaña et al, 2011).

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).

Figure 4: X-ray and MRI of the bull shark specimen.

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.

Figure 5: Embryos of Galeus atlanticus: (a-c) double-headed embryo and (d,e) normal embryo (Sans-Coma, et al., 2016).

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.

 

Figure 6: Poster of movies starring 3-, 5- and 6-headed sharks.