An octopus with what appeared to be nine arms was collected in an artisanal fishery in Puerto Angel, Oaxaca, back in 2012. This species, Octopus hubbsorum, occurs throughout the tropical Pacific, and it’s the primary target of octopus fisheries in Mexico. This 275-gram octopus was among a sample of 115 octopuses collected by scuba divers, and based her oocytes (which go on to become eggs), she was an immature female.
Upon closer inspection, María del Carmen Alejo-Plata and Omar Valencia Méndez of Universidad del Mar discovered that the female had an abnormally bifurcated second arm -- it had divided into two branches. The findings were described in American Malacological Bulletin.
The length of the arm before the split was 70 millimeters, and it was divided into two branches: The longer arm was 215 millimeters, the shorter one was 140 millimeters. The shorter arm also had a 5-millimeter localized knob and a regenerated arm tip. Additionally, there was an unusually well-developed web between the divisions -- the same kind found uniting the bases of normal arms.
Bifurcation (or even polyfurcation) of the arms aren’t all that uncommon in octopuses, especially in cases of injury. These have been reported since 1900, and in one extreme case, seven of the eight arms were branched multiple times, resulting in a total of 90 terminal branches.
However, the arm branching seen here doesn’t appear to be the result of an injury. Instead, the duo of researchers think that these nine arms are the result of a Hox genes mutation. A large family of genes called homeobox genes direct the formation of many body structures -- from limbs to organs -- during early embryonic development. Mutations in these genes are responsible for several developmental disorders throughout the animal kingdom.
Images: Alejo-Plata et al., Amer. Malac. Bull. 2014