Phylogenetic relationships of Octopoda

FIG. 1. Maximum-parsimony tree obtained from analysis of the unweighted COI data Constraint of monophyly of the Octopodidae resulted in a significantly longer tree, whereas enforcement of monophyly of the Cirrata did not yield a significantly longer tree. Source-(Carlin D.A et al, 2001)

FIG. 2. Maximum-likelihood tree derived from a heuristic search of the COI data under the assumption of a general time-reversible model of substitution with site-specific rates (GTR ) estimated according to the gamma distribution. Branch lengths are drawn proportional to the probabilities of change occurring along each branch under the GTR model.

In today’s blog, let us explore phylogenetic relationships of the Octopoda comparing results from molecular data analyses with classification based on morphological features.

The Octopod phylogeny is complex and full of controversies. Research utilizing Molecular data did not support the Octopodidae as a monophyletic group (based on morphological features).(Carlin D.A et al, 2001)

 Notable disparities were seen at the order of branching.(Fig.1 and 2) At the base, Argonauta branched off first in the MP tree. Argonauta grouped based on smaller mature male compared to mature female, hectocotylus remain coiled inside a pocket until use, and hectocotylus break off during mating. (Nesis, 1987), Whereas in the ML tree, a clade consisting of Argonauta and three members of subfamily octopodinae (O. tetricus, O. bimaculoides, and Hapalochlaena sp.) initially branched off from the others.Based on the various positioning of  O. bimaculoides ,it’s evolutionary history is unconfirmed.

O. tetricus ;O. bimaculoides and Hapalochlaena were shown as sister taxa, in clade subfamily octopodinae .A sister taxon relationship based on shared trait of ink sac and two sucker rows. This was also supported by data from ML and MP trees.

In both tree,O. californicus was closely related to Benthoctopus sp. than O. bimaculoides, although O. californicus shared possession of ink sac and double  sucker rows with O. bimaculoides. Benthoctopus sp. on the other hand possess double sucker row with no  ink sac .(O’Shea S.1999)

MP tree was a  significantly longer tree compared to the ML tree especially since branch lengths were constructed proportionally to the number of parsimony steps between nodes.(Carlin D.A et al, 2001)

REFERENCE

Carlin D.A et al (2001) A molecular phylogeny of the Octopoda (Mollusca: Cephalopoda) evaluated in light of morphological evidence. Molecular phylogenetics and evolution, ISSN: 1055-7903, Vol: 21, Issue: 3, Page: 388-97 .

Nesis, K. N. (1987). “Cephalopods of the World: Squids, Cuttlefishes, Octopuses, and Allies,” Tropical Fish Hobbyist Publ. Inc., Neptune City, NJ.