Octopoda Leach, 1818

Introduction

Octopods have rather short, compact bodies and only eight arms; no trace of the missing second arm pair remains even during embryonic development. Many species are benthic (bottom-living) and crawl over the ocean floor with the mouth facing the substratum. Others alternate between a benthic and a pelagic (free-swimming) habitat and some species are completely pelagic. The two suborders of Octopoda are very different in appearance but there is little doubt that it is a natural group as the monophyly of the Octopoda is supported by a large variety of characters. The Cirrata is a group of deep-sea octopods commonly known as the "finned octopods" due to their large, wing-like fins. The Incirrata contain the common (benthic), shallow-water octopods as well as many deep-sea benthic and pelagic species.

Brief diagnosis:

An octopodiform ...

• with one arm pair, presumaably true arms II, absent.
  
• with suckers on proximal halves of arms.
  

Characteristics

1. Arms
   1. Suckers with cuticular lining.
      
   2. Sucker stalks are broad, muscular cylinders.
      
      
2. Head
   1. Nuchal cartilage absent.
      
   2. Inferior frontal lobe system present; superior buccal and posterior buccal lobes fused; suprabrachial commissure separate from brain (absent in Japetella).
      
      
3. Funnel
   1. Funnel valve absent.
      
      
4. Viscera
   1. Visceropericardial coelom reduced.
   2. Oviducal glands act, in part, as spermathecae.
   3. Oviducts with oviducal glands subterminally located on oviducts.
   4. Photosensitive vesicles located on stellate ganglia.
      
   5. Dorsal mantle cavity present.

Discussion of Phylogenetic Relationships

The monophyly of both suborders is well established. Young and Vecchione (1996) found two apomorphic character states that defined the monophyly of the Incirrata:

• Presence of a cornea.
• Hectocotylization of one of the third arms.

Another character can probably be added to the list of apomorphies although it hasn't been included in a cladistic study:

• Chorion of egg drawn out into a stalk (Young, et al., 1999).

Young and Vecchione (1996) found three character states that defined the monophyly of the Cirrata:

• Spermatophores as sperm packets without ejaculatory apparatus.
• Presence of horizontal arm septa.
• Buccal position of posterior salivary glands.

Voight (1997) also found morphological, cladistic support for the Cirrata and Incirrata but her study is controversial. Carlini (1998) found molecular analyses supported the monophyly of both groups.

Other Names for Octopoda Leach, 1818

• octopuses
• octopi
• Octopods or devilfishes

References

Carlini, D. B. 1998. The phylogeny of coleoid cephalopods inferred from molecular evolutionary analyses of the cytochrome oxidase I, muscle actin, and cytoplasmic actin genes. Ph.D. diss. Coll. William and Mary, 273 pp.

Young, R. E. and M. Vecchione. 1996. Analysis of morphology to determine primary sister taxon relationships within coleoid cephalopods. Amer. Malac. Bull. 12: 91-112.

Young, R. E., M. Vecchione and D. Donovan. 1999. The evolution of coleoid cephalopods and their present biodiversity and ecology. South African Jour. Mar. Sci. (in press).

Voight, J. R. 1997. Cladistic analysis of the octopods based on anatomical characters. J. Moll. Stud., 63: 311-325.