Key to Class Cephalopoda

Modified from  Kozloff, 1987, 1996 p. 291 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
  With some notes added from Anderson, Roland C., 2006.  On West Coast Octopuses including a field key to west coast species.  The Festivus 38:1 pp 5-6 (San Diego Shell Club).
  Dosidicus gigas is also added, and the description of Benthoctopus leioderma (now Muusoctopus leioderma) is slightly modified based on our observations of live specimens.
 

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1a	With 8 arms; arms with sessile suckers; body ovoid and without fins; without an internal shell or "pen"	2
1b	With 2 tentacles in addition to 8 arms; arms with stalked suckers (photo) and/or hooks (photo); body rounded to elongate, with fins; with an internal shell or "pen"	4
2a	Skin of body smooth except for a ridge that borders the mantle on both sides; ability to change color limited to lighter and darker shades of brick-orange; white spots not present on the dorsal  mantle or on the web in front of the eyes; mantle length (measured from the body apex to the midpoint between the eyes on the dorsal  mantle) less than 5 cm [note added: This may not be true in larger specimens]; arms 2-3 times the body length; males without special enlarged suckers on the arms; without a planktonic larval stage.  With 10-13 lamellae on the outer demibranch of each gill.  Generally found deeper than 500 feet. (Benthoctopus leioderma in original key)	Muusoctopus leioderma
2b	Skin of body not smooth, but with extensible folds and/or papillae; capable of changing color freely; color red to red-brown, often mottled with white; with conspicuous white spots on the dorsal  mantle and on the web in front of the eyes; mantle length usually greater than 5 cm; arms 3-5 times the body length; males with 1 or 2 enlarged suckers on the arms; with a planktonic larval stage	3
3a	Mantle length frequently greater than 20 cm and with deep longitudinal wrinkles, weight sometimes exceeding 50 kg; body with skin folds and large truncate, paddle-like papillae; hectocotylus large, about 1/5 the length of the male's third right arm; with 12-15 lamellae on the outer demibranch of each gill; eggs 6-9 mm long; planktonic larva with a single row of chromatophores on each arm.  (Does not have 3 papillae under each eye but has 1 white spot centrally located between the eyes which makes an equilateral triangle with the eyes and a white streak from each eye to the second arm on each side.)	Enteroctopus dofleini
3b	Mantle length less than 10 cm, weight less than 200 g; body with small, pointed papillae [but not the skin folds]; hectocotylus is small, about 1/10 the length of the male's 3rd right arm; with 11-13 lamellae on the outer demibranch of each gill; eggs 3-4 mm long; planktonic larva with a double row of chromatophores on each arm (This species also has three prominent cirri visible below the eye (photo  photo), and 2 white spots in front of the eyes, which help distinguish it from small specimens of E. dofleini)	Octopus rubescens
4a	Body rounded; mantle length (measured from the body apex to the free edge of the dorsal  mantle) less than 5 cm; mantle free all around; fins semicircular, almost as long as the mantle, with a broad free lobe; arms short, circular in transverse section; tentacles retractile into special pits; dorsal arms of male modified for spermatophore transfer	Rossia pacifica
4b	Body elongate and tapering; mantle length usually greater than 5 cm; mantle attached; fins triangular, generally about half as long as the mantle; arms long, angular in transverse section; tentacles not retractile into special pits; dorsal arms of male not modified for spermatophore transfer	5
5a	Tentacle clubs narrow, with 2 rows of hooks and with a distinct cluster of modified suckers that form a "fixing apparatus"; length of fins greater than their width, and more than half the mantle length; mantle covered with fleshy longitudinal ridges; mantle length commonly exceeding 150 cm	Moroteuthis robusta
5b	Tentacle clubs without hooks and without a "fixing apparatus"; length of fins equal to or less than their width, and generally less than half the mantle length; mantle smooth, without ridges.	6
6a	Tentacle clubs narrow, the suckers large; arms with 2 rows of suckers; arms of unequal length, the ventral pair long and broad, and the left ventral arm of the male modified for spermatophore transfer; eyes covered by a continuous membrane (cornea); length of fins equal to their width.	7
6b	Tentacle clubs expanded, with numerous minute suckers, 15-20 in each row; arms with 4 rows of armature, the outer rows with suckers, the inner rows with hooks on all but the ventral arm pair; arms nearly equal in length, and the left ventral arm of the male not modified for spermatophore transfer; membrane over the eyes perforated; width of fins greater than their length; mantle length up to 30 cm (specimens collected at floating docks are usually small, with a mantle length of less than 10 cm)	Berryteuthis magister
7a	Tentacle clubs narrow, the suckers large and in 4 rows; arms with 2 rows of suckers, without hooks; arms of unequal length, the ventral pair long and broad, and the left ventral arm of the male modified for spermatophore transfer; eyes covered by a continuous membrane (cornea); length of fins equal to their width; mantle length not exceeding 19 cm	Doryteuthis opalescens
7b	Suckers on tentacle clubs in 2 rows, arm suckers have hooks (photo), mantle length up to 150 cm.	Dosidicus gigas

  Dosidicus gigas
  Enteroctopus dofleini
  Muusoctopus leioderma
  Octopus rubescens
  Rossia pacifica