Overview

Brief Summary

Introduction

Incirrate octopods are about 85% of all octopodan species. They are very small to large in size. The members of the Octopodidae, the family with the most species and individuals, are benthic. The other seven incirrate families have pelagic species. The benthic incirrate octopods live from the intertidal zone to at least 4000 m, and bathypelagic forms may be found down to 2000 m. The octopodids exhibit sophisticated behavior. The brain of benthic octopuses, especially that of Octopus vulgaris, has become a model for relating brain structure to function (e.g., J. Z. Young, 1971). Incirrate octopods are commercially fished throughout the world.

Fins are absent and the arms bear one or two rows of suckers and lack cirri. The shape of the body is saccular and rather broad. The arms are often of about equal length, but sometimes the dorsal or lateral or ventral arms are distinctly longer than the others.

Brief diagnosis:

An octopod ...

  • without fins.
  • without cirri on arms.

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Comprehensive Description

Nomenclature

Naef (1923) placed the argonautoid families within a single family, the Argonautidae. Robson (1932) placed these families in the Tribe Argonautida and the remaining families in the Tribes Heteroglossa and Ctenoglossa based on the structure of the radula. The latter tribe contained the families Bolitaenidae and Amphitretidae. However the placement of these families in their own clade has been questioned (e.g., Thore, 1949; Voight, 1997).

Idioctopus gracilipes described by Taki (1962, 1964) and placed in its own family is probably a synonym of Amphitretus pelagicus (see Hochberg, et al, 1992).

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Characteristics

  1. Arms
    1. One member of Arms III hectocotylized.
    2. Cirri absent from arms.
    3. Arms without internal horizontal septa (except Japetella).
    4. Suckers uniserial or biserial.

  2. Eyes
    1. Cornea present (greatly reduced in some pelagic species).

  3. Fins
    1. Absent.

  4. Gills
    1. Gills with branchial canals.
    2. Gills asymmetrical in cross-section.

  5. Viscera
    1. Paired oviducts present.
    2. Egg chorion with stalk.
    3. Typical spermatophores present.
    4. Posterior salivary glands located posterior to cephalic cartilage.
    5. Radula and ink sac usually present.

  6. Shell
    1. Shell a pair of stylets or absent.

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Life History and Behavior

Life Cycle

Life history

Where known, incirrate octopods have a life span of 6 months to 4 years; one year is typical. Those living to 4 years are large and/or cold water species. Most species appear to be terminal spawners, i. e. they lay all eggs toward the end of their life-span and die shortly after the last of the brooded embryos hatch. Some species, however, lay several egg batches and feed between spawning periods. Spawning in the epipelagic incirrates is in this latter mode but egg batches overlap (i.e. a second batch is deposited before the first one hatches).

All incirrate octopods, benthic and pelagic, brood their eggs until the young hatch. Egg strings are mostly fixed to a substrate although some species carry their eggs on the arms. Specialized brooding habits are present in pelagic incirrates. In Argonauta the dorsal arms secrete a "shell" that functions as a brood chamber and in Tremoctopus and the bolitaenids the arms form brood-chambers. In Ocythoe, the eggs develop in the oviducts and in Vitreledonella, apparently, in the mantle cavity.

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Evolution and Systematics

Evolution

Discussion of Phylogenetic Relationships

Voight (1997) attempted to unravel the relationships among incirrate genera using cladistic methods based on morphology. Her family-level phylogeny is as follows:

    ----------- Cirrata
/ ---------- Bolitaenidae
/ / --------- Amphitretidae
\/ / / \----- Vitreledonellidae
\/ / ------ Alloposidae
\/ \ / ----- Argonautidae
\ \/ /
\ \/ Ocythoidae
\ \
\ \----- Tremoctopodidae
\------- Octopodidae

Thore (1949) considered the Amphitretidae and Vitreledonellidae to be closely related due to the shared position of the stomach and caecum relative to the digestive gland. Young et al. (1999) caution that the unusual position of the stomach and caecum serves to reduce the silhouette in upper mesopelagic waters and that the similarity could be convergent. In addition, the very different structure of their radulas has complicated the understanding of their relationships. Nevertheless, in a first look at relationships via a cladistic analysis based on morphology, Voight (1997) found these families grouping together, and placed the Vitreledonellidae in the synonomy of the Amphitretidae. This study, however, requires confirmation.

Molecular data from the mitochondrial Cytochrome Oxidase I gene and the nuclear Actin II gene, while without strong bootstrap support, suggest that our present classification of the Incirrata may be in error (Carlini, 1998; Carlini and Graves, 1999). Indeed, the possibility exists that all incirrate families are derived from the Octopodidae.

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