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The Cycloteuthidae contains two rather dissimilar genera.

Containing group: Oegopsida

Introduction

Cycloteuthids are mesopelagic squids of moderate size, the largest reaches about 60 cm ML, with a cosmopolitan distribution in tropical and subtropical waters (Nesis, 1982). In Cycloteuthis the mantle and fins are drawn-out into a slender tail. A tail is lacking in Discoteuthis and the large, oval (disc-like) combined fins extend virtually the full length of the mantle. Very little is known about the biology of any species in the family.

Brief diagnosis:

An oegopsid ...

• with triangular funnel locking-apparatus.
• with mantle component of funnel locking-apparatus not reaching anterior margin of mantle.
  

Characteristics

1. Arms
   1. Arm suckers biserial.
   2. Buccal-crown connectives attach to ventral borders of Arms IV.
      
      
2. Tentacles
   1. Tentacular club with quadraserial suckers.
      
      
3. Funnel/mantle locking apparatus
   1. Funnel component with sub-triangular groove.
   2. Mantle component does not reach anterior mantle margin.
   

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   Figure. Ventral views of the funnel locking-apparatus of C. serventyi (left) and the two species of Discoteuthis (right).

Comments

The genera are easily separated by body and fin shape (see title illustrations).  In addition the presence of a large visceral photophore only in Cycloteuthis allows genera to be separated when badly damaged.

Nomenclature

A list of all nominal genera and species in the Cycloteuthidae can be found here. The list includes the current status and type species of all genera, and the current status, type repository and type locality of all species and all pertinent references.

Discussion of Phylogenetic Relationships

The features that unites these two genera is the similar triangular shape of the funnel locking-apparatus and the position of the mantle component. Other than these unusual features, there is little morphological evidence for their close relationship. Molecular evidence (Carlini, 1998; Lindgren, 2010) supports their close relationship. A more recent study, however, found the two genera only distantly related (Lindgren, 2012).

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.

Lindgren, A.R. 2010.Molecular inference of phylogenetic relationships among Decapodiformes (Mollusca: Cephalopoda) with special focus on the squid Order Oegopsida. Mol. Phylogenet. Evol. 56: 77-90.

Lindgren, A. R., M. S. Pankey, F. G. Hochberg and T. H. Oakley. 2012. A multi-gene phylogeny of Cephalopoda supports convergent morphological evolution in association with multiple habitat shifts in the marine environment. BMC Evolutionary Biology 2012, 12:129-144.

Nesis, K. N. 1982. Abridged key to the cephalopod mollusks of the world's ocean. 385+ii pp. Light and Food Industry Publishing House, Moscow. (In Russian.). Translated into English by B. S. Levitov, ed. by L. A. Burgess (1987), Cephalopods of the world. T. F. H. Publications, Neptune City, NJ, 351pp.

Young, R. E. and C. F. E. Roper. 1969. A monograph of the Cephalopoda of the North Atlantic: The family Cycloteuthidae. Smithsonian Contributions to Zoology No. 5:1-24.