Onykia aequatorialis (Thiele 1920)

Introduction

This specimen was found floating dead at the surface and described by Thiele (1920) as a new species, distinct from the two other then-known species of "Moroteuthis" (M. ingens and M. robusta) based on its size, the rostrum of the gladius, and several other bodily proportions. 

Diagnosis

An Onykia ...

• found in the equatorial Atlantic and Gulf of Mexico.
• with rhomboidal fins (ie fins not drawn-out posteriorly).
• with warts on the skin.
• with rostrum of gladius <15% GL and oval in cross-section.

Characteristics

1. Tentacles 
1. Unknown - lost in both known specimens (spent females).
   
   
2. Arms
1. 48-60% ML, with Arms IV distinctly longest.


3. Head
1. Lower beak rounded, length and depth approximately equal.
   


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Figure.  Lower beak of Onykia aequatorialis Holotype.  Left - left profile view.  Right - left oblique view.  Photographs by K. Bolstad.

4. Mantle
   1. Epidermis with soft, irregular warts.
      
      

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      Figure. Dorsolateral mantle skin of O. aequatorialis Holotype.  Photograph by K. Bolstad.

5. Fins
   1. Rhomboidal shape, not drawn out into a tail.
   2. Length ~55% of ML.
   3. Width ~70% of ML.
   
   
6. Gladius
   1. Maximum width of vanes ca. 13% of gladius length (to tip of conus).
   2. Extant portion of rostrum ~12% of the ML, oval in cross-section.
      

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      Figure. Ventral view, gladius of Onykia aequatorialis. Line drawing by K. Bolstad. 
      

Comments

Due to previously uncertain deposition of the Holotype, this species has been considered a nomen dubium (Tsuchiya & Okutani 1991).  However, examination of the type has revealed that it is distinct from other known Onykia species.  The beak is readily distinguishable from both O. robsoni and O. ingens.  The shorter rostrum of the gladius and the rhombic (not drawn-out) fins also separate O. aequatorialis from O. robsoni, while oval cross-section of the rostrum of the gladius and the distinctly longest Arms IV separate it from O. ingens.

Both known specimens of this species are spent females.

Other Names for Onykia aequatorialis (Thiele 1920)

• Moroteuthis aequatorialis

References

Thiele, J. 1920. Die Cephalopoden der Deutschen Südpolar Expedition. Deutsche Südpolar Expedition, Zoologie, 16: 433–465, Pl. 53, 54.

Tsuchiya, K.; Okutani, T. 1991. Growth Stages of Moroteuthis robusta (Verrill, 1881) with the Re-evaluation of the Genus. Bulletin of Marine Science 49(1/2): 137–147.