Stoloteuthis sp. A

Introduction

Figure.This was the title photograph when Stoloteuthis sp. A
was originally described as Heteroteuthis sp. A. 

When this species was first described on the Tree of Life, it was known from only a single female which had the basic shape of a species of Heteroteuthis especially with its anteriorly located ventral shield and, therefore, was descibed as an unrecognized species, Heteroteuthis sp. A. Now with several more specimens including mature males, the species clearly does not belong in Heteroteuthis. Its placement, however, is uncertain as it shows strong similarities to both Stoloteuthis and Sepiolina but is placed in the latter for reasons stated under "Phylogenetic relationships" below.

Brief diagnosis:

A Stoloteuthis with ...

• deep, angled posterior pit in funnel-locking groove.
• fused nuchal cartilage but free dorsal mantle margin.
• ventral shield restricted to anterior third of mantle.
  

The second feature is unique.

Characteristics

1. Arms (female)
   1. Arm formula: 3>1=2=4.
      
   2. Tips (distal 10% of arm) without suckers on arms I, III and IV; arms II with suckers to tip.
      
   3. Arms I and III with keels especially broad distally. Arms II virtually without keels, arms IV with lateral membrane.
      
   4. Suckers of dorsal and ventral series on arms identical to one another.
      
   5. Web ...
      
      

      Click on an image to view larger version & data in a new window

      
      

      Figure. Ventrolateral view of left arms I-IV of Stoloteuthis  sp. A female, 13 mm ML. Note bare arm tips on arms IV, III and I. Note the broad, distal, aboral keels  on arms I and III. Photographs by R. Young.

2. Arms (male).
   
   1. Arm formula: III>I=II>IV. Arms I and IV rather blunt; arms II and III attenuate; arms I very broad broad (hectocotylized); all arms without broad distal keels.
      
      

      Click on an image to view larger version & data in a new window

      
      

      Figure. Oral views of the arms of Stoloteuthis sp. A, males. Left - The entire brachial crown, 16 mm ML,note the modified arms I and the stubby arms IV with modified suckers at their tips; tentacles lost during capture. Right - A larger specimen, 20 mm ML, but in poor condition, showing the modified arms I and the very long but narrow web that joins them (top arrow) and the very short web that connects to arms II (bottom arrow).

   2. Hectocotylus - Both arms I very thick, and with complex structure. Protective membranes larger on dorsal margins; secondary membranes passing from protective membrane, between suckers to midline of arm. Long, slender structures beneath surface on both lateral and medial sides of arms (apparently elongate sucker pedicles).  Suckers in 2 series at arm base but in 3 or more irregular series in mid-arm (most apparent in largest male). 
      
   3. Arm webs - Arms I with narrow, web between them that extends to the arm tips nearly binding them together.
   4. Suckers - Suckers of arms IV smaller than on other arms in mid-arm regions, but suckers at arm tips not greatly diminished in size; 4 terminal arm-tip suckers with opaque white tissue surrounding inner sucker rings. Arm II-IV without enlarged suckers. Most suckers globular with cup-shaped outer rings. Hectocotylus suckers globular at base but, distally, become more tubular and curved.
      
   

   Click on an image to view larger version & data in a new window

   
   

   Figure. Stoloteuthis sp. A, male, 16 mm ML. Left - Side view of web between arms I near arm tip (top red arrow) and much lower between arms I and II (lower red arrow). Left middle - Oral view of basal 2/3 of arms I showing curvature of some suckers. Right middle - Arm II segment at 2/3 of arm from base showing typical globular suckers. Right - Tip of arm IV showing 4 white, distal suckers.
   

3. Head
   
   1. Small ocular pore on secondary eyelid just anterior to lens (usually difficult to locate).
      
   
   
   

   Click on an image to view larger version & data in a new window

   
   

   Figure. Dorsolateral view of the head of Stoloteuthis sp. A, female, 12 mm ML, showing the ocular pore (arrow). Photograph by R. Young.
4. Funnel
   1. Funnel component of the funnel/mantle locking-apparatus slightly boomerang-shaped with deep, angular pit at bend and narrow grooves on either limb. Mantle component with slender ridges extending to a high point at bend which is in posterior half of lock. Anterior edge does not reach mantle margin.
      
   2. Funnel connected to head by broad funnel adductor muscles that attach to funnel near distal sides of funnel lock.
      
   

   Click on an image to view larger version & data in a new window

   
   

   Figure. Funnel/mantle locking-apparatus of Stoloteuthis sp. A, female, 12 mm ML. Left - Side view of mantle component. Right - Frontal view of funnel component. Photographs by R. Young.

5. Mantle
   1. Mantle margin free but nuchal cartilage fused to mantle.
      
   2. Ventral mantle shield confined to anterior half of mantle (see photograph under "Photophores").
      
   

   Click on an image to view larger version & data in a new window

   
   

   Figure. Dorsolateral view of the nuchal fushion (arrow) in Stoloteuthis sp. A, female, 12 mm ML. Photograph by R. Young.

6. Pigmentation/iridophores
   1. Thick iridophore layer present on posteroventral surface of funnel.
      
   

   Click on an image to view larger version & data in a new window

   
   

   Figure. Ventral view of the funnel of Stoloteuthis sp. A female, 12 mm ML, showing iridophore layer (arrow). Photograph by R. Young.

Comments

More details of the description of Stoloteuthis sp. A can be found here. 

A better understanding of the structure of the male hectocotylus along with that of other species of Stoloteuthis and of Sepiolina nipponensis is needed before the relationships of Stoloteuthis sp. can be properly determined.

Discussion of Phylogenetic Relationships

The following table shows how some of the more phylogenetically important features in ggenera compare with those characters in Stoloteuthis sp. A. This is a simplified look at the genera and is simply to indicate that placing Stoloteuthis sp. A in a known genus is not easy. We have placed most emphasis on hectocotyliation which, as defined here shows similarities between Stoloteuthis sp. A and other  Stoloteuthis spp. and Sepiolina in the special modification of arm I (although the type of modification may prove to only superficially similar). The structure of the club in the latter prevents placing Stoloteuthis there. We are basically using Stoloteuthis as a catch-all genus as it is already difficult to define with the inclusion of S. weberi.