Introduction

Gonatus middendorffi was described by Kubodera and Okutani (1981) based on a holotype (225 mm ML) and two male paratypes (183, 245 mm DML) with three additional juvenile specimens (40-60 mm ML) collected from the subarctic waters of the northwestern North Pacific. Morphological changes with growth of this species were well described. G. middendorffi is one of a few gonatids that can be identified almost throughout its life.

Nesis (1973, 1997) treated G. middendorffi as a synonym of G. kamtschaticus described by Middendorff in 1849. Based on the original description (Onych. Kamtschatica n. sp. in Middendorff's paper), G. kamtschaticus (24-26 cm ML) has a slender mantle, short fins about 1/3 of body length, long tentacles equal to body length, and the large central club hook is 8mm in length with a distal hook of 4 mm in length.

G. middendorffi specimens of nearly the same size as G. kamtschaticus have relatively larger fins (FL=1/2 ML), shorter tentacles and smaller clubs with much smaller central (ca. 4 mm) and distal hooks (ca. 2mm) on manus than G. kamtschaticus. We consider that G. middendorffi is not synonymous with G. kamtschaticus.

Diagnosis

A Gonatus with ...

• long slender muscular mantle (MW=18% of ML); small fins (FL=1/2 ML), sagittate with round sides (FW=77% of FL) and long tail (ca. 18% of ML).
• relatively short arms (AL<1/2 ML), short tentacles and small tentacular club (ca. 12% of ML).
• central large hook on tentacular club, medium-sized hook distal to central hook and five small suckers some of which are modified into small hooks in a line proximal to the central hook.