The average size of adult H. pfefferi is 100 mm (3.9 in) in mantle length (ML).

The body consists of a large funnel with small paddle-like fins.

[citation needed] Paddle-shaped fins are attached to a part of the gladius.

[3] As paralarvae (<30 mm ML), they live near the surface of the oceans, between 100 m and 200 m deep.

They descend to the mesopelagic zone as they mature, but exhibit a diel vertical migration pattern.