Getting a grip on the squid hectocotylus: Sexually dimorphic glandular system underlies spermatophore transfer in Doryteuthis pleii (Blainville, 1823).

During mating, male cephalopods generally use a modified arm (the hectocotylus) to transfer spermatophores to the female. To shed some light on the functional morphology of such an important organ in cephalopod reproduction, we have thoroughly investigated the male and female arms of the loliginid squid Doryteuthis pleii (Blainville, 1823) by applying light and electron microscopy. In D. pleii, the distal region of the left ventral arm of mature males is modified, with the suckers of the dorsal row being greatly reduced in size and supported by narrow pedicels. Moreover, the epithelia between the dorsal and ventral sucker rows and between the ventral row and its protective membrane have two types of exclusive secretory cells. These cells secrete neutral sugars and basic proteins and can be distinguished mainly by the size of their granules as well as their affinity for hematoxylin. Additionally, they display a particular distribution: type I cells are predominant in the area between dorsal and ventral rows, while type II cells are prevalent in the region between the ventral row and ventral protective membrane. Several hypotheses may explain the function of this sexually dimorphic glandular system, e.g.: control over spermatophore eversion by coating them with a viscous substance; manipulation of the female physiology/behavior through chemical signaling; and production of an adhesive secretion that would enable the hectocotylus to firmly hold spermatophores without damaging them. The external morphology of the hectocotylus of other species suggests a similar glandular system is likely widespread across decapodiform cephalopods.