title: Spermatangium formation and sperm discharge in the Japanese pygmy squid Idiosepius paradoxus.
abstract: In cephalopods, sperm discharge is an important event not only for sperm transfer but also influencing sperm storage capacity of attached spermatangia (everted spermatophores). To investigate sperm discharge from spermatangia and the condition of naturally attached spermatangia in Japanese pygmy squid (Idiosepius paradoxus) we (i) investigated the morphology of spermatophores and spermatangia, and the process of spermatophore evagination and sperm discharge from spermatangia obtained in vitro; (ii) observed spermatangia that were naturally attached to female squids at 6, 12, 18, 24 and 48 h after copulation to investigate alterations in naturally attached spermatangia with time. The spermatophore of I. paradoxus is slender and cylindrical and consists of a sperm mass, a cement body and an ejaculatory apparatus, which is similar to those of loliginid squids. The spermatangium is fishhook-shaped, its distal end being open and narrow. After the spermatangium is formed, the sperm mass gradually moves to the open end of the spermatangium, from where sperm are released. Sperm discharge is a rapid process immediately after the beginning of sperm release, but within 5 min changes to an intermittent release of sperm. Although the volume of residual spermatozoa differed among spermatangia that were naturally attached to a single individual, the probability that spermatangia would be empty increased with time. Most naturally attached spermatangia discharged almost all of their spermatozoa within 24h after copulation, and no spermatangia were attached to females 48 h after copulation. These results suggest that sperm transfer from the spermatangium to the seminal receptacle must occur within 24h, and that the spermatangium functions as a transient sperm storage organ in pygmy squids.
authors.affiliation: Division of Biosphere Science, Graduate School of Environmental Science, Hokkaido University, Hokkaido 0600810, Japan; Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, Nagasaki 8528521, Japan. Electronic address: norico3000@gmail.com.
authors.forename: Noriyosi
authors.initials: N
authors.lastname: Sato
authors.affiliation: Port of Nagoya Public Aquarium, Aichi 4550033, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kasugai
authors.affiliation: Usujiri Fisheries Station, Field Science Centre for Northern Biosphere, Hokkaido University, Hokkaido 0411613, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Munehara
mesh_terms: D000818:Animals; D003307:Copulation; D049832:Decapodiformes / Q000166:cytology*; D005260:Female; D008297:Male; D013093:Spermatogonia / Q000166:cytology*; D013094:Spermatozoa / Q000166:cytology
keywords: Cephalopoda; Idiosepius paradoxus; Sperm storage; Sperm transfer; Spermatophoric reaction