# Ultrastructural Evidence for Dual Sperm Morphotypes in Hormone-Induced Japanese Eel (Anguilla japonica): Implications for Sperm Maturation

## TL;DR

Japanese eel sperm can have two different shapes, which may help improve artificial breeding techniques.

## Contribution

The discovery of two distinct sperm morphotypes in Japanese eel provides new insights into sperm maturation and reproduction.

## Key findings

- Two sperm types were identified: round and eyebrow-shaped, with distinct nuclear and axonemal structures.
- Round sperm had a '9 + 2' axoneme pattern, while eyebrow-shaped sperm had a '9 + 0' pattern.
- The findings suggest morphological diversity in eel sperm, relevant to reproductive biology and artificial breeding.

## Abstract

Two distinct morphological structures of Japanese eel sperm were observed by means of light microscopy and electron microscopes. The cell nucleus of one type of sperm was round, the type of sperm was smaller in size, and the axoneme displayed a typical “9 + 2” pattern. Another type was the eyebrow-shaped sperm, characterized by an elongated head and an axoneme with a “9 + 0” microtubule pattern. These findings demonstrated morphological diversity in Japanese eel sperm, providing fundamental data and scientific support for further research in sperm biology and artificial reproduction.

The microstructure and ultrastructure of the sperm of Japanese eel, Anguilla japonica, artificially induced with weekly injections with carp pituitary (CP) and human chorionic gonadotropin (HCG), was studied, and milt from 10 out of 20 mature fish was collected. Two distinct morphological structures of A. japonica sperm had been observed with optical microscopy. The cell nucleus of one type of sperm was round or nearly round, the sperm was smaller in size, with 2.57 ± 0.62 μm of the long diameter of the cell nucleus, 2.11 ± 0.59 μm of the short diameter, and 37.35 ± 7.71 μm of the flagellum length. Another type was the eyebrow-shaped sperm, the sperm was relatively larger in size, with 7.66 ± 1.09 μm of the long axis, 2.54 ± 0.46 μm of the short axis, and 38.26 ± 9.02 μm of the flagellum length. By means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the anterior end of the round sperm lacked an acrosomal structure. The implantation fossa was located in the center of the base of the sperm head, and it was in the shape of a channel along the long axis of sperm. The centriolar complex was situated within the implantation fossa. There were 2–3 mitochondria distributed at the basal end of the nucleus. Sperm flagellum prolongated from the sleeve cavity, and the initial part of axoneme connected to the distal end of basal body. The axoneme displayed a typical “9 + 2” pattern. There was a spherical structure in the curving area for eyebrow-shaped sperm, and the axoneme formed a “9 + 0” pattern. The discovery of two different types of sperm in the Japanese eel has provided new perspectives for research on its artificial reproduction.