Monthly changes in the swim bladder morphology of the female Japanese eel Anguilla japonica in the coastal waters of Mikawa Bay, Japan

Abstract

In the coastal area of Mikawa Bay, central Japan, specimens of the female Japanese eel Anguilla japonica could be divided into two groups according to the degree of swim bladder development. In one case, they were undeveloped, and in the other, they were highly developed with large rete mirabile, thick submucosa, and a well-developed gas gland. The morphology of the swim bladder in the latter group was comparable with that in the artificially fully maturated eel. The specimens with an undeveloped swim bladder were caught during all months, although their number was small. The specimens with a highly developed swim bladder were most abundant in November and December. During these months, catch of the specimens also increased sharply, by more than 10 fold compared to that in other months. These observations indicate that most of the eel appearing in coastal Mikawa Bay from October to January have a highly developed swim bladder adapted for a deep-sea environment. It was also conjectured that these specimens most likely migrated from rivers feeding into Mikawa Bay, toward spawning grounds in the open sea, and that this occurred after development of their swim bladders was completed. Actually, the specimens caught in the upstream of the Toyo River feeding into Mikawa Bay from late August to early October already had highly developed swim bladders.Key words Adaptation to deep-sea environment · Japanese eel · Swim bladder development · Spawning migration swim bladder wall of the silver eel was more abundant than that of the yellow eel (Kleckner, 1980a,b). The gas storage capability in the swim bladder of the silver eel was higher than that of the yellow one (Kleckner, 1980b).However, these studies are limited to the comparison between yellow and silver eel and are not focused on the issue of where and when the eels adapt their swim bladders so that they can acquire adequate buoyancy in the deep-sea environment. In reference to the issue, based on our study on the relationship between morphological changes in the swim bladder and sexual maturity (Yamada et al., 2001), we previously postulated that the Japanese eel Anguilla japonica leaves the coastal area of the sea after completing its swim bladder development.In the present study, we observed the seasonal changes in the swim bladder of Japanese eel caught in the coastal area of Mikawa Bay and in the Toyo River feeding into the bay, central Japan, to investigate where the eel develops its swim bladder for its spawning migration.

Differences in Swim Bladder Histology of <i>Anguilla bicolor bicolor</i> at Various Stages of Sexual Maturity

“…The highest GGI (2.37 ± 0.77) was observed in silver male, while the lowest (0.73 ± 0.32) was found in the yellow undifferentiated (Table 2). These results are lower compared to those of Japanese eel, in which the GGI can reach up to 10 ( Yamada et al 2004).…”

“…Based on histological observations, silver stages (male and females) had significantly thicker layers of swim bladder than other stages as shown by the histological result (Figure 3) and measurements (Figure 5). These data suggested that an increase in the thickness of the gas gland allows a greater contribution of gas to gas secretion, and that the submucosa thickness reduces gas conductivity from swim bladder wall (Yamada et al 2001;Yamada et al 2004). Mucosa layer contains bundle sheets of smooth muscles that play an important role in the mechanical movement of the swim bladder and are closely associated with gas glands (Fange 1958).…”

“…Histological observations were carried out based on the thickness of gas gland cells (GG), mucosa (M), and submucosa (SM) layers of the undifferentiated yellow, yellow eels, silver males, and silver females. The GGI, MI, and SMI were measured as in Yamada et al (2004) as follows: Gas Gland Index (GGI) = 100 GG TL -1 Mucosa Index (MI) = 100 M TL -1 Submucosa Index (SMI) = 100 SM TL -1 GGI, MI, and SMI were calculated from 1-6 slides for each sample, and every slide was measured from 5 directions as replications.…”

Bathymetric segregation of spawning stages in the Atlantic sardine Sardina pilchardus

“…Even though the relationship between swimbladder dynamics and reproductive state has not been widely explored, it has been documented for a few fish species, e.g. Japanese eel Anguilla japonica (Yamada et al 2004). Furthermore, Ona (1990) found for several fish species that changes in swimbladder volume are inversely related to changes in the size of various visceral organs, including the gonads.…”