| The ricefield eel(Monopters albus) is a high economic value freshwater fish in China. Since Liu C K et al. found it has the sex reversal phenomenon in 1944, ricefield eel has become the model organism of sex changing research in fish. A large number of scholars have researched it from all kinds of aspects, such as morphology, physiology, biochemistry and molecular biology, however, there are still some problems on sex reversal mechanisms, scale farming and artificial breeding technology of it. This is the first time that the artificial breeding eel is as the research object in this study, and we continuously acquired the eel samples of each gender(female, intersex and male) from 1 to 4 years old. We investigated the embryonic development, Gonadal differentiation and development, Sex reversal, Gamete quality and growth characteristics of each age of the artificial breeding eel using the methods of histology, statistics and CASA. Our major results are as follows:(1) We continuously collected the artificial breeding eel samples from embryo to 1 year old according to the developmental time points and investigated the process of gonadal ontogenesis, differentiation and position changing. The results showed that: under hydrostatic condition in the laboratory(26~28℃), the primordial germ cell(PGC) was first seen at the left of the nephridium area as single cells at 12 h after hatching; at 24 h after hatching, the PGCs moved into the genital primordium, which meant the formation of primordial gonad; at 72 h after hatching, the primordial gonad divided into two symmetrical parts; at 25 d after hatching, the connective tissue membrane appeared to wrap the primordial gonad, meanwhile, there emerged the ovarian cavity at one part of the primordial gonad which marked the differentiation of ovarian cavity; one side of PGCs proliferated rapidly and differentiated into the primary oocytes, leading to squeeze the other side; the part extruded was going to become genital fold which is the primordium of testis.(2) We found that there were some differences on body height around the cloaca among the different gender of ricefield eel, which leaded to various abdominal shapes among them. In female, the abdominal shape was like “W” and the body height in the back of the cloaca was higher than that in front of the cloaca, but not obviously; In intersex, the abdomen was flat and cloaca was not sunk; In male, the body height in the back of the cloaca was obviously higher than that in front of the cloaca. The abdominal shape and cloacal state has a certain association with the gonadal development status, thus we could use the abdominal shape and cloacal state as a crucial morphological index to identify the gender of ricefield eel.(3) In the ricefield eel of the same age: there were no significant differences between the intersex and male on the body length(P > 0.05), however, the body length of the them were all longer than that of female significantly(P < 0.05); the weight of the intersex and male were heavier than that of female as well(p < 0.05). Gamete quality analysis showed that the relative fecundity of 2+ years old female eel was the largest and the egg diameter of it was relatively small. Additionally, the proportion of females in the 2+ years old eel was higher than that in the 3+ or 4+ years old one. As a whole, the sperm motorial speed of the ricefield eel was relatively slow, and the sperm motility was low.(4) Under hydrostatic condition in the laboratory(26~28℃), at 140±4h, the fertilized egg began to hatch, and the hatchability was 5%; at 158±2h, the hatchability reached the peak(41%); the whole process of embryonic development lasted for about 151 h. In this study, we divided the growth stage of ricefield eel which from incubation to young fish into three periods according to the gonadal development, early growth characteristics and feeding behavior: larva was the period from 1 to 10 d after hatching; juvenile was the period from 10 to 30 d after hatching; young fish was the period from 30 d after hatching to first sexual maturity.(5) The one years old artificial breeding eel appeared allometry(WWL = 0.0109L2.3195, R 2 = 0.956). We speculated this phenomenon was a survival strategy to store energy for ovarian development and sex reversal.(6) In summary, it is better to choose 2 years old female eel and 3 or 4 years old male eel as the parent fish for artificial propagation; additionally, the larvae should be cultivated indoor until 30 d after incubation, after that, transfer them to the foster cages for subsequent cultivation. |
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