Abstract:Liparis tanakae is a dominant species of the Yellow Sea ecosystem, and its biological characteristics—such as high seasonal variations in population structure, rapid growth, relatively short lifespan, and pronounced response to environmental changes—substantially contribute to population maintenance mechanisms. To date, relevant ecological studies of L. tanakae have mainly focused on seasonal and interannual changes in its spatial distribution, feeding ecology, genetic classification, and osteological and myological characteristics. However, research on its reproductive ecology remains insufficient, limiting our scientific understanding of the mechanisms underlying its dominance. As a pivotal life history phase, reproduction is essential for the survival and persistence of fish populations. Furthermore, the reproductive capabilities of species form the basis for broader ecological adaptations and success within their environments. To investigate the biological reproductive characteristics of L. tanakae, histological studies of the ovaries at various developmental stages were conducted using paraffin sections and hematoxylin and eosin staining. Samples were obtained from fishery resource surveys conducted in June and August 2022 and October 2023 in the Yellow Sea as well as during the winter (January and February 2022, December 2023, and January 2024). In total, 71 specimens were collected, 50 of which were used for ovary sample sectioning. The morphology and diameter distribution characteristics of oocytes at different developmental stages were described in detail, as well as the spawning type. The results showed that oocyte development could be divided into six phases. In phase Ⅰ, the oocyte is just differentiated from the oogonium, with a small cell volume and relatively large nucleus; the oocyte in phase Ⅱ enlarges, and a follicle membrane appears around the cells; in phase Ⅲ, yolk vacuoles start to appear in the cytoplasm of the cell and yolk begins to accumulate; zona radiate appears between the cytoplasm and follicle membranes, and the follicle membrane becomes two-layered; in phase Ⅳ, yolk begins to fill up the yolk vacuoles, gradually forming yolk platelets; in phase Ⅴ, yolk merges into a single large yolk ball, and the nucleus dissolves; in phase Ⅵ the oocytes degenerate, the yolk is gradually absorbed to form a cavity, and some oocytes are irregular in shape. Ovary development can also be divided into six stages, and there are obvious differences in oocyte composition at different ovarian development stages. In stage Ⅱ, the ovary is mainly composed of oocytes in phase Ⅰ and Ⅱ with the percentages of 34.92% and 65.08%, respectively. In stage Ⅲ, the ovary is mainly composed of oocytes in phase Ⅱ (49.47%) and Ⅲ (34.73%). In stage Ⅳ, the percentage of oocytes in phase Ⅲ account for 51.55%, whereas that of oocytes in phase Ⅳ increase to 32.99%. In stage Ⅴ，the ovary is mainly composed of oocytes in phase Ⅲ (34.62%) and Ⅳ (32.28%). In stage Ⅵ, the percentages of oocytes in different phases are relatively uniform, with some degraded oocytes and empty follicles. The egg diameter in stages Ⅳ and Ⅴ exhibited a unimodal distribution with the dominant groups between 0.90–1.00 and 1.70–1.80 mm, respectively. The egg diameter in stage Ⅵ showed a bimodal distribution, with dominant groups between 0.50–0.70 and 1.10–1.20 mm, and high oocyte proportions in the small-growth and degraded phases were observed. Compared to previous results (1985–1986 and 2011–2012), our study (2022–2024) revealed a notable increase in both the distribution range of egg diameters and the proportion of large egg diameter groups. Hatchability was positively correlated with the egg diameter, and an increase in the egg diameter of L. tanakae improved the hatching rate. This may be an adaptive response of L. tanakae to the multiple pressures from a rapidly changing external environment, caused by climate change and fishing activities. In addition, other economically important fish species in the Yellow Sea ecosystem, such as Larimichthys polyactis and, Gadus macrocephalus have declined in the past few decades. This has decreased the inter-specific competition pressure faced by L. tanakae to some extent, and the relatively abundant prey environment might have contributed positively to the egg diameter increase of L. tanakae. The results indicate that L. tanakae ovaries exhibited considerable developmental potential and sustainability, and we consider L. tanakae to be a non-synchronized multiple batch spawner. Our study reveals the developmental characteristics of L. tanakae ovaries and provides a theoretical reference for further enrichment of the reproductive ecology of this species.
