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This study describes the first thorough investigation of oocyte and embryonic development as well as histologically confirmed size at sexual maturity of female Charybdis bimaculata. A total of 1211 females were collected from the central part of Kagoshima Bay, southern Japan, from October 2018 to January 2020. Six sequential oocyte developmental stages were confirmed by histological examination. C. bimaculata exhibited asynchronous-type ovarian development. Therefore, ovarian maturity status (immature, maturing, and mature) was determined from the most advanced oocyte found in the ovary. Females having oocytes evidencing either germinal vesicle breakdown or migratory nucleus were defined as mature. Mature females ranged from 19.0 to 30.6 mm in carapace width. The size at sexual maturity was estimated to be 21.8 mm in carapace width based on the 50% maturity size. The spawning season was determined from May to November when mature females occurred. Microscopic observation confirmed four sequential embryonic developmental stages. The majority of females with final-stage embryos had mature ovaries signifying continuity of reproduction. Asynchronous-type ovary, continuous reproduction, and consecutive broods suggest multiple spawning in a single spawning season. The incubation period was estimated to be under 1 month for C. bimaculata. [ABSTRACT FROM AUTHOR]

Morphological changes in the oocytes of the Japanese eel, Anguilla japonica, induced to undergo ovarian development by repeated injections of salmon pituitary homogenate, were examined using electron microscopy. Oil droplets were closely associated with organelles, especially mitochondria, and increased in number as oocyte growth proceeded. They fused at the migratory nucleus stage. During vitellogenesis, two types of cortical alveoli were distinguished, one having filamentous contents, the other having latticated contents. As oocytes reached maturity, the structure of the cortical alveoli was exclusively filamentous. Yolk globules were homogeneous and highly electrondense, but electrondensity decreased during hydration. The structure of the zona radiata of previtellogenic oocytes consisted of two layers, and an additional reticular network structure was formed on the inside of the zona radiata during the vitellogenic stage. The zona radiata lost the reticular network structure and assumed a layered structure of uniform electrondensity at the migratory nucleus stage. These structural changes during oocyte development were mostly comparable to those in other teleosts. Results of the present study should assist in developing improved methods for full control of artificial maturation in the Japanese eel. [ABSTRACT FROM AUTHOR]

Oikopleura dioica is a ubiquitous marine zooplankton of biological interest owing to features that include dioecious reproduction, a short life cycle, conserved chordate body plan, and a compact genome. It is an important tunicate model for evolutionary and developmental research, as well as investigations into marine ecosystems. The genome of north Atlantic O. dioica comprises three chromosomes. However, comparisons with the genomes of O. dioica sampled from mainland and southern Japan revealed extensive sequence differences. Moreover, historical studies have reported widely varying chromosome counts. We recently initiated a project to study the genomes of O. dioica  individuals collected from the coastline of the Ryukyu (Okinawa) Islands in southern Japan. Given the potentially large extent of genomic diversity, we employed karyological techniques to count individual animals' chromosomes in situ using centromere-specific antibodies directed against H3S28P, a prophase-metaphase cell cycle-specific marker of histone H3. Epifluorescence and confocal images were obtained of embryos and oocytes stained with two commercial anti-H3S28P antibodies (Abcam ab10543 and Thermo Fisher 07-145). The data lead us to conclude that diploid cells from Okinawan O. dioica  contain three pairs of chromosomes, in line with the north Atlantic populations. The finding facilitates the telomere-to-telomere assembly of Okinawan O. dioica genome sequences and gives insight into the genomic diversity of O. dioica from different geographical locations. The data deposited in the EBI BioImage Archive provide representative images of the antibodies' staining properties for use in epifluorescent and confocal based fluorescent microscopy., Competing Interests: No competing interests were disclosed., (Copyright: © 2021 Liu AW et al.)

Oikopleura dioica is a ubiquitous marine tunicate of biological interest due to features that include dioecious reproduction, short life cycle, and vertebrate-like dorsal notochord while possessing a relatively compact genome. The use of tunicates as model organisms, particularly with these characteristics, offers the advantage of facilitating studies in evolutionary development and furthering understanding of enduring attributes found in the more complex vertebrates. At present, we are undertaking an initiative to sequence the genomes of Oikopleura individuals in populations found among the seas surrounding the Ryukyu Islands in southern Japan. To facilitate and validate genome assemblies, karyotyping was employed to count individual animals' chromosomes in situ using centromere-specific antibodies directed against H3S28P, a prophase-metaphase cell cycle-specific marker of histone H3. New imaging data of embryos and oocytes stained with two different antibodies were obtained; interpretation of these data lead us to conclude that the Okinawan Oikopleura dioica has three pairs of chromosomes, akin to previous results from genomic assemblies in Atlantic populations. The imaging data have been deposited to the open-access EBI BioImage Archive for reuse while additionally providing representative images of two commercially available anti-H3S28P antibodies' staining properties for use in epifluorescent and confocal based fluorescent microscopy., Competing Interests: No competing interests were disclosed., (Copyright: © 2020 Liu AW et al.)

Spiophanes uschakowi is a common polychaete living in tubes in sandy sediments in shallow waters of the Sea of Japan. Females and males release their gametes into the water where fertilization and holopelagic, planktotrophic larval development occur. In females, oogenesis is intraovarian: vitellogenesis occurs when the oocytes grow in paired ovaries attached to genital blood vessels in fertile segments. The developed oocytes are accumulated in the coelomic cavity prior to spawning. The newly released oocytes are lentiform, 185-200 μm in diameter, with honey-combed envelopes 5-7 μm thick. Each oocyte has 41-49 cortical alveoli regularly arranged in a peripheral circle, a nucleus 80-83 μm in diameter, and a single nucleolus about 30 μm in diameter. In males, spermatogonia proliferate in testes and the rest of spermatogenesis occurs in the coelomic cavity. During spermiogenesis, the acrosomal vesicle migrates from the posterior to the anterior part of the spermatid. The spermatozoa are ect-aquasperm with a plate-like acrosome 0.58 ± 0.06 μm thick and 2.14 ± 0.13 μm in diameter, barrel-shaped nucleus 2.23 ± 0.13 μm long and 3.18 ± 0.13 μm in diameter, short midpiece 0.93 ± 0.09 μm long with five spherical mitochondria, two centrioles and one small lipid droplet, and a flagellum 62-63 μm long with 9 × 2 + 2 organization of microtubules. The acrosome is a complex heterogeneous structure with 4-6 subspherical apical bodies, and numerous small branched basal cisternae. The anterior end of the nucleus is truncate, while its posterior end has wide shallow depressions accommodating the mitochondria. The centrioles are situated in the center of the midpiece between mitochondria and oriented obliquely to each other. The structure of the gametes of broadcast-spawning spionids is reviewed and the roles of surface granules in species-specific attraction of sperm toward eggs by releasing chemical signals (sperm chemotaxis), and cortical alveoli as a place of penetration of spermatozoa into oocytes (micropyle) are suggested. The lentiform oocytes of Spiophanes spp. are unique among Spionidae by their shape, while spermatozoa are unique by their plate-like acrosomes., (Copyright © 2018 Elsevier Ltd. All rights reserved.)