{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,5]],"date-time":"2026-10-05T21:50:55Z","timestamp":1791237055190,"version":"4.1.0"},"reference-count":77,"publisher":"Zoological Society of Japan","issue":"4","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Zoological Science"],"published-print":{"date-parts":[[2026,5,29]]},"abstract":"<jats:p>Reconstructing ancestral reproductive systems is crucial for understanding the evolution of bilaterian body plans. Yet, despite extensive research on some organ systems of the last common bilaterian ancestor, the evolution and development of reproductive systems remain poorly understood. To address this gap, we investigated postembryonic sexual development in the xenacoelomorph Hofstenia atroviridis, a basal acoel species belonging to the infraorder Prosopharyngida. We reared H. atroviridis from eggs to adults and described the ontogeny of its reproductive organs using histology and immunohistochemistry with muscle and neural markers, along with observations of mating behaviors in each size class. Hofstenia atroviridis is a protandrous simultaneous hermaphrodite whose sexual maturity correlates with body size rather than age, as observed in Hofstenia miamia. The male copulatory system comprises a seminal vesicle, granular vesicle, penis, and a single copulatory stylet, surrounded by regionally specialized musculature and innervation. In contrast, the female system consists of paired, asaccate ovaries with large, follicle cell-bound oocytes, lacking a discrete gonopore or copulatory organ. Fertilization occurs internally through traumatic insemination, and eggs are likely released through the mouth. Our findings reveal that the male reproductive system of H. atroviridis exhibits unexpected complexity and structural differentiation despite the organism's overall simplicity. We propose that sexual selection acting on a hermaphroditic reproductive system may have driven the evolution of a multifunctional copulatory organ in the Urbilateria. This study establishes H. atroviridis as a promising and comparative model for exploring the evolutionary origins of animal reproductive systems.<\/jats:p>","DOI":"10.2108\/zs250119","type":"journal-article","created":{"date-parts":[[2026,6,8]],"date-time":"2026-06-08T19:49:41Z","timestamp":1780948181000},"page":"349-363","source":"Crossref","is-referenced-by-count":0,"title":["Morphology and Development of Reproductive Organs in the Acoel Hofstenia atroviridis"],"prefix":"10.2108","volume":"43","author":[{"given":"Rupandey","family":"Parekh","sequence":"first","affiliation":[{"name":"Misaki Marine Biological Station, School of Science, The University of Tokyo, Miura, Kanagawa 238-0225, Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Coline","family":"Hermine","sequence":"additional","affiliation":[{"name":"Center for Interdisciplinary Research in Biology, Coll\u00e8ge de France, Universit\u00e9 PSL, CNRS, INSERM, France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yohei","family":"Otomo","sequence":"additional","affiliation":[{"name":"Misaki Marine Biological Station, School of Science, The University of Tokyo, Miura, Kanagawa 238-0225, Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Toru","family":"Miura","sequence":"additional","affiliation":[{"name":"Misaki Marine Biological Station, School of Science, The University of Tokyo, Miura, Kanagawa 238-0225, Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"848","reference":[{"key":"51","doi-asserted-by":"crossref","unstructured":"JM Mart\u00edn-Dur\u00e1n  and A Hejnol  \"Dev Biol.\"  475, \"A developmental perspective on the evolution of the nervous system.\" 181 (2021)","DOI":"10.1016\/j.ydbio.2019.10.003"},{"key":"31","doi-asserted-by":"crossref","unstructured":"G Genikhovich  and U Technau  \"Development.\"  144, \"On the evolution of bilaterality.\" 3392 (2017)","DOI":"10.1242\/dev.141507"},{"key":"1","doi-asserted-by":"crossref","unstructured":"S Abalde  and U Jondelius  \"Syst Biol.\"  74, \"A phylogenomic backbone for Acoelomorpha inferred from transcriptomic data.\" 70 (2025)","DOI":"10.1093\/sysbio\/syae057"},{"key":"4","doi-asserted-by":"publisher","unstructured":"M \u00c1lvarez-Presas, I Ruiz-Trillo, J Paps,  \"Res Sq.\"  \"Novel genomic approaches support Xenacoelomorpha as sister to all Bilateria.\" (2024)","DOI":"10.21203\/RS.3.RS-5529390\/V1"},{"key":"8","doi-asserted-by":"crossref","unstructured":"G Balavoine  and A Adoutte  \"Integr Comp Biol.\"  43, \"The segmented Urbilateria: A testable scenario.\" 137 (2003)","DOI":"10.1093\/icb\/43.1.137"},{"key":"9","doi-asserted-by":"crossref","unstructured":"S Beltagi,  \"J KAU: Mar Sci.\"  12, \"Anaperus ornatus N. sp.: A new interstitial acoelan Turbellaria from marine sediments in the Red Sea.\" 163 (2001)","DOI":"10.4197\/mar.12-1.12"},{"key":"10","doi-asserted-by":"crossref","unstructured":"N Blackstock  and AD Pickering  \"Cell Tissue Res.\"  210, \"Acidophilic granular cells in the epidermis of the brown trout, Salmo trutta L.\" 359 (1980)","DOI":"10.1007\/BF00220194"},{"key":"11","unstructured":"S Bock,  \"Uppsala Universitets Arsskr Mat Och Nat.\"  1, \"Eine neue marine turbellariengattung aus Japan.\" 1 (1923)"},{"key":"12","doi-asserted-by":"crossref","unstructured":"SJ Bourlat  and A Hejnol  \"Curr Biol.\"  19, \"Acoels.\" R279 (2009)","DOI":"10.1016\/j.cub.2009.02.045"},{"key":"13","unstructured":"BC Boyer  and GW Smith  \"Pac Sci.\"  36, \"Sperm morphology and development in two acoel turbellarians from the Philippines.\" 365 (1982)"},{"key":"14","unstructured":"RC Brusca, W Moore, SM Shuster,  \"Invertebrates.\"  \"Introduction to the Bilateria and the Phylum Xenacocoelomorpha. 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