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Melatonin has been shown to regulate reproduction and neoblast proliferation in triclad flatworms, yet the extent to which these mechanisms operate in other platyhelminths remains unknown. We examined light sensing, endogenous melatonin production, and circadian control of asexual reproduction in the early\u2010branching catenulid\n                    <jats:italic>Stenostomum virginianum<\/jats:italic>\n                    . Microscopy revealed refractile bodies with photoreceptor\u2010like structures, blue\u2010light autofluorescence, and PAX6 immunoreactivity in cells anterior to the brain that support the presence of a photoreceptive system. High\u2010performance liquid chromatography detected endogenous melatonin production in\n                    <jats:italic>S. virginianum<\/jats:italic>\n                    . Exogenous melatonin administered under continuous darkness suppressed reproduction and neoblast mitosis. The same treatment applied at ZT\u20096 during a 12\u2009h:12\u2009h light\u2013dark cycle failed to affect reproduction, implying a masking or circadian compensatory effect. Phylogenetic analysis identified two broadly conserved bHLH\u2010PAS genes (ARNT) in a transcriptome for\u2009\n                    <jats:italic>S. virginianum<\/jats:italic>\n                    ; although CLOCK and CRY were not detected, they are present in the closely related\n                    <jats:italic>Macrostomum lignano<\/jats:italic>\n                    , indicating that this absence may be due to an incomplete transcriptome. These findings suggest that circadian rhythms influence reproduction and cell division in\n                    <jats:italic>S.\u2009virginianum<\/jats:italic>\n                    , extending our understanding of melatonin\u2010mediated, light\u2010responsive regulation in distinct branches of the platyhelminths.\n                  <\/jats:p>","DOI":"10.1111\/php.70100","type":"journal-article","created":{"date-parts":[[2026,5,8]],"date-time":"2026-05-08T12:33:38Z","timestamp":1778243618000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Circadian regulation of reproduction and neoblast cell division in the catenulid flatworm\n                    <i>Stenostomum virginianum<\/i>\n                    (Platyhelminthes)"],"prefix":"10.1111","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3238-4523","authenticated-orcid":false,"given":"Daniel L.","family":"Stanton","sequence":"first","affiliation":[{"name":"Department of Biological Sciences University of North Carolina at Charlotte  Charlotte North Carolina USA"},{"name":"Department of Animal Sciences University of Florida  Gainesville Florida USA"},{"name":"Department of Biology Winthrop University  Rock Hill South Carolina USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ian D.","family":"Deas","sequence":"additional","affiliation":[{"name":"Department of Biology Winthrop University  Rock Hill South Carolina USA"},{"name":"Office of the Dean of Undergraduate Studies Princeton University  Princeton New Jersey USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"J. 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Torrance California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5734-7118","authenticated-orcid":false,"given":"Adam M.","family":"Reitzel","sequence":"additional","affiliation":[{"name":"Department of Biological Sciences University of North Carolina at Charlotte  Charlotte North Carolina USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8263-6445","authenticated-orcid":false,"suffix":"III","given":"Julian P. 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