Invertebrate Biology. 2020;00:e12302. wileyonlinelibrary.com/journal/ivb | 1 of 14 https://doi.org/10.1111/ivb.12302 © 2020, The American Microscopical Society, Inc. 1 | INTRODUCTION Polyplacophorans, or chitons, are relatively common across the benthic habitats, where >1,000 species (MolluscaBase, 2019; Schwabe, 2005) are found from rocky intertidal shores to marine trenches (Schwabe, 2010). Although chitons are widely recog- nized as ecologically important herbivores on rocky shores world- wide (e.g., Hawkins & Hartnoll, 1983; Lubchenco & Gaines, 1981; Otway, 1994), much less is known of their life histories and repro- ductive cycles, particularly in the southern Pacific region. As in nearly all molluscs, most chitons are gonochoric (dioecious) and free spawning with external fertilization, although some genera include brooders. At least two species are hermaphrodites, possibly with self-fertilization (e.g., Eernisse, 1988), whereas others show low levels of hermaphroditism (e.g., Ramírez-Álvarez et al., 2014). The gonad of chitons, which is formed by the fusion of two gonadal pri- mordia, is located in front of the pericardial cavity and develops as a single dorsal structure, which, when fully developed, usually ex- tends between the second and seventh shell plates (Pearse, 1979; Strathmann & Eernisse, 1987; Eernisse & Reynolds, 1994). It has been Received: 16 August 2019 | Accepted: 16 August 2020 DOI: 10.1111/ivb.12302 ORIGINAL ARTICLE First comparative assessment of the reproductive cycle of three species of Chiton on a temperate rocky shore of the southeastern Pacific María J. Brito 1 | Patricio A. Camus 1,2 | Felipe I. Torres 3 | Javier Sellanes 4,5 | Christian M. Ibáñez 3 1 Departamento de Ecología, Facultad de Ciencias, Universidad Católica de la Santísima Concepción, Concepción, Chile 2 Centro de Investigación en Biodiversidad y Ambientes Sustentables (CIBAS), Universidad Católica de la Santísima Concepción, Concepción, Chile 3 Departamento de Ecología y Biodiversidad, Universidad Andres Bello, Santiago, Chile 4 Millennium Nucleus for Ecology and Sustainable Management of Oceanic Islands (ESMOI), Universidad Católica del Norte, Coquimbo, Chile 5 Departamento de Biología Marina, Facultad de Ciencias del Mar, Universidad Católica del Norte, Coquimbo, Chile Correspondence Christian M. Ibáñez, Departamento de Ecología y Biodiversidad, Facultad de Ciencias de la Vida, Universidad Andrés Bello, Avenida República 440, Santiago, Chile. Email: ibanez.christian@gmail.com Funding information Fondo Nacional de Desarrollo Científico y Tecnológico, Grant/Award Number: 1130266; Universidad Católica de la Santísima Concepción Abstract The present study provides the first assessment of the reproductive cycle of three common and widespread species of the genus Chiton, sympatric in rocky shores of Coquimbo, northern Chile (~30°S): C. granosus, C. cumingsii, and C. magnificus. From specimens seasonally sampled during a 2-year period, a histological study was per- formed in order to describe the gonad structure and stages of gonadal development for each species. The morphological structure of the gonad was similar in the three species, with sexual cells developing from the gonad wall towards its center. The three species were gonochoric and iteroparous, with asynchronous gamete matura- tion at population level; in all seasons, sexual maturity was evident in small to large individuals of each species (≥25 mm in total length for C. cumingsii and C. granosus, and ≥47 mm for C. magnificus). KEYWORDS gonad development, histology, oogenesis, Polyplacophora, reproductive cycle, sexual maturity