Larvae belonging to the family Sabellariidae (Polychaeta) were collected from the Gulf of California and in the Bahía de Banderas, on the west coast of Mexico. The oldest stages were selected for identification. The presence of three parathoracic segments and one pair of opercular
hooks led to the genus Idanthyrsus. Identification at the species level was more difficult, although I. armatopsis is a serious possibility. This is the first description of the planktonic larval stage of a representative of the genus Idanthyrsus. The authors draw attention
to the usefulness of a precise description of certain features, such as the morphology of uncini, or of the primary opercular paleae, which simplifies the task of identifying larvae at the species level. Comparison inside the family, between species whose development is known, confirms the
homogeneous ontogenesis and suggests strong phyletic constraints. Then emerge the questions of reproduction for specimens living over a large bathymetric range and the origin of larvae collected in the open sea.
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