Why genetic validation is crucial for identifying larval stages of Arctica islandica (Venerida, Arcticidae).

The ocean quahog Arctica islandica is a long-lived marine bivalve widely distributed in the western Baltic Sea and commonly used as a model organism in ecological and climatic research. Despite its importance, aspects of its early development remain poorly understood. This study attempts to provide a morphological description of A. islandica larvae. Plankton samples were collected in Kiel Bay (Baltic Sea) in June 2024, September 2024, and April 2025, using a WP2 net (mesh size 100 µm). Larvae were identified by sequencing the mitochondrial cytochrome oxidase subunit I (COI) gene and subsequently documented using light and scanning electron microscopy (SEM). A benthic juvenile was additionally examined for comparison. Larvae of A. islandica were detected exclusively in June and September, yielding a total of 17 COI sequences. Genetically confirmed veliger larvae were detected in September (shell length 143-185 µm), whereas larger larvae interpreted as pediveligers were detected in a separate sampling event in June (shell length 217-505 µm). Larval anatomical structures and the apparent absence of certain organs are discussed. This study provides the first genetically confirmed description of the veliger and pediveliger stages of A. islandica and demonstrates that morphology alone is insufficient for reliable identification of A. islandica larvae and emphasizes the necessity of genetic validation.