Unusual development of an accessory otolith in herring ( <i>Clupea harengus</i> ) larva

Abstract Otoliths are key to reconstructing fish life history, as their structure records individual and environmental information. These inner ear structures grow incrementally, revealing growth rates, migration patterns, and environmental conditions. However, internal factors (e.g., genetic mutations and stress) and external factors (e.g., temperature changes and pollution) can cause structural anomalies, such as shape deformities, asymmetry, and altered composition. This study describes a unique anomaly, an unusual accessory otolith, in the sagittal otolith of an Atlantic herring ( Clupea harengus L.) larva collected from the Vistula Lagoon in the southern Baltic Sea, a vital spawning area. A 40 mm larva exhibited a rare otolith anomaly (found in only one of ~2000 specimens analyzed) characterized by a main otolith measuring 0.47 × 0.39 mm with an average of 64 daily increments, and an attached accessory measuring 0.20 × 0.10 mm with an average of 25 daily increments. This anomaly, previously unreported in Baltic herring larvae, may reflect environmental stressors and is significant for understanding developmental abnormalities in fish.