The study focused on plankton samples collected from Kiel Bay in the Baltic Sea during three distinct periods: June 2024, September 2024, and April 2025. Using a WP2 closing net with a mesh size of 100 micrometers, the team hauled samples vertically through the entire water column to capture organisms from all depths. The sampling strategy allowed for a comprehensive examination of larval presence across different seasonal conditions. In addition to the planktonic larvae, a benthic juvenile specimen was examined to provide a reference for shell development and to trace the ontogenetic changes that occur during metamorphosis. This multi-temporal approach was essential for capturing the variability in larval abundance and developmental stages throughout the year.

To ensure accurate species identification, the researchers employed DNA barcoding, specifically sequencing the mitochondrial cytochrome c oxidase subunit I (COI) gene. A total of 85 larvae were processed for molecular analysis, with 51 specimens carefully pierced to facilitate DNA release and 34 kept intact to preserve their morphological features for illustration. The PCR amplification and subsequent Sanger sequencing yielded 17 high-quality COI sequences, all of which confirmed the identity of the specimens as Arctica islandica. These sequences were deposited in the Barcode of Life Datasystems and GenBank, providing a valuable genetic baseline for future studies. The success of PCR on both pierced and intact specimens suggests that shell perforation is not a strict requirement for successful DNA extraction in these small organisms.

Morphological analysis revealed distinct differences between the larvae collected in different months. The September sample contained smaller larvae, ranging from 143 to 185 micrometers in shell length, which were identified as veligers. In contrast, the June sample yielded larger larvae, measuring between 217 and 505 micrometers, interpreted as pediveligers. The veliger stage is characterized by the formation of the prodissoconch II and the presence of a velum for motility, while the pediveliger stage involves the development of a foot, preparing the larva for settlement and metamorphosis. Detailed imaging using light and scanning electron microscopy allowed the researchers to document the intricate surface structures of the larval shells, including the sculpture of irregular shallow pits and commarginal rims. These observations provide the first detailed anatomical record of these stages in the Baltic Sea population.

A key finding of the study is the significant morphological overlap between Arctica islandica larvae and those of other bivalve species, such as Mytilus edulis and Geukensia demissa. Despite detailed examination, no single diagnostic morphological character was found that would allow for unambiguous species-level identification based on shape or size alone. The larvae lacked distinctive features like cardinal teeth, which develop later in ontogeny, making them visually similar to many other planktonic bivalves. This finding underscores the limitations of traditional taxonomic methods in larval ecology and highlights the necessity of genetic validation for accurate species assignment. The study challenges previous assumptions that size or shell proportions could serve as reliable taxonomic criteria, emphasizing that environmental factors and developmental timing can influence morphological traits.

The seasonal occurrence of larvae in Kiel Bay provides insights into the reproductive phenology of Arctica islandica in the Baltic Sea. Veliger larvae were detected in September, while larger pediveligers were found in June, indicating that reproduction and larval development occur over an extended period. The absence of larvae in the April sample, collected at a lower water temperature of 5.6 degrees Celsius, suggests that colder conditions may delay reproductive activity or slow larval development. In contrast, the higher temperatures recorded in June and September, at 10.4 and 13.2 degrees Celsius respectively, align with the range associated with active spawning in other populations. This extended reproductive season reflects the variability in gonadal development and spawning timing observed among individuals, which may be influenced by local hydrographic processes and food availability.

The study also highlights the impact of Baltic Sea conditions on larval development. The larvae were collected under salinity conditions of 23 to 24 grams per liter, which is substantially lower than the salinity experienced by North Atlantic populations. This reduced salinity, along with other environmental stressors such as deoxygenation and warming, may affect growth rates and survival during the early life stages. While adult Arctica islandica show considerable tolerance to hypoxic conditions, larval stages are assumed to be more sensitive to environmental fluctuations. The discrepancy between the larval sizes observed in this study and those reported from laboratory cultures suggests that environmental factors play a significant role in shaping larval development. Understanding these dynamics is crucial for predicting how the species will respond to ongoing climate change in the semi-enclosed Baltic Sea.

Despite the significant contributions of this study, several limitations remain. The sampling was not specifically designed to investigate reproductive phenology and was limited to three occasions at two locations, which restricts the ability to draw definitive conclusions about the timing and frequency of spawning events. Furthermore, larval abundance could not be quantified due to the inability to reconstruct sampling volume. The interpretation of the pediveliger stage is based on a suite of morphological characteristics rather than the direct observation of a foot, which could not be resolved in the examined material. Future research combining genetic identification with advanced histological or imaging techniques may help resolve these uncertainties and provide a more comprehensive understanding of late larval development and metamorphosis.

In conclusion, this study establishes a critical baseline for the study of Arctica islandica’s early life history. By providing the first genetically confirmed description of its larval stages, the researchers have demonstrated the essential role of molecular tools in modern marine biology. The findings emphasize that morphology alone is not sufficient for reliable identification of bivalve larvae, advocating for the integration of genetic validation in future ecological and conservation studies. As Arctica islandica serves as a vital indicator of long-term environmental changes, accurate knowledge of its larval ecology is indispensable for assessing population persistence and recruitment dynamics in a changing climate. This work opens new avenues for investigating the complex interactions between environmental conditions and early life stages in one of the world’s longest-lived marine invertebrates.

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

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

Article References: Chaibi, C. A., Kniesz, K., & Zettler, M. L. (2026). Why genetic validation is crucial for identifying larval stages of Arctica islandica (Venerida, Arcticidae). ZooKeys, 1291, 55-73. https://doi.org/10.3897/zookeys.1291.195659

Image Credits: AI Generated

DOI: 10.3897/zookeys.1291.195659

Keywords: genetic, validation, crucial, identifying, larval, stages, Arctica, islandica, Venerida, Arcticidae, scientific research