Molecular Detection of European Shore Crab (Carcinus maenas) During Larval Release in the Coast of Southern Norway

Abstract

Although the eDNA method has proven effective for detecting aquatic species, its efficiency can vary depending on the species in question. Crustaceans are believed to shed a low amount of eDNA due to the presence of hard-shelled exoskeletons, thus reducing the efficiency of eDNA detection. However, at a specific time of the year, particularly in summer, the zoea larvae of benthic crab species hatch and exhibit a vertical migration depending upon the diel or tidal cycle. Lab experiments have shown that the eDNA detection rates in water samples tend to be significantly higher at such times when the brood hatches, and thus, the density of crab zoea is high. The primary aim of this study is to investigate the efficiency of the eDNA detection method during larval release in European shore crabs (Carcinus maenas). This study also aims to determine the larval release pattern in C. maenas in the micro-tidal area along the coast of southern Norway. Water and plankton samples were collected 2 hours after every expected high and low tide for 48 hours in a three-day period in June (28-30) and August (7-9). Designed primers and probe was used for the qPCR of isolated DNA from water samples. The findings revealed the 100% detection efficiency of C. maenas in the channel. However, the microscopic analysis of the plankton sample failed to determine the clear larval release pattern. Nevertheless, the presence of larvae in the night samples and the subsequent morning samples indicate that shore crabs release their larvae at night, regardless of tidal conditions. Additional water samples were collected in November to assess whether the 100% DNA detection during June and August was facilitated only by the presence of larvae in the water column. The results from November samples were consistent with previous findings indicating that, while zoea larvae contributed to the efficiency of eDNA detection, other biotic and abiotic factors may also play a significant role. Therefore, it is crucial to consider other factors when attempting to study or detect crustaceans using eDNA in the field.