Effectiveness of Sampling Techniques in Collecting the Polyp Stage of the Invasive Freshwater Hydrozoan Craspedacusta sowerbii

Abstract

Current sampling methods for detecting the presence of the invasive freshwater hydrozoan Craspedacusta sowerbii rely mainly on visual confirmation of the medusa stage. Confirming the presence of the polyp stage is equally important for observing medusae since typical late summer/early fall occurrences or observations of medusae are sporadic though are becoming more frequent. The polyp stage is important as it is the organism’s primary stage and is present throughout the year depending on water temperatures. Therefore, sampling methods for the polyp stage are, commonly, the collection of substrates such as rocks, plants, or pieces of wood in a given body of water, and these can be cumbersome to examine. Polyps are also small, transparent, and difficult to see on natural substrates. Based on a preliminary culturing of the polyp stage on glass and plastic microscope slides in the laboratory, we designed a sampling methodology based on submerging four substrate types (glass and plastic microscope slides, Hester-Dendy discs, and small glass Petri dishes) to confirm the presence of C. sowerbii polyps in the field. We tested this method in three lakes in the Illinois–Indiana region (USA). Two of the lakes have recorded sightings of medusae but the third has no record of polyps or medusae. The sampling method we designed was effective in that C. sowerbii polyps were found on both plastic and glass slides. While this method can be sufficient for detection of the polyp stage, it also shows potential for improvement; we highlight abiotic and biotic ecological parameters as significant factors influencing the collection of C. sowerbii polyps to be considered for future methodologies.