Discussion:
This study is the first to report on the detection of small cubozoan jellyfish in marine systems using an eDNA approach. Species-specific primers were designed for four cubozoans and eDNA analyses confirmed detection in the field against visual observations or known presence of jellyfish in proximate locations at the time of water sampling. The study is also the first to highlight the potential for eDNA to detect cubozoan polyps based on C. sivickisi as a model, thus providing a genetic tool to locate source reefs that provide recruits to adult medusae populations. This particular finding is significant, as many cubozoans are dangerous to humans and currently there is a paucity of knowledge on the source areas for medusae and therefore how to monitor and better potentially manage jellyfish envenomation risks.
Understanding the rate at which DNA degrades in the aquatic environment can help identify if a species has been present within a certain time. Based on a simple laboratory trial, which only focused on microbial decay on eDNA, our findings indicated a fast degradation of jellyfish DNA within the first 3 days of the animal being removed from the treatment, with only a residual signature left after 9 days in some replicates. Our findings were similar to studies on other taxa where it is has been concluded that DNA can remain in aquatic environments for 2 to 10 days, depending on dilution and aquatic factors (Thomsen et al. 2012; Pilliod et al. 2014). In the oceanic environment, currents, UV radiation, dilution and microbial decay all affect the degradation of DNA further confirming close proximity of an organism if eDNA is detected. These findings suggest that due to the rapid decay of DNA the target organisms we sought to detect in the field were, or had been in the area recently, and/or eDNA has been carried by currents from proximate locations. Furthermore, our data is consistent with the findings from Minamoto et al. (2017), suggesting the rate of DNA degradation in jellyfish is rapid, with fastest decline in the first 3 days and that oceanic forces act on the eDNA of jellyfishes and rapidly disperse and eliminate DNA. To determine the true effect currents have on eDNA dispersal, modelling is required to determine the distance that eDNA can be advected from a source.
An environmental DNA approach was successful in the detection of jellyfish DNA in both the laboratory and the field. For most jellyfish taxa we knew the target species was present in the field before sampling. Copula sivickisi , C. xaymacana and C. fleckeri were detected at multiple sites where they had been visually observed. The poor relationship between quantity of DNA and abundance of those species where we were able to obtain visual countswas probably due to the following: (1) the heterogenous distribution of DNA in the water column due to the clumping nature of eDNA (Furlan et al. 2016); (2) the source(s) of all eDNA in samples is not known, be that from organisms nearby or some distance away;
(3) where samples have been taken some distance from source organisms, this leaves more time for microbial and physical decay; though this may not be the case in the current study as jellyfish were observed close to where the samples were taken; (4) the speed of dilution of eDNA from the source (Gargan et al. 2017); (5) small scale oceanography may have a role in concentration and dispersion; (6) vertical stratification of the water column preventing the eDNA from being transported through the thermocline; (7) sea water temperature, as detectability from the source may increase with temperature (Lacoursière‐Roussel et al.2016). The clumped nature of eDNA (Furlan et al 2016) also emphasises the importance of replication at multiple levels, among locations, sites within location and replicates within sites.
JellyCams (JCAMS) (Schlaefer et al. 2020) and counts of jellyfish within a few metres of the lights were useful for determining the presence/absence of jellyfish, as well as abundance. Clearly the JCAMs provided a more accurate estimate of relative abundance than eDNA, but the two techniques combined are useful, particularly when jellyfish are rare and are less likely to be detected in lights. For example, C. xaymacana was relatively rare in JCAMS imagery and visual counts, but positive detections were revealed using eDNA techniques, even without visual detection. Samples where jellyfish were known to inhabit a site provided a true positive value of detection.
An eDNA approach was shown to have utility to detect the proximate presence of C. sivickisi polyps. We were able to detect an eDNA signature in water samples taken close to the substratum during winter, when adult medusae were absent. At all of the sites we sampled for polyps, adult medusae were present during the previous jellyfish season (Sept-Nov). Following the mating of adults, females drop a bundle of embryos on the substratum (Garm et al. 2015) as their home ranges are in the tens of metres (Schlaefer et al. 2020) and we assume they release planulae at the time. Accordingly, any eDNA of C. sivickisi that was detected could only be explained by the proximate presence of polyps of the species. Critically, the benthic polyps could only be detected in depth stratified sampling that including samples taken within 0.5 m of the substratum. It is likely the thermocline acted as a barrier reducing or preventing small amounts of DNA from the substratum reaching the surface, as thermoclines are well known for blocking the vertical passage of particles (Gray & Kingsford 2003) and therefore may inhibit the movement of DNA particles through the water column. Location of polyp beds in situ for most cubozoans has eluded scientists to date, due to their cryptic nature, small size, complex habitats and low water visibility. Our study is the first to detect the likely presence of polyps in situ and thus shows eDNA provides a strong predictor for where polyps will be located. This in turn allows for the identification of polyp reefs that serve as sources for adult medusae in the summer season. This ability to now detect all life history stages will provide new opportunities to understanding the ecology and habitat use of cubozoans.
In conclusion, in the current study is was demonstrated that an eDNA approach is an effective technology to detect cubozoans in marine systems, both medusae and putatively the cryptic polyp life-stage. Environmental DNA provides a cost-effective and less labour-intensive way to detect jellyfish that have a broad spatial and temporal variation. A major finding was that we could detect eDNA of C. sivickisi polyps in situ . Environmental DNA therefore can be used to detect the source locations seeding adult cubozoans and potentially fast-track our understanding of jellyfish ecology. This approach will also be of high utility to detect potential range shifts of cubozoans as a result of climate change.