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.