3.2. Dinophysis/Gymnodinium
The size of the obtained PCR fragments did not always
correspond to the expected 565 bp 28SrDNA size: in
addition, another 700 bp amplicon was sometimes
detected. The amplified 700 bp fragment (KJ499997) of
the sponge Haliclona sp. (OTU QM4715) had a similar
sequence (94%) to the free-living dinoflagellate Dinophysis
lativelata (AB473665) [29] (Fig. 1). The sponge Phycopsis.
sp. (OTU QM1640), which was expected to be Symbiodinium-free,
was found to host species (KJ499996) with a 28S
gene sequence similar to a genus of a new planktonic
mixotrophic dinoflagellate Paragymnodinium shiwhaense
n. gen., n. sp. isolated from coastal waters off Western
Korea (AM408889) (Fig. 1). The latter has recently been
described by Kang et al. [30].
The partial 28S DNA sequence (KJ500002) amplified
from the nudibranch Flabellina sp. suggests that it could
either be a new genus or a new species within the order
Gymnodininales or genus Gymnodinium (Fig. 1).
Two possible hypotheses could explain these new
observations: (i) Dinophysis and Gymnodinium are freeliving
and use host tissues as habitats to aggregate and
reproduce or (ii), host tissues contain symbiotic lineages of
Dinophysis and Gymnodinium. Histological analyses should
shed more light on this. To date, only planktonic Dinophysis
and Paragymnodinium are referenced. It is known that
dinoflagellates Gymnodinium spp. and Pfisteria piscicida
kleptoplastids (notably chloroplasts) are photosynthetically
active for only a few days, while kleptoplastids in
Dinophysis spp. can continue to function for two months
[31] and therefore could be an alternative pathway to
provide energy to the host.
3.3. Associations
Correlation between the clades present in various hosts
from a single location reveals that clades A, C and D were
found in corals from Moorea’s lagoon [7]; clade B and C
were found in the studied nudibranch and sponge specimens
from Moorea’s and/or Tahiti’s lagoon (Table 1). Since
nudibranchs can move from one location to another and
release healthy symbionts to the environment [15], this
study reveals their ability to act as potential reservoirs and
vectors of viable Symbiodinium for the whole reef ecosystem.
They may be able to provide the corals or anemones
they feed on with Symbiodinium, by discharging them in
their vicinity, allowing, for example, symbiont switching
after a bleaching event or in case of changing environmental
conditions. A more comprehensive set of data
concerning the clades present, their uptake, turnover
and seasonal flexibility in nudibranchs and sponges,
together with their coral substrates or preys, will enable
the assessment of their role in buffering the coral reefs’
response to climate change and ocean acidification.
To conclude, this work provides numerous new
elements, which will serve to better characterize the
nature of the relationships involving Symbiodinium and
invertebrate host species. Other areas of interest should
now be considered, such as the study of metabolic
exchanges between host and symbionts: the rate and
nature of nutrient exchanges has not yet been studied in
sponges and nudibranchs. The extent to which they rely on
the dinoflagellates for survival is still under investigation.
Also, the question of the stability and specificity of the
relationship needs to be studied more extensively. Indeed,
it has been recently demonstrated in corals [7] that flexible
hosts exhibit a higher sensitivity to environmental
changes; they have a higher propensity to bleach and
higher mortality rates. Hosts, which have a high degree of
fidelity towards their symbionts, may be more environmentally
resilient. Compared to corals, sponges and
nudibranchs have alternate and quite efficient feeding
mechanisms, which make the role of Symbiodinium clades
less important in their acclimatization to changing
environments. Further investigations on the sponges and
nudibranchs’ ability to switch from one Symbiodinium
clade to another linked to their fitness in a changing
environment will determine if specific associations are
required for survival.