High photosynthetic plasticity may reinforce invasiveness of upside-down zooxanthellate jellyfish in Mediterranean coastal waters

Host starvation and in hospite degradation of algal symbionts shape the heat stress response of the Cassiopea-Symbiodiniaceae symbiosis

BACKGROUND

Global warming is causing large-scale disruption of cnidarian-Symbiodiniaceae symbioses fundamental to major marine ecosystems, such as coral reefs. However, the mechanisms by which heat stress perturbs these symbiotic partnerships remain poorly understood. In this context, the upside-down jellyfish Cassiopea has emerged as a powerful experimental model system.

RESULTS

We combined a controlled heat stress experiment with isotope labeling and correlative SEM-NanoSIMS imaging to show that host starvation is a central component in the chain of events that ultimately leads to the collapse of the Cassiopea holobiont. Heat stress caused an increase in catabolic activity and a depletion of carbon reserves in the unfed host, concurrent with a reduction in the supply of photosynthates from its algal symbionts. This state of host starvation was accompanied by pronounced in hospite degradation of algal symbionts, which may be a distinct feature of the heat stress response of Cassiopea. Interestingly, this loss of symbionts by degradation was concealed by body shrinkage of the starving animals, resulting in what could be referred to as "invisible" bleaching.

CONCLUSIONS

Overall, our study highlights the importance of the nutritional status in the heat stress response of the Cassiopea holobiont. Compared with other symbiotic cnidarians, the large mesoglea of Cassiopea, with its structural sugar and protein content, may constitute an energy reservoir capable of delaying starvation. It seems plausible that this anatomical feature at least partly contributes to the relatively high stress tolerance of these animals in rapidly warming oceans. Video Abstract.

The Zooxanthellate Jellyfish Holobiont Cassiopea andromeda, a Source of Soluble Bioactive Compounds

Cassiopea andromeda (Forsskål, 1775), commonly found across the Indo-Pacific Ocean, the Red Sea, and now also in the warmest areas of the Mediterranean Sea, is a scyphozoan jellyfish that hosts autotrophic dinoflagellate symbionts (family Symbiodiniaceae). Besides supplying photosynthates to their host, these microalgae are known to produce bioactive compounds as long-chain unsaturated fatty acids, polyphenols, and pigments, including carotenoids, with antioxidant properties and other beneficial biological activities. By the present study, a fractionation method was applied on the hydroalcoholic extract from two main body parts (oral arms and umbrella) of the jellyfish holobiont to obtain an improved biochemical characterization of the obtained fractions from the two body parts. The composition of each fraction (i.e., proteins, phenols, fatty acids, and pigments) as well as the associated antioxidant activity were analyzed. The oral arms proved richer in zooxanthellae and pigments than the umbrella. The applied fractionation method was effective in separating pigments and fatty acids into a lipophilic fraction from proteins and pigment-protein complexes. Therefore, the C. andromeda-dinoflagellate holobiont might be considered as a promising natural source of multiple bioactive compounds produced through mixotrophic metabolism, which are of interest for a wide range of biotechnological applications.

Reproductive cycle and gonadal output of the Lessepsian jellyfish Cassiopea andromeda in NW Sicily (Central Mediterranean Sea)

Knowledge of the reproductive strategy is a key prerequisite to predict population dynamics and potential invasiveness of both native and non-indigenous outbreak-forming species. In 2014 the Lessepsian upside-down jellyfish Cassiopea andromeda reached the harbor of Palermo (NW Sicily, Thyrrenian Sea), to date its established westernmost outpost in the Mediterranean Sea. To predict C. andromeda reproductive success in its novel habitat, gonad histology was carried out to record the number and size of mature and immature oocytes. Both male and female simultaneously presented gametes at all stages of development suggesting an asynchronous, yet apparently continuous, reproduction strategy. Indeed, oogenesis was observed throughout the year from pre-vitellogenic, vitellogenetic, and late-vitellogenetic to mature oocytes suggesting multiple reproductive events, as known in other Mediterranean Rhizostomeae. Oocytes were found from May to December, with two seasonal peaks of abundance (late spring = 392 and autumn = 272), suggesting imminent spawning events. Further, jellyfish size varied significantly throughout the year, with maximum diameter (up to 24 cm) in summer, and minimum diameter (6 cm) in winter. Small-sized jellyfish in winter belong to the new cohort, most probably arising from intense summer strobilation of polyps. Late spring fertilization, planula development, and metamorphosis, followed by polyp strobilation in the summer months, may explain the late appearance of a new jellyfish cohort, likely coincident with that recorded throughout winter.

Characterization of the populations of upside-down jellyfish in Jardines de la Reina National Park, Cuba

Upside-down jellyfish are a group of benthic scyphozoans belonging to the genus Cassiopea, whose members are in symbiosis with dinoflagellates and inhabit tropical and subtropical waters. Although there are some studies of the genus in the Caribbean, these are limited. In Cuba, the group's studies are restricted to reports on taxonomic lists and, as far as we know, no one has performed any analyzes of the densities of these jellyfish in seagrass or mangrove ecosystems in Cuba. In this work, the populations of Cassiopea spp. in Jardines de la Reina National Park (JRNP) were characterized, for the first time for this Marine Protected Area and Cuba. One hundred 1m² square frames were placed at 14 JRNP sites. For each site, the species, density, size of the individuals and abiotic factors were determined. Density and diameter comparisons were made between sites, zones and regions within the JRNP. The percentage of the benthic cover was determined and a correlation was made between density and diameter. A total of 10,803 individuals were recorded, of which 7,618 belong to Cassiopea xamachana and 3,185 belong to Cassiopea frondosa. Both species share a niche and no evident segregation was detected according to abiotic variables. Significant differences were detected in comparisons of density and size across sites and zones. Density and size in the JRNP were negatively correlated, and higher aggregations of the species were observed at lower sizes. Density mean values ranged from 2.18 to 14.52 ind. /m² with maximum values of 79 ind. /m². Cayo Alcatraz was the site found to have the highest density while Cachiboca was the site with the lowest density. The average bell diameter size of the individuals ranged from 9.34 to 15.31 cm for the sampled sites, with minimum and maximum values of 2.5 cm and 32.6 cm. The smallest size was recorded at Cayo Alcatraz while the largest size was reported for Boca de las Anclitas. The environmental factors evaluated showed no significant relationship with the density or diameter of Cassiopea, while the Thalassia testudinum cover was negatively correlated with Cassiopea density at all fourteen sites in the JRNP. The percentage of Cassiopea coverage was higher than those reported in the literature, with four sites exceeding 20% coverage. In general, the populations of Cassiopea spp. in the JRNP did not differ greatly, although a higher density was observed towards the eastern region of the park. It was shown for the first time for the species that density and size have a negative correlation. Future studies are required to quantify the impact of Cassiopea on coastal marine ecosystem processes, and to further determine how anthropogenic changes may be altering the function of these tropical ecosystems.

Cophylogeny and specificity between cryptic coral species (Pocillopora spp.) at Mo'orea and their symbionts (Symbiodiniaceae)

The congruence between phylogenies of tightly associated groups of organisms (cophylogeny) reflects evolutionary links between ecologically important interactions. However, despite being a classic example of an obligate symbiosis, tests of cophylogeny between scleractinian corals and their photosynthetic algal symbionts have been hampered in the past because both corals and algae contain genetically unresolved and morphologically cryptic species. Here, we studied co-occurring, cryptic Pocillopora species from Mo'orea, French Polynesia, that differ in their relative abundance across depth. We constructed new phylogenies of the host Pocillopora (using complete mitochondrial genomes, genomic loci, and thousands of single nucleotide polymorphisms) and their Symbiodiniaceae symbionts (using ITS2 and psbA^ncr markers) and tested for cophylogeny. The analysis supported the presence of five Pocillopora species on the fore reef at Mo'orea that mostly hosted either Cladocopium latusorum or C. pacificum. Only Pocillopora species hosting C. latusorum also hosted taxa from Symbiodinium and Durusdinium. In general, the Cladocopium phylogeny mirrored the Pocillopora phylogeny. Within Cladocopium species, lineages also differed in their associations with Pocillopora haplotypes, except those showing evidence of nuclear introgression, and with depth in the two most common Pocillopora species. We also found evidence for a new Pocillopora species (haplotype 10), that has so far only been sampled from French Polynesia, that warrants formal identification. The linked phylogenies of these Pocillopora and Cladocopium species and lineages suggest that symbiont speciation is driven by niche diversification in the host, but there is still evidence for symbiont flexibility in some cases.

Snapshot of the Distribution and Biology of Alien Jellyfish Cassiopea andromeda (Forsskål, 1775) in a Mediterranean Touristic Harbour

Simple Summary

Alien species are an important cause of biodiversity loss and changes to ecosystems. Harbors are hotspots for the introduction of these species, and, usually, the impacts and pathways of invasion of the host populations are poorly known. Since 2014, an alien jellyfish, Cassiopea andromeda, coming from the Red Sea, has invaded a Mediterranean touristic harbor and established a population there. In this study, the distribution and trophic behavior of C. andromeda were investigated to improve knowledge on this species within the Mediterranean. The preliminary results highlight and confirm that C. andromeda is a perfect invader thanks to its nutritional strategy and capacity to adapt to heavily anthropized areas. Therefore, its potential impact on the local biodiversity and thus on the ecosystem’s structure and functioning is worth considering.

Abstract

Harbors are hotspots for the introduction of alien species, and, usually, investigations on their host populations help fill the knowledge gap in their pathways of invasion and in their impacts on marine biodiversity and ecosystems. In 2014, the upside-down alien jellyfish Cassiopea andromeda invaded a Mediterranean touristic harbor (“Cala”), and its abundance has since increased over time. In the present study, the distribution and trophic behavior of C. andromeda in Cala were investigated for the years 2017–2018 through visual sampling, and GIS-based statistical and stable isotope analyses. Since Cala is a hard-to-reach area (with many anchor cables and boats), Megabenthos Underwater Video was used to count the number and estimate the size of jellyfishes. The variations in size throughout the study period suggest that the population of C. andromeda is quite established in Cala at depths lower than 7.5 m. The ranges of the environmental parameters recorded (temperature, salinity, and transparency) were consistent with the ideal conditions for maintaining a Cassiopea population, but they did not seem to influence aggregation. Additionally, the carbon and nitrogen isotopic signatures studied highlight the mixotrophic behavior of this species. These preliminary results confirm the capacity of C. andromeda to live and reproduce in heavily anthropized areas.