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= Cassiopea = Not to be confused with Cassiopeia (disambiguation).

Cassiopea is a genus within the Scyphozoa, or true jellyfish, and the only members of the family Cassiopeidae [1, 2]. The first species in this genus was described in 1774 [2]. These jelly fish are commonly referred to as the “upside-down jellyfish”, “mangrove jellyfish”, and “zooxanthellae jellyfish” [4]. They are found in tropical and sub-tropical coastal regions around the world, including shallow mangrove swamps, mudflats, canals, and turtle grass flats [2, 3]. The medusa usually lives upside-down on the bottom, which has earned them their common name. These jellyfish partake in a symbiotic relationship with photosynthetic dinoflagellates and therefore, must lay upside-down in areas with sufficient light penetration to fuel their energy source [2, 4].

Contents:
1.    Species

2.    Habitat and Distribution

3.    Life Cycle/History

4.    Behavior

5.    Relation to Humans (Defense System, bioindicator)

6.    References

Species
To date, 24 ostensible species have been proposed, but only 10 species have been validated through molecular means [2]. There is still discrepancy about whether C. xamachana is a new species or an introduction of C. andromeda to a new location [2].

According to the World Register of Marine Species, this genus includes 11 species [1]:

•           Cassiopea andromeda

•           Cassiopea depressa

•           Cassiopea frondosa

•           Cassiopea maremetens

•           Cassiopea medusa

•           Cassiopea mertensi

•           Cassiopea ndrosia

•           Cassiopea ornate

•           Cassiopea picta

•           Cassiopea vanderhorsti

•           Cassiopea xamachana

Habitat and Distribution
Species within the genus Cassiopea can be found in tropical and subtropical regions of the world [4]. These jellyfish are restricted to shallow, marine habitats as a result of their mutualistic relationship with an photosynthetic symbiont [4]. Cassiopea species reside in mangrove, lagoons, and seagrass flats, as well as reefs and other shallow habitats [4, 10]. Oftentimes, Cassiopea plays a vital role in the nutrient cycling of a given habitat. [5].

Some Cassiopea species are deemed alien and invasive in coastal habitats like the Mediterranean Sea, Brazil and the Hawaiian Islands [4, 11]. Studies indicate that Cassiopea are capable of tolerating heat more than cold, which could be a driver of success in new coastal habitats [4]. Cassiopea species have been documented in high abundances in places like the Red Sea and the Caribbean [8]. Furthermore, significantly human-impacted, eutrophicated coastal environments host ample numbers of Cassiopea [11].

Life Cycle
Cassiopea species are capable of both sexual and asexual reproduction [5]. The jellyfish in this genus exhibit fixed sex determination and gonochorism [2]. For some species, fertilization occurs internally within the female’s gastrovascular cavity [2]. After internal fertilization, the embryos utilize the brachial canal of the female to move from the gastrovascular cavity onto the oral disc [2]. A couple hours post-settlement on the oral disk, the embryos begin to cleave and then approximately 48 hours after fertilization, the embryos are ciliated [2]. 98 hours following fertilization, planulae larvae have developed and capable of swimming, attaching and producing polyps [2]. For other Cassiopea species, fertilization occurs in the water column following simultaneous release of sperm and eggs [5]. In this case, fertilized eggs become free-living, ciliated larvae and upon finding suitable substrate, will also settle and become a scyphistoma, or polyp [5]. Thus far, the timing of development seems to be dependent on temperature [2]. Planula larvae of certain species prefer different substrate, for example the larvae of C. xamachana prefers to settle shaded by degrading mangrove leaves for the nutrient availability and shelter [9]. Oftentimes, cnidarian larvae are provoked to settle by chemical cues, but Cassiopea species tend to be substrate driven [9]. The polyp stage of Cassiopea species express the ability to asexually reproduce by generating planuloid buds, which can then settle and produce more polyps [2, 5]. Both settlement and metamorphosis in Cassiopea is impacted by the presence of specific bacteria [2].

In both asexual and sexual reproduction cycles, the larva are aposymbiotic, meaning they are not infected with a symbiont [7]. Cassiopea polpys must obtain an endosymbiont to spark the transition into the adult stage of their life cycle [2]. Without colonization by Symbiodinium, Cassiopea species cannot undergo strobilation, the progression from polyp to medusa [5]. During strobilation, the scyphistoma produces ephyrae which is released into the water column as a medusa, the adult stage of a jellyfish [5]. Experiments with bleaching and reinfection of Cassiopea polyps result in a better understanding for coral reef post-bleaching restoration [12].

Behavior
Cassiopea earned the nickname “upside-down jellyfish” as a result of their tendency to lay arms up on the sea floor [2, 3]. This benthic life style is unique to Cassiopea; no other genus in the Rhizostomidae exhibit this [10]. These jellyfish exhibit this abnormal behavior because they host an endosymbiont, called Symbiodinium, which require adequate sunlight in order to photosynthesis and provide the jellyfish with energy in exchange for shelter [3]. Cassiopea host the symbionts within their tissues, proboscis, oral arms, and the subtentacular region of their bodies [7]. This interaction is known as an obligate symiobiotic relationship, which means the dinoflagellate and the jellyfish depend on this relationship for survival and mutualistically benefit from it [7]. Furthermore, this relationship is species-specific during the medusa stage, meaning each Cassiopea species hosts a certain species of symbiont [7]. This relationship allows Cassiopea species to function as photoautotrophs [10]. Similar to corals, exposure to extremely high temperatures results in the bleaching of polyps, or the release of their endosymbiont [12]. Without an endosymbiont, a polyp metamorphose into an adult [2].

In order to withstand this intense solar exposure, Cassiopea produce mycosporine acids that protect them from ultraviolet radiation [10]. They also sport a peculiar blue protein in their mesoglea that acts as a protection from radiation related damage without blocking sunlight from the photosynthetic dinoflagellates [10]. Additionally, Cassiopea excrete a mucus that is thought to help maintain their upside-down benthic orientation [10]. This mucus has also been explored as a potential feeding mechanism in addition to photoautotrophy [10]. It is thought that microorganisms get caught in the mucus and ingested as a mechanism to provide more nutrients for the photosynthesis to occur [10].

While lying on the bottom, Cassiopea contract and relax their bell to create a steady pulse that initiates small currents in the water [3]. They utilize these currents to circulate nutrients from the benthos, filter feed, disperse gametes and release waste [3, 6]. The pulsing mechanism seen in this genus is regulated by an organ called the rhopalia that sense gravity an light [3]. Often times, the carbon fixed by the photosynthetic symbiont exceeds the requirements of the Cassiopea which encourages growth and procreation [6]. Cassiopea endosymbionts have the ability to compensate for 160% of their metabolic demand for carbon [6]. In addition to their mutualistic relationship with bacteria, Cassiopea species are often found in commensalitistic relationships with crustaceans, like the mysid Idiomysis tsurnamali [8].

Jellyfish within the Cassiopea genus can play a key role in coral reef food webs [4, 5]. Their pulsing mechanism releases organic matter which becomes food for the reefs [4]. This suggests that Cassiopea species have the potential to be bioindicators for coral reef health [8]. In many benthic environments, Cassiopea is considered a key stone species for the given ecosystem, like coral reefs in the Northern Red Sea for example [8].

Cassiopea jellyfish have been observed to enter a sleep state, indicated by a period of dormancy [3]. This quiescent period is characterized by a decreased pulsation rate, reduction in response to stimuli and increase inclination to sleep when sleep deprived [3].

Relation to Humans
Characteristics like copious asexual reproduction, and tolerance to a wide range of temperatures and salinities give Cassiopea the potential to be successful bioinvaders [2]. Cassiopea species are often documented to be well-established in human-impacted ecosystems, like in the Bahamas [2, 4]. Studies have shown that medusae in significantly impacted areas obtain higher densities of endosymbionts [4]. Extreme, unforeseen procreation events in Cassiopea may be linked to ecosystem health, therefore Cassiopea has been explored as a potential biomonitor species in relationship to management for coastal ecosystems, specifically water quality management [2].

Cassiopea jellyfish have been found to accumulate metals at higher concentrations than that of the surrounding water, as well as actively regulate concentrations of other metals [2, 4]. The intense accumulation of metals in Cassiopea species has the potential to indicate metals being transferred up the food web [2].

The venom found in Cassiopea species is being explored as a source of natural compound extraction for the treatment of Alzheimer’s disease [13].