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Cassiopea andromeda, the upside down jellyfish.


Dylan Wesarg 2015

Summary

Cassiopea andromeda belong to the family Cassiopeidae, or better known as the upside down jellyfish. As their names suggests, these small Scyphozoans spend the majority of their lives laying upside down of the ocean floor. This unusual behaviour enables their symbiotic dinoflagellates, located throughout their bodies to photosynthesis and produce carbohydrates with which the jellyfish can utilise in metabolic activities. In return the dinoflagellates gain a favourable environment free from predators. This relationship has strong implications on the jellyfish itself, regulating where Cassiopea andromeda is found and their rate of growth. It even plays a major role in their life cycle and behaviour. All of this will be further explained throughout this webpage along with information on their other features and closely related relatives.

Physical Description

The genus Cassiopea belongs the order Rhizostomeae and based off studies can grows to a size of about 18cm in diameter (Lampert et al. 2012). They can be distinguished apart from other jellyfish by their unusual behaviour of laying upside down on the seafloor. This in turn, exposes their oral arms to light allowing for photosynthesis to occur in their symbiotic zooxanthellae. The genus is described to lack both tentacles and a central mouth opening. Instead this genus and many other members of its order have a root-like manubrium at the base of their oral arms. This manubrium has many additional outgrowths, increasing its surface area. It is loaded with many nematocysts, mucus secreting cells and has many brachial canals breaking through the outer epidermis to form secondary mouths (Ruppert, Fox & Barnes 2004; Bigelow 1900).

Cassiopea have a total of eight unfused oral arms with the edition of numerous vesicles that hold large quantities of zooxanthellae (see figure 1). These vesicles can differ in colour from individual to individual. Experiments examining these colour differences determined no correlation exists between the colour pigment and levels of light/UV-radiation experienced by the individuals. With the cause of the colour pigment found in individuals being unexplained. (Lampert et al. 2012).

The last major section of the jellyfish is it gelatinous bell, which is often concave on the dorsal side. This allows for the jellyfish to effectively suction itself onto the ocean floor. The bell will then pulse regularly to create water flow over its oral arms for respiration and to assist in food capture.

Cassiopea syphistomas do differ from the other members of their family in the fact that they undergo monodisc strobilation as opposed to polydisc strobilation (see development and behaviour). Their syphistoma (figure 2) is made up of the pedal disk, which functions as an anchor attaching the animal to a substrate. The pedal disc is connected to the stalk which links to the calyx or head. It is here where the tentacles protrude to function in food capture. The hypostome with the mouth opening is located within the oral disc on top of the calyx (Hofmann, Neumann & Henne 1978).
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Figure 1
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Figure 2

Ecology
Like many Cnidarians, the genus Cassiopea live in symbiosis with dinoflagellates which belong to the genus Symbiodinium. The dinoflagellates are photosynthetically active, providing their hosts with a source of carbon in exchange for nutrients and shelter from predation (Fruedenthal 1962; Rowan 1998; Stat & Grates 2008). There are a total of eight clades of Symbiodinium and interestingly Lampert et al. (2012) found that Cassiopea andromeda have a symbiosis with differing clades depending on their global positioning. The jellyfish house their symbionts intracellularly, either in the subtentacular region, oral arms or in the bell itself (Mellas et al. 2014). The jellyfish will rest inverted on the seabed exposing their dinoflagellates to the light allowing for photosynthesis to occur. This mutualistic relationship is widely believed to give both organisms survival advantages over competitors.

Due to this symbiosis, Cassiopea are typically found in warm, shallow waters for maximum light penetration. Allowing for more effective photosynthesis and a greater production of carbohydrates as a consequence. See section ‘Biogeographic Distribution’ for more on where these animals are found.
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Figure 3

Life History and Behaviour

Reproduction & Development
Cassiopea andromeda has the typical triphasic life cycle, common to medusoid forms of the Cnidarians (see figure 4). Mayer (1906) found in his research that individuals from the Red Sea to be gonochoristic. Interestingly, observations by Hofmann and Hadfield (2002) in Hawaii however, found individuals that were hermaphroditic. Both studies found that C. andromeda brood their eggs on specialised vesicles located in the center of the oral disk, until a planula larvae hatches.

The planula larvae can then spend several days in the water column in order to find a suitable area on which to settle and metamorphose into a scyphistoma. Settlement of the planula larvae is actually dependent exogenous cues given off by bacteria located on the substrate (Neumann 1979). In this study it was found that settlement was significantly reduced when the larvae were in seawater containing antibiotics and it was concluded that a bacterium product was the inducing factor for settlement and metamorphosis. However it must be noted that this was determined under experimental conditions and conditions individuals encounter in their natural habitat may vary considerably.

Once metamorphosed into the scyphistoma, the individual can reproduce asexually in two different ways. Firstly, a bud can form on the calyx (see figure 4) which will pinch off, forming a larva-like planula. The detached bud can then move through the water via ciliary movement, seeking the bacterium exogenous inducer that facilitates settlement and metamorphosis (Hofmann & Gottlieb 1991). In the second instance, the tentacular region can separate from the stalk via monodisk strobilation, to form an ephyra. The ephyra will then develop into a sexually reproducing adult. In this case the remaining stalk can regenerate the tentacular region and the process will begin again (Thieme & Hofmann 2003).

A paper published by Hofmann, Neumann and Henne (1978) showed that strobilation in C. andromeda can induced by a raising water temperature to 24°C. They then went on to indicate that strobilation may be further dependent on the rapid multiplication of zooxanthellae located throughout their epidermis and mesoglea. They found that scyphistoma deprived of the symbionts were unable to develop ephyra. The symbiotic relationship is therefore an important part of the jellyfish’s life cycle and questions as to whether this relationship is under threat via climate change will be addressed in the section conservation and threats.
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Figure 4

Behaviour
Cassiopea andromeda effectively lay upside down on the ocean floor to expose their oral arms to the sun and enable the zooxanthellae present in the body to photosynthesis. Contractions of striated muscles controls movement of the individual and water flow across its body for both respiration and food capture (explained in more detail under Anatomy and Physiology). These are relative simple behaviours when compared others seen in the animal kingdom. However Niggl and Wild (2010) have indicated that the upside jellyfish can exhibit more complex behaviours, such as environmental preference linked to water clarity and levels of bare sediment. They also speculated that the abundance of copepods (a potential food source) could affect abundance in certain habitats.

This raised the question as to whether C. andromeda changes its immediate behaviour when in the presence of potential food. Individuals were placed in a plastic container with two liters of sea water and monitored for an extended period of time (around 10 minutes). It was noted that in the first couple of minutes their beating was irregular, most likely due to the stress of being transfer from the main tank, in which they were housed and into the plastic container for observation. However after this initial period beating of the bell became steady and individuals seemed to relax.

Over the next five or so minutes each of the jellyfish seemed to protruded their oral arms upwards and away from their main body (see figure 5). This behaviour was seen across all jellyfish tested to a vary degree, with the most extreme case seen in figure 5 A. It seems likely that this is an advantageous behaviour, in which their appendages are exposed to larger amounts of light, promoting photosynthesis by their symbionts.

Mysids or small crustaceans were then added into the water so see whether their behaviour would change with the presence of food in the surrounding water. Within a matter of seconds the jellyfish would pull in there oral arms effectively holding them within their bell. It can be assumed that this behaviour would attribute to a more effective means of capturing prey. As the muscles contract forcing the bell to pulse, water it forced up through the nematocyst harboring oral appendages. If these appendages are held further out from the body of the animal, there is a higher chance prey could slip through and escape capture as oppose to holding them in together in a clumped fashion.

It can be speculated that these two behaviours seen in C. andromeda, effectively work to maximise the animals ability to acquire nutrients. One of which promotes photosynthesis in their symbionts whilst the other increases the likelihood of capturing food.
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Figure 5

Anatomy and Physiology

Nervous System
Compared to other invertebrates, Cnidarians have a relatively simple nervous system. However they still need to be able to sense and react to different environment cues. In the case of the Scyphozoans, they have 2 distinct nerve nets called the giant fiber nerve-net (GRNN) and the diffuse nerve-net (DNN) which lay at the base of the gastrodermis and epidermis. These are connected at the animal’s ganglion by neurons that span the mesoglea (Rupert, fox & Barnes 2004; Passano 2004). The striated swimming muscles that control the contractions of the bell and thus movement are interconnected with both nerve nets. In some species of Scyphozoans, impulses in either nerve-nets cause contraction of the striated muscles. This is not the case in Cassiopea however. As Passano (2004) explains in his paper, DNN impulses are used to facilitate or increase the strength at which the muscles respond to the GRNN impulses. It’s interesting to note that the simplicity of Cassiopea’s nervous system is able to control quite complex behaviours from feeding and swimming to even reacting to changes in water turbidity.

Digestion
The process of acquiring food as a heterotroph is very similar to that of other Cnidarians. Nematocysts are used to immobilise and capture any potential food that may brush against them. From here on however, the process of food capture and digestion differs slightly. As mentioned in physical description, Cassiopea lacks a central mouth opening. They instead have many brachial canals along their oral arms that effectively break through the outer epidermis of the manubrium to form secondary mouths. Food is passed along these canals to where they meet in the center of the oral dick. Four slit-like passages then connect the brachial channels to the stomach where food can be digested (Bigelow 1900). It must be noted that Verde and McCloskey (1998) found that C. andromedas close relative C. xamachana can solely survive on the carbohydrates produced by the zooxanthellae they house in their bodies and is probably the same C. andromeda. This demonstrates once again the importance of this relationship for the jellyfish, allowing it to survive through periods of low food availability and therefore aiding in its survival.

Respiration & Excretion
These diploblastic organisms rely on diffusion across their tentacles and general body wall for respiration. As a byproduct of this respiration, Cnidarians produce ammonia which will then diffuse across their body wall and be quickly dispersed into the water column (Ruppert, Fox & Barnes 2004). Interestingly, Cates and McLaughlin (1976) found evidence suggesting that Cassiopeia’s symbionts (Symbiodinium) aid in this process of excretion by removing excess ammonia and will also assist in nitrogen recycling.

Evolution and Systematics
The phylum Cnidaria has huge amounts of diversity within itself, including corals and sea anemones from the class Anthozoa, true jellyfish from Scyphozoa, Hydrozoa and some of the most venomous animals in the world, the Cubozoa. The possession of nematocysts groups these animals together and therefore was most likely present in their common ancestor (Collins 2002).

Cassiopea andromeda ultimately fall under the class Scyphozoa, due to their possession of rhopalia (also present in Cubozoa), gastric filaments and the process they undergo during asexual reproduction, strobilation (Ruppert, Fox & Barnes 2004). C. andromeda are further placed under the order Rhizostomeae due there loss of a central mouth opening and the development of many secondary mouths derived from complex canal network in their manubrium. They are further placed in their own family Cassiopeidae or better known as the upside down jellyfish.

Within the genus Cassiopea belong C. xamachana, C. frondosa and C. andromeda. The separation of these species have been based of morphological differences and also variations in global distributions between C. andromeda and C. xamachana. However, research into global phylogeography of this genus has suggested a review of these taxonomic groupings based on molecular data. The data attained supported the existence of at least six species within the genus Cassiopea (Holland et al. 2004). These three species however, are still currently recognised.

Biogeographic Distribution
Holland et al. (2004) describes the genus Cassiopea as a globally distributed animal. Due to their symbiotic relationship with zooxanthellae they are typically found in shallow waters on reef flats, mud flats and around mangroves.

Cassiopea andromeda however, is typically found throughout the western Pacific and as far east as the Hawaiian Islands (see figure 6). Interestingly C. andromeda weren’t documented in the Hawaiian Islands in 1906 during surveys of scyphozoans in the area (Mayer 1906). It wasn’t until the mid-1940’s that they were documented in this part of the world. It is believed that the jellyfish arrived in Hawaiian waters via US navy vessels from other parts of the Pacific during the Second World War. C. andromeda can also be found in the Red Sea and in 2010 the first record of this species was documented in the Mediterranean off the north-east coast of Malta (Schembri, Deidun & Vella 2010).
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Figure 6

Conservation and Threats
At the current time there are little to no indications that this species is under any significant threat and therefore conservation of this species is of great importance. However it is important to note that the threat of climate change and the associated temperature rises are threatening other animals that have a similar symbiotic relations with zooxanthellae. This is no more apparent than in bleaching events seen in some corals. These bleaching events have been attributed to elevated stress levels, such as increased water temperature, increased UV radiation or variations in salinity (Goulet, LaJeunesse & Fabricius 2008). McGill and Pomory (2008) studied the effects of bleaching of the weights of Cassiopea xamachana and found that bleaching due to temperature reduced wet weight significantly. It is important to note that even the non-bleached jellyfish lost weight. This was attributed to increased levels of stress in the aquarium. It is also important to remember that this was observed under experimental conditions and not in their natural habitat where these animals have the capacity to move. These findings to point at the potential danger these animals could face in the future, however it seems to be negligible at the current time.

References
Bigelow, RP 1900, The anatomy and development of Cassiopea xamachana, Memoirs of the Boston Society of Natural History, Boston.

Cates, N, McLaughlin, JJA 1976, ‘Differences of ammonia metabolism in symbiotic and aposymbiotic Condylactus and Cassiopea spp.’, Journal of Experimental Marine Biology and Ecology, Vol. 21, pp. 1-5.

Collins, AG 2002, ‘Phylogeny of Medusozoa and the evolution of cnidarian life cycles’, Journal of Evolutionary Biology, Vol. 15, pp. 418-432.

Freudenthal, HD 1962, ‘Symbiodinium gen. nov. and Symbiodinium microadriaticum sp. nov., a zooxanthella: taxonomy, life cycle, and morphology’, Journal of Protozoology, Vol. 9, no. 1, pp. 45-52.

Goulet, TL, LaJeunesse, TC, Fabricius, KE 2008, ‘Symbiont specificity and bleaching susceptibility among soft corals in the 1998 Great Barrier Reef mass coral bleaching event’, Marine Biology, Vol. 154, pp. 795-804.

Hofmann, DK, Gottlieb, M 1991, ‘Bud formation in the scyphozoan Cassiopea andromeda: epithelial dynamics and fate map’, Hydrobiologia, Vol.216/217, pp. 53-59.

Hofmann, DK, Hadfield, MG 2002, ‘Hermaphroditism, gonochorism, and asexual reproduction in Cassiopea sp. – an immigrant in the islands of Hawai’I’, Invertebrate Reproduction and Development, Vol. 41, pp. 215-221.

Hofmann, DK, Neumann, R, Henne, K 1978, ‘Strobilation, budding and initiation of scyphistoma morphogenesis in the Rhizostome Cassiopea andromeda (Cnidaria: Scyphozoa)’, Marine Biology, Vol. 47, pp. 161-176.

Holland, BS, Dawson, MN, Crow, GL & Hofmann, DK 2004, ‘Global phylogeography of Cassiopea (Scyphozoa: Rhizostomae): molecular evidence for cryptic species and multiple invasions of the Hawaiian Islands’, Marine Biology, Vol. 145, pp. 1119-1128.

Lampert, KP, BÜrger, P, Striewski, S, Tollrian, R 2012, ‘Lack of association between color morphs of the jellyfish Cassiopea andromeda and zooxanthellae clade’, Marine Ecology, Vol. 33, pp. 364-369.

Mayer, AG 1906, ‘Medusae of the Hawaiian Islands collected by the steamer Albatross in 1902’, Bull US Fish Comm, Vol. 23 pp. 1131–1143.

McGill, CJ, Pomory, CM 2008 ‘Effects of bleaching and nutrient supplementation on wet weight in the jellyfish Cassiopea xamachana (Bigelow) (Cnidaria: Scyphozoa)’, Marine and Freshwater Behaviour and Physiology, Vol. 41, no. 3, pp. 179-189.

Mellas, RE, McIlroy, SE, Fitt, WK, Coffroth, MA 2014, ‘Variation in symbionts uptake in the early ontogeny of the upside-down jellfish, Cassiopea spp.’, Journal of Experimental Marine Biology and Ecology, Vol. 458, pp. 38-44.
Neumann, R 1979, ‘Bacterial induction of settlement and metamorphosis in the planula larvae of Cassiopea andromeda (Cnidaria: Scyphozoa, Rhizostomeae)’, Marine Ecology, Vol. 1, pp. 21-28.

Niggl, W, Wild, C 2010, ‘Spatial distribution of the upside-down jellyfish Cassioepea sp. within fringing coral reef environments of the Northern Red Sea: implications for its life cycle’, Helgoland Marine Research, Vol. 64, pp. 281-287.

Passano, LM 2004 ‘Spasm behavior and the diffuse nerve-net in Cassiopea xamachana (Scyphozoa: Coelenterata)’, Hydrobiologia, Vol. 530/531, pp. 91-96.

Rowan, R 1998, ‘Diversity and ecology of zooxanthellae on coral reefs’, Journal of Phycology, Vol. 34, no. 3, pp. 401-417.
Ruppert, EE, Fox, RS, Barnes, RD 2004, Invertebrate Zoology: A Functional Evolutionary Approach, 7, Brookes/Cole, USA.

Schembri, PJ, Deidun, A & Vella, PJ 2010, ‘First record of Cassiopea andromeda (Schyphozoa: Rhyzostomeae: Cassiopeidea) from the central Mediterranean Sea’, Marine Biology Records, Vol. 3, pp. 1-2.

Stat, M, Gates, RD 2008, ‘Vectored introductions of marine endosymbiotic dinoflagellates into Hawaii’, Biological Invasions, Vol. 10, pp. 579-583.

Thieme, C, Hofmann, DK 2003, ‘Control of head morphogenesis in an invertebrate asexually produced larva-like bud (Cassiopea andromeda; Cnidaria: Scyphozoa)’, Development Genes and Evolution, Vol 213, pp. 127-133.

Verde, AE, McCloskey, LR 1998, ‘Production, respiration, and photophysiology of the mangrove jellyfish Cassiopea xamachana symbiotic with zooxanthellae: effect of jellyfish size and season’, Marine Ecology Progress Series, Vol. 168, pp. 147-162.