Sea anemone? Soft coral? A dead jellyfish? An upside-down jellyfish?
If your guess was the latter, you would be right.
Cassiopea is a genus of jellyfish that live their lives standing on their heads. Why would they do that?
Cassiopea spp. house thousands of photosynthetic zooxanthallae within their mesoglea (Berryman, n.d.). Zooxanthallae provide carbon for the jellyfish, and in return, the jellyfish provide the zooxanthallae with minerals, shelter (Tan, 2009) and presumably protection. Cassiopea spp. therefore live upside-down to allow their zooxanthallae companions access to sunlight. Also for this reason, Cassiopea spp. are found mainly in shallow marine waters – for example in the seagrass meadows of our very own Pulau Semakau (Tan, 2009).
The symbiotic relationship between Cassiopea jellyfish and zooxanthallae is not dissimilar to the one between zooxanthallae and corals. Unlike with the corals however, the zooxanthallae do not provide for all of the carbon required by the jellyfish. Instead of being able to wait for food to be put on their figurative tables, the jellies do actually still need to feed themselves to an extent (Vodenichar, 1995), either through filter feeding, absorption of nutrients from the water, or by capturing prey with their nematocysts (Berryman, n.d.).
Upside-down jellyfish do not stay put on the seafloor all of the time though! If disturbed, a swarm of upside-down jellyfish can launch upwards (Maui Ocean Center, n.d.).
Under normal circumstances however, these jellyfish prefer to be upside-down, and if you try to flip them over, they will slowly turn themselves upside-down again (Tan, 2009)!
But wait! There is a third party in this relationship…
Sometimes, Cassiopea spp. may be carried on the backs of urchin crabs (Dorippe frascone) for defense against their predators.
It seems like it’s mutualism with a dash of an exploitative interaction thrown into the mix, here.
Which makes the ecological interactions in Nature all the more fascinating.
Second source: Holland, B. S., et al., 2004. Global phylogeography of Cassiopea (Scyphozoa: Rhizostomeae): molecular evidence for cryptic species and multiple invasions of the Hawaiian Islands. Marine Biology, 145(6): 1119-1128.
“Upside-down Jellyfish,” by Matt Berryman. Marine Invertebrates of Bermuda, n.d.. URL: http://www.thecephalopodpage.org/MarineInvertebrateZoology/Cassiopeaxamachana.html (accessed on 14 April 2010).
“Upside-down Jellyfish,” Maui Ocean Center Marine Life Profile. Maui Ocean Center, The Hawaiian Aquarium, n.d. URL: http://www.mauioceancenter.com/marinepdf/upside-down_jellyfish.pdf (accessed on 14 April 2010).
“Upsidedown jellyfish,” by Ria Tan. Wild Factsheets, March 2009. URL: http://www.wildsingapore.com/wildfacts/cnidaria/others/jellyfish/upsidedown.htm (accessed on 14 April 2010).
Vodenichar, J.S., 1995. Ecological physiology of the scyphozoan Cassiopea xamachana. M.S. Thesis, University of Georgia, Athens, USA.