Cassiopea jellyfish on seafloor
Cassiopea Jellyfish, Credit: Dive and Seek

If you’ve been snorkeling or wading around Lido Key this past spring, you may have noticed some odd, cauliflower-like animals on the bay bottom. These are a type of jellyfish called the upside-down jellyfish (or Cassiopea species). As the name suggests, these creatures usually don’t swim in open water like most other jellyfish. Instead, Cassiopea spend their adult lives on the seafloor with their “arms” facing upward.

In April 2019, Florida Fish and Wildlife Commission researchers discovered large numbers of Cassiopea around Lido Key in Sarasota Bay. Up to 121 individuals were found per square meter, which is about the size of a coffee table. Populations this large are rarely found so far north in the animal's range. So why are they here? Unfortunately, there isn't a simple answer. Studies have found that large Cassiopea numbers occur more frequently in coastal areas with a high human populations and elevated nutrient levels. Warmer water temperatures may also play a role. Upside-down jellyfish tend to grow larger when exposed to warmer water, and higher water temperatures have been related to large populations in other species of jellyfish. Additional factors like salinity, how mixed or murky the water is, changes in climate patterns (i.e. El Niño), and numbers of predators or competitors may also contribute to changes in jellyfish populations.

Did You Know?

Jellyfish and corals are distantly related cousins. Remember that song from science class? It starts with domain, kingdom, phylum, class, order... and goes through all the taxonomic groups. Jellyfish and corals are part of the same phylum, called cnidarians. Upside-down jellyfish especially exhibit a lot of traits that are similar to their coral cousins.

All jellyfish and corals start their lives as larvae floating with ocean currents. Eventually, they settle on the seafloor or a hard surface, where they become polyps. As polyps, both corals and jellies can reproduce asexually to form more polyps, or clones, and grow into a colony. This is why a coral is considered a single organism even though it can consist of many individual polyps. For jellyfish, each polyp breaks off from the group to form a single jellyfish. When you see separate jellyfish floating around looking like different animals, they may actually be clones of the same organism. Crazy, right?!

Coral Life Cycle

Life Cycle of coral
Coral Life Cycle, Credit: Sinauer Associates Inc. 2014

Jellyfish Life Cycle

Jellyfish Life Cycle
Jellyfish Life Cycle, Credit: Concepts in Biology

After each jellyfish clone breaks off from the group, it usually flips around and starts swimming with its tentacles pointing down. However, the upside-down jellyfish doesn't flip around nor take to pulsing through the water column often. It instead, settles on the seafloor with its arms up.

Similar to coral, Cassiopea, or upside-down jellyfish, also form symbiotic relationships with tiny algae (ocean plants) called dinoflagellates, which make food for the jellyfish. Instead of having a single mouth like other jellies, upside-down jellyfish have eight arms with folds that catch floating food and send it through a canal-like system to one of their many, tiny mouths. When they are disturbed, these jellyfish exude a slime filled with stinging cells into the water column.

Next time you're out in the bay, keep an eye out for these interesting animals. It's not everyday you find yourself in the presence of gelatinous clones.

Upside-down Jellyfish Credit Andriy Nekrasov
Upside-down Jellyfish Credit Andriy Nekrasov
Cassiopea Xamachand Jellyfish Credit Don DeMaria Sea Samples FL Keys
Cassiopea Xamachand Jellyfish Credit: Don DeMaria, Sea Samples FL Keys

The Immortal Jellyfish (Turritopsis dohrnii)

In the 1990s, scientists discovered a behavior in a species of jellyfish that dubbed it the immortal jellyfish.

Immortal Jellyfish Credit American Museum Of Natural History

Credit: Takashi Murai/The New York Times Syndicate/Redux


When this jellyfish faces stress, it can change back into a polyp after already being a floating adult.

Polyp Colony Of Turritopsis Dohrnii

Credit: Peter Schuchert

Once it's a polyp again, it can reproduce asexually, creating an endless number of clones of itself.

Turritopsis Dohrnii jellyfish

Credit: Onedio

In fact, identical individuals of this species have been found in oceans across the globe.

Satellite Map Of The World

Credit: NASA, 2014


Aljbour, Samir & Zimmer, Martin & Kunzmann, Andreas. (2017). Cellular respiration, oxygen consumption, and trade-offs of the jellyfish Cassiopea sp. in response to temperature change. Journal of Sea Research. 128. 10.1016/j.seares.2017.08.006.

Florida Fish and Wildlife Commission, 2019.

"The Immortal Jellyfish". 2015. American Museum of Natural History.

Purcell, Jennifer. (2005). Climate effects on formation of jellyfish and ctenophore blooms: A review. Journal of the Marine Biological Association of the United Kingdom. 85. 461 - 476. 10.1017/S0025315405011409.

Purcell, Jennifer. (2011). Jellyfish and Ctenophore Blooms Coincide with Human Proliferations and Environmental Perturbations. Annual Review of Marine Science. 4. 209-235. 10.1146/annurev-marine-120709-142751.

Stoner, Elizabeth & Layman, Craig & Yeager, Lauren & Hassett, Heather. (2011). Effects of anthropogenic disturbance on the abundance and size of epibenthic jellyfish Cassiopea spp. Marine Pollution Bulletin. 62. 1109-14. 10.1016/j.marpolbul.2011.03.023.