Part 2:  Menagerie a trois

Story & Photos By Eric S. Cole, Biology Department, St. Olaf College, Northfield, Minnesota

The “upside-down jellyfish (Cassiopea) is the “creature” focused on in this article.

The life cycle of a jellyfish, and in particular the upside-down jellyfish (Cassiopea) involves something called alternation of generations, going between a swimming form, (the “medusa” that most of us recognize as a jellyfish), and a tiny anchored form that most of us never see, the “polyp” which resembles a minute coral or anemone. Evolution has produced a remarkable creature that spends half its life anchored to a solid substrate snaring passing zooplankton with stinging tentacles, and half of its life swimming as a pulsating “bell” while harboring photosynthetic microalgae through both life stages.

This symbiotic relationship provides shelter to the microalgae and energy to the jellyfish. The tiny polyps grow, and divide by budding off other tiny polyps. The “head” of a polyp consists of a mouth encircled by a ring of stinging tentacles. When the time is right, this “head” pinches off and swims away, and a juvenile jellyfish (or “ephyra”) is born.

The medusae exist in both male and female forms and when mature, begin producing sperm and eggs. Changes in water temperature can drive sexual reproduction in congregating jellyfish. The eggs of female Cassiopea jellies are fertilized and brooded internally. Planktonic planulae (a type of larvae specific to Cnidarians) are subsequently released, swim around, and are eventually attracted to a substrate for settlement where they metamorphose into the tiny polyp forms. 

This figure depicts the life cycle of the “upside-down” jellyfish, Cassiopea.

During their life-cycle, the planulae that are shed into the water column join other planktonic life forms where they remain suspended for some time. They appear to be choosey about where to settle before turning into feeding polyps. Somehow, planulae sense chemical cues in the environment, discerning the most productive place to land. Previous researchers described infant polyps anchoring to rotten mangrove leaves. By analyzing the leaves, they hypothesized that a “biofilm” of living cyanobacteria (blue-green algae) coats the decaying plant matter, releasing tiny peptides that the jellyfish larvae are attracted to. 

Andrew Hinrichs, our research team’s jellyfish expert, found polyps anchored to similar decaying leaves in one of the Turks & Caicos’ island ponds. Later, while exploring a saltwater pond on Providenciales, we found a rich bed of upside-down jellies. Though we hunted diligently, we could not locate their corresponding polyps.

This adult lugworm was pulled from the sediments by team member and redoubtable lugworm wrangler, John Campion.

What we did find was a second denizen of the inland ponds, the giant Atlantic Lugworm (Arenicola). These fleshy beasts burrow beneath the sediment that provides substrate for the maturing jellyfish. One finds them by groping blindly beneath the muck, until one’s fingers encounter their pulsating mass. Evidence of their existence appears in the form of ballooning egg masses that rise from their burrows. These are the size of grapefruits, and house thousands of baby lugworm embryos. The surface of the lugworm egg mass appears dusty, as algae seems to settle on them and stick.

Lugworm egg sacs play a peculiar role in our studies, in that they appear to attract the juvenile “medusa” (the smallest actual jellies) as well as snails that graze the algae layer on top. It was intriguing that the lugworm egg mass seemed to attract the juvenile jellies (the “medusae”), but we were searching for juvenile polyps (the tiny anchored form).

This clutch of baseball-sized jelly masses houses eggs.

Andy began looking more closely at the snails grazing both the algal coat on the egg masses, and the surrounding beds of green feather algae (Caulerpa). To our amazement, Andy found Cassiopea polyps anchored to the shells of the grazing snails. These snails (a poorly studied group of “horn snails” or Cerithids) represent the third animal in our mysterious menagerie. Andrew taught us how to see them, and we were able to study them more carefully. “Clean-shelled” snails seemed unattractive to the jellyfish polyps, whereas snails whose shells were encased in a dark, calcareous plaque (think of what your dentist works on while cleaning your teeth, only nastier) appear irresistible to the planulae that metamorphosed into the juvenile polyps. 

In corresponding with other scientists Dr. Aki Ohdera, William Fitt, and Monica Medina, we realized that we had discovered a potentially intense source of a bio-attractant that calls jellyfish larvae to settle on this layer of encrusting microbes that cover the Cerithid snails.

Our life history Trio includes: lugworm (in light brown), Cerithid snails (dark brown), Cassiopea polyps (pink) and Cassiopea medusa (turquoise).

To recap, (it is hard to keep up with this story) lugworms lay egg masses (giant gelatinous orbs) on the bottom of our favorite Provo pond. Single-cell algae settle on these egg jellies, dusting their surface like frosting on a cupcake. This algal frosting attracts Cerithid snails that plow lanes, grazing the algal film that covers the egg mass and in turn, the snails attract microbes that cover their shell, creating a dark, calcareous crust. Nearby, adult jellyfish spawn, releasing their tiny larvae into the pond water. The larvae drift until they smell something (or bump into something) “good.” As it turns out, the most desirable real estate in the entire lake (if you are a larval jellyfish) is this calcareous crust decorating the shell of the snails seen cruising over the gelatinous egg jellies of the giant lugworm. Oddly, microbial crusts on these snails may offer a potential tool for coral-restoration. 

On our most recent trip (February 2025), we found the snails again, and gently scraped away some of their encrusting biofilm with stainless steel picks provided by my dental hygienist. We applied for and got a permit to collect and test this material, both for bio-activity (does it attract jellyfish babies?) and to learn the genetic identity of the microbes involved.

Evgenia Roth and Brooke Ellis are two of our students both working for one of St. Olaf’s professors, Dr. Anne Gothmann, who is studying the deep-water coral Balanophyllia elegans. Her living corals also shed planulae that Evgenia and Brooke have been collecting. We are currently testing to see if our snail’s encrusting biofilm can attract larvae from across the phylum. If so, identifying this bio-active compound could help efforts to grow and settle corals in the laboratory as researchers work to restore natural populations where successive waves of brutalizing heat have beaten them back.

One unexpected perk from our recent adventure was discovering the Turks & Caicos Reef Fund office at the South Bank Marina in Providenciales. This facility has a spectacular collection of live local corals maintained by a team of coral enthusiasts. It is well-worth a visit, especially on Wednesdays when the corals are fed. We were given an exceptional tour of the collection by one of the facility’s resident researchers, Gracie Perry Garnette, during some of our down time and a break from snorkeling the “black lagoon.” Maybe down the road our snail discoveries might help TCI restore hard corals to their local reefs. (See www.visittci.com/providenciales/tci-reef-fund).

Eric Cole is a Biology Professor at St. Olaf College, in Northfield, Minnesota.