Jellies. Of course.

An aquarium crowd pleaser and anathema of the beach, jellies are by no means rare animals, yet they never cease to fascinate and terrify their human audience. Their simple design and alien appearance combine in awe-inspiring beauty – a diaphanous bag of cells operating at the most basic level of functionality. It is this simplicity of jellies that amazes. It is hard for us to believe that these are actually animals, yet they manage to thrive with the most primitive tissue structure, lacking viscera, and possessing only a rudimentary nervous system. The thick, brown sea nettles that contrast so attractively with a blue aquarium background and the heavy, dinner-plate shaped moon jellies that wash up on the beach are actually far more complex than many jellies in the ocean. Although they resemble their stinging cousins, the animals of Phylum Ctenophora, are usually classified separately from the more well-known jellies of Phylum Cnidaria.

At the New England Aquarium we distinguished the groups as true and comb jellies. The true jellies, members of Phylum Cnidaria, tend to have more robust bodies and possess venomous cells called nematocysts which can paralyze prey and sting a human interloper. The animals of Phylum Ctenophora are totally painless yet proliferate so aggressively that reaching into a tank feels like dipping your hand into a jar of, believe it or not, jelly. Comb jellies can bloom in extremely dense masses, flowing in to shore with the high tide where we would meet them with buckets, scooping them up with sore arms by the thousands. After an accidental introduction in the Black Sea, one type of comb jelly, Mnemiopsis, even caused the collapse of local fisheries by eating both fish larvae and fish prey and expanding to a devastating population size.

Comb jellies come in all shapes and sizes – from the marble sized gooseberry (Pleurobrachia) to the nearly flat, meter-long Venus’ Girdle (Cestum). The relatively few species of the phylum all have transparent bodies that appear as fine as a snowflake. The Venus’ Girdle, for instance, is so thin that divers can look through its body to see undistorted images on the other side. The only observable action of these animals is the rapid flutter of tiny cilia, organized in eight long stripes, which propel the animal through the water. Their swimming movement appears nearly effortless compared to the strenuous flex of a true jelly’s bell. The flutter of the cilia can also scatter light, creating a prismatic effect in many species. Most Ctenophore species are capable of bioluminescence, lighting up the dark ocean with flashes of blue and green. These nearly invisible oceanic lanterns add to the extraordinary experience of diving at night, which is at once strange and beautiful.

Read more about Phylum Ctenophora and one diver's encounter with the nighttime glow of Venus' Girdle here.

Photo: Mnemiopsis photographed by Herb Segars