Most jellyfish never touch a partner. Males and females release sperm and eggs into open water at the same moment, and fertilization happens out there.1eLife. Reproduction: Shedding light on spawning in jellyfish The drifting jellyfish you see at the beach is usually only half the story, because its young spend part of life fixed to a rock as a tiny plant-like form that can copy itself.2PubMed Central. Changes of cell-type diversity in the polyp-to-medusa metagenesis of the scyphozoan jellyfish Aurelia coerulea (formerly sp.1) One spawning pair can eventually give rise to swarms of clones, which helps explain why jellyfish sometimes appear in huge numbers all at once.
How Jellyfish Time Their Spawning
Light is the shared clock. In many jellyfish, the brightening of sunlight at dawn makes males and females release sperm and eggs at the same time.3eLife. Reproduction: Shedding light on spawning in jellyfish The eggs and sperm then meet in the same patch of water while both are fresh, which raises the odds of fertilization.4eLife. Reproduction: Shedding light on spawning in jellyfish Other species spawn in the evening, triggered when light switches to dark.5PubMed Central. A light-entrained clock mechanism in a hydrozoan jellyfish synchronizes evening gamete release The sex organs themselves can sense light and respond by releasing a hormone that ripens and frees the eggs.6eLife. A gonad-expressed opsin mediates light-induced spawning in the jellyfish Clytia Even with tight timing, many eggs go unfertilized or get eaten, and releasing huge numbers makes up for the losses.
In some small jellyfish, males begin releasing sperm slightly before females release eggs.7PubMed Central. A light-entrained clock mechanism in a hydrozoan jellyfish synchronizes evening gamete release An early start lets sperm spread farther, which may help when jellyfish are scattered thinly in the sea.8PubMed Central. A light-entrained clock mechanism in a hydrozoan jellyfish synchronizes evening gamete release
Some species follow the moon. The Hawaiian box jellyfish arrives at the shoreline on specific nights each lunar month to spawn.9UH News. Study reveals moon and darkness trigger Hawaiian box jellyfish spawning Its cue is a set number of hours of darkness after sunset before the moon rises.10UH News. Study reveals moon and darkness trigger Hawaiian box jellyfish spawning That makes beach appearances predictable, about 8 to 10 days after a full moon.
The egg takes part too. In some jellyfish it gives off chemical signals that attract and steer sperm toward it.11Flagellar Mechanics and Sperm Guidance. Sperm Guidance in Other Animals and Phylla (Such as Other Fish, Jelly-Fish, or Amphibian) Once fertilized, the egg stops attracting sperm within about 5 minutes.12PubMed. Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum It also turns sticky and begins dividing.13PubMed. Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum
From Fertilized Egg to a Polyp on a Rock
Some mothers guard their eggs first. Moon jellies hold fertilized eggs on the frilly arms under the bell until the young are ready to go.14Two Oceans Aquarium. The life cycle of a jellyfish: Spawning, cloning and immortality
Eventually the egg develops into a tiny hair-covered larva that swims to find a surface to attach to.15Marine Lakes Research. Jellyfish life cycle (Scyphozoa) Larvae settle on the sides or undersides of rocks, rotting logs, and dead leaves.16Marine Lakes Research. Jellyfish life cycle (Scyphozoa) Undersides are shaded and protected from grazers, and less silt settles on downward-facing surfaces. Then the larva changes into a polyp, a small stalk with tentacles fixed in place.17Marine Lakes Research. Jellyfish life cycle (Scyphozoa) This polyp, not the egg, will later produce the next free-swimming jellyfish.18Marine Lakes Research. Jellyfish life cycle (Scyphozoa)
A polyp is only a few millimeters tall and looks like a tiny sea anemone, with its mouth facing outward, ringed by tentacles.
So a jellyfish’s life runs in two parts. Most species alternate between a polyp that reproduces by cloning and a swimming jellyfish that reproduces with eggs and sperm.19PubMed Central. Changes of cell-type diversity in the polyp-to-medusa metagenesis of the scyphozoan jellyfish Aurelia coerulea (formerly sp.1) In one upside-down jellyfish, polyps can only make jellyfish if they host tiny algae inside them.20The International Journal of Developmental Biology. Checkpoints in the life-cycle of Cassiopea spp.: control of metagenesis and metamorphosis in a tropical jellyfish
How One Polyp Clones Itself
Moon jelly polyps use several ways to clone, mainly budding off new polyps from their sides.21Frontiers in Ecology and Evolution. Asexual reproduction strategies in the moon jellyfish Aurelia (Cnidaria: Scyphozoa) They also send out creeping runners that grow new polyps.22Frontiers in Ecology and Evolution. Asexual reproduction strategies in the moon jellyfish Aurelia (Cnidaria: Scyphozoa) Even broken-off pieces of those runners can become polyps.23Frontiers in Ecology and Evolution. Asexual reproduction strategies in the moon jellyfish Aurelia (Cnidaria: Scyphozoa) Polyps can form resting cysts as well, and even make swimming larva-like clones without any jellyfish stage.24PubMed Central. The elusive life cycle of scyphozoan jellyfish – metagenesis revisited One settled larva can become a whole carpet of genetically identical polyps. Cloning needs no partner, and the colony can wait out bad seasons as polyps, which can live for years in one spot. Budded polyps share the parent’s genes exactly; only eggs and sperm mix genes.
How Polyps Turn Into Baby Jellyfish
In moon jellies, a polyp reshapes into a stack of discs.25PubMed. Regulation of polyp-to-jellyfish transition in Aurelia aurita The top disc pulses and swims off first, and each disc pops off as a star-shaped baby jellyfish a few millimeters wide. One polyp can release many babies in a season. A temperature-sensitive protein acts as a timer that builds up a hormone triggering the change.26PubMed. Regulation of polyp-to-jellyfish transition in Aurelia aurita
Winter matters a great deal. Moon jelly polyps from southern England produced almost no baby jellyfish after winters of only 2 to 4 weeks.27Hydrobiologia. Shorter, warmer winters may inhibit production of ephyrae in a population of the moon jellyfish Aurelia aurita They made none when winter stayed at 10°C.28Hydrobiologia. Shorter, warmer winters may inhibit production of ephyrae in a population of the moon jellyfish Aurelia aurita Longer and colder winters led to faster release and more babies.29Hydrobiologia. Shorter, warmer winters may inhibit production of ephyrae in a population of the moon jellyfish Aurelia aurita So shorter, warmer winters may mean fewer moon jellies in some places, not more.30Hydrobiologia. Shorter, warmer winters may inhibit production of ephyrae in a population of the moon jellyfish Aurelia aurita
Release often happens in late winter or spring. Within 3 to 4 weeks the flat star grows several times wider and rounds into a bell.31PubMed Central. First description of the life cycle of the jellyfish Rhizostoma luteum (Scyphozoa: Rhizostomeae) The arms of the star are absorbed as the bell grows outward, and muscles reorganize into the rings that power swimming.32Invertebrate Biology. Organization of the subumbrellar musculature in the ephyra, juvenile, and adult stages of Aurelia aurita Medusae Over weeks the babies become visible adults, so summer swarms trace back to winter polyps.
Box Jellyfish That Really Do Mate
A few box jellyfish species have evolved true mating with internal fertilization.33PubMed. Mating in the box jellyfish Copula sivickisi–Novel function of cnidocytes In Copula sivickisi, a small Indo-Pacific species, males and females tangle their tentacles together.34PubMed. Mating in the box jellyfish Copula sivickisi–Novel function of cnidocytes The male hands over a sperm package that the female swallows, fertilizing her eggs inside her.35PubMed. Mating in the box jellyfish Copula sivickisi–Novel function of cnidocytes He even uses stinging cells in a new way, to attach the package.36PubMed. Mating in the box jellyfish Copula sivickisi–Novel function of cnidocytes
Partners must find and touch each other, but far fewer sperm are wasted. This is rare among jellyfish. Not all box jellyfish mate this way; the Hawaiian box jellyfish spawns in the water instead.
Jellyfish That Skip Stages
Not every jellyfish has a polyp. The mauve stinger, Pelagia noctiluca, spends its whole life drifting, developing from larva straight into a baby jellyfish.37PubMed Central. Comparative muscle development of scyphozoan jellyfish with simple and complex life cycles Open-ocean jellyfish have no rock bottom to settle on, and the polyp stage was lost at least twice in open-ocean groups.38PubMed. Loss of metagenesis and evolution of a parasitic life style in a group of open-ocean jellyfish Even species that usually have polyps can sometimes skip them, with larvae turning directly into baby jellyfish.39PubMed Central. The elusive life cycle of scyphozoan jellyfish – metagenesis revisited
Other jellyfish skip the swimmer instead. Stalked jellyfish stay attached to seaweed or rocks and never have a free-swimming stage.40Elsevier. Morphological analyses of the adult and juvenile stages of the stalked jellyfish Craterolophus convolvulus (Johnston, 1835) (Cnidaria: Staurozoa: Stauromedusae: Craterolophidae) using micro-CT They still spawn eggs and sperm, producing crawling larvae.41Elsevier. Morphological analyses of the adult and juvenile stages of the stalked jellyfish Craterolophus convolvulus (Johnston, 1835) (Cnidaria: Staurozoa: Stauromedusae: Craterolophidae) using micro-CT
The Immortal Jellyfish
Turritopsis jellyfish can turn from adult jellyfish back into polyps at any age.42PubMed. Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa) Injury, harsh conditions, or old age trigger the change.43PubMed Central. Transcriptome Characterization of Reverse Development in Turritopsis dohrnii (Hydrozoa, Cnidaria) The jellyfish shrinks into a ball of simple cells, which then grows into a polyp.44PubMed Central. Transcriptome Characterization of Reverse Development in Turritopsis dohrnii (Hydrozoa, Cnidaria) Its cells switch jobs, a skin cell becoming another type, which is extremely rare in animals.45PubMed. Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa) That means it can avoid death from aging, though predators and disease still kill it.46PubMed Central. Cellular Reprogramming and Immortality: Expression Profiling Reveals Putative Genes Involved in Turritopsis dohrnii’s Life Cycle Reversal Each reversal resets its age, and it may happen repeatedly, but most individuals in the wild are still eaten.
Why Jellyfish Appear in Sudden Swarms
How many larvae and polyps survive strongly shapes how many adult jellyfish appear.47PubMed. Jellyfish life histories: role of polyps in forming and maintaining scyphomedusa populations Abundant food can push moon jelly populations from cloning toward producing more swimming jellyfish.48bioRxiv. Ecological drivers of jellyfish blooms – the complex life history of a cosmopolitan medusa (Aurelia aurita) Food-rich years can therefore bring dramatic boom-and-bust swarms.49bioRxiv. Ecological drivers of jellyfish blooms – the complex life history of a cosmopolitan medusa (Aurelia aurita) Warmer winters may boost blooms in some populations while reducing them in others, as noted for southern England.50Hydrobiologia. Shorter, warmer winters may inhibit production of ephyrae in a population of the moon jellyfish Aurelia aurita Blooms tend to recur seasonally in the same places, and swarms usually fade as adults spawn and die.
People have a hand in this too. Docks and piers give polyps new homes, nutrient runoff increases plankton food, and overfishing removes competitors.
Breeding Jellyfish in Aquariums
Aquariums use round, gently circulating tanks so delicate jellyfish don’t get trapped.51Frontiers in Marine Science. Versatile aquarium for jellyfish: A rearing system for the biomass production of early life stages in flow-through or closed systems Mauve stingers raised in captivity produced their own eggs after about 85 days.52Frontiers in Marine Science. Successful Culture of Pelagia noctiluca (Cnidaria: Scyphozoa) Over Time: A Continuous Supply of the Holoplanktonic Jellyfish for Research and Industrial Applications They were raised to a third generation, and feeding shrimp helped them form eggs.53Frontiers in Marine Science. Successful Culture of Pelagia noctiluca (Cnidaria: Scyphozoa) Over Time: A Continuous Supply of the Holoplanktonic Jellyfish for Research and Industrial Applications Farmed jellyfish now supply aquariums and research, and possibly food.54Reviews in Aquaculture. An overview of jellyfish aquaculture: for food, feed, pharma and fun