This post is part of an occasional series on local marine species compiled and photographed by New England Aquarium aquarist Peter Gawne. When he's not diving locally for these posts or researching coral reefs in Belize, Peter takes care of the touch tank's sharks and rays and the jellies exhibit.

Atlantic sea nettles are predatory sea jellies of the phylum Cnidaria, and the class Scyphozoa.

Atlantic sea nettle (Chrysaora quinquecirrha)

Atlantic nettles can be found throughout the western Atlantic Ocean, from southern New England to Brazil. These jellies have a potent sting, and are responsible for numerous uncomfortable stings to swimmers during the summer months.

Atlantic nettles are quite variable in coloration. They can range from opaque white, to pale pink or yellow. Individuals often possess more deeply colored stripes radiating on the surface of their bells.

The long, trailing tentacles of Atlantic sea nettles, like those of all cnidarians, are studded with stinging cells called nematocysts. These tentacles possess a potent sting, and are used to capture ctenophores, jellies and other zooplankton from the water column. Jellies are indiscriminate feeders, their tentacles stinging nearly anything organic they come into contact with, which unfortunately includes humans.

Nematocyst cells from an Atlantic sea nettle cover every inch of its tentacles. Microscopy reveals these lethal cells, coiled and waiting to fire their hollow harpoon-like threads.

A combination of chemoreceptors and mechanoreceptors control the firing mechanism for the nematocyst. When the right combination of stimuli exist, the nematocysts fire, sending microscopic hollow harpoons into their prey. After penetration, the toxic contents of the nematocyst are delivered to the target through these hollow threads.

A discharged nematocyst from an Atlantic sea nettle has penetrated tissue. Its hollow thread will deliver powerful toxins to the jelly’s prey.

The venom of jellies contains a cocktail of potent compounds, including proteinaceous porins, neurotoxic peptides, bioactive lipids and other small molecules. This venom stuns or kills small prey, allowing the jelly to digest the incapacitated prey. Porins in jelly venom are largely responsible for the potency of the sting on humans. Porins are a cellular membrane pore-forming toxin, and in jelly venom are fast-acting. These proteins will indiscriminately punch holes in whatever cells they come into contact with, including blood, skin and nerve cells. In humans this results in pain, redness or blistering.

In the case of extremely potent jellies, such as the Australian box jelly (Chironex fleckeri), the high levels of porins in the venom create tiny holes or pores in the red blood cells. Potassium leaks from the ruptured cells and into the bloodstream. Potassium is an electrolyte which plays an important role in keeping the heart beating at a normal rhythm. Typically, potassium levels are held in check by our kidneys, but they cannot manage the massive influx of potassium from the rapidly rupturing cells. When the plasma potassium concentration gets too high it disrupts the electromotive force and the heart cannot beat, resulting in cardiac arrest.

While the sting of an Atlantic nettle is unpleasant, it is only life threatening in the case of an allergic reaction. “I’ve Been Stung: What Should I Do?” by Paul S. Auerbach, M.D., M.S., is an excellent resource for the treatment of marine animal injuries, including those by jelly stings.

Brine shrimp have fallen prey to the ephrya of an Atlantic sea nettle.  Their digesting bodies can be seen through the transparent body of the nettle.

The life cycle and early stages of Atlantic sea nettles are remarkably similar to their cousins, moon jellies (read more about it here). The major difference being the eggs and sperm of the medusoid adults are left to the currents to find one another, rather than ingested and brooded by the female.

Special thanks to NEAq Senior Aquarist Eric Payne for assistance with microscopy.

This post is part of an occasional series on local marine species compiled and photographed by New England Aquarium aquarist Peter Gawne. When he's not diving locally for these posts or researching coral reefs in Belize, Peter takes care of the touch tank's sharks and rays and the jellies exhibit.

Moon jelly (Aurelia aurita) can be found off the New England coast. They belong to the phylum Cnidaria, and the class Scyphozoa.

Moon jellies are found throughout the Atlantic Ocean, and are a common sight in New England waters. Like all cnidarians, moon jellies possess stinging cells (nematocysts), but the sting of moon jellies is mild enough to be imperceptible to most humans. Moon jellies are comprised of 95 percent water, and lack anything resembling a heart or brain.  Despite their lack of substance, moon jellies are carnivorous, and use their fine, fringe-like tentacles to capture prey and transport it to their oral arms for digestion.

Moon jellies provide a source of food for a variety of animals, including sea turtles, ocean sunfish, other jellies, birds, and even humans.  Jellies, including moon jellies, have long been on the menu in Asian cuisines.  The United States helps to satisfy the demand for edible jellies, which are dried and then cut into noodle-like strips.  Following the first official commercial season in 2013, jelly fishing became the third largest fishery in the state of Georgia.  Commercial plants in Georgia are able to process more than 5 million pounds of jellies per week.  The entire catch is exported to Asian markets.

The life cycle of moon jellies is similar to that of most scyphozoan jellies, but that makes it no less extraordinary.  Adult moon jellies are either male or female, with the occasional hermaphrodite.  The males release sperm strands, which are ingested by female moon jellies.  Once ingested, the sperm strands fertilize the female's eggs.  These two genetic-halves combine, resulting in a planula.  Unlike most scyphozoan jellies, which simply release their eggs into the water column, female moon jellies brood planulae beneath their oral arms.  Once the planulae are fully formed, they are released from the brooding sacs to swim, via small hairs called cilia, until they reach a suitable substrate.  When a spot has been found, the planula hooks onto the substrate, and begins its first metamorphosis into a polyp.

Tiny moon jelly polyps cling to a rock. The polyp farthest to the right is in the initial stages of strobilation.

Polyps are the benthic (ground-associated) stage of the jelly life cycle. Jelly polyps resemble the adult form of their relatives, sea anemones, in that they possess a point of attachment to a hard surface, and a crown of tentacles at the opposite end. Polyps can survive months or even years, by using their tentacles to capture small prey from the water column.

When environmental conditions are right, a metamorphosis begins with a process called strobilation. The strobilating polyp begins to undergo morphological changes, where the animal’s body becomes segmented, transforming it into a sequence of disks.  Each of these segments will eventually detach from one another and become a free-swimming phase, called an ephyra.  Thus one polyp can generate multiple free-swimming individual animals.  Typically, a portion of the polyp remains adhered to the seafloor, where it regenerates the body and prepares to strobilate anew.

Pictured above is a moon jelly ephyra cultured at the New England Aquarium. A 2014 National Institute of Health-funded program made advances in tissue engineering by emulating jellyfish ephryae cultured at the New England Aquarium (original study, video).

The ephyra is the free-swimming precursor to the final stage of the jelly life cycle.  Ephyrae pulse about through the water column, capturing prey, and growing.

Showing up in staggering numbers, smacks (a term for a group of jellies) of moon jellies can dominate New England harbors and estuaries in the summer months. The benthic polyp stage slows or becomes dormant during the coldest months of the year. Water temperatures, sunshine, and plankton increase in the spring, propelling the polyps into action. The polyps, all driven by the same environmental cues, begin to produce ephyrae en masse.  This mass influx of jellies into the environment often leads to incredible “blooms” of jellies, where thousands of adult jellies can be found in huge aggregations.

In a matter of weeks to months, the ephyrae will have grown, and gathered enough energy, to develop into their adult form—the medusa. The jelly medusa is final phase of the jelly life cycle, and the most recognizable form. These adult jellies will drift though the seas for up to a year or more, reproducing and beginning their extraordinary life cycle anew.

Large plastic sheets are kept as optimal growth sites for moon jellies at the New England Aquarium.

Not all ephyrae released from polyps become juvenile jellies; some may drop to the sea bottom and develop into new polyps, which will eventually bud off their own ephyrae. Before strobilating, and when conditions for growth are good, polyps may bud off new polyps. Polyps may live for years, so densities in areas favorable for growth can be quite high.  It should be noted that all polyps are either male or female, and all ephyrae produced from a single polyp are genetically identical, and thus the same gender.

Each year, New England Aquarium staff culture and raise thousands of sea jellies. They can be found on display at the New England Aquarium’s Jellies exhibit. In 2014 alone, the New England Aquarium supplied more than 1,000 jellies to other public aquariums around the globe, including institutions in Las Vegas, Dubai and Australia.

On the third floor of the Aquarium next to the octopus exhibit is a diverse Pacific tank that is glowing with vibrant colors. One of its more subdued residents sports handsome stripes and distinct spots. Can you spot the spot shrimp (Pandalus platyceros)?

Spot shrimp

Two spot shrimp!

• Playtime for the octopus
• The oft-overlooked sharks and rays around the Aquarium
• The charming goosefish is an egg-veil master
• Plankton, they're kind of a big deal  

It should be no surprise to learn that the dwarf seahorse (Hippocampus zosterae) is one of the smallest seahorse species. The adults on exhibit in the Yawkey Coral Reef Center are only about an inch long!

Dwarf seahorses are impossibly tiny. They inhabit shallow grass beds.

Like other seahorse species, the males have an abdominal pouch where they brood eggs delivered by females. Although males become “pregnant,” seahorses do not display sex role reversal. Males compete with one another for access to a mate and form monogamous pair bonds. Many of the seahorses in the exhibit have paired off and you know what happens next—babies!

If you look closely at the photo below you'll see some of the babies clinging to the grassy habitat in the exhibit. They look like the adults only much, much tinier. It doesn't take too long for the juveniles to grow up however. In fact, some of the adults on exhibit now were born on exhibit!

Can you see the baby clinging to the grasses on the middle right?

The dwarf seahorses exhibit in the Yawkey Coral Reef Center deserves a closer look—because those babies are so tiny you'll need to take a good long look to find them!

The adult and baby seahorses cling to the grasses in the exhibit.

You may think you know the Aquarium, but take a look at any exhibit and you'll see there's a lot more going on than you may think. Take the Living Corals exhibit, for example. Everyone loves to watch the clownfish cuddle with its anemone, or the Banggai cardinalfish posing for pictures with their distinctive stripes. But take a closer look in the sand, the rocks the quiet corners of the exhibit—you never know what you'll see.

For example, a hermit crab found itself in a bit of a sticky situation when a passing urchin latched on to its shell. These two invertebrates are not the focal points of this exhibit, but they certainly exhibited some fascinating behavior worthy of a closer look! Here's the whole interaction sped up in a timelapse video.



One reason urchins carry around shells and seaweed may be for camouflage. The hermit crab's shell would have made for a dashing addition. But the crab was able to eek out of its shell enough to pluck the tube feet from its shell so it could scurry away.

So the next time you visit, get macro. Pause to look at the tiny animals around the Aquarium and get ready for a little bit of awesome.

Here's more about these humble heavyweights in our ocean communities:

• Meet the urchins in our cold-water tidepool touch tank
• Aquarists actually caught one of our hermit crabs hatching her eggs
• The BBC filmed hermit crabs in Belize while researcher Randi Rotjan was there
• Check out this unbelievable video of hermit crab migrations
• See more hermit crabs in Belize, plus other amazing ocean animals
• Here's a hermit crab with a most unusual shell