Sea Walnut Comb Jelly: A New World Native That Has the Old World by Its Tentacles

Mnemiopsis leidyi is commonly known as sea walnut comb jelly, Leidy's comb jelly, or warty comb jelly. Other scientific synonyms include Mnemiopsis gardeni and Mnemiopsis mccradyi.

Its genus name, Mnemiopsis, is derived from two Greek words, Μνήμη, mneme, “memory”, and ὄψις, opsis, “act of seeing, sense of sight”, which, in combination, usually are translated as meaning "unforgettable sight." And, indeed, especially in the dark, sea walnuts are unforgettable.

Renowned scientist Alexander Agassiz (December 17, 1835 - March 27, 1910) selected its species epithet, leidyi, to honor Joseph Leidy (September 9, 1823 - April 30, 1891), a physician, vertebrate paleontolgist, and naturalist from Philadelphia, Pennsylvania. Dr. Leidy studied marine invertebrates along the coasts of Rhode Island and New Jersey. President of the Academy of Natural Sciences of Philadelphia for the last eight years of his life, Joseph Leidy published more than 800 scientific papers in his lifetime.

A sea walnut is a sea dweller in the genus Ctenophora (Greek χτένα, khtena, "comb" + φέρω, pherō, "carry"). Ctenophores characteristically have cilia, or bristles, which are grouped in four double combs, or rows. Movement of the ciliated combs serves to propel the lobed body of the sea walnut through its watery world. Because of their transparency and their distinctive combs, ctenophores are popularly referred to as comb jellies.

With water comprising over 95 percent of their body, sea walnuts often delicately disintegrate upon capture and removal from water. Soon after contact with air the only remnants are comb sections with the cilia still beating.

Another feature of sea walnuts is their ability to regenerate as long as each fragment represents at least one-fourth of the entire body.

Their common name of sea walnut derives from the resemblance of their bodies to a walnut (genus Juglans) both in size and in shape.

The sea walnut is a New World prehistoric native that hugs the western Atlantic Ocean from subtropical southern Argentina in South America northward to temperate Massachusetts in North America. Its domain largely is demarcated by a UNESCO World Heritage Site (1999), the Valdes Peninsula (Península Valdés), at 42° 30′ South latitude, 63° 56′ West longitude, to the south and by Cape Cod, at 41° 41′ 20″ North latitude, 70° 17′ 49″ West longitude, to the north.

Sea walnuts live in the ocean's photic (Greek: φῶς, phos, "light"), or sunlit, region, which is the topmost horizontal layer where the energy of sunlight converts carbon dioxide into organic compounds through the chemical process of photosynthesis (Greek: φώτο-, photo-, "light," + σύνθεσις, synthesis, "putting together").

The sea walnut's sunlit, marine habitat encompasses both the bountiful littoral (Latin: litus, "beach, shore") zone and the teeming pelagic (Greek: πέλαγος, pélagos, "open sea") zone. Thus, sea walnuts, not being bottom dwellers, are not found in either the benthic (Greek: βένθος, benthos, "depths of the sea") or the demersal (Latin: demergere, "to dip, sink") zones, the two bottommost layers of the deep ocean's aphotic (Greek: prefix ἀ-, a-, "without" + φῶς, phos, "light") zone.

Despite the seeming fragile collapsibility of their appearance, sea walnuts are adaptable, stalwart marine denizens. They do not have a narrowly defined ecology. Their euryoecious (Greek: ευρυς, eurys, "wide" + οἰκία, oikia, “house, dwelling”) nature allows them to tolerate a variety of environmental conditions and habitats.

euryhaline:

Sea walnuts are euryhaline (Greek: ευρυς, eurys, "wide" + ἅλς, hals, "salt"), meaning that they are not intimidated by varying levels of salinity. Sea walnuts tolerate salinity levels as low as 3.4 and as high as 75.

eurythermic:

As eurytherms (Greek: ευρυς, eurys, "wide" + θέρμη, thermē, "heat"), sea walnuts survive in a wide range of temperatures, from 36° to 90°Fahrenheit (2° to 32° Celsius).

hypoxic:

Sea walnuts are not distressed by hypoxia (Greek: ὑπό, hypó, "under" + , ὀξύς, oxys, "sharp"), which is inadequately low or reduced levels of oxygen.

As a product of the marine (Latin: mare, "sea") environment, sea walnuts are saltwater organisms. Nevertheless, they are not restricted to salty waters. Sea walnuts thrive in brackish (Dutch: brak, "salty") waters, a mixture of fresh water with the sea's saline waters. Estuaries (Latin: aestus, "tide, boiling [of the sea]"), which are partly enclosed, river- or stream-fed coastal bodies of water with influxes of saline water from the open sea, transition from fresh water to brackish water to salt water. An estuary favored by sea walnuts is the Chesapeake Bay. The largest estuary in the eastern United States, the Bay covers a drainage basin of 64,299 square miles (166,534 square kilometers) touching Washington, D.C., and six mid-Atlantic states (Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia).

Sea walnuts have a length ranging from 3 to 5 inches (7 to 12 centimeters). Their width usually halves their length.

Two lobes open at one end to form a mouth, which in turn segues into a flattened, elongated pharynx. A small esphagus links the pharynx with the stomach funnel. Canals branch off from the stomach, running from there throughout the body. At the aboral pole, which is the end opposite from the oral pole of the mouth, canals open to the outside through anal pores.

Two main feeding tentacles, which are retractable into pouches, are each located laterally on opposite sides of the body. Embedded in the tentacles are colloblasts (Greek: κόλλα, kolla, “glue” + βλαστός, blastos, "bud, sprout"), which are specialized lasso cells for ensnaring prey. Two filaments protrude from each cell. One straight filament is paired with a spiraled filament. Both are topped with a granular head from which a sticky substance is secreted that adheres to their prey, which are then transported to the sea walnut's mouth.

Bristly cilia (Latin: cilium, "eyelid, eyelash"), which are hairlike cellular projections, are arranged in double-tiered rows known as combs because of the overall resemblance to fine-tooth combs. Coordinated movement of these cilia propel sea walnuts through their watery world. Sea walnuts primarily move with their mouth forward and open.

Sea walnuts and other comb jellies are favorite photographic subjects. As light is diffracted between the cilia, the iridescence of the combs sparkles and shimmers against the transparent body.

Also sea walnuts appear magically in the nighttime waters as they bioluminesce, or glow blue-green, in the darkness, especially when disturbed, for example, by choppy waves roiling the watery world or by canoe paddles slapping reverberations nearby.

As a simultaneous hermaphrodite, each sea walnut exhibits both male and female reproductive organs. Reproduction is achieved efficiently by self-fertilization. Eggs, which usually average 2,000 to 3,000 per day, are spawned (Latin: expandere, "spread out"), or shed, into the water. Spawning, which requires plentiful food and water temperature between 66.2° to 73.4° Fahrenheit (19° to 23° Celsius), is only triggered at night. Fertilization takes place externally in the aqueous environment.

Within only twenty hours of fertilization, a sea walnut embryo is fully developed.

Thirteen days after hatching, a sea walnut is able to reproduce.

A sea walnut's lifespan ranges from several months to one year.

In the presence of abundant food, sea walnuts are intemperately enthusiastic. They do not observe satiety, or fullness, in a feast. As long as food is accessible, sea walnuts are able to keep consuming by way of a flow-through digestion system that has three distinct, consecutive phases. In this way, prey items are treated simultaneously in each phase. Apart from a small area of overlap, consumption and elimination occur along separate pathways. Food enters in the center of the mouth, and large indigestible fragments are ejected in the corners of the mouth. Digestive waste is excreted through the anal pores. Thus, consumption, digestion, and elimination can occur simultaneously.

As generalized feeders, sea walnuts consume an array of organisms in their aquatic environment, from fish eggs and larvae to zooplankton (Greek: ζῴον, zoon, "animal" + πλαγκτος, planktos, "wanderer, drifter"), which are tiny animals in the first link of the marine food chain. Zooplanktonic prey include fellow marine invertebrates, such as:

• younger sea walnuts,
• cnidarians (Greek: κνίδη, knide, "nettle"), which characteristically have cells for stinging their prey and a single opening for both consuming food and eliminating wastes, and crustaceans (Latin: crusta, "crust"), which have an exoskeleton, or external skeleton, segmented body, and jointed limbs.

Sea walnuts regularly consume such crustaceans as:

• brine shrimp (Artemia salina), with a lifespan of up to one year and an adult length of about 0.5 inches (1 centimeter), and
• copepods (subclass Copepoda), one-eyed, ubiquitous, with a lifespan of up to two years and an adult length averaging from 0.04 to 0.08 inches (1 to 2 millimetres).

Thus, sea walnuts' primary diet is carnivorous, piscivorous, and zooplanktivorous.

Through aquatic glidings, sea walnuts propel water, which is replete with edible organisms, over their mucous-covered lobes and thereby trap their prey.

Sea walnuts are able to consume prey even after filling their stomach chamber because they simultaneously eject excess food as a ball of mucous.

Sea walnuts tackle famine by reducing their body size, which allows them to survive for up to one month without food.

Synecology (Greek: σύν, syn, “with” + oικoλoγία, ecologia, “house” + “study”) identifies organisms that regularly coexist with sea walnuts in their native habitats. Persistent cohabitants include not only sea walnuts' prey but also sea walnuts' predators, which is the other aspect of prey-predator trophic (Greek: τροφή, trophē, "food, feeding") levels in the food chain.

Natural predators, whose native waters in eastern North America overlap with sea walnuts' native territory, serve to control this comb jelly's population. Influenced by such factors as dearth of preferred prey, generalized feeders may or may not consume sea walnuts in their vicinity and thus are whimsical predators of uncertain, although unlikely, concern to these comb jellies.

On the other hand, natural predators  which specialize in preying upon sea walnuts include:

• Atlantic butterfish (Peprilus triacanthus), averaging about 6 to 9 inches in length, with a short head, blunt snout, and a coloration of leaden blue, with pale sides dotted with dark irregular spots, and silvery belly;
• Atlantic sea nettle (Chrysaora quinquecirrha), dome-shaped body, usually white in color, occasionally with radiating bands of pale pink, red-brown, or yellow along their upper surface, and with up to 24 long, thin, trailing tentacles;
• lion's mane jellyfish (Cyanea capillata), an eight-lobed bell- or dome-shaped body ranging in color from crimson or purple to light orange or tan, with eight clusters of sticky, long, trailing tentacles in bundles of over 100 per cluster, and featured as the killer in "The Adventure of the Lion's Mane" by Sir Arthur Conan Doyle (May 22, 1859 – July 7, 1930);
• spiny dogfish (Squalus acanthias), a small schooling shark with spines in front of both dorsal fins, distinguished by scatterings of small white spots against its slate grey to brown back and light grey to pure white belly; and
• pink comb jelly (Beroe ovata), a fellow ctenophore with a voracious proclivity for devouring sea walnuts, having a mitten-shaped body with a length of around 4 inches (10.16 centimeters) and a width of 2 inches (5.08 centimeters), and having a milky coloring tinted pink or reddish brown.

Along the U.S. mid-Atlantic coast, these five natural predators are attracted to sea walnuts as prey. Predators and prey cluster in the same marine environments.

Sea walnuts especially thrive in the Chesapeake Bay as well as further north in Rhode Island's Narragansett Bay and in Massachusett's Cape Cod Bay.

A plant or animal is not necessarily restricted to living and flourishing in its native habitat forever. Relocation may occur over vast expanses. In the waning decades of the twentieth century sea walnuts experienced a considerable, easterly relocation. In their new locations, sea walnuts flourished glaringly and were labeled undesirably as alien invasive species.

The International Union for Conservation of Nature (IUCN), a global organization focused on the conservation of biodiversity and the sustainable, equitable use of natural resources, defines an alien invasive species as a non-native species "which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens native biological diversity." Invasive species may be introduced, intentionally or unintentionally, on land or in water.

Introducing invasive, non-native marine plants and animals into different, new environments through ship ballast water poses one of the four greatest threats to the oceanic world. (The other three threats are identified as marine pollution, overexploitation of living marine resources, and physical alteration and/or destruction of marine habitats.) Over 80 percent of the world's commodities are moved by ships. With each tonne equaling 2,204.62 pounds (1,000 kilograms), approximately 10 billion tonnes of ballast water are transferred internationally through shipping each year.

By providing balance and stability to cargoless ships, ballast water is an operational necessity for modern shipping. Unweighted by cargo, a ship flounders. Thus, after a ship's cargo is unloaded, unsteadiness is corrected by opening intake ports and pumps in the hold, which is the area between the bottom of the ship and the lowermost deck. As cargo is loaded, an imbalance of weight is offset by opening discharge valves to release water from the ballast tanks.

Water that is being pumped into the hold contains an abundance of visible and microscopic marine plants and animals. Water that is being discharged from the hold releases its stowaways into a different marine environment. Ballast water transports approximately 7,000 species every day from one part of the globe to another location.

A problem, with an ensuing threat, is presented by species that, acclimating to their new habitat, proliferate excessively due to the absence of the natural predators of their native ecosystem.

Such is the nightmare situation in European and Eurasian seas, where disproportionate, unnatural population excesses of sea walnuts recently have wreaked economic and environmental havoc or potential are poised to create ecological and economic imbalances.

The Black Sea, one of Eurasia's major inland seas, lies well over 6,050 miles (9,735 kilometers; 5,257 nautical miles) to the east of the Chesapeake Bay, where sea walnuts abound natively. Sometime in the early 1980s sea walnuts were unintentionally introduced into the Black Sea, presumably through ship ballast water. The environment in which sea walnuts found themselves was weakened by irregularities in the food chain through overfishing and by compromised water quality through agricultural runoff. Unfettered by the absence of pink comb jellies (Beroe ovata), their natural predators in the Chesapeake Bay, sea walnuts ate their way into top food predator status in the unprepared and overwhelmed food chain.

Feasting on the eggs and young of European anchovies (Engraulis encrasicolus), sea walnuts single-handedly depleted the Black Sea population of this snoutnosed forage fish. Preyed upon by larger fish, European anchovies are critical components of the Black Sea food chain. Additionally, European anchovies are commercially valuable as bait and in fresh, dried, smoked, canned, and frozen commodities to area fishing industries operated by Bulgaria, Georgia, Romania, the Russian Federation, Turkey, and the Ukraine. By 1989 the monopolistic presence of sea walnuts in the Black Sea was estimated at around one billion tonnes, and area fishing industries losses soared into the equivalent of several hundred million dollars.

Their devastating crash of the Black Sea fishery industry qualify sea walnuts within the top 100 of the world's worst invasive species, as determined by the Invasive Species Specialist Group (ISSG). The detrimental impact of sea walnuts on biological diversity and their devastating collapse of fishing industries illustrate critical issues concerning ecological invasion by non-native marine species.

A natural solution to the reign of terror by sea walnuts in the Black Sea presented itself with the unintentional introduction of a ferocious predator from the western Atlantic Ocean, pink comb jelly (Beroe ovata). Presumably, these jellies were pumped into ship ballast tanks in eastern North America and discharged beyond the eastern extent of the Atlantic Ocean into this distant inland sea.

As sea walnuts now are spreading throughout European and Eurasian seas, affected countries are reviewing the recent drastic situation in the Black Sea and are considering strategies for confronting the escalating distribution of sea walnuts as they run amok in nonthreatening ecosystems. As a result of the fortuitously accidental invasion of the Black Sea by pink comb jellies, the global strategy is focused largely on biological control by a natural predator.

Atlantic butterfish (Peprilus triacanthus):

The First International Meeting on the Invasion of the Caspian Sea by Mnemiopsis leidyi was held in Baku, Azerbaijan, at the end of April 2001. Participants closed the meeting with the decision to suspend consideration of intentionally introducing Atlantic butterfish, another major predator of sea walnuts, into the Caspian Sea as a biological control. Intentional introduction of non-native species is oftentimes a controversial measure which requires extensive precautions.

Spiny dogfish (Squalus acanthias):

Spiny dogfish have a worldwide distribution, appearing natively in the Atlantic, Indian, and Pacific oceans, and inhabiting as well the Mediterranean Sea and the Black Sea. Nevertheless, this natural predator of sea walnuts poses a small threat to its prey because of its own declining worldwide population as a result of overfishing by target and bycatch fisheries. In 2006 the International Union for the Conservation of Nature (IUCN) added spiny dogfish to its Red List of Threatened Species, categorizing the once abundant species as vulnerable to the endangerment of extinction.

Pink comb jellies (Beroe ovata):

Participants at the First International Meeting on the Caspian Sea invasion by sea walnuts concluded that pink comb jellies are the best choice for biological control. Pink comb jellies are expected soon to follow the sea walnuts' route into the Caspian Sea by way of ship ballast water.

Sea walnuts are now regarded as the most studied ctenophore in the world. The attention thath these durable water denizens have garnered stems from their explosively successful adaptation to European and Eurasian seas. Their colonization of the Old World was an accident of history that continues to this day and beyond. Through ecological imbalances, an innocuous, inconspicuous creature that is closer to the bottom of the food chain wreaked havoc throughout the oceanic food chain and devastated human economies as well.

Unsuspectingly pumped from their territorial waters and then unceremoniously dumped into an alien environment in a distant quarter of the globe, sea walnuts first sought to survive and then flourished because the Black Sea, their new home, was less threatening and more accommodating than their native waters. Unfortunately, their success created a massive deficit in the defenseless, unprepared Black Sea ecosystem. Then while the Black Sea staggered to recover, sea walnuts staged a similar, albeit swifter, takeover of the delicate Caspian Sea.

Transported from the New World to the Old World, sea walnuts have enthralled the world's seas by their tentacles. The problem is how to loosen their sticky grip. And the dramatic lesson that sea walnuts are teaching the world is that there are no easy answers to disruptions in the oceanic food chain.

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Mnemiopsis leidyi; Bergen, southwestern coastal Norway: CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Sea_Walnut.tif

ca. 1870 portrait by Gilbert Studios, Philadelphia: Public Domain, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Joseph_Leidy_by_Gilbert_Studios_c1870.jpg

tail of a whale: Edith Schreurs, CC BY-SA 2.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Tail_of_a_whale_near_Valdes_Peninsula.jpg?uselang=fr

Cape Cod Railroad Bridge: Pete (Peter Gene), CC BY-SA 2.0, via Flickr @ https://www.flickr.com/photos/petebackwards/328208684/

"Hypoxic event in Greenwich Cove"; photo by Chris Deacutis, IAN (Integration and Application Network, Univ. Maryland Center for Environmental Science): eutrophication&hypoxia, CC BY 2.0, via Flickr @ https://www.flickr.com/photos/48722974@N07/4523955262/

"The Thomas Point Shoal Lighthouse in Chesapeake Bay, Maryland": PA1 Pete Milnes/US Coast Guard, Public Domain, via Wikimedia Commons @ http://en.wikipedia.org/wiki/File:Thomas_Point_Lighthouse_Chesapeake_Bay.jpg

"Sea walnut, Boston Aquarium": Steven G. Johnson/Flickr, CC BY-SA 3.0, via Wikimedia Commons @ https://fr.wikipedia.org/wiki/Fichier:Sea_walnut,_Boston_Aquarium.jpg

Figure 22 broad side; a=auricles; f=furrow; o=actinostome (oral opening); Figure 23 narrow side: Public Domain, via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/page/28070415

Northern quahogs thrive in estuarial muddy sands, Winyah Bay Estuarine Research Reserve, South Carolina: NOAA Photo Library, CC BY 2.0, via Flickr @ http://www.flickr.com/photos/noaaphotolib/5114728190/

brine shrimp (Artemia salina): Hans Hallewaert, CC BY-SA 4.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Artemia_salina_1.jpg

copepod: Uwe Kils, CC BY-SA 3.0, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Copepodkils.jpg

Dermochelys coriacea: Hannes Grobe/AWI, CC BY 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Lederschildkr_dermochelys-coriacea-senkenberg_hg.jpg

Atlantic sea nettle (Chrysaora quinquecirrha); Grey's Reef National Marine Sanctuary, Georgia; photo by Greg McFall, Gray's Reef NMS, NOS, NOAA: NOAA Photo Library, CC BY 2.0, via Flickr @ https://www.flickr.com/photos/noaaphotolib/5029593133/

lion's mane jellyfish: Erik Solheim (erikso), CC BY-SA 2.0, via Flickr @ https://www.flickr.com/photos/eirikso/4848845443/

view of Atlantic Ocean from Beavertail State Park, southern Conanicut Island, Rhode Island: Roger LeJeune (dadofliz), CC BY-ND 2.0, via Flickr @ https://www.flickr.com/photos/dadofliz/5487032207/

concept, content and design: Steve Raaymakers and Daniel West and Associates, London: International Maritime Organization, CC BY 2.0, via NOAA @ http://stateofthecoast.noaa.gov/invasives/ballast_water.html

How the comb-jelly (Mnemiopsis leidyi) is spreading through European seas (invasive species): Philippe Rekacewicz, CC BY NC SA 2.0, via UNEP/GRID-Arendal @ http://www.grida.no/graphicslib/detail/how-the-comb-jelly-mnemiopsis-leidyi-is-spreading-through-european-seas-invasive-species_727d

European anchovies (Engraulis encrasicolus), Ligurian Sea, Italy: Etrusko25, Public Domain, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Acciughe_2.jpg

Comb-jelly (Mnemiopsis leidyi) spreading through the Caspian Sea (invasive species): Hugh Ahlenius, CC BY NC SA 2.0, via UNEP/GRID-Arendal @ http://www.grida.no/graphicslib/detail/comb-jelly-mnemiopsis-leidyi-spreading-through-the-caspian-sea-invasive-species_11bd

Beroe ovata on the Black sea offshore: CristianChirita, GNU 1.2, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Medusa010.jpg

western Caspian Sea along Baku Boulevard (Azerbaijani: Dənizkənarı Milli Park): Samir Rəsulov, Public Domain, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Baku_Bulvar.jpg

view of Atlantic Ocean from Caleta Valdés, inlet on Valdes Peninsula's east coast, near Punta Norte (northernmost point): Zuarin, CC BY 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Caleta_Valdés_2.JPG

aboral (away from or opposite mouth) view of adult Mnemiopsis leidyi, caught off southern New Jersey coast: The jt, CC BY SA 4.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:MLeidyi_headon_10082015.png

Baltic Sea's southern waters off Royal City of Darłowo, northwestern Poland: Argonowski, CC BY 3.0, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Baltic_Sea_(Darlowo).JPG

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