The Sea Walnut (aka The warty comb jelly)
The Sea walnut is an animal, which as first glance looks like a jellyfish, and is shaped more or less like a walnut. It is technically known as Mnemiopsis leidyi, and belongs to the group Ctenophora. It has travelled far from its native home in the Atlantic Sea on the American coast to become a major pest on Eurasian waters.
The Misplaced
Appearance
Sea walnuts are a marine soft bodied species of belonging to a taxonomic group known as the ctenophores which are visually akin to jellyfish. Unlike jellyfish, they have no stinging cells.
Their bodies contain a very high percentage of water, and have a gelatinous texture with no hard parts. They are translucent, but may exhibit bioluminescence - glowing coloured lights on its body.
They grow to a maximum length of 100 -120 mm and are about 25 -35 mm in width. The body is made up of sets of lobes and it moves by the action of cilia - it is the largest organism to use this method of moving
An interesting feature of Mnemopsis leidyi is that it has a transient anus, which means that it only forms during defecation.
Reproduction
M. leidyi is hermaphroditic, possessing gonads that produce both sperm and eggs. The fecundity of M.leidyi depends on the size of the organisms and the temperature of the water. It only takes about 2 weeks from hatching for a sea walnut to be capable of producing eggs. Optimal temperatures for spawning are around 20 to 25 degrees Celsius. Two to three thousand eggs per day production by adults at high food concentrations is common.
Ecology and Feeding
The sea walnut is able to inhabit areas that display a wide range of salinity levels, 4-39 parts per thousand.
And it is very tolerant of a range of temperatures, inhabiting waters over a range of 6 to 31 degrees Celsius.
Sea walnuts are voracious carnivores and feed on a wide range of species.
Adult forms feed primarily on invertebrates - often very small crustaceans such as copepods (1 to 5 mm in length) and cladocera (water fleas). The also ingest mollusc larvae and eggs, and young fish larvae.
Larval forms feed on microzooplankton and protozoa.
The Displaced
Due to their voracious appetites and their high reproductive capability, the presence of sea walnuts has had a drastic negative effect on zooplankton and fish populations in the invaded areas. Biomass and biodiversity have both suffered. These changes have been so drastic that even populations of species high up the food chain such as the Caspian seal, and dolphins have been gravely damaged.
The sea walnuts are responsible for eating massive amounts of zooplankton and fish larvae. And this also puts competitive pressure on other organism that feed on the same sources. So M.leidyi is displacing natives from the ecosystem by both predation and competition for food.
The Consequences
Sea walnuts have been able to spread via waterways throughout many seas and saline inland seas in Europe and Western Asia.
According to Shiganova (2022)1, this map shows the chronology of Mnemiopsis leidyi invasion and dispersal in the seas of Eurasia.
They have caused major disruptions to a number of ecosystems and also to human communities in this region.
Case study - The Black Sea
The Black Sea is bordered by the countries of Russia, Ukraine, Romania, Georgia, Bulgaria, and Turkey, and has an area of 436,402 sq. km
Changes in the Black Sea ecosystem over the last 70 years
Controlling the invaders
Biological control of sea walnut numbers has been tried with some success.
As this pest can easily spread via the discharge ships' ballast waters, some tighter control of this practice is warranted.
The most prominent parasite of the sea walnut is a sea anemone named Edwardsiella lineata. Little research has been done on whether this parasite has the same set of wide tolerances of its host. If it does not, only selective areas could use it as a biocontrol agent.
Beroe ovata is another member of the Ctenaphora phylum and it feeds on M. leidyi almost exclusively.
In the video below, the eating interaction can be seen at about the 50 second mark.
Dig Deeper
- Temperature and Salinity Affect Development of the Parasitic Sea Anemone Edwardsiella lineata Potentially Limiting Its Impact as a Biological Control on the Ctenophore Mnemiopsis leidyi
- Population Dynamic and Distribution of Mnemiopsis leidyi in the Southern Black Sea
- Bacterial degradation of ctenophore Mnemiopsis leidyi organic matter
- https://www.researchgate.net/figure/Chronology-of-Mnemiopsis-leidyi-invasion-and-dispersal-in-the-seas-of-Eurasia-sources_fig1_365444440 [↩]