While the lobster young seem to successfully ward off jellyfish defences, how do they digest their toxic prey?
Many species of lobsters feed on jellyfish. How do they avoid getting knocked out by their prey’s venom? Japanese researchers may have finally found the answer.
Jellyfish trail their long tentacles in the sea to snag small crustaceans. Each tentacle is a streamer of venom tubules armed with barbs. As soon as a barb touches a shrimp, it jabs the body and the tubule of venom explodes, letting it seep into the vulnerable innards. Their trigger-happy venomous arms ought to make jellyfish invincible. But they are eaten alive by phyllosomas, the larvae of lobsters.
These tiny flat-bodied predators have long legs with pointed tips, the better to latch on to the gelatinous bell of jellyfish. From this safe perch, they haul up the dangerous tentacles one at a time and consume them. Jellyfish secrete copious amounts of mucus to swamp the larva’s breathing systems. To rid themselves of this suffocating substance, the phyllosomas groom themselves with a pair of long legs. Until they completely devour the jellyfish, they use their prey as shelter and to surf the high seas. While the lobster young seem to successfully ward off jellyfish defences, how do they digest their toxic prey?
Lobster digestive systems have three parts. A gastric mill in the foregut grinds food into finer particles. The midgut is the main digestive organ, and the hindgut performs the excretory functions. The foregut and hindgut are lined with the same hard chitin as the body surface. This armour plate prevents venom from permeating the soft tissues. If phyllosomas have an Achilles heel, it has to be the unarmoured and vulnerable midgut.
Other creatures like aeolid nudibranchs, or sea slugs, also eat venomous sea anemones and jellyfish without suffering any ill-effects. They swallow nematocysts, or the venom apparatus, without letting them explode. Whole tubules slide through the digestive system and move to the top of their bodies. If any predator tried to take a bite of the soft slugs, it would have to contend with the barbed nematocysts just below the surface. Thus sea slugs have aced the defence game – not only do they render their prey’s venom delivery system useless, but they deploy it for their own use.
However, the phyllosomas’ digestive system is different. It operates like a filter-press, squeezing the body fluids of their prey and sieving it through the digestive gland. This filter prevents large nematocysts from travelling into the gut of lobster young. If they squeeze the venom out of the nematocysts, are they in danger of being envenomed?
Japanese researchers from Tokyo University of Marine Science and Technology and Hiroshima University bought an egg-bearing female smooth fan lobster from a fisherman. When the young hatched, they fished them out of her tank and reared them.
Smooth fan lobsters don’t look like the spiny or rock lobsters we see in restaurants in India. No more than half a foot long, they live along the sandy coasts of the western Pacific and east Africa.
When the larvae were about 30 days old, and between a centimetre and two in length, the researchers began the experiment.
After starving the phyllosomas until their digestive tracts were empty, the researchers fed each with a 10-foot-long tentacle from Japanese sea nettles, a species of jellyfish, fished from the Sea of Japan. The larva excreted thread-like feces that the researchers suctioned from the bottom of their tanks. They peered at the droppings through a microscope only to see the nematocysts were empty of venom. A film-like coating called peritrophic membrane wrapped the feces.
Although the researchers suspected that phyllosomas may produce the membrane, they were surprised. “No nematocyst tubules were observed on the outer surface of the membrane,” Kaori Wakabayashi, the main author of the study, told The Wire.
The membrane cling wraps food so it doesn’t touch the surface of the vulnerable midgut. The researchers say that this barrier probably saves the lives of phyllosomas.
However, when venom is swallowed, the degree of risk it poses to the organism is minimal. Venom is toxic when injected under the skin. In the stomach, digestive enzymes digest it without causing any harm to phyllosomas.
In that case, why do lobster larva secrete peritrophic membrane at all?
“We think that the membrane protects the surface of the digestive tract from the nematocyst stinging,” says Wakabayashi. “We do not know when the nematocysts discharge during the process of digestion, but some of them possibly discharge in the digestive tract. Phyllosomas seem to be capable of digesting jellyfish venom, but they will be killed once the venom comes into the other tissues and/or organs such as muscles.”
Many mammals like mongooses, hedgehogs, and opossums are immune to snake venom. Could the blood of phyllosomas include factors that neutralise jellyfish venom?
For this part of the experiment, researchers used the large bluish Nomura’s rhizostome jellyfish. They made a slurry of the creature’s short tentacles, and after centrifugation and filtration, they extracted crude venom. They injected this potent extract into the abdominal muscle of ten phyllosomas. To control for other factors, they injected a phosphate-buffered saline solution in others. Nine of the ten phyllosomas that got a venom injection died. All the ones that got the phosphate buffer survived. The researchers conclude that phyllosomas are not impervious to jellyfish venom.
“It is not clear to me exactly what dose of active toxin was administered to the phyllosoma in the experiment and whether this represented what might result from consumed nematocysts releasing toxin into the alimentary canal or even piercing the wall of the alimentary canal,” says Andrew Jeffs, a lobster biologist at University of Auckland, New Zealand. “The crude extract might also have contained other materials from the jellyfish other than toxins that might have triggered the observed response in the phyllosoma.” Jeffs wasn’t involved with the study.
Without the peritrophic membrane, barbs on the nematocysts could puncture the phyllosomas’ midgut. If jellyfish venom came in contact with such ulcers, phyllosoma lives would be under dire threat since they have no immunity. The peritrophic membrane protects the lobster youngsters not by isolating venom but by protecting the gut walls.
“The significance of this study is that researchers have to a large extent ignored the role of jellyfish and other gelatinous animals in food webs and the cycling of nutrients and carbon in oceans,” says Richard O’Rorke, an environmental geneticist at the University of Auckland, New Zealand. “Gelatinous organisms are a massive component of the pelagic ecosystem but are too often overlooked in studies and can be treated as a nuisance that clogs gear. This publication goes a step further than considering jellyfish as a component of the food chain by attempting to answer how they are ingested by members of the food chain.”
The paper was published in the journal Plankton and Benthos Research in August 25, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.