Parasitic Crabs May Reduce Amount of Jellyfish Off Israel's Coast

To predict whether beaches will have fewer jellyfish, researchers still need to know how many crabs live on each creature and how fast it can heal, not to mention its nutritional value

Jellyfish on the beach in Ashdod.
Eyal Toueg

The nomad jellyfish arrives on the Israeli coast every summer and proliferates like mad. The good news for swimmers is that tiny crabs are living off these creatures, but the bad news is that it’s not yet clear if the little terrors will have any effect on the jellyfish population.

The connection between the crab and the jellyfish was made by Bella Galil, curator of the Steinhardt Museum of Natural History at Tel Aviv University, Baruch Rinkevich’s lab at the National Institute of Oceanography in Haifa, and an Australian researcher. Their findings have been published in this month’s issue of the journal Zootaxa.

Galil says that a year ago she took an ordinary sample of nomad jellyfish and saw many dark spots on their blueish domes. It turned out that between the jellyfish’s sting-cell-studded tentacles were tiny crabs each about 6 millimeters long, about a quarter of an inch. These crabs are known to be parasites on jellyfish but had never before been seen on the animal near Israel’s shores.

The crabs feed on the food the jellyfish collect and on their bodies As far back as the mid-19th century, scholars began exploring the relationship between crabs and jellyfish. The crabs have limbs close to their mouths that let them feed on their host’s soft tissue. They also have spurs on their other limbs by which they can cling to their slimy host.

Galil explains that the nomad jellyfish might be able to survive despite the crabs. Their survival may depend on several elements including the number of crabs that live on each jellyfish, how much food they can accumulate, how fast the creature is able to heal and the nutritional value of its tissues for the crab.

A nomad jellyfish off Israel's coast.
Shevy Rothman

According to Galil, because of the size of this species of jellyfish, it might be able to go on reproducing as usual and balance out the damage by the crabs, as long as the latter’s numbers remain low.

The nomad jellyfish is one of dozens of invasive species originating in the Red Sea that reached the Mediterranean via the Suez Canal. One explanation for the jellyfish’s success in their new environment is that they have fewer predators, parasites and disease to restrict their proliferation.

Nomad jellyfish do extensive damage to Israel’s coast. They sting swimmers, clog seawater pumps and harm local wildlife. Galil says that to limit such damage, the mechanisms by which they spread must be better understood, including the effect of the parasite crab.

But Galil adds that there is little evidence showing that natural enemies regulate the population of invasive species, especially invertebrates like jellyfish. According to Galil, more research is needed on the natural enemies and their affect in a changing environment.