About a year ago I was in the Gulf of California, Mexico, studying the whale sharks that aggregate there each spring. While patrolling the area of Bahia de La Paz by panga looking for whale sharks, we found ourselves in the midst of an amazing number of what appeared to be bright blue floating softballs. Lifted briefly out of the water, they were beautifully dense and marked with small black spots. We later learned these were blue cannonball jellyfish (Stomolophus meleagris), a common area resident, but thought little about finding them in the whale shark habitat.
It seems we should have. A recent paper by Lopez-Martinez et al reports the frequent co-occurrence of whale sharks with cannonball jellyfish in the Gulf of California. Stomolophus meleagris is an abundant species in the Gulf, and it occurs in large numbers in certain coastal areas. Cannonball jellies are actively fished in the region, with landings on the Eastern shore of the Gulf of 43,000 tons in 2017. They are shipped for food to some Asian countries, and also collected for the aquarium trade and for certain pharmaceutical uses.
The authors describe repeatedly finding whale sharks feeding among large numbers of jellyfish (200+ individuals) in Bahia de La Paz. Cannonball jellies are also planktivorous, feeding on the same nutrient source as the whale sharks. This suggests the two species may compete for food, but can the jellies consume enough plankton to impact the whale sharks? A single jellyfish eats less than one gram of plankton per day, but groupings of millions of individuals are known. Additionally, climate change is causing ocean warming that favors increases in the numbers of jellyfish. The collective plankton utilization of large numbers of cannonball jellies might constitute significant food competition for whale sharks.
Whale sharks may occasionally turn the tables on the jellyfish, however. While these authors did not observe whale sharks consuming any of the S. meleagris, whale sharks are known to feed on jellyfish in other areas.
The paper is: López‐Martínez J, Porchas‐Quijada M, Álvarez‐Tello FJ, Porchas‐Cornejo MA. 2018 Association of the whale shark Rhincodon typus with the cannonball jellyfish Stomolophus meleagris. Journal of Fish Biology. Accepted Manuscript. DOI:10.1111/jfb.13725