Spider crabs riding sea nettles (jellies) - video

Jelly species fluctuate seasonally in Virginia waters due to their temperature tolerances and prey availability. The warmest months of the year feature the (stinging) sea nettle, the species many swimmers and beach goers are intimately familiar with.

An adult sea nettle Chrysaora quinquecirrha

Adult sea nettles begin to show up in shallow creeks around May depending upon water temperatures and invade the beaches by mid June and are usually gone entirely by late September. Though sea nettles are relatively passive swimmers, they are still very effective predators. Sea nettles use stinging cells called nematocysts on their long trailing tentacles that fire off when contacted to ensnare and incapacitate small fishes and invertebrates, as evidenced by the the sting people may feel when coming in contact with their tentacles in the water. The sting is powerful enough for humans to feel, but only to be irritating, to small prey they  are deadly. So it seems unusual that jellies would have other animals willing to swim in and among their tentacles, even ride on top of the jellies! Yet there are fishes and invertebrates who use jellies to their advantage. A significant percentage of the sea nettles we collected this year have had spider crabs in or on them, and the obvious question would be why? Or how did they get there?

 Juvenile spider crabs Libinia sp. removed by hand from our exhibit jellies 

A spider crab rides on the bell of a jelly

Spider crabs are known to ride on not only larger sea nettles but also cannonball jellies, another warm water jelly. They have been observed eating the jelly itself, but also picking at detritus the jelly may accumulate during its travels. Spider crabs are opportunistic omnivores so the strategy makes sense, but spider crabs are bottom dwellers and cannot swim, so how do they get there?

Spider crabs are often called decorator crabs for placing sponge or vegetation as camouflage

Spider crabs have zoae larvae, a planktonic swimming larvae which could easily latch on to a drifting jelly; an excellent refuge from predators, a mode of transportation, and a source of food. Spider crabs could also board their host jellies as fully formed, albeit tiny versions of the hard non-swimming form, through direct contact along the numerous bridges, piers, docks, pilings etc. where the spider crabs settle and grow and the jellies often pass against in their travels. Once they are on the jelly, they can hitch a free ride, feed and simply drop off whenever necessary.

A juvenile harvestfish (about the size of a nickel) we caught under a nettle

There are also fishes who also take advantage of jellies as floating food/shelter. We often witness harvestfish Peprilus alepidotus or closely related butterfish Peprilus triacanthus swimming among the nettles' tentacles. Both these species have a more obvious relationship with the jellies, they use the jellies for protection against larger predators and they eat the tentacles. In both cases, the lowly jelly provides food and shelter for other species which may somewhat improve the image of a jellies being simply a pest.