 While sightings have been rare, when they are seen it is often as part of a massive swarm of the creatures, such as those that occurred in surface waters off the coast of Baja California and southern California in 1989, 1999 & 2010.These sightings seem to coincide with incidents of red tides, which consist of the zooplankton that black sea nettles feed upon.Black sea nettles are carnivorous.