Do medusa jellyfish live alone or in groups?

Answer 1

Both. Most medusa jellyfish are planktonic (free drifters) that are largely at the whim of currents and tides of the marine environment. These somewhat arbitrary forces often result in individuals becoming scattered and therefore solitary.

However, several factors also allow gatherings of many individuals, as well. The life cycle of most jellyfish starts with sexual reproduction in a gathering of mature adult medusae in the fall, with asexual reproduction in the spring, resulting in a 'bloom' of jellyfish starting their medusa stage of life in relatively close proximity. Second, especially those jellies whose habitats are close to shore, tidal action can actually tend to gather jellyfish into 'swarms' within those protected bays and inlets. Third, most jellies do have an ability to direct their very limited propulsive movements towards or away from things that they can sense, such as light, salinity, and temperature. This can result in huge swarms of jellies that occasionally can even clog power plant cooling water intakes.

Btw, scientists tend to avoid the lay term 'smack' for a group of jellies. Although the gathering ability of the shallow-water moon jellyfish (genus Aurelia) has been studied and somewhat understood, we still have much to learn about most jellyfish gatherings, especially the method jellyfish use to gather into a swarm for sexual reproduction. It is obviously essential that the free drifting released gametes have the best chance of 'bumping into' the gametes from other individual jellyfish of the opposite sex.