Moon jellies (Aurelia aurita) are probably familiar to anyone in Northern Europe or Northeast America who has spent time at the seashore. They are also known in British waters as common jellies, for very good reason. Moon jellies occasionally occur in dense aggregations, such as here, partly due to successful reproductive years and partly due to them being simply pushed together by wind and tide. Although they do have some control over their direction of travel, by pulsing movements of their bell, they are largely at the mercy of wind and tide. This can result in large numbers of them being pushed up against the shoreline, especially in embayments such as here, in Tobermory Bay, Isle of Mull, Scotland.

A swarm of moon jellies, Aurelia aurita, Tobermory Bay, Isle of Mull. @colinmunro

Moon jellies do sting, but the stinging cells (known as nematocysts) within their tentacles have barbs that cannot penetrate human skin. Consequently we can’t feel their stings. Moon jellies feed on a wide range of planktonic animals that can be immobilised and captured by their stinging tentacles. I’m writing this in mid-September. At this time of the year moon jellies are approaching their maximum size, commonly around 10 centimetres across, though some grow much larger. In another month or two (in UK waters) they will begin to reproduce. Moon jellies reproduce both sexually and asexually, at different stages of their life cycle. The medusa stage (that’s the jellies that we see floating around) reporduces sexually; the polyp stage (read on to find out about that) reproduces asexually by a process known as strobilating (I’ll explain) and simply budding to produce more polyps. let’s start with the medusa stage we’re familiar with. The male jellies release strings of sperm which are ingested by the females. The fertilised eggs then hatch as tiny larvae, which are brooded by the femalein their oral arms (underneath the bell). These planulae larvae fall to the seabed, where they attach and form tiny polyps (looking rather like tiny sea anemones, to which jellies are related). These polyps grow into long cylinder shapes, which then start to divide horizontally (and begin to look like a stack of plates). These ‘plates’ or ephyra as they are scientifically known, then swim off, one at a time, each one becoming a tiny new jelly – and this is what’s known as strobilating. If it survives long enough the tiny ephyra will grow to become a full size moon jelly in less than one year and repeat the process. Of course only a few will do so. Many of us will be aware that leatherback turtles each jellies, but leatherbacks are generally pretty rare. A much more common predator of moon jellies is – another jellyfish. The lions mane jelly is a pretty voracious predator of moon jellies. You can read about it in my marine biology blog here.