Jellyfish, or sea jellies as they are now often called (clearly they are not fish) are amongst the most ancient of multi-organ animals. Fossils of jellyfish (or scyphozoans, to give them their scientific name) are found only rarely as they contain no hard structures within their bodies, which are 95% water. However, under the right conditions fossils of soft bodied creatures will form; current fossil evidence suggests they first evolved at least 500 million years ago.
The lion’s mane jellyfish (Cyanea capillata) common throughout the North Atlantic, epitomises this image of a large, slowly pulsing, gelatinous bell (or medusa) and long trailing tentacles that pack a powerful sting, but this is in fact only one stage of a complex life cycle. Lion’s mane medusae begin to appear in April or May in the Northern Atlantic, but are quite tiny at that stage. These jellies are voracious predators and grow rapidly through the summer. By August the medusae are commonly one third to half a metre across, with trailing tentacles many metres long. However there is considerable variability; large specimens have been reported at over two metres across with tentacles up to 37 metres long, though these generally occur within the more northern parts of their range. As they grow large in late summer they will often drift, under the influence of wind and tides, in to sheltered bays where they may aggregate in large numbers. This is when sperm is release and egg fertilisation takes place. In common with most scyphozoans (the taxonomic group to which jellyfish belong) the sexes are separate; lion’s mane jellies are either male or female. Sperm is released from the mouth of male jellies and drifts in the current, some reaching female jellies, where the eggs are fertilised. Fertilised eggs are stored in the oral tentacles of the female, where thy develop in to tiny planulae larvae. Once fully developed the planulae larvae detach and, after drifting for a short time, settle on the seabed. Here they metamorphose into a polyp, not dissimilar to tiny sea anemones or coral polyps (both of which are relatives of jellyfish). These polyps then grow, taking on a layered appearance until they resemble a stack of wavy-edged pancakes. Each one of these ‘pancake layers’ will then separate from the parent polyp, once again becoming free living and drifting with the currents. The ‘pancakes’, more properly ephyra larvae, will grow throughout the summer into the giant lion’s mane jellies and the cycle is complete. With a lifespan on only one year, during which they can grow to be as long (possibly even longer) than blue whale, lion’s mane jellies need to catch and consume considerable amount of prey. Each trailing tentacle is packed full of vast numbers of stinging cells, known as nematocysts. When touched these cells fire out a harpoon-like structure which pumps toxins in to the hapless victim (this is what causes the painful sting from jellyfish). These toxins incapacitate the prey, which is then drawn up towards the mouth of the jellyfish. A large lion’s mane may have over 1,000 tentacles trailing far behind them. Many SCUBA divers in Scotland and Scandinavia have experienced the situation where, having completed their dive on a sunken wreck and returned to the buoy line they planned to ascent to the surface, only to look up and see numerous lion’s mane jellies strung out along the line. As the current sweeps the jellies along so their tentacles catch on the buoy line, leaving the divers with the unpleasant prospect of ascending through thousands of jellyfish tentacles.
Not every creature lives in fear of lion’s mane jellies however. Leatherback turtles, the only species of marine turtle that can tolerate the cold waters these jellies inhabit, consume them with relish, apparently oblivious to the stinging tentacles. Lion’s mane jellies can make up 80-100% of a leatherback’s diet. When you consider that a full grown leatherback weighs up to 800kg and may consume up to its own weight in jellyfish daily (bear in mind jellyfish are 95% water) then that equates to pretty large numbers of jellyfish being eaten.
As summer wanes and autumn approaches the lion’s mane jellies begin to die. This provides a feeding bonanza for many scavengers. On the surface seabirds will peck away at the gelatinous bell, whilst those that sink are often torn to shreds by shore crabs (Carcinus meanus) and velvet swimming crabs (Necora puber).
At the other end of the scale these deadly tentacles can provide refuge to some unlikely creatures. Juvenile whiting (Gadus melangus) have long been known to swim underneath the bell of lion’s mane jellies, apparently unconcerned by the curtain of tentacles they weave between. In fact they have been observed to rush into the mane of tentacles when startled by predators. A series of fascinating experiments by the Swedish zoologist Erik Dahl in the late 1950s showed that, compared to other fish species, juvenile whiting were able to adapt their movements such that even when surrounded by tentacles they rarely came in to contact with them. Also, unlike other fish species, when they did brush against them it seemed to cause them little concern. Biopsies of the tissue of whiting where they had contacted tentacles showed that very few if any stinging nematocysts had fired into the fish’s body; this compared to hundreds per square millimetre for other fish species. We still don’t understand the mechanism behind this protection. So does the lion’s mane get anything in return for the refuge afforded the young whiting? Well another creature found on lion’s mane jellies is the tiny planktonic amphipod (a type of crustacean) Hyperia galba. Hyperia is, for the jellies, a rather irritating ectoparasite. It lives on the outside of the jellies’ bell, nibbling away at it. Now whiting don’t appear to like the taste of lion’s mane jellies, instead they are rather partial to planktonic crustaceans; in particular (you’ve guessed this already) Hyperia galba. It is these elegant little symbiotic collaborations that make nature so beautiful.
This article was originally published on my photography website blog www.colinmunrophotography.com/blog, but I’ve reproduced it here because it is essentially a marine biology blog.