A newspaper headline in April 2019 read: “Invasion of the
monster jellyfish! British resort harbour is inundated by swarm of huge sea
creatures the size of dustbin lids” [1]. This sensationalist journalism described
an influx of barrel jellyfish (Rhizostoma
pulmo) into Torquay harbour, where their appearance caused quite a stir on
social media.
R. pulmo (see above for a photoshopped image by Ales Kladnik) commonly
“swarms” in the Mediterranean and north-eastern Atlantic coast and their appearance
en masse often results in much curiosity and many subsequent questions:
(i) Why do these jellyfish form swarms?
(ii) Why do they come into shallow water?
(iii) Do they sting [humans] and what do they eat?
(iv) What feeds on them?
(v) Do humans eat jellyfish?
It isn’t easy to get answers, but a bit of searching gave me
some information:
(i) Whatever the primary cause of swarming, it usually results
in increased breeding success and the start of another cycle in the fascinating
life history of these animals. Male barrel jellyfish release sperm into the
water where they fertilise eggs that have been released by females. The resulting
larvae are called planulae and they are very small (just a few mm in length)
and covered with cilia (hair-like extensions of the body wall) that enable them
to swim in the plankton, along with the larvae and adults of many other animals.
Each planula transforms into a polyp when it encounters the sea bed and these
polyps may produce new polyps by budding, or they may form a strobila. The process
of strobilation produces mini-jellyfish (ephyrae) that, when sufficiently formed,
break away and begin a free-swimming life, eventually growing into the jellyfish
with which we are so familiar.
The life cycle of the barrel jellyfish, so little known to
most of us, is described in detail in a research paper by Fuentes et al. [2]
that contains this summary figure (individuals not drawn to scale):
(ii) Migration into shallow water increases the chances of
planulae being able to locate a substratum to begin their
transformation into polyps and it may also serve to concentrate adult jellyfish prior
to reproduction. It seems improbable that they can perceive when water is
shallow (they are very primitive animals with nerve fibres but no nervous system
[3]) and their location in shallow water may be a consequence solely of currents.
It is also likely that shallow water promotes the concentration of their
planktonic food; this, too, being effected by water currents.
(iii) Some jellyfish are very harmful to humans, with long
trailing tentacles bearing stinging cells that discharge barbs loaded with toxin.
This adaptation was present for hundreds of millions of years before humans
appeared on the scene and these “poisonous jellyfish” use their stings to
capture animals, especially fish, that are then drawn to the mouth by the tentacles.
Barrel jellyfish are quite different and they feed on plankton which are
captured by small stinging cells and also by mucus, cilia then being used to
transfer bound food packages (largely from the bell) to the mouth. Their stings
are not strong enough to penetrate human skin.
(iv) Barrel jellyfish are eaten in large numbers by
leatherback turtles (Dermochelys coriacea) - this is one of the reasons
why floating plastic bags are such a menace to these animals, as they perceive
them to be food. Some R. pulmo are taken by other animals, but they are not
subject to human “fisheries”, as are other types of jellyfish, especially in
the East.
(v) Methods of preparing
jellyfish are described by Hsieh et al. [4] and the process takes time and is
labour intensive. The first stage involves separating the bell from the oral arms
and both are washed in sea water. They are then sprinkled liberally with salt
and with alum; the former to reduce the high water content of the jelly between
the two tissue layers of the body (there are only two, most animals have three [3]), and the latter to act as an anti-bacterial
agent. The drying jellyfish are turned every few days and eventually become “crunchy
and crispy” [4]. The finished product can be kept for up to one year and strips
are commonly cooked as a stir-fry with sesame (see below).
Jellyfish could
become a major food in the future. They are abundant, tolerant of low oxygen
tension and high temperatures, and can be scooped up when they from swarms. However,
not all jellyfish are likely to be edible and some method of reducing the
labour-intensive drying process will be needed before we get commercial
production. When that is achieved, maybe we will all be eating jellyfish?
[2] Fuentes, V., Straehler-Pohl, I., Atienza, D., Franco,
I., Tilves, U., Gentile, M., Acevedo, M., Olariaga, A. and Gili, J.-M. (2011)
Life cycle of the jellyfish Rhizostoma pulmo (Scyphozoa: Rhizostomeae) and
its distribution, seasonality and inter-annual variability along the Catalan
coast and the Mar Menor (Spain, NW Mediterranean). Marine Biology 158:
2247-2266.
[3] Ruppert, E. E. and Barnes, R.D. (1994) Invertberate
Zoology. Sixth Edition. Fort Worth, Saunders College Publishing.
[4] Hsieh, Y.-H. P., Leong, F.-M. and Rudloe, J. (2001)
Jellyfish as food. Hydrobiologia 451: 11-17.
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