The word “jellyfish” is a popular term defining what marine biologists call gelatinous
macrozooplankton. The word “gelatinous” refers to the general consistency of these animals:
their body is mostly made of extracellular matrix (often called mesoglea), i.e. the matrix that
holds cells together and that is present in all animals, including us, but that, in these
organisms, is the greatest portion of the whole body. Jelly refers just to gelatine. This body
architecture is shared by animals that are very far from each other, in terms of evolutionary
history. The fossil record tells us that true jellyfish are the oldest animals among those that are
still living today, being represented in fossils that date back to the Pre-Cambrian. They are
referred to the phylum Cnidaria (Cartwright et al., 2007). Vertebrates, including us, are
referred to the phylum Chordata, and some chordates, namely the Tunicata, are also members
of gelatinous macrozooplankton, with the Thaliacea and the Appendicularia. Gelatinous
macrozooplankton, furthermore, comprises also the Ctenophora, or comb jellyfish. The
representatives of these three phyla are the bulk of gelatinous macrozooplankton and,
together, make up what we call “jellyfish” (Boero et al., 2008). The following paragraphs
contain a textbook-knowledge account of the three phyla, summarizing the information that is
relevant for the scopes of this report.
Cnidaria
The true jellyfish are the planktonic stages of three cnidarian classes: the Hydrozoa, the
Scyphozoa, and the Cubozoa. Most Scyphozoa and all Cubozoa fall within the category of
macro- and even megazooplankton, since they are large enough, as adults, to be perceived by
the naked eye, ranging from 2 mm (e.g. some small medusae) to 2 m in bell diameter, and
several metres of tentacle length, of the largest medusae. Some Hydrozoa are macroplankters
too, but many species belong to the mesozooplankton, being smaller than 2 mm. Gelatinous
mesozooplankton is usually not perceived by a casual observer, unless when its
representatives reach high densities.
Jellyfish move by jet propulsion, contracting their bells, or umbrellas. The umbrella usually
carries tentacles on its margin and has a manubrium hanging in its cavity. The mouth is at the
end of the manubrium. The tentacles catch the prey and bring it to the manubrium.
Cnidarians do have stinging cells, i.e. cells armed with cnidocysts, little capsules containing
an inverted filament that can be everted to inject a venom into their victims (either preys or
predators or... us). With very few exceptions, cnidarian jellyfish are carnivores, and use their
cnidocysts to kill their prey that, according to the species, can be either other jellyfish, or
crustaceans, or fish eggs and larvae, or anything reaching a viable size for the predator. Some,
however, are microphagous or even contain zooxanthellae. Cnidarian jellyfish, also called
medusae, have complex life cycles that often involve a benthic stage: the polyp. Jellyfish life
histories often involve larval amplification. The adult medusae reproduce sexually, and each
fertilization leads to the formation of a planula larva (Fig. 1).
The larva settles and leads to a colony that can become quite large, feeding on other animals.
A single colony, through asexual reproduction, can produce thousands of small medusae that,
then, will grow to maturity. “Amplification” means that each fertilization event does not lead
to a single adult but, instead, to many adults, due to asexual reproduction in the polyp stage.
The sexually competent medusa is the adult, whereas the polyp stage, where the amplification
occurs, is a larva. Hence: larval amplification.
Many Hydrozoan species have suppressed the medusa stage and are sexually mature as
polyps. Whereas some Hydrozoans and Scyphozoans do not have a polyp stage, and spend
their whole life as medusae. The Hydrozoa produce medusae by lateral budding, the
Scyphozoa by strobilation, and the Cubozoa by complete metamorphosis of a polyp into a
medusa.
Besides medusae, the Cnidaria can contribute to gelatinous macrozooplankton as floating or
swimming colonies, such as the hydroids Velella and Porpita, or siphonophores like Physalia.
1.1.2. Ctenophora
Gelatinous macrozooplankton is usually equated to stinging jellyfish, and its presence causes
major concern about own safety in non-marine biologists, due to fear of potential stings.
Many members of gelatinous zooplankton, however, are not Cnidaria, and do not sting. The
Ctenophores (Fig. 2) do not have a bell and a manubrium, and do not move by pulsations, they
just share a gelatinous appearance with the Cnidaria. Ctenophores move by ciliary propulsion,
through what zoologists call “ctenes” or combs. Hence the popular name: comb jellies. They
3
can be a few centimetres, or even 50 or more centimetres, being globular, or similar to a
dirigible, or ribbon like. Ribbon like ones, of the genus Cestum, can move also by snake like
movements, but the other members of the group usually glide, appearing motionless and, in
spite of that, moving. Their bodies are characterized by iridescent glows that are caused just
by the flapping combs, the propulsors of the animal. Ctenophores have two tentacles armed
with colloblasts, cell organelles that, instead of containing a venom, as the cnidocytes of
Cnidaria, contain a glue that holds on
their victims. Like cnidarian jellyfish,
they also feed on other gelatinous
plankters, on crustaceans, or on fish eggs
and larvae, being comparable to true
jellyfish in their feeding habits.
Ctenophores have no impact on human
health, and cannot cause any direct harm
to us. Ctenophores are holoplanktonic
(some are benthic, but will not be
considered in the present account), there
whole life cycle taking place in the water
column.
1.1.3. Chordata
Pelagic tunicates (Fig. 3) are members of the
phylum Chordata; they comprise the Thaliacea and
the Larvacea, or appendicularians. The Larvacea
are of small size, but can be present in very high
quantities. The Thaliacea, namely salps, doliolids
and pyrosomes, are of much larger size, pyrosome
colonies and salp chains reaching several metres in
length. Pelagic tunicates are much different from
both Cnidaria and Ctenophora in their feeding
habits, they are filter feeders upon protists (usually
phytoplankton), bacteria and even viruses. Their
life cycles are holoplanktonic and involve both
sexual and asexual reproduction, with the
possibility of high biomass increases due to
formation of large colonies. Apparently, just as for
Ctenophora, the pelagic tunicates do not have
benthic stages.
Figure 2. A ctenophore: Leucothoea (art by
A. Gennari).
Figure 3. A pelagic tunicate: Salpa (art by A. Gennari).
4
1.2. The blooms
The whole functioning of marine ecosystems is based on blooms, i.e. on pulses of primary
and secondary production due to the sudden increase in the population size of some key
species. The spring bloom of phytoplankton, in temperate seas like the Mediterranean and the
Black Seas, is determined by a peak of primary production of planktonic protists (the
phytoplankton) that are usually diatoms or flagellates. The phytoplankton pulse is followed by
a zooplankton pulse that takes advantage of the phytoplankton. Crustaceans, especially
copepods, are the main representatives of herbivorous zooplankton. The zooplankton peak
sustains the rest of the food web, being predated upon by carnivorous plankters. Among these,
fish larvae and juveniles are prominent, eventually to become the well-known representatives
of nekton: the fish. The pathway phytoplankton → herbivorous crustacean zooplankton →
carnivorous zooplankton → fish (Fig. 4) is the backbone of marine production and sustains
also our exploitation of marine resources, through fisheries. The species forming the nodes of
this pathway are part of a system that functions due to production pulses (the blooms). If the
pathway is sustained, the ecosystem produces fish that, in their turn, realize complex
pathways within the fish universe. Small fish are fed upon by larger fish, and most of the
nekton seems to be self-sufficient. But this is just an impression. Primary production must be
at the base of food webs, and primary production is mainly the phytoplankton pulses. The
impression of self-sufficiency of the fish domain reveals its weakness if we consider fish as
life cycles, and not just as the adults we feed upon. Fish larvae and juveniles are often
carnivorous, but they feed on preys that are herbivorous: the copepods and other crustaceans
that rely on the phytoplankton pulses. An ecosystem cannot function with carnivores only!
Figure 4. The pathway phytoplankton → herbivorous crustacean plankton
→ carnivorouszooplankton → fish (art by A. Gennari, graphics by F. Tresca).
5
1.3. Ecosystem “malfunctioning”
The term “malfunctioning” is obviously anthropocentric. All ecosystems do function,
otherwise they would cease to exist. If they function so as to satisfy our expectations, they are
considered as functioning well, whereas if they cease to do so, then they are labeled as
functioning in a bad way (malfunction means just this: bad functioning).
Jellyfish are the oldest animals, among the ones that are currently present on the planet. They
were present since the Pre-Cambrian and are not so different from their ancestors. Having
passed through more than 500 millions of years of natural selection, with no big changes in
their body organization, these animals are simply perfect! Simple and perfect. They also
express their populations in pulses, like most of the representatives of marine systems.
Jellyfish blooms, thus, are a quite normal phenomenon. The evolution of highly efficient
animals, such as fish, however, probably posed a limit to their prevalence in the oceanic
realm, with the triumph of the phytoplankton → herbivorous crustacean zooplankton → fish
pathway that we like so much. A system based on pulses, however, is almost reset at each
seasonal cycle. Such systems have been called “lottery systems” (see Boero, 1994; Fraschetti
et al., 2003 for reviews). There is a “prize”, represented by the primary production pulse, and
the winners are those who better utilize it, channeling its energy into their representatives, so
as to build another pulse. For the fish to be the winners, their larvae and juveniles must tap
from the secondary production of crustaceans. Jellyfish compete with the fish larvae and
juveniles for the use of this resource. Furthermore, they can also feed on the eggs and larvae
of the fish. We have seen that jellyfish have life cycles with larval amplification (Fig. 1). They
can be produced in great quantities, so as to rapidly build huge populations. Hence: jellyfish
blooms.
The lottery game in marine systems is based on the match or mismatch of the secondary or
tertiary producers with the pulses that are at the base of marine ecosystems (Cushing, 1990).
If the jellyfish produce a pulse with a good match with the pulse of crustaceans, and the fish
do not, then the jellyfish can take over, and their bloom is reinforced. The bloom of jellyfish
will compete with the fish larvae
and juveniles and limit their
growth, but it can also impact
directly on the fish, since the
blooming jellyfish will predate
also on their eggs and larvae
(Moller, 1984). When this
happens, the phytoplankton →
herbivorous crustacean
zooplankton → fish pathway is
disrupted, with the onset of the
phytoplankton → herbivorous
crustacean zooplankton →
jellyfish pathway (Fig. 5).
Figure 5. The pathway
phytoplankton → crustacean
plankton → jellyfish (art by A.
Gennari, graphics by F. Tresca).
6
The fish, however, can rely on their “internal” pathways and most of them can stand the
failure of one cohort, since they are long lived and can spawn for several years. The loss of
one cohort can be buffered by the adult individuals that, usually, are invulnerable to jellyfish
or that even feed upon them. Jellyfish, instead, are short lived and the individuals that make
up a single pulse cannot persist and must reproduce successfully, starting from scratch, to
produce another pulse in the subsequent favorable season. Fish, instead, can “hold their
breath” and try again a year later.
When systems work in this way, jellyfish blooms are “accidents” that do not disrupt in a
radical way the functioning of the phytoplankton → herbivorous crustacean zooplankton →
fish pathway. Hence they can be disregarded, as they have been so far by fisheries biologists.
They have an impact, of course, but of limited entity.
The “jellyfish” considered here are the carnivorous ones, namely Cnidaria and Ctenophora.
The same pattern can be present also for herbivorous jellyfish, namely the Chordata. They
feed directly on the phytoplankton and when they are particularly abundant they compete with
the copepods, depleting the phytoplankton → herbivorous crustacean zooplankton → fish
pathway, with the production of a short circuit in it: the phytoplankton → herbivorous
gelatinous zooplankton pathway (Fig. 6).
At the end of their peak, pelagic tunicates usually contribute to what we call marine snow and
fall to the benthos, almost skipping the pelagic trophic pathways (besides the bacteria that
feed on them while they are falling towards the bottom).
Figure 6. The pathway phytoplankton → herbivorous gelatinous zooplankton
(art by A. Gennari, graphics by F. Tresca).
7
1.4. The grand picture
Marine ecosystems functioning, thus, takes place through three main pelagic pathways: the
phytoplankton → herbivorous crustacean zooplankton → carnivorous zooplankton → fish
pathway, the phytoplankton → herbivorous crustacean zooplankton → carnivorous
gelatinous zooplankton pathway, and the phytoplankton → herbivorous gelatinous
zooplankton pathway (Fig. 7). These pathways are not mutually exclusive, but one can prevail
over the others. Usually, the first one (ending up with fish) prevails and determines what we
consider as a “normal” situation (Fig. 4). The other two pathways, one ending up with
carnivorous gelatinous zooplankton (Fig. 5) and the other with herbivorous gelatinous
zooplankton (Fig. 6), from time to time can go through episodic success that, normally, cannot
disrupt the prevailing pathway, ending up with fish. These blooms might even enhance the
diversity in the nekton, as hypothesized above. The scientific literature is replenished of
records of “anomalous” blooms of gelatinous plankton that, traditionally, have been
considered as freaks in the functioning of marine systems. As a matter of fact, they are not
freaks, they are part of the manifold possibilities in which marine ecosystems work. The
evolutionary lineages interacting in these systems coexist since millions of years and can cope
with each other.
Figure 7. The three main pathways determining marine ecosystem functioning
(art by A. Gennari, graphics by F. Tresca).
8
1.5. The impact of gelatinous plankton on fish populations
Summarizing, the impact of gelatinous zooplankton on fish populations can be: i) positive,
due to a keystone effect that prevents the monopolization of overly successful fish species at
the expenses of others, so maintaining fish biodiversity high. This effect occurs when fish and
jellyfish coevolved in the same environmental context and if the jellyfish are abundant just for
short periods; ii) negative, due to predation on and competition with fish larvae and juveniles
(predation occurs also on fish eggs) if the jellyfish are not coevolved with the resident fish or
if the fish populations are not “healthy”, due to overfishing, and the jellyfish blooms are
abnormally large and long-lasting. A different kind of competition might be exerted by
thaliaceans, since they overexploit the phytoplankton and deplete resources for the crustacean
grazers that are fed upon by fish larvae and juveniles.
1.6. Measures and estimates of predation impacts of gelatinous plankton on
fish
The species of gelatinous plankton are in the thousands, and most of them are Hydromedusae
(see Bouillon et al., 2004; Bouillon et al., 2006), followed by the Scypozoa and Cubozoa (see
Arai, 1997), the Tunicata (see Bone, 1998), and the Ctenophora (see Harbison et al., 1978). In
comparison to the very high diversity of this compartment of plankton, the number of species
whose biology and ecology have been investigated is exceedingly small. For most of them we
barely know that they exist, and often even their life cycles are unknown.
These predators, furthermore, are very opportunistic since they are equipped with tentacles
armed with cnidocysts or colloblasts that can catch almost anything, from unicellular
organisms to much larger prey. Some are very specialized in their diets, but most of them feed
on anything they can find.
The study of the trophic role of gelatinous plankton, and especially the carnivorous one, is
made in two ways. The simplest one consists in collecting animals in the field and inspecting
their gut, listing all the food items they contain. Feeding rates are measured in the laboratory,
offering food to the animals and evaluating their clearing rates from a given volume of water
and the time of digestion of the offered prey. These studies have been made on few species
and at specific places (Tab I and II for Aurelia aurita). If a jellyfish species lives both in the
North Sea and in the Mediterranean Sea, as is the case of Pelagia noctiluca (Tab. III), the study
of its diet in the North Sea does not necessarily reflect its diet in the Mediterranean Sea, since
the available food items might be very different. So, what has been found at one place cannot
be automatically extended to all the places where a given species occurs.
