Itchy jellyfish, appendicularians with luxury chalets, filtering barrels, and other extraordinary creatures
Jellyfish is perhaps the most popular plankton group, although I am not sure everybody would classify it as plankton. Jellyfish clusters together with other organisms to form what we call gelatinous plankton. In this hodgepodge, we can find chordates, such as appendicularians or salps, some fish eggs, jellyfish, and even some seaweed. Here, however, we will focus on those groups of metazoan zooplankton whose body structure is made up of some jelly. The main groups are appendiculars, pyrosomes, salps, doliolids, jellyfish, and ctenophores.
The appendicularians are no more than a few millimeters long and are shaped like a tadpole. Although they seem very simple and little evolved, these plankton animals, like other tunicates of (salps, doliolids, and pyrosomes), are of those nearest to us evolutionarily speaking (outside, of course, of fish larvae). They are chordates (they have a notochord or nerve dorsal cord), and with a bit of imagination, one could find some resemblance to them in the early stages of an embryo. But the most fantastic thing about these creatures is not this fact, but the house where they live. A few times a day, the appendicularians build a gelatinous house that is a highly efficient filtration device, allowing them to filter particles of thousandths of a millimeter. The houses are usually less than a cm long, and the appendicularians that live inside, with rhythmic beats of the tail, create water currents to swim while attracting food (small protists and bacteria); with this system, they can filter a few liters of water a day.
Salps and doliolids
Although they are different groups, salps and doliolids share many characteristics, and I will explain them together. Both are tunicates, like the appendicularians and filter-feeders. They look more or less like a barrel and can often be mistaken for a piece of clear plastic. They range from a few millimeters to a few centimeters in length. We often find them alone, but they can form colonies of several meters long. They inhabit virtually all seas and oceans, but in Antarctica, particularly salps, they play a key role in the ecosystem, consuming and packaging the algae that proliferate in the spring. Actually, in the Southern Ocean, they share relevance with the famous krill, to the point of talking about years of salps or years of krill.
Pyrosomes are one of the rarest and most difficult groups to find in gelatinous plankton. They are also tunicated, and consist of colonies of clonal organisms of a few millimeters, which joined by a gelatinous matrix can form structures from a few centimeters to about 20 meters long, always in the shape of a wind sleeve. All the individuals in the colony collaborate to swim and seek food. We can find them in surface layers of tropical seas, but also at great depths. As if all this were not enough, they are also bio-luminescent.
Everyone knows jellyfish. What not everyone may know is that they are close relatives of corals. In fact, many species during their life cycle go through a polyp phase (such as those of corals), sessile, from which small jellyfish (ephyra) will emerge (strobilize). There are many types, shapes and colors of jellyfish. According to the taxonomy, we have 4 classes: schifozoa, cubozoa, hidrozoa and staurozoa. The schifozoa would be the real jellyfish, with their umbrella (umbrella) and tentacles. Many of them sting with the action of stinging cells called cnidocysts. By the way, never touch a jellyfish; even if it is dead on the beach, its cnidocytes could still be functional. The Cubozoa, cubic in shape, as the name implies, and small but very dangerous. Many Australian box jellyfish, despite being a few inches long, can be deadly. A species of box jellyfish has been described in the Mediterranean, but while it can do a lot of harm, it is not as dangerous as those in Australia. Hydrozoans are colonial organisms, Portuguese man of war or tiny blue jellyfish that often invade the beaches of the Catalan coast are examples. One of the individuals in the colony acquires buoyancy and becomes the nectophore, others engage in reproductive functions, and others hunt and digest prey. A complicated colony where everyone has their role. Finally, we have the Staurozoa, not very common, small in size, and sessile.
The presence of jellyfish on the Catalan coasts has long been a topic of research and debate. The main question is predicting whether a year will be jellyfish or not? Well, the thing is not simple, because it depends on many factors. In Catalonia, perhaps the most common jellyfish in summer is the Pelagia noctiluca, which has a life cycle without a sessile phase, reproducing in the open sea, far from the coast. This suggests that their presence will depend on currents and whether or not open seawater can reach the coast. On the Catalan coast, these water exchanges between the open sea and the coast rely on a density front (like a water barrier) formed parallel to the shore by the currents that flow through it. This front is stronger or smaller depending on the freshwater inputs from the rivers. Years of heavy rainfall make the differences in salinity on either side of the front strengthen, and the front prevents jellyfish from entering coastal areas from the open sea. Drier years, or those in which, for whatever reason, less freshwater reaches the sea, make the density front weak and the jellyfish reach the beach. This is one explanation, but I guess there are others, and the presence of jellyfish on our coasts depends on many factors.
In our country, fortunately, jellyfish only represent a problem for tourism and certain water activities. However, there are places, such as the seas of Japan, where much more problematic species have proliferated, as they affect fishing activities. Nomura jellyfish, which have recently increased their abundance in these waters, can be more than two meters in diameter. There are so many that they often collapse fishing nets and the whole catch has to be thrown back to the sea. In addition, they compete with fish for food (zooplankton) when they are small and eat fish when they are grown up. Not a very good picture, indeed.
Ctenophores are similar to jellyfish, but apart from not biting (or not much), they have other characteristics that differentiate them. To begin with, its locomotion is due to eight bands of ciliated combs that beat together to generate displacement. They are bioluminescent and of various shapes, predominantly spherical with or without tentacles or ribbon-shaped. Perhaps the best-known species is Mnemiopsis leidyi, an invasive species that can wipe out fisheries wherever it is introduced. The crudest example is the Black Sea, where the species was accidentally introduced in the 1980s. In a decade, M. leidyi reached about 400 individuals per cubic meter and decimated local fish species. To remedy the ecological (and economic) disaster, another ctenophore, Beroe ovata, was introduced, which preys on M. leidyi. It seems to be working, and although M. leidyi has not been completely eradicated from the Black Sea, its abundances are under control. The problem is that for some years now, we have M. leidyi in the Mediterranean.
After this review, one wonders about the usefulness of grouping such different beasts into what we call gelatinous plankton. The life, food, evolution, and ecology strategies of each group we’ve seen are so different that this unification may not make much sense. Of course, they all look like jelly desserts.