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Jellyfishes: what are they and how can we identify them?

Jellyfishes generally are marine animals that, like anemones, gorgonians and corals, like the red coral of the Mediterranean, are part of the cnidarians. In this article, we will see what is a jellyfish and how can we identify the most common ones. Moreover, we will know its danger. If you arrive at the end of the post, you will find a little surprise. 

INTRODUCTION

Cnidarians are one of the most ancient animals that inhabit in the Earth, as they appeared 600 million years ago. They are characterized by the presence of a cells called cnidocytes, which have urticating organelles. It is thought that nowadays there are more than 9,000 species, classified in four classes: Anthozoa, Scyphozoa, Cubozoa and Hydrozoa. Despite they have a simple structure and functionality, they inhabit almost all the aquatic environments, mainly marine. There are two basic forms in the life cycle of a cnidarian: polyp, in which the animal is sedentary, with a tubular body and that reproduces asexually; and medusa, which can freely swim, with a bell-like shape and that reproduces sexually. There are organisms that only are one of this two stages, while others are polyp first and then medusa.

cnidarian body plan
Cnidarian body plan (Picture from College of DuPage BIO1151)

WHAT IS A JELLYFISH?

Like we have seen, jellyfishes are a morphological type of cnidarians and they don’t constitute a taxonomical group by themselves. Despite its shape is a little bit variable, they are much less variable than polyps because all of them live in a similar way. Almost all of them have a free life, but there are some cases in which they are retained in the polyp’s colony, acting as reproductive structures.

Its shape is bell-like, plate-like or umbrella-like, with a thick jellied layer. The external surface (exumbrella) is convex and the internal (subumbrella) is concave. Hydrozoa’s jellyfishes have the mouth in the central, lower part of the umbrella, in the end of a tubular extension called manubrium, while in the Scyphozoa this is very reduced. Tentacles hang from the umbrella and they are full of cnidocytes. Jellyfishes never form colonies, but they can live in shoals. Many people confuse jellyfishes with ctenophora, but with this features you can’t confuse them.

IN WHICH CLASSES ARE THERE JELLYFISHES?

Anthozoa never produce jellyfishes.

Scyphozoa constitute the group of the biggest jellyfishes. The fact that they don’t have velum is what allows to differentiate Scyphozoa’s jellyfishes from Hydrozoa’s jellyfishes. The margins of the mouth form oral arms, which can be very long.

Many hydrozoa produce jellyfishes. These are almost transparent and small. Different from Scyphozoa’s jellyfishes, they present velum in the margin of the umbrella, which is a withdrawal of the tissues. 

General morphology of an Hydrozoan's jellyfish (Picture obtained from Systematic Biology).
General morphology of an Hydrozoa’s jellyfish (Picture obtained from Systematic Biology).

Jellyfishes of the Cubozoa class have cube-like shape, with one or more tentacles in each edge. They are usually very poisonous.

KEYS TO IDENTIFY JELLYFISHES FROM THE MEDITERRANEAN SEA

Despite the main species of the Mediterranean Sea can be easily identified by their looks, here we are exposing a simplified dichotomous keys  in order to recognise the 8 most common species.  

  1. Jellyfishes with velum (Hydrozoa’s jellyfish)
    • Pelagic colony in which the individuals are specialized on doing different functions (Subclass Siphonofora)
      • The central part of the colony is a flatten disc with a jellied consistency (Order Anthomedusae): Velella velella
      • Centre without a disc-like shape (Order Siphonophora): Physalia physalis
    • Jellyfish with a little contractile umbrella and a very mobile velum:
      • Entire margin of the umbrella: Olindas phosphorica
      • Margin of the umbrella with vertical lines that are divided into lobes: Solmissus albescens
  2. Jellyfishes without velum (Scyphozoa’s jellyfishes)
    • Jellyfishes with just one oral opening:
      • Short tentacles: Aurelia aurita
      • Long tentacles: Pelagia noctiluca
    • Jellyfishes with the mouth plugged by tentacles:
      • Long tentacles: Rhizostoma pulmo
      • Short tentacles: Cothylorhiza tuberculata

JELLYFISHES OF THE MEDITERRANEAN

By-the-wind-sailor (Velella velella)

By-the-wind-sailors (Velella velella) are organisms with a diameter of the disc between 1 and 8 cm. This disc is circular or oval, blue and has an small sail. In the periphery, there is a ring of polyps with a tentacle-like shape. It means that they actually aren’t jellyfishes, but they are colonies with appearance of a jellyfish. This jellyfish is frequently seen in our coasts, whose danger is law, almost non-existent.

velella velella
By-the-wind-sailor (Velella velella) (Picture: Denis Riek)

Portuguese man of war (Physalia physalis)

The Portuguese man of war (Physalia physalis) present a buoyant part that measure 30 cm long and 10 cm wide, which is purple and transparent. In the submerged part, there are the tentacles, which are thin and long, so long that can measure 20-30 m (yes, metres!). Despite it is a rare species, it is highly dangerous due to neurotoxic, citotoxic and cardiotoxic toxins. Their bites are very painful and in some cases can produce death. Like the previous one, it is a colony of polyps, so it is neither a jellyfish.

physalia physalis
Portuguese man of war (Physalia physalis) (Picture from Madrimasd)

Olindias phosphorica

Olindias phosphorica is a jellyfish with a yellow and pink-blue umbrella, that present several channels towards the centre. Gonads are very patent and have a dun and reddish colouration. It has a high danger because its bite is painful, similar to a wasp.

Olindias phosphorica (Foto de Apena Team Torino)
Olindias phosphorica (Picture of Jordi Regàs)

Solmissus albescens

Solmissus albescens is characterised by the presence of 12-16 white tentacles and many quadrangular lobes. The umbrella is transparent and it looks like crystal. Measure between 2,3 – 3 cm of diameter.

Solmissus albescens (Foto de Jordi Regàs).
Solmissus albescens (Picture of Jordi Regàs).

Common jellyfish (Aurelia aurita)

Common jellyfish (Aurelia aurita) is an animal with an umbrella similar to a plate, with 25 cm of diameter, transparent but spotted in blue. It has 4 oral and long tentacles and other shorter in the margin. The four reproductive organs are purple-violet and has a shape similar to horseshoe. It is frequently seen, with a low danger because its poison is little toxic.

Borm (Aurelia aurita) (Foto de Alexander Vasenin, Creative Commons)
Common jellyfish (Aurelia aurita (Picture of Alexander Vasenin, Creative Commons)

Pink jellyfish (Pelagia noctiluca)

Pink jellyfish (Pelagia noctiluca) is the most frequent jellyfish in the Mediterranean. It can be recognized for the presence of a pink to red umbrella of 5-10 cm, from which hangs 4 oral tentacles and 16 marginal tentacles that can measure 2 m long. The surface of the umbrella has brown spots. Its danger is high because its poison is powerful, although it is not lethal. Curiosity: it is luminescent during the night.

pelagia noctiluca
Pink jellyfish (Pelagia noctiluca) (Picture of Jordi Regàs).

Compass jellyfish (Chrysaora hysoscella)

Compass jellyfish (Chrysaora hysoscella) is an animal with flatten umbrella that can achieve 30 cm of diameter, which is reddish white and with 16 brown strips. It has 4 oral and long tentacles and 24 marginal ones. Its danger is high, similar to pink jellyfish, despite is much less frequent than this.

Chrysaora hysoscella
Compass jellyfish (Chrysaora hysoscella) (Picture obtained from Daily Desktop Wallpapers)

Shiff arms jellyfish (Rhizostoma pulmo)

Shiff arms jellyfishes (Rhizostoma pulmo) are animals with an umbrella that measures between 10 and 40 cm of diameter, bell-like shaped, blue white and with a violet margin. They just have 8 oral, joined, not branched and blue white tentacles. It is frequently seen and its danger is high because it causes irritation and burning.

Rhizostoma pulmo
Shiff arms jellyfish (Rhizostoma pulmo) (Picture of Jordi Regàs)

Cotylorhiza tuberculata

Cothylorhiza tuberculata has a look similar to fried egg. The umbrella is flatten, measures between 20 and 35 cm of diameter, yellowish brown and with an orange protuberance in the middle. They have 8 oral tentacles covered with button-like appendixes in the end that are blue or white. Its danger is low and it is one of the most common.

Cotylorhiza tuberculata
Cotylorhiza tuberculata (Picture of Jordi Regàs)

BONUS TRACK: CITIZEN SCIENCE

The Institute of Marine Sciences (CSIC) is doing a research about jellyfishes, which is at the same time a citizen science project in the context of Seawatchers. If you want to collaborate, here there is the information.

REFEREES

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This publication is licensed under a Creative Commons:

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