Mediterranean: The Earth’s Mini-Ocean

Charting our journey’s first leg through one of the most eclectic seas in the world.

To define points of interest over the course of our journey, you can’t just take a map, point at a location that seems nice and go for a dive there. We won’t prevent anyone from doing that, of course, but your chances of seeing something ground-breaking or meeting a nifty jellyfish there would be, quite frankly, pretty slim. If you do make the necessary preparations, however, it becomes much more likely that you dive will be intriguing and productive.

The preparations themselves are a whole separate story. You need to account for a whole variety of factors in order not to miss anything important and get the most out of every stop. Some of these factors are: currents, winds, water temperature, the varying water composition at different depths, seasonal particularities of how phytoplankton flowers and a whole range of other issues. All these things need to be just right to create the best possible conditions for different types of plankton. And so, in order to prepare the map for the first leg of our journey, we dug through a huge pile of specialist literature, analysing them and studying the fauna of the Mediterranean Sea. Experts who have worked in these waters for many years also help us in charting our route.

On to the first leg of the journey then! We’ll set sail in Marmaris, which is in Turkey. The starting point itself means that we will have to cross the Mediterranean. You would think that this sea is so well-known by now that there is no point in detailed study. We even though of making this part of our expedition a “training stage” before our big start across the Atlantic! When we had a look at a few hundred scientific articles on this area, however, we discovered that gelata in these parts are still eagerly awaiting a group of scientists with nice big cameras (us!) There are no pictures of these species in their natural habitat and a large chunk of information about their lives is missing. Not to fear though, we’re on it.

So what’s so special about the Mediterranean Sea? After all, it constitutes only 1% of the total surface area of the Earth’s oceans. Well, for one, it’s the most isolated basin in the World Ocean. The Mediterranean is part of the Atlantic region, but its only connection to it is the narrow Strait of Gibraltar, the depths of which reach up to 320 meters. The sea’s complex composition and the continental shelves, islands, deep basins and trenches is encompasses make some scientists describe the Mediterranean as a small ocean. Indeed, it’s considered to be one of the most intricate marine systems in the world.

The Mediterranean Sea as seen from space.

The Strait of Sicily separates the Mediterranean into two large parts — East and West. Each of these includes even smaller seas: Aegean, Cyprus, Adriatic, Ionian, Tyrrhenian, Ligurian and others. These are all different from one another in a variety of ways which affect, among other things, the types of plankton which inhabit it. Out of these, one of the most important factors is the hydrological regime, or stream conditions.

There are many types of currents in the Mediterranean Sea: permanent main currents and secondary ones. The currents are separated into surface ones, mid-depth ones and deep ones. There are temporary currents brought on by the wind. Several zones of permanent upwelling exist — these are vertical currents that go from the bottom of the sea to its surface. There are also seasonal currents [1, 2, 3]. Here is a nice map showing what’s happening:

Ocean Current Flows around the Mediterranean Sea.
NASA/Goddard Space Flight Center Scientific Visualization Studio.

There are many other factors we need to consider for our studies in addition to the currents. A very important variable is seasonality. In the Mediterranean, plankton development peaks during the early part of spring. You can encounter only a few species during the autumn and winter [4]. The surge in plankton begins with phytoplankton flowering. In about a month, a mass of small zooplankton appears. These small animals feed on phytoplankton. After these animals reach a certain peak, the time for gelata comes, since they’re the next link in the food chain [5].

It’s important to know how the water’s chemical composition, temperature and salinity affect plankton. Water in the Mediterranean evaporates rapidly. Because of this, its salinity is fairly high, around 39%. This is higher than the oceanic average which is about 35%. Due to this aspect, the Mediterranean is home to many endemic species, which means some animals can only be found there. Italian planktonologist Ferdinando Boero notes that about 20% of Mediterranean hydrozoan jellies can’t be found in any other sea or ocean [6]. We also can’t ignore that fact that the Mediterranean basin is a densely populated area (22 countries line it!) Human beings affect the ecosystems they inhabit by default. All these particularities of the Mediterranean region affect the species composition and abundance of gelata.

Hydrozoan jellyfish Aglantha digitale

Plankton is also capable of daily vertical migration which is sometimes as significant as 2 kilometres. The Mediterranean is home to several species of siphonophores, jellyfish and tunicates which are capable of going 500 meters deep and returning to the surface in the space of a day. Therefore, in order to investigate the maximum amount of animals and encounter the more interesting inhabitants of the deep, we will need to conduct observations around the clock, during the day, as well as at night [7]. Some planktonic organisms cannot be found in surface waters no matter how hard you try. For example, crown jellyfish Periphylla periphylla and Paraphyllina ransoni, as well as the Rhizophysa filiformis siphonophore like to swim at depths of over 300 meters [8]. To observe these animals, we will use our remotely operated underwater robot.

Our route through the Mediterranean Sea. The red dots mark the start (Marmaris) and finish (Porto) points. Blue represent currents,
green arrows show our path. Silly diver boxes mean that the locations are interesting for diving and filming gelata.

At this stage of our preparations, we have already drafted a preliminary route through the Mediterranean with preferred diving locations. The main regions we investigate have intensive year-round upwellings: Eastern Aegean and Ionian seas, as well as the Gulf of Lion [3]. Black Sea waters seep through into the north-eastern parts of the Aegean Sea, passing by the Dardanelles, the Sea of Marmara and the Strait of Bosphorus. This really affects the planktonic species composition in the area, making it much richer than in other parts [9]. We will also dive in the central parts of circular currents. According to the studies we analysed, the most diverse and numerous quantity of plankton can be encountered within the vortexes [10]. There are about a dozen of such places in the Mediterranean. We are also interested in the regions which lie to the west and south-west of Crete, Sicily, Malta, Sardinia, Corsica and Balearic Islands since these areas have strong currents practically all year round and are also prone to upwellings, notably those caused by strong winds [2, 3]. We definitely need to explore the coast of Southern France and the North of Italy, particularly the Tyrrhenian Sea. Both these regions have specific seasonal conditions which affect the presence of a number of gelata favourably.

Of course, we cannot predict some factors. Weather condition will come as a surprise, for example. We therefore cannot guarantee that we will encounter certain gelatinous inhabitants of the Mediterranean basin. However, our preparations really increase our chances of meeting cool gelata. With all the hard work we have put into planning our journey, we’re sure that no storm will prevent us from having an interesting and productive sail through the Mediterranean!


  1. Circulation in the Mediterranean Sea. 2005. C. Millot, I. Taupier-Letage.
  2. Surface circulation in the Eastern Mediterranean using drifters (2005-2007). 2009. R. Gerin., P.-M. Poulain, I. Taupier-Letage, C. Millot, S. B. Ismail, C. Sammari.
  3. Seasonal patterns of wind-induced upwelling/downwelling in the Mediterranean Sea. 2001. A. Bakun, V.N. Agostini.
  4. Zooplankton assemblages and influence of environmental parameters on them in a Med coastal area. 1998. I. Siokou-Frangou, E. Papathanassiou, A. Lepretre, S. Frontier.
  5. Modeling of the plankton ecosystem of the Aegean Sea in relation to mixed layer dynamics. 2006. N. Skliris. S. Sofianos, A. Mantziafou, I. Keramitzoglou, N. Adaktilou, C. Kartalis, A. Lascaratos.
  6. Zoogeography and life cycle patterns of Mediterranean hydromedusae (Cnidaria). 1993. F. Boero, J. Bouillon.
  7. The pelagic domain Pelagos, the Whale Sanctuary. 2007. Edited by G. Relini.
  8. Medusae, siphonophores, and ctenophores of the Alboran Sea, south western Mediterranean. 1996. C.E. Mills, P.R. Pugh, G.R. Harbison, S.H.D. Haddock.
  9. Mesozooplankton distribution in relation to hydrology of the Northeastern Aegean Sea, Eastern Mediterranean. 2006. A. Isari, A. Ramfos, S. Somarakis, C. Koutsikopoulos, A. Kallianiotis, N. Fragopoulu.
  10. Meso- and macrozooplankton composition patterns related to hydrodynamic structures in the Ligurian Sea (Trophos-2 experiment, April-June 1986). 1995. S. Pinca, S. Dallot.