THE EASTERN MEDITERRANEAN
ECOLOGY & THREATS
Though it may be the largest sea, and provides a home to over 17,000 marine species, the Mediterranean actually makes up less than 1% of the global surface area of the world's oceans. As a semi-enclosed, highly saline habitat, it is home to specially adapted species and has the world's highest rate of endemism; 20-30% of Mediterranean species are found nowhere else in the world. It is also an important breeding and nursery ground for a host of the well-known iconic species, like bluefin tuna, a distinct population of Mediterranean fin whales, and it is even suspected to be a stronghold for the enigmatic great white shark.
​
Unfortunately, this unique ecosystem is struggling. A meagre 0.04% of the Mediterranean's surface is covered by no-take fishing zones despite 75% of Mediterranean and Black Sea stocks being fished at unsustainable levels. Over the span of six decades, between the 1950s and the early 21st century, the Mediterranean also lost 41% of its top predators, and is now the most invaded marine ecosystem in the world, with more than 600 established non-native species. The Eastern Mediterranean Basin is especially vulnerable to the potential threats posed by alien species, due to its man-made connection with the Red Sea via the Suez Canal.
​
POSIDONIA OCEANICA
Diving in the Mediterranean, we are lucky enough to see the dark green leaves of lush Posidonia oceanica meadows swaying with the water beneath us. This species, also called Neptune grass or Grass of Poseidon, is endemic to the Mediterranean and is one of the most important and biodiverse marine habitats in this sea. These dense meadows serve as an important home, nursery, and breeding ground for a huge range of marine vertebrates, invertebrates, and even other plant life. Not only are they valuable to marine animals, but also to us. Growing very slowly, over hundreds of years this species builds huge biogenic reefs that protect our coastlines from storm and wave action, and also store tonnes of sequestered carbon in its rhizomes. In a year, the meadows along the coastline of Crete alone absorb the equivalent carbon emissions of around 4,100 passenger vehicles. A square metre of healthy Neptune grass can also produce 14-20L of oxygen in a day.
​
A lot of commercially valuable species rely on Posidonia oceanica as a key habitat, but this is also where a lot of its threats come from. Fisheries trawling Neptune grass meadows for catch can destroy these fragile natural reefs, which then only grow back at a rate of up to 2cm a year. Mediterranean countries are also a popular tourist destination, and anthropogenic devlopment along the coast, as well as the dragging anchors of recreational boats, destroy large expanses of meadows. Along with climate change, these combined pressures have resulted in huge decreases in this keystone species. Projections estimate 70% Posidonia oceanica loss by 2050, and functional extinction by 2100 if current rates of decline remain constant. This is why we launched our seagrass restoration project in 2022, and campaign for the active conservation of Neptune grass across Mediterranean waters.
​
Painted comber, Serranus scriba
A beautiful sea-bass with intricate face markings, the highly territorial painted comber is a hermaphrodite; both male and female. They are a favourite among divers as they can point you towards octopus or eels by staring at them. They are often found around the outskirts of Neptune grass meadows, with big vertical bands that help them blend in among the leaves.
Mediterranean damselfish, Chromis chromis
In the late summer and early autumn, the neon blue, tiny juveniles of the endemic Mediterranean damselfish can be seen swimming in and around Posidonia oceanica. While adults form free-swimming shoals above rocky areas, juveniles rely on seagrass nurseries for protection and food.
Colonial sea-squirt, Trididemnum spp.
Seagrass leaves create a 3D habitat in patches of sand, and more room for other life to grow, hide, and breed. Other plants and animals, like this colonial tunicate in the genus Trididemnum, use Posidonia oceanica leaves as a substrate to host their life cycle.
SESSILE ANIMALS
Underwater, the line between plant and animal can seem a bit blurry. A lot of the most strange and exciting animals living in the Mediterranean have no eyes or legs, and can easily be mistaken for underwater plants. Shining a torch into the caves and crevices abundant in the rocky volcanic walls beneath the surface reveals a world full of colour hidden in plain sight.
​
Visibility in the Mediterranean is crystal clear, with anything less than 30m being considered a low-vis day. This is largely due to the abundance of filter feeding sessile organisms growing on our vast rocky substrates. Sponges, fan worms, and tunicates filter pollutants and excess nutrients from the water, and form clusters that add dimension to their habitat - changing the physical and biological characteristics of the environment as 'ecosystem engineers'. Some species of sponge and sea-squirt are even used by scientists as a source of anti-inflammatory and even potentially anti-cancer biological compounds.
Mediterranean fanworm, Sabella spallanzanii
Often called a 'flower of the sea', this beautiful fan worm is actually a species of polychaete worm with specially adapted feeding tentacles that form this pretty spiral. They filter suspended particles from the water, and can slow water flow. Their self-secreted calcareous tubes also form a habitat for a variety of epibionts.
Red sea squirt, Halocynthia papillosa
This bright red little animal has two siphons, an incurrent and an excurrent, and feeds by pumping water through its body via these siphons. It is a suspension feeder, removing organic particles and detritus from the water.
Sunset cup coral, Leptopsammia pruvoti
A species of stony coral that grows in solitary polyps, the sunset cup is a favourite of ours to find. Corals belong to a group of stinging animals called cnidarians, although many think they are plants.
Not relying on symbiotic algae like tropical corals, the bright yellow polyps can be found in dark caves and crevices. Extended tentacles filter particles from the water.
ICONIC INVERTEBRATES
Some of the world's most loved and iconic marine species are invertebrates like cephalopods and nudibranchs. The Mediterranean has both in abundance, with many guests coming to catch a glimpse of our bright pink Flabellina affinis, or see several different species of octopus, cuttlefish and squid at night. Our rocky shores and bright seagrass meadows make the perfect shelter for a wide variety of bold and beautiful molluscs.
​
Cephalopods are a highly evolved and intelligent group of animals. Some find it hard to believe they are in the same phylum as slugs and snails. Lacking the hard protective shell of their relatives, instead many cephalopods protect themselves by outwitting their enemies. It was believed for a long time that higher cognition and intelligence only evolved in the vertebrate lineage, but cephalopods have challenged this. They show remarkable problem-solving, pattern and face recognition abilities, as well as novel tool use, advanced spatial and perceptual learning abilities, and an incredible memory. Rather than being centralised in one brain like our intelligence, the arms of an octopus can even respond to stimuli, act, and 'think' independently of a central nervous system, and of each other.
​
Their soft-bodied nudibranch cousins have evolved different defence mechanisms. Some species of brightly coloured sea-slugs like our famous flabellina can take on the stinging ability of other animals. With a digestive system that extends into their cerata, aeolid nudibranchs can store stinging nematocysts in the appendages on their back for protection. Some sacoglossan slugs also 'steal' abilities from their food. Solar-powered slugs, they sequester and use chloroplasts from the algae they eat to be able to gain energy from the sun themselves in a phenomenon known as kleptoplasy.
Pink flabellina, Flabellina affinis
A celebrity of Mediterranean waters. the bright pink flabellina is a species of aeolid nudibranch. It feeds on sessile, stinging-celled hydropolyps, and can transport the stinging nematocysts into the tips of its appendages without being harmed. In this way, flabellinas protect themselves and gain the ability to sting even though it is not inherent.
Green elysia, Elysia timida
The tiny green elysia is only a few milimetres in size. You need to look very closely at algae covered rocks in the shallows to see these little creatures 'dancing' their way through sunlit waters. The green on the back of this slug is actually photosynthetic chloroplasts it has
'stolen' from the algae it eats.
Common octopus, Octopus vulgaris
A favourite among divers, the common octopus is one of the most intelligent and well-known icons of our waters. They have three hearts, a doughnut shaped brain, and eight arms capable of tasting, touching and moving independently of one another. With no bones and only one hard 'beak', they can squeeze into impossibly tiny spaces, change colour, and even adapt the shape and texture of their skin to mimic their surroundings.
OUR ROLE
Our committment lies in safeguarding the persistence of this special environment through proactive outreach efforts and restoration initiatives. Through education and research, we aim to protect the underwater world that we are so lucky to be able to dive in every day. We strive to make all of our guests environmentally conscious, offering completely free twice-weekly presentations on Mediterranean ecology and biology, teaching PADI AWARE specialties, our very own marine biology activities and partnering with initiatives like TRSHBG and Plastic Free Boy to collect and recycle marine debris and expand our message, as well as running important habitat restoration projects.
​
Find out more about the role of our marine field station.
​