The Brainless Animal That Rules the Mediterranean

When we take you out to explore the crystal clear waters of Crete, your eyes are

immediately drawn to the groupers hiding in the crevices, the playful octopuses, or

the sea turtles crossing our dive group. Yet right under our noses is a still, quiet, and

totally fascinating inhabitant that we too often tend to ignore during our pre-dive

briefings. Far from being simple rocks or some kind of seaweed, sea sponges play an

absolutely vital role underwater and our team of instructors is convinced that they

fully deserve to become the true stars of our next dive excursions.

An Animal From Another Time

To begin with, they hide their game well because when we show them to you on the

seafloor, they are not plants at all but real animals. They are actually ultra-ancient

invertebrates that have been traveling our oceans for more than six hundred million

years, long before the first fish ever appeared. Their way of living is surprisingly

simple since they have no heart, no brain, and no eyes, possessing instead a

wonderfully designed porous body made of specialized cells that work as a team to

ensure their survival at the bottom of the Mediterranean. This skeleton which allows

them to stand upright against the currents varies from one species to another, with

some manufacturing spicules of limestone or silica, a kind of biological glass that

forms tiny invisible needles making the sponge abrasive and uneatables for most

predators. The historical Mediterranean bath sponge, which we sometimes cross

paths with in deeper waters, chose another strategy by developing a skeleton made

entirely of spongin, a flexible and soft protein that explains why humans have used it

since ancient times. Even more incredible, this organism possesses a regeneration

capacity unique in the animal kingdom, because if you completely separated all its

cells, they would be able to seek each other out, regroup, and rebuild a new

functional sponge right before our eyes.

The Crystal Lungs of Crete

If we are lucky enough to have such blue and transparent water around Crete for our

try dives and explorations, it is largely thanks to the daily work of these little animals.

Sponges function like real natural filters by sucking in seawater through their small

holes to catch the bacteria and microparticles they feed on, before pumping out

perfectly clean and clear water. A single medium sized sponge can filter several

thousand liters of water per day, guaranteeing that incredible visibility we enjoy on

every single dive. Beyond this cleanup, they manage what scientists call the sponge

loop by absorbing tons of invisible dissolved organic matter to recycle it into perfect

nutrients for the snails and crabs of the reef, while regulating the silicon cycle

necessary for the microalgae that produce the oxygen we all breathe.

The Secret Architects of the Reef

In addition to purifying our playground, sponges are superb architects for the marine

environment. Their cavities form perfect hiding spots and real nurseries for the

shrimps, crabs, and tiny blennies that we often have fun spotting together and that

are fleeing from predators, and all this activity ends up attracting bigger fish,

transforming our dive sites into bustling and lively places. To defend this habitat

without being able to move, the sponge has become a formidable chemical factory,

capable of releasing toxic substances to push back its invading neighbors, which are

molecules that fascinate modern medicine for creating new antibiotic treatments.

Sometimes, the rooming situation becomes total since some species host millions of

microalgae and bioluminescent bacteria with which they share their nutrients, lighting

up in shades of green or red in the darkness of the caverns we explore, while in

these dark corners, other species have even abandoned filtration to become true

carnivores, capturing small crustaceans using sticky filaments.

A Fragile Heritage to Protect

Diving in Greece is also a real journey back in time, because our country is the

historical birthplace of natural sponge harvesting, a tradition that goes back to

antiquity. In the past, local divers went down on breathhold to impressive depths by

simply letting themselves be weighted down by a heavy marble stone in order to

harvest these precious specimens used for bathing all over Europe. These animals

grow so slowly that they are among the oldest living things on the planet, with some

specimens living through centuries while only gaining a few millimeters per year.

Today, these creatures remain highly vulnerable, because a sudden kick of a fin or a

poorly dropped boat anchor can destroy decades of growth in a single second.

Added to this is the invisible threat of marine heatwaves which cause terrible necrosis

epidemics at the bottom of our sea, making the protection of this ecosystem essential

for the future of the Mediterranean. During your next outing with our school, take the

time to admire their incredible shapes and colors while keeping the good habits of an

ecoresponsible diver, by mastering your buoyancy and your buoyancycontrol to open

your eyes wide and avoid touching this treasure of glass and life.