THE CALCAREOUS WORM
Beneath the surface of sheltered estuaries and lagoons, the calcareous tube-forming worm creates reefs from mineral secretions. From these white, stony tubes rise crown-like fans striped in rose and pearl like a underwater flower.
Habitat
Calcareous worms seek out the quiet places of the world, including harbours, lagoons and river mouths, where salt and fresh waters meet and unite. It settles on rocks, pilings and sometimes even the hulls of ships. It usually anchors in soft sediment with a love for shelter and calmness. It thrives in warm, temperate climates across several continents.
Diet
Being a filter-feeder, calcareous worms dine on the unseen; plankton, detritus and drifting organic matter are captured with its branchial crown. While feeding, these worms filter and purify the water around.
Behaviour
Each worm lives in a self-made calcareous tube, which is mostly thin, white and curved and form clusters with each other. The worms emerge when waters are calm with their crowns opening like blooming flowers. In the blink of an eye they vanish at the occurrence of ripple or shadow.
Conservation Notes
The calcareous tube-worm itself is not endangered. Quite the contrary: Several species of this kind have become invasive intruders in reefs they are not native to. There it influences the sedimentation patterns and water chemistry, with a risk of decreasing the phytoplankton occurrence.
LIMPETS
Among the sharp rocks of the intertidal zone, where waves crash and recede with incredible force, one animal finds its perfect habitat: the limpet. A limpet is a small marine animal with a simple, conical shell that looks like a tiny, flattened volcano or a pointed hat.
Habitat
Limpets are found all around the globe and mostly inhabit shorelines. There are multiple kinds of limpets, varying in size depending on the region, with a maximum diameter of up to 12 cm. These mollusks anchor themselves to hard surfaces using a muscular foot, often in zones where waves are so brutal that few other animals can survive.
Diet
Even though they look simple, limpets are remarkably efficient grazers. Using a radula — a tongue lined with tiny teeth — they scrape diatoms, microalgae, and biofilm from rocky surfaces. Some species even consume cyanobacteria.
Behaviour
Some species create shallow holes in the rock, known as home scars. These serve as precise resting spots, to which limpets return after foraging by following their own mucus trail.
Conservation Note
While the specific effects of coastal pollution, habitat loss, and ocean acidification on limpets still need further study, these environmental factors are known to impact shell formation in marine mollusks — and may therefore affect limpets as well.
PADINA PAVONICA
Rimmed with white or gold, Padina is one of the few genera of brown algae that features calcareous deposits on its surface. In the shallow coastal waters of Croatia, especially around Istria and Dalmatia, species like Padina pavonica add both texture and ecological function to rocky shores and seagrass meadows.
Habitat
Padina pavonica prefers warm, shallow marine environments, typically between 0,5 and 10 metres deep. It grows attached to rocks, shells and other hard substrates in well-lit, low-energy zones. Padina is often found coexisting with seagrass beds and other macroalgae in clear, nutrient-poor waters. It is a good indicator of relatively undisturbed coastal ecosystems.
Structure
Padina belongs to the brown algae and is unique in being partially calcified, with calcium carbonate deposited on the outer layers of its fan-shaped thallus. This feature offers protection from herbivory and desiccation, especially in intertidal zones. Like many brown algae, Padina alternates between sporophytic and gametophytic stages in its life cycle.
Conservation note
Padina pavonica is not currently listed as threatened, but it is an important bioindicator of coastal ecosystem health due to its sensitivity. It relies on clear water and is vulnerable to eutrophication and acidification. As ocean acidification progresses, the alga’s ability to protect itself from herbivory is impaired.
PRAWNS
Beneath the gentle waves of the Mediterranean coast, a striped prawn moves gracefully over soft sediments, its long antennae waving through the water: Penaeus kerathurus, also known as the caramote prawn. Recognized as a key species in its environment, this prawn has a semi-transparent body marked with distinctive brownish or bluish longitudinal stripes, which help camouflage it on the seafloor.
Habitat
The caramote prawn inhabits shallow coastal waters, typically between 5 and 40 meters in depth, though it has occasionally been recorded as deep as 75 meters. It prefers muddy and sandy bottoms, often found near lagoons or river mouths, where nutrients are abundant. Its geographic range includes the entire Mediterranean Sea, parts of the Black Sea, and stretches along the eastern Atlantic coast, from Portugal down to Angola.
Diet
P. kerathurus is a nocturnal and opportunistic predator. It spends daylight hours buried in the seabed to avoid predators, then emerges at night to forage. Its diet includes a variety of benthic organisms, such as mollusks, polychaetes (marine worms), crustaceans, and echinoderms. This broad diet supports its adaptability and success in various coastal habitats.
Behaviour
This species displays sexual dimorphism, with females typically growing larger than males. It follows a nocturnal rhythm, hiding in the substrate during the day and becoming active at night to search for food. Its reproductive behaviorinvolves a precopulatory courtship, and mating occurs via indirect sperm transfer—a process common among many marine crustaceans.
Conservation Note
P. kerathurus is of high economic importance and is extensively fished across its distribution range. Although it has not yet been assessed by the IUCN Red List, there are growing concerns about the impact of overfishing and habitat degradation, particularly in more enclosed and sensitive areas like the Amvrakikos Gulf in Greece. Furthermore, the spread of non-native prawn species introduced through the Suez Canal may increase competition, potentially threatening native prawn populations like P. kerathurus.
THE BLACK-FACED BLENNY
In the quiet corners of rocky reefs, a flash of fire-red darts between shadows. Tripterygion delaisi, the black faced blenny. This vibrant triplefin ist a vivid symbol of life thriving at the water’s edge, where sunlight dances on stone.
Habitat
These fish inhabit shallow, rocky coastal waters and are most commonly found around 5-12 metres in the Mediterranean. They prefer crevices, overhangs and shaded rock faces, providing shelter and ambush spots. Their range spans the eastern Atlantic and throughout the Mediterranean Sea.
Behaviour
The black faced blenny is a carnivore, that primarily consumes small crustaceans like amphipods and copepods. Males defend small territories during breeding season, performing a distinctive figure-8 courtship swim. The male guards the eggs, which the female lays upon rock surfaces or algae.
Conservation note
The black faced blenny is considered stable by the IUCN due to its wide distribution and lack of major threats. However, its reliance on rocky coastal habitats makes it vulnerable to habitat degradation.
SEA CUCUMBER
In the twilight zones of coral reefs and sandy sea floors, the sea cucumber lies like a soft-bodied relic of a slower age. These echinoderms, with their leathery skin and elongated forms, appear unassuming but they are essential engineers of the benthic world.
Habitat
Sea cucumbers are found across a wide range of marine environments—from shallow tropical lagoons to abyssal depths over 5,000 meters. They favor soft sediment and coral rubble, particularly in regions rich in organic material. Many species inhabit Indo-Pacific reefs, while others adapt to polar seas and abyssal plains.
Diet
As detritivores, sea cucumbers consume organic matter deposited in sediments. Using feathery oral tentacles, they scoop or filter detritus, algae, and micro-organisms from the ocean floor. Their digestion plays a crucial role in nutrient recycling and sediment bioturbation.
Behaviour
Sea cucumbers often move slowly across the substrate using tube feet or by peristaltic motion. When threatened, many species expel sticky Cuvierian tubules or even their internal organs, a defense mechanism known as evisceration. Remarkably, they can regenerate lost organs within weeks.
Conservation Notes
While some sea cucumber populations remain stable, overexploitation, especially in Asia for traditional medicine and cuisine, has driven others into decline. Certain high-value species, such as Holothuria scabra, are now considered vulnerable.
CHROMIS FISH
In the sunlit upper layers of coral reefs, Chromis fish weave between branching corals in glittering shoals. These small damselfish shimmer in shades of blue and green, like mobile fragments of the sea itself.
Habitat
Chromis species flourish in warm, tropical and subtropical marine environments, particularly in the Indo-Pacific and Caribbean. Preferring shallow reef crests and lagoons, they seek refuge among Acropora corals and rocky outcroppings. These habitats provide shelter from predators and swift access to food carried by gentle currents.
Diet
Omnivorous and opportunistic, Chromis feed primarily on zooplankton suspended in the water column. Hovering just above the reef, they pluck copepods and other microcrustaceans from the drift. In doing so, they play a modest but essential role in controlling plankton populations and cycling reef nutrients.
Behaviour
Social by nature, Chromis swim in tightly packed schools, creating a united front against predators. By day, they venture upward to feed; by night, they retreat to coral branches. Males guard nest sites fiercely, tending to adhesive egg clutches deposited on coral rubble or rock. Their courtship involves flickering body movements and rapid ascents toward the surface.
Conservation Notes
Though not currently threatened, Chromis populations are sensitive to coral reef degradation. As obligate reef dwellers, their well-being depends on coral health and water clarity. Climate change, ocean acidification, and destructive fishing practices can all diminish the habitats they rely on.
SEAHORSE
In the seagrass meadows and quiet coastal shallows of the Adriatic Sea, the short-snouted seahorse floats upright like a drifting leaf. Its curved tail coils gently around stems, and its knobby head, crowned with a low crest, surveys the underwater grassland with slow, deliberate motion.
Habitat
The Adriatic seahorse prefers shallow, sheltered waters along the eastern coast of the Adriatic, especially areas rich in seagrass or algae. Found from lagoons and bays to man-made structures like harbors, this species thrives in slow-moving waters with soft sediment. It relies heavily on camouflage and calm surroundings to survive and feed.
Diet
A stealthy ambush predator, the seahorse waits patiently for tiny prey to pass. Using a long, tube-like snout, it sucks in small crustaceans, larvae, and plankton. It lacks teeth and a stomach, so it must feed constantly to survive. Its diet helps regulate microfauna populations in the seagrass ecosystem.
Behaviour
Unlike most fish, seahorses are poor swimmers. They rely on a dorsal fin to move and a prehensile tail for anchoring. Monogamous pairs may share small territories, performing daily greetings by changing colour or curling tails. Most remarkable, however, is the male’s role in reproduction: females deposit eggs into a brood pouch on the male, who then incubates and later expels fully formed juveniles.
Conservation Notes
Populations of the Adriatic seahorse face pressure from habitat loss, pollution, and disturbance from coastal development and tourism. Although not globally endangered, the species is regionally vulnerable, with limited data on population trends. Conservation efforts focus on habitat protection and improved monitoring in Croatian and Mediterranean waters.
written by Carina Zettel