There are approximately nine hundred species, distributed throughout the planet's seas and oceans, where they play a fundamental role in the balance of all marine habitats. These are sea urchins, organisms as small as they are complex, which over hundreds of millions of years have been able to adapt to the most diverse living conditions in nature. However, they are highly sensitive to climate change and, due to their calcareous dermal skeleton, to increasing ocean acidification. This is without even considering the effects of indiscriminate harvesting for food on the most exposed and declining populations.

Sea urchins are invertebrates belonging to the echinoderm phylum, along with starfish, sea lilies, brittle stars and sea cucumbers. They share, at some stages of their life, the characteristic of pentaray symmetry and the presence of a dermal skeleton composed of calcareous plates that, in sea urchins, are welded together to form a rigid shell. The plates are also capable of regenerating if partially damaged. The dermal skeleton features the long, mobile spines characteristic of sea urchins, while the ventral ambulacral pedicels protrude from the sea urchin's abdomen, allowing the animal to move, thanks to an internal aquifer system "fed" by seawater.

Sea urchins are classified into regular and irregular echinoids. Originally, they were all regular, meaning they had a spherical shape, pentaradial symmetry, anus facing upwards and mouthparts symmetrically downwards, to allow the animals to graze on the (widely varying) seabeds where they live, as benthic organisms. Regular echinoids are still the most numerous, but starting in the Jurassic, a differentiation occurred. This gave rise to the irregular echinoid species, with heart-shaped or flattened shapes and a characteristic bilateral symmetry, with the anus and mouth positioned obliquely rather than mirroring each other. The mouth, however, is in contact with the sandy or muddy substrate, in which they burrow and take refuge.

Regular echinoids are characterized by a spherical theca that is externally divided into ten sectors: five ambulacral areas, from which the pedicels emerge, and another five interambulacral areas, from which the gametes emerge. The largest of these areas is called the madreporite and has holes to allow the passage of seawater. This serves to operate the aquifer system, which allows the urchin to move, activating the release of the numerous ambulacral pedicels from the theca, which may end in suckers for better adhesion to the substrate. In irregular echinoids, the ambulacral areas are called "petaloid" due to their distinctive petal shape.

The downward-facing mouthparts contain the animal's feeding organ. This organ is called "Aristotle's lantern" because the Greek philosopher was the first to identify and describe it. It consists of five calcareous plates, shaped like a pyramid with the base pointing upward, connected by muscle bundles. The five teeth, located in the pyramids, protrude through these plates. These teeth are used by the animal to scrape the substrate, literally grazing on rocky seabeds, among seaweed beds, or in seagrass meadows. The gills are located in the area around the mouth.

Sea urchins feed primarily on plants, bits of algae, or Posidonia oceanica leaves and other marine plants, but, depending on the food availability in their areas, they may also consume small animals or animal debris. The primary ecological function of sea urchins in the world's seas is linked precisely to their role in the food chain: by feeding on plants, especially algae, they prevent the proliferation that is harmful to the balance of coral reefs and coralligenous seabeds. They are also important as food for other marine animals. Given the hardness of their shells, sea urchins are preyed upon only by sharp-toothed fish such as gilt-head bream and white sea bream, but they can also be eaten by large crustaceans, mollusks, and even starfish, their relatives.

Males and females are identical, making them indistinguishable. Both release their gametes simultaneously into the sea, where fertilization occurs. The larvae, known as "plutei," are characterized by bilateral symmetry and have between six and eight pairs of arms equipped with ciliated bands with which they feed and move through the water column: they are, in fact, planktonic organisms. Only at the end of this phase, which lasts a few months, do the plutei settle on the seabed, where their adult life as benthic organisms begins.

It is usually the first bird that comes spontaneously to mind with the marine environment. A faithful companion during sea voyages or in coastal areas, but increasingly often, now, also a constant presence in inland areas and in large cities, where with its peers it is permanently settled near landfills or waste collection points.

It is an asteroid. However, it does not belong to the sky, but to the depths of the sea. Like any other benthic organism, it spends almost all of its life on the seabed, whether it is composed of sand, debris or rock. And in the green of the long leaves of Posidonia stands out, with the color that distinguishes and identifies it, the red starfish or, according to its scientific name, Ephinaster sepositus.

The first, the Sphyraena viridensis, is in all respects an “alien”, considering that its area of​​origin is the north-eastern Atlantic Ocean, although, in addition to being known as the yellowmouth, it is also known as the Mediterranean Barracuda, the sea in which it has spread only a decade or so, as a result of rising temperatures.

They are small marine animals. Extremely small. And yet, they have an enormous and unparalleled importance in the complex and delicate ecological balances of the oceans. And therefore of the entire planet, considering that the blue expanse occupies over 70 percent of its surface.

It is the largest. But also the most harmless among jellyfish. The Barrel Jellyfish,

A formidable jumper, its brightly colored livery makes it stand out on the blue surface of the sea.

It is a fish. But with such a particular shape that it is difficult to classify it as a fish.

Among the inhabitants of the sea, it is one of the most popular subjects for those who photograph the beauties of the blue planet. And it is one of the symbols of the Mediterranean, although it is also present in the Atlantic Ocean. Belonging to the Serranidae family, which has about eighty species, eight of which are at home in the Mare Nostrum, the Dusky Grouper (Epinephelus marginatus) is certainly the most widespread and familiar. Among the largest bony fish in the Mediterranean, carnivorous and formidable predator at the top of the food chain, it plays a delicate and fundamental role in the ecological balance of the sea. Therefore, the progressive decrease found throughout the Mediterranean basin represents a serious problem, confirmed by the "vulnerability" assessment that the IUCN, the International Union for Conservation of Nature, has attributed to the species.

Groupers are territorial animals, living alone in their burrows among the rocks, but it is not uncommon to find them nesting among the sandy seabeds where Posidonia and Zostera grow. It prefers to live between 10 and 200 meters, but some specimens have been found even at greater depths. It hunts its prey during the day, mostly cephalopods and fish. It is not aggressive with species that are not among its prey, but tends to be aggressive with marine animals that share its territory and with other male groupers.

The large head is a peculiarity of groupers. Like the large mouth, with a mandible protruding from the maxilla, equipped with canines and other teeth, inclined inwards, some mobile and depressible, functional to the variety of prey it feeds on. The body is robust. The color can vary during the breeding season and in particular situations, such as when the animal is scared. In general, the back has a coloration between dark brown and dark red with numerous gray or yellowish spots, distributed everywhere even on the head; the belly is lighter, tending towards yellow. The odd fins are dark brown with a white edge at the tip, the pelvic and pectoral fins are dark, yellowish at the base. The tail, with a rounded fan shape, is streaked with white. It has spines on the long dorsal fin and on the anal fin. It can reach one meter and twenty in length and an average weight of sixty kilos in the male. The female is smaller.

Groupers can live up to fifty years. They are protogynous hermaphrodites, meaning they are born female and reach sexual maturity at 5 years of age. Then, around 12 years of age, and in any case between 10 and 15, they become male. While this is the general “rule,” there are also particular situations, mostly related to the needs of the species, in which the sex change does not occur.

In the Mediterranean, it is during the summer season that grouper specimens temporarily give up solitary life and gather together for reproduction with other similar ones, in sites at a depth of between 15 and 30 meters. The species has been subject to excessive hunting, especially in the last century, which has led to its reduction everywhere, even making it rare in some parts of the Mediterranean where it was once very widespread. Since the adults and larger specimens, therefore the males, are preferred as prey, imbalances have been created in the population between males and females, thus interfering with the reproductive capacity of the species. But also the pollution of the sea and increasingly the effects of global warming represent threats to groupers. In recent decades, various protection projects have been dedicated to them and in any case in the Marine Protected Areas active protection is starting to bear fruit. Considered an endangered species in the Mediterranean and in the rest of the world, it is protected by the Bern and Barcelona Conventions. And it is a precious indicator of the ecological balance of the sea: where there are groupers, it means that the food chain works, that there is biological variety, that protection is effective.

The Italian Marine Protected Areas also include the rarer Red Grouper (Mycteroperca rubra), reddish brown, darker on the back and much less on the belly, with dark lines and white spots on the sides, and the Golden Grouper (Epinephelus costae), also known as the ductus, two-coloured with a brown and beige body with a characteristic ovoid golden spot near the gill cover and very visible bands on the sides.

It is the natural “secret” of the red-pink color of flamingos.

A tiny animal of that same color, whose small size is inversely proportional to its presence on earth. In fact, the maximum length of an adult specimen of Artemia salina, the species that appeared on the planet in the Triassic, is just one centimeter, therefore a good one hundred million years ago, when dinosaurs also inhabited the planet. And since then, the tiny crustacean, the only representative of the Artemia genus and the Artemiidae family, has retained its main morphological characteristics, thanks to which it has had the ability to adapt to the most varied and extreme environmental conditions and to become a true model of resilience in the animal kingdom.

Artemia lives in every part of the globe, so it is a so-called cosmopolitan species, except in the oceans, and survives between 5 and 40 degrees in almost fresh to very salty waters, having as its habitat the more or less brackish coastal ponds and salt marshes, from which the epithet of its scientific name "salina" derives. The safest places, since the high salinity keeps away most of the potential predators. But more commonly, due to its appearance, it is known as sea-monkey.

The body features include the head, with three eyes and four antennae; the thorax with eleven limbs and a series of lamellae, which allow it to breathe, but also to get rid of excess salt introduced with the water from which it filters nutrients; and the almost transparent abdomen from which the intestine can be seen. The species presents a notable sexual dimorphism, since the males have a larger jaw. Another characteristic of the species, to facilitate nutrition through water, is swimming with the back facing downwards.

Since they are extremely small, they feed on bacteria, phytoplankton and all possible nutrients that do not exceed the size of 50 microns. And it is precisely from food that their particular pink and orange color comes, in fact the small green algae Dunaliella salina (which also lives in the same habitats with a high percentage of salinity) that they feed on, produces beta-carotene. The substance that then ends up in the diet of flamingos that, in turn, feed on Artemia. The more or less pronounced pink or red-orange color of the waders that populate the same brackish environments depends, in fact, on the percentage of Artemia present in their diet: the more crustaceans they swallow, the more brightly colored their legs, beak and plumage are, making them unmistakable in their beauty.

Artemia crustaceans have a lifespan of around seven hundred days, during which they go through various larval stages, before becoming adults and, therefore, entering the reproductive stage, which is another highly characteristic element of the species. Artemia reproduces sexually at temperatures between 20 and 35 degrees and it is necessary for there to be water in the ponds in which it lives. When this condition is lacking, due to evaporation of the water and in periods of prolonged drought, in dry ponds Artemia reproduces by parthenogenesis, laying cysts, or eggs with a particularly resistant shell, which remain "suspended" even for long periods (cryptobiosis), even up to ten years, ready to hatch as soon as the environmental conditions are favorable again. From the eggs, larvae called nauplii of a few millimeters are born, which draw nourishment from the yolk sac and, therefore, can remain alive without the supply of other nutrients for two or three days after hatching. Nauplii are prey for numerous species of marine animals, which carry out a considerable selection of organisms before they develop into the subsequent larval stages. Furthermore, when a reproductive emergency occurs, for various reasons, it can also reproduce asexually.

The versatility of this species, its enormous capacity for resilience in adapting to the most varied environmental conditions in which it lives, even the most extreme and least favourable, has made it the totem animal of the aboriginal populations of Australia.

Its common name in several languages ​​including Italian associates it with the Gorgons. And one of them, Euryale, is evoked by the order of the Euryales to which it belongs. But it is certainly not a monster, as the mythological figures with hair formed by snakes were considered. Quite the opposite.

The Gorgon star or Astrospartus mediterraneus is a marine animal of astonishing beauty. A living lace, when at night it completely opens its spectacular tentacles to get food. And it is in that form that the reference to the Gorgons appears evident. And the origin of its scientific name from aster, star, and spartus, shrub, for the numerous ramifications of its arms. Instead, during the day, when it is not busy feeding, and folds its tentacles, it forms a sort of ball or basket that justifies the name in English of basket star.

The Astrospartus lives at depths between 30 and 800 meters in the western Mediterranean and the North Atlantic, along the coasts of Spain and West Africa up to Senegal. Widely distributed off the African coast of the Mediterranean, it is less so around Italy, where, although it is present from the sea of ​​Sicily to those of Tuscany and Sardinia, it is quite rare to meet it. Since it is a photophobic animal, which avoids light, it is active at night and prefers poorly lit and deep sites.

With its light color, which can vary between gray and yellow with pink hues, it stands out against the dark red of the red gorgonian (Paramuricea clavata), also a filter-feeding animal, on which it prefers to settle, but it also does so on the white gorgonian (Eucinella singularis) and on various species of sponges. To attach itself to the animals it hosts, it uses small hooks present at the ends of the numerous branches of its ten, thin tentacles. They all start from the central body, formed by a disk with a maximum diameter of around 8 centimeters, in which the five symmetrically arranged sectors can be distinguished. They are the distinctive element of the Echinoderms, of which the Astrospartus is also a part.

The body has a mouth, to which the tentacles bring food, as they capture it by filtering the plankton present in the water column, but also by wrapping and "imprisoning" small fish and marine animals. The width of the crown of tentacles with all their ramifications, when fully unfolded, can reach 80 centimeters in diameter, thus tenfold the size of the central body and serves to procure as much food as possible. Then, when the need to feed is satisfied, the scenic offshoots are folded one after the other, until they form the basket shape typical of the resting phase of the beautiful sea gorgon.

He never gives them up. And every time he moves to a larger shell, he takes care to prepare an adequate accommodation in the new home also for his faithful traveling companions. And life companions. The only ones with whom he agrees to share the “domestic” space in which he notoriously does not like intrusions, not even from his peers.

It is no coincidence that, together with the recovery shell adapted from time to time to the new measurements of its growing body, the Paguro Bernardo carries with it that nickname of “hermit” borrowed from its solitary existence. The only exceptions are the anemones anchored on its shell, belonging to the species Calliactis parasinica, which for this consolidated symbiosis is naturally associated with the Pagurus bernhardus.

Community life, the result of a long and complex adaptation, has advantages for everyone. For the hermit crab, which always carries them with it, those polyps represent a precious life-saving protection. Sea anemones, in fact, are cnidarians, anthozoans, hexacorals. Cnidarians, as they have stinging cells that, placed on white filaments called acontia, come out as soon as they are attacked by predators. In this way, the anemones defend themselves and also the hermit crab that hosts them. Furthermore, their presence on the shells, favors their camouflage on the seabed, helping the hermit crab to hide more effectively from its attackers.

Even for sea anemones, the advantage of cohabitation is notable. First of all, thanks to the hermit crab that carries them on its shell, they can move around without any effort and with movement they have a greater chance of feeding. And then they can always make use of the leftovers from their host's meals. The host really takes care of them, willingly accepting their presence on the shell in which it is installed. And furthermore, every time it is forced to change it, it takes care to move one by one the anemones it lives with onto the new shell. Bernardo and his anemones are one of the most astonishing examples of mutualistic symbiosis that is established between the inhabitants of the sea depths.

For decades, the monk seal (Monachus monachus) has been practically a ghost, the protagonist of a few sightings in the open sea so rare as to deserve official announcements and prominent newspaper headlines.

 

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