2.6 Life below and in the sand

Plants and animals living on the sea bottom are part of the benthos. Nearly 16% of all living animals are benthic species. Some of them walk on the sea bottom, others are buried in the sand or attached to the rocks; many of these animals, however, spend the first part of their life cycle as fluctuating animals (plankton, FIG1_SES2.6).
The sandy bottom seems almost desert, but this is only because living organisms are less eye-catching and better hidden.

FIG1_SES2.6 Phytoplankton_diatoms. By Prof. Gordon T. Taylor, Stony Brook University – corp2365, NOAA Corps Collection, Public Domain, https://commons.wikimedia.org/w/index.php?curid=246319

The most common invertebrates are molluscs, especially bivalves, which slip with their foot and project only with the syphons (FIG2_SES2.6) from which they enter the water to breathe and filter the food. But there are also echinoderms, like sea stars and sea cucumbers; crustaceans and polychaete.

FIG2_SES2.6 Bivalves underwater with their siphons visible. By Stefan Didam – Schmallenberg – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6830892

Fish living among seabeds have developed evolutionary adaptations that have often led to the modification of their body as a flattened form, the development of appendages to “walk”, to perceive the sunken prey or to reach a shelter. Their skins also change to become camouflage and blend with the sand.

For example, the sole (Solea spp., FIG3_SES2.6) has both eyes on the right side of the body, a practice characteristic of an animal that spends most of its time lying on the seabed or in the tub gurnard (Chelidonichthys lucerna) the ventral fins have been modified in a sort of paws to walk on the bottom without sinking.

FIG3_SES2.6 Solea solea. By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=3388340

2.6.1 The species that live underground
Benthic organisms are the organisms that live associated with the sea bottom. Examples include burrowing clams, sea urchins and barnacles.

2.6.2 The species that are camouflaged
The coloration of marine organisms is extremely diversified and it depends on many factors while its meaning is simpler to understand: mimicry, confusing themselves with the environment. The color of the skin can be a weapon or defense ant it is given by presence of pigments in the epidermis that are dispersed or concentrated according to need. Physical phenomena of diffraction and interference of light act on the material that makes up the body structures of marine organisms (ex. shells or skin coatings). Some colors are due to the presence of vegetable symbionts (ex. algae); the stomach content or blood is visible for the transparency of the body and the tone varies according to diet.

2.6.1 The main characteristics of the bivalves living in the Mediterranean Sea

The term bivalves refers to all the molluscs belonging to the Bivalvia class, whose shell is divided into two parts, called precisely valves, joined together by a movable hinge that, with the adductor muscles ,allow the mollusc to open and close the two valves and, in case of danger, keep them locked. Some bivalves manage to move simply by opening and closing the two parts of the shell very quickly. The species that live in the sand are equipped with a foot that serves to dig, which possesses sensory and motor organs. These molluscs are filtering animals and their nervous system is very simplified. They feed and breathe thanks to a visceral mass, which contains organs for digestion and reproduction. A mantle wraps and encloses the visceral mass that secretes the shell. The food particles are filtered through the gills that are able to retain the organic substances present in suspension in the water. In fact they have an open circulation because the blood does not circulate entirely inside the vessels, but is collected by the gills, pumped through the heart and released directly into the spaces between the tissues, from which it returns to the gills and then to the heart. In nature there are about 30 thousand different species of bivalves and the most common species are mussels, clams and oysters. The widespread ones in the Mediterranean are: the tellines (Donax Trunculus), the planking tellines (Tellina Planata), donax (Donax), mussels (Mytilus galloprovincialis), clams (Chamelea gallina).

2.6.2 Molluscs: Mytilus edulis(description and characteristics)

The etymology of the term Mollusco derives from the Latin nŭcem mollŭsca (m), "nut with a soft shell", in turn from mollis, "springs" and refers to the characteristic of the body without vertebrae.Their structures include some characteristic constant elements:
the head with a pair of eyes, two tentacles, nerve ganglia and a mouth that encloses the radula (a kind of tongue covered by chitinous teeth);
the muscular organ foot through which the animal crawls and sometimes swims;
the mantle that overhangs the foot and contains the visceral mass;
between the mantle and the foot there remains a furrow, or a cavity in which the gills are located.
The mantle, in the most advanced classes, that is the class of Conchiferi, provides a limestone shell that guarantees mechanical protection.
There are about 80,000 species and are classified into eight classes, those groups with the largest number of species are:
1. Gastropods that include animals characterized to have only one shell or not to have it.
2. Bivalves whose name is due to the particular conformation of the shell, which contains the body, divided into two valves joined at the back.
3. Cephalopods that may be shell-free or internal.
The other species are classified in:
4. Solenogastri are mollusks with a flattened body without a shell;
5. Caudofoveati are organisms without shells;
6. Polyplacophora are known as chitons and have the shell formed by many plaques;
7. Monoplacophora is a class whose ancestors probably gave rise to the various classes of mollusks;
8. Scafopods are characterized by the calcareous shell-shaped shell.
Most molluscs live in the sea; some inhabit lakes and rivers and others on the ground, usually in wetlands.
Reproduction is always sexual, in some species individuals are hermaphrodites.
The development can be:
direct, from the fertilized egg is born an individual similar to the adult;
indirect, there is formation of a larva and metamorphosis.

2.6.3 Echinoderms: Sea Stars, a special constellation (description and characteristics).

The echinoderms belong to the class of invertebrates that are characterized by a skeleton of spiny calcareous plaques.The skeletal system is held together by a mutable connective tissue that now allows for a rigid structure, now the softer, allowing the animal to assume particular positions without any physical effort.They consist of an aquifer system with a series of channels for water circulation. This system has a triple function: respiratory, nutritional and ambulatory. The echinoderms are predators and go hunting for molluscs and small fish. There are five species of echinoderms: asteroids, echinoids, olotouroids, crinoids and ofiurides. These are marine animals, almost all of them live in salt water: starfish, sea urchins, sea lilies, serpentine stars and sea cucumbers. Starfish have a flat body and five arms and have a mouth turned downwards. The dairy is capable of regulating amputated parts of the body. Sea urchins have poisonous spines all over their bodies. Sea lilies have a calyx-shaped body and are characterized by bright colors. Serpentine stars have the same structure as starfish but with delicate arms. Finally, sea cucumbers have a cylinder-shaped body and mouth is surrounded by a crown of tentacles.Finally the production occurs through the deposition of eggs from the female, which are then fertilized in the water by the male. Almost all have sexual dysmorphism and are sexually separated animals. This means that each specimen is either male, or female. We find exceptions, but they are rare and not particularly known.

2.6.4 Echinoderms: Sea Cucumbers(description and characteristics).

The Echinoderms are a phylum of marine deuterostomes. The name derives from the fact that they are often covered with calcareous plates.This phylum consists of 5 main types of animals; starfish, sea urchins, sea cucumbers, fragile stars, crinoids.
They are bilateral organisms while adults present an apparent radial symmetry and they have only one system of aquifer channels that serves them to breathe ,to capture prey and to move.
The olotours or holothurians called sea cucumbers are a class of echinoderms widespread on the sea. They are characterized by an elongated cylindrical body; their name derives from the morphology similar to the vegetable, there are about 1250 different species.
They feed themselves about corpuscles like algae and small fish but also of form.
The animal releases both the eggs and the sperm in water and fertilization takes place; also the contractions of circular and longitudinal muscles contribute to movement.
They have great regenerative capabilities: they can "eviscerate" limestone plaques. Their lenght is 10/15 cm.
Sea cucumbers have a high economic value, they are in fact fished for food in the Mediterranean, in the waters of the Far East, in the Indian Ocean and in the Pacific. In some countries, sea cucumbers are considered delicacies, especially in the East, in China they have a price ranging between $ 10 and $ 600 per kilo, while some particularly valuable species can cost up to three thousand dollars a kilo.

2.6.5 Chelidonichthyslucerna(description and characteristics).

Chelidonichthyslucerna, also called tub gurnard belongs to the phylum of the Chordates, to the subphylum of the Vertebrates, to the class of the Osteichites on the order of the Scorpiformes and to the family of the Triglides. The tub gurnard is a species of bottom-dwelling coastal fish with a spiny armored head and fingerlike pectoral fins used for crawling along the sea bottom. It has a large and massive head with a well visible and protruding snout, the spine on the operculum is not very large. The profile of the forehead is straight, not sunken. The pectoral fins are large and black-blue in color with intense electric blue patterns and a black spot at the hairline. The lateral line is evident and each flake on the lateral line has a dent turned backwards , and a pointed dorsal fin. The color of the body is brownish or reddish on the back on the sides turns on the salmon pink. The belly is white. It reaches 70 cm in length. The Chelidonichthyslucerna is different from similar species and in particular from the congener Chelidonichthys lastoviza for the profile of the less steep and concave snout, to have the most massive head and to have the last of the squared muzzle. It lives in the Oriental Atlantic Ocean, from Ireland to Morocco and in the Mediterranean Sea. We can find it at a depth between 20 and 200 metres. It can emit grumbling sounds with the swim bladder. When the Chelidonichthyslucerna is chased, it can roil the water to confound it . It eats mainly shellfish. It is traversed with trawl nets, gillnets, lobs and lines. Chelidonichthyslucerna's white and tender meats are very popular.

2.6.6 Benthic organisms: the sea urchins (description and characteristics).

The animals and plants that live on the seabed are known as Benthic communities.
Benthic means pertaining to the sea bed.
These consist of small crustaceaans, echinoderms, molluscs, worms, and many other colourful and wonderful beasts. The sea urchins is a part of the Echinoidea class of the phylum Echinodermata .
It’s very common in the Mediterranean and in the Eastern Atlantic and lives in rocky bottoms on the prairies of the Poseidonia oceanica.
The newborn hedgehog is in the form of a small gelatinous larva of a few millimeters , bilaterally symmetrical , called Pluteo , whose body branch off from 4 to 6 arms .
These stages last for 4 to 6 weeks , during which the pluteus is carried by the sea currents.
It has a shell with long , movable quills and on the ventral surface , it has small extraflexion that allows it to move .
The colour can be purple , brown or greenish .
The buccal apparatus consists of five mobile skeletal elements , each provided with a tooth.
To breathe, it uses small gills.
It feeds on algae, animals, sponges and Poseidonia oceanica leaves.
It needs shade and then covers itself with shells, pebbles and Poseidonia leaves that are held with peduncles.
The sea urching is a species much sought for the delicacy of its eggs called gonads.
The sea urchins has among its main competitors the sarago and the sea bream, which despite the squills , thanks to the powerful teeth that these species have, they are able to break shells and devour the contents.

2.6.7 Benthic organisms: barnacles, a permanent guest (description and characteristics).

The name of the barnacle is due to the six pairs of ends in the form of Cirri, bifid starting from the base, so that they look like twelve. These strongly curved spiral appendices are generally retracted within the cavity of the mantle.
Barnacles are a subclass of shellfish. They are exclusively marine animals, fixed to the substrate, often colonial or can some adherents to rocks, not only coastal, but also of considerable depth; Several species live as ectoparasites on the integument of some large cetaceans.
Generally, the egg of the barnacle, when it opens, gives rise to a free-floating nauplius. After many wetsuits, the last Nauplius gives rise to a larva, called Cipridiform.
In the barnacle parasites, the cipridiform larva attaches to the abdomen of the host and undergoes the metamorphosis, assuming a globular form with abundant ramifications that are distributed in the body of the host itself.
The body shape of the non-parasitic barnacle follows two fundamental models: in the barnacles type the body consists of a part (chapter) covered with plates, usually free, and a stalk, naked, more or less elongated, very muscular and flexible.
Among these parasitic forms there are the sacculins, parasites of the crabs, on whose belly they form a tumor. The antennas, through which the larva is fixed, bring a spike that perfores the carapace. This is one of the most amazing mechanisms invented by nature, a real syringe through which the parasite injects itself, by first passing the head then all its substance within the body of the crab, leaving out of the point where it is only the sex glands occurred. The head and the body are reduced to simple roots that branching into the body of the host making him die. From what derives the name of this zoological suborder: Rizocefali, like"head to Root".