Lesson 1, Topic 1
In Progress

Underwater Life


The term algae refers to a vast group of organisms, single-celled or pluricellular, of which they are part of the microscopic algae, called phytoplankton, to the algae existing on the beaches.

In common they share the fact that in their cells there are chlorophyll, a class of pigments with photosynthetic capacity. For a long time they were considered as plants, however they have no roots, stems or true leaves. Only green algae have an evolutionary relationship with terrestrial plants, all other algae result from independent lines of evolutionary development.

Algae are now part of the Protista kingdom. Its ecological role is essential, since phytoplankton forms the basis of marine food chains, thereby sustaining almost all marine life. Phytoplankton is also the main primary producer, responsible for half of the planet’s photosynthetic production. Algae also provide refuge to a wide variety of organisms. Some species are also of economic importance, as they are used in the food, cosmetics and pharmaceutical industries.


Green algae mostly live in continental waters and can colonize very diverse habitats. It comprises single-celled and pluricellular forms, and the latter can reach macroscopic dimensions, with well-developed fronds. Among the most abundant green algae are Ulva lactuca (sea lettuce) and Codium tomentosum.


The brown algae are almost exclusively marine, with only three species of freshwater. They are mainly found in cold and temperate water regions. They are pluricellular plants normally fixed to the substrate. Some of the species of this class are the Laminaria hyperborea (laminárias) and fucus vesiculosus (bodelha). These plants form an excellent habitat for various organisms, whether vegetables or animals and, if some of them can live on the frond of laminials, most prefer the cracks and holes of their fixation organs.


These algae are mainly marine and are distributed throughout the seas of the world, mainly in warm and calm waters. Some of the most abundant species in our waters are Gelidium sesquipedale (gelide), Callophyllis laciniata and Halurus equisetifolius.


Lichens result from the symbiosis between an alga and a fungus. The splint of a lichen usually consists of a flattened and fouling leaf part, clinging to plants, rock or other substrates. The constituent fabric can be soft or rough, smooth or rough, tufted, branched, erect or prostrate. Some representatives of this group are Verrucaria maura and Ochorolechia parella.


Angiosperms, or flowering plants, occur on our coast in typically marine environments. This very homogeneous group is characterized by presenting an external organization where you can distinguish root, stem, leaves and flower. The most common representative of this sub-kingdom is the Zostera marina (seagrass).


SPONGES (Filo Porifera)

Sponges are animals with very variable shapes and colors.

Its body is pierced by several small pores through which the water enters, allowing exchanges with the middle. The water outlet is made by a larger hole called the osculum.

Sponges live in almost every type of habitat, from the tidal zone to the great depths of the ocean. Their shape depends largely on the degree of exposure of the places where they are fixed. Thus, in areas of great exposure the sponges are rounded or flattened against the substrate, but in sheltered places they assume a different growth pattern, presenting upright rods with delicate branches.

An interesting feature of sponges is the ease with which they are associated with other organisms, such as versed and crabs.

Some of the sponges easily observed during a dive are Myxilla incrustans and fasciculated Ircinia (belonging to the Class Demospongiae).


These animals do not have a rigid internal skeleton, their shape being maintained by the pressure of the water inside. Its body is characterized by having an opening, which serves as a mouth and anus, surrounded by tentacles. The main feature is the presence of urticating cells, called cnidocytes.

In this phylum are included organisms with distinct appearances, although with the same general organization of the body:

  • Hydrozoans (Hydrozoa Class)
  • Jellyfish or Jellyfish (Scyphozoa Class)
  • Anemones and corals (Class Anthozoa)

Some representatives of these classes can be seen frequently, such as the alforreca Rhizostoma pulmo (Class Scyphozoa), the gorgonian Paramuricea chamaeleon or the anemones Corynactis viridis and Actinia equine, with various shapes and coloring (Class Anthozoa).


Phyllodoce Rose

The arachnids (worms) have a true body cavity filled by a fluid. The body is divided longitudinally into a recognizable number of segments each of which with chitinous dins. The head is well developed and is provided with various sensory organs. The mouth opens in the second segment and the anus opens in the terminal segment.


Molluscs represent the second largest group of animals, with about 50,000 living species. These animals already have structured organs in systems although simple. The body divides into head, visceral mass and foot. The main characteristic of molluscs is their shell, although in some groups it can be reduced or even absent.


These molluscs are asymmetrical and have a well developed head and a wide foot. Its shell is univalve and usually coiled in a spiral. Among the most common marine gastropods are Patella lusitanica (lapa) and Gibbula umbilicalis (burrié). Haliotis lamellosa (ear of the sea) and Aplysia punctata (vinaigrette) can also be observed with some ease.


These molluscs have bilateral symmetry, and a body compressed laterally and encased in a shell of two valves that are joined dorsally by a ligament and a charneira, closing with the aid of one or two adductor muscles. They also have a mantle consisting of two flattened wolves, whose posterior margin usually forms siphons that control the entry and exit of water.

The bivalves are benthic and many live buried in the bottom sediment. The easiest to see in diving are the Mytilus edulis (mussels), which live fixed to hard substrates.


Cephalopods have a cylindrical, sac-shaped body, and have a foot that divides into a number of tentacles provided with suction cups, surrounding the mouth and joining the head.

In most species there is an inner shell, more rarely external and sometimes absent. The mouth leads to a pair of corneal jaws that resemble a parrot beak.

In this class individuals are grouped by the number of tentacles. Cephalopods with ten most common tentacles are Sepia officinalis (cuttlefish), Loligo vulgaris (squid) and Alloteuthis subulata (pota). The most common cephalopod with eight tentacles is Octopus vulgaris (octopus).


It is the largest of all animal phylums with about one million known species. Their representatives have a chitin exosequeleto, so they have to make seedlings to grow. They present bilateral symmetry, segmented body and articulated paws with distinct functions. This Phylum is divided into a large number of classes, the most important being the Crustacean Class.


Crustaceans have the body divided into three parts: head, thorax and abdomen.

The exoskeleton is formed by calcium salts and can be thick and heavy. The head has two pairs of antennae and jaws in the fourth segment. Crustaceans have very diverse body shapes and life cycles. Among the best known, due to its commercial value is scalpellum scalpellum (you notice), Crangon vulgaris (shrimp), Cancer pagurus (shoemaker), Maja squinado (santola) or Palinurus vulgaris (lobster).


Echinoderms are one of the most related groups with the cordados. They are exclusively marine and have in the adult state a special type of structure: they have radiated symmetry, with the mouth in the center of one of the faces of the body and the anus normally in the central part of the opposite face.

The body may be globular or have more or less distinct extremities. In the first case are sea urchins, such as diadema antillarum (long-thorn edifice), Echinus esculentus (sea urchin) or Paracentrotus lividus (puddle hedgehog). Starfish, such as astropecten aranciacus (digring star) or marthasterias glacialis (starfish ), or ophioderma longicauda, are among the most common distinct extremity equinoderms.

Looking completely different, holothuria forskali (sea cucumber) has very small skeletal elements, giving the animal a soft texture.

The sea cucumber moves only with the aid of the anterior part of the body, that is, the animal moves like a worm.

FISH, BIRDS, MAMMALS (Filo Chordata)

The cordados are animals that have a dorsal nerve cord. They form the most familiar set of all marine animals.

Vertebrates , in addition to the characteristics of the cordo, have a backbone composed of many bone parts articulated (vertebrae) and arranged in a longitudinal chain. The head is distinct and in it are very developed sensory organs. Vertebrates often have forethet (pectoral) and posterior (pelvic) peer appendages.

In terms of known species, vertebrates are only a small fraction of the Animal Kingdom. However, they include many of the most familiar organisms, from lampreys to man.

CARTILAGINOUS FISH (Chondrichtyes Class)

This class includes some of the largest and most voracious marine animals and is represented by a wide variety of organisms that includes from the small Scyliorhinus canicula (purple paw) to the huge Rhincodon typus (whale shark), which despite its size feeds on plankton, passing through the various types of rays, from the great manta (also feeds on plankton) to the tremelgas, rounded body and and that produce electrical discharges.

The electrical discharge of the tremelga poses no danger to man, but the spike at the base of the tail of some rays, such as uge, can cause painful injury to a more careless diver.

These vertebrates with a true jaw and a carizophenous skeleton do not have bone rays on their fins. The mouth is located on the ventral face of the head, and the skin is usually rough and rough.

BONY FISH (Osteichthyes Class)

It encompasses fish with true jaws and bone skeleton. They are more evolved fish with bone rays on the fins and operculum covering the gills, with gaseous bladder in the abdominal cavity and with symmetrical caudal fin.

This group includes the largest number of existing fish species (more than 25,000 species), constituting the largest class of vertebrates on the planet.

This high specific diversity translates a great adaptive plasticity that results from its trophic and locomotor adaptations that allowed them to colonize literally all biotypes of the aquatic environment, from the pelagic oceanic environment to estuarine and freshwater habitats.

The coastal strip is a preferred habitat for a large number of species and it is impossible to make an exhaustive description of the species that occur here. Some can be easily observed during a swim along the coast.