Mollusks . The word mollusc comes from the Latin mollis or the Greek malakos (soft), is used to designate the largest group of the animal kingdom after the arthropods , with more than 200,000 living species and several thousand fossils.
These animals have developed incredible habits of life to conquer habitats as dissimilar as those found from the tidal lines to the great marine depths, and from the very level of the tides to the highest mountains of the earth.
The mollusks present the soft body, without segments or joints, generally formed by the head, in the anterior portion; a strong muscular foot, located centrally and a dorsal mass formed by viscera.
Shell or protective skeleton
Most of these species have the body covered by the mantle, thin, easily dilatable membrane that secretes the conchiolin, shell-forming substance or external protective skeleton that has defensive function, this shell can also be internal, protecting the viscera, the animals that lack it present, as alternatives, unique methods of defense, among which are included the cripsis or camouflage in the environment and the chemical defenses .
The fleshy mantle presented by some mollusks is formed by two folds of the dorsal wall of the body, which enclose a cavity where the respiratory organs are and which adheres to the shell, when it exists, secreting calcium carbonate to make it; the shape, coloration and structure of the shells have a great taxonomic value, and in some species they acquire architectural and chromatic designs that surpass the imagination of the most creative of the plastic artists. Except for the bivalves , in the other molluscs there is usually a differentiated head, where another anatomical structure is found.
Most of the bivalves are sedentary and feed by filtering the water where they live, from which they retain the nutritive particles, be they detritus, phyto or zooplankton; their hairy gills (which generate water currents), covered with mucilage, capture and carry food to the buccal palps, where it is selected and taken to the mouth, these respiratory organs are more complex than those of other organisms, as they act in two vital processes, respiratory and food.
In the scaphopods, it is their ciliated tentacles that check the sand of their environment in search of food, especially foraminifera, their favorite prey.
The mollusks with radula, chitons , snails and slugs , use this tool to scrape the substrate from which they feed and obtain their daily ration of plant or animal cells that make up their diet.
Because of the structure of the radulas can be speculated on the diet of the wearer, long ribbons and numerous teeth often correspond to omnivorous molluscs or vegetarians, while tapes with few teeth in their transverse rows are associated with carnivorous species and Sometimes very specialized like that of the cones. A few marine marine slugs break this rule, the sacoglosos, whose radula has a single hook-shaped tooth or cobbler’s awl with which they perforate the walls of the algae and then suck their internal fluids with the help of a muscular suction pump.
The forms of the radular teeth of the different species are invariable and characteristic of them, thus having a high systematic value for the identification of these, which is not easy, since sometimes it requires a powerful microscopy to be able to see them in detail.
Excretory and respiratory system
All molluscs have a well developed excretory system and it is in the respiratory system that there are great adaptations: in aquatic animals, the exchange of gases is carried out through the surface of the body, as in other lower invertebrates, also developing specialized respiratory organs, called gills (ctenidia), which reach a singular development in nudibranchs; in some terrestrial snails and freshwater it is the mantle that has a similar function to a respiratory lung.
The size range between the mollusks is very wide, the smallest, even having a shell, can measure less than a millimeter and present very elaborate micro sculptures such as the microcapsules of the Pickwortidae family, or be smooth and opaque or crystalline, as is the case of the genera Rissoella and Granulina that look like tiny grains of sand in movement. Among the largest, the giant clam of the Pacific Ocean (Tridacna gigas) has a shell that exceeds 125 cm wide, used in the Middle Ages as a baptismal font and sometimes as a gardener or fountain, which uses today.
Special mention deserve giant squid, the largest known invertebrates, whose body can reach 20 meters. with the tentacles extended and weigh more than 450 kg.
Among Cuban mollusks with external shells, the maximum size is reached by snails such as cobo (Eustrombus gigas), triton (Charonia variegata) and pink quinconte (Cassis madagascariensis) that can exceed 30 cm in length, although the record in size of The Cuban mollusks have a cephalopod, the octopus pelagic parasol (Tremoctopus violaceus), which can reach 1.8 m in length, and whose presence on the outer edge of the Cuban island platform is relatively common.
Sex and Reproduction
In most of the species the sexes are separated and the fertilization takes place in the water, there is only hermaphroditism in the terrestrial snails themselves, in the marine slugs and in some bivalves , being the fertilization crossed; sex changes also occur from male to female in some oysters and in the same reproductive cycle. In the cephalopods, the courtships prior to mating are frequent, using then the males a specialized arm (hectocotylus arm) for the transfer of their spermatic packets into the cavity of the mantle of the female.
The spawning of the different species of mollusks are of very varied forms, especially in the gastropods and sometimes very complex forming cases or capsules of gelatinous consistency with numerous eggs, or with only one, the latter very frequent when the animal is of development direct. In marine slugs , the sunsets are usually white or colored gelatinous strings or ribbons, with abundant eggs and roll up characteristically in each species. In some cases they contain masses of lipids accompanying the eggs that serve as first food to newly hatched larvae.
The octopuses take care of their clusters of eggs, cleaning them and aerating them until their hatching; some chitons and freshwater clams incubate the eggs until their descendants are born.
In terrestrial snails eggs are deposited independent, clustered or wrapped in mucilage, buried under leaf litter or mosses, and some freshwater species deposit them out of the water, clinging to the soles of the shore, forming clusters of white eggs, calcareous, very characteristic, around the ears that come out of the water; from these eggs are born small well formed snails, without going through larval stages in the water, as it happened with the cephalopods, and without suffering a complex metamorphosis.
There are mollusks of sessile life (spondylus, oysters), others are able to move through a muscular ventral foot (gastropods and chitons) or through a coverlet turned into a nail (the cobos). Other mollusks expel jets of water under pressure that impel them (the file), and in the case of the octopuses they move contorting the arms.
Structure and organization
Currently the Mollusca phylum is considered divided into 10 classes, eight living (2 without fossil record) and 2 extinct (Rostroconchia and Hyolitha):
Caudofoveata class (= Chaetodermomorpha)
Vermiform (2 mm to 14 cm long), body fully covered with scales, with a front surface of the ventral surface that functions as an adhesive plate. Separate sexes, ovules and sperm fall freely into the water. Some 90 species are known, all marine, distributed in a single order (Chaetoderma Tiida) and three families. They live in muddy sedimentary bottoms where they dig galleries, from the infralittoral zone (10 m) to 7,000 m depth; They feed on small animals and organic matter. They are predated by polychaetes, nemerteans and other similar carnivores.
Solenogastres Class (= Neomeniomorpha)
Vermiform (0.8 mm to 30 cm), with the mantle covered with cuticle and scales and / or spicules; foot in the form of a median longitudinal groove with a fold; cavity of the subterminal to terminal mantle, without pinnate gills; medium stomach straight, without separate glands. Hermaphrodites, sometimes with internal fertilization. Some 185 described species, grouped in 4 orders according to the cover of the mantle, all marine (5 – 6850 m), epibenthic and predatory Cnidarians.
Polyplacophora class (= Placophora)
Flattened (3 mm – 43 cm), with the dorsal region covered by 8 plates, mantle with scales and / or calcareous spicules in its peripheral belt, wide and flat foot, very adherent, if detached they can be rolled up. Separate sexes, ovules and sperm fall freely into the water. Three large groups are recognized: Multi Placophora (extinct) with shells of 17 plates, Paleoloricata, also extinct and Neoloricata, which gathers some 600 recent marine species (0-7000 m), classified into 3 orders (Lepidopleurus, Chitanda and Acanthochitona) and a fossil order (Chelodida).
Class Monoplacophora (= Tryblidia)
Shell with conical shape, flattened and bilaterally symmetrical (1.5 to 37 mm). Head with two pairs of appendages, peripedal mantle cavity with 5 or 6 pairs of modified gills, 5 or 6 pairs of excretory organs, two pairs of gonads and heart with two pairs of atria. There are 12 to 15 marine species known (distributed between 175 – 6,500 m depth), grouped into a family. They feed by filtering detritus.
Class Bivalvia (= Pelecypoda, Acephala, Lamellibranchiata)
Laterally compressed body, protected by two valves (1mm to 1.35m), extended back mantle forming siphons, head with buccal palps, reduced buccal mass, without radula. Sexes generally separated, external fertilization. Some 6,000 marine species (0 – 10 700 m depth) and 2 000 limnícolas.
Class Scaphopoda (= Solenoconcha)
Body protected by an elongated shell, cylindrical, with both ends open (2 mm to 13.5 cm). Head with two tentacles (captures), radula well developed, lack gills. Some 350 species (with two orders), all marine and mining (0 – 7,000 m depth), micro carnivores.
Large size variation (0.3 mm to more than one meter long) and shapes. Of the 60,000 species of current gastropods, half are terrestrial, about 25,000 are marine and the rest live in freshwater. In general they have great ecological extension.
Class Cephalopoda (= Siphonophora)
Octopuses, cuttlefish and squids of variable size (1 cm to more than 18 m), are distinguished by the number of arms or tentacles that surround the mouth (8 – 10 or 90). Some 600 recent species and more than 1,000 fossils, all marine (0 – 4 500 m depth).
Mollusks live in all aquatic and terrestrial environments, from mangroves and rocky or sandy coasts , in the area of ??tides, to the abyssal depths, including the water column, and in the most varied environments, whether lake, wall calcareous up to 7,000 m altitude, forests, deserts and swampy mud, only the air environment escapes its colonization.
Geographical scope of marine molluscs in Cuba
The marine mollusks of Cuba are distributed in a wide area of ??the neotropical region, which present physical, chemical and biological conditions very similar in all their extension; it covers, in the western Atlantic Ocean, from the Bermuda Islands to near Cabo San Roque , Brazil and includes the central, south and southeast portions of the Gulf of Mexico , the archipelagos of the Bahamas and the Greater and Lesser Antilles, the coasts Central America and South America Bathed by the Caribbean Sea and the coasts of Brazil to Cabo San Roque.
Being so abundant and diversified, mollusks have numerous predators and in turn, in the set of marine species, they themselves become the largest trophic range of consumers, with species with diets ranging from detritus, carrion or microalgae from which the filter feeders are nourished, to specialized diets such as the slime of some fish , sponges or gorgonians and corals , besides cannibalism, the consumption of eggs or the capture of other mollusks, in which the octopuses are true teachers.
As for its predators, in addition to octopuses and some gastropods, starfish , crabs and lobsters stand out , fish such as rays or chimaeras with their teeth adapted to grind clams , birds that love snails land and freshwater and above all man, which acts systematically on the more than 500 edible species that exist throughout the world, which include some terrestrial snails, although the importance of their farms is negligible compared to those that are have developed in the marine environment, especially those of bivalve molluscs, where mussels , oysters and oysters .
The utilitarian value of molluscs
They present different utilitarian groups for society. The first interest that man found in them was their nutritional value, from the primitive stage to the current times when thousands of tons of molluscs are obtained per year from the waters of the oceans.
In the towns of Asia , Africa and Polynesia , they were used as coins, even to buy slaves; in ancient Greece .
The shells of mollusks, because of their varied forms and intense colors, are transformed into ornaments for the body or home, but also in utilitarian items such as containers, musical instruments and others. The pearls are the perfect jewels of nature, they are produced only by mollusks, the artificial cultivation of them involves, as a source of dissimilar jobs to many people.
Enzymes that are used in biochemistry and medicine are extracted from the digestive tract of several species; the nerve cells of others are used in neurophysiological investigations.
Given the ease with which the shells of mollusks are fossilized, due to their calcareous nature, geologists and paleontologists take them as indicators of the different eras and periods of the history of the earth, since it is the only group of animals whose genealogy and evolution has been known complete from the oldest layers of rocks to modern ones.
Not all molluscs are beneficial to man, because some gastropods can feed on clams in their artificial offspring, there are a few that can parasitize man, there are bivalves like teredos and lithophagas that drill the wood and stone, creating a network of galleries that can weaken concrete or wooden structures, jetties of piers and hulls of boats, there are gastropods from the family of cones that can be potentially dangerous because of the active poison they possess, can kill people if they are handled carelessly, some can be toxic by ingesting them at certain times of the year.