General characteristics. The subclass of foraminifera (Latin foramen, genus foraminis - hole, hole, fero - to wear) includes a large group of sarcodes, numbering up to 20,000 modern and fossil species, the cytoplasm of which is enclosed in an organic, agglutinated or calcareous shell. Foraminiferal pseudopodia consist of thin, branched, root-like, interconnecting (anastomosing) filaments emerging from the shell either only through the aperture, or through the aperture and canals penetrating the shell wall. Foraminifera are mostly marine benthic or planktonic, free-living or attached forms. Not most of foraminifera have adapted to life in brackish water basins and only a few are known in fresh water bodies. They have been known in fossil form since the Cambrian.

Body structure. Foraminiferal cytoplasm is usually colorless, sometimes pink, orange or yellow colors. Ectoplasm, quite homogeneous in structure, exchanges substances with the external environment and serves as a place for the formation of pseudopodia. Under an electron microscope, pseudopodia appear as a bundle of fibers of different diameters; each fiber is surrounded by a sheath. The ability of pseudopodia to extend and retract is based on the property of the cytoplasm to change its state of aggregation, moving from liquid state(sol) into a viscous (gel). Pseudopodia, not connected to the substrate, branch, are connected by bridges and form a kind of trapping network into which larvae, various microorganisms and organic detritus fall (Fig. 26). Digestion of food often occurs outside the sink.

Shell structure. The overwhelming majority of foraminifera have a shell, and only a small part have a cytoplasm surrounded by a thickened elastic organic shell - a membrane. The shell can be relatively simple or reach great complexity (Fig. 27). Its dimensions range from 0.02 to 110-120 mm. The shell wall can be organic, agglutinated and calcareous. The most poorly organized foraminifera (allogromiids) have a wall consisting of tectin, which is a combination of proteins and carbohydrates. In many foraminifera, the tectin wall includes foreign particles of various mineral and chemical composition: grains of quartz, various heavy minerals, carbonates, mica plates, sponge spicules, organic detritus (fragments of sponge spicules, shells of other foraminifera, radiolarian skeletons, fragments of mollusk shells) and other " construction material".

In this case, foraminifera, like testate amoebae, usually “swallow” this “building material” inside. After some time, the protoplasm swells and the “building material” is pushed to the surface, where it is cemented with tectin, calcium carbonate, oxides or iron carbonate.

Thus, agglutinated shells appear.

It was previously assumed that, in rare cases, the cement in some foraminifera could be silica. However, the presence of flint cement in modern foraminifera has not yet been established. Many researchers believe that the flint skeleton observed in a number of fossil foraminifera is secondary and developed in the process of fossilization on calcium carbonate. The question of where ferruginous cement comes from, whether foraminifera have the ability to secrete iron from the cytoplasm or whether it is brought in from the outside in the form of fragments of ferruginous minerals also remains unclear. The cytoplasm of some foraminifera has a peculiar selective ability - to build a skeleton, it “selects” material only of a certain size, color and even composition, for example, only quartz grains or spicules of flint sponges, or mica leaves. But most often, any suitable debris scattered at the bottom of the reservoir is used. Cement and agglutinated particles are included in the shell in different proportions: in some forms the particles are tightly adjacent to each other, in others they are separated by sections of cement, sometimes cement completely predominates. The microstructure of the wall of agglutinating foraminifera has not been sufficiently studied. Many have an internal organic lining.

Most foraminifera have a secretory calcareous shell, the wall of which consists of a tectin base impregnated with mineral salts; An important role here is played by calcium carbonate (calcite or aragonite) with varying amounts of admixture of magnesium carbonate (up to 18%) and calcium and magnesium phosphate. The structure of the wall of calcareous shells is quite diverse. There are three main types of wall microstructures: microgranular, porcelain and hyaline (vitreous). Recently, cryptocrystalline has also been isolated. The used names “porcelain-like” and “vitreous” are not very suitable, since they reflect not the specifics of the microstructure itself, but the general appearance of the wall, but these names are generally accepted and still exist in the literature.

The microgranular type of wall is observed in Paleozoic endothyrids, fusulinids and in some Meso-Cenozoic orders; it is characterized by the presence of grains of microgranular calcite ranging in size from 1 to 5 microns, the absence of cement and a variable admixture of agglutinated particles. A shell with this type of wall microstructure does not have sculpture or additional skeletal formations; The internal skeleton is presented in the form of wall outgrowths. The surface of the shell is dull, light or grayish-yellow in color.

The porcelain-type wall is characterized by a random arrangement of crystals and their crystallographic axes; The crystals have different shapes, their sizes range from 0.5 to 5 microns. In reflected light, the wall is white, porcelain-like, sometimes shiny. The shell wall contains an organic base. This type of wall is characteristic of the miliolid order.

The glassy, ​​or hyaline, type is divided into two subtypes: glassy-granular and glassy-radial. In the first subtype, calcite or aragonite crystals are of a uniform round or angular shape, tightly adjacent to each other; crystal sizes 0.5-10 microns; the optical axes are oriented randomly or with a predominance of a certain orientation with the C axis at an angle to the wall surface. In the glassy-radial subtype, the crystals of calcite or aragonite are highly elongated, located mainly perpendicular to the wall surface; The optical axis C is also located.

The cryptocrystalline type of wall microstructure is characteristic of Paleozoic foraminifera; the wall consists of calcite crystals with unclear boundaries.

Often, in the process of fossilization of secretory calcareous shells, secondary microstructures associated with recrystallization processes arise. In some cases, crystal enlargement occurs, in others, elongated crystals disintegrate into small subisometric grains.

The macrostructure of the shell wall is formed by morphologically isolated layers, intrachamber linings, and secondary layers on outer surface shells and on the surface of septa.

The primary wall of the shell can be single-layered or consisting of two or more layers. Primary single-layer walls are developed predominantly in representatives with a porcelain-like microstructure, as well as in many agglutinated and tectin shells. Foraminifera with a glassy and microgranular structure have both single-layer and multilayer walls; in a multilayer wall, individual layers are separated by thin layers of organic matter; The layers that make up the walls usually differ from each other in their structural features. For some groups (fusulinids), these layers have special names: the primary wall is called proteca; it consists of an outer thin layer - the tectum and a main inner layer that carries, various names. In Schwagerina it has a cellular structure and is called keriotheca (see Fig. 39). In glassy multilayer shells, it is proposed to call the three-layer primary wall bilamellar, since it initially distinguished between the inner and outer (or main) layers.

The inside of the shell wall is lined with a thin organic film. On the outer surface of the shell and on the internal whorls, secondary layers of shell walls are developed; they are formed after the formation of a new chamber in the form of subsequent layers on the outer or inside a previously formed wall (they are sometimes called layers of growth, or thickening, or secondary multilayers).

In the simplest case, when a new chamber is formed, the entire open part of the shell is covered with new shell substance and its old part thickens significantly (Fig. 28), while the newly formed septum and all previous septa remain single-layered (Fig. 28, 1); this type of structure is observed in nodosariids, buliminids and the simplest families of rotaliids. In the second case, when a new chamber is formed, the shell substance covers the entire open part of the shell and overlaps the previous septum in such a way that it becomes double, and the newly formed aperture septum remains single-layered (Fig. 28, 3). In such double septa, a system of septal canals develops in the cavities remaining between the two layers. This type of double septa with a system of intraseptal canals is characteristic of the rotaliid order and is called rotaloid septa. In the third case, the newly formed chamber with a final aperture septa is primary-double and, in its method of formation, resembles the first case (Fig. 28, 2). Similar double septa, also equipped with a system of canals, are characteristic of shells of some groups of the orders buliminids and nummulitids (orbitoids).

Wall porosity. Many foraminifera have a porous wall. Pores can be simple or complex. Simple pores are represented by cylindrical tubules with a diameter of 0.2-0.5 µm; complex pores are characterized by the union of small pore tubules into larger ones (keriothecal porosity in fusulinids).

Some Meso-Cenozoic foraminifera have an alveolar wall structure formed by various outgrowths that make up additional intracameral skeletal formations. All pore channels are usually covered with an organic lining. Shape and frequency of pores on the sink in last years intensively studied using an electron scanning microscope.

Shell shape. The foraminifera shell can be one-, two-, or multi-chambered (Fig. 29). With continuous growth, a shell is formed that is not divided into chambers; such a shell is called single-chamber. In the simplest case, a single-chamber shell has the shape of a ball or flask, with one mouth (Saccammina, Lagena) or with several openings (Astrorhiza). It may be agglutinated or calcareous. With increased growth along the mouth edge, a tube-shaped shell appears, open on one side or both.

Rice. 29. Scheme of the structure of foraminiferal shells: 1 - single-chamber; 2 - two-chamber; 3-5 - multi-chamber: 3 - single-row, 4 - spiral-planar: 4a - from the side, 4b - from the mouth, 5 - spiral-conical: 5a - from the dorsal side, 5b - from the mouth, 5c - from the ventral side; AA - winding axis, D 1 - large diameter, D 2 - small diameter, knl, pp - plane of symmetry, s - septal sutures, sp - septal surface, ssh - spiral suture, T - shell thickness, y - mouth

Two-chamber shells consist of a spherical initial chamber and a second, long, undivided, tubular, separated from the first by one partition. The second chamber can be straight or branched, or curled into an irregular coil-shaped, flat or conical spiral.

A shell in which the internal cavity is divided by partitions, or septa, into chambers is called multi-chambered (Fig. 29, 3-5). The emergence of multilocularity is associated with a change in the growth pattern of the cytoplasm and shell. Growth changes from constant to periodic, with periods of intense growth separated from each other by periods of rest. Each period of growth corresponds to the formation of a new chamber, which, as a rule, is larger than the previous one; the shape and location of the new chamber and the aperture septum separating the newly formed chamber from external environment, depend on the physicochemical properties of the cytoplasm, on the magnitude of the contact angles formed by diverging pseudopodia with the walls of the previous chamber, and on the nature of the surface of the latter. The emergence of growth periodicity had great importance in the development of foraminifera, since it freed them from the need to continuously build a shell. Traces of such periodicity can already be observed on some one- and two-chamber tubular shells bearing light constrictions.

The simplest form of a multi-chamber sink can be considered uniaxial or single-row, when each subsequent chamber, having a spherical shape as the most advantageous, having the largest volume with the smallest surface area, is built up over the previous one. But in such single-row forms there is a fairly high risk of fracture, especially in places of pinching, so improving the shape leads to the fact that the new chamber covers part of the previous chamber with its main part, as if moving onto it.

Another way to strengthen the shell is to twist it into a spiral. The most primitive type will be irregularly ball-shaped, in which the whorls coil randomly in several directions. When such coiling is ordered, plectogyric shells or milioline-type shells appear. In the first case, the winding axis of the subsequent turn deviates by a certain angle from the position of the axis of the previous turn. In the second case, the chambers form a spirally coiled ball, located in several mutually intersecting planes. This is explained by the fact that the direction of the winding axis changes with the growth of the shell by a certain angle. The length of each chamber is usually half a turn. In some forms, the chambers are spaced 144° from each other and are located in five planes (Quinqueloculina), intersecting at an angle of 72° (see Fig. 42), in others, the chambers are located in three planes (Triloculina), mutually intersecting at an angle of 120° , and finally, in the third, each chamber is located 180° from the previous one (Pyrgo, or Biloculina).

The spiral-planar type is considered as a modified uniaxial type, in which the main axis spirally curls in one plane. The contact lines between adjacent whorls of the shell spiral are called spiral sutures. The imaginary straight line around which the shell whorls are wound is called the winding axis. The thickness of the shell is measured along the winding axis of spiral-planar. The diameter of the shell is drawn perpendicular to the winding axis through the initial chamber. The cross section of the shell perpendicular to the diameter is equatorial. The plane of symmetry coincides with the equatorial section. The shape of spiral-planar shells is varied and depends on the diameter and thickness (see Fig. 41, 3). With a diameter significantly greater than the thickness, the shell has a disc-shaped or lenticular shape. With a diameter almost equal to the thickness, the shell takes on a spherical shape. When the thickness significantly exceeds the diameter, a spindle-shaped shape appears. If, when viewing a spiral shell from the side, all whorls are visible, it is called evolute (see Fig. 35, 1). If the last whorl covers all previous whorls, then the shell is called involute (see Fig. 48, 5). Between these two extreme types of structure there is big number forms occupying an intermediate position (semi-evolute and semi-involute).

The degree of increase in speed varies. In most spiral-planar shells, the increase in whorls occurs gradually, but in some forms the whorls increase very quickly and the shell takes on the appearance of a “cornucopia” or even becomes fan-shaped. Sometimes a rapid increase in revolutions can lead to the closure of the opposite ends of the fan and the appearance of a cyclic type of shell. In cyclic shells, the chambers are located in concentric circles in the same plane (see Fig. 49).

In the spiral-conical type (rotalium), the chambers are arranged along a cochlear, or trochoid, spiral (Fig. 29, 5). The side corresponding to the base of the cone, where usually only the last whorl is visible, is usually called ventral, or ventral. The side corresponding to the apex of the cone, where all whorls are visible, is called dorsal, or dorsal. The spiral seam separates the spiral whorls from each other.

The spiral-helical type of shells is distinguished by the fact that the height of the growth of the chambers occurs in a high spiral, which significantly exceeds the diameter of the base (see Fig. 37). Typically, such shells have a spiral arrangement of chambers that looks like a two-, three-, or multi-row arrangement of chambers, and therefore the names two-row, three-row, or multi-row shells are more often used for them. In attached foraminifera, the shell takes on a tree-like or irregularly branched shape (see Fig. 34, 4).

The shape of the chambers is very diverse. Chambers are distinguished: spherical, oval, tubular, cyclic, radially elongated, angular (conical, diamond-shaped, truncated-conical), roll-shaped.

However, the main types of shell structure discussed above do not exhaust the variety of their forms.

Heteromorphism. Often, during the process of individual development (ontogenesis), a change in the type of shell structure occurs, which leads to a heteromorphic structure. For example, the initial shell may be spiral-planar, the next section may consist of two sparsely spaced chambers, and the final section may be single-row. Such a shell is called trimorphic. If the shell combines only two types of structure, then it is bimorphic (see Fig. 37, 2b, c), and, finally, if it is of the same type in its structure, then it is called monomorphic. The most pronounced heteromorphic structure of the shell is expressed in microspherical individuals (schizonts).

Aperture, or mouth. The opening through which the cytoplasm communicates with the external environment, located at the end of a single-chamber shell or in the last septum of a multi-chamber shell, is called the mouth, or aperture. The last septum forms the septal, or aperture, surface. When a new chamber is formed, the mouth of the previous chamber becomes an opening connecting adjacent chambers. This hole is called a foramen (opening, hole); hence the entire subclass received the name foraminifera. The orifice (Fig. 30) is located in the center, eccentrically or at the base of the aperture septum; it can be simple, i.e., consist of one hole of various shapes: round, oval, slit-shaped, cross-shaped, branched, radial. A complex orifice consists of several openings. The most common type of complex orifice is the sieve orifice, which consists of numerous small openings. In many foraminifera the structure of the mouth is complicated additional education, which include special outgrowths called dental plates, or teeth. They have important taxonomic significance and apparently serve to strengthen the edge of the shell and attach the bundle of emerging pseudopodia.

In addition to the main mouth, various openings in the shell serve as outlets for ectoplasm. These include thin channels piercing the wall of some agglutinated and calcareous microgranular and radiate shells; additional orifices are located in different places: along the peripheral edge, along the seam, etc.

Channel system. The most highly organized foraminifera (rotaliids, nummulitids) have a system of canals inside the shell (Fig. 31). The main elements of this system are the spiral and interseptal canals. The spiral canal is connected to the ventral lobe of each chamber; interseptal canals extend from it, located in the cavities of double septa and opening with thin pores in the sutures. In some rotaliids, the canal system is very complex: not one, but two spiral canals are observed, from which the umbilical and interseptal canals extend.

Rice. 31. The canal system in rotaliids: 1a - view from the ventral side; 1b - internal cast along a longitudinal section; vk - intraseptal canal, k - chambers, sk - spiral canal, y - mouth, y" - mouth of the spiral canal

Additional skeleton. The additional skeleton includes those formations that complicate the structure of the shell and septa. They can be internal and external. Internal formations include calcareous deposits located in endothyrids and fusulinids at the edges of the equatorial aperture (chomata) or on the sides of additional apertures (parachomata), or intermittently only near the septa (pseudochomata). These also include conical columns of nummulitids that penetrate the shell. On the surface of the whorls they look like tubercles - granules and serve to strengthen the shell.

External additional skeletal formations include various sculptural elements in the form of ribs, cells, carinae, tubercles, needles, spines and various outgrowths on the shell.

In some foraminifera with a spiral shell, the umbilical region is closed by a kind of sleeve or disk consisting of glassy calcite; often this disc is penetrated by tubules associated with internal system channels. Many shells of planktonic foraminifera have thin, long spines, which significantly increase their overall surface and make it easier to soar in the water column.

Reproduction and development. Foraminifera have a complex life cycle of development (Fig. 32), accompanied by alternation of asexual and sexual generations. During sexual reproduction, at some stage of development in an individual that has reached adulthood, the nucleus is divided into a huge number (thousands) of particles, around which a small particle of cytoplasm is separated. In this way, mononuclear cells appear, equipped with two bundles. These are sex cells, or gametes. In their structure they are completely identical and, thanks to their flagella, they have mobility. After the fusion of two gametes (fertilization), usually originating from different individuals, a fertilized cell arises - a zygote, which has a diploid set of chromosomes. The first (embryonic) calcareous chamber stands out around the zygote. From it, in multi-chambered foraminifera, the second, third, etc. chambers are formed. The zygote gives rise to the microspherical generation, or schizont. Schizont (form B) comparatively for a long time remains mononuclear, but with a diploid set of chromosomes. Then, at some stage of growth, reduction division occurs and the nucleus becomes haploid (with a single set of chromosomes). When a schizont reaches an adult state, the nucleus divides sequentially several times and the schizont temporarily becomes multinucleated; Dozens and sometimes over hundreds of small nuclei are formed, around which the cytoplasm is separated. In this case, so-called “embryos” or amoeba-shaped embryos appear. A fairly large embryonic chamber is formed around each “embryo”. "Embryos" leave the mother's shell and move on to independent existence. This process is asexual reproduction. The emerging individuals gradually grow, build new chambers and give rise to a macrospherical generation, called gamonts (form A).

Rice. 32. Scheme of alternation of generations in foraminifera: a - microspherical form (schizont B) with daughter “embryos”; b, b" - megaspherical forms (gamonts A 1, A 2); d - gamete with a haploid (p) set of chromosomes, h - zygote with a diploid (2p) set of chromosomes, pp - reduction division, e - daughter "embryos"

A study of the ontogenesis of foraminifera has shown that a regular alternation of gamonts and schizonts is usually observed. But sometimes this natural alternation is disrupted; one schizont (form B) is followed by two generations of gamonts (forms A 1, A 2). In some cases, gamonts are almost indistinguishable or slightly different in size, in others, gamonts are larger than schizonts and have a large number of chambers, in still others, gamonts and schizonts differ in the size of the initial chambers. In macrospherical specimens the initial chamber is usually large in size, the shell is relatively small and the number of chambers is smaller than in microspherical specimens. The latter are distinguished by the small size of the initial chambers, a relatively large shell and a generally large number of chambers. The phenomenon associated with the formation of two types of shell structure in foraminifera is called dimorphism. The study of dimorphism (or trimorphism) is important not only from the point of view of systematics, but also for studying the origin and family ties between foraminifera. In this case, individuals that arose as a result of the sexual process and more fully reflect ontogenetic development are more important.

Fundamentals of taxonomy and classification. Important for taxonomy, foraminifera have the structure and composition of the shell wall, the structure of the cytoplasm and nucleus, features of the alternation of generations and other characteristics. On this basis, D. M. Rauzer-Chernousova and A. V. Fursenko (1959) identified 13 orders. American researchers A. Leblik and E. Tappan (1964) proposed dividing the foraminiferal order into five suborders. In accordance with the textbook rank of foraminifera as a subclass, these suborders are raised to the level of superorders. The subclass of foraminifera, based on the structure of the shell wall, is divided into five superorders: Allogromioidea, Textularioidea, Fusulinoidea, Miliolidoidea, Rotalioidea.

Phylum Foraminifera.

Foraminifera are marine shell rhizomes. This is the largest group of sarcoids. Foraminifera are found in all seas and are especially diverse at depths of 100-200m. They are part of the benthos and lead a crawling lifestyle. Rare species foraminifera, for example from the genus Globegirina, lead a planktonic lifestyle.

Foraminifera shells come in three types: organic, made of pseudochitin, encrusted, mainly with grains of sand, and calcareous. This is the exoskeleton secreted by the ectoplasm of the cell. The most common are calcareous shells. The sizes of the shells vary from 20 microns to 5 cm. Calcareous foraminiferal shells can be single-chambered or multi-chambered with an aperture. The partitions between the chambers are permeated with holes, and the cytoplasm of the cell is a single whole. The walls of the shells may be perforated or unperforated.

Thin branching rhizopodia protrude through the mouth of the shell and holes in its wall. Rhizopodia perform two functions: locomotor and food capture. Foraminifera, using rhizopodia, attach to the substrate and slowly move on these flowing thin threads, and also use them to capture food. They feed on bacteria, small protozoa and even multicellular organisms. Foraminifera have one or many nuclei. Some species of foraminifera have various symbionts: bacteria and unicellular algae.

Life cycles of foraminifera. In most foraminifera species, in the process life cycle alternation of sexual and asexual reproduction is observed. The figure depicts the developmental cycle of the unicameral foraminifera Myxotheca arenilega, which reflects typical features of the development of testate rhizomes.

The asexual generation of shell rhizomes - agamonts, through multiple divisions, form daughter agamete cells. These amoeboid cells leave the mother shell, grow, secrete a new shell around themselves and give rise to another generation of shell rhizomes - gamonts, which reproduce sexually.

Gamonts undergo multiple division (gamogony), and at the same time small cells with flagella are formed - gametes. Gametes are formed during gamogony significantly more (hundreds) than the number of agametes during agamogony (tens). Gametes are released into the water, where they copulate. Most foraminifera exhibit isogamous copulation of gametes that are identical in size and shape. This is the most primitive form of the sexual process. From the zygote, agamonts are formed, secreting a shell around themselves.

The alternation of sexual and asexual reproduction in the life cycle of species is called metagenesis.

In the life cycle of foraminifera, there is an alternation of haploid and diploid generations (the only case in the animal kingdom). Agamonts developing from a zygote are diploid. During the process of agamogony, one of the first divisions of the nucleus is meiosis. Thus, unlike multicellular animals, in which meiosis occurs during the formation of gametes (gametic reduction), in foraminifera, chromosome reduction is observed during the formation of agametes. In contrast to zygotic reduction, in foraminifera the reduction of chromosomes is called intermediate, since it does not occur immediately after the formation of the zygote, but only during the formation of agametes.

The meaning of foraminifera. Foraminifera shells make up layers of limestone, chalk and some other rocks. Foraminifera have been known in fossil form since the Cambrian. In total, about 30 thousand fossil species of foraminifera are known. Nummulite limestones are composed of shells of large species of foraminifera - nummulites, the size of which reached 5-16 cm. Fusuline limestones, consisting of smaller fusuline shells, are more widespread. Cretaceous deposits consist of the smallest foraminiferal shells, as well as limestone shells of flagellates - coccolithophores.

Each geological period was characterized by special mass species of foraminifera, which serve as guiding forms in stratigraphy for determining the age of geological strata.

Also, fossil foraminifera are used by geologists as indicators of oil-bearing formations based on the relationship of location individual species foraminifera with oil occurrence.

Order Foraminifera

Foraminifera are the largest order of Sarcodidae.

amoeba protozoan phagocytosis

More than 1000 species of foraminifera are known as part of the modern marine fauna. A small number of species, probably representing the remnant of marine fauna, live in the subsurface salt waters and brackish wells of Central Asia. In the oceans and seas, foraminifera are ubiquitous. They are found in all latitudes and at all depths. However, only a very few species living in the seawater column are planktonic organisms.

Structure: Foraminifera have a shell - an exoskeleton. Most shells are calcareous, sometimes forming chitinoid or consisting of foreign particles glued together by cell secretions. Foraminifera ingest foreign particles and then secrete them onto the surface of the body, where they are anchored in the thin outer leathery layer of the cytoplasm.

However, most of them have calcareous shells consisting of calcium carbonate. The sizes of calcareous shells of different species of foraminifera can be very different. Most calcareous rhizome shells are not single-chambered, but multi-chambered.

The internal cavity of the shell is divided by partitions into a number of chambers, the number of which can reach several tens and hundreds. The partitions between the chambers are not solid, they have holes, due to which the protoplasmic body of the rhizome is not divided into parts, but represents a single whole.

Nutrition : The walls of the shells are not all, but many foraminifera are permeated with tiny pores, which serve to allow the pseudopodia to exit. Foraminiferal pseudopodia are thin, long and filamentous. They are often connected to one another by anastomotic bridges, forming a trapping network into which small organisms are caught that serve as food for foraminifera.

Food captured by pseudopodia is digested outside the bulk of the cytoplasm. Food vacuoles are formed in a bunch of pseudopodia surrounding food objects. Nutrients are absorbed and enter the cytoplasm of the foraminifera.

Reproduction : Foraminifera reproduce both asexually and sexually, and in some forms these two methods of reproduction alternate with each other. Asexual reproduction begins with the nucleus dividing several times in succession, resulting in the formation of many small size cores.

Then, around each nucleus, a section of cytoplasm is isolated and the entire protoplasmic body of the rhizome breaks up into many mononuclear amoeba-shaped embryos, which emerge through the mouth to the outside.

Immediately around the amoeba-shaped embryo, a thin calcareous shell stands out, which will become the embryonic chamber of the future multi-chambered shell.

Thus, with asexual reproduction in the first stages of its development, the rhizome is single-chambered.

However, very soon, more cameras begin to be added to this first chamber.

It happens like this: a certain amount of cytoplasm immediately protrudes from the mouth, which immediately secretes a shell. Then there is a pause, during which the protozoan feeds intensively and the mass of its protoplasm increases inside the shell.

Then again part of the cytoplasm protrudes from the mouth and another calcareous chamber forms around it. This process is repeated several times: more and more new chambers appear until the shell reaches the dimensions characteristic of this species.

As a result of asexual reproduction, individuals of the macrospherical generation are obtained, which differ significantly from the microspherical generation that gives rise to them.

Representatives : Representatives of this order are: Globigerina, Elphidium strigilata, Elphidium crispum, Nodomorphina compressiuscula, Ammodiscus incertus, Peneroplis planatus.

Rays, or radiolarians, - rich in species a group of exclusively marine sarcoids. It has more than 6000 species. They lead only a planktonic lifestyle. Sizes from 40 microns to 1 mm or more. Skeleton made of silica or strontium sulfate.

Among the huge army of living organisms inhabiting our planet, there are foraminifera. This name seems a little unusual to some people. The creatures wearing it also differ in many ways from the creatures we are used to. Who are they? Where do they live? What do they eat? What is their life cycle? What niche did they occupy in the animal classification system? In our article we will cover all these issues in detail.

Group Description

Foraminifera are representatives of a group of protists, single-celled organisms with a shell. Before we begin to study foraminifera, let us become familiar with the group to which they belong.

Protists are a set of organisms that are part of a paraphyletic group, which includes all eukaryotes that were not part of the plants, fungi and animals familiar to us. He introduced this name in 1866, but it acquired a modern understanding only when it was mentioned in 1969 by Robert Whittaker, in his author’s work on the system of the five kingdoms. The term "protists" comes from the Greek "proti", which means "first". These are the organisms with which, one might say, life began on our planet. According to traditional standards, protists branch into three branches: algae, fungi, and protozoa. All of them have a polyphyletic nature and cannot assume the role of a taxon.

Protists are not distinguished according to the presence positive characteristics. Most often, protists are a common set of single-celled organisms, but at the same time, many of their varieties are capable of building the structure of a colony. Some representatives may be multicellular.

General phenotypic data

The simplest foraminifera have an exoskeleton in the form of a shell. Their predominant amount is made up of limestone and chitinoid structures. Only sometimes do we come across creatures with a shell made of foreign particles glued together through the activity of the cell.

The cavity located inside the shell communicates with the environment around the body through numerous pores. There is also an orifice - a hole leading into the cavity of the shell. Through the pores, the thinnest, external and branching pseudopods grow, which form a connection with each other using reticulopodia. They are necessary for the movement of the cell along the surface or in the water column, as well as for the extraction of food. Such pseudopods form a special mesh, the diameter of which extends far beyond the shell itself. Particles begin to stick to such a network, which in the future will serve as food for foraminifera.

Lifestyle

Foraminifera are classified as protists, mainly marine type. There are forms that inhabit brackish and fresh waters. You can also meet representatives of species that live at great depths or in loose muddy bottoms.

Foraminifera are divided into planktonic and benthic. In plankton, the shell is considered the most widespread "organ" of their biogenic activity, which takes the form of sediments on the ocean floor. However, after the mark of 4 thousand m they are not observed, which is due to the rapid process of their dissolution in the water column. The silt from these organisms covers about a quarter of the planet's total territory.

Data obtained through the study of fossil foraminifera make it possible to determine the age of sediments formed in the distant past. Modern species have very small sizes, from 0.1 to 1 mm, and extinct representatives could reach up to 20 cm. Most shells appear to be sandy fractions, up to 61 microns. The maximum concentration of foraminifera present in sea ​​water. There are a lot of them in the waters near the equator and in the waters of high latitudes. They were also found in the Mariana Trench. It is important to know that the diversity of species and the complexity of their shell structure are characteristic only of the equatorial region. In some places, the concentration indicator can reach one hundred thousand specimens in the thickness of one cubic meter water.

The concept of benthic protists

Benthos is a collection of animal species that inhabit the thickness of ordinary soils and those at the bottom of reservoirs. Oceanology considers benthos - as organisms that live on the sea and ocean floor. Researchers of the hydrobiology of fresh water bodies describe them as inhabitants continental type reservoirs. Benthos are divided into animals - zoobenthos and plants - phytobenthos. Among this variety of organisms, a large number of foraminifera are observed.

In zoobenthos, animals are distinguished by their habitat, mobility, penetration into the ground or method of attachment to it. In accordance with their feeding method, they are divided into predators, herbivores and organisms that feed on particles of organic nature.

The concept of planktonic protists

Species of foraminifera of the planktonic type are tiny organisms that drift in the water column and cannot resist the current (swim where they want). Such specimens include certain types of bacteria, diatoms, protozoa, mollusks, crustaceans, fish larvae, eggs, etc. Plankton serves as food for a large number of animals inhabiting the waters of rivers, seas, lakes and oceans.

The word “plankton” was introduced into circulation by the German oceanologist W. Hensen in the last years of the 1880s.

Features of the design of sinks

Foraminifera are animals whose shells are classified according to the method of their formation. There are two forms - secretory and agglutinated.

The first type is characterized by the fact that the formation of the shell occurs through the combination of mineral and organic substances that the animal itself secretes.

The second (agglutinated) type of shell is formed by capturing a number of fragments from the skeletons of other organisms and sand particles. Bonding is carried out by a substance secreted by a single-celled organism.

School chalk contains a large percentage of foraminiferal shells, which are its main element.

Based on their composition, the following types of protists are distinguished:

• Organic foraminifera are the oldest form, occurring at the beginning of the Paleozoic.
• Agglutinated - consisting of various kinds of particles, up to carbonate cement.
• Secreted calcareous - composed of calcite.

Foraminifera shells vary in structure by the number of chambers. The “house” of an organism can consist of one chamber or many. Multi-chamber sinks are divided according to the linear or spiral method of construction. Winding of curves in them can occur in a ball-shaped and planospiral, as well as in a trochoid way. There were foraminifera with an orithoid type of shell. In almost all organisms, the first chamber is the smallest in size, and the last one is the largest. Secretion-type shells often have “stiffening ribs” that increase the mechanical strength.

Cycles of life

The class of foraminifera is characterized by a haplo-diplophase life cycle. In a generalized scheme, it looks like this: representatives of haploid generations undergo as a result of which a similar series of gametes with two flagella appears. These cells fuse in pairs and form the entire structure of the zygote. From it, an adult individual will subsequently develop, belonging to the agamont generation.

The fact that during fusion the chromosome set is doubled determines the formation of a diploid generation. Inside the agamont, the process of nuclear division takes place, which occurs thanks to meiosis. The space around the haploid nucleus, which became so due to reduction division, is separated by cytoplasm and forms a shell. This leads to the formation of agamonts, which are similar in purpose to spores.

Protozoa in nature

Let us consider the role and significance of foraminifera in nature and human life.

Feeding on bacterial organisms and the remains of organic nature, protozoa do a great job of preventing pollution.

Protozoa, among which there are many foraminifera, have a high rate of fertility under certain conditions environment. They serve as food for the fry.

Euglena, in addition to serving as food for other inhabitants of water bodies and cleaning them, carries out photosynthesis processes, reducing the concentration of CO2 and increasing the O2 content in the waters.

The degree of pollution can be determined by analyzing the number of euglena and ciliates in the water column. If the reservoir contains a huge amount of organic compounds, then an increased number of euglena will be observed there. Amoebas are most often concentrated where the content of organic substances is low.

The “houses” of protozoa participated in the formation of limestone and cretaceous fossils. Therefore, they play an important role in industry, as they have formed substances that are widely used by humans.

Taxonomy data

In our time, about ten thousand species of foraminifera are known, and the number of known fossils exceeds the forty thousand mark. The most famous examples are the amoebas of foraminifera, miliolides, globigerina, etc. In the hierarchical table of taxonomic elements of living nature, they were given the title of class, which is also called the phylum of the simplest organisms of eukaryotes. Previously, this domain consisted of five suborders and was included in the single order Foraminiferida Eichwald. A little later, researchers decided to raise the status of foraminifera to an entire class. The classification identifies them as having 15 subclasses and 39 orders.

Results

Based on the material in the article, it can be understood that foraminifera are representatives of protists, single-celled organisms that are part of the superkingdom of eukaryotes. They have shells that are formed from two main materials, namely, from grains of sand and from minerals, as well as from substances secreted by them. Foraminifera occupy an important place in the food chain. They had a huge influence on the formation of the modern picture of the planet’s soils.

The most extensive order among rhizomes are the inhabitants of the sea - foraminifera(Foraminifera). More than 1000 species of foraminifera are known as part of the modern marine fauna. A small number of species, probably representing the remnant of marine fauna, live in the subsurface salt waters and brackish wells of Central Asia.

In the oceans and seas, foraminifera are ubiquitous. They are found in all latitudes and at all depths, from the coastal littoral zone to the deepest abyssal depressions. Nevertheless, the greatest diversity of foraminiferal species is found at depths of up to 200-300 m. The vast majority of foraminiferal species are inhabitants of the bottom layers and are part of the benthos. Only a very few species live in the thickness of sea water and are planktonic organisms.

Let's get acquainted with some of the most characteristic forms of the foraminifera skeleton (Fig. 32).

Among the huge variety of foraminiferal shell structures, two types can be distinguished by their composition. Some of them consist of particles foreign to the body of the root - grains of sand. Just as we saw with diffusion(Fig. 30), foraminifera possessing such agglutinated shells swallow these foreign particles and then secrete them onto the surface of the body, where they are anchored in the thin outer leathery layer of the cytoplasm. This type of shell structure is typical for frequently occurring representatives of the genera Hyperammina, Astrorhiza (Fig. 32, 3-7), etc. For example, in some areas of our northern seas (Laptev Sea, East Siberian Sea) these large foraminifera, reaching 2-3 cm length, cover the bottom with an almost continuous layer.

The number of foraminiferal species with agglutinated shells is relatively small (although the number of individuals of these species can be enormous). Most of them have calcareous shells consisting of calcium carbonate (CaCO3).

These shells are secreted by the cytoplasm of rhizomes, which have the remarkable property of concentrating in their body calcium contained in sea water in small quantities (calcium salts in sea water are slightly more than 0.1%). The sizes of calcareous shells of different species of foraminifera can be very different. They vary from 20 microns to 5-6 cm. This is approximately the same size ratio as between an elephant and a cockroach. The largest of the foraminifera, the shell of which is 5-6 cm in diameter, can no longer be called microscopic organisms. The largest (genus Cornusspira and others) live at great depths.

Among the calcareous shells of foraminifera, two groups can be distinguished.

Unilocular foraminifera have a single cavity inside the shell, which communicates with the outside environment through the mouth. The shape of single-chamber shells is varied. In some (for example, Lagena), the shell resembles a bottle with a long neck, sometimes equipped with ribs (Fig. 32, 2).

Very often, a spiral twisting of the shell occurs, and then its internal cavity becomes a long and thin canal (for example, Ammodiscus, Fig. 32, 8, 9).

Most calcareous rhizome shells are not single-chambered, but multi-chambered. The internal cavity of the shell is divided by partitions into a number of chambers, the number of which can reach several tens and hundreds. The partitions between the chambers are not solid, they have holes, due to which the protoplasmic body of the rhizome is not divided into parts, but represents a single whole. Not all shells, but in many foraminifera, are permeated with tiny pores, which serve to allow the pseudopodia to exit. This will be discussed in more detail below.

The number, shape and relative arrangement of chambers in a shell can be very different, which creates a huge variety of foraminifera (Fig. 32). In some species, the chambers are arranged in one straight row (for example, Nodosaria, Fig. 32, 12), sometimes their arrangement is double-rowed (Textularia, Fig. 32, 22). The spiral shape of the shell is widespread, when individual chambers are arranged in a spiral, and as they approach the chamber bearing the mouth, their sizes increase. The reasons for this gradual increase in the size of cameras will become clear when we consider the course of their development.

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In spiral foraminiferal shells there are several turns of the spiral. The outer (larger) whorls can be located next to the inner whorls (Fig. 32, 17, 18) so that all chambers are visible from the outside. This is an evolute type of shell. In other forms, the external (larger) chambers completely or partially cover the internal chambers (Fig. 33, 1). This is an involute type of shell. We find a special form of shell structure in foraminifera miliolide(family Miliolidae, Fig. 32, 19). Here the chambers are strongly elongated parallel to the longitudinal axis of the shell and located in several intersecting planes. The entire shell as a whole turns out to be oblong and somewhat reminiscent of a pumpkin seed in shape. The mouth is located at one of the poles and is usually equipped with a tooth.

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Shells belonging to the cyclic type (genera Archiacina, Orbitolites, etc., Fig. 33, 2, 34) are distinguished by their highly complex structure. The number of chambers here is very large, with the inner chambers arranged in a spiral, while the outer ones are arranged in concentric rings.

What is the biological significance of such a complex structure of multi-chambered rhizome shells? A special study of this issue showed that multi-chamber sinks are much stronger than single-chamber sinks. The main biological significance of the shell is the protection of the soft protoplasmic body of the rhizome. With the multi-chamber structure of the shell, this function is carried out quite perfectly.

How is the soft protoplasmic body of foraminifera structured?

The internal cavity of the shell is filled with cytoplasm. The nuclear apparatus is also placed inside the shell. Depending on the stage of reproduction (which will be discussed below), there may be one or several cores. Numerous very long and thin pseudopodia protrude from the shell through the mouth, branching and anastomosing among themselves. These special foraminiferal false legs are called rhizopodia. The latter form a very fine mesh around the shell, the total diameter of which usually significantly exceeds the diameter of the shell (Fig. 34). In those species of foraminifera that have pores, the rhizopodia protrude out through the pores.

The function of rhizopodium is twofold. They are organelles of movement and food capture. Various small food particles “stick” to the rhizopodia, very often these are unicellular algae. Their digestion can occur in two ways. If the particle is small, it gradually “slides” along the surface of the rhizopodium and is drawn into the shell through the mouth, where digestion occurs. If the food particle is large and cannot be drawn into the shell through the narrow mouth, then digestion occurs outside the shell. In this case, cytoplasm collects around the food and a local, sometimes quite significant thickening of the rhizopodium is formed, where the digestion processes take place.

Studies carried out in recent years using time-lapse filming have shown that the cytoplasm that makes up the rhizopodium is in continuous motion. Cytoplasmic currents flow quite quickly along the rhizopodium in the centripetal (towards the shell) and centrifugal (away from the shell) directions. On the two sides of the thin rhizopodium, the cytoplasm seems to flow in opposite directions. The mechanism of this movement still remains unclear.

Reproduction of foraminifera is quite complex and in most species is associated with alternation of two different forms reproduction and two generations. One of them is asexual, the second is sexual. Currently, these processes have been studied in many species of foraminifera. Without going into details, let's look at them using a specific example.

Figure 35 shows the life cycle of the foraminifera Elphidium crispa. This species is a typical multichambered foraminifera with a spirally twisted shell. Let's begin our consideration of the cycle with a multi-chambered rhizome, which has a small germinal chamber in the center of the spiral (microsphere generation).

The cytoplasm of the rhizome initially contains one nucleus. Asexual reproduction begins with the nucleus dividing successively several times, resulting in the formation of many small nuclei (usually several dozen, sometimes over a hundred). Then, around each nucleus, a section of cytoplasm is isolated and the entire protoplasmic body of the rhizome breaks up into many (according to the number of nuclei) mononuclear amoeba-like embryos, which emerge through the mouth to the outside. Immediately around the amoeba-shaped embryo, a thin calcareous shell stands out, which will be the first (embryonic) chamber of the future multi-chambered shell. Thus, with asexual reproduction in the first stages of its development, the rhizome is single-chambered. However, very soon, more cameras begin to be added to this first chamber. It happens like this: a certain amount of cytoplasm immediately protrudes from the mouth, which immediately secretes a shell. Then there is a pause, during which the protozoan feeds intensively and the mass of its protoplasm increases inside the shell. Then again part of the cytoplasm protrudes from the mouth and another calcareous chamber forms around it. This process is repeated several times: more and more new chambers appear until the shell reaches the dimensions characteristic of this species. Thus, the development and growth of the shell is stepwise. The dimensions and relative position of the chambers are determined by how much protoplasm protrudes from the mouth and how this protoplasm is located in relation to the previous chambers.

We began our examination of the life cycle of Elphidium with a shell that had a very small embryonic chamber. As a result of asexual reproduction, a shell is obtained, the embryonic chamber of which is much larger than that of the individual that began asexual reproduction. As a result of asexual reproduction, individuals of the macrospherical generation are obtained, which differ significantly from the microspherical generation that gives rise to them. In this case, the offspring turns out to be not quite similar to the parents.

How do individuals of the microspherical generation arise?

They develop as a result of sexual reproduction of the macrospheric generation. This happens as follows. As with asexual reproduction, the sexual process begins with nuclear division. The number of nuclei formed in this case is much greater than during asexual reproduction. A small area of ​​cytoplasm is isolated around each nucleus, and in this way a huge number (thousands) of mononuclear cells are formed. Each of them is equipped with two flagella, thanks to the movement of which the cells swim actively and quickly. These cells are sex cells (gametes). They merge with each other in pairs, and the fusion affects not only the cytoplasm, but also the nuclei. This process of fusion of gametes is the sexual process. The cell formed as a result of the fusion of gametes (fertilization) is called a zygote. It gives rise to a new microspherical generation of foraminifera. Around the zygote, immediately after its formation, a calcareous shell stands out - the first (embryonic) chamber. Then the process of development and growth of the shell, accompanied by an increase in the number of chambers, is carried out according to the same type as during asexual reproduction. The shell turns out to be microspherical because the size of the zygote secreting the embryonic chamber is many times smaller than the mononuclear amoeboid embryos formed during asexual reproduction. Subsequently, the microspherical generation will begin asexual reproduction and again give rise to macrospherical forms.

Using the example of the life cycle of foraminifera, we encounter an interesting biological phenomenon of the regular alternation of two forms of reproduction - asexual and sexual, accompanied by the alternation of two generations - microspherical (develops from a zygote as a result of fertilization) and macrospherical (develops from mononuclear amoeboid embryos as a result of asexual reproduction).

Let us note another interesting feature of the sexual process of foraminifera. It is known that in most animal organisms, sex cells (gametes) are of two categories. On the one hand, these are large, rich in protoplasm and reserve nutrients immobile egg (female) cells, and on the other - small motile sperm (male reproductive cells). Sperm motility is usually associated with the presence of an actively moving filamentous tail. In foraminifera, as we have seen, there are no morphological (structural) differences between sex cells. They are all identical in structure and, due to the presence of flagella, have mobility. There are still no structural differences that would allow us to distinguish between male and female gametes. This form of the sexual process is the original, primitive one.

As already mentioned, the vast majority of modern foraminiferal species are benthic organisms found in the seas of all latitudes from the coastal zone to the greatest depths of the world's oceans. A study of the distribution of rhizomes in the ocean showed that it depends on a number of environmental factors - temperature, depth, salinity. Each zone has its own typical foraminifera species. Species composition foraminifera can serve as a good indicator of habitat conditions.

Among foraminifera there are a few species leading a planktonic lifestyle. They constantly “float” in the thickness of the water mass. A typical example of planktonic foraminifera is different types globigerin(Globigerina, fig. 36). The structure of their shells differs sharply from the structure of the shells of bottom rhizomes. Globigerina shells are thinner-walled, and most importantly, they bear numerous appendages diverging in all directions - the thinnest long needles. This is one of the adaptations to life in plankton. Due to the presence of needles, the surface of the body, namely the ratio of surface to mass - a value called specific surface area, increases. This increases friction when immersed in water and promotes “floating” in the water.

Foraminifera, widespread in modern seas and oceans, were also richly represented in previous geological periods, starting with the most ancient Cambrian deposits. Calcareous shells, after reproduction or death of the rhizome, sink to the bottom of the reservoir, where they become part of the silt deposited at the bottom. This process takes place over tens and hundreds of millions of years; As a result, thick sediments are formed on the ocean floor, which include a myriad of rhizome shells. During the mountain-building processes that took place and are taking place in the earth's crust, as is known, some areas of the ocean floor rise and become dry land, and land falls and becomes the bottom of the ocean. Much of modern land has been the bottom of the ocean at various geological periods. This fully applies to the territory Soviet Union(with the exception of a few northern regions of our country: the Kola Peninsula, most of Karelia and some others). Sea bottom sediments on land turn into sedimentary rocks. All marine sedimentary rocks contain rhizome shells. Some deposits, such as the Cretaceous ones, consist mostly of shells of rhizomes. Such a wide distribution of foraminifera in marine sedimentary rocks is of great importance for geological work, and in particular for geological exploration. Foraminifera, like all organisms, did not remain unchanged. During the geological history of our planet, the evolution of the organic world took place. Foraminifera also changed. For different geological periods The history of the Earth is characterized by its own species, genera and families of foraminifera. It is known that the geological age of these rocks can be determined from the remains of organisms in rocks (fossils, imprints, etc.). Foraminifera can also be used for this purpose. As fossils, due to their microscopic size, they present very great advantages, since they can be found in very small quantities rock. In geological exploration of mineral resources (especially in oil exploration), the drilling method is widely used. This produces a column of rock of small diameter, covering all the layers through which the drill passed. If these layers are marine sedimentary rocks, then microscopic analysis always reveals foraminifera. Due to its great practical importance, the question of the association of certain types of foraminifera with certain sedimentary rocks of calcareous age has been developed with a high degree of accuracy.

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