INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

SARCODINA

PHYLUM FORAMINIFERA

Marine, predominantly benthic, less commonly planktonic free-living Protozoans whose Cell is enclosed in a unilocular or multilocular shell. Pseudopodia are slender, branched, and anastomosing (reticulopodia). Life cycles proceed with alternation of sexual and asexual generations, with intermediate reduction. One part of The life cycle may be uninucleate, and the other multinucleate. Approximately 4 thousand extant species have been described. In the Black Sea, 26 species have been found, and in the Sea of Azov, 10.

Structure. Shell sizes range from 20 µm to 5-6 cm. The shell consists of an organic matrix impregnated with CaCO₃ or encrusted with sand grains, sponge spicules, or other foreign particles. It can be unilocular or multilocular (most commonly 6-30 chambers, less frequently several hundred) (Fig. 27). The shell has one or more apertures (mouths) for the exit of reticulopodia. In multilocular forms, the chambers communicate with each other through openings called foramina (hence the phylum name, from Lat. foramen - opening and ferre - to bear), or through complex canals and tunnels in more advanced forms. Here, a complex integration system arises, uniting individual chambers into a single whole, through which ectoplasm circulates. The function of this integration system can be compared to that of a Circulatory system with calcareous walls. In addition, the shells of most species are penetrated by tiny pores closed from the inside by sieve-like plates with extremely minute micropores. These porous plates are permeable to oxygen, carbon dioxide, salt ions, and low-molecular-weight organic substances, thus playing a vital role in Respiration, osmoregulation, and environmental exchange.

Class="center">

Fig. 27. Foraminifers (after Polyansky): a - Spiroloculina depressa; b - Nonion umbillicatulus;

c - Globigerina sp.; d - Textularia sagittula; e - Orbitolites complanatus

The foraminiferal shell is externally covered with a layer of Cytoplasm—thin in benthic species and thick in planktonic ones. A network of thin, branched, and anastomosing threads—reticulopodia—extends from this outer cytoplasmic layer. The outer layer of cytoplasm in extended reticulopodia moves rapidly in opposite directions: along one side of the reticulopodium from its base to the tip, and along the other in reverse. The inner, axial part of the reticulopodium is denser and contains longitudinal microtubules. Reticulopodia perform diverse Functions: capturing food, locomotion, forming new capsules, etc.

Foraminiferal Cells possess one or multiple nuclei depending on the stage of the life cycle. In some multinucleate species at certain stages, nuclear dualism occurs: the nuclei are differentiated into vegetative and generative ones, meaning they differ in their functions.

Nutrition in foraminifers is diverse: they can consume Water-soluble Amino Acids, as well as solid detritus particles, Bacteria, unicellular Algae, Yeasts, Protozoa, and invertebrate eggs and larvae. The cytoplasm transports food to the aperture and draws it inside the shell, where digestive vacuoles are formed; undigested remains are transported by the opposing cytoplasmic current to the tips of the reticulopodia and expelled outward.

The life cycle (Fig. 28) exhibits metagenesis with an intermediate type of chromosome reduction. The sexual generation—the gamont—consists of uninucleate haploid individuals that, through repeated mitotic divisions, produce several thousand biflagellated Gametes. The gametes leave the shell and enter the water, where they fuse in pairs (copulate) to form diploid zygotes.

Fig. 28. Life Cycle of the foraminifer Elphidium crispa (after Polyansky, modified):

1 - gamont (macrospheric form); 2 - release of gametes; 3 - gamete copulation;

4 - zygote; 5 - agamont (microspheric form); 6 - release of agametes; 7 - young gamont

The zygote is surrounded by a unilocular shell (the embryonic chamber), to which subsequent chambers are later added, and grows into a multinucleate diploid agamont (the asexual generation). Inside the shell, the zygote Nucleus divides mitotically numerous times, producing several hundred diploid nuclei. Among them are vegetative nuclei, which regulate METABOLISM, and generative nuclei, which participate in reproduction. In the adult agamont, a reduction division of the generative nuclei (Meiosis) takes place, resulting in The formation of numerous haploid nuclei. A portion of cytoplasm segregates around each of them, and the agamont cell breaks down into hundreds of haploid uninucleate cells—agametes—which emerge from the maternal shell into the water, with each building its own shell and transforming into a uninucleate gamont. The shells of gamonts and agaments differ morphologically: the embryonic chamber of the gamont is quite large ("macrospheric shell"), whereas that of the agamont is small ("microspheric shell").

Foraminiferal growth occurs by adding new chambers: a second, third, and so on are built onto the first embryonic chamber until the shell reaches the size characteristic of the species. This process varies among different species, but reticulopodia always play an active role in the formation of subsequent

chambers. For example, in Textularia candeiana, whose shell is encrusted with detritus particles, this process proceeds as follows: reticulopodia emerge through the aperture of the last chamber in the shape of a fan and form a cyst around themselves composed of densely packed detritus particles.

Inside the cyst, in front of the aperture, a space is formed that matches the size of the future chamber. Next, the reticulopodia pick up soil particles, distribute them evenly over the entire surface of the prospective chamber, and cement them with organic matter, upon which biocrystallized calcium carbonate is subsequently deposited. A thin organic layer is formed on the inner wall of the chamber. The cyst breaks down, the new chamber fills with cytoplasm, and reticulopodia extend through its aperture.

Typical Representatives of the marine benthos are the genera Polystomella and Peneroplis; certain species of the genus Globigerina lead a planktonic lifestyle.

Foraminifers play a major role in the formation of sedimentary rocks. Calcareous deposits on sea floors consist of the shells of deceased foraminifers. As a result of mountain-building processes, these deposits can be uplifted to land. Thus, the Pyrenees, Alps, and Himalayas largely consist of foraminiferal limestones, which have long been used as a building material. The British Isles were named Albion by the Romans (from Lat. albus - white) due to the color of the chalk cliffs. Foraminifers belong to so-called "index fossils"—organisms whose remains are well preserved in geological strata and are used to determine the age of rocks.