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PROTOZOOPLANKTON The nonphotosynthetic, single-celled organisms historically referred to as Protozoa are important components of the marine plankton. This is a diverse assemblage encompassing many unrelated groups. Our treatment of protozooplankton is limited to the larger, singlecelled heterotrophic groups likely to be retained and noticed in zooplankton collections: ciliates, foraminiferans, radiolarians, acantharians, and heliozoans. Recent studies show how abundant these heterotrophic protozoans are in estuarine and coastal areas where they form a vital connection in the microbial loop, linking bacterial and nanoplankton production to larger zooplankton and fishes. At times, protozooplankton biomass may exceed that of mesozooplankton. This fauna is relatively unstudied in our area, except in a few localities . Heterotrophic dinoflagellates appear under phytoplankton in the previous section. AMOEBOID PROTOZOA: FORAMINIFERA, RADIOLARIA, ACANTHARIA, AND HELIOZOA Foraminifera, Radiolaria,1 Acantharia, and Heliozoa are all single-celled heterotrophic protozoans belonging to the Sarcodina. Like the more familiar sarcodine, Amoeba, they use their adhesive cellular extensions (pseudopods) to catch prey. Unlike Amoeba, these three protozoan groups possess a characteristic hard outer covering. Lacking cilia, flagella, or other means of locomotion, these planktonic species drift passively in the water. Although Radiolaria, Acantharia, and Heliozoa are unlikely to be abundant in nearshore waters, they are common, distinctive, and sometimes large enough to be noticed in zooplankton collections . The naked amoebae, while sometimes abundant in estuarine areas, are too small and delicate to be noticed without special collection and observation procedures. The Foraminifera, generally called “forams,” are covered by an outer shell (the test) usually composed of calcium carbonate. The shell is often divided into interconnected chambers added in distinctive patterns as the foram grows. Shells can have spines, visible pores, perforations, or some combination of these features. Some harbor photosynthetic symbionts. About 40 of the 4,000 species are planktonic and can be abundant, especially in the open ocean. The Radiolaria, Acantharia, and Heliozoa have spherical cells with fine, stiff cytoplasmic projections known as actinopods that radiate outward, giving them a starlike appearance . The Radiolaria have a glassy skeleton made of silica through which the actinopods 1. We use the traditional name Radiolaria, but its status is disputed. PROTOZOOPLANKTON 77 extend. These beautiful plankton are strictly marine and prefer clear, warm oceanic waters . They are found nearshore when Gulf Stream eddies or Loop Current intrusions come ashore. Radiolarian cells sometimes aggregate into large colonies. The similar Acantharia have strontium sulfate skeletons. Acantharia are common in deep ocean waters but can occur in nearshore waters. They are most commonly reported from the Gulf of Mexico but may occur along the Atlantic Coast when intrusions of Gulf Stream waters arrive nearshore . Most heliozoans live in freshwater, a few species thrive in oligo- and mesohaline reaches of estuaries, and one species is exclusively marine. Heliozoa should be observed while alive because they are difficult to preserve. Each of these three groups uses its own particular type of pseudopod to catch food, ranging from bacteria to algal cells to surprisingly large ciliate or copepod prey. Food items are ingested by phagocytosis, much like ingestion seen in Amoeba. Some warm-water radiolarians and forams retain algal cells (or their chloroplasts) and thus become functionally mixotrophic. Limited information on reproduction suggests that forams and Actinophrys sol, the common brackish heliozoan, reproduce asexually and sexually. Among the few radiolarians in which the life cycle is known, asexual reproduction (binary fission) is common ; sexual reproduction has been identified in some colonial species. The life cycle is typically two to four weeks. CILIATES Often overlooked because of their small size, ciliates are ubiquitous in coastal and estuarine waters where they play a crucial role in planktonic food webs. Of the several groups of ciliates, tintinnids, with their hard outer coverings, are often conspicuous in plankton samples . Other ciliate groups that lack a protective skeleton are less likely to be collected in recognizable form with plankton nets. Despite the active reclassification of higher groupings within both ciliates and sarcodines, many genus and species names have remained relatively stable for a century or more. Ciliates form a vital link, connecting bacteria and pico-/nanophytoplankton to larger zooplankton. Up to 50% of both bacterial and phytoplankton production may be removed by ciliates each day. In turn, ciliates are eaten by larger zooplankton, including copepods, decapod larvae, fish larvae, rotifers, and ctenophores. The trophic link provided by ciliates is especially crucial when tiny cells dominate algal productivity because few of the larger zooplankton feed on particles as small nano- or picophytoplankton. Ciliates are capable of rapid growth...


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