specific. the distributions of symbiodinium species comprising different ecological guilds in a coral reef ecosystem. a. host cnidarians can expel millions of symbiont cells (viable and necrotic) per day into the surrounding environment (a). in turn these animals pass large volumes of water through their gastrovascular system for respiration and cellular waste removal, a process that introduces numerous small particles including food and various other symbiodinium spp. (indicated by differences in cell color) (b). b. the ecological niche, from the viewpoint of functional groups, differs among species of symbiodinium. several ecological guilds exist, including abundant host-specific and host-generalist species (1), low background and potentially opportunistic species (2), and non-symbiotic species closely associated with the coral's biome (3) and/or occupying unrelated habitats (4) species diversity assigned to different ecological guilds the large diversity of symbiodinium revealed by genetic analyses is distributed non-randomly and appears to comprise several guilds with distinct ecological habits. of the many symbiodinium characterized genetically, most are host-specific, mutualistic, and dominate their host. others may represent compatible symbionts that remain as low-abundance background populations because of competitive inferiority under the prevailing external environmental conditions (e.g. high light vs. low light). some may also comprise opportunistic species that may proliferate during periods of physiological stress and displace the normal resident symbiont and remain abundant in the host's tissues for months to years before being replaced by the original symbiont. there are also those that rapidly infect and establish populations in host juveniles until being replaced by symbionts that normally associate with host adult colonies. finally, there appears to be another group of symbiodinium that are incapable of establishing endosymbiosis yet exist in environments around the animal or associate closely with other substrates (i.e. macro-algal surfaces, sediment surface) symbiodinium from functional groups 2, 3, and 4 are known to exist because they culture easily, however species with these life histories are difficult to study because of their low abundance in the environment. free-living and "non-symbiotic" populations there are few examples of documented populations of free-living symbiodinium. given that most host larvae must initially acquire their symbionts from the environment, viable symbiodinium cells occur outside the host. the motile phase is probably important in the external environment and facilitates the rapid infection of host larvae. the use of aposymbiotic host polyps deployed as "capture vessels" and the application of molecular techniques has allowed for the detection of environmental sources of symbiodinium. with these methods employed, investigators may resolve the distribution of different species on various benthic surfaces and cell densities suspended in the water column. the genetic identities of cells cultured from the environment are often dissimilar to those found in hosts. these likely do not form endosymbioses and are entirely free-living; they are different from "dispersing" symbiotic species. learning more about the "private lives" of these environmental populations and their ecological function will further our knowledge about the diversity, dispersal success, and evolution among members within this large genus. culturing certain symbiodinium strains and/or species are more easily cultured and can persist in artificial or supplemented seawater media (e.g. asp–8a, f/2) for decades. the comparison of cultured isolates under identical conditions show clear differences in morphology, size, biochemistry, gene expression, swimming behavior, growth rates, etc. this pioneering comparative approach initiated a slow paradigm shift in recognizing that the traditional genus sensu lato comprised more than a single real genus. culturing is a selective process, and many symbiodinium isolates growing on artificial media are not typical of the species normally associated with a particular host. indeed, most host–specific species have yet to be cultured. samples for genetic analysis should be acquired from the source colony in order to match the resulting culture with the identity of the dominant and ecologically relevant symbiont originally harbored by the animal. life cycle symbiodinium life cycle the life cycle of symbiodinium was first described from cells growing in culture media. for isolates that are in log phase growth, division rates occur every 1–3 days, with symbiodinium cells alternating between a spherical, or coccoid, morphology and a smaller flagellated motile mastigote stage. while several similar schemes are published that describe how each morphological state transitions to other, the most compelling life history reconstruction was deduced from light and electron microscopy and nuclear staining evidence. during asexual propagation (sometimes referred to as mitotic or vegetative growth), cells undergo a diel cycle of karyokinesis (chromosome/nuclear division) in darkness. the mother cell then divides (cytokinesis) soon after exposure to light and releases two motile cells. the initiation and duration of motility varies among species. approaching or at the end of the photoperiod the mastigotes cease swimming, release their flagella, and undergo a rapid metamorphosis into the coccoid form. as cultures reach stationary growth phase, fewer and fewer motile cells are observed, indicating slower division rates. large tetrads are occasionally observed, particularly when cells in stationary growth phase are transferred to fresh media. however, it is unknown whether this stage is the product of two consecutive mitotic divisions, or perhaps a process that generates sexually competent motile cells (i.e. gametes), or is the end result of meiosis (i. e. as meiotic tetrads) following gamete fusion. there is no cytological evidence for sexual recombination, and meiosis has never been observed, but population genetic evidence supports the view that symbiodinium periodically undergo events of sexual recombination. how, when, and where the sexual phase in their life history occurs remains unknown. morphology the life stages of dinoflagellates in the genus symbiodinium. (a) electron micrographs of a symbiodinium mastigote (motile cell) with characteristic gymnodinioid morphology (s. natans) and (b) the coccoid cell in hospite. as free-living cells the mastigote allows for short-range dispersal and can exhibit chemotaxis toward sources of nitrogen. once within the host, these symbionts rapidly proliferate in and in many cases dominate the cytoplasm of the host cell. fine scale features and organelles found among symbiodinium species. (a) close-up of divided cells of a tetrad from a culture of s. corculorum that is genetically similar to s. pilosum (both are type/subclade a2). this particular lineage is unusual among described symbiodinium by possessing mucocysts, and chloroplasts with parallel and peripheral thylakoid arrangements. flagella are seen in cross section in the space between daughter cells. (b) a thick cellulosic cell wall is observed for many cultured isolates. (c) tem of a cross-section through a chloroplast lobe at the periphery of the cell with parallel and peripheral thylakoids grouped in sets of three. tem of cross-section through a dividing cell (the doublet). the light micrograph inset depicts a doublet in culture. flagella are present from the daughter mastigote cells (white arrowheads) prior to emerging from the mother cell wall. n = nucleus; acc = accumulation body; pyr = pyrenoid. (robert k. trench et al. 1981) the morphological description of the genus symbiodinium is originally based on the type species (holotype) s. microadriaticum. because these dinoflagellates possess two major stages in their life history (see above), namely the mastigote (motile) and coccoid (non-motile) stages, the morphology of both is described in order to provide a complete diagnosis of the organism. flagellated (mastigote) cell the motile flagellated form is gymnodinioid and athecate ("nude"). the relative dimensions of the epicone and hypocone differ among species. the alveoli are most visible in the motile phase but lack fibrous cellulosic structures found in thecate ("armored") dinoflagellates. between the points of origin of the two flagella is an extensible structure of unknown function called the peduncle. in other dinoflagellates, an analogous structure has been implicated in heterotrophic feeding and sexual recombination. in symbiodinium, it has been suggested that the peduncle may be involved in substrate attachment, explaining why certain cells seem to spin in place. compared to other gymnodinioid genera, there is little or no displacement at the sulcus where the ends of the cingulum (or cigulum) groove converge. the internal organelles of the mastigote are essentially the same as described in the coccoid cell (see below). the transition from mastigote to coccoid stage in symbiodinium occurs rapidly, but details about cellular changes are unknown. mucocysts (an ejectile organelle) located beneath the plasmalemma are found in s. pilosum and their function is unknown, but may be involved in heterotrophic feeding. coccoid cell the coccoid cell of symbiodinium is spherical and ranges in average diameter from 6 to 13 µm, depending on the species (blank et al. 1989). this stage is often wrongly interpreted as a dinocyst; hence, in published literature, the alga in hospite is often referred to as a vegetative cyst. the term cyst usually refers to a dormant, metabolically quiescent stage in the life history of other dinoflagellates, initiated by several factors, including nutrient availability, temperature, and day length. such cysts permit extended resistance to unfavourable environmental conditions. instead, coccoid symbiodinium cells are metabolically active, as they photosynthesize, undergo mitosis, and actively synthesize proteins and nucleic acids. while most dinoflagellates undergo mitosis as a mastigote, in symbiodinium, mitosis occurs exclusively in the coccoid cell. cell wall the coccoid cell is surrounded by a cellulosic, usually smooth cell wall that contains large-molecular-weight proteins and glycoproteins. cell walls grow thicker in culture than in hospite. the cell membrane (plasmalemma) is located beneath the cell wall, yet little is known about its composition and function in terms of the regulation of trans-membrane transport of metabolites. during karyokinesis and cytokinesis, the cell wall remains intact until the mastigotes escape the mother cell. in culture, the discarded walls accumulate at the bottom of the culture vessel. it is not known what becomes of the walls from divided cells in hospite. one species, s. pilosum, possesses tufts of hair-like projections from the cell wall; this is the only known surface characteristic used to diagnose a species in the genus. chloroplast most described species possess a single, peripheral, reticulated chloroplast bounded by three membranes. the volume of the cell occupied by the chloroplast varies among species. the lamellae comprise three closely appressed (stacked) thylakoids, and are attached by two stalks to the pyrenoid surrounded by a starch sheath. in three of the described species, the thylakoids are in parallel arrays, but in s. pilosum, there are also peripheral lamellae. there are no thylakoid membranes invading the pyrenoid, which is unlike other symbiotic dinoflagellates. the lipid components of thylakoids include the galactolipids as monogalactosyl-diglycerides (mgdg) and digalactosyl-diglycerides( dgdg),; the sulpholipid sulphoquinovosyl-diglyceride (sqdg), phosphatidyl glycerol, and phosphatidyl choline. associated with these are various fatty acids. the light-harvesting and reaction centre components in the thylakoid membrane include a water-soluble peridinin-chlorophyll a-protein complex (pcp or percp), and a membrane-bound chlorophyll a-chlorophyll c2-peridinin-protein-complex (acppc), along with typical photosynthetic electron transport systems such as the photosystem ii reaction centre and the chlorophyll-a-p700 reaction centre complex of photosystem i. also associated with the thylakoids are the xanthophylls dinoxanthin, diadinoxanthin, diatoxanthin and the carotene, β-carotene. the pyrenoid contains the nuclear-encoded enzyme type ii ribulose-1,5-bis-phosphate-carboxylase-oxygenase (rubisco), which is responsible for the catalysis of inorganic carbon dioxide (co2) into organic compounds. all cultured isolates (i.e. strains) are capable of phenotypic adjustment in their capacity for light harvesting (i.e. photoacclimation), such as by altering the cellular chl. a and peridinin quota, as well as the size and number of photosynthetic units. however, the ability to acclimate is a reflection of genetic differences among species that are differently adapted (evolved) to a particular photic environment. for example, s. pilosum is characterized as a high light-adapted species, while others are low light-adapted (s. kawagutii) or adapted to a larger range in varying light fields (s. microadriaticum). the reticulated chloroplast structures of (a) cladocopium goreaui (formerly s. goreaui type c1), (b) symbiodinium fitti (type a3), and (c) effrenium voratum (formerly s. californium type e1) imaged in 3-d using confocal microscopy of chlorophyll autofluorescence. nucleus in general, the nucleus is centrally located and the nucleolus is often associated with the inner nuclear membrane. the chromosomes, as in other dinoflagellates, are seen as ‘permanently super-coiled’ dna in transmission electron micrographs (tem). the described species of symbiodinium possess distinct chromosome numbers (ranging from 26 to 97), which remain constant throughout all phases of the nuclear cycle. however, during m-phase, the volume of each chromosome is halved, as is the volume of each of the two resulting nuclei. thus, the ratio of chromosome volume to nuclear volume remains constant. these observations are consistent with the interpretation that the algae are haploid,