COMMUNITY STRUCTURE DYNAMICS OF EPILITHIC AND EPIPHYTIC DIATOMS IN A MISSISSIPPI STREAM1

Epilithic and epiphytic diatoms were collected monthly from 18 June 1979 through 19 May 1980 from both shaded and unshaded sites of Sessums Creek, a shallow, oligotrophic and strongly alkaline stream in northeastern Mississippi. The dominant macroalgae were filamentous green algae belonging to the non-mucilage producing genera Cladophora and Rhizoclonium. Ninety taxa were identified with Navicula minima Grun. and Nitzschia dissipata (Kütz.) Grun. accounting for 50% of all valves counted. Other important taxa were Navicula menisculus var. upsaliensis (Grun.) Grun., N. cryptocephala var. veneta (Kütz.) Rabh., Achnanthes minutissima Kütz. and Cymbella turgida (Greg.) Cl. With few exceptions, the more dominant taxa were equally abundant in the shaded and unshaded sites and also in the epilithon and epiphyton. Species diversity (H') and the number of taxa in a sample in all four habitats showed an identical pattern, being lowest in winter and highest in fall. No one pair of habitats was more structurally similar than any-other pair when compared by a similarity index (SIMI). Apparently, differences in light intensity and substrate type had little effect on diatom distribution in Sessums Creek and it was concluded that the locality sampled supported a single, nearly homogeneous diatom community.     

Geothermal diatoms: a comparative study of floras in hot spring systems of Iceland,New Zealand,and Kenya

Diatom floras were examined from geothermal environments in three contrasting tectonic settings. These included subduction-related acid and alkaline springs in New Zealand; alkaline springs along a divergent plate boundary on Iceland; and alkaline springs in the Kenya Rift. These shallow (<1 cm) aquatic environments vary considerably (e.g., temperature: 21.3–99°C; pH: 2.1–9.65; 56.41–643 mg l⁻¹ SiO₂). Diatoms form an important component of geothermal floras at temperatures of <45°C. The floras from New Zealand are distinguished by the common occurrence of Pinnularia. Icelandic springs have a variety of Fragilariaceae. Navicula and Anomoeoneis are most common in the Kenyan springs. Statistical analyses suggest that the diatoms cluster into seven major groups. The most common taxa include: Achnanthidium exiguum v. heterovalvum (Kras.) Czarn., Anomoeoneis sphaerophora (Ehrenb.) Pfitz, Brachysira brebissonii f. thermalis Grun., Caloneis bacillum (Grun.) Cl., Craticula cuspidata (Kütz.) Mann, Diadesmis confervacea Kütz., Epithemia argus (Ehrenb.) Kütz., Frustulia rhomboides (Ehrenb.) DeT., Hantzschia amphioxys (Ehrenb.) Grun., Navicula tenelloides Hust., Nitzschia amphibia Grun., Nitzschia inconspicua Grun., Nitzschia invisitata Hust., Nitzschia frustulum (Kütz.) Grun., Nitzschia sigma (Kütz.) W, Smith., Pinnularia chapmaniana Fog., Pinnularia appendiculata (Ag.) Cl., Pinnularia molaris (Grun.) Cl., Pinnularia acoricola Hust., Rhopalodia gibberula (Ehrenb.) O. Müll., Staurosira construens v. venter (Ehrenb.) Ham., Staurosira elliptica (Schum.) Will. & Round, and Staurosirella pinnata (Ehrenb.) Will. & Round. Canonical correspondence analysis shows clear correlations between species, alkalinity, pH, and conductivity, with less strong correlations for silica and temperature. Other factors include substrate type, current velocity, and light conditions. The preservation potential of host deposits varies considerably, being lowest for springs on clastic deltas and highest where travertine or sinter is accumulating. Handling editor: J. Padisak     

THE DISTRIBUTION OF EDAPHIC DIATOMS ALONG ENVIRONMENTAL GRADIENTS OF A LOUISIANA SALT MARSH1

Edaphic (sediment-associated) diatoms were collected from five sites along a gradient from slightly brackish to saline marsh on 30 November 1977 and 29 April 1978. Several environmental factors change along this gradient in addition to salinity, including tidal flushing and percent organic content of the soil. A total of 112 taxa were identified, with Amphora exigua Greg., Navicula phyllepta Kütz., N. salinarum Grun., and N. tripunctata (Müll.) Bory being among the five most abundant taxa on both sampling dates. The taxonomic composition of the diatom flora was very similar to those observed for other salt marshes located along the Gulf and Atlantic coasts. Diatom taxa were generally euryhaline and distributed continuously along the gradient from brackish to saline marsh. Species diversity (H¹) and the number of taxa in a sample (S) were also similar to those reported for other North American salt marshes. The number of taxa in a sample was higher in the saline than in the brackish area on both sampling dates while species diversity was greater in the saline area only in April.     

SCANNING ELECTRON MICROSCOPY OBSERVATIONS OF DEFORMITIES IN SMALL PENNATE DIATOMS EXPOSED TO HIGH CADMIUM CONCENTRATIONS1

Different types of malformations are likely to affect the morphology of diatoms when exposed to particularly unstable environmental conditions, the most easily identifiable being distortion of the whole frustule. In the present study, we investigated, by means of SEM, valve abnormalities induced by high cadmium contamination (100 μg · L^?1) in small pennate diatoms. Changes in the shape of Amphora pediculus (Kütz.) Grunow and anomalous sculpturing of the cell wall of many species, such as Encyonema minutum (Hilse) D. G. Mann, Mayamaea agrestris (Hust.) Lange‐Bert., Gomphonema parvulum (Kütz.) Kütz., or Eolimna minima (Grunow) Lange‐Bert., were observed, which were not, or almost not, noticeable in the LM. With consideration to current knowledge of diatom morphogenesis, metal uptake by the cell would induce, directly or indirectly, damage to many cytoplasmic components (e.g., microtubules, cytoskeleton, Golgi‐derived vesicles) involved in the precisely organized silica deposition. This study confirms that many species, whatever their size, are likely to exhibit morphological abnormalities under cadmium stress, and that this indicator may be valuable for the biomonitoring of metal contamination, even if SEM observations are not necessary for routine studies.     

BENTHIC MARINE DIATOM ASSOCIATIONS: UPPER FLORIDA BAY (FLORIDA) AND ASSOCIATED SOUNDS1

Models of the diatom associations found in upper Florida Bay and adjoining sounds have been constructed utilizing Q modal factor-vector analysis and ecologic diversity indices (Shannon index, number of species, evenness). Four distinct associations were defined using Q-mode factor-vector analysis. Two associations were epiphytic, occurring on Thalassia testudinum König—Association I was characterized by Cocconeis placentula Ehr., Association III by Cylindrotheca closterium (Ehr.) Reim. & Lewin and Cocconeis placentula Ehr. The other two associations were epipelic, occurring on the carbonate mud substratum—Association II was characterized by Cyclotella striata (Kütz.) Grun., Rhopalodia gibberula (Ehr.) O. Müller and Surirella fastuosa (Ehr.) Kütz., Association IV by Fragilaria crotonensis Kitton and Cyclotella striata (Kütz.) Grun. The majority of the 161 species identified were present in both the epiphytic and epipelic assemblages. Only 33 species were restricted to the epiphyton, and 18 species restricted to the epipelon. The epipelic assemblage was significantly more diverse than was the epiphytic assemblage. A general trend of increased diversity away from terrestrial environs toward more open areas of water in both the epipelon and epiphyton was also found.     

黑龙江省七星河湿地硅藻植物的初步研究

报道了2004年对黑龙江省七星河湿地国家级自然保护区的硅藻门植物研究的结果,共发现155个分类单位,包括113种39变种3变型,分别隶属于2纲6目9科26属,其中中国新记录2种2变种1变型共计5个分类单位,初步分析了七星河湿地硅藻植物群落的种类组成及生态类型,大部分的硅藻属于普生种类,同时出现了一些喜酸种类如：Eunotia pectinalis var. ventralis(Ehr.)Hust., Tabellaria flocculosa(Roth) Kütz.,Gomphonema augur Ehr.,Navicula subtilissima Cl.等,还有少量的嗜碱性种类出现。     

SUBSTRATUM CONDITIONING AND DIATOM COLONIZATION IN DIFFERENT CURRENT REGIMES1

Short-term (3 day) diatom colonization of clean ceramic tiles and tiles conditioned with a thin, nonalgal biofilm was examined in fast- and slow-current outdoor experimental stream channels to assess effects of organic conditioning and current regime on diatom colonization. Colonization rates onto unconditioned tiles were 10 times lower in fast current that in slow. Substratum conditioning reduced the effects of current because organic conditioning of tiles significantly enhanced colonization in fast current, but not in slow current. Two diatom taxa (Nitzschia acicularis W. Sm. and Synedra radians Kütz) colonized unconditioned tiles more rapidly than conditioned tiles in slow current, suggesting the existence of negative interactions between these diatoms and bacteria and/or organics in conditioning films.     

PERIPHYTON COLONIZATION OF ROCK SURFACES IN A BOREAL FOREST STREAM STUDIED BY SCANNING ELECTRON MICROSCOPY AND TRACK AUTORADIOGRAPHY1

Periphyton colonization of natural rock surfaces and granite tiles was followed experimentally in the Matamek River, an acidic (pH 5.5) sixth order boreal stream in northeast Quebec, Canada. Accumulation of chlorophyll a and freshweight algal biomass was logarithmic over a 25 day colonization period. The major colonizers were Tabellaria flocculosa (Roth) Kütz., T. fenestrata (Lyngb.) Kütz., Mougeotia sp., and Eunotia pectinalis (Kütz) Rabh., and its varieties. The microcolonization sequence on granite tiles, followed over 27 days with scanning electron microscopy, showed an initial accumulation of algal cells on the upstream and downstream margins and in depressions, followed by a gradual filling-in of the flat surfaces. It is hypothesized that the observed slow rate of colonization was due to the high surface tension of the granite substratum and the absence of preconditioning by an organic film. It is further hypothesized that the increase in cellular carbon fixation rates of T. flocculosa measured over a 23 day period using nuclear track autoradiography parallels the development of an algal-detrital microcosm on the granite tile, and is evidence for the establishment of localized tight nutrient spiralling. It is suggested that the periphyton community may be regarded as a nutrient recycling system at a microenvironmental level which may be especially important in oligotrophic systems.     

A weighted—averaging regression and calibration model for inferring total phosphorus concentration from diatoms in British Columbia (Canada) lakes

1. The relationship between surficial sediment diatom taxa (Bacillariophyceae) and measured limnological variables in forty-six British Columbia lakes was explored using canonical correspondence analysis (CCA). Lake-water total phosphorus concentration (TP), maximum lake depth, conductivity, and calcium concentration each accounted for independent and statistically significant directions of variation in the distribution of diatom taxa. 2. Weighted-averaging (WA) models were developed to infer lake-water TP from the relative abundances of 131 diatom taxa in the surficial sediments of thirty-seven lakes. WA regression and calibration with classical deshrinking provided the best model for TP reconstructions. 3. Our quantitative inference model has two major advantages over existing multiple linear-regression models: (i) inferences are based on the responses of individual taxa to TP, and do not involve grouping the taxa into a small number of ecological categories; and (ii) the model assumes that diatoms respond to TP in a unimodal, rather than a linear, fashion. 4. The WA model can now be used to infer past lake-water TP, within the range 5–28νgr1^?1, from diatoms preserved in the sediments of British Columbia lakes. The model can provide quantitative estimates of the onset, rate, and magnitude of lake eutrophication in response to natural processes and human disturbances.     

EFFECTS OF WATER VELOCITY ON THE ARCHITECTURE AND EPIPHYTES OF CLADOPHORA GLOMERATA (CHLOROPHYTA)1

The architecture of the filamentous green alga Cladophora glomerata (L.) Kütz. and the composition of this alga's epiphytes [primarily the diatoms Epithemia turgida (Ehrenb.) Kütz., Epithemia sorex Kütz., and Cocconeis pediculus Ehrenb.] were examined in different velocity regimes. After transferring algal-bearing cobbles among velocities, the effects of changes in velocity were also examined. Cladophora branching pattern did not initially differ among slow, medium, and fast velocities, indicating that stable water velocities did not affect branching pattern. Two weeks after cobble transfer, Cladophora in fast velocity had fragmented more (i.e. had fewer filaments and fewer branch points per length of filament and had a higher percentage of unbranched filaments) than Cladophora in slow velocity. Fragmentation was greatest in tufts moved from slow velocity, suggesting velocity-associated differences in susceptibility to breakage. Epiphytic assemblage composition differed among slow, medium, and fast velocities and between locations on the filament (base and apex). Cocconeis pediculus dominated where exposure to high velocity was greater (filament apices in medium and fast velocities), whereas the Epithemia spp. dominated where lower velocities occurred (filament bases in all velocities and apices in slow velocity). Two weeks after the cobble transfer, the translocated diatom assemblages had changed and the original pattern of diatom distribution was restored.     

POLYMORPHISM IN THE DESMID MICRASTERIAS PINNATIFIDA AND ITS TAXONOMICAL IMPLICATIONS1

A case of polymorphism in the desmid Micrasterias pinnatifida (Kütz.) Ralfs is described. It is based on material collected from the overflow channel of a dam in Caeté, Minas Gerais State. After careful examination of nearly 500 specimens, 26 different morphological expressions of the alga were found which led the authors to make the following deductions: (1) for delimitation of species and infraspecific taxa in desmids it is absolutely necessary to analyze sample populations; (2) the morphological characteristics presently used, for delimination of some varieties of M. Pinnatifida (Kütz.) Ralfs seem to have no taxonomical value if only isolated individuals are examined; (3) the varieties granulata, inflata, and tridentata must be considered with some caution, until further studies are carried out, because they can be merely distinct morphological expressions of M. pinnatifida (Kütz.) Ralfs var. pinnatifida.     

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

RELATIONSHIPS OF SEDIMENTED DIATOM SPECIES (BACILLARIOPHYCEAE) TO ENVIRONMENTAL GRADIENTS IN DILUTE NORTHERN NEW ENGLAND LAKES1

Limnological gradients of small, oligotrophic, and low conductance lakes in northern New England were defined by principal components analysis; relationships of sedimented diatom species to the gradients were investigated by correlation analysis. Diatom distributions were most strongly related to the gradient of pH and alkalinity and the covarying variables, conductance, Mg, Ca, total Al, and exchangeable Al. Weaker relationships to lake morphology, dissolved organic carbon and water color, altitude and marine aerosol inputs, and the distinctive water chemistry of some New Hampshire lakes were also present. Results for 16 taxa of importance in our studies of lake acidity are given in detail and are compared to results from other regions of eastern North America. Planktonic taxa were absent below pH 5.5, with the exception of the long form of Asterionella ralfsii var. americana Korn. The two forms of this taxon differed ecologically: the long form (>45μm) had an abundance weighted mean (AWM) pH 4.90 and occurred mostly in lakes that were deep relative to transparency; the short form (<45μm)had an AWM pH and occurred on lakes that were shallow relative to transparency. The ecological advantage of a “splitter” approach to diatom taxonomy was demonstrated by examination of other taxa as well, including Tabellaria flocculosa (Roth) Kütz. These results have important implications for paleolimnological interpretations.     

AN “ENVIRO‐INFORMATIC” ASSESSMENT OF SAGINAW BAY (LAKE HURON,USA) PHYTOPLANKTON: DATA‐DRIVEN CHARACTERIZATION AND MODELING OF MICROCYSTIS (CYANOPHYTA)1

Phytoplankton and Microcystis aeruginosa (Kütz.) Kütz. biovolumes were characterized and modeled, respectively, with regard to hydrological and meteorological variables during zebra mussel invasion in Saginaw Bay (1990–1996). Total phytoplankton and Microcystis biomass within the inner bay were one and one‐half and six times greater, respectively, than those of the outer bay. Following mussel invasion, mean total biomass in the inner bay decreased 84% but then returned to its approximate initial value. Microcystis was not present in the bay during 1990 and 1991 and thereafter occurred at/in 52% of sample sites/dates with the greatest biomass occurring in 1994–1996 and within months having water temperatures >19°C. With an overall relative biomass of 0.03 ± 0.01 (mean + SE), Microcystis had, at best, a marginal impact upon holistic compositional dynamics. Dynamics of the centric diatom Cyclotella ocellata Pant. and large pennate diatoms dominated compositional dissimilarities both inter‐ and intra‐annually. The environmental variables that corresponded with phytoplankton distributions were similar for the inner and outer bays, and together identified physical forcing and biotic utilization of nutrients as determinants of system‐level biomass patterns. Nonparametric models explained 70%–85% of the variability in Microcystis biovolumes and identified maximal biomass to occur at total phosphorus (TP) concentrations ranging from 40 to 45 μg · L^?1. From isometric projections depicting modeled Microcystis/environmental interactions, a TP concentration of <30 μg · L^?1 was identified as a desirable contemporary “target” for management efforts to ameliorate bloom potentials throughout mussel‐impacted bay waters.     

ABSOLUTE CONFIGURATION ANALYSIS OF THE FLAGELLAR APPARATUS IN CLADOPHORA AND CHAETOMORPHA MOTILE CELLS,WITH AN ASSESSMENT OF THE PHYLOGENETIC POSITION OF THE CLADOPHORACEAE (ULVOPHYCEAE,CHLOROPHYTA)

Absolute configurational analyses of flagellar apparatus components were performed on the motile cells produced by three species of Cladophora, Cl. dalmatica Kütz., Cl. flexuosa (Dillw.) Harv., and Cl. glomerata (L.) Kütz., and by Chaetomorpha aerea (Dillw.) Kütz. There was little variation among the species. All of the flagellar apparatuses demonstrated the ulvophyceous features of 180° rotational symmetry, counterclockwise absolute orientation, and basal body overlap, as well as the alignment of the basal bodies perpendicular to the long axis of the cell. Diagnostic features included the nearly complete absence of C tubules from the basal bodies and the presence of a coarsely striated component dorsal to the two-membered rootlets in all cells, as well as, in quadriflagellate cells, a tetralobate distal fiber, the coaxial arrangement of the lowermost pair of basal bodies, and the presence of a characteristic array of basal-body-associated striated bands. The distal fiber architecture, the presence of a “wing” in the X-membered rootlets, and the “flattening” of the flagellar apparatus components suggests a close relationship of the Cladophoraceae to the Dasycladales, and indicates that these two groups may have shared a common ancestor, possibly ancient in terms of the geological time scale but relatively recent in the context of ulvophyceous evolution. A sizeable phylogenetic gap exists between the Cladophoraceae and uninucleate-celled, presumably primitive members of the Ulvophyceae.     

EFFECTS OF CHIRONOMID (INSECTA: DIPTERA) TUBE-BUILDING ACTIVITIES ON STREAM DIATOM COMMUNITIES1,2

Chironomid retreats, constructed out of sand grains upon submerged wood debris, increase the surface area available for diatom colonization. The three dimensional substratum afforded by chironomid tubes supports up to twelve times the diatom biavolume found upon adjacent, unmodified substratum in a northern Michigan stream. Diatom enumeration within scrapings from small defined areas on artificial substrata, combined with examination of intact natural substrata through scanning electron microscopy (SEM), reveals distinct, microdistribution patterns. The larval retreats of two major taxa of tube-dwelling chironomid's (Micropsectra sp. and Pseudodiamesa cf. pertinax Garrett) display significantly different diatom communities relative to adjacent masonite substratum. Substratum without chironomid tubes is primarily colonized by Achnanthes minutissima Kütz. and Cocconeis placentula Ehr., exhibiting the lowest species diversity of microhabitats examined. The diatom flora upon sand tubes of Micropsectra sp. is dominated by Opephora martyi Herib., as is the flora of sand grains collected from the stream sediment load. These two micro-habitats exhibit a high community similarity (SIMI). The SIMI index also suggests that the flora of P. pertinax tubes is highly similar to that of sand grains. Diversity, however, is almost three times greater on P. pertinax tubes and SEM observations reveal that this microhabitat is characterized by a more spatially complex flora; Nitzschia and Navicula spp. dominate the upperstory, and O. martyi is located on underlying sand grains. Results indicate that tube-building chironomids in Carp Creek affect diatom microdistribution by: (1) stabilizing sand grains and associated flora within their retreats, (2) providing a ‘refugium’ for upperstory diatom taxa from the mayfly grazer, Baetis vagans McDunnough (Insecta: Ephemeroptera), and (3) through local nutrient enrichment.     

STRUCTURE,LIFE HISTORY AND SYSTEMATICS OF RHOICOSPHENIA (BACILLARIOPHYTA). IV. CORRELATION OF SIZE REDUCTION WITH CHANGES IN VALVE MORPHOLOGY OF RH. GENUFLEXA1

Rhoicosphenia Grun. is a relatively isolated genus among the biraphid diatoms. Morphological changes in an isopolar member of the genus, Rh. genuflexa (Kütz.) Medlin, were investigated using light and scanning electron microscopy. The fully raphid valve showed changes in its flexure that could be correlated with size reduction during its life history from the initial cells to the smallest cells found in the population. Bands showed changes in number (from three to one) related to size reduction. Rh. genuflexa is morphologically similar to Rh. abbreviata (C. Ag.) Lange-Bert. (=Rh. curvata (Kütz.) Grun.), although the two are distinct taxa. These observations support previous contentions that Rhoicosphenia is a natural taxonomic grouping.     

MORPHOLOGICAL VARIABILITY OF EUNOTIA PECTINALIS (BACILLARIOPHYCEAE) IN A SOFTWATER RHODE ISLAND STREAM AND IN CULTURE1

The diatom Eunotia pectinalis (O. F. Müll.?) Rabh. exhibited considerable morphological variability in samples collected over a 13-month period from a softwater Rhode Island stream. All observed morphotypes were classified into three main complexes based upon their valve configuration. These groupings were confirmed by discriminant analysis using quantitative valve characteristics and were named in accordance with their associated varieties: minor (Kütz.) Rabh., pectinalis and ventricosa Grun. Clonal cultures were subjected to a defined series of temperatures, agitation rates and light intensities in order to assess inherent morphological variability. Similar trends were evident for all complexes. Over time, valve length decreased, valve breadth enlarged, and striae number per 10 μm at valve center and apex increased. Morphological plasticity was evident in culture and appeared to be more a function of the clone genotype than the environmental conditions to which it was subjected. One clone originally identified as E. pectinalis var. minor assumed certain morphological features of E. vanheurckii Patr. after four months in culture.     

ULTRASTRUCTURE OF THE FILAMENTOUS HABIT IN THE DIATOM NAVICULA CONFERVACEA (KÜTZ.) GRUN.1

Frustules of the filamentous diatom Navicula confervacea (Kütz.) Grun. are united by the overlapping of marginal teeth located alongside the junction of valve face and mantle. Scanning and transmission electron microscopy show this connecting mechanism to differ from the previously reported inter-digitating spines or teeth in the genera Melosira, Stephanodisus, and Fragilaria.     

Life history of Cladophora surera sp. nov. (Cladophorales, Ulvophyceae)

The life history of Cladophora surera sp. Nov., described from the south of the province of Buenos Aires, Argentina, was found to be diplobiontic and isomorphic with haploid gametophytic (n= 24) and sporophytic (2n= 48) plants. Other freshwater species, namely C, suhriana Kutz, and C. callicoma Kütz. have also been reported to be diplobiontic and isomorphic but differing from C. surera by their ploidy level. Plants exhibit great morphological variation, as the number of branches/mm² tends to increase with higher water velocity, thus adopting morphotypes that resemble very different species, that is, C. vagabunda (L.) Hoek, C. glomerata (L.) Kutz., C, laetevirens (Dillw.) Kutz., C. brasiliana Martens, C. dalmatica Kütz., C. vadorum (Aresch.) Kütz. and C. rivularis (L.) Hoek., but clearly differing from them by other morphological parameters; apical cell diameters, The ploidy level 24/48 has not been established for a species of Cladophora. Autogamy is the normal conjugation method and can take place even inside the gametangia before the gametes are released. Isogametes conjugate in a slow behavioral anisogamy, in which the contents of one migrate to the other. Tetra-flagellate zoospores result from meiosis.