The Spermatozoon of Brachionus plicatilis(Rotifera,Monogononta) With Some Notes on Sperm Ultrastructure in Rotifera

The spermatozoon of B. plicatilisis a thread–like cell with an anterior flagellar portion and a posterior cell body. The flagellum has a lateral ‘undulating membrane’, containing a folded longitudinal cisterna and an axoneme. The basal body of the axoneme is at the anterior tip. The axoneme lacks outer dynein arms and extends through the entire flagellar region and most of the cell body. The main portion of the flagellum and of the cell body contains a series of vesicles with tightly packed tubules that may serve as a cytoskeleton. The cell body contains a partly condensed nucleus, several mitochondria and some cytoplasm. Some elongated mitochondria are arranged in the postnuclear region. When the spermatozoon moves, the undulations propagate from the basal body at the flagellar tip. Late spermatids can be recognized by the nucleus and the flagellum being coiled and enclosed within a common cell membrane. As in other rotifers, there are cigar–like cell products (‘rods’) in the testes. The general organization of the cell, including the absence of an evident acrosome, resembles that of the other known monogonont sperm types.     

Glycosidases in Brachionus plicatilis (Rotifera)

1. Tests for glycosidases were performed in homogenates of Brachionus plicatilis. 2. Hydrolytic activity was detected with the following substrates: (a) with synthetic substrates (NP = 4-nitrophenyl): NP-alpha- and NP-beta-D-glucopyranoside, NP-alpha- and NP-beta-D-galactopyranoside, NP-N-acetyl-beta-D-glucosaminide, NP-N-acetyl-beta-D-galactosaminide, NP-alpha- and NP-beta-D-mannopyranoside and NP-alpha-L-fucopyranoside; (b) with disaccharides: sucrose, maltose, trehalose, isomaltose, cellobiose, gentiobiose and lactose; (c) with polysaccharides: laminarine, carboxymethyl-cellulose, avicel, Micrococcus luteus (for lysozyme) and 4-nitrophenyl-alpha-D-maltoheptaoside (for amylase). 3. The pH dependence of the glycosidase activities was determined. 4. The distribution of enzyme activities within fractions from the homogenate was studied in order to localize them within the cell. 5. Proteins from Brachionus homogenate were separated by SDS-gel electrophoresis and the positions of the following glycosidase activities were detected by assays performed on the gels (estimated molecular weights in parentheses): alpha-glucosidase (250,000); beta-glucosidase (200,000); beta-galactosidase (70,000); N-acetyl-beta-glucosaminidase (60,000).     

Ultrastructure of the rotifer Brachionus plicatilis

The ultrastructural study of digestive organs andintegument of the rotifer, Brachionusplicatilis, was performed by transmission of electronmicroscopy to elucidate the relationship between theirstructure and function. The integument of the rotiferis composed of a thick external and a thin internallayer, in which many pores are regularly distributed.The contents of secretory bulb are excreted throughthose pores to the external surface. The inner surfaceof the digestive tract is lined with relatively denseand regularly spaced cilia for propelling food along.The cells of stomach and the intestine contains manyendocytotic vesicles, digestive vacuoles, and lipidinclusions, indicating its active endocytoticfunction. The gastric gland cells have abundant roughendoplasmic reticulum, Golgi complexes and secretorygranules for producing digestive enzymes. Thisultrastructural study clarifies the morphologicalcharacters of the integument and the digestive organsthat are closely related to its function.     

SEM of internal structures of Brachionus plicatilis (Rotifera)

Examination by scanning electron microscopy of sectioned rotifers provides views on their internal structures which complement the results of other techniques. Thus, additional information has been obtained for example, on the digestive tract, on nerve connections and on the morphology of the mastax. Our observations confirm that nearly all organs are connected in some way to the integument, suggesting that integument structures may be responsible for the holding together of the whole rotifer body.     

Coexistence of rotifer (Brachionus plicatilis) clones in Soda Lake,Nevada

Patterns of genetic variation among Brachionus plicatilis clones collected at different times from Soda Lake, Nevada (U.S.A.) were determined using polyacrylamide gel electrophoresis of isozymes. Six of the eleven enzymes assayed were variable. Three of these — malic dehydrogenase (MDH), cholinesterase (CHO), and phosphoglucomutase (PMG) - were selected to screen the 138 clones established from the field collections. Each enzyme revealed variation both within and between field samples. Thirty-eight of the 100 potential composite electromorphs formed by combining isozyme variants were found in the 138 clones. A substantial amount of genetic variation is therefore present. Patterns of electromorph frequency change resembled those reported for cladocerans. No evidence was found for a seasonal succession of distinct electromorphs. We suggest that the B. plicatilis in Soda Lake constitute a single population.     

Survival analysis of three clones of Brachionus plicatilis (Rotifera)

Age-specific survival schedules of females from three genetically different clones of Brachionus plicatilis were analyzed at several environmental conditions in the laboratory.Lifespan showed the expected decrease with increasing temperature, but a general trend with salinity or genotype was not observed. Probability of death increased with age, as tested by polynomial regression analysis of the survival curve and using theoretical mortality distributions. Three two-parameter models (linear-exponential model, Weibull model, and Gompertz model) were fitted to the survival data. Fitting of these models to data was rather poor, but the Gompertz model and, to a lesser extent, the Weibull model fitted the data better than the linear-exponential model. Parameters obtained from the survival curve analyses were related to other demographic parameters. A significant relationship between the shape parameter of the Gompertz model and the cohort generation time was detected, suggesting, but not proving, an effect of reproductive effort on aging.     

Enzyme activity measurements on isolated organs of Brachionus plicatilis (Rotifera)

A method is described by which the integument of Brachionus plicatilis, together with its intracellular lamina, is quickly dissolved before other parts or tissues of the animal are destroyed. After removing the integument several parts of the body can be separated and fractionated in a more or less intact state by centrifugation in a Percoll gradient. The measurement of enzyme activities has indicated that this procedure might provide a way of localizing enzymes within the rotifer body.     

Nomenclatural consequences of some recent studies on Brachionus plicatilis (Rotifera,Brachionidae)

Recent studies on morphology, karyotype, genetics including allozyme constitution and reproductive behaviour of S- and L-type B. plicatilis showed that these types are best treated as different species. A reexamination of existing available names revealed B. plicatilis O. F. Müller, 1786 and B. rotundiformis Tschugunoff, 1921 as the correct names for the L- and S-type, respectively.     

Ecological factors affecting gene flow in the Brachionus plicatilis complex (Rotifera)

We investigated how adaptation to salinity and temperature acts as reproductive barriers in three sympatric species from the Brachionus plicatilis species complex. These species co-occur in a salt marsh in Spain, and a previous electrophoretic study of variation revealed no hybrids between them. A factorial experiment was designed to test for differences in population growth rates and patterns of bisexual reproduction. The design combined representative strains from each species in different salinity and temperature conditions, representing the range over which these rotifers are found in their natural environment. We found differences in the growth response of the three species to both factors and in the pattern of bisexual reproduction. These differences help to explain patterns of succession observed in the field. We conclude that these ecological factors, together with mate recognition systems, account for the absence of gene flow in these sympatric species. Received: 22 July 1996 / Accepted: 14 March 1997     

Aging and Loss of Fertility in Male and Female Brachionus plicatilis (Rotifera)

Summary

The decline with age of mictic female susceptibility to fertilization and male capacity for fertilization is characterized for the rotifer Brachionus plicatilis. All mictic females were susceptible to fertilization until age 4 hr. Susceptibility then declined non-linearly according to the quadratic equation Y = 140.6—14.3X + 0.36X ². By age 24 hr, sexual females no longer could be fertilized. Only 83% of newborn males were capable of fertilization. This level of fertility held until age 8 hr, then declined linearly. The age when 50% of individuals were no longer fertile was termed the length of fertilizability ₅₀(LF₅₀) and is 7.9 hr (16.7% of lifespan) and 18.8 hr (26.1% of lifespan) for females and males, respectively. Newborn males had an average of 30.1 ± 1.40 motile sperm. Males transferred a mean of 2.3 motile sperm into the pseudocoelom of females on each insemination. Sperm inseminated per copulation closely corresponds to the mean number of resting cysts produced by fertilized females. It is not likely that resting cyst production is limited by sperm availability.     

Production of the rotifer Brachionus plicatilis for aquaculture in Kuwait

Macrophytes, plankton samples, salinity and depth data were collected from wetlands in October 1981. Species diversity and distributions are recorded and relationships drawn between macrophytes and plankton occurrence. An examination of the data of Geddes et al. (1981) enables comparisons of species composition in a similar range of Western Australian lakes in 1978 at a different stage in the seasonal cycle. Comparison with the biota of salt lakes in Australia's eastern states and elsewhere lead to biogeographical speculations for the macrophytes and for some of the planktonic species.     

Precopulatory mate guarding and mating behaviour in the rotifer Epiphanes senta (Monogononta: Rotifera)

Epiphanes senta is a littoral rotifer species that occurs in temporary waters and displays a mating behaviour which has not, to my knowledge, so far been described for monogonont rotifers. Monogonont rotifers show distinctive periods within their life cycle during which mictic females appear. Mictic females produce haploid eggs that develop into males or into diapausing eggs if fertilized. The females of E. senta are mostly stationary on the substrate while males are more active swimmers. If they encounter eggs with female embryos of their own species, they attend them and mate with the hatching female. Experiments showed that males are able to discriminate between male, female and diapausing eggs. They exhibit a strong preference for female eggs that are only a few hours away from hatching compared with eggs in early developmental stages. Further experiments did not show any significant differences in male attendance of mictic and amictic eggs. It is hypothesized that males judge the age of a female egg by sensing a chemical that is produced by the growing embryo and diffuses through the egg shell. The male mating behaviour is similar to precopulatory mate guarding known from arthropods but it lacks the monopolization of the female by the male.     

Rotifera from Burundi: the Lepadellidae (Rotifera: Monogononta)

We studied the distribution of Lepadellidae (Rotifera) in freshwater habitats in the floodplain of the River Rusizi in northwest Burundi. Twenty-three species belonging to ColurellaBory de St. Vincent, 1824 (3 species), Lepadella Bory de St. Vincent, 1826 (18 species) and Squatinella Bory de St. Vincent, 1826 (2 species) are recorded, 22 of them are new to Burundi. One of the taxa encountered probably represents an unnamed species. Lepadella arabicaSegers & Dumont, 1993 is recognised as junior subjective synonym of Lepadella eurysterna Myers, 1942 (syn. nov.). Most of the taxa recorded are cosmopolitan or tropicopolitan, two are restricted to the tropical regions of the Old World and Australia, and one, Squatinella lunata Segers, 1993 is an Ethiopian endemic.     

Influence of the concentration of oxygen on the swimming path of Brachionus plicatilis (Rotifera)

Brachionus plicatilis females were maintained for > 24 hours in water where the concentration of oxygen was precisely controlled (spherical flasks with 6 mg l^–1 or < 0.5 mg l^–1; food = dead Tetraselmis sp.). Each female was randomly taken from one flask and quickly placed in an observation chamber containing the same experimental conditions. The swimming path was videotaped (5 minutes); then the size of the female was measured. The tape was analyzed by automatic tracking (25 x,y coordinates of the center of the animal, in a 512 × 512 pixels space). The swimming path was analyzed for 45 females in both treatments. The speed (mm s^–1 body length^–1) was calculated for all trajectories, or only for those segments where females swam in a horizontal plane. This relative speed significantly decreased when the concentration of oxygen was very low. There was a negative correlation between the linear speed and the angular speed. The spatial sinuosity (S of Bovet & Benhamou, 1988) was calculated. The trajectories were significantly more sinuous when the concentration of oxygen was very low. These results could explain the accumulation of some rotifers in the oxycline; rotifers may spend more time in very low concentrations of oxygen by slowing and by turning more.     

Factors influencing the biomass of the rotifer Brachionus plicatilis in culture

The contribution of the egg weight and the population size structure to the body mass have been studied in two strains of Brachionus plicatilis of different size. A mathematical model was developed in order to obtain a reliable estimate of the dry mass from two single easily determined parameters; the egg/female ratio and the mean lorica length.     

Ecological genetics of the rotifer Brachionus plicatilis in Soda Lake,Nevada, USA

In an earlier study we found that the rotifer Brachionus plicatilis in Soda Lake, Nevada (USA) maintains extensive genetic variation for three isozymes. This variation was present both within and between different collections. No evidence was found that there was a seasonal succession of distinct electromorphs. In this paper we ask: is the genetic variation maintained by selective neutrality among electromorphs? Parthenogenetic females from ten clones representing seven different composite electromorphs were selected for life table analysis. Substantial main effects of genotype or interactions of genotype with experimental food quantities and temperatures, were found for the intrinsic rates of increase, net reproductive rates, and mean life spans of these clones. We conclude that these fitness differences permit us to reject the hypothesis that genetic variation is maintained because of selective neutrality among clones. Instead, it seems likely that directional selectional may be either too weak or of too brief a duration in Soda Lake for clonal exclusion to occur.     

Production of the rotifer Brachionus plicatilis (Ploimida: Brachionidae) in a Brazilian coastal lagoon

Secondary production, density and biomass fluctuations, and some reproductive aspects of the rotifer Brachionus plicatilis were studied weekly for two years in a Brazilian coastal lagoon. Food (and secondarily temperature) seemed to be the main factor involved in the summer increments. In the first summer, a mass mortality of fish caused changes in the phytoplankton, which switched from the dominance of Cyanophyceae to Chlorophyceae and other groups. For two months, production, density and biomass were relatively high until the recovery of the Cyanophyceae dominance. In the second summer, peaks of shorter duration and higher values of production, in comparison to the first one, were found. The contribution of resting eggs hatching, in the first summer, and the increment in parthenogenetic reproduction, in the second summer, would account for the population increases observed. Males and mictic females occurred when densities were high.     

Continuous culture of the rotifer Brachionus plicatilis fed recycled algal diets

A culture system for the rotifer Brachionus plicatilis was designed to maintain higher food conversion rates and stable population densities. Two 2001 plastic tanks were employed in the culture experiments, tank A for feedback culture and tank B for a control culture. The experiments were carried out for 70 days at 24 °C, light intensity, 1500 lux, and a photoperiod of L:D 15:9. B. plicatilis were fed once a day on baker's yeast and Chlorella.Food conversion rates in tanks A and B were 24.7% and 10.1%, respectively. Population density of B. plicatilis in tank A was consistently stable at 100–150 ind. ml^–1 throughout the culture period. Density in tank B, however, showed large fluctuations after 40 or 50 days and by the end of the experiment, declined to zero.