Abstract
The study of sexual reproductive behavior supported by ultrastructural evidence is important in rotifers to describe differences among potential cryptic species. In this research, the morphology of the rotifer Brachionus bidentatus is described at the ultrastructural level, using electronic microscopy, together with a brief description and discussion of its sexual reproductive behavior. The characteristics of the (a) male,(b) the female, (c) the sexual egg or cyst, (d) the partenogenic egg, (e) the no-fecundated sexual egg (male egg), and (f) the trophi, were described. Another part of this research is dedicated to the ultrastructure of the sex cells of the male rotifer B. bidentatus. Samples were obtained from La Punta pond in Cosio, Aguascalientes, Mexico (22°08’ N - 102°24’ W), and a culture was maintained in the laboratory. Fifty organisms, from different stages of the rotifer Brachionus bidentatus, were fixed in Formol at 4% and then prepared; besides, for the trophi, 25 female rotifer Brachionus bidentatus were prepared for observation in a JEOL 5900 LV scanning electronic microscope. In addition, for the observation of male sex cells, 500 males of Brachionus bidentatus were isolated, fixed and observed in a JEOL 1010 transmission microscope. Females of B. bidentatus in laboratory cultures had a lifespan of five days (mean±one SD=4.69±0.48; N=13), and produced 4.5+3.67 (N=6) parthenogenetic eggs during such lifespan. In the case of non-fertilized sexual eggs, they produced up to 18 eggs (mean±one SD=13±4.93; N=7). Sexual females produced a single cyst on average (mean±one SD=1±0; N=20). For the sexual cycle, the time of copulation between male and female ranged from 10 to 40 seconds (mean±one SD=17.33±10.55, N=7). The spermatozoa are composed of a celular body and a flagellum, the size of the body is of 300nm while the flagellum measures 1 700nm. The rods have a double membrane. Their mean length is almost 2.45μm±0.74, N=6; and their mean wide is 0.773μm±0.241, N=11. The evidence on the specific ultrastructural characteristics of the rotifer B. bidentatus is notorious, even more in the male and in the cyst cell. Regarding the ultrastructure of the spermatozoa and the rods, compared to other species they only differ in size, despite their structural resemblance. Our study of the ultraestructure of this species adds useful information that along with molecular data will help clarify the taxonomy of brachionid rotifers.
Highlights
The study of rotifer ultrastructure is still important for two reasons: (1) there is still a paucity of information regarding basic rotifer morphology, (2) detailed study of rotifer trophi using scanning electron microscope (SEM) is an useful tool to help differentiate cryptic species (Segers, Murugan & Dumont, 1993)
The study of B. bidentatus ultrastructure and sexual features have shown its importance as it allows to determine subtle, yet important, differences in sexual reproductive behavior and structure that contribute to the general knowledge of this genus, the best studied in rotifers
The cysts are part of the sexual cycle, but they are capable of withstanding unfavorable conditions, being reservoirs of genetic diversity (Gómez & Carvalho, 2000). They are characterized by differences in morphology and a darker color with respect to the parthenogenetic eggs based on observations of light microscopy; many other differences are clear when they are observed with electronic microscopy (Pourriot & Snell, 1983)
Summary
The study of rotifer ultrastructure is still important for two reasons: (1) there is still a paucity of information regarding basic rotifer morphology, (2) detailed study of rotifer trophi using SEM is an useful tool to help differentiate cryptic species (Segers, Murugan & Dumont, 1993). Brachionus bidentatus is a freshwater rotifer that has several polymorphic characteristics (Koste, 1978; Silva-Briano 2000). One of these variants, having two anterior wing-like projections, was described as a new species after experiments demonstrated that these projections were not the result of phenotypic plasticity Two types of cells have been identified in the sperm vesicle: spermatozoa and rods (Clément & Wurdak, 1991). The rods are cells that lack flagellum and nucleus and have great secretory activity They are composed of a dense homogeneous substance originated in the Golgi apparatus (Clément & Wurdak, 1991). We analyzed the trophi and the spermatic cells, and compared our findings with those previously published
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