| Phylum:
Rotifera
Sub-phylum: Trochhelminthes Class: Monogononta Order: Rotatoria Genus: Brachionus Species: B. calyciflorus The male / female
rotifer photographs are to scale --->
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Amictic female B. calyciflorus with one (two) egg(s) attached |
Male B. calyciflorus
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Members of the class Monogononta occur in both the plankton and benthos of fresh-water lakes and streams. Monogonont rotifers are simple morphologically, with a body of three parts: a head, trunk, and foot. Locomotion is facilitated by a circular band of cilia surrounding the head called corona. A stiff outer shell called lorica covers the body giving it a distinctive shape. The lorica sometimes has anterior and posterior spines that function as predator defense. In B. calyciflorus cyclomorphosis (spine length variation) is common. The posterolateral spine length may exceed 100 m length on individuals exposed to a predatory rotifer Asplanchna brightwelli. Rotifers are composed of about 950 cells and have fully specialized nervous, digestive, excretory and reproductive sytems. They generally exhibit constancy of cell number (eutelic). This means that its growth during ontogenic development is achieved through the enlargement of its fixed number of cells or syncytium. All cell division occurs before hatching. Subsequently, the majority of individual's growth occurs before the first reproduction. The number of eggs which may be laid is determined by the number of egg nuclei present in the germarium at birth (Hoff and Snell 1989, King 1967).
Four periods in the rotifer life cycle may be distinguished: (1) the embryonic period from egg formation to egg hatching, (2) the pre-reproductive period from egg hatching to first reproduction, (3) the reproductive period from first egg production till the last egg hatching, (4) the post-reproductive period from cessation of reproduction to death. While the duration of these periods varies with the environmental conditions and from species to species, the characteristics of individuals in any one period are similar (King 1969). Brachionid rotifers have rather short life spans. Female life span at 25oC is about 6-8 days. Males live only about 2 days.
In B. calyciflorus,
elongated foot at the posterior is sometimes used for attachment to a substrate.
However, most of the time is spent swimming continuously throughout the
water column, filter feeding on suspended bacteria and algae. The early
post-hatching stages are typified by vigorous feeding and moving activities.
Parthenogenetic females achieve remarkable reproductive capacity. They
produce their first offspring when they are about 18 hours old, and continue
producing throughout most of their lives. Life-time fecundity for a single
amictic
(asexual) female is temperature dependent. In our Florida strain of B.
calyciflorus the average was 14, 15, and 18 daughters at 15oC,
22oC and 29oC respectively. Under ideal conditions
we observed the maximum number of offspring produced was 27 at 22oC
and 48 at 29oC. During peak reproduction (days 1-4 of female's
life), eggs are extruded every 4-6 hours and hatch after another 12 hours.
Such a high fecundity coupled with short developmental time give rotifers
some of the highest reproductive rates recorded for animals (r=0.7 to 1.4
offspring per female per day)(Hoff and Snell 1989). Another characteristic
of brachionid rotifer reproduction is that eggs are extruded sequentially
i.e. clutch size is always 1 (Stelzer 2005) even though female may be carrying
two or more eggs at a time. As the end of the reproductive period approaches,
these functions and their vigor successively degenerate. The effects of
senescence are mostly obvious in post-reproductive animals which are readily
identified by their slow movement and rates of feeding as well as such
gross morphological changes as the acquisition of optically less dense
body cavity and gut (King 1969). 
Rotifers of class Monogononta reproduce by cyclical parthenogenesis. In its natural environment, a typical reproductive cycle starts in the spring when a diploid resting egg that has remained dormant through the winter hatches to produce an amictic female. This female reproduces by diploid ameiotic parthenogenesis and, assuming no mutation, her descendants constitute a genetically identical clone. Under certain environmental conditions, the amictic female may form eggs that hatch into morphologically identical mictic females. If unfertilized by already present males, smaller haploid eggs are produced which hatch into males. If fertilized, thick shelled resting eggs (cysts) are formed which eventually hatch into a new line of amictic females.
Rotifer cysts are encysted embryos with arrested metabolism which are capable of dormancy of up to 20 years. At 110 mm long, the tough shell of rotifer cyst allows the rotifer to endure extreme environmental conditions, including desiccation. Because of these properties, rotifer cysts are easily dispersed by birds and other animals (Hoff and Snell 1989). This characteristic undoubtedly contributes to the cosmopolitan distribution of rotifer species throughout the world. Rotifer cysts are likely to hatch sporadically over weeks and months. As more than one resting egg cohort in a sediment egg bank is likely to hatch in a given season, natural populations in the plankton are therefore likely to be multiclonal (Gilbert, John J. 2002).
The cues triggering
sex are poorly understood for most species of rotifers, but food, the presence/absence
of physiological stress as well as genetics, all play a role. Rotifer density
is the cue for mixis in B. calyciflorus (Gilbert 2004), but the
propensity for the response to mixis stimulus and to production of mictic
females is low in the first generations after hatching from cysts and gradually
increases with each subsequent generation. It reaches its maximum after
about 12 generations of parthenogenesis (Schröder and Gilbert 2004).
Female parthenogenesis permits rapid population growth of clones without
recombination load and production of non-reproducing males. Sexuality in
rotifers, on the other hand, produces new genotypes and a sediment bank 
of
diapausing eggs. Thus the repression of rotifer sexual reproduction in
early generations maximizes rotifers' fitness by prolonging the phase of
female parthenogenesis and increasing the population size before the initiation
of sexual cycle (Gilbert, John J. 2002).
REFERENCES:
Additional Rotifer Information: University of California at Berkeley
Gilbert, John J. 2002. Endogenous regulation of environmentally induced sexuality in a rotifer: a multigenerational parental effect induced by fertilization. Freshwater Biology 47:1633-1641
Gilbert, John J. 2004. Population density, sexual reproduction and diapause in monogonont rotifers: new data for Brachionus and a review. J. Limnol. 63:32-36
Hoff, Frank H. and Snell, Terry W., 1989. Plankton Culture Manual. Florida Aqua Farms, Dade City Florida.
King, Charles E., 1967. Food, Age, and the Dynamics of a Laboratory Population of Rotifers. Ecology 48(1):111-128
King, Charles E., 1969. Review Article: Experimental Studies of Ageing in Rotifers. Exp. Geront. 4:63-79
Schröder, T. and Gilbert, J.J. 2004. Transgenerational plasticity for sexual reproduction and diapause in the life cycle of monogonont rotifers: intraclonal, intraspecific variation in the response to crowding. Funct. Ecol. 18:458-466
Stelzer, Claus-Peter,
2005. Evolution of rotifer life histories. Hydrobiologia 546:335-346
