CYCLOMORPHOSIS IN ROTIFERS

CYCLOMORPHOSIS IN  ROTIFERS

Introduction

The Morphological change those occur in certain species of invertebrates in accordance to

environmental conditions is called Cyclomorphosis. The term Cyclomorphosis was coined by Lauterborn (1904) but the actual concept came to lime light only after Coker (1939). The phenomenon has been noted in the dinoflagellates, cladocerans, and to a much striking degree in the copepods. Organisms that reproduce during most of the year by asexual or parthenogenesis methods appear to exhibit this phenomenon. The degree to which the Cyclomorphosis is developed within different populations of the same species is variable. Although the seasonal incidence of the change is clearly determined by environmental factors, there may also be inherited diversity in the capacities of different races of a species to react to these factors. The seasonal changes of form are so striking that the summer and winter forms of the same species would certainly be supposed to represent different species by an observer, unacquainted with the facts. It involves the alternation of different morphological units in a species in accordance to the climatic as well as environmental changes. Total body size may increase, decrease or remain the same throughout the cycle, depending on the species.

Rotifers are one such group of invertebrates found abundantly in any fresh water body throughout the globe. Popularly known as “the wheel animalcules”, these are although small in number but

large populations are found in a particular environment. Among the Rotifers the monogonants especially that of order Ploima having the genus Brachionus are unique for their polymorphic forms and exhibition of Cyclomorphosis. Several investigation carried by earlier workers also reveal the same fact. Cyclomorphosis in rotifera was described first by Weisenberg-lund (1926) and later by several workers like Beauchamp (1952), Gallagher (1957), Hutchinson (1967), Gilbert (1973), Dodson (1974) also by Indian contributors like Arora (1965), Nayar (1968) and Dhanapathi (1980).

Place of investigation

To understand the phenomenon of Cyclomorphosis in Rotifers in different seasons as well as in

different ecological conditions, present work was carried out in two different perennial ponds in the city of Vizianagaram belonging to the state of Andhra Pradesh, India. Of these two ponds, one is highly polluted (Pond-1) and the other a bit clean and fresh water in nature (Pond-2) since no municipal or domestic wastages are allowed to pass in to this pond. Plankton samples were collected periodically (weekly once) from both pond-1 and pond-2. Species related to Brachionus are collected separately and measurements were being taken. Three species of Brachionus were only investigated, they are e.g., Brachionus calyciflorus, B.caudatus and Keratella cochlearis.

Brachionus calyciflorus Pallas

Brachionus calyciflorus is an extremely variable species, the variability is pronounced in the size,

length of posterolateral spines as well as occipital spines. Examination of all the data from the localities surveyed, the data from Big Pond (Pond-I) was found continuous enough to explain the phenomenon of Cyclomorphosis, in B. calyciflorus. Although this species was observed in the samples collected between April, 2009 and January, 2010, the continuous data could be obtained only from 7th October, 2009 to 9th January, 2010. On 23rd September, 2009 when the surface temperature was 32.4oC there were few forms of this species in the samples. On 7th October the number of specimen appeared a bit increased when the temperature was 28.5oC. The quantitative study of the rotifer revealed that there were two peak periods of abundance when the temperature was 21.0oC ON 26-11-09 and the second peak when it was 20.0oC ON 02-01-10. The rotifer stated disappearing from the samples when the temperature rose to 23.0oC on 16th January, 2010 and found very few on 23rd January 2010.During the period of occurrence of these species, the hydrogen-ion concentration (pH) fluctuated between 8 and 9 and turbidity between 11 and 72 ppm.

Morphological variations:

  Measurements were taken for thirty specimens from each sample collected once in a week. Length

of lorica (TBL), Maximum breadth (B) of lorica and the length of Postero-lateral spines (PLS) were

measured and their mean values were given in table. The respective points used in making the

measurements were shown in Graphs. The lengths of the lorica ranged from 230 to 328 microns while the breadth from 200 to 228 microns. In case of Postero-lateral spines the length of the right posterolateral spine which was more variable was taken. Its length ranged from 21 to 53 microns while the length of left posterolateral spine ranged from 23 to 50 microns.Thus we can conclude that the right spine is elongated than the left one.

The specimens with and those without posterolateral spines were almost equal in the samples of 7th

and 14th October. The specimens without posetrolateral spines gradually decreased and they disappeared completely in the samples collected from 4th November to 18th December. Again few specimens without posterolateral spines had re- appeared on 25th December and continued till 9th of January 2010. In the earlier collections the specimens without posterolateral spines (var. dorcas) are larger in size than those with posterolateral spines (var. dorcas f. spinosa). In the samples from 4th November till 18th December only var. dorcas forma spinosa was present. But the lateral Collections showed an assemblage of varied forms (var. dorcas, var. pala and var. dorcas f. spinosa ). Some specimens without posterolateral spines were found smaller than those with posterolateral spines in the collection during 25th December, 2009 to 9th January, 2010. Increase in the length of the postero-lateral spine (right) with the decrease in the size of the lorica has been observed.

Brachionus caudatus Barrios and Daday

 Although this species occurred for most part of the year in both the ponds but for morphological

variances, samples were collected from Pond – 1 were taken for present study. Only the measurements of the population collected from 05th March to 21st May 2009 were only analyzed for drawing the following conclusions.

Morphological Variation

 This rotifer showed a variation in length of posterior spines, arising on either side of the foot. The

measurements were taken for twenty specimens from each weekly sample. The length of lorica (TBL), maximum breadth of lorica (B) and the length of each posterior spine (PS) were measured and the mean values were presented in the Table. The respective points used in the making the measurements were shown.

 The lengths of Lorica ranged from 98 to 128 microns while the

Breadth from 87 to 112 microns. The Posterior spines are almost

equal except in very few. As the right one showed much variation,

measurements of this spine were taken for illustration. Its length varied from 12 to 63 microns.With the increase in the temperature the length of lorica increased and there was also a corresponding increase in the length of spines. But the rate of increase in the length of posterior spine is greater than the rate of increase in the length of lorica.

Conclusion

 The proximal causes including Cyclomorphosis can be mooted with the environmental conditions

those change within no time along with the climate. A relatively warm or warming temperature, turbulence presence of light, female principle and predation are some of the prime reasons. Exuberant forms of rotifers have been correlated with starvation for Branchious or with dense food or cold water. Gilbert had the opinion that incase of Branchious the abundance of Asplanchana (a predator) in the same locality also influence Cyclomorphosis. Turbidity appears to be also an important factor influencing the reproduction and abundance of both the species. B.calyciflorus was observed in abundance when the turbidity was low. Another important factor which was observed in the present observation was dissolved oxygen. With the increase in the dissolved oxygen content there was a corresponding increase in the abundance of both the species. Nayar (1956) was of the opinion that these physico-chemical factors may not have direct influence on the rotifer B.calyciflorus. This may be true to certain extent but in the present investigation it is clear that the occurrence of Cyclomorphosis is basically due to the physico-chemical changes in the environment.

  In both these ponds the maximum size of the lorica was observed for all the species during the

period of high temperature. In B.calyciflorus with the decrease in the temperature, there was a

corresponding decrease in the length of the lorica and increase in the length of postero-lateral spine. But in B.caudatus with the increase in temperature, there was a corresponding increase in the length of lorica and length of posterior spines. It may be concluded that no single factor can account for this seasonal polymorphism but a combination of many factors like temperature, turbidity and hydrogen-ion (pH) Concentration, dissolved oxygen and feeding behavior, etc. are all responsible for these variations which might act in a cumulative manner.

Brachionus is typical for its polymorphic forms and specialized spiny outgrowths. It was also

noticed that the species of Brachious examined from pond-I are much enlarged, healthy in look and much spinous comparatively to the samples brought and examined from pond-II. One of the biggest reasons for such variance may be attributed in the form of domestic sewage pollution which is opened into the pond-I in large accounts.

 After a fresh spell of monsoon, the distribution of phytoplankton is plenty. The malleate or malleo-

ramate mastex of herbivorous rotifers such as Brachionus and Keratella is specialized to capture such

nanno planktons, while the incudate trophy of Asplancha are specialized for capturing small rotifers. This suggests that prey of the right size but wrong shape is never accepted in predation, hence many be rejected after being caught. For example, the presence of long spines on B.calyciflorus does not affect the rate of collision with Asplancha but decreases the probability of being eaten once caught. Hence, a spine or any such thing at the right place at right time makes the prey unsuitable for predation.

n the present investigation, a correlation between the distribution and species composition is also observed during the period of elongation of outer ornamentation. More the spine elongations less was the species composition. It can be finally be said that the morphological structures, the lorica , abundance as well as the size and population of one species is inter related to the other especially to that of predator

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