SOME OBSERVATIONS ON THE CLADOCERAN 369
It was concluded that although clearly concentrated near the
surface during the day, animals are more randomly distributed,
probably at all levels, at night.
The tendency to come to the surface and to remain in close
association with it is probably a phototactic response, or at least
partly so. This would weaken, presumably, as daylight failed.
The term 'migration' has been avoided in this context, since
phototaxis would appear to be the only directed movement.
Summary
Scapholeberis mucronata (O. F. Muller) was studied in a small
ornamental pond. Although heavier than water Scapholeberis is
able to remain suspended from the surface film, and is able to
swim in this position. The interchange between the surface film
and lower levels regularly takes place. In that the surface film
not only provides no advantage to the majority of Cladocera, but
may be fatally dangerous, it is all the more remarkable that
Scapholeberis should use the physical properties of the film to its
advantage. This habit therefore represents an impressive feat of
evolution and involves suitable adaptations in morphology, colora-
tion and locomotion.
The animals appear to feed upon organic matter derived partly
from the surface film, and probably also from lower levels, and
comprising mainly algae and detritus, depending upon relative
abundance.
Variations in the length of the head and tail spines were seen
to take place during the year: the head spine was longest in
spring, while the tail spine reached its maximum in the summer.
Parthenogenetic females were seen to carry a maximum of five
eggs, with a progressive reduction through the season. Ephippial
females were noted immediately prior to the sudden collapse of
the population in September 1964, but males were not seen.
Scapholeberis was also shown to possess a characteristic circa-
dian rhythm in its vertical distribution. Daylight hours were
spent at, or close to, the surface as positive phototactic response;
deeper water was entered in darkness.
Acknowledgments
I wish to thank Dr J. Rzoska for his most valuable help and
advice, and Dr G. Fryer, who has kindly read and criticised the
manuscript.
References
Green, J. (1963). Seasonal polymorphism in Scapholeberis mucronata
(O. F. Muller). J. Anim. Ecol, 32: 425-439.
Ruttner, F. (1962). Grundriss der limnologie. Walter de Gruyter & Co.,
Berlin, 332 pp.
Scourfield, D. J. (1894). Entomostraca and the surface-film of water.
J. Linn. Soc. Zool, 25: 1-19.
Scourfield, D. J., and Harding, J. P. (1958). Key to the British Fresh-
water Cladocera. (Freshwater Bio. Ass. Sci. Publ., No. 5, 54 pp.).