Rate of metabolism and sex determination in Cladocera.

Abstract

Cladocera reproduce mostly by parthenogenesis, and for long periods of time only female young may appear. Sometimes, however, males arise from parthenogenetic eggs, which eggs are indistinguishable from the parthenogenetic eggs ordinarily giving rise to females. At times sexual eggs appear and, if pairing occurs, these larger eggs pass into specially modified egg cases, the ephippia, within which they are cast off, undergo the earlier developmental stages, and enter a resting or latent period. This latent period is much prolonged (probably for months or even years with some forms) unless the eggs undergo desiccation or freezing or both. After desiccation or freezing if returned to water at 10° to 25° C. the sexual eggs develop into female young.

Experience in rearing several species of Cladocera lead the senior writer to conclude 1 that different environmental conditions are responsible for the occurrence of parthenogenetic females, or of males, or of sexual eggs. In the collaborative work (most experiments were with Moina macrocopa) it was found that crowding the mothers caused the production of a variable percentage of males, while sisters given identical treatment, but uncrowded, produced only females. It was also found that CO₂ or uric acid treatments slightly increased the percentage of males in crowded or semicrowded bottles. However by far the majority of treatments (including other excretory or related products), which altered the chemical condition of the medium in crowded or semicrowded bottles, reduced male production. These experiments suggested the accumulation of excretory products as associated with male production.