Photoperiod and temperature as inducers of gamogenesis in a dicyclic population of Scapholeberis armata Herrick (Crustacea: Cladocera: Daphniidae)

Abstract

The induction of gamogenesis in a dicyclic population of Scapholeberis armata from a small pond at 40 ° N in Pennsylvania, USA, was studied . Animals were collected at the time of gamogenesis in mid-October (fall) 1988 and early June (spring) 1989) . 10 clones were established from isolated females for each season and maintained at low density under standard conditions of 20 °C and 13 .5L/10.5D, with Chlamydomonas reinhardtii diluted with pond water to 3 . 105 cells ml -1 as a food source. For each clone, mothers of pairs of test animals were acclimated to 4 sets of conditions : 20 'C/15 .5L (spring), 15 'C/11 .5L (fall), 20 °C/1 1 .5L (control), and 15 °C/15 .5L. Test animals were observed bidaily or daily for up to 37 days . Occurrence of molts and of parthenogenetic broods or ephippia was recorded . At short photoperiod, ephippial females developed, usually after one or more parthenogenetic broods, among both spring and fall animals . The response was stronger (higher % sexual females) among fall clones, and was enhanced in both groups by 15 °C water . No animals responded to long photoperiod in warm or cool water . It is likely that additional factors, such as prior photoinduction, rapid depletion of food supply, and increased inter-animal contact due to high population density are required for the induction of spring gamogenesis . Differences in the responsiveness of spring and fall clones to inducing conditions suggest some differences in genotypes of spring and fall subpopulations in this pond . It is proposed that variable responses among genotypes to factors inducing gamogenesis may be the basis for the differences and clines in the timing of gamogenesis found among ecologically and latitudinally separated populations . 269

Impacts of male and food density on female performance in the brackish cladoceran Daphniopsis australis

“…Besides food availability, changes in photoperiod, temperature and population density also trigger the switch from asexual to sexual reproduction (Banta et al, 1939;Berner et al, 1991;Kleiven et al, 1992). Among these proximate factors, the initiation of sexual reproduction seems to be more responsive to the changes in population density (Innes, 1997) and food availability than other factors (Gibson et al, 1998;Martinez-Jeronimo et al, 2007).…”

Thermal effects

“…At high temperatures (32°and 36°C), development of eggs was inhibited, and fingerlings exhibited abnormal behavior after 48 hours of exposure to sublethal levels of urea. Berner et al ( 1991) studied the induction of gamogenesis in a dicyclic population of Scapholeberis armata from a small pond at 40 0N in Pennsylvania. Ten clones were established from isolated females for each season and maintained at low density 588 under standard conditions of 20°C and 13.5L (light)/ 1O.5D, with Chlamydomonas reinhardtii diluted with pond water to 3 X 10 5 cells/ml, as a food source.…”