Induction of diapausing amictic eggs in Synchaeta pectinata

Amictic females of a clone of S. pectinata from Star Lake (Norwich, Vermont) may produce diapausing as well as non-diapausing (subitaneous) eggs. The proportion of diapausing eggs produced in cultures was unaffected by temperature (12 vs 19 degrees C) or rotifer population density (minima of 0.33 vs 3 ind. ml(-1)) at 19 degrees C. However, at 19 degrees C this proportion was higher in cultures maintained at a low food level suppressing reproduction (5 x 10(3) cells ml(-1) Cryptomonas erosa) than in those maintained at a high food level (2 x 10(4) cells ml(-1)); the treatment effect was marginally significant (p = 0.067). Consistent with the effect of low food availability, a period of starvation was very effective in inducing the development of diapausing eggs. None of 19 females cultured individually from hatching at 19 degrees C on C. erosa (2 x 10(4) cells ml(-1)) in 1-ml volumes produced any diapausing eggs in 4 days (0 out of 349 eggs), while 13 out of 16 females subjected to a 15-hour starvation period 6 hours after birth produced one or more diapausing eggs during that time (34% of the 158 eggs produced by the 16 females were diapausing). Diapausing eggs produced and left at 19 degrees C hatched after 4 to 13 days. Those produced in cultures with a low food level took significantly longer to hatch (9.7 days) than those produced in cultures with a high food level (8.1 days) (p = 0.022). In natural communities, S. pectinata should be able to respond directly and rapidly to poor food conditions by producing eggs that undergo an obligatory dormant period before resuming development.