Gigantism in the rotifer <i>Asplanchna brightwellii</i>: dietary control, reproduction, propensity for sex and ecological significance

Abstract Asplanchna brightwellii naturally eats algae, protozoans and rotifers in the 5–200 μm size range. On these foods, it reproduces asexually, and also sexually if they contain traces of tocopherol synthesized by photosynthetic organisms. Here, three clones of A. brightwellii were cultured on two tocopherol-rich foods, the alga Cryptomonas erosa or the rotifer Brachionus rubens feeding on that alga, to compare their effects on Asplanchna’s body size and population biology. Adult females fed Brachionus were 27–49% larger, often reaching lengths of 1 mm. Amictic females (producing diploid eggs) fed Brachionus produced more daughters and reproduced more rapidly; mean net reproductive rates (R0) were 17.1 vs. 8.9, and mean intrinsic rates of natural increase (r day−1) were 0.78 vs. 0.50. Mictic females (producing haploid eggs) fed Brachionus produced more males (14.3 vs. 9.8) or fertilized diapausing eggs (6.5 vs. 2.8), and larger males (440 vs. 397 μm). Diet had no effect on propensity for sex in two clones, but a pronounced effect in another: two sets of populations fed Brachionus contained 3.5 and 5.8 times fewer mictic females. Diet-induced variation in Asplanchna’s body size, demographics and commitment to sex has important implications in natural communities.