Epiphanes brachionus spinosa from a temporary habitat in the Chihuahuan Desert: Morphological response to Asplanchna and control of sexual reproduction

Abstract

Abstract The rotifer Epiphanes brachionus spinosa occurs in shallow temporary waters with hydroperiods up to several months in the Chihuahuan Desert (U.S.A. and Mexico). Clonal populations from one such habitat were initiated from stem females hatched from fertilised resting eggs in flooded dried sediment, and cultured with the alga Cryptomonas erosa at 19°C in a photoperiod (L:D 16:8). Experiments show that the rotifer has: (1) a morphological response to the predatory rotifer Asplanchna, with which it can co‐occur; and (2) an early and constant propensity for sex and resting‐egg production during population development. Stem females, and females from later generations, have two posterior spines. Measurements of live juveniles and adults indicate that spine length increases isometrically during postnatal growth. Presence of live Asplanchna brightwellii, or its kairomone in Asplanchna‐conditioned medium, significantly increase spine length. In one clone, spine lengths of juvenile Epiphanes from cultures with or without live Asplanchna were, respectively, c. 40 and c. 27 μm. In that clone, spine lengths of adult Epiphanes in cultures with or without Asplanchna kairomone were, respectively, c. 54 and c. 37 μm. In another clone, these spine lengths were, respectively, c. 44 and c. 36 μm. Asplanchna‐induced spine elongation may act as a mechanical defence against capture and ingestion by the predator. Sexual reproduction in E. brachionus spinosa populations is initiated soon after populations develop from stem females and then continues at a rate allowing both population growth by female parthenogenesis and production of resting eggs. Resting eggs hatched after 6–12 days at 19°C. Stem females were amictic (producing daughters parthenogenetically) but produced some daughters that were mictic (producing haploid males parthenogenetically or fertilised resting eggs). In subsequent generations tested 2–7 weeks after hatching of stem females, amictic females from three clonal populations produced similar proportions of mictic daughters (mean c. 0.25) when cultured singly in large volumes (15 and 40 ml) or a small one (1.5 ml). Mictic‐female production is probably automatic and controlled by an endogenous mechanism, rather than specifically induced by crowding or some other environmental factor. Early and continuous sexual reproduction assures production of resting eggs throughout short and unpredictable hydroperiods.