Microgravity and hypergravity effect on survival and reproduction of microinvertebrates

Preliminary to carry out long-term experiments on the International Space Station (ISS) using living orgainisms, the capacity of the experimental organisms to cope with perturbations of gravity should be tested. Actually, animals have evolved under gravity, because on earth gravity force cannot be eluded, and several features that influence life-history traits may be affected by the presence of gravity. Among the other features, feeding efficiency may be affected by gravity if the animals feed by filtering suspended particles, creating currents that carry the particulate food to their mouth opening. In presence of gravity the food particles tend to sink to the bottom and filter-feeders must be able to suspend and collect the particles with some apparatus such as ciliary wreaths. It can be predicted that hypergravity, increasing the particle sedimentation rate, will reduce the animal filtering efficiency, while microgravity will increase filtering rate. Differently, some bacteriophagous animals do not possess structures to collect their food, but commonly live and move into sediment and feed on the bacteria upon encounter. Hypergravity will apply higher pressure on their bodies, and could force them to adhere to some surface and to reduce their displacement, and microgravity could impede adhesion to the surface and make food item encounters improbable. Thus, gravity perturbations may affect animal life-history traits, such as survival or fecundity, by influencing their feeding efficiency. In this study we exposed a filter-feeding organism (Macrotrachela quadricornifera, Rotifera Bdelloidea) and a bacteriophagous one (Panagrolaimus rigidus, Nematoda) to both microgravity and hypergravity to test their reproduction capacity under such stressful conditions, and their suitability as models for experiments on the ISS.