Ecological benefits of consistent expression of posterolateral spines in Brachionus calyciflorus

Abstract

Plasticity of spine morphology is a universal phenomenon in rotifers. Some Brachionus calyciflorus clones consistently develop two posterolateral spines (PS) with similar length (Type A), but others (Type B) not. The underlying mechanisms remain unknown. In the present study, the resting eggs were collected from the same B. calyciflorus strain, their hatchlings (stem rotifers) were categorized into types A and B, and then, clonally cultured for eight generations at different algal densities or temperatures. The results showed that the PS varied in length among different temperatures and food levels, but inherent differences between types A and B were always observed, indicating that differentiation between types A and B might not associate with temperature or food availability. Next, we compared the population growth and PS length between types A and B at the general condition (control), in responses to Asplanchna kairomone, Keratella tropica and Moina macrocopa. The results showed that type B had higher population growth rate than type A, suggesting the higher adaptability of type B in the general condition. In response to Asplanchna kairomone, the PS length of both types A and B was significantly induced, and their PS length was similar, but type B showed higher population growth rate than type A. Coculture with M. macrocopa significantly lengthened the PS of type B but not type A, and types A and B showed similar population growth curves. Coculture with K. tropica did not affect significantly the PS length of types A and B. However, type A showed significantly and greatly higher population density and population growth rate than type B, indicating a much stronger competition ability of type A than type B against K. tropica. Overall, these results indicated that consistent development of PS might facilitate B. calyciflorus to defend against small competitors.