<i>Intraspecific space competition in Halichondria panicea</i> (Porifera: Demospongiae)

Abstract The spatial competition in the White Sea’s Halichondria panicea sponge was studied through a field experiment assessing growth in isogeneic and allogeneic sponge fragments of equal or different sizes. After 3 months and 1 year in seawater, growth was evaluated using ImageJ software on photographs. Intraspecific competition among allogeneic H. panicea individuals led to a decrease in relative growth, with the size of interacting individuals influencing competitive strategy. Optimal growth occurred when competitors were larger, minimal when sizes were equal, suggesting an alternative competitive strategy in the latter case. Competition between isogeneic individuals of H. panicea was weak or even absent; fusion of isogeneic fragments increased the growth intensity and substrate coverage by the sponge. Analysing the growth directions of sponges, we have found a phenomenon that may be interpreted as an attempt to ‘avoid’ physical contact with a competitor. In the neighbourhood with an allogeneic individual of larger or smaller size, the growth towards the competitor was lower than in other directions, regardless of whether the neighbouring individuals reached contact with each other or not. This may indicate that growth was redirected due to some distant communication mechanisms. The growth of allogeneic and isogeneic explants before contact occurred in a similar manner. Apparently, H. panicea cannot recognize the genetic nature of a competitor at a certain distance.