Nontransitive xenogeneic interactions between four common Red Sea sessile invertebrates

Available substrate for settlement is. a limited resource for many sedentary marine organ- isms, and hence the acquisition and maintenance of space is involved with an array of competitive interactions. Here we examine the competitive hier- archies between 4 common sessile reef inhabiting invertebrates in the Gulf of Eilat, Red Sea: Stylo- phora pistillata (scleractinian coral), Parerythro- podium fulvum fulvum (alcyonarian coral), Mille- pora dichotoma (hydrocoral), and the red sponge Diplastrella gardineri. Xenogeneic interactions among these species are expressed as a panel of effector mechanisms which involve nontransitive hierarchies and include patterns of overgrowth, cy- totoxicity, bleaching and allelopathy. Overgrowth is the most common mechanism in this panel of interactions, found in 4 out of the 6 pairwise tested combinations. Several species such as Stylophora exhibited different types of species specific and col- ony specific effector mechanisms when confronted in inter- and intraspecific encounters. Additionally, these common sedentary organisms were found to interact simultaneously in a network of 3 species or more.