Remarkably, when the extrusomes are discharged, the cell remains intact and functional. They can display differences in structure and morphology, but they share the common characteristic of discharging their contents to the outside of the cell in response to mechanical or chemical stimuli. They are membrane-bound organelles usually located in the cell cortex, attached to the cell membrane. The term “extrusome” was proposed, for the first time, by Grell in 1973 for extrusive (ejectable) bodies, which occur widely in protists. Extrusomes, the specialized organelles for predator-prey interaction Finally, raptorial ciliates are able to directly catch other organisms using peculiar organelles to paralyze and/or kill their prey, generally called extrusomes.Ģ. Suctorial-feeding ciliates are represented by sessile or sedentary species that for most of their lives remain attached to other organisms or various substrates, intercepting the food particles with their specialized tentacles. In filter-feeding ciliates, the food, represented by small organisms or edible debris of various types, was pushed into the ciliate buccal cavity by the rhythmical beats of the cilia located in its adoral apparatus. The authors compare them to the strategies used in fishing, like netting, trapping, and harpooning. Verni and Gualtieri describe three main phagocytotic processes in ciliates: filter feeding, suctorial feeding, and raptorial feeding. Different techniques of phagocytosis have been described in various protists, where they have especially been investigated in ciliates. In this context, a common feeding mechanism found in heterotrophic protists is phagocytosis, a process which requires specific organelles for food assimilation and which occurs in three steps: food capture, phagosome formation, and food digestion. General scheme of predator-prey interactions. This scheme should be considered functional for both animals and protists, and indeed several studies have shown that the food recognition and the offense-defense mechanisms adopted by some groups of protists can be compared, in terms of complexity and variability, with those observed in animals. On the other hand the prey recognizes the presence of the predator (step 1′) and it can organize its defense mechanisms (step 2′). Figure 1 shows a general scheme of predator-prey interactions, where the predator recognizes the presence of the prey (step 1) and can attack it (step 2). In this regard, it is known that protists have developed a variety of strategies of feeding behaviors especially in response to different environmental factors, together with a diverse kind of food available in micro-habitats. Among these functions, self-nonself recognition mechanisms are represented by a large repertoire in protists and can trigger either autocrine or paracrine processes in some ciliates (see for a review), together with the capability to detect prey (food) or predators in others. Whittaker originally defined protists as those “organisms which are unicellular or unicellular-colonial and which form no tissues,” and for this reason they must carry out at the cellular level all the basic functions which can be observed in multicellular eukaryotes.

Indeed, especially in the last 30 years, a lot of studies have been devoted to describing the predator-prey interactions among unicellular eukaryotic organisms like protists. A common definition for predatory behavior describes it as the process through which one animal, the predator, captures and kills another animal, the prey, before eating it in part or entirely however, according to the opinion of a number of microbiologists and protistologists, this definition should be also extended to different organisms included in other life Kingdoms, with particular regard to microorganisms.

Predator prey