**1. Introduction**

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 [1]; 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. 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. Whittaker [2] 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. 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 [3] for a review), together with the capability to detect prey (food) or predators in others. 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. **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). On the other hand the prey recognizes the presence of the predator (step 1′) and it can organize its defense mechanisms (step 2′) [4]. This scheme should be considered functional for both animals and protists, and indeed several studies have shown that the food recognition and the

**Figure 1.**

*General scheme of predator-prey interactions. Redrafted from [4].*

offense-defense mechanisms adopted by some groups of protists can be compared, in terms of complexity and variability, with those observed in animals.

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 [5]. Different techniques of phagocytosis have been described in various protists, where they have especially been investigated in ciliates [5–7]. Verni and Gualtieri [5] describe three main phagocytotic processes in ciliates: filter feeding, suctorial feeding, and raptorial feeding. The authors compare them to the strategies used in fishing, like netting, trapping, and harpooning. 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. 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. Finally, raptorial ciliates are able to directly catch other organisms using peculiar organelles to paralyze and/or kill their prey, generally called extrusomes.

### **2. Extrusomes, the specialized organelles for predator-prey interaction**

The term "extrusome" was proposed, for the first time, by Grell in 1973 for extrusive (ejectable) bodies, which occur widely in protists [8]. They are membranebound organelles usually located in the cell cortex, attached to the cell membrane. 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. Remarkably, when the extrusomes are discharged, the cell remains intact and functional. Studies on extrusomes and related organelles have been reviewed by Hausmann [9], Dragesco [10], Kugrens et al. [11], Hausmann and Hülsmann [12], and Rosati and Modeo [13]. Typical examples of these organelles include toxicysts, trichocysts, mucocysts, cortical, or pigment granules in ciliates and flagellates, haptocysts in suctorians, and kinetocysts in heliozoan actinopods. Some extrusomes are known to be related in predator-prey interactions, for example, to catch and kill the prey (such as toxicysts, haptocysts, kinetocysts, and some cortical granules), or used as defensive organelles (such as the trichocysts and various cortical or pigment granules), but the role of other kinds of extrusomes such as the mucocysts in *Tetrahymena* or the trichites in Strombidiidae [13] still remains obscure.

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**Figure 2.**

*Predator-Prey Interactions in Ciliated Protists DOI: http://dx.doi.org/10.5772/intechopen.78622*

Offensive extrusomes generally possessed by raptorial protists and located usually at or near the feeding apparatus are discharged after contact with a possible prey, which is immobilized, damaged, or firmly bound to the predator. Among these, organelles, certainly the most widely studied, belong to the category of toxicysts (toxic extrusomes) and they play an essential role in capturing and killing prey [7, 13]. Toxicysts are synthesized in Golgi or ER vesicles and are usually localized in the cell cortex attached to the cell membrane. Most of them are observed in species belonging to the class Litostomatea and subclass Haptoria, but they are also present in other predatory ciliates. They are usually positioned in a specific region of the cell, near the oral apparatus, and generally in the first portion which contacts the prey during the raptorial feeding [13]. Independently of the specific differences in the morphology of the cytostome, the toxicysts are always present in an appreciable number, for example, in the genera *Didinium*, *Dileptus*, *Prorodon*, *Litonotus*, *Colpes*, *Homalozoon*, and many others. In resting position, the toxicysts appear generally as rod-like elements (**Figure 2**), and could be discharged in milliseconds, if exposed to an appropriate stimulus such as contact with a prey (**Figure 3**) [7]. In this case, the tubules of the toxicysts are suddenly introduced into the cytoplasm of the prey's body, like hypodermic needles, to inject the toxic material. Hausmann [7] reports essentially two ways by which the toxicysts may be discharged: in the first case, there is a fusion of the toxicyst's membrane with the plasma membrane, followed by the tubule discharge via evagination; in the second, observed in certain ciliate species, a telescopic discharge of the tubules was observed. During or near the end of the toxicysts' discharge, the toxic secondary metabolites were secreted by the tubules. It is worth noting that this mechanism of discharging toxic substances shows the structural and functional similarities that can be found between the toxicysts in ciliated protists and nematocysts in Cnidaria, despite the substantial

In contrast with recent and less recent studies about the nature of the toxic secondary metabolites used by ciliates in chemical defense, no exhaustive data are yet available about the composition of the toxins stored in the toxicysts of predatory ciliates. This is essentially due to the difficulty in separating the content of extrusomes from other molecules produced by the ciliate, in order to purify them at

*Transmission electron microscope (TEM) picture of the toxicysts in a dividing cell of a ciliate* Didinium nasutum*. Scale bar = 1* μ*m. Original picture by R. Allen from http://www.cellimagelibrary.org/images/10010.*

homogeneity for subsequent chemical and structural analyses.

**3. Offensive extrusomes**

differences in size [7].
