Nicola Ricci

The behaviour of ciliated protozoa

Abstract This paper reviews the literature on the behaviour of ciliated protozoa, describes the locomotion of five species and examines studies of its causation, development, function and evolution. The significance of the very small size of ciliates and their unicellularity for the motor patterns is discussed. Several elements are involved in the causation of ciliate behaviour: depolarization of the membrane potential; adhesion to the substrate; cell size; individual variation; and metabolic state. The development and inheritance of mating behaviour and the adaptive consequences of thigmotaxis are also described. The hypotrichs are shown to have the most sophisticated behaviour and the fastest, most stereotyped locomotion of all the ciliates. The ciliated protozoa are suggested as suitable organisms for behavioural research on basic problems of how animals adapt to their environment.

The ethogram of Euplotes crassus (ciliata, hypotrichida): I. The wild type

The behavior of Euplotes crassus has been studied qualitatively and quantitatively in an attempt of throwing new light onto the problem of the relationships occurring among motor behavior, systematic position and evolutionary strategies of the ciliated protozoa. It was found that both single and paired Euplotes creep along piecewise pathways whose geometric elements (arcs, segments, angles) have been measured. E. crassus performs different delicate maneuvers to shift from one element of its trajectory to the next: beyond the Continuous Trajectory Change, the Smooth Trajectory Change and the Rough Trajectory Change, described elsewhere for other ciliates, three different kinds of Side-Stepping Reaction were recognized and carefully described. The average velocity is 850 μm/sec during creeping and 800 μm/sec during swimming. Finally, also the mating behavior has been analyzed and the successive steps leading to pair formation thoroughly described. The data are compared with those already available for different species.

Rhythmic spontaneous depolarizations determine a slow-and-fast rhythm in walking of the marine hypotrich Euplotes vannus

Summary In the marine hypotrich Euplotes vannus, the coupling of behavioural elements to shifts in the freely fluctuating membrane potential was investigated by use of a computerized locomotion track evaluation system. Potential shifts were attributed to beating of cirri by single-image analysis of simultaneous video-tape-recordings. Walking speed rhythmically decreases for 150–200 ms, and increases for 550–670 ms with a frequency of ~ 1.3/s. This pattern is caused by repetitive, spontaneous depolarizations of ~ 200 ms duration from about − 25 mV to about − 15 mV with a similar frequency. The speed is decreased by the combined reduction of the stroke amplitude of the fronto-ventral cirri and anteriad bending of the transverse cirri. Both the depolarizations and the walking pattern persist in a solution with a reduced Ca2+ activity of 50 μmol/l. Arhythmic spontaneous long depolarizations of 600–1200 ms duration induce a deceleration until movements stop and a fast backward jerking upon reversal of the beat direction of all cirri. This “side-stepping” (“avoiding”) reaction requires a high extracellular calcium concentration. Presumably, the depolarization rhythm is based on a pacemaking sequence of conductance changes of potassium.

The effects of 2 μM Hg++ on the ethogram of Euplotes vannus (Ciliata, Hypotrichida)

Summary Strains of Euplotes vannus (a marine hypotrichous ciliate) were treated with 2 μM Hg++. The ethograms of treated and control populations differ due to several traits, which are qualitatively described and quantitatively measured: (a) the creeping of treated populations is more uniform and continuous than that of controls; (b) typical behavioural patterns of controls (the “slow down” and the bursts of “Side Stepping Reaction” — SSR) disappear completely in treated ciliates; (c) the SSR of treated organisms has an average rotation angle of 163° (controls: 84°); (d) the velocity decreases significantly in treated cells (340 μm/s vs 450 um/s). These results are discussed from the point of view of their survival significance.

Behavioural modifications imposed to the ciliate protist Euplotes crassus by caulerpenyne: the major toxic terpenoid of the green seaweed, Caulerpa taxifolia

Summary Caulerpenyne is the most abundant, toxic terpenoid produced by Caulerpa taxifolia , one of the few toxigenic, green seaweed endemic in the tropics. Owing to the bewildering high concentration of this harmful secondary metabolite, associated with largely magnified homeostatic and reproductive potentialities, the strain(s) of C. taxifolia invading the Mediterranean Sea are a potential risk for biodiversity and ecotoxicity. The effects of caulerpenyne on the biology of the marine ciliate protist Euplotes crassus were studied by means of the ethogram, that is, by the qualitative and quantitative analysis of its creeping behaviour. Concentrations as low as 1.5 μg/ml (= 4 μM) sufficed to induce several clear-cut changes in the creeping of E. crassus and 6 μg/ml (=16 μM) affected significantly the entire ethogram: (a) the cell “activity” was reduced decreasing velocities as well as lengths of the “Long Lasting Elements” (LLE) and largely increasing the percentage of motionless Euplotes ; (b) the adhesion to the substrate was relaxed causing disappearance of the “Smooth Trajectory Change” (STC); (c) the electrophysiological states were altered modifying the Side Step Reaction (SSR) which lasted longer (prolonged Side Step Reaction, p-SSR). It follows that the Euplotes ’ adaptive behaviour becomes aberrant dooming individuals to a short-term death even under conditions of largely sublethal concentrations of caulerpenyne. The complex of the foregoing induced, behavioural variations is discussed in view of clues to the underlying mechanisms.

Protozoa as tools in the pollution assessment

The points discussed here, in their scientific importance, concrete usefulness, adaptability to practical applications and their fresh approach to the problems of marine pollution, are a demonstration of how protozoa offer a virtually infinite range of valuable instruments for the analysis of pollution in the sea with modern methods

A Quantitative Approach to Movement, Displacement, and Mobility of Protozoa

ABSTRACT Diffusion theory can completely describe the movement of a ciliate along a track of a certain length (L), travelled in a time (t), and with the extremes lying at a distance D. Three important descriptors of this behavior are: (1) the kinetic index (Ik= L/t), namely the average velocity in μm/s, which expresses the state of the “accelerator” of the ciliate; (2) the geometric index (Ig= D/L) measuring the straightness of the track by a dimensionless number. 0 ≤ Ig≤ 1, which expresses the state of the “steering wheel” and represents a sort of “directional efficiency”; and (3) the displacement rate (Rd= D/t), integrating the first two indices and expressing the combined effect of the “accelerator” and the “steering wheel” of the organism with a unique measure (in μm/s), which defines the average displacement rate or the effectiveness of the track in displacing the organism in space. A weighted estimate of general mobility is given by the mobility rate [Rmo= (R̄d.f)creeping‐ (R̄d.f)swimming], obtained by multiplying the average Rd of the creeping organisms and the average Rd of the swimming organisms by their relative frequencies of occurrence (f), and adding the two products. Values for experimental populations of Oxytricha bifaria (Ciliata, Hypotrichida) maintained at 24, 19, 14, and 9° C demonstrated both the appropriateness and the usefulness of these indices and rates to describe the tracks a posteriori, and to provide measures to reason about their possible adaptive significance.

The Effects of Cooling Conditions on the Behavior of Oxytricha bifaria (Ciliophora Hypotrichida)

The new analytic approach to the behavior of ciliates represented by the ethogram was used to study the locomotion of Oxytricha bifaria at different temperatures, isotropically applied according to a new protocol. It was shown that under these experimental conditions as the temperature dropped in stages from 24°C to 19°C to 14°C to 9°C, the general mobility of experimental populations decreased, as indicated by 1) the decreasing percentage of mobile organisms, 2) their decreasing velocity, 3) their prolonged backward creeping, and 4) the increasing length of their immobilization periods. The ethogram more particularly revealed that decreasing temperatures induced 1) the appearance of rightward arcs (several of them being travelled by specimens sliding on the substrate). 2) the reduction of both the radius and the velocity along the normal leftward arcs (A‐) and the segments (S), and 3) the symmetric increase of both the central angle and the duration of the A‐ and S. These changes were reversibly induced: they disappeared when the temperature returned to 24°C. Moreover. a new behavioral pattern, the prolonged Side Stepping Reaction, was found. Helicoidal swimming, occurring only at 14°C, was analyzed. Among all of the behavioral parameters, nine were shown to change dramatically between 14°C and 9°C, demonstrating that the linear cooling of the populations induced clear non‐linear effects.

The crowding effect: an ethologic analysis

Summary The phenomenon of crowding of ciliates in rest areas has been described previously [23]: these organisms tend to collect on the bottom of an experimental apparatus under objects interfering with the water-air interface. The behaviour of Oxytricha bifarta has been analyzed as a basic element possibly accounting for the effect itself: to monitor the phenomenon properly a well-defined time (1 h 15 min) was chosen during the formation of the overcrowded population; in addition, well-defined areas were TV recorded, the populations within them studied and their behaviour analyzed and compared. The results obtained are the following: (a) the number of idle cells is significantly larger in the Shelter-Area (S) than in the Open (O); (b) there is a net inward flow of oxytrichas, constantly enhancing the cell density in S; (c) the ethogram of oxytrichas creeping into S is quite similar to that of the populations in O: only the creeping velocity in S is smaller than in O; (d) the frequency of Side Stepping Reactions performed by O. bifaria at the level of the border between S and O is 5–6 times higher for oxytrichas creeping outwards than for those entering S. The combined effect of these phenomena well accounts for the formation of the crowding conditions.

Reversible carnivory in Oxytricha bifaria: a peculiar ecological adaptation

The adaptive significance of the giant form of Oxytricha bifaria has been studied. It proved to be a carnivorous phase of a bacterivorous ciliate, rather than a cannibalistic one. By such a dramatic reversal in the feeding behavior O. bifaria, a primary consumer, can reversibly become a secondary one.