[The adaptive value of cyclomorphic spine production in Brachionus calyciflorus pallas (rotatoria) : I. Predator-prey relationships in short term experiments].

In the rotifer Brachionus calyciflorus there occurs a cyclomorphic variation concerning the absence or the presence of a pair of postero-lateral spines. If present, these spines may vary up to lorica length. The production of these spines is induced by starvation, low temperature, or a peptide substance released into the medium by the predacious rotifer Asplanchna. The first two factors apparently induce spines only up to a relative short length, while extremely long spines appear to be induced only by the Asplanchna substance. 1. Existing hypotheses with regard to possible adaptive values of the cyclomorphic spine production are discussed and rejected with the exception of two, which are tested by laboratory experiments. 2. The hypothesis of Erman, that spined Brachionus calyciflorus swim more slowly and filter the nourishment more effectively as a consequence of the resistance of the spines to friction, is disproved experimentally (see Table 1 and 2). 3. The hypothesis of Beauchamp and of Gilbert, that spines form a protection against the predacious Asplanchna, is tested in detail. a) The components of the predator-prey relationship of Asplanchna and the influences of the spines on these components are studied in short term experiments (stopped after the first catch; see Fig. 2 and Table 3): The number of accidental collisions with Asplanchna is not influenced by spine length. However, by increasing spine length the rate of catches per collision and the rate of swallowed prey per catch decrease, and the time spent in swallowing the prey increases. The timespan from the start of the experiment to the moment when the first prey is just being swallowed shows a marked increase with growing spine length (see Table 3: g). b) In experiments of longer periods of exposition (up to one hour; see Table 5) the long-spined B. calyciflorus are again nearly completely unassailable. In these cases the advantage of short-spined animals (compared with spineless ones) is reduced. The reason for this is a more rapid saturation of the Asplanchna by unarmed prey. This causes a quicker relaxation of the predator's activities (decrease of the swimming speed, number of prey caught per collision, and number of prey swallowed per catch). In Asplanchna feeding on short-spined prey, saturation is slower but at the same time the predator's activities decrease more slowly, too (see Fig. 4). By extending the periods of exposition the survival chances of spined and spineless animals become more similar. c) In the presence of spineless animals (Brachionus rubens) as an alternative prey, the protection of the spines (in relation to spineless controls) becomes more important, because in this case the unarmed alternative prey makes a higher proportion of the predator's diet. This is true over a wide range of predation pressure (see Tables 6 and 7). d) Protective spinesara also present in juvenile B. calyciflorus. However, the juveniles are in all of their categories of spine length more sensitive to predator attacks than are corresponding adult females. The reason for this is the smaller overall body size of these animal (Tables 3, 4, 5, 6, Fig. 4). 4. The connexions between the inductive factors and the adaptive values of the cyclomorphic spine production are reviewed and discussed.