Interactions between the Anomopod Cladocerans Ceriodaphnia Dubia, C. Cornuta, Simocephalus Vetulus and S. Serrulatus, the Aphanoneurid Worm Aeolosoma sp., and the Fish Skiffia Lermae: Predation or Competition, or Both?

Abstract

We tested the reciprocal effects of water conditioned by exudates of Aeolosoma sp. (Aphanoneura, Oligochaeta) and fish (Skiffia lermae: Goodeidae) and by the presence of Aeolosoma sp. on the population dynamics of four species of anomopod cladocerans (C. cornuta, C. dubia, S. serrulatus and S. vetulus) at 25 °C for 26 days using Chlorella vulgaris(1 × 10⁶ cells ml⁻¹) as a basic food for all. We found that, regardless of treatment, C. cornuta and C. dubia had a long initial phase of slow growth, followed by a rapid increase after the second week and that, regardless of treatment, C. dubia had a higher rate of population increase per day than C. cornuta. S. vetulus had a longer lag phase than S. serrulatus. Fish-conditioned water resulted in growth inhibition in S. serrulatus, but had no effect on S. vetulus, while Aeolosoma-conditioned water inhibited growth of S. vetulus but not that of S. serrulatus. Our results suggest a scale of responses of cladocerans to exposure to fish- and worm-conditioned water and to the presence of worms, ranging from a mild stimulation to no effects to inhibited growth. Kairomone-effects on body size of C. cornuta were not significant, but beak length was strongly influenced, and more by fish than by worm kairomones. In S. vetulus, differences in size were not significant. However, individuals grown in the presence of worms had higher biomass while those grown in the presence of fish and worm kairomones showed a decreased weight. Effects of worm exudates and of their live biomass on cladocerans were thus opposite in Simocephalus. In Ceriodaphnia dubia, in contrast, they were additive. Aeolosoma was depressed by all four cladocerans, although worms tried to penetrate the valves of the cladocerans to feed on the tissue inside. In doing so, they suffered significant casualties, especially from blows of the powerful post-abdomen of Simocephalus. The nature of the cladoceran-worm interaction is therefore far from simple: in addition to the 'chemical communication' that is present, it has a predatory component (worms trying to feed on cladocerans) but the reverse might also be true (cladocerans filtering out tissues of disrupted Aeolosoma). Since both worms and cladocerans feed on algae, exploitative competition also seems to be involved. In this, the worms appear to be the inferior partner, although none of the experiments lasted long enough to drive any of the competing partners to extinction.