Suppression of the rotifer Polyarthra remata by the omnivorous copepod Tropocyclops extensus: Predation or competition

Abstract

Indirect evidence from observations in the field suggested that a common and often abundant cyclopoid copepod, Tropocyclops extensus, despite its small size (0.5 mm) and largely algal diet, is an important predator of the rotifer Polyarthra remata. Laboratory experiments showed that copepodids and adults, but not nauplii, markedly suppressed the population growth of this rotifer. The suppression could be attributed entirely to predation, rather than to exploitative competition for shared food resources or to interference. Mortality rates predicted from a separate feeding-rate experiment were sufficient to account for the decline in population size observed in the Tropocyclops treatment of culture experiments. Also, monitoring the availability of cryptomonad food in treatments with and without Tropocyclops throughout these experiments showed that cryptomonad densities always remained high and were just as high or significantly higher in the copepod treatments. Furthermore, direct videographic observations of P. remata from cultures with and without Tropocyclops demonstrated no significant differences in swimming velocity, tendency to deviate from a straight-line path, or frequency and length of spontaneous escape responses. Per capita ingestion rates of T. extensus on P. remata varied from 2 to 8 day-1, were significantly higher for adult females than adult males or copepodids V, and increased significantly with rotifer density. A tendency of copepods to have higher ingestion rates on young than adult rotifers was not significant. Copepods cultured since birth with cryptomonads and P. remata ate significantly fewer rotifers than those cultured only with cryptomonads; this may be explained by a more sated condition of the former and provided no evidence for the idea that previous experience with this rotifer might increase predation efficiency. The results show that T. extensus in natural communities has the potential to deplete natural populations of susceptible rotifer prey. Accordingly, it may shift the species structure of rotifer assemblages in favour of resistant species, and provide selection pressure for avoidance responses.