Distribution Patterns and Seasonal Occurrence of Rhabdostyla sp. (Peritricha: Epistylididae) on Dero nivea (Oligochaeta: Naididae)

Abstract

Rhabdostyla sp. preferentially infested Dero (Dero) nivea in an alkaline bog stream from June 1982 to December 1983. Rhabdostyla occurred infrequently on two other species of naidid oligochaetes but were absent from nonliving substrates, algae, macrophytes and other invertebrates. Seasonal abundance peaks of Rhabdostyla occurred in October of 1982 and September-October of 1983 coincident with abundance peaks of D. nivea. Prevalence of host infestation ranged from 7.5-100% when infestations were present. Overall mean number of Rhabdostyla, 1 SE per D. nivea, was 4.8-i-0.2; means of individual sampling periods ranged from 2.0/worm to 13.1/ worm. Up to .36 Rhabdostyla were present on individual D. nivea. Frequency distributions were significantly (P<0.05) fitted to the negative binomial; k values ranged from 0.02 to 3.22. Rhabdostyla were preferentially distributed on the anterior 20% and the posterior 30 % of their hosts presumably because food and/or oxygen availability was greater on these regions. Niche breadth values ranged from 0.107 to 0.413. Prevalence was significantly correlated to host size (P <0.001; r2 = 0.61). INTRODUCTION Peritrich protozoa are common constituents of freshwater ecosystems and are often attached to external surfaces of invertebrate metazoa such as Porifera, Coelenterata, Turbellaria, Oligochaeta, Hirudinea, Bryozoa, Mollusca, Crustacea and Insecta (Corliss, 1979; Kudo, 1966; Lust, 1950; Matthes, 1950; Stephenson, 1930). Epizoic protozoa of zooplankton have received considerable attention recently (e.g., Evans et al., 1979, 1981; Henebry and Ridgeway, 1979), yet comparatively little quantitative data have been published on epizoic protozoa associated with benthic fauna. During a study of the population dynamics of naidid oligochaetes (Smith, 1984, 1986), I observed these worms infested with the peritrich Rhabdostyla sp. and undertook a study of the host-epibiont association. Specific objectives of the study were to determine seasonal abundance patterns of Rhabdostyla, host specificity, prevalence of host infestation and distribution patterns of Rhabdostyla on worm populations and on individual hosts. MATERIALS AND METHODS The study site was a sluggish, alkaline bog stream situated within Cedarburg Bog, Ozaukee Co., Wisconsin, at lat 43 ?22 'N and long 87?59 'E. Physical and chemical parameters of the stream were measured concurrently with biological samplings using APHA (1976) methods. Smith (1984, 1986) provides a detailed list of these parameters measured over the study period. Naidid oligochaetes were collected monthly from June 1982 to March 1983 and twice monthly from April 1983 to December 1983, using four multiple-plate samplers (Weber, 1973) suspended adjacent to the stream bottom. Colonization period for the samplers was 4 weeks. Naidids were scraped from the samplers, counted and sorted alive under a dissecting microscope at 80X in the laboratory within 4 hr of collection. Worms were preserved in 70% ethanol and mounted whole in CMC-9. Confirmation of naidid species, enumeration of Rhabdostyla and location of Rhabdostyla on individual worms were accomplished using light microscopy. Size of Dero nivea was determined by counting the number of segments present in individual worms.