THE OBLIGATE COMMENSAL CILIATES OF STRONGYLOCEN-TROTUS DROBACHIENSIS : OCCURRENCE AND DIVISION IN URCHINS OF DIVERSE AGES; SURVIVAL IN SEA WATER IN RELATION TO INFECTIVITY C. DALE BEERS Department of Zoology, University of North Carolina, Chapel Hill, North Carolina, and the Mount Desert Island Biological Laboratory, Salisbury Cove, Maine Seven species of ciliated protozoa have been reported from the alimentary tract of the sea urchin Strongylocentrotus drobachicnsis (O. F. Miiller) in the coastal waters of Mt. Desert Island, Maine (Powers, 1933a). Three of them are holo-trichs which have no known free-living congeners and are restricted to echinoid hosts. They are Entodiscus borealis (Hentschel, 1924) Madsen, 1931 ; Madsenia indomita (Madsen, 1931) Kahl, 1934; and Biggaria gracilis (Powers, 1933) Kahl, 1934. In the words of Kirby (1941, p. 921), such ciliates "may be supposed to have evolved in the shelter of these hosts" and they are thus regarded as obligate commensals. The relation of the remaining four to their host is not entirely clear, owing to inadequate study. Powers (1933a, p. 119) regards them "as chance or vagrant ciliates, which, after being engulfed with food, are able to survive" and multiply as entozoic commensals. Two of the four are holotrichs, namely, Plagio-pyla minuta Powers, 1933, and Cyclidium stercoris Powers, 1935 ; one is a hypo-trich which Beers (1954) identified as Euplotes balteatus (Dujardin, 1841) Kahl, 1932, and the final and least common is an undetermined species of the peritrich Trichodina. Reference may be made to Beers ( 1948) for further details concerning the taxonomy of the ciliates. In order to avoid the constant repetition of the unwieldy binomial Strongylocentrotus drobachiensis, the terms "urchin" and "urch-ins" are substituted in the following pages and refer without exception to this echinoid. To return to the three obligate commensals, which are the subject of the present study, Power (1933a) notes that adult urchins at Mt. Desert Island are almost invariably infected with them and indeed may harbor them in almost incredible abundance. In the summer of 1947, Beers (1948) extended Powers' investigations by making a quantitative study of the occurrence and morphogenetic condition of the ciliates in 182 urchins, the tests of which varied in diameter from 30 to 60 mm. All the urchins were infected with E. borealis and M. indomita, and 181 of them with B. gracilis. Counts of the ciliates in fresh samples of enteric fluid showed that the vast majority of the urchins harbored infections of each species that varied in intensity from "moderate" (M) to "heavy" (H), M meaning 50-500 individuals of the species per 0.1 ml. of fluid and H meaning 500-1000 or more per 0.1 ml. The remaining infections were designated as "light" (L), meaning fewer than 50 individuals of a species per sample. A regional distribution of the ciliates was also 69
THE OBLIGATE COMMENSAL CILIATES OF STRONGYLOCENTROTUS DROBACHIENSIS: OCCURRENCE AND DIVISION IN URCHINS OF DIVERSE AGES; SURVIVAL IN SEA WATER IN RELATION TO INFECTIVITY
BioStor
