THE OBLIGATE COMMENSAL CILIATES OF STRONGYLOCENTROTUS DRÖBACHIENSIS: OCCURRENCE AND DIVISION IN URCHINS OF DIVERSE AGES; SURVIVAL IN SEA WATER IN RELATION TO INFECTIVITY

¹ Department of Zoology, University of North Carolina, Chapel Hill, North Carolina, and the Mount Desert Island Biological Laboratory, Salisbury Cove, Maine

1. The first part of the study concerns certain relationships of the ciliates Entodiscus borealis, Madsenia indomita and Biggaria gracilis to their host, the sea urchin Strongylocentrotus dröbachiensis. It is based on an examination of 152 urchins taken at Mt. Desert Island, Maine, in the summer of 1960. The respective ages of the urchins are estimates based on size (diameter of test). The second part concerns the survival of the ciliates in sea water, since their survival is inseparably related to the infection of new hosts.

2. Nine urchins measuring 8-14 mm. in diameter (age 1.5 years) were either uninfected or very lightly infected, and none of the ciliates was dividing. Urchins evidently acquire their ciliates at this age (second summer).

3. All the urchins of the remaining age-groups were infected with all 3 ciliates. Of 12 urchins that measured 15-23 mm. in diameter, all contained dividing specimens of B. gracilis, 9 contained dividing individuals of E. borealis, and 7 contained M. indomita in division. The results indicate that the respective ciliate populations build up rapidly to maximal densities in the third summer of the urchin's life (age about 2.5 years).

4. The remaining urchins were assigned by size to 4 age-groups. The number of urchins in each group, their range in size, and their estimated ages follow: 32 urchins, 24-39 mm., 3.5 years; 35, 40-49 mm., 4.5; 37, 50-59 mm., 5.5; 27, 60-65 mm., 6.5. All the urchins harbored dividing specimens of B. gracilis; thus this ciliate remains in constant division once infection is well established. But in each group only a small percentage of the urchins (3 to 8%) contained dividing specimens of E. borealis and M. indomita. Thus, their division, though evidently cyclical, could not be correlated with the age of the urchins.

5. In a natural population of urchins, the division of E. borealis and M. indomita does not affect simultaneously any large percentage of the urchins. Except in 2.5-year-old urchins, it appears to occur randomly. Since the urchins of a population practice dissimilar food habits, it is possible that division is correlated with the nature of the food and the subsequent intestinal flora.

6. In pure sea water most specimens of E. borealis and M. indomita can survive about 48 hours, and their death is due to starvation. Individuals of B. gracilis can survive no longer than 6-12 hours, and death does not result from starvation but seemingly from the properties of the medium. It is suggested that the constant voiding of B. gracilis among the fecal pellets of the host compensates for its relatively brief period of survival in sea water.