Investigations on the ecophysiology of Geleia nigriceps Kahl (Ciliophora, Gymnostomata) inhabiting a sandy beach in Bermuda

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Abstract
The dominant ciliate species Geleia nigriceps inhabiting a sheltered beach in Bermuda was characterized by its horizontal and vertical distribution on the beach in correlation with sediment composition and grain size, temperature, pH, oxygen, redox potential, and water coverage, its tolerance of high and low temperature under normoxic and oligoxic conditions, and its tolerance of extreme pH-values. From tolerance experiments it can be concluded that G. nigriceps is able to tolerate relatively high temperatures (up to 37.3°C) for only short time (e.g., when low tide occurs around noon on hot days). This species, inhabiting both the oxidized and the reduced milieu below the redox potential discontinuity layer of the beach, is only to a small degree dependent upon oxygen availability and can be regarded as a more or less habitual dweller of anoxic or oligoxic sediments. The functions relating LT50 and exposure time were almost identical for normoxic and oligoxic conditions. Tolerance experiments revealed that pH values from 8.2 to 9.6 present no obstacle to G. nigriceps in its occupation of the upper millimeters of the sediment. In addition, it could be demonstrated that pO2 has no effect on the tolerance of alkaline conditions. The vertical pattern of distribution of G. nigriceps correlated with water coverage during one period of outgoing tide suggests vertical migration. The upper sediment layer is more densely colonized than the lower ones at stations where water coverage exceeds 45%. A highly significant correlation exists between the population density of the upper centimeter and water coverage.