Redescription of Zoothamnium niveum (Hemprich & Ehrenberg, 1831) Ehrenberg, 1838 (Oligohymenophora, Peritrichida), a ciliate with ectosymbiotic, chemoautotrophic bacteria

A very large and fast growing species of Zoothamnium Bory de St. Vincent, 1826 (Oligohymenophora, Peritrichida), Z. niveum (Hemprich & Ehrenberg, 1831) Ehrenberg, 1838 has been discovered in mangrove channels of the Caribbean Sea, the Gulf of Mexico, and the Red Sea. This colonial, sessile ciliate is remarkable for its obligatory association with ectosymbiotic, chemoautotrophic, most likely sulfur-oxidizing bacteria covering almost the entire body of the host. Two distinct morphotypes of bacteria - rods and cocci presumably representing only one species, are present. A redescription of Z. niveum is made based on live observations, silver carbonate stainings after Fernandez-Galiano [29], and scanning electron microscopy. Z. niveum is characterized by a feather-like colony, a stalk which is often divided, branches occurring alternately on the stalk, and an alternate arrangement of zooids on the branches. Three different types of zooids are present: microzooids and undividing terminal zooids with a slender, strongly asymmetric oral side; roundish to ellipsoid macrozooids; and ellipsoid dividing terminal zooids. All have a striped silverline system with pellicular pores and a macronucleus with a unique shape. The oral ciliature turns 1 1/4 in a clockwise direction when viewed from inside the cell. The ciliature of the microzooids consists of a paroral membrane, three adoral membranelles (polykineties), one stomatogenic kinety orally, and one circular irregular row of barren kinetosomes aborally (trochal band). The ciliature of the macrozooids is the same as in the microzooids, except for an additional long epistomial membrane orally and a telotrochal band of several circular rows of kinetosomes aborally. A comparison of morphological characters within the genus Zoothamnium is provided, with special attention given to different forms of epigrowth on various representatives of this genus.