ASEXUAL REPRODUCTION IN THE COLONIAL TUNICATE, BOTRYLLUS SCHLOSSERI (PALLAS) SAVIGNY, WITH SPECIAL REFERENCE TO THE DEVELOPMENTAL HISTORY OF INTERSIPHONAL BANDS OF PIGMENT CELLS

Abstract

1. The most striking feature of many Botryllus colonies is the localization of special light-reflecting pigment cells between the oral and atrial siphons of each zooid. Collectively these pigment cells constitute an intersiphonal band. 2. Individual zooids of a Botryllus colony are distributed radially around a common cloaca as a system of zooids. Consequently all the intersiphonal bands within one system constitute a star-shaped pattern called an intersiphonal pattern. 3. Any Botryllus colony is characterized by a specific intersiphonal pattern for only a very brief period of its existence. Any given intersiphonal pattern is then totally destroyed within a few hours and is replaced by an entirely different pattern or group of patterns. The formation and subsequent destruction of intersiphonal bands is described in detail for the oozooid and first seven generations of blastozooids in living colonies established from isolated larvae. 4. The progressive remodeling of the intersiphonal patterns is intimately associated with the changes involved in asexual reproduction. An interval of four to five days occurs between the maximum development of one intersiphonal pattern and the maximum development of the next. During this brief period a typical series of changes occurs in the appearance of a colony. a. The zooids become highly sensitive and contractile and the intersiphonal pattern becomes very irregular. b. Zooids then undergo maximum and permanent contraction, the intersiphonal bands disintegrate and the pigment cells are released into the circulatory system and are carried passively by the blood stream. c. Simultaneously the new generation of zooids is growing and the new intersiphonal bands form in them. At first these are solid bands, but they tend to split lengthwise until they become double intersiphonal bands, especially in older generations of zooids. d. Reflecting pigment cells then disappear from the circulatory system. 5. There is no evidence that the pigment cells released by the break-down of the intersiphonal bands of one generation of zooids can be utilized in the formation of new bands in the next generation of zooids. 6. If the oozooid develops a strong intersiphonal band of pigment cells (i.e., a broad, almost solid band of reflecting cells between the two siphons) only strong intersiphonal bands appear on all later generations of zooids; if it develops a weak intersiphonal band (i.e., only a few reflecting cells near the dorsal midline with no obvious arrangement into a band) only weak bands appear on all later generations of zooids. This difference between weak and strong expression is constant and is probably genetically determined. 7. Intersiphonal patterns vary in different colonies even though they are in the same stage of development. These variations are discussed in terms of a. differences in the number of buds per zooid in different colonies, b. the premature atrophy of developing buds in some colonies, c. the possible functional persistence of parent zooids after their buds become functional, d. the fusion of adjacent colonies. 8. The form of an intersiphonal pattern is determined by several factors: a. Whenever possible zooids tend to come into contact with another zooid on each side and throughout as much of their length as possible; b. the number of zooids available for system formation is a determining factor; c. the size of the individual zooids likewise affects the form of the pattern. If these three factors are kept in mind it is possible to predict the form of any intersiphonal pattern providing the number of zooids and the generation of zooids is known. When more than one system is present in the same colony the form of the intersiphonal pattern is always modified due to crowding of zooids at the contact points between adjacent systems. 9. When a colony contains sufficient zooids to form more than one system the new system does not contain only the few excess zooids which can no longer be accommodated by the old, but rather the old system is replaced by two systems, each with approximately equal numbers of zooids. As far as can be determined there are no inherent differences between colonies in the number of zooids which constitute a system. 10. The appearance of the intersiphonal bands or patterns should never be used as a characteristic for classifying Botryllus into species unless one is fully aware of the range of variation that occurs even in the same colony and of the factors responsible for such variations.