The asexual life history of the colonial rotifer, Sinantherina socialis (Linnaeus)

The rotifer Sinantherina socialis (Linnaeus) normally lives in colonies of 30 to 200 individuals, all of which are the same age and, barring colony fusion, are probably genetically identical. Colonies do not grow by the addition of new members, but rather reproduce as a unit, with juvenile colonies forming within a parent colony, then leaving the parent to live independently. During an initial planktonic phase, juvenile colonies that encounter one another often fuse into larger colonies. The distinctive characteristics of this species pose a number of questions concerning the costs and benefits of coloniality and the mechanism and consequences of colony fusion. To address these questions a number of laboratory techniques were developed including methods for culturing this species on artificial substrata and manipulating colony size.; The costs and benefits of a colonial lifestyle were examined for S. socialis because members of this species are found in colonies despite the fact that they are capable of settling and growing as solitary individuals. The feeding (i.e. clearance) rates, intrinsic rates of increase, sizes, and field survivorships of solitary and colonial S. socialis were compared in controlled experiments using laboratory reared animals. These experiments revealed that colonies have a higher field survivorship than solitary individuals and that a contributing factor is that colonies are better than solitaries at withstanding the attacks of predatory snails and amphipods. The higher survivorship of colonies translated into a higher intrinsic rate of increase in the field. In contrast, laboratory experiments showed that, in the absence of predators and other field conditions, the intrinsic rate of increase of colonies is lower than that of solitaries. The reduced intrinsic rate of increase of colonies is most likely due to the fact that they have a greater surface area than solitaries and therefore have increased growth, and presumably maintenance, costs. These costs are high enough to offset the seeming advantage of the increased feeding rate of colonies relative to solitary individuals.; The spontaneous fusion of juvenile S. socialis colonies has often been observed; fusion experiments were therefore designed to determine whether this species has an allorecognition system to protect against the fusion of unrelated colonies. First, fusion experiments using colonies from a single laboratory line were performed under controlled conditions to determine when in ontogeny fusion is possible. Next, controlled experiments were performed to determine whether laboratory colonies derived from the same geographic location are more likely to fuse with each other than with colonies derived from distant locations. These experiments revealed that the main determinant of colony fusion is ontogenetic stage. Only unsettled juvenile colonies fuse and they fuse indiscriminately with other juvenile colonies regardless of the location from which they were collected. There is thus no evidence for the existence of an allorecognition system in S. socialis.