Coloniality and Inducible Polymorphism

Abstract

Although zooid polymorphism is a common phenomenon among colonial marine invertebrates, the proximate mechanisms governing transformations and coordination among the different units of a colony are largely unknown. Zooids in these often genetically homogeneous colonies may be coordinated by intrinsic, centralized, signaling mechanisms at the colony level (e.g., by giant axons in siphonophores) or by more decentralized mechanisms at the zooid level, such as environmental cues activating same-aged zooids. This second class of mechanism is responsible for coordinated defense in colonies of the marine bryozoan Membranipora membranacea. Membranipora membranacea colonies are initially populated by identical looking zooids, some of which can rapidly differentiate into an organized pattern of defensive zooids on the periphery of the colony in response to chemical cues from predators. To test the hypothesis that the age of the zooids rather than their position within the colony determines which zooids develop spines, I manipulated the normal center-to-edge age gradient. With these manipulated colonies and localized chemical cues, I established that spination is evoked independently by local action of inducers on each zooid of the appropriate age group, irrespective of its position within the colony. Thus, spines were produced in the centers of colonies as well as on the edge when the central zooids were the same age as the edge zooids. Similar age-dependent, "self-organizing" mechanisms regulate the distribution of specialized tasks within colonies of social hymenopterans. For example, honey bees form colonies of related units, and, in these, the principal basis for specialization of labor among the units is ontogeny; similarly aged individuals respond similarly to the same environmental cues. Inducible defensive polymorphisms such as those shown by bryozoans, hydrozoans, and anthozoans may be one pathway by which fixed polymorphisms in zooid form and function arise. If the transition from facultative to fixed is common, then processes such as genetic assimilation may be more important than has been thought. It seems possible that the relative ubiquity of both facultative and constitutive polymorphisms in colonial invertebrates may be a result of the compartmentalized, iterated nature of modular organisms, which allows the production of variant zooids through slight changes in developmental timing.