Inducible morphology, heterochrony, and size hierarchies in a colonial invertebrate monoculture

Conditional or inducible strategies are a powerful tool for analyzing the evolution of aggressive behavior. Structures such as stolons and nematocyst-laden tentacles, induced to deter encroachment by competitors, are proportionately better represented in clonal and colonial marine invertebrates than in aclonal animals. Stolons can be produced by colonies of Membranipora membranacea (Bryozoa) within 48 hr after contact with conspecifics. Absolute size and relative size of interacting colonies determine whether stolons will be produced. Although individual stolons are eventually overgrown by conspecifics, they reduce the size of affected zooids by 27%. Since stolon production is primarily a strategy of large colonies, we suggest that stolons function to limit space occupied by small colonies and may also trigger early and localized reproduction. Thus large colonies can surround multiple small mates and, because they reproduce only locally where induced by contact with small colonies, still maintain high growth rates on free colony perimeters. Stolons appear to be juvenilized zooids and to originate through a process of heterochrony. These induced facultative polymorphisms may be one pathway by which fixed polymorphisms arise in colonial invertebrates. We attribute the unique production of inducible structures against competitors by clonal and colonial invertebrates to both unusually high levels of developmental plasticity and an energetically favorable architecture for perimeter defense.