Morphogenetic basis for phenotypic differences in hydroid competitive behaviour

Abstract

FOR many plants, fungi and sessile invertebrates, the availability of habitable space often limits their size, reproductive output and survival. Intraspecific and interspecific variation in the ability to compete for space is common^1–3, but the way in which competitive ability is regulated remains largely unknown. We describe here a system of intraspecific competition in the hydroid Hydractinia symbiolongicarpus in which the principal mechanisms underlying variation in competitive ability have proven amenable to experimental analysis. Competitive interactions can be aggressive, involving the induction of a specialized fighting organ called a hyperplastic stolon, or non-aggressive, in which no hyperplastic stolons are produced. Colonies display continuous variation in the ontogenetic appearance of tissues that differ in their competence to be induced to produce hyperplastic stolons. We find that the outcome and kind of competitive interaction between strains are predictable given knowledge of their morphologies. In this hydroid, complex competitive behaviour arises from a coupling of discrete morphogenetic potential of differing tissue types with continuous variation in the ontogenetic and astogenetic appearances of these tissues.