Reference: Biol. Bull. 166: 54-67. (February, 1984) BIOLOGY OF HYDRACTINIID HYDROIDS. 1. COLONY ONTOGENY IN HYDRACTINIA ECHINATA (FLEMMING) CATHERINE S. MCFADDEN,' MARGARET J. MCFARLAND, 2 AND LEO W. BUSS DepartmentofBiology,YaleUniversity,NewHaven,Connecticut06511 ABSTRACT The colonial marine hydroid, Hydractinia echinata, exhibits a wide range of growth morphologies during ontogeny, from sheet-like colonies of uniform ectodermal mat to colonies which produce complex networks of stolons. Colony ontogeny was quantified under uniform environmental conditions for 70 colonies of H. echinata, with data collected on the growth rates of the three major colony parameters: mat tissue, stolon tissue, and polyps. Analysis of the relative growth rates of the tissues clearly illustrates that variability in colony form between colonies can be attributed to continuous variation in just one of the parameters governing colony ontogeny. INTRODUCTION Despite their phylogenetic and ecological diversity, the many colonial phyla share a limited number of general colony forms. Much recent work has focussed on the geometrical properties and mechanical constraints of particular morphologies (Riedl, 1971; Kaufman, 1973; Wainwright, et ai, 1976; Alexander, 1977; Jackson, 1979; Buss, 1979; Cheetham et al, 1980, 1981 and many others). Mechanical constraints clearly limit the range of physical regimes suitable for occupation by a given mor- phological type. For instance, highly branched upright forms may not be able to withstand areas of strong water movement, where sheet-like forms of low vertical relief may thrive; calmer waters where arborescent forms are more abundant may not be suitable for sheet-like organisms due to higher rates of sedimentation. Likewise, one can infer varying degrees of susceptibility to biological threats from geometric considerations of different morphologies. For instance, a sheet-like organism, with a large surface area in contact with the substratum, may be more susceptible to a surface-bound predator or competitor than the arborescent form. Conversely, the tree-like organism may be more susceptible to a generalized water-column-dwelling predator than the sheet-like form. Several authors have attempted, with considerable success, to predict the distribution and abundance of various colonial organisms as a function of morphological type (Wainwright and Dillon, 1969; Kaufmann, 1973; Chamberlain and Graus, 1975; Wainwright and Koehl, 1976; Brakel, 1976; Winston, 1976; Graus, et al., 1977; Chamberlain, 1978; Jokiel, 1978; Buss, 1979; Foster, 1979; Jackson, 1979, and many others). Parallelling this ecological interest in patterns of gross colony morphology, there has been an increase in interest in the phyletic distribution and fossil record of different morphologies (Boardman and Cheetham, 1973; Thomsen, 1977; Chamberlain, 1978; Van Valen, 1978; Jackson, 1979; Larwood and Taylor, 1979; Cheetham, et ai, 1980, Received18August1983;accepted23November1983. 'Present address: Department of Zoology NJ-15, University of Washington, Seattle, Washington 98195. 2Presentaddress:DepartmentofBiology,PrincetonUniversity,Princeton,NewJersey08544. 54