Reference: Biol Bull, 177: 277-286. (October. 1989) Variation in Growth Rate and Reproduction of the Bryozoan Bugula neritina MICHAEL J. KEOUGH Department of Zoology. University of Melbourne. Parkville. \ 'ictoria 3052 Australia Abstract. Colonies of the arborescent cheilostome bryozoan Bugula neritina vary dramatically in their growth rate even when in apparently identical microhab-itats. Comparison of growth rates of juveniles derived from four parent colonies at each of two sites showed only weak effects of parental colony on juvenile growth. These effects accounted for at most only 5.4% of total variation in growth. Variation in growth, and hence age at first reproduction, is interpreted as a plastic response of colonies to fine-scale environmental variation. Bryozoans from seagrass meadows mature at a smaller size than those colonies from nearby rocky reefs ( 1 200, vs. 3500 zooids at first reproduction, respectively). When juveniles from both of these habitats were grown in a common garden, there was, again, no variation among parental groups, but a highly significant effect of origin of juveniles. Juveniles matured at a size similar to that seen in their parental population, indicating that genetic or very early maternal effects influence timing of repro-duction. A post hoc test of the effect of onset of reproduction on colony growth showed no reduction in growth rate. Instead, colonies that reproduced grew faster than sim-ilar aged and sized colonies that did not reproduce. Introduction A growing body of empirical evidence suggests that, for modular organisms, many demographic variables de-pend more strongly on size than on age. Three of the most important such variables are mortality rates, timing of onset of reproduction, and reproductive output. Mor-tality rates are often size-dependent in two ways; first, the probability of a colony being eaten or overgrown may decrease with increasing size (e.g., Reiswig, 1974; Wer-Received 15 March 1989; accepted 31 July 1989. nerandCaswell, 1977; Gross, 1981; Antonovics and Pri-mack, 1982; Russ, 1982; Sebens, 1982; Hughes and Jackson, 1985; Hughes and Connell, 1987). Second, small colonies may be killed completely when attacked, while a similar attack on a larger colony may only cause damage ("partial mortality"), from which the colony subsequently recovers (Bak et al.. 1981; Palumbi and Jackson. 1982). Size, rather than age, may determine the onset of reproduction for many clonal animals and plants (Inouye and Taylor, 1980; van Duyl et a/.. 1981; Gross, 1981; Wahle, 1983; Augspurger, 1985; Keough, 1 986; but see Harvell and Grosberg, 1 988). For a number of clonal organisms, size is correlated positively with re-productive output (Hayward, 1973; Inouye and Taylor, 1980; van Duyl et al.. 1981; Nakauchi, 1982; Sebens, 1983; Wahle, 1983; Augspurger, 1985), which in turn is thought to be an important component of relative fit-ness. A negative correlation between size and mortality and a positive correlation between size and reproductive out-put both favor rapid initial growth of juveniles. However, many clonal organisms show extensive variation in growth rate. Harper ( 1977; 1985) reviews data for terres-trial plants, and Hughes and Jackson (1985), Jackson and Winston (1982), and Keough (1986) provide exam-ples of such variation in clonal marine animals. For most marine organisms, the causes of this variation have not been examined in enough detail to estimate relative con-tributions of phenotypic responses to fine-scale environ-mental variation and genetic or maternal factors. For many plants, however, considerable variation in mor-phology and growth can be attributed to phenotypic plasticity in response to small-scale environmental varia-tion (Silander, 1985). In animals, both genetic and envi-ronmental influences on growth rate are reported com-monly (e.g., Travis et al.. 1987). The cosmopolitan bryozoan Bugula neritina appears 277
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