this pattern. If a planar colony doubles in area and mass, both its energy intake 

 and metabolic costs could double as well. If Cj >_ c 2 there is no size asymptote 

 predicted on energetic arounds (Fig. 2B). A similar argument can, however, be used 

 to predict the size of units within a colony (polyps, zooids, etc.) which often show 

 guite determinate growth and a size asymptote (Sebens 1979). On energetic arounds, 

 individuals or colonies (where c^ < C2) might thus grow to an asymptotic size that 

 maximizes their reproduction under a given set of habitat conditions. This asymp- 

 tote will be higher in habitats that are 'better' for either prey availability or 

 physical conditions affecting metabolic rate (Fig. 2C). Mortality rates may be high 

 enough, however, in some habitats that the energetically predicted asymptote is 

 never reached. 



Mechanisms other than energetics will lead to a habitat-dependent maximum size 

 that is non-asymptotic (growth does not slow and cease). Size-dependent fission 

 (e.g. Sebens 1982) could have such an effect. Size-dependent mortality will also 

 produce a maximum size that can differ between habitats (Fig. 2D). Birkeland (1973) 

 found that seafans in Panama have smaller maximum sizes in habitats with higher wave 

 energy because storm waves tear off large colonies. Octocoral colonies ( Alcyonium 

 siderium) at wave exposed sites in New England are larger than at calmer sites, but 

 there is no obvious growth cessation in large colonies at the most exposed sites; 

 storm waves may also set the maximum size in this case. Size-selective predation 

 would produce a similar pattern. In fact, Paine (1976) showed that a bimodal 

 distribution of mussel sizes can arise because of constant seastar predation on most 

 size classes; an escape in size occurs for the few mussels that, by chance, grow 

 large enough that they cannot be consumed by seastars. 



Figure 3 A. Growth rate differences in two habitats, Hj and H 2 , with equal 



asymptotic sizes (SA). SI = initial size, S2 = size after one time interval. 



B. Similar growth rates in two habitats, Hj and H 2 , with unequal asymptotic 

 sizes (SA1, SA2). 



C. Ford-Walford Plot of A. D. Ford-Walford Plot of B. 



E,F. Comparison of mean size increments by size class in two habitats (see 

 text). Bars along abscissa indicate size classes 1-6. 



12 



