SIZE STRUCTURE AND GROWTH RATES IN POPULATIONS OF 

 COLONIAL AND SOLITARY INVERTEBRATES 



Kenneth P. Sebens 



Biological Laboratories and Museum of Comparative Zoology 



Harvard University, Cambridge, MA, USA 02138 



ABSTRACT 



Benthic invertebrate populations on coral reefs and in other marine communities 

 often occupy several habitat types and display very different size structures in 

 those habitats. The explanation for such size differences may lie in recruitment, 

 mortality, or growth rate differences. These possible causal factors can be 

 separated by carefully monitoring populations of marked individuals or colonies. 



INTRODUCTION 



Even a brief examination of a coral reef indicates that such a community is 

 easily divided into obvious zones or habitat types. Certain coral species, other 

 sessile invertebrates, and many mobile ones, have distributions that span a number 

 of distinct habitats. In such cases, the sizes of individual organisms or colonies 

 can have radically different distributions as can the maximum sizes attained. The 

 same is true on temperate rocky shores (Fig. 1B,C,D). However, a single period of 

 samplinq, resulting in a series of size-frequency histograms, is insufficient to 

 explain the inter-habitat size differences and can serve only to illustrate the 

 pattern. Conclusions arrived at by linking particular distribution types to size or 

 aae-dependent mortality patterns (e.q., Grassle and Sanders 1973) can be hypotheses 

 at most, especially when qrowth is indeterminate or colonial. 



A size-frequency histogram skewed to the right (Fig. 1A) reflects a population 

 dominated by large or old individuals and may result from: 1.) constant high rates 

 of juvenile mortality, 2.) infrequent recruitment followed by good juvenile 

 survivorship and arowth that slows and approaches an asymptotic size or 3.) rapid 

 juvenile nrowth with low mortality, growth cessation at a size asymptote, and high 

 rates of mortality only for large individuals, just the opposite of the first expla- 

 nation. A skew to the left, as in a population dominated by small or young indivi- 

 duals, could result from either constant high mortality at all sizes or from size- 

 selective predation on large individuals (e.g., Grassle and Sanders 1973). Such a 

 distribution could also be found shortly after an infrequent high recruitment event, 

 and be followed by high juvenile mortality and an eventual switch to a distribution 

 skewed toward large individuals. When clonal or colonial organisms are considered, 

 another explanation arises; periods of colony or individual fragmentation (Highsmith 

 1980, Hughes and Jackson 1980, Sebens 1983) could also produce such a pattern. 



If individuals have very indeterminate growth, reaching different size maxima 

 under various microhabitat conditions, a left-skewed distribution might result if a 

 population spans microhabitats, within one of the distinct zones or habitats, that 

 are mostly of poor quality (producing small individuals) with a few patches of 

 better quality (thus larger individuals). This pattern would result even if 

 mortality and recruitment rates were equal in all microhabitats and if mortality was 

 size-independent. Distributions approaching a Gaussian curve could also result from 

 any of the above processes. Finally, size-frequency histograms sometimes exhibit 

 strona mul timodol ity. A bimodal distribution (Fig. 1A) is often taken as evidence 

 of distinct age classes. Yet, the same distribution would be produced by indeter- 

 minate growth and a habitat comprising two microhabitat types 'poor' and 'good'. It 

 should thus be clear that, the shapes of size-freouency histograms do little more 

 than illustrate existing patterns. Causal interpretations are impossible from these 

 alone, especially when size and age are at least partially uncoupled. 



