Variability Of The Biomass-density Relationship And Self-thinning In Space-limited Rocky Intertidal Barnacle Systems

Growth of sessile marine intertidal invertebrates is limited by space availability. As a result, suppression of weaker individuals due to density-dependent mortality is accompanied by an increase in biomass of survivors during growth in the so-called process of self-thinning. Self-thinning in sessile invertebrates has been widely studied especially in the cultured mussels. In barnacles, self-thinning has drawn less attention despite their importance in marine intertidal community dynamics and their potential in aquaculture. Acorn barnacles were assumed to have a self-thinning exponent close to-3/2widely acknowledged for plant populations and underlain by isometric growth. However, in addition to evidence of allometric growth when space is limited, empirical works showed inconsistency of barnacle functional mechanisms such as the pattern of individual variation in growth with the dominance and suppression model proposed for thinning plant populations. Furthermore, the effect of competing neighbors on food availability proved to have less relative importance in barnacles and elevation of individuals in a growing dense population may even expose them to more food. Different recruit densities between tidal levels should result in different geometry of space occupation with adult individuals morphologically different and a different biomass-density relationship. There is a trade-off between shell and tissue growth in barnacles and self-thinning lines for these individual parts would differ as a result of different biomass allocation along the vertical intertidal gradient. The objective of this work was to test the variability of the self-thinning exponent in barnacles and its deviation from the’-3/2power law’.Using Tetraclita squamosa, a sub-tropical intertidal acorn barnacle as a target species, I investigated the biomass-density relationship along a vertical intertidal gradient of a marine rocky intertidal habitat in Zhujiajian island of Zhoushan archipelago in East Sea, China. Three intertidal levels were designated:0.4to1.1m,1.1to1.8m and1.8to2.6m above the mean lower low water. However, only the two lower intertidal heights have been considered for the analysis of individual parts biomass-density relationship since self-thinning occur in these tidal heights. Quadrats with different size were set according to the size and density of individuals on sampling plots. Photographs were used for uncrowded plots in order to estimate dry biomass from barnacle basal diameter whereas for plots with high density, individuals were scraped off the rock. The sampled individuals were dried, counted and weighed in aggregate and before drying, shell and tissue from samples for the analysis of biomass-density relationship of barnacle individual parts were separated. Then the density and the dry mean individual biomass were determined for every sampled quadrat. Individuals have been collected for the estimation of mean tissue:shell ratio which were used in the explanation of biomass-density relationship of individual parts in relation to biomass allocation. The whole individual self-thinning slope was steeper than the-3/2power and did not differ among lower tidal levels with an intercept significantly higher in the low intertidal than in the mid-intertidal, whereas there was no significant relationship in the high intertidal. The self-thinning slope for tissue in low intertidal was steeper than the shell one in mid-intertidal and all individual part slopes were higher than-3/2. Growth in height of crowded barnacles along with the weaker effect of competition for food may retard mortality and result in a self-thinning slope steeper than-3/2. In higher heights, individuals are subject to more desiccation stress and would allocate more biomass to shell than their counterparts in lower heights which allocate more biomass to tissue, thus self-thinning slope was higher for tissue in low intertidal. Since intercept is an index of characteristics of morphology and growth, the greater intercept in the low intertidal indicated that more biomass per unit area is packed lower in the intertidal habitat. Hence, columnar growth (allometric growth) is more pronounced at lower than at higher intertidal levels.The results of this work suggested that self-thinning exponent in barnacles is steeper than-3/2found for plant populations and allometric growth may not be the only factor causing this deviation from the "self-thinning law". Furthermore, this scaling exponent is not a constant. Self-thinning exponent for barnacle individual parts is not constant and these different exponents would be explained by different biomass allocation to shell and tissue in response to environmental conditions. As one progresses higher in the intertidal, self-thinning processes disappear owing to reduced competition.