The relative importance of mode of intraspecific competition, growth rate, and recruitment time on size structure of populations of the bryozoan Membranipora membranacea was examined experimentally. ...Size distributions of populations of solitary colonies (1/225 cm2), low density colonies $(<15/225\ {\rm cm}^{2})$, and high density colonies $(>25/225\ {\rm cm}^{2})$ were compared to determine if mechanisms that generate size hierarchies in plant populations also operate on populations of sessile invertebrates. Degree of inequality increased from solitary to low density populations, indicating that colony growth rate alone does not determine inequality. However, no difference in inequality was observed between low and high density populations, and degree of inequality neither increased nor decreased through time, indicating that the dominant mode of competition in these populations was resource depletion. Late-recruiting colonies were smaller than early recruits, but did not suffer disproportionately from competition relative to their size or recruitment time. This latter result lends further support to the hypothesis that resource depletion is the major determinant of size distributions in these populations. We conclude that processes similar to those that produce hierarchical size distributions in plant populations also structure populations of colonial invertebrates.
The rare marine snail Cyphoma signatum is a trophic specialist on gorgonians Plexaurella spp. A comparative study of this snail species with its generalist congener C. gibbosum showed that feeding ...behavior differs between the 2 species. Feeding by the specialist often damages the polyps and scleratinized matrix of Plexaurella or bares its proteinaceous axis, while the generalist tends to feed primarily on the polyps of Plexaurella. As a result, C. signatum ingests more total Plexaurella tissue (including sclerites) and more organic material, though fewer polyps per time, than does C. gibbosum. Differences in radular morphology are implicated as the proximal cause of species-specific feeding behavior. The lateral teeth of C. signatum are wider and their median teeth have a long central point not present in C. gibbosum, which may enable more extensive wounding of colonies. The feeding behavior of the specialist results in a higher amount of damage to Plexaurella, from which the coral recovers more slowly, as gorgonian regeneration times are inversely proportional to wound depths. Permanent damage to Plexaurella may result when bared proteinaceous axes are fouled by algae or other encrusting organisms. Although the shallow wounds of C. gibbosum are common on Plexaurella, they regenerate within 6 d. Gorgonian censuses at 4 sites showed that high proportions of permanently damaged Plexaurella (≥ 50% permanent damage per colony) were correlated with areas of high C. signatum density. In the short term, predation by C. signatum appears to increase permanent damage and mortality of Plexaurella. The specialist snail may therefore impose greater selection on the gorgonian than does the generalist.
A design conflict exists in passive suspension feeding colonies between maximizing surface area for feeding and minimizing drag-related forces on the colony. The importance of colony flexibility as a ...homeostatic mechanism was demonstrated experimentally on the scale of both the entire colony and the polyp. On the colony scale, flexibility reduces the relationship between drag and water velocity from a square to a first power dependence. This finding is consistent with the discovery that flexion in trees also reduces drag to a linear function of velocity. On the polyp scale, colony flexibility strongly damps flow velocity changes at the polyp over at least an order of magnitude change in ambient velocity. This previously unappreciated consequence of flexibility may be an important selective force affecting the evolution of colony form. The separate consequences of flexion at the polyp and whole colony level are considered in a simple conceptual model incorporating polyp feeding success and colony detachment probability over a range of flow velocities. Inspection of the model reveals that the lower velocity limit at which a colony can survive is likely to be constrained by polyp feeding success, while the upper velocity limit may be constrained by either polyp feeding success or the probability of colony detachment.
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