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Date of Award


Document Type


Degree Name

Master of Science (M.S.)


Graduate School

First Advisor

Steven Obrebski

First Committee Member

Edmund H. Smith

Second Committee Member

James A. Blake


A dominant organism on intertidal Pacific Coast sandflats is the lophophorate Phoronopsis viridis Hilton. This suspension feeder builds a stiff, sandy tube, 3 mm wide and up to 20 cm long and usually lying within 6 cm of the surface (Johnson 1967a). Phoronopsis viridis is found over large areas and tidal ranges of sandflats in aggregate densities of over 21,000 phoronids per m2 (Ronan 1978). On Lawson's Flat, the location of the present study, the areas of highest density of P. viridis have been known to persist for over 20 years (Steven Obrebski, personal communication).

The factors accounting for the persistence of these dense phoronid populations have not been studied although four hypotheses have been proposed (see Ronan 1975, 1978). These are: 1) dense clusters of adult phoronids increase the probability of successful gamete fertilization; 2) high densities of phoronids stablize the sediment, thus limiting the movement of large burrowing in fauna that are potentially destructive to phoronids (Ronan 1975); 3) a dense canopy of lophophores slows currents over the sediment surface, increasing the deposition of food items; and 4) clustering limits the effects of predation. The sudden retraction of a lophophore in response to a disturbance results in withdrawal of neighboring phoronids in the area of the disturbance (Ronan 1978).

In order to evaluate the adaptive significance of population persistence it is crucial to understand the mechanism by which dense populations :persist. Therefore the purpose of this study is not to assess the adaptive significance of clustering, but rather to determine what mechanisms might explain the persistence of dense phoronid populations.. Consequently, this study is more applicable to an "ecological" time scale than an *evolutionary" one. In this study the population dynamics and recruitment pattern of P. viridis is described with reference to the mechanism which enables persistence of dense phoronid areas. In addition, the effects of food abundance and the reworking activity of the surface deposit feeder, Axiothella rubrocincta Johnson, a maldanid polychaete, on P. viridis density and recruitment are measured.



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