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.

Die vorliegende Arbeit befasst sich mit Aspekten der frühen Entwicklung der Phoronida („Hufeisenwürmer“). An drei Arten wird der Furchungsprozess untersucht (Phoronis pallida, Phoronis muelleri, Phoronis vancouverensis). Dies erfolgt sowohl mithilfe der 4D-Mikroskopie als auch anhand von immunocytochemischen Markierungen der Mitosespindeln und konfokaler Laser-Scanning-Mikroskopie. Verschiedene morphologische Merkmale des Furchungsprozesses werden quantitativ erfasst und innerhalb sowie zwischen den Arten verglichen. Die Ergebnisse zeigen eine weitgehend übereinstimmende Furchung bei P. pallida und P. muelleri Embryonen: Ab dem dritten Zellzyklus teilen sich die Blastomeren meist schräg – und alternierend dextral und sinistral – zur animal-vegetativ Achse. Dieses Muster zeigt überraschende Übereinstimmungen mit dem Muster der Spiralfurchung. Dies kann als morphologische Unterstützung molekular-phylogenetischer Befunde einer Stellung der Phoronida innerhalb der Spiralia/Lophotrochozoa interpretiert werden. Die Furchung bei P. vancouverensis unterscheidet sich von der Furchung der anderen beiden Arten; sie weist jedoch auch Unterschiede zu einer Radiärfurchung auf. Generell zeigt die Furchung aller drei Arten einen gewissen Grad an Variabilität. Anhand von in-vivo Einzelzellmarkierungen untersucht die Studie darüber hinaus das Schicksal der Blastomeren früher P. pallida Embryonen bis zu späten Gastrulationsstadien. Diese Analysen zeigen, dass die ersten beiden Furchungsteilungen durch die spätere Achse Blastoporus-Apikalplatte, jedoch in keinem konstanten Orientierungsverhältnis zur Ebene der Bilateralsymmetrie der Gastrula verlaufen. Dies unterscheidet sich von der Situation, wie sie von spiralfurchenden Tieren bekannt ist. Die Unterschiede und die beobachtete Variabilität des Furchungsprozesses werden im Licht unterschiedlicher Mechanismen der Spezifizierung von Zellschicksalen und Körperachsen bei verschiedenen Taxa der Spiralia und den Phoronida diskutiert. / This study addresses aspects of the early development of Phoronida (“horseshoe worms”). The cleavage process is analyzed for three species (Phoronis pallida, Phoronis muelleri, Phoronis vancouverensis). These investigations are performed using 4D-microscopy as well as immunocytochemical stainings of the mitotic spindle apparatuses in combination with confocal laser-scanning microscopy. Different morphological features of the cleavage process are quantified and compared within as well as between the species. The results reveal a highly consistent cleavage of P. pallida and P. muelleri embryos: from the third cell cycle onward, the blastomeres divide mostly obliquely – and alternatingly dextral and sinistral – with respect to the animal-vegetal axis. This cleavage pattern shows surprising correspondences to the pattern of spiral cleavage. The finding can be interpreted as morphological support for recent molecule-based phylogenies, which indicate a position of Phoronida within the Spiralia/Lophotrochozoa clade. The cleavage of P. vancouverensis differs from the cleavage in the other two species; however, it also shows differences to a radial cleavage pattern. In all three species, the cleavage process also involves some degree of variability. Furthermore, the study traces the cell fates of early P. pallida embryos up to the state of late gastrulation, by the use of fluorescent in-vivo single cell markings. These analyses reveal that the first two cleavage divisions both pass through the later axis blastopore-apical plate of the gastrula, yet they do not pass in a constant relationship with respect to the later plane of bilateral symmetry. This differs from the situation known from spiral cleaving animals. The differences and the encountered variability of the cleavage process are discussed with respect to different mechanisms of the specification of cell fates and body axes in different taxa of the Spiralia and the Phoronida.