Graduate Thesis Or Dissertation

Dynamics of crab larvae (Anomura, Brachyura) off the central Oregon coast, 1969-1971

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  • Bimonthly plankton samples were collected from 1969 through 1971 along a transect off the central Oregon continental shelf (44° 39. l'N) to document the species of crab larvae present, their seasonality, and their onshore-offshore distribution in relation to seasonal changes in oceanographic conditions. A comprehensive key with plates is given for the 41 species of crab larvae identified from the samples. Although some larvae occur every month of the year, the larvae of most species were found from February through July within ten nautical miles of the coast. Sea surface temperatures reached their highest annual values in May-June, coincident with the period of peak larval abundance. Many species of larvae which appeared in late winter-early spring of 1970 appeared later and more abundantly in late spring-early summer during 1971, apparently because of the colder winter-spring the latter year. The highest densities of crab larvae (Pachycheles pubescens, Fabia subquadrata, Pinnixa littoralis, Cancer oregonensis, Pugettia spp., and Pagurus ochotensis) were observed at stations one and three miles from shore. Adults of these inhabit the intertidal and sublittoral zone. The major mechanism retaining larvae within three to five miles of the coast is believed to be the strong onshore component of the surface currents regardless of the season. The behavior of the larvae in determining their position in the water column and the conservative effect of tidal currents also play an important role in their nearshore retention. Evidence is given relating the transport of the more abundant larvae to the intensity of coastal upwelling, although offshore transport is generally limited to within ten miles of the coast, Those few nearshore species (Cancer oregonensis, Cancer magister, Pugettia spp., Lophopanopeus beilus Emerita analoga), whose larvae are found consistently offshore (beyond ten miles), usually occur in comparatively high densities during late-spring and summer when the prevailing sea surface flow is offshore, have a relatively long pelagic life, and may exhibit a strong photopositive response during the early mega.lopa stage. The larval dynamics of the economically important Dungeness crab, Cancer magister, were given special attention. An apparent mass mortality occurred in the early zoeal stages during the 1971 season. This was associated with sea surface salinity and temperature in analyses of covariance, but larval survival predicted through response surface techniques and gut-fullness analysis did not substantially explain their sparseness. Multiple regres sion analyses indicated that environmental conditions during the critical larval period explained 60-70% of the variability in crab landings for some ports four years later. February precipitation, indicative of coastal surface salinity, for southern Oregon and northern California coastal stations was the environmental variable most highly predictive of crab landings for ports of southern Washington and northern Oregon. The evidence suggests that a prime factor limiting the strength of the Dungeness crab's harvestable year-class four years later is low salinity during the early larval phase in February. Various pairs of species whose larvae are similar in morphology, seasonal occurrence, and distribution are compared and contrasted in their pelagic niches. The role of the pelagic larva as a dispersal mechanism, the contribution f its morphology and behavior to the success of the species, and reproductive strategies such as brood size, number, and timing are discussed.
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