Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Pelvetiopsis (Dwarf Rockweed)

Pelvetiopsis limitata (Gardner 1910)

Kingdom Chromista, phylum Ochrophyta, class Phaeophyceae, order Fucales, family Fucaceae

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Description

This perennial brown alga stands between 4-8cm tall and is light tan to olive in color arising from a small discoid holdfast. The densely branched thallus is cylindrical at the base, becoming flattened to cylindrical in the upper fronds. The dichotomously divided branches tend to arch inward and lack midribs (Abbott and Hollenberg 1976).

Habitat and Geographic Range

Found on tops of rocks, rarely on sides, in the upper intertidal zone; frequents more wave-exposed sites. Range extends from Vancouver Island, British Columbia, to Cambria (San Luis Obispo County), CA (Abbott and Hollenberg 1976).

Synonyms

Pelvetia fastigiata limitata

Similar species

Can be confused with the other common rockweeds: Fucus gardneri, Hesperophycus californicus, and Silvetia compressa. Fucus and Hesperophycus generally have wider fronds with midribs, whereas Silvetia and Pelvetiopsis have narrower fronds that lack a midrib. Silvetia has a longer basal stipe and is typically found at lower tidal levels than Pelvetiopsis (Abbott and Hollenberg 1976).

Natural History

Pelvetiopsis limitata is considered a good indicator organism of exposed rocky coasts. It forms extensive zones in the high intertidal region and is fed on by limpets and other invertebrate grazers. Pelvetiopsis is most closely related to Hesperophycus (Serrão et al. 1999), with both genera producing one large egg per oogonium. Two species of Pelvetiopsis occur in California, P. limitata and P. arborescens. The former species more closely resembles a dwarf Fucus, whereas the latter is similar in appearance to a small Silvetia due to its more cylindrical branches (Abott and Hollenberg 1976). In central California, Pelvetiopsis can co-occur with Silvetia compressa although Pelvetiopsis is generally found at higher tidal elevations. When identification is in doubt, specimens can be examined microscopically to determine the number of eggs per oogonium. Pelvetiopsis has only one egg per oogonium while Silvetia has two and Fucus eight. Little scientific attention has been given to Pelvetiopsis, leaving much of its reproductive periodicity, longevity, and ecology unknown.  Pelvetiopsis may be an indicator species of human traffic. A study on human trampling effects showed that P. limitata was markedly absent from the most heavily trampled sites and suggested that it may be highly susceptible to breakage especially when growing on the edges of rocks (Beauchamp and Gowing 1982).  Pelvetiopsis also becomes detached from the substrate during winter storms which are predicted to increase in intensity and frequency due to climate change. Recruitment and survival of Pelvetiopsis embryos are higher under the canopy of adults especially in higher tidal elevations (Skene 2009). Predicted effects of climate change and the resulting sea level rises on this high zone species include increased rates of adult mortality and limited ability to shift its distribution to higher elevations (Skene 2009).

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