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PHYCOLOGICAL SOCIETY OF AMERICA

Ecology

of what used to be called Pelvetia, now

Silvetia compressa (J. Agardh) Serrão, Cho, Boo et Brawley

Habitat

Silvetia compressa grows primarily in the Northern Hemisphere on the west coast from Coos Bay, Oregon to Lower California (Ensenada.) It is found either isolated or in aggregations forming thick beds on top of somewhat protected rocks in the mid-littoral belt. The ability of the fertilized egg to settle and germinate under the environmental conditions present determine its geographical distribution and location on shore; movement of water may affect the attachment process. (See Demography.) It can withstand freezing temperatures and summer highs; its level above the high tide line is proportional to the amounts of fucoidan present--the more it has, the higher it can survive.

Silvetia's habitat at high tide:

Silvetia's habitat at low tide:

Animal Interactions

S. compressa plays an important role in intertidal ecology due to its interactions with animals and other algae. F.C. Gunnill has performed many studies on these interactions. At Hopkins Marine Station at Cabrillo Point on the Monterey Peninsula, California, USA, qualitative observations of the S. compressa beds around Hopkins show that there seems to be more invertebrates living on the plants on protected beaches and fewer out at more exposed beaches.

Below are some images of these invertebrates and epiphytes. If the species mentioned in the captions are not readily apparent, click on the image to see a labeled picture.

This image illustrates epiphytes on a Silvetia thallus. The darker red spots are epiphytes while the green spots are cellular debris from Silvetia. On certain thalli Silvetia lie dark spots. By using a razor blade and scraping the surface, one may prepare a slide of the epiphytes. This one is probably a red alga, perhaps Erythrotrichia.

In this aggregation of Silvetia, another member of the order Fucales may be seen. At the lower side of the image lies Fucus which tends to grow near Silvetia but in smaller density.

Many gastropods make their homes under SIlvetia fronds at low tide. Here, the fronds trap moisture and provide protection against the sun and wind. Many gastropods are grazers upon the epiphytes growing on SIlvetia. In this image are limpets and the black turban snail, Tegula funebralis.

Chitons are another type of mollusc which inhabit Silvetia. Next to the chiton in this image are small structures made of sand. These are the tubes of a tube worm, Phragmatopoma californica. This polychaete worm makes its tube out of sand grains which it first inspects and then cements down to form a small tube. Though the group of tubes looks like a colony, each tube is the result of one individual worm settling near other worms and building its home; the group is not formed by any asexual reproduction as many metazoan colonies are.

Other gastropods are predatory like this tiny Nucella or Acanthina. These prey upon barnacles and limpets.

© 1996 R. H.  Lin