Coastal outcroppings pounded by waves and exposed at low tide. Photo: Ashley Smith
Organisms thrive at this rocky confluence of land and sea because they are supplied with nearly unlimited amounts of oxygen, sunlight, nutrients and food. The high diversity of life along the rocky shores may also be attributed, in part, to the unusual mix of substrate – such as the soft shale at Duxbury Reef and hard shale at Estero de San Antonio – and the alternating estuaries and lagoons that line the sanctuary's shores.
The rocky shore habitat within the Gulf of the Farallones includes areas such as Bodega Head, Duxbury Reef, the Point Reyes Headlands, the rocky shores of Tomales Bay and the intertidal shores of the Farallon Islands. Duxbury Reef receives additional protection by the state of California as a Marine Conservation Area and an Area of Special Biological Significance (ASBS). Bodega Head and the Point Reyes Headlands are also ASBSs.
The sanctuary's rocky shores are divided into a series of zones that are defined by the amount of time the rocks are exposed to air and water. Different algal and invertebrate species inhabit each of these zones.
The Splash Zone
Few species have adaptations to survive in the splash zone. Organisms that survive here are almost always exposed to the air and are rarely submerged by water. Marine invertebrates such as periwinkle snails, barnacles, limpets and a type of green algae, Ulva intestinalis, are among the few taxa that can survive here.
The High Zone
Organisms that inhabit this zone are exposed to air for about 72 percent of the time. The lined shore crab, Pachygrapsus crassipes, survives in this zone by positioning its flat body in rock crevices, out of direct sunlight and hidden from larger predators. Limpets, chitons and black turban snails, Tegula funebralis, form a watertight seal onto the rocks with their shells to protect themselves from desiccation. Some algal species such as rockweed, Fucus gardneri, have moisture-retention adaptations that enable them to survive for many hours exposed to sunlight and air.
The California aglaja (Navanax inermis) is a species of predatory sea slug, a marine opisthobranch gastropod mollusk in the family Aglajidae and the order Cephalaspidea, the headshield slugs and bubble snails.
The California mussel (Mytilus californianus) can withstand strong wave action due to its tough shell and strong attachment to the rocks with bissel threads.
The mid zone is marked by a high density of living organisms. Black turban snails and aggregating anemones, Anthopleura elegantissima, are common here. The California mussel, Mytilus californianus, also occupies this zone and can form large beds that provide important refuge and habitat for a variety of invertebrates and algae.
The Low Zone
In this zone, organisms may be exposed to air just a few times a month. Species that survive here are more resilient to waves and less resilient to air exposure. The giant green anemone, Anthopleura xanthogrammica, the purple sea urchin, Strongylocentrotus purpuratus, and the sunflower star, Pycnopodia helianthoides, are invertebrates that frequent the sanctuary's lower intertidal and subtidal regions. The sea palm, Postelsia palmaeformis, a beautiful and edible alga, is also common in the exposed low zones of the sanctuary’s rocky shores.
Oil spills pose a significant threat to the health and balance of life on the sanctuary’s rocky shores. Past spills, such as the 2007 Cosco Busan oil spill in San Francisco Bay, have deposited oil on the sanctuary's rocky shores, including Duxbury Reef. Oil can smother mussel beds and kill acorn barnacles, limpets and other species. Monitoring programs are vital in addressing the potential impacts, restoration and recovery rates from spills.
Non-native invertebrates have made their way to the Gulf of the Farallones. These introductions are a major concern, due to the sanctuary’s close proximity to the highly invaded San Francisco Bay.
To date, almost 150 species of introduced marine algae and animals have been identified in the sanctuary. Invasive invertebrates, such as the green crab, Carcinus maenas, make up more than 85 percent of all introductions in sanctuary waters. They threaten the abundance and/or diversity of native species, disrupt ecosystem balance and threaten local marine-based economies.
Other threats include:
- contaminated water from streams and culverts that drain onto the sanctuary's rocky shores,
- trampling and the collection of organisms for food, bait or “souvenirs” by visitors, and
- accidental boat groundings that can scrape animals off rocks, release fuels and other contaminants into the water, and produce marine debris.
MonitoringMonitoring programs are essential to understanding short- and long-term natural variability and for assessing the health of the sanctuary’s rocky shores. Several programs monitor rocky intertidal species abundance and distribution in sanctuary waters. The following list includes some of the projects underway in the sanctuary. Please click on the Projects tab at the top of this page for more information.
The Duxbury Reef Restoration Program
This sanctuary program analyzes visitor data, determines baseline species diversity patterns and abundance on the reef, and identifies high- and low-impact areas regarding visitor use.
Long-term Monitoring Program and Experiential Training for Students (LiMPETS)
LiMPETS is a program for middle schools, high schools and other volunteer groups; it was developed to monitor rocky intertidal and sandy beach areas of California’s national marine sanctuaries.
The sanctuaries and their partners monitor the abundance and distribution of major intertidal biota to improve awareness and stewardship of these ecosystems. Various data collection procedures include transects, random quadrat counts, total organism counts, sex determination and size measurements. Data are collected so that interested groups working with the sanctuaries’ education staff can follow long-term changes.
The Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO)
PISCO is a collaboration of scientists from four universities (Oregon State, Stanford, UC Santa Barbara and UC Santa Cruz) that explores how individual organisms, populations and ecological communities vary over space and time.
PISCO has initiated a massive effort to monitor the rocky shores in and around the sanctuary. It is involved in a large-scale, long-term study of the patterns of species diversity in these habitats and the physical and ecological processes responsible for structuring these communities.
Sanctuary Ecosystem Assessment Surveys – Rocky Intertidal Habitat
As part of the SEAS Monitoring Programs, GFNMS has conducted long-term monitoring of the rocky intertidal habitats of the Farallon Islands since 1992. Data collected include percent cover, density counts, and species inventories. Over 200 taxa have been documented at the monitoring sites; five are rare and seven are extended ranges. Surveys are conducted annually during late summer (August), fall (November) and winter (February) months.