Survey of microhabitats and fauna at Garrison Bay

To the casual observer, the muddy intertidal of Garrison Bay may appear relatively depauperate of macrobenthic invertebrates in terms of abundance and species richness. C loser examination, however, reveals a taxonomically-rich array of intertidal organisms adapted to life in, on, and around the mud.

A variety of microhabitats, ranging from cobble to soft sediment, structure the distribution of different organisms at Garrison Bay.  We focused our quantitative survey on animals found in soft sediment along a continuous transect from the upper to lower intertidal .  Other microhabitats, which will be discussed more briefly here, also play critical roles in the presence and distribution of organisms at the site. The complete faunal list generated for the site is biased toward organisms that occur in association with the soft sediment, though we attempted to generate as complete a list as possible by surveying other microenvironments as well.

Garrison Bay looking west from the Howe property
toward the island


The environment at Garrison Bay consists generally of a shallowly dipping shore face of intermixed fine-grained sediment and molluscan shell material. Within this apparently homogeneous environment a number of microhabitats can be recognized.

Associated with the soft sediment are infaunal polychaete worms (e.g., F. Hesionidae) and bivalves (e.g., Macoma nasuta), as well as epifaunal or shallowly infaunal nemerteans (Paranemertes sp.), gastropods (e.g., Margarites sp.), and decapods (e.g., Cancer magister).   Some species are deeply infaunal, with burrows that extend ~ 12 cm down to a hard clay layer below the sediment/water interface (e.g., Tresus capax ). Distributions of these organisms are heterogeneous across this microhabitat, owing to differences in tidal height (high temperature and desiccation risk) and the concentration of coarse material  (which may influence oxygenation as well as temperature and moisture).

Most coarse material consists of molluscan skeletal material, primarily valves from just a few species of bivalves (e.g., Macoma nasuta, Protothaca staminea and Clinocardium nuttallii) located on average within the upper 10 cm of sediment.  We found that the concentration of coarse material positively covaries with the density of burrows along a gradient of tidal height.

Nereid polychaete (F. Nereidae)

Succulent Garrison Bay sediment

Siphon of the gaper clam (Tresus capax )

Rocky substrate environments found at Garrison Bay include boulders, cobbles, and small exposures of bedrock patchily distributed across the intertidal zone. These hard substrates provide habitat for chitons ( Mopalia muscosa), limpets (Lottia pelta), whelks (Nucella sp.), sponges (e.g., Haliclona permollis ), and barnacles (Balanus glandula), among other organisms. Bedrock exposures found east of our primary transect provide habitat for the organisms mentioned above and are littered with large accumulations of broken mollusc shells derived from the feeding habits of sea gulls (which fly above the rocks and drop molluscs in order to fracture their shells and feed upon them).

Rocky substrate with large Balanus

Lottia sp. on small cobbles in the upper intertidal

Haliclona permollis

Gull on bedrock exposure, which is littered with light-colored broken shell material

Wooden planks and pilings associated with docks in Garrison Bay provide substrates for epifaunal organisms such as tube-dwelling polychaetes (F. Sabellidae), and mussels (Mytilus sp.). Tree branches and trunks along the bay margins provide additional (if somewhat ephemeral) woody substrates for these organisms as well as barnacles (Balanus sp.). Egg masses of the bubble snail Haminoea vesicula are also found in association with partially emergent branches in the lower intertidal.

Egg masses of Haminoea vesicula

Dead shells of cockles (Clinocardium nuttallii) and other bivalve molluscs are conspicuous in the surficial sediment throughout the intertidal at Garrison Bay. These dead shells provide small islands of hard substrate in a surrounding sea of mud.  In addition, dead shells oriented concave up in the sediment may act as small catchments, retaining water during low tides and thus potentially reducing the exposure of some organisms to desiccation.

Clinocardium nuttallii
shell providing a substrate for phoronid tubes, errant polychaetes, and bubble snail egg masses

Although not typical residents of soft sediment environments, anemones (Metridium sp.) are found attached by their pedal disks to shell material low in the intertidal at Garrison Bay. Phoronids ( Phoronis sp.) in mud tubes can also be found along the margins of partially buried Clinocardium nuttallii valves, as seen in the photograph to the right.  Dead shells may also play important roles in protection and reproduction .

Metridium on the half shell

[Garrison Bay Main Page] - [Home]