Virtually every square inch of submerged rubber on the FHL floating dock tires has been colonized by an incredible variety of organisms, the majority of which are sessile÷that is, fixed in place. The most common kinds of sessile animals living on the tires and taking up most of the primary surface area, are sponges, tunicates, bryozoans and barnacles. Other immobile tire inhabitants include hydrozoans, sea anemones and several different kinds of tube-dwelling polychaete worms. Although these organisms do not have the ability to relocate themselves on the tires once they've settled, they are nonetheless in constant battles with one another for the additional tire space needed for growth. In our study of this floating community, we found abundant evidence of organisms overgrowing one another in these desperate competitions for space.

Colonization of tire surfaces by sessile organisms allows these floating habitats to support an additional, less conspicuous, contingent of creatures that have the distinct advantage of mobility. Locomotor capacity enables food to be searched for rather than waited for; predators can be escaped from or hidden from rather than succumbed to; environments that become unfavorable for any number of reasons, physical or biological, can be left behind for the promise of a new home tire÷maybe one that's seen fewer than 60,000 miles.

On a floating dock community in particular, mobility allows animals to make the most of the density and variety of sessile tire inhabitants, which provide them with an abundance of food and a topographically complex little environment with lots of hiding spots. Mobile creatures that live on the tires also have the option, if they're fast enough, to abandon ship whenever their home is heaved up out of the water and onto the dock by eager FHL graduate students.

Of course, some kinds of locomotion are impractical or impossible in the floating dock habitat. Animals that require soft substrates in which to burrow (e.g. many of the worms found in False Bay) have no way to make a living, locomotory or otherwise, on the tires and are thus not present. Large mobile animals are, in general, rarely found on the tires, partly because they sometimes escape before the tire is brought up out of the water but also partly because of both the limited extent and vertical orientation of most of the tire surface area. Large crustaceans, for instance, would have to suspend themselves from those surfaces on a nearly continuous basis. One of the largest mobile organisms to come up during our surveys was a crab called Hapalogaster mertensii,whose carapace measured only about 5 cm across.
 
 

Very small crustaceans and other kinds of arthropods thrive in the tire habitat. Their agility and small body sizes are ideally suited to life on a suspended (i.e. floating) and topographically intricate surface. The isopod Idotea wosnesenskii,about 2 cm long, makes its way very quickly over the alga Ulva fenestrata but manages to stay firmly attached all the while. Many tiny amphipods (~1 mm) inhabit the minute bryozoan forests that blanket the tires. Their activity, barely visible to the naked eye, turns the bryozoan colonies into veritable hubs of motion when viewed under a microscope.

 

Many tire inhabitants move more slowly and less frequently than the arthropods. The chitons (Tonecella lineata) and limpets that are common on the outside tires seem to prefer rubber surfaces that are nealy free of encrusters, moving about on them over secreted mucous films. Although these species are mobile, both of them can attach themselves to their steel-belted substrate with such tenacity that only a prying tool can remove them. This chiton is about 5 cm long.

 

The nudibrachs also move relatively slowly through their habitats, as sea slugs are wont to do. Phidiana crassicornis, about 1 cm long, is often seen creeping lethargically over tire sponge and tunicate mats and it is much more easily collected than chitons or limpets. The nudibranch's ability to traverse roughened landscapes allows it to seek out food on all parts of the tire.

 

The various methods of locomotion used by animals inhabiting this floating dock community reflect both its diversity and its obvious limitations. The density and variety of sessile organisms found on the tires provides both food and shelter to the mobile animals that can navigate it at will, such as small nudibranchs, crabs and other crustaceans. Most large animals are excluded; others are restricted to areas of the tire that have yet to be recruited by the dominant sessile species.