Feeding in Floating Dock Habitats
Most species found on the floats have sessile lifestyles and feed by removing suspended particles from sea water, or suspension feeding.  In some animals like ascidians, a filter is used to strain particles from the water.  However, a few common sessile organisms such as Metridium senile, bryozoans, and hydroid colonies like Garveia and Plumularia use their tentacles to actively capture food.  Other non-sessile animals use different modes of feeding and can be either herbivorous or carnivorous.  Floats are favorable for filter feeders because plankton is continuously available and is the main source of food for many organisms that have sessile lifestyles.  Floats are also favorable environments for many kinds of seaweeds that may act as a substrate for animal settlement and growth (for example, the bryozoan Membranipora membranacea) as well as being the main food source for herbivores.

Among the filter feeders are members of the annelid family Serpulidae. These worms are equipped with special ciliated feeding tentacles that collect detritus and plankton from the water. These structures have a feathery appearance and  are usually red in color.  The collected food particles are transported into the mouth along ciliary tracts and then enter the digestive tract.
View of the oral disk of Metridium senile.The mouth  
is in the center and is surrounded by tentacles that 
contain nematocysts.
Lateral view of Metridium senile (15cm).
Metridium senile, a cnidarian, is one of the sessile organisms that relies on active food capture by tentacles. The mouth of Metridium senile is surrounded by these frilly tentacles, which contain nematocysts, or stinging cells.  Nematocysts are used for immobilizing prey, and the tentacles transport the captured prey to the mouth. Click here to view Metridium nematocysts in action: look for thin filaments as they curlicue away from the tentacle edge.
Among the raptorial feeders is a nudibranch species called Phidiana crassicornis. This nudibrach is colorful and easily spotted. Phidiana crassicornis feeds on cnidarians, and the fine cerata that cover the dorsal surface are packed with the intact and functional nematocysts of the sea anemones and other cnidarians it feeds on.  These nematocysts are used to protect the animal from predators or potential competitors. Recent studies have shown that the aggressive nature of these creatures increases when food is scarce and may even lead to cannibalism (Ricketts et al.,p.140). 


Another species of nudibranch, Doridella steinbergae, can be found on the surface of the alga Laminaria saccharina whenever the alga is covered with the encrusting bryozoan Membranipora membranacea. Doridella lives and feeds on Membranipora.  These nudibranchs are translucent gray in color and reach a maximum length of 10mm.

Common bryozoans found on the floats are Bugula pacifica and Bugula californica. These two species live in colonies and have zooids that are specialized for feeding. The ciliated tentacles of the lophophore collect food particles by generating water currents and filtering out food particles. Click here to view feeding zooids of Bugula pacifica.

Floats are favorable spots for herbivores such as chitons. One chiton species commonly found on the floats is Katharina tunicata, which feeds on diatoms and on the brown and red algae that encrust the floats. Chitons scrape off food from the substrate using the radula, a unique structure that bears  teeth in transverse rows.
Dorsal view of Katharina tunicata (5cm). The shell is divided into eight overlapping plates, the main distinguishing feature of chitons. Partial view of a radula removed from the mouth of a chiton.  The radula is a long structure bearing teeth that is used for scraping food from substrates.

To learn more about feeding patterns of organisms in different habitats, visit the following web pages:
Argyle Creek                          Cattle Point                   False Bay                       Plankton