Who�s Cleaning the Bay?

The answer may surpriseyou!� But if you have attended our MarineLife Inventory days and participated in analyzing the mud samples, you probablyhave some clues.� The bottom of the Baysupports huge populations of numerous kinds of invertebrates, most of whichlive by filter feeding.� Themost abundant are various types of bivalve mollusks (clams, mussels andoysters) and polychaete worms.� They filter out and consumehuge amounts of phytoplankton, as well as bacteria and other particles, therebymaking an enormous contribution to maintaining water quality. In addition tothe clams in the mud, various rocks, pilings and other solid substrates in thebay support a rich ecosystem containing sponges, anemones, sea squirts,mussels, and many kinds of crustaceans.�Many of these creatures also live by filter feeding and so help tomaintain the bay�s water quality.

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There are two kinds offilter feeders, which I will call internal and external filter feeders.

Internal filter feedershave a basket-like filter inside a body cavity which opens to the outsidethrough two siphons.� They bring in waterthrough one opening (the �incurrent siphon�), pump it through the filter to removemicroscopic food particles, and discharge it through another opening (the�excurrent siphon�). Mechanisms move the food particles from the filter itselfto the animal�s mouth.

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Internal filter feeders:� Mussel with a wide incurrent siphon on theleft and a smaller, oval excurrent siphon in the center; Clam with two siphonson the right, and a muscular foot on the left; Sea Squirt with the incurrentsiphon a little above the excurrent siphon.

Mussels, which are commonin the bay and even more abundant on our rocky areas of coastline, are amongthe most important of the internal filter feeders. Their shells close up whenthey are left dry by the tide, but when submerged they spread apart the twohalves of the shell (the two �valves� in the bivalve) to reveal a wide incurrent siphon surrounded by pinktentacles that prevent the entry of items that are too large.� Inside the shells the gills do the job offiltering out food particles, and then the water is discharged through asmaller, oval, excurrent siphon.�The water is moved through the animal by a poorly understood �bivalvepump� with the pumping force generated by bands of lateral cilia that runalong the sides of the gill filaments. The food is wiped off the gillsby a pair of appendages called palps, and is then transferred to the mouth deepinside the shell. Similar arrangements can be seen in the oysters and scallops.Studies have shown that an individual mussel or oyster can filter about 5liters (~2 gallons) of water per hour.

In many other bivalves,especially the burrowing ones including all the clams, both siphons are simpletubes, and in some cases they are much longer than the animal is wide.� This allows the animal to live in safety deepin the mud while the siphons emerge above the surface (although those siphonsare often nibbled by hungryfish and other carnivores!). Bivalves feed on plankton, as wellas benthic algae and detritus, and in turn they provide food for echinoderms,fish, birds and other animals.

Other filter feeders use anexternal filter.� This strategy is usedby all the barnacles, both acorn and goose, as well as several kinds ofpolychaete worms.� Barnacles are actuallygreatly modified crustaceans, in effect standing on their heads and using theirlegs for filtering.� But instead of pumpingwater over the filter, these animals use a grasping motion, rhythmicallyextending their feet upwards into the water, and then quickly bringing themback inside the shell along with any captured food.

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External filter feeders: Goosebarnacle, Feather Duster, Sandcastle Worm

A similar external butretractable filter is used in the tube-dwelling polychaete worms, often called�feather dusters�.� Some of these live intubes made of mucus and sand; others make a harder, calcified tube. They areable to retract and close a door (operculum) when threatened by low tide orpredation.

A unique type if filterfeeding has evolved in a species called the Fat Innkeeper Worm.� This animal constructs and lives in aU-shaped burrow, and it secretes a net of slime that filters out food as theworm pumps water through the tube.� Whenthe net is fully loaded with food, the worm swallows the food along with thenet, and then makes a new net. The burrow of the Fat Innkeeper Worm makes anexcellent home for a variety of commensal animals, including a small fishcalled a goby, a pea crab, a clam and a scale worm, all of which feed on theInnkeeper�s leftovers.� The regularpresence of these guests is what gives the animal its name!

All of the filters providemechanisms for collecting microscopic food particles from the water, butadditional mechanisms are needed to carry the collected food into the animal�smouth.� This is usually accomplished byfields of waving microscopic tentacle-like structures called cilia.� In some cases a string of mucus is producedby the animal to keep the food in place while it is in transit.

Oneof our local filter feeders takes advantage of wave action to move water overits filters.� This is the Pacific SandCrab (Mole Crab) which is very common and familiar on our sandy beaches insummer and has two distinct filter feeding mechanisms. Its legs have hairymargins for filtering food and transferring it to the mouth. But when the crabburies itself in the sand it extends its two antennae on the surface where theyfilter out food particles brought in by wave action. After the antennae collectthe particles, they transfer them to another pair of appendages, theantennules, and then to the mouth.

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Pacific Sand Crab: on the sandy bottom; buried with both eyes andantennae exposed; and buried with the filtering antennae exposed.

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Some of our filter feeders are colonial,and the individual members of a colony often make, amazingly regularpatterns.� A colonial tube-buildingpolychaete builds huge smoothly rounded masses on rocks in the intertidal areasof our beaches, where it earns its name �sandcastle worm!� In the bryozoans (also called ectoprocts ormoss animals), the individuals (called zooids) are microscopic and in perfectlyregular arrays.� One of these colonialanimals is responsible for the gray patches you often see on seaweeds washed upon the beach, but other bryozoans form patches on mussels, sea squirts andother solid surfaces.� Each zooid has aring of tentacles that are withdrawn into a box-shaped skeleton when the colonyis taken from the water; when submerged the tentacles are extended to trap foodparticles and pass them into the central mouth.�Some sea squirts (tunicates) are also colonial, but they take thecolonial philosophy one step further: they have individual incurrent siphons,but a group of animals shares a single excurrent siphon.

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Colonial Filter Feeders: Sandcastle Worm,Bryozoan, Colonial Tunicate (excurrent siphon just left of center).

Like many other bays andestuaries, Upper Newport Bayis affected by a condition called eutrophication.� This refers to a process where the bayreceives excess chemical nutrients (nitrates and phosphates, usually fromfertilizer runoff) that fertilize the growth of excessphytoplankton.� The phytoplankton eventuallysinks to the bottom and provides fuel for bacterial decomposition, leading toanoxic conditions in bottom waters.�Since filter feeders consume phytoplankton, they play an enormouslyimportant role in limiting eutrophication and maintaining water quality.� But there is another element to consider - although bivalves consume largeamounts of phytoplankton, in the process they generate �pseudofeces� which actsas a fertilizer to promote the production of more phytoplankton as well asmacroalgae (seaweed)! Of course, a certain level of phytoplankton is necessaryto support the filter-feeding animals.�Therefore, the maintenance of good water quality by filter feedersrequires a steady-state level of both phytoplankton and filter feedingpopulations. The filter feeders are also a majorfood source for many kinds of fish and birds, so they are critically importantfor the bay�s functions as a nursery for fish and as a feeding station for hugenumbers of migratory birds.

The loss of oyster populations from the Chesapeake Bay in Maryland/Virginia, mainly as a result of overharvestingcombined with loss of oyster reef habitat by destructive harvesting methods,led to a dramatic decline of filter feeding activity and consequent unhealthyeutrophication.� Efforts are under way toimprove water quality in the bay by increasing the commercial production ofoysters and clams.� Startlingimprovements in the water quality of the River Mersey in England, one of the most polluted estuaries in Europe, have been attributed to filter feeding by densepopulations of mussels. Water clarity in was dramatically increased in Lake Champlain, Vermont, when it was,unfortunately, invaded by the exotic zebra mussel. Water clarity is attractive,of course, but we need to remember that when water is completely clear it maynot be providing a healthy and appropriate level of phytoplankton to supportthe filter feeders.� The long-termecological health of Newport Bay and every other estuary will depend criticallyon the survival of active populations of benthic filter feeders, especiallybivalves, as well as appropriate levels of phytoplankton.�

Learn more! Lookup the Intertidal Life of Orange County, California at http://muzic-ivan.info/Intertidal.htm