Ok, so after a couple of weeks off for the holidays, MLS is back to continue this series on filter feeding animals. If you need to get caught up, part I, which identified the organisms filter feeders actually feed on, is here.
In this edition we’ll be discussing siphons, which is the main method used by both bivalve mollusks and tunicates to extract these tiny creatures from the water column.
Most filter feeders are sessile or semi-sessile, meaning that they either cannot move or, as far as food gathering goes, are severely limited in movement. Sessile animals in the oceans must have some mechanism in place to take advantage of the vast amount of floating food, plankton, suspended in the water.
Perhaps the most common method of feeding on plankton is using structures known as siphons. These are basically hollow straws, and they usually occur in pairs. One siphon, known as an incurrent siphon, sucks water into the animal’s body. The other, known as an excurrent siphon, spits the filtered water (and wastes) back out.
Perhaps the group of animals that has taken the greatest advantage of feeding using siphons are bivalve mollusks. Nearly every species in the class Bivalvia are filter feeders. Bivalves can be either epifaunal, meaning they live on the ocean bottom like scallops and mussels, or they can be infaunal, meaning they actually live burrowed under the surface as clams do (in fact, the generic term "clam" refers to any bivalve that is infaunal).
Although all of these mollusks have tubular siphons, the length of the siphons determines how far underground the animal digs. Epifaunal mussels and scallops have siphons that barely extend past their shells. Shallow digging clams, such as quahogs, have short siphons that may only be a half inch long while deeper dwellers, like steamer clams, although only two or three inches in shell length, may have siphons (called a "neck" when you eat them) that can extend up to a foot long.
The depth a clam can dig is determined
by the length of its siphons.
In most cases the siphons are long enough to extend up into the water column during feeding, but short enough that the clam can retract them completely back into the shell for protection. Some of the deepest digging mollusks, such as the pacific goeduck, which can dig up to four feet into the substrate, have siphons so massive they are unable to fit them back into the shells. These deep-diggers never leave the infaunal community and are vulnerable to predators if dug up and placed on the surface of the mud. As an aside, this is one thing that bothers me about size regulations regarding my local soft-shell clam ("steamers"). Those that are below the legal size limit must be released, however simply throwing them back into the water basically condemns them to be eaten by crabs or starfish before they can dig themselves back into the bottom.
The giant goeduck may live for over 150 years.
(No snickering.)
When a filter feeding mollusks needs to eat, the shell is gaped enough to allow the siphons to protrude from between the shells. The siphons are then extended as far as needed to reach open water. Cilia on the gills create a current within the clam’s body which pulls water (and plankton) into the incurrent siphon and then back out of the excurrent siphon. During this process the water is carried through the gills, which not only exchange gasses during respiration, but also filter out the plankton by trapping it in strands of mucous. The plankton-mucous mixture is then transferred to the stomach by small paddle-like organs called "palps". In this way a relatively small clam is capable of filtering nearly ten gallons of water per hour.
"Steamer" clams with siphons extended.
Another group of animals that specializes in filter feeding via siphons are the tunicates, or "sea squirts". Although they would appear to be very simple invertebrates, these animals are actually closer to vertebrates. You would never guess this looking at an adult, which is usually a soft, jelly-like animal which grows attached to rocks and pilings. However the free-floating larval stage, known as a tadpole larvae, posseses all three distinguishing chordate characteristics: A dorsal nerve chord, a notochord and a post-anal tail (yes, you too had a post-anal tail during your early development). These structures are lost as the larvae develops into an adult.
Like with clams, although sea squirts aren’t remotely related to them, tunicates have an incurrent siphon, excurrent siphon and gills that trap the plankton. In the tunicate’s case this specialized gill structure is known as a pharyngeal basket, and again it’s a sheet of mucous that actually does the food collecting. Here is a good cutaway graphic showing the siphons and the large pharyngeal basket of a tunicate.
In the image below you can see the intestines through the body wall of a sea squirt (this species is called a "sea vase") as it travels up towards the excurrent siphon. In addition to filtered water, wastes from the digestive system and carbon dioxide from the gills are also ejected from the outgoing siphon. By the way, the photo that opened this essay is of a colony of tropical violet tunicates.
Other diaries in this series can be found here.