Reading path: Marine organisms and body plans →
A new science guide at a historical Coldwater address. About the earlier site →
Photographic reference
Credit: Dr. Dwayne Meadows, NOAA/NMFS/OPR. Original source · CC BY 2.0. None; original photograph displayed proportionally.
What to look for
| Feature | Observation | Comparison or limit |
|---|---|---|
| Attached ascidian | Two siphons route water through the filtering body. | A sea squirt is neither a sponge nor a coral. |
| Pelagic diversity | Salps and larvaceans are also tunicates. | Not every adult tunicate attaches to a surface. |
| Outer structures | The tunic is a body covering. | A larvacean mucus house is an external feeding structure. |
A chordate connection that changes with age
Tunicates belong to Chordata, the phylum that also includes vertebrates. In many tunicates, the larva has a notochord and a tail used for swimming. During metamorphosis, an ascidian larva settles and reorganizes; the adult loses much of the tail-associated anatomy. The chordate connection is therefore easiest to recognize in one life stage, rather than by comparing an adult sea squirt with a fish. [3]
A hypothetical observation illustrates the contrast: a small tadpole-like larva swims briefly, then attaches to a hard surface and becomes a sac-like adult. The change is not a transition from plant to animal, nor from one unrelated organism to another. It is development within one animal lineage, with different structures serving different phases of life.
The sea squirt’s two-way current
An adult ascidian, often called a sea squirt, is typically attached. Water enters through an incurrent siphon, passes through a filtering pharynx, and exits through an excurrent siphon. Food particles are retained and moved toward the digestive tract; water flow also supports gas exchange and carries waste away. The tunic is the animal’s outer protective covering, not the filter itself. [3]
This body plan can look like a sponge or a small coral at a distance, but those comparisons describe appearance, not identity. A sponge filters with specialized cells and a different organization; a coral is a cnidarian, often living in a colony. An ascidian is a chordate with its own inlet-and-outlet flow path. If a diver sees two openings on an attached, soft-bodied animal, that is a useful observation to record, while a species identification requires additional traits.
Tunicates are not all attached
The subphylum includes several major groups, among them Ascidiacea, Thaliacea, and Appendicularia. That diversity matters: the familiar attached sea squirt is not a template for every tunicate. Salps are pelagic tunicates that move through open water, while larvaceans retain a tail and live as small swimmers. Their adult forms do not all settle onto rock or harbor structures. [1]
Larvaceans feed using an external mucus structure often called a house. Water and particles pass through its filtering passages; when the structure clogs, it can be discarded and replaced. The house is not the animal’s tunic: one is a temporary feeding apparatus outside the body, the other is a covering associated with the body. Confusing the two would blur the distinction between protection and food capture. [2]
Filters link small animals to ocean processes
A larvacean’s filter can concentrate tiny particles from a much larger volume of water. Researchers have used laser imaging and particle-flow measurements to reconstruct the geometry of giant larvaceans’ delicate inner filters in place, because such mucus structures can collapse or be damaged when removed. The work shows how observing both animal and surrounding flow can reveal function that a still image alone may miss. [2]
Discarded mucus structures sink with trapped material, so filtering can also move carbon-rich particles downward through the water column. The size and effect of that transport depend on conditions and must be measured; the general mechanism does not establish a particular rate at a given site. A useful hypothetical field record would distinguish the animal, its tail, the surrounding filter house, and any observed sinking material instead of calling all of them a single “tunic.” [2]
Sources & further reading
Have a correction or a source to suggest? Send a note →