Ascidians are suspension feeder that filter food particles such as phytoplankton from the surrounding sea water. They transport water through the branchial sac (pharynx) which is perforated with small, ciliated slits, the stigmata. The ciliary tracts on each side of the stigmata create a water current that pulls water through the branchial siphon into the branchial sac, through the stigmata, and into the atrial cavity from which the water leaves the body as a stream through the atrial siphon. When the water is transported across the branchial wall, suspended particles are trapped on a mucous net that is created by the endostyle, a mucus secreting structure on the ventral wall.
The feeding mechanism is similar in different ascidians regardless of the complexity of the structures. Styela clava belongs to a group of solitary ascidians with a structurally complex branchial sac that is folded. The folding increases the internal surface area, thereby increasing the number of stigmata and filtration capacity. A mucus secreting organ on the vental surface, the endostyle, secretes a mucus net that moves across the surface of the sac by cilia on the pharyngeal bars. The mucus net with its retained food particles then moves toward the esophagus. Thus, the amount of food consumed is determined by the concentration of food particles in the sea water, the efficiency of particle retention, and the volume of water transported. At a structural level, the endostyle is a longitudinal ciliated groove on the ventral wall and is composed of 8 different types of cells, a single band at the base of the groove with very long cilia that extend toward the surface of the groove and 7 pairs of bands on each side of the endostyle wall.
Diagram of the Branchial Sac of S. clava in Relation to the Digestive Tract
The digestive tract of Styela clava from the left side with the internal surface of the branchial sac exposed. bs, branchial siphon; bt, branchial tentancles; en, endostyle; bb, branchial bar; s, stigmata; i, intestine; as, atrial siphon; dt, dorsal tubercle; r, rectum; dl, dorsal lamina; es, esophagus; rb, retropharyngeal band; st, stomach.
Cross Section through the Branchial Sac of Styela clava
View of the Internal Surface of the Branchial Basket of Styela clava
Internal View of 3 Stigmata in the Branchial Sac of Styela clava
Section through the Stigmata and a Branchial Bar of Styela clava
Section through the stigmata and a branchial bar of Styela clava. Cilia prevent food particles from leaving the branchial cavity but allow filtered sea water to pass through. Ciliated bars transport food-containing mucus along the longitudinal and transverse bars to the dorsal lamina and thereafter to the esophagus. The cilia on the bars actually pull the mucus net forward, whereas the stigmata cilia act only as filters. sb, stigmata bar, ve, ciliated ventral epithelium; ue, unciliated epithelium.
Histologic Section through the Stigmata of Styela clava
Cross section of the Endostyle of Styela clava
Cross-sectional diagram of the different cell types of the endostyle of Styela clava. The cells at the base of the groove have very long, flagella-like cilia. The remaining cells are paired with semi-alternating glandular and ciliated cells.
Godeaux, J.E.A. Functions of the endostyle in the tunicates. Bull. Mar. Sci. 45: 228-242, 1989.
Holmes, N. Water transport in the ascidians Styela clava Herdman and Ascidiella aspersa (Mueller). J. Mar. Bio. Ecol. 11: 1-13, 1973.
MacGinitie, GE. The method of feeding of Tunicates. Biol Bull 77: 443-447, 1939.
Petersen, J, & Svane, I. Filtration rate in seven Scandinavian ascidians: implications of the morphology of the gill sac. Mar. Biol. 140: 397-402, 2002.
Petersen, JK, Mayer, S, & Knudsen, MÅ. Beat frequency of cilia in the branchial basket of the ascidian Ciona intestinalis in relation to temperature and algal cell concentration. Mar. Biol. 133: 185-192, 1999.
Riisgird, HU. The ascidian pump: properties and energy costs. Mar. Ecol. Prog. Ser. 47:129-134, 1988. (Studied in Styela clava).
Shimek, RL. Tunicates or Sea Squirts: A Wet Link. Reefkeeping Online Magazine.