Foods, feeding, & growth
   
 
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Research study 1
drawing of water currents through the branchial basket of a solitary tunicate
Blue arrows show the direction of water flow from its entrance into the branchial siphon, through the filtering surface of the branchial (pharyngeal) basket, and its exit from the atrial siphon

photograph of fecal matter in the intestine of a tunicateTunicates feed by filtering small organic particles, including phytoplankton and bacteria, from seawater.  Water enters via the branchial siphon, passes through a sieving structure known as the branchial basket, and exits via the atrial siphon.  Cilia on the basket beat to provide the propulsive force.  Food particles are caught up in a sticky, fine-mesh mucus net that is produced on one side of the basket by the endostyle and moved by cilia along the inside surface of the basket to a vertical trough on the other side.  In the trough, the food-bearing mucous net is is rolled into a ropey strand and directed into the esophagus (located at the bottom of the branchial basket).  Undigested wastes are released from the anus into the atrial siphon and carried away in the water flow (see the fecal-laden intestine in the photograph). Studies at the Kerckhoff Marine Laboratory in Corona del Mar, California show that in open-ocean areas foods mainly consist of plankton, but often enriched by algal spores from seaweeds.  In estuaries the food has a larger component of stirred-up detritus.  A ring of interlaced tentacles across the branchial siphon acts to prevent the intake of larger particles.  Large particles that strike the tentacles or find their way into the branchial basket are forcibly ejected by sudden contraction of the body wall; hence, “sea squirt”.  MacGinitie 1939 Biol Bull 77: 443.

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Research study 2
 

photograph of the solitary tunicate Ascidia paratropa

Studies at Victoria, British Columbia on the branchial basket of photograph showing close view of the mesh in the branchial basket of a tunicate Ascidia paratropa courtesy Pennachetti 1984 Zoomorph 104: 216the solitary tunicate Ascidia paratropa reveal the presence of 2 adjacent and connected mucous nets (one of the nets is featured in the photgraph). The filtering net has elastic and adhesive properties, and has a microscopic pore size (0.5µm in Ascidia paratropa). Movement of the mucous net across the stigmata (openings) of the branchial basket is primarily by ciliary beating, but muscular activity in the stigmata of the branchial basket is apparently also involved. The net is small enough in mesh size to enable bacteria to be caught, but is resilient enough to withstand the thrashings of small crustaceans and other invertebrates that may enter with the water flow and get caught up in it. Pennachetti 1984 Zoomorph 104: 216.

 

Solitary tunicate Ascidia paratropa 0.7X
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Research study 3
 

With filtration rates of 2-5Lh-1 in some of the larger solitary tunicates, it is no surprise that many histogram comparing larval prey eaten by a solitary tunicate Chelysoma productum with larval prey available in the planktoninvertebrate larvae are sucked in and eaten.  Gut analyses of solitary tunicates Chelysoma productum, Pyura haustor, and Ascidia callosa in San Juan Islands, Washington reveal that several types of invertebrate larvae are eaten. If the mean numbers of each type of larvae eaten are compared with their numbers present in the plankton at the time of the study, we see something interesting; namely, that a few types of larvae (e.g., copepod nauplii, bivalve veligers, and ascidian tadpoles) are quite abundant in the plankton, but are not equally represented in the gut contents (see histograms).  Could this be because these types are relatively fast, agile, and large in size, or perhaps because they taste bad?  Alternatively, in the case of the tadpoles, perhaps tunicates do not eat their own or other species’ larvae? Bingham & Walters 1989 J Exp Mar Biol Ecol 131: 147.

NOTE data are shown here graphically only for Chelysoma productum - this species represents over half of the specimens analysed and thus provides the best data set. Note that the Y-axes are set on a log scale so that all the data could fit conveniently in the space available

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Research study 4
 

Field studies on growth and colony senescence of Botryllus schlosseri at the Monterey Marina, California shows that from settlement of the graph showing growth and senescence in a colonial tunicate Botryllus schlosseri over a period of 141dfounding larva, a colony grows exponentially to a size of 1400 zooids in 69d (at 14oC).  Reproduction begins after about 50d (shown by the purple arrow).  After production of 10 egg clutches per zooid over the next 70d a colony abruptly senesces and dies, even while bearing a full clutch of eggs.  Senescence progresses through 4 stages over 1-2wk and culminates in the simultaneous death of all zooids in the colony.  The 4 stages of senescence are: narrowing of blood vessels and decreased blood flow, shrinking of zooids with dense pigmentation, disorganisation of zooid groupings, and softening and disintegration of the tunic and tissues.  Life spans vary from 3mo for spring-born colonies to 8mo for autumn-born ones.  In the graph, shown for a July-born cohort, age in days and number of cycles (budding events) are indicated.  Autumn-born colonies overwinter as juveniles and commence reproduction in spring at an age of about 150d.  The authors note that their study is the first to document senescence in the field for Monterey B. schlosseri.  Chadwick-Furman & Weissman 1995 Biol Bull 189: 36; see also Boyd et al. 1986 Biol Bull 170: 91.

NOTE  colonies are grown “from scratch” from settled larvae on glass plates. The plates are transferred to wooden racks at the marina field site at 0.5-1m depth, seasonal temperatures 11-17oC.  The authors create 4 cohorts, in Jan, May, Jul, and Oct.  The plates are removed for study and cleaning every few days

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