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
 

photograph of colonial tunical Polyclinum planumdrawing of oozooid of colonial tunicate Polyclinum planum showing parts that will bud off new components for the initial blastozooid of the new colonyA researcher at the Long Marine Laboratory, California reports on an interesting (and complicated) growth pattern for the compound ascidian Polyclinum planum. This common low-intertidal species grows to 8cm diameter and is supported on a short stalk. The tadpole larva settles in the usual way and metamorphoses to a special zooid, known as an oozooid1, that represents the progenitor zooid for the entire colony. After settling, the oozoid feeds for 7-8wk (at 14oC), then commences colony building by first separating into 3 parts, representing the original thorax, abdomen, and post-abdomen of the oozooid (see drawing on Right). Each of these 3 sections now begins to bud2 off sections of itself, the final number of buds depending upon the original length of each section. Within a few days from commencement of budding, each bud (usually 1-4) produced from the post-abdomen part begins to differentiate into a new zooid, termed a blastozooid. The process involves regenerating a new thorax and abdomen. In the same manner as the post-abdomen, the original thorax and abdomen also bud off several smaller sections of themselves, but these later atrophy and disappear3. The author thinks that they may provide energy and nutrients to the newly developing blastozooids. As they regenerate the parts required to be a functioning zooid, the 1-4 new blastozooids migrate within the original tunic and orient their thoraces such that their atrial (exhalent) siphons empty into a common, central cloacal opening (see drawing below), with their endostyles and inhalent siphons lieing on the outside. Initially, as just noted, there are only a few of these “founder” blastozooids, but each has the capability to bud off new zooids and so the colony becomes larger. The author does not describe later colony development, other than to mention change to a flattened rather than hemispherical zooid-bearing shape, and appearance of a supporting peduncle. Holyoak 1992 Biol Bull 183: 432. Photograph courtesy James Watanabe, Stanford University, California seanet.

drawing of young colony of colonial tunicate Polyclinum planum showing component zooidsNOTE1 special name given to the first zooid derived from a sexually produced larva that divides asexually to form a new colony

NOTE2 each of these is termed a strobila, with the process being known as strobilation. The choice of these terms by earlier French and Japanese researchers seems unfortunate, as they are commonly used to describe medusae production in jellyfishes, and the word strobila also refers to the chain of proglottids making up the back end of a tapeworm. In the case of jellyfishes, strobilation produces junior but near perfect replicas of the adult, while in tapeworms the process of proglottid replication leads to a kind of segmentation, but not in the true sense. Neither process is the same as seen here for these ascidians. Perhaps “budding” would be a better, less confusing, descriptor

NOTE3 the author notes that the disappearance of these parts in P. planum differs from that described by earlier researchers on related polyclinid species

 

A young colony showing 6 component zooids
within. The thoraces of 3 of these, along with
their endostyles, can be seen through the tunic

  Research study 4.1
 

In a later study at Hopkins Marine Station, California, the same author investigates colony growth and shape in Polyclinum planum. The initially globose-shaped colony is supported on a tough and flexible peduncle that raises it off the substratum. Results of in situ monthly graph showing relationship of survival and age in the colonial tunicate Polyclinum planumobservations of 211 colonies in a wave-impacted intertidal habitat over a 23mo period reveal that the colony flattens as it grows larger (see graphs). This increases relative surface area of zooids, thus increasing potential feeding and reproductive output, and at the same time presumably diminishing drag effects in waves and currents. Holyoak 1997 Biol Bull 192: 87.

graph showing growth of colonies of the tunicate Polyclinum planum over 23mo, along with shape-changes during an arbitrary 5mo periodNOTE density of colonies in the study area averages about 8 per 0.25 m2. Recruitment occurs throughout the year. The author follows growth of selected recruits, expressed as increase in silhouette area in photographs, until their dislodgement and death after about 18mo

NOTE the author does not test this directly, but does show that comparably sized colonies are dislodged significantly more in strong-surge winter conditions than in calmer summer conditions



The inset silhouettes show change in colony
shape over an arbitrary 5mo period . Note
the flattening tendency over time

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Research study 5
 

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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Research study 6
 

graph showing relationship of pumping power to body size in ascidiansphotograph of early juvenile ovozooid of colonial tunicate Distaplia occidentalisIn a study done at Friday Harbor Laboratories, Washington, researchers investigate scaling of pumping rates in 4 species of west-coast tunicates.  Given that the siphon dimensions in earlier developmental stages are relatively smaller than in later stages, the authors predict that there will be an ontogenetic shift from relatively lower volumetric flow rates in early juveniles to relatively higher rates in adults.  Results show, indeed, that the juveniles have relatively lower flow rates than adults.  The allometric increases (see graph), expressed here as pumping power, occur rapidly in the 2 solitary species.  Note in the graph the less efficient pumping power of the juveniles, but an isometric relationship with body size is quickly reached.  Within 2-3wk after settlement size-specific pumping rates in the juveniles are comparable to those in the adults.  The relatively lower volumetric flow rates in the juveniles seems mostly to originate with the higher siphonal resistance of their relatively narrower siphons and not with changes in sigmatal shape (see photograph).  Interestingly, juveniles of one of the colonial species, D. occidentalis, show superior pumping performance in comparison with juveniles of both solitary species.  The authors note that theirs is the first study of feeding-flow rates in early juvenile marine invertebrates. Sherrard & LaBarbera 2005 Mar Ecol Progr Ser 287: 139.

NOTE  measured as the product of average incurrent velocities of particles times siphonal cross-sectional area

NOTE  these include 2 solitary species, Corella inflata and Ciona savignyi, and 2 colonial species, Distaplia occidentalis and Botrylloides violaceus

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