Monitoring at MacMillan Wharf, Provincetown, MA

Monitoring at MacMillan Wharf, Provincetown, MA
Marine Invasive Species (MIS) Monitoring at MacMillan Wharf, Provincetown, MA.

Monday, May 28, 2012

The Invasive Club Tunicate

An Abundant Resource for Ascidian Research

Styela clava and its cousin Styela plicata have been getting some pretty bad press over the last 10 years due to their circumglobal distribution and classification as Marine Invasive Species.  As a Styela biologist, I'd like to spend some time discussing some of the positive aspects to the Styela story. They may be be ugly ducklings on the outside (literally), but because of their recent world-wide distribution, they have also been the subject of scientific investigation by several research labs.  They are an attractive research model because they are readily available to researchers around the temperate world (in both hemispheres), and they are an important group of ascidians biologically in terms of their relationships to colonial botryllid species and other solitary ascidians.  In Asia (Korea), S. clava is part of the regional cuisine (Mideodok-chim) and is grown in aquaculture. The biology of Styela has become important for understanding diseases that have threatened commercially-cultured ascidians (soft-tunic syndrome in Halocynthia roretzi).  Inside, they are fascinating to study because their internal organs are bright orange and easy to examine.   S. clava is particularly useful because it is slender with a relatively thin, easily-cut tunic, whereas S. plicata is globular in shape and has a thick, less pliable tunic.  S. clava cohabitates with S. plicata in bays where their ranges overlap on both the East and West US coasts. 

Successful MIS Species:  S. clava and S. plicata
Styela clava and Styela plicata have made a home in Marinas and Bays around the world and are two of the most commonly studied Styela species.  In North America, both species have been established on both coasts for several decades.  S. clava has a more northern distribution than S. plicata, and the two co-habitate where their ranges overlap.  

Native American Styelids (Pacific Coast):  S. montereyensis, and S. gibbssi.
Styela montereyensis and Styela gibbsii, two Styela species from the Pacific coast were the two Styela research species of choice before the two MIS species became established in North America and Europe.  S. montereyensis and S. gibbsii have stayed within their ranges in the North American Pacific and prefer the cooler waters of the open coast. 

Styela clava showing orange-colored internal organs
Dissection of Styela clava with branchial sac (bs) on left and digestive tract on the right.  All the internal organs are located in the upper portion of the individual and not in the stalk whose function is essentially to raise the siphons and body above the growth of encrusting invertebrates and algae.  st, stomach; int, intestine;  os, oral siphon. as, atrial excurrent siphon. 

Diagram of the internal organs of Styela clava
Diagram of the internal organs of Styela clava.  Branchial sac, digestive tract consisting of esophagus, stomach, and intestine; reproductive organs in the body wall (both sides); and heart.   en, endostyle; bf, branchial folds; dl, dorsal lamina; rb, retropharyngeal band; ht, heart; es, esophagus; st, stomach; in, intestine; ov, ovary; t, testis.

Styela clava has a number of advantages as a model system for the study of solitary ascidians.  It is abundant, grows up to 4-5 inches (12 cm), and is easy to dissect.  It is adaptable for a wide range of studies and has a firm, flexible tunic that supports the internal organs during dissection.  In coastal marinas, S. clava individuals on docks show a range of sizes depending on the season and whether the substrate has been recently cleared.  During my 2011 MIS monitoring from Cape Cod to Salem Harbor, the population density was kept at a moderate level by competition with other encrusting invertebrates and colonial ascidian species, especially Didemnum vexillum and Diplosoma listerianum, which can aggressively overgrow solitary ascidians.  When fully mature, large individuals are easy to spot (unless totally covered by Didemnum).  However, small, young individuals may not be counted in the field when if they are not recognized. This is particularly important in marinas with new or cleaned docks. 

Young and Fully-Grown Individuals of Styela clava
Size distribution of Styela clava (the field is approx 5 inches [12.5 cm] high). The 6 smallest individuals were covered with Diplosoma listerianum, which was manually removed, revealing a smooth, light-tan tunic. The 3 medium sized individuals show typical bumpy brown corrugated tunic. The 3 largest individuals are covered with colonial ascidians and other attaching invertebrates. Left, complete coverage with Didemnum vexillum except for the siphons.  Center, patch-work collection of orange Botrylloides and Botryllus. Right, coverage by Didemnum and Botrylloides. Siphons and the upper body can contract within the tunic of the colonial species maintaining water flow and inhibiting overgrowth.

Styela has been used as a model system for studying filter feeding, the anatomy, histology, and ultrastructure of tunicates, cell biology (such as the function of the endostyle), blood cell formation, and the production of anti-bacterial peptides (styelins, clavanins, and clavaspirin).  The ovaries and testes are located in the body wall on both sides of the body instead of along the digestive tract and are arranged in radiating rows that are ideal for morphological and developmental studies.  Styela is a good control solitary ascidian for the study of coloniality in botryllid ascidians and is therefore important from an evolutionary point of view.  The styelids are located at a cross road linking more distantly related solitary ascidians such as Ciona, Ascidia, and Ascidiella, and other more closely related advanced ascidians such as Pyura (on the West Coast), Halocynthia (sea peach), and Boltenia (sea potato).  Some of these topics will be discussed in more detail in upcoming posts.

Granulocytes in Blood-Forming Tissue of Styela clava
Granulated blood cells in ascidians contain antimicrobial peptides that are important in fighting off bacterial and fungal infections.  The peptides are made in the cytoplasm and are concentrated into secretion granules that are released from the cell when they detect microorganisms.  This process is a part of the innate immune response and serves a similar function to the granulated blood cells in vertebrates that fight infections.  

Antibacterial Peptides from Blood Cell Granules of Styela clava
Styelins share amino acid sequences with each other and with cecropins in the domestic pig Sus scrofa domesticus (P1) and Drosophila virilis (Dv1).  Clavanins also share similar amino acid sequences.   

Styela, branchial sac, filter feeding, endostyle, stigmata, cilia, mucus, digestive tract, stomach, blood cells, hemocytes, granulocytes, cell renewal, reproduction, ovary, testis, innate immunity, styelin, clavanin, etc.

1.  Ermak TH. Cell proliferation in the digestive tract of Styela clava (Urochordata: Ascidiacea) as revealed by autoradiography with tritiated thymidine. J. Exp. Zool., 194: 449-466, 1975.
2.  Ermak TH. The hematogenic tissues of tunicates. In, The Phylogeny of Thymus and Bone Marrow-Bursa Cells, RK Wright and EL Cooper, Eds., Elsevier/North Holland, Amsterdam, pp. 45-56, 1976.
3.  Ermak TH. The renewing cell populations of ascidians. Amer. Zool. 22: 795-805,1982.
4.  Jiang AI, Lin J, Wang CH. Physiological energetics of the ascidian Styela clava in relation to body size and temperature. Comp. Biochem. Physiol. 149: 129-136. 2006.
5.  Kumagai A, Suto A, Ito H, Tanabe T, Takahashi K, Kamaishi T, Miwa S. Mass mortality of cultured ascidians Halocynthia roretzi associated with softening of the tunic and flagellate-like cells. Dis. Aquat. Org. 90: 223-234, 2010.
6.  Lambert G. New records of ascidians form the NE Pacific: a new species of Trididemnum, range extension and redscription of Aplidiopsis pannosum (Ritter, 1899) including is larva, and several non-indigenous species. Zoosystema 25: 665-67X, 2003 (includes an updated description of Styela clava).
7.  Lee IH, Cho Y, and Lehrer RI.  Styelins, broad-spectrum antimicrobial peptides from the solitary tunicate, Styela clava.  Comp Biochem. Physiol. 118B: 515-521, 1997.
8.  Lehrer RI, Lee IH, Menzel L, Waring A, and Zhao C.  Clavanins and styelins, alpha-helical antimicrobial peptides from the hemocytes of Styela clava.  Adv. Exp. Med. Biol. 484: 71-76, 2001.
9.  Lehrer RI, Tincu JA, Taylor SW, Menzel LP, and Waring J.  Natural Peptide Antibiotics from Tunicates: Structures, Functions and Potential Uses. Integr. Comp. Biol. 43: 313-322, 2003.
10.  Menzel LP, Lee IH, Sjostrand B, and Lehrer RI.  Immunolocalization of clavanins in Styela clava hemocytes.  Dev. Comp. Immunol. 26: 505-515, 2002.
11.  Raftos DA, and Cooper EL. Proliferation of lymphocyte-like cells from the solitary tunicate, Styela clava, in response to allogeneic stimuli. J. Exp. Zool., 260: 391–400, 1991.
12.  Raftos DA, Stillman DL, and Cooper EL. In vitro culture of tissue from the tunicate Styela clava.  In Vitro Cell. Dev. Biol. 26, 962-970, 1990.
13. Sawada T, Zhang J, and Cooper EL. Classification and characterization of hemocytes in Styela clava. Biol. Bull. 184:87-96,1993.
14. Thorndyke MC. Observations on the gastric epithelium of ascidians with special reference to Styela clava.  Cell Tiss. Res. Volume 184, 539-550, 1977.
15. Thorndyke MC.  Comparative studies on the effects of cholecystokinins, caerulein, bombesin 6-14 nonapeptide, and physalaemin on gastric secretion in the ascidian Styela clava.  Gen. Comp. Endocrinol., 1984. 

Korean Mideodok-chim (Steamed Styela with beef, clams, vegetables, and ground rice over noodles)
iTunicate Newsletter (Invasive Tunicate Network in Alaska):  Plate Watch.  Includes a recipe for steamed Mideodok-chim.