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Styela plicata (Lesueur, 1823)


SzeWen Chan 2019

Summary

Styela plicata is an ascidian commonly known as pleated sea squirt due to its unique rounded fold on the surface. First described by Lesueur in 1823, this species has a long history of spreading via ship routes. As a result, it can be found in shallow water habitats of nearly every continent with capabilities to tolerate wide range of temperature, salinity as well as polluted environments.  Styela plicata is a sessile, benthic filter-feeder which takes water into pharyngeal basket via oral siphon that filters out food before pumping water out again via the atrial siphon. It belongs to the subphylum Urochordata which is believed to have shared a common ancestor with vertebrates due to the presence of notochord at larval stage. Recent studies have focused on their role as a bioremediator of estuarine contamination due to their high filtering rate and widespread occurrence.

Physical Description
This species is a relatively large solitary species within the family Styelidae with individuals up to 93mm recorded previously (Van Name, 1945). It varies in external appearance mainly for its tunic, a cuticular covering protecting the body inside. The tunic is generally firm and tough. However, on the tunic surface, some individuals have longitudinal furrows are separated by broad, rounded ridges running parallel to the siphons (Van Name, 1945), forming a tidy pattern resembling a ploughed field (Fig. 1). In other individuals, the ridges are randomly broken into dome-shaped elevations, as if rounded cobblestones are laid on the tunic. The species does not exhibit any great variation in other morphological features (Kott, 1985). 

The general outline of the body is oval and upright with the attachment to substratum occurs on the posterior end (Van Name, 1945). The ventral side is more convex thus bringing both oral and atrial siphons closer together. The oral siphon is nearly terminal at the anterior end while the atrial siphon is located slightly down back on the dorsal side (Kott, 1985). When not discoloured, the greyish white tunic is slightly translucent but often this species appeared dirty brownish in colour. This is likely because of the deposition of sediments along the furrows.

Both siphons are short and marked with brownish purple radial stripes and a faint orange circular line when extended. This external feature is characteristic of the species (Kott, 1985) but is inconspicuous when siphons are contracted (Fig. 2).   
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Figure 1
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Figure 2

Ecology
Styela plicata is a well-known example of introduced species with poorly known origins (Lambert, 2001). Nowadays they are commonly found inhabiting man-made structure like ship hull, marinas and harbours but also present on rocky substrate. The species has incredible capabilities to withstand high level of pollution (Pineda et al., 2011) and wide range of temperature and salinity (Thiyagarajan and Qian 2003) in its lifetime. These enable Styela plicata to thrive in diverse ecosystems. Moreover, Styela plicata reach sexual maturity in less than 2 months in summer and produce two to three generation in a year (Yamaguchi, 1975). As a result, Styela plicata outcompetes successfully for space (Lambert, 2001) with other benthic invertebrates and sometimes threatens indigenous communities, thus is considered as invasive. 

Styela plicata is a highly efficient filter feeder likely because of its complex pharyngeal basket (Fiala-M´edioni, 1978). Its high filtration rate in turn processes large volume of water over time especially from polluted areas such as fishing harbours. Styela plicata not only is able to filter suspended particulate matter including phytoplankton and algae, but also to accumulate toxins such as heavy metals in their tissue (Aydın-Önen, 2016). As a result, recently many researchers have turned their focus on investigating the feasibility of using this species as bioremediator of algae and bacteria contamination (Draughon et al., 2010) or bioindicator for water quality (Aydın-Önen, 2016; Parrinello et al., 2017).

Life History and Behaviour
The eggs of Styela plicata are surrounded by a fluid-filled sac (Villa & Patricolo, 2001) that continues to supply the non-feeding larvae with nutrition after fertilization. Once hatched, Styela plicata can have an extended swimming period of over 2 days to find a suitable substrate without a cost to metamorphosis (Thiyagarajan & Qian, 2003). Larval settlement is most successful in the spring and fall (Fisher, 1976), when there is less predation. After settlement, larvae metamorphose into a functional adult form within 4 days (Yamaguchi, 1975) and live as sessile, filter-feeding tunicate we commonly see. Styela plicata can live between 2-3 years (Lambert & Lambert, 1998).
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Figure 3

Anatomy and Physiology
As a member in the order Stolidobranchia, Styela plicata has evolved to contain all bodily organs in its pharyngeal basket. The oral siphon opens internally into the pharyngeal basket with oral tentacles  which prevent ascidians from ingesting large objects in the water current (Ruppert and Barnes, 1994). The pharyngeal basket has four sharply defined folds (Van Name, 1945). On the folds are numerous gill slits that allow water to pass from the pharyngeal cavity to the atrial siphon. 

The stomach of Styela plicata is elongated in connection with a very narrow and curved gut loop (Fig. 4). This feature is distinctive and consistent in all individuals of the species (Kott, 1985). There are usually multiple gonads with two on the left and 4 to 6 gonads on the right side of the ascidian (Kott, 1985). Gonads exhibit slight variation in size and each gonad consists of a central, tubular ovary surrounded by many lobes or branching male glands (Kott, 1985). There are lots of endocarps from the body wall between the gonads and along the intestine (Fig. 5). The leaf-like endocarps are thickened projections of the body wall to protect and stabilize the gut loop and gonads (Kott, 1985), especially when these organs are not embedded in the body.

Reproduction and Growth

As described above, Styela plicata carries both male and female gonads within its body. Therefore, they are called hermaphrodites and they reproduce sexually by both self-fertilization (Pineda et al., 2011) and cross-fertilization. Styela plicata normally spawn all year round in conjunction with annual temperature changes, and spawning can occur between 11°C to 28°C (West & Lambert, 1976). Styela plicata releases their gametes simultaneously by light stimulation after a period of darkness (West & Lambert, 1975). External fertilization occurs in the water column once eggs and sperm are released (Ruppert et al., 1994). After fertilization and development, free-swimming tadpole larvae of 1.3mm on average settles and a functional ascidian is formed in 4 days (Yamaguchi, 1975)
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Figure 4
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Figure 5

Biogeographic Distribution
Styela plicata is a shallow-water species which prefer sheltered habitats in tropical and temperate oceans. Its geographical distribution is broad (Fig. 6), mainly due to commercial shipping (de Barros, 2008). In fact, the first description of this species by Lesueur in 1823 was based on an individual found on a ship’s hull in Philadelphia (Van Name, 1945). No other individual was found in the surrounding natural substrata. It is hypothesized that Styela plicata is native to northwestern Pacific region (de Barros, 2008). The species arrive to new locations through ship’s hull, in ballast water etc and physiologically adapt to wide ranges of environment as mentioned in Ecology. Anthropogenic maritime activities will likely increase the rate of introduction of Styela plicata all over the world, invasion of non-indigenous species could be a major threat to biodiversity (Pineda et al., 2011).
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Figure 6

Evolution and Systematics
While ascidians do not resemble algae, nor do their appearance cryptic, their positions in the animal kingdom has been a question for people. As their tunics were once mistaken as shell of Molluscs, or the leathery skin of a sea cucumber from the phylum Echinodermata. With the discoveries of tailed ascidian larvae with a notochord, nerve cord and muscle in 1866 (Holland, 2016), ascidians were first considered basal in the phylum Chordata. However, their relationship to vertebrates remained debated. A consensus to the phylogenetic relationships between ascidians and vertebrates and within groups in ascidians was difficult to reach. 

Recently, phylogenetic trees were constructed by analysing rRNA sequences concurred amongst members within subphylum Urochordata and of other subphyla (Holland, 2016). Other phylogenetic trees suggest placing Larvacea basal in Urochordata (Cañestro et al., 2003), or putting Urochordata as sister group to vertebrates, making cephlochordates basal in the phylum (Putnam, 2008).
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Figure 7

Conservation and Threats
There is currently no conservation concern about this invasive species. In contrary, its highly adaptive capabilities allow it to survive in many habitats. When in high density, Styela plicata excludes other species from the community and its larvae are capable of invading space occupied by other species (Sutherland, 1978). It also destabilizes the community because of the annual slough-offs, taking other attached organisms with it (Sutherland, 1978). It is recommended that boat owners could clean their boats by water blasting in order to prevent the spread of invasive species. Local governments should manage the discharge of ballast water by ships as well.

References

Aydın-Önen, S. (2016). Styela plicata: a new promising bioindicator of heavy metal pollution for eastern Aegean Sea coastal waters. Environmental Science and Pollution Research, 23(21), 21536-21553.

Cañestro, C., Bassham, S., & Postlethwait, J. H. (2003). Seeing chordate evolution through the Ciona genome sequence. Genome biology, 4(3), 208.

de Barros, R. C., da Rocha, R. M., & Pie, M. R. (2009). Human-mediated global dispersion of Styela plicata (Tunicata, Ascidiacea). Aquatic Invasions, 4(1), 45-57.

Draughon, L. D., Scarpa, J., & Hartmann, J. X. (2010). Are filtration rates for the rough tunicate Styela plicata independent of weight or size?. Journal of Environmental Science and Health Part A, 45(2), 168-176.

Fiala-Médioni, A. (1978). Filter-feeding ethology of benthic invertebrates (ascidians). IV. Pumping rate, filtration rate, filtration efficiency. Marine Biology, 48(3), 243-249.

Fisher, T. R. (1976). Oxygen uptake of the solitary tunicate Styela plicata. The Biological Bulletin, 151(2), 297-305.

Holland, L. Z. (2016). Tunicates. Current Biology, 26(4), R146-R152.

Kott P (1985) The Australian Ascidiacea - Phlebobranchia and Stolidobranchia. Memoirs of the Queensland Museum 23, 1-440

Lambert, G. (2001). A global overview of ascidian introductions and their possible impact on the endemic fauna. In The biology of ascidians (pp. 249-257). Springer, Tokyo.

Parrinello, D., Bellante, A., Parisi, M. G., Sanfratello, M. A., Indelicato, S., Piazzese, D., & Cammarata, M. (2017). The ascidian Styela plicata hemocytes as a potential biomarker of marine pollution: in vitro effects of seawater and organic mercury. Ecotoxicology and environmental safety, 136, 126-134.

Pineda, M. C., López-Legentil, S., & Turon, X. (2011). The whereabouts of an ancient wanderer: global phylogeography of the solitary ascidian Styela plicata. PLoS One, 6(9), e25495.

Putnam, N. H., Butts, T., Ferrier, D. E., Furlong, R. F., Hellsten, U., Kawashima, T., ... & Benito-Gutierrez, E. (2008). The amphioxus genome and the evolution of the chordate karyotype. Nature, 453(7198), 1064.

Ruppert, E. E., & Barnes, R. D. (1994). Invertebrate zoology (Vol. 1). Fort Worth: Saunders College Publishing.

Sutherland, J. P. (1978). Functional roles of Schizoporella and Styela in the fouling community at Beaufort, North Carolina. Ecology, 59(2), 257-264.

Thiyagarajan, V., & Qian, P. Y. (2003). Effect of temperature, salinity and delayed attachment on development of the solitary ascidian Styela plicata (Lesueur). Journal of Experimental Marine Biology and Ecology, 290(1), 133-146.

Van Name, W. G. (1945). North and South American ascidians. Bulletin of the American Museum of Natural History, 84, 1-462.

Villa, L., & Patricolo, E. (2001). Follicle cells of Styela plicata eggs (Ascidiacea). In The Biology of Ascidians (pp. 67-73). Springer, Tokyo.

West, A. B., & Lambert, C. C. (1976). Control of spawning in the tunicate Styela plicata by variations in a natural light regime. Journal of Experimental Zoology, 195(2), 263-270.

Yamaguchi, M. (1975). Growth and reproductive cycles of the marine fouling ascidians Ciona intestinalis, Styela plicata, Botrylloides violaceus, and Leptoclinum mitsukurii at Aburatsubo-Moroiso Inlet (Central Japan). Marine Biology, 29(3), 253-259.