Stalked Sea Squirt, Clubbed Tunicate, Asian Tunicate, Leathery Sea Squirt, Rough Sea Squirt

Type Description

S. clava is a solitary tunicate. Including both the club-shaped body and peduncle, larger specimen of S. clava can have a maximum length of around 130 mm (5.1 in) and smaller specimen only reaching 30 mm (1.2 in) in length. Smaller specimen tend to have no distinct peduncle. S. clava has a tough, wrinkled or irregularly grooved skin and comes in two variations of color dependent on size. Larger specimen have a light brown body and a darker brown peduncle while smaller specimen are yellow-brown.

Habitat

"S. clava is a marine invertebrate animal. Adults are entirely sessile, growing attached to hard subtidal substrata as deep as 25 meters (82 ft). They can be found on virtually any hard surface such as rocks, buoys, pilings and shells of mussels. S. clava is predominant in the littoral zone, preferring sheltered localities free of strong wave action and floating objects, making artificial surfaces in harbors and marinas exceptional habitat. It is a hardy species that can live in a wide range of temperatures from -2 °C to 27 °C and can tolerate high salinity waters (26% - 28% salinity) that would be lethal to other tunicate species.

Since the mid-1900s, S. clava has been unintentionally introduced globally to temperate coastal waters outside of its native range. It has successfully established stable populations on both coasts of North America, Europe, New Zealand, Australia, and Argentina.
The earliest sighting of S. clava outside of its natural range was on the United States’ west coast in Californian coastal waters in the early-1900s. Since then, the invasive tunicate has spread as far south as Baja, Mexico and as far north as Vancouver Island, Canada. S. clava populations in North America’s Atlantic waters is believed to have been introduced around the 1970s.
In the mid-1900s, the next sighting of S. clava was recorded in European waters in Britain. In the span of 25 years, S. clava populations have expanded their range in the coastal waters of the United Kingdom and to mainland Europe. The current European countries with established S. clava populations are England, Ireland, Belgium, Netherlands, Denmark, France, Portugal and Spain."

Reproductive Information

Like most tunicates, S. clava is hermaphroditic and produces short-lived pelagic lecithotrophic larvae. They reproduce externally via broadcast spawning, and the reproductive period is highly dependent on sea surface temperatures reaching a critical temperature threshold, between 16 °C and 20°C. The reproductive period can vary from 4 to 10 months depending on location. Along the Californian coast in the United States, the reproduction period occurs for 4 months from June to September, while in Denmark and England, the reproduction period is also 4 months but occurs from July to October.

Lifecycle

Nutrition Information

S. clava is a suspension feeder that consumes matter such as phytoplankton, zooplankton, oyster larvae and other suspended organic materials (NIMPIS, 2002). S. clava has a high filtration rate (Clarke and Therriault, 2007).

General Impact Information

"Successful introduction and establishment of populations outside of its natural range can cause dramatic changes in the structure and composition of benthic communities. It is dominating fouling communities, leading to population declines in other filter-feeding species, leading to lower biodiversity. S. clava is a solitary species, but with optimal conditions can reach high densities, up to 500 - 1,000 individuals, fouling man-made substrates resulting in boat and fishing gear difficulties.
They also pose a threat to aquaculture, which is being seen in European waters. In Bassin de Thau, France (Étang de Thau), S. clava is becoming a management problem as they pose a threat to oyster and mussel farming by outcompeting the shellfish for food and substrate space."

General Management Information

"Prevention: Monitoring water samples for species of invasive ascidians can help ensure early detection and response. However current methods (use of recruitment plates and dissection microscopes) are costly and time-consuming and it is difficult to distinguish different species from observing egg and larval stage under a dissecting microscope. Use of polymerase chain reaction (PCR) based detection is being investigated and could provide easier and earlier identification of S. clava (including of egg and larval stages) (Stewart-Clark et al. 2009). Control of potential vectors (e.g. fishery and shipping vessels and fouled aquaculture equipment) can also help prevent spread. (See Locke et al. 2009 for vector control efforts in Prince Edward Island; see MAF Biosecurity New Zealand, 2008 for suggestions on S. clava vector management in New Zealand).
Exposure, temperature and salinity: NIMPIS (2002) states that, ""In some power plants, raw water systems, reservoirs, locked marinas and impoundments, water levels can be lowered (drawn-down) to expose fouling infestations to the air. Subsequent freezing or desiccation due to ambient temperatures may kill a large proportion of the exposed population."" The authors go on to state that this method has been successful in controlling S. clava. Various combinations of salinity, temperature and exposure to air have proved successful in killing S. clava fouled on oysters without harming the oysters (NIMPIS, 2002).
The dipping of dredged oysters, and associated species, in saturated or strong salt solutions is extremely effective in killing ascidians without harming the oysters. Brine dipping of oysters fouled with Sargassum muticum, Codium fragile ssp. tomentosoides and S. clava was found to be an effective control. Brine dipping infested oysters is considered the cheapest, safest and most effective method of control of fouling species, however, this requires collection of all the fouled oysters to place them in a bath as it is not possible to implement in the open environment (NIMPIS, 2002).
Chemical control: Hydrated lime (calcium hydroxide) and acetic acid are natural chemicals that can be used to remove fouling tunicates. Spraying or immersion treatments with a saturated solution of hydrated lime or 5% acetic acid are effective at removing fouling tunicates, but are also biocidal to a variety of non-target organisms. There is also some concern about alteration of estuarine pH if the chemicals were to be heavily used (Locke et al., 2009). The synthetic chemical medetomidine can reduce S. clava larval mobility and settlement and may have potential as a management tool to control S. clava fouling. (Willis et al., 2011).
Physical control: High pressure water blasting has been used as a removal strategy for other tunicates but has been of limited use for S. clava because of its tough tunic (Clarke and Therriault, 2007).
Alternative approaches: There is interest in marketing S. clava as a food item – they are already eaten as a delicacy in Korea. Marketing S. clava (removed from aquaculture operations) for the Korean community in North America could offset some of its damaging economic effects on the aquaculture industry (Karney and Rhee, 2009)."

General Pathway Information

Notes

Outside of its native range, S. clava has proven to be an increasingly successful invasive species due to physiological adaptations and environmental tolerances. S. clava's thick tunic, relative to native tunicates, provides better protection from possible predators and helps prevent desiccation. It can withstand subzero to 23 °C sea temperatures and high salinity water, giving it strong tolerance to environmental changes in water. The lack of a natural predator already gives S. clava an advantage over native tunicates, but their large size as well allows them to outcompete other filter-feeding species such as oysters or mussels for food and substrate space.

References