Pyura stolonifera is a species of ascidian or sea squirt, regularly used by fishermen as bait, hence the common name red bait (or rooiaas in Afrikaans). They are advanced animals that are close to vertebrates on an evolutionary scale. Red bait plays a crucial role in marine ecosystems. These ecosystem engineers can create, modify, or maintain habitats, often promoting greater biodiversity in their environments.
How to recognize red bait
Despite being ‘solitary’ creatures, they are commonly found forming dense aggregations on rocky beaches, much like groups of mussels. Redbait is the largest ascidian found on the South African coast, reaching up to 20 cm in height and 15 cm in diameter. They are characterized by a massive, globular body covered by a thick, leathery structure known as the test or tunic that often hosts epibionts. The body wall or mantle, which lies beneath the test, displays a red-orange colour. Protruding from the body are two siphons. At low tide, the ascidians can be seen squirting a stream of water from their exhalant siphons, which is why they are also known as sea squirts.
Getting around
Individuals remain sessile throughout their adult stage, permanently attaching themselves to hard substrates in marine environments. However, their larval form, known as ascidian tadpoles, is free-swimming for a short time, typically lasting a few hours to a few days. The larvae propel themselves through the water by undulating their tails, following a helical path. Ascidian tadpoles display phototaxis, moving in response to light stimuli. This influences their settlement patterns, with Pyura stolonifera larvae preferring to attach to well-lit surfaces. Ascidians can also be transported artificially in the ballast water, on aquaculture and fisheries equipment, and via marine debris.
Distribution
Red bait exhibits a disjunct distribution across the temperate zones of southern Africa. The known species’ range extends from Namibia to Durban.
Habitat
They primarily inhabit rocky reefs, from mid-intertidal to the subtidal zones and are known to occur up to depths of approximately 15 meters. The species thrives in artificial marine environments, frequently colonizing harbours and marinas, where they are found attached to submerged chains and ropes.
Food
Ascidians are sessile filter feeders that extract detrital organic matter, diatoms and other phytoplankton from seawater. Research has revealed that the diet of red bait is more diverse than previously thought. Potential other food items include developing stages and larvae of other invertebrate species and early forms of its own species.
SEX AND LIFE CYCLE
Species within the genus are hermaphroditic, meaning individuals possess both male and female reproductive organs. Eggs and sperm are produced simultaneously and expelled through the exhalant siphon into the surrounding water where fertilization occurs. This process is known as spawning. Fertilized eggs develop quickly into free-swimming larvae, which settle within a short time onto a suitable substrate and metamorphose into sessile adults.
THE BIG PICTURE
Friends and foes
Redbait is known to host several crustacean species that may function as either symbionts or ectoparasites. They also harbour endosymbionts such as nemerteans, copepods, and amphipods. In addition to being harvested by humans for bait, redbait is preyed upon by sea snails, as well as by fish, sea stars, and crabs.
Smart strategies
Spawning in Pyura populations is synchronized and is initiated at low tide when individuals are exposed to air. To increase fertilization success, both eggs and sperm are released simultaneously by multiple individuals. Once fertilized, larvae will settle close to the parent population. These reproductive and developmental strategies help ensure the ecological dominance of Pyura species within their habitats.
Poorer world without me
Red bait forms a fundamental component of their environment, acting as ecosystem engineers and contributing to biodiversity and ecosystem functioning. A decline in their populations could have significant cascading effects on marine ecosystems. Additionally, as filter feeders, they play a crucial role in particle removal. A decline in their numbers could adversely affect water quality, leading to increased turbidity and a higher risk of algal blooms.
People and I
Ascidian research holds potential for industrial and pharmaceutical applications. Over 500 diverse chemical compounds produced by ascidians have been isolated. These compounds serve various functions, including anti-fouling, anti-inflammatory and antimicrobial activities. Ascidians also produce metabolites with potential therapeutic properties, such as antibiotic activity, and immunosuppressive effects. Additionally, ascidians can accumulate dangerous toxins, mainly heavy metals and microplastics, from the marine environment, making them useful bioindicators of environmental pollution. However, pollution can lead to the bioaccumulation of toxins and microplastics in the food chain when predators consume ascidians, posing risks to human consumers of marine organisms.
Ascidians can quickly occupy artificial facilities in marine environments such as buoys, seabeds, and cages, posing a fouling challenge to the aquaculture industry. They directly compete with economically valuable shellfish (e.g., mussels, scallops, pearl oysters, and oysters) for food and habitat space and damage aquaculture infrastructure. Ascidians are routinely removed from affected infrastructure and discarded into the sea, contaminating the water and causing eutrophication issues.
Conservation status and what the future holds
Due to their abundance and wide range, red bait populations are considered stable. Consequently, the IUCN does not regard red bait to be a species of conservation concern and their status has not been assessed by the IUCN Red List.
Ascidian species are known to frequently invade and establish themselves in marine environments outside their native range. Pyura stolonifera exhibits significant invasive potential, indicating a potential future range expansion. This species may spread through human-mediated activities such as ship hull fouling, discharge of ballast water, and the transfer of aquaculture stock and equipment. There is evidence of successful introductions of closely related species to non-native regions. For example, Herdman’s red bait (Pyura herdmani) has established populations in Europe, doppelganger cunjevoi (Pyura doppelgangera) has been introduced to New Zealand, and cunjevoi (P praeputialis) has colonized coastal areas in Chile. Therefore, future research is needed to determine the invasive potential of red bait, identify invasion pathways, and mitigate the risk of invasions to protect vulnerable marine ecosystems.
Relatives
Pyura stolonifera was initially considered a single, morphologically variable species widely distributed across the southern hemisphere, including Australia, New Zealand, South Africa, and Chile. However, molecular and morphological analyses have led to a reassessment of this species complex. Subsequent research has revealed that P. stolonifera comprises closely related species that are often difficult to distinguish from one another. These species include Pyura praeputialis, Pyura doppelgangera, Pyura herdmani, and Pyura dalbyi. Additionally, African populations recognize P. stolonifera and P. herdmani as two distinct species endemic to South Africa.
Scientific Name and Classification:
Kingdom: Animalia
Phylum: Chordata
Class: Ascidiacea
Order: Stolidobranchia
Family: Pyuridae
Genus: Pyura
Species: P. stolonifera Heller, 1878
Common names: Red bait (English); Rooiaas (Afrikaans)
References and further reading
- Branch, G., Branch, M., Griffiths, C., Beckley, L., 2022. Two Oceans: A guide to the marine life of southern Africa. Penguin Random House South Africa.
- Bruno, J., 2001. Habitat modification and facilitation in benthic marine communities. Mar. Community Ecol.
- Buschbaum, C., Dittmann, S., Hong, J.-S., Hwang, I.-S., Strasser, M., Thiel, M., Valdivia, N., Yoon, S.-P., Reise, K., 2009. Mytilid mussels: global habitat engineers in coastal sediments. Helgol. Mar. Res. 63, 47–58.
- Castilla, J.C., Collins, A.G., Meyer, C.P., Guiñez, R., Lindberg, D., 2002. Recent introduction of the dominant tunicate, Pyura praeputialis (Urochordata, Pyuridae) to Antofagasta, Chile. Mol. Ecol. 11, 1579–1584.
- Dalby, J., 1996. Nemertean, copepod, and amphipod symbionts of the dimorphic ascidian Pyura stolonifera near Melbourne, Australia: specificities to host morphs, and factors affecting prevalences. Mar. Biol. 126, 231–243.
- Grave, C., 1920. Amaroucium pellucidum (Leidy) form constellatum (Verrill). I. The activities and reactions of the tadpole larva. J. Exp. Zool. 30, 239–257.
- Hartmeyer, R., 1911. Die ascidien der deutschen Sudpolar-Expedition, 1901-1903. Dtsch. Sudpolar-Exped. 12, 403–606.
- Kott, P., 1985. The Australian Ascidiacea part 1, phlebobranchia and stolidobranchia. Mem Qld Mus 23, 1–440.
- Monniot, C., 2001. South African ascidians. South African Museum.
- Monniot, C., Monniot, F., 1994. Additions to the inventory of eastern tropical Atlantic ascidians; arrival of cosmopolitan species. Bull. Mar. Sci. 54, 71–93.
- Monniot, C., Monniot, F., Laboute, P., 1991. Coral reef ascidians of New Caledonia. IRD Editions.
- Monteiro, S., Chapman, M., Underwood, A., 2002. Patches of the ascidian Pyura stolonifera (Heller, 1878): structure of habitat and associated intertidal assemblages. J. Exp. Mar. Biol. Ecol. 270, 171–189.
- Red Bait (Rocky Shores of Algoa Bay) · iNaturalist, n.d. . iNaturalist. URL https://www.inaturalist.org/guide_taxa/999936 (accessed 7.29.24).
- Rius, M., Branch, G.M., Griffiths, C.L., Turon, X., 2010. Larval settlement behaviour in six gregarious ascidians in relation to adult distribution. Mar. Ecol. Prog. Ser. 418, 151–163.
- Rius, M., Teske, P.R., 2011. A revision of the Pyura stolonifera species complex (Tunicata, Ascidiacea), with a description of a new species from Australia. Zootaxa 2754, 27–40.
- Rius, M., Teske, P.R., Manriquez, P.H., Suarez-Jimenez, R., McQuaid, C.D., Castilla, J.C., 2017. Ecological dominance along rocky shores, with a focus on intertidal ascidians.
- Rius, M., Turon, X., Morán, P., Pérez, J., Almón, B., Pahad, G., Teske, P.R., Vázquez, E., 2024. A morphogenetic characterisation of a potentially dominant African marine species in Europe. Biol. Invasions 1–8.
- Strother, J., McHenry, M., 2005. The effects of morphology and behavior on phototaxis in ascidian larvae. Presented at the Integrative And Comparative Biology, Oxford Univ Press Inc Journals Dept, 2001 Evans Rd, Cary, NC 27513 USA, pp. 1200–1200.
- Teske, P.R., Rius, M., McQuaid, C.D., Styan, C.A., Piggott, M.P., Benhissoune, S., Fuentes-Grünewald, C., Walls, K., Page, M., Attard, C.R., 2011. “ Nested” cryptic diversity in a widespread marine ecosystem engineer: a challenge for detecting biological invasions. BMC Evol. Biol. 11, 1–13
Author: Janine Baxter