Learn About Sea Squirts: Predators & defenses

Predation on adults

Predators of larval sea squirts include all manner of suspension-feeding invertebrates and fishes, while predators of adult sea squirts include flatworms, sea stars, opisthobranchs, and tritons. PREDATION ON ADULTS is considered here, while PREDATION ON LARVAE is dealt with in another section.

Predators of adult tunicates include flatworms, snails, and sea stars, considered in subsections below. A putative alarm pheromone has been described in one west-coast species and an account given in the last subsection below. photograph of a sea star Orthasterias koehleri photograph of several top shells Calliostoma ligatum feeding on a colonial tunicate Distaplia occidentalis

Several top shells Calliostoma ligatum feeding on a colonial tunicate Distaplia occidentalis 0.7X

A sea star, Orthasterias koehleri, straddles two colonial tunicates and some coralline algae 0.5X


Research study 1: Flatworms

photograph of a flatworm Eurylepta leoparda feeding on a tunicate Corella willmeriana courtesy Ron Long, SFU, Burnaby

A predator of the solitary tunicate Corella willmeriana is the flatworm Eurylepta leoparda. The worm bypasses the tough outer covering of tunicin by rolling into a tube and slipping into the branchial siphon. After consuming the branchial basket it proceeds to eat all of the remaining internal organs. After 3-7d it leaves behind an empty husk of tunicin. Eurylepta prefers C. willmeriana as prey and apparently does not eat other species of sea squirts. Lambert 1968 Biol Bull 135: 296. Photograph courtesy Ron Long, Simon Fraser University, Burnaby.

One or more Corella willmeriana being consumed by several flatworms Eurylepta leoparda. One worm at about 7 o'clock can be distinguished by the brown "leopard" spotting on its dorsal surface 2X


Research study 1: Snails

photograph of a vellutinid snail Lamellaria diegoensis with a photo of its prey tunicate Cystodytes lobatus.  Photo of Lamellaria courtesy Jeff Goddard, Univ Calif Santa Barbara

In the low intertidal zone around Pacific Grove, California velutinid snails Lamellaria diegoensis feed on compound tunicates, specifically, Cystodytes lobatus. The snail is often cryptically coloured against the background of its prey and is uncommon enough that in a 6mo period the author could find only a single specimen to study. Lamellaria feeds on the zooids by rasping through the tunic and sucking them up, spicules and all. Lambert 1980 Veliger 22: 340; see also Lambert 1979 Biol Bull 157: 464. Photograph of Lamellaria from an Oregon cave courtesy Jeff Goddard, University of California, Santa Barbara.

A velutinid snail Lamellaria diegoensis (0.9X)and a colonial tunicate Cystodytes lobatus (1.3X)

Research study 2: Snails

photograph of hairy triton Fusitriton oregonensis courtesy Ron Long, Simon Fraser University, BurnabyThe hairy triton Fusitriton oregonensis eats several species of solitary ascidians, some large in size, and can consume an individual every 2d. Laboratory tests at Friday Harbor Laboratories, Washington show that preference varies for different species, from ones readily eaten to ones never eaten, as shown below. Some species have no known defenses and are readily eaten, while others are readily eaten in laboratory experiments, but may be protected in nature by living out of reach of Fusitriton on the surfaces of other tunicates - that themselves are protected from being eaten.At least one species, Cnemidocarpa finmarkiensis, is not eaten but has no known defenses, and 2 others, Pyura hausor and Halocynthia igaboja, are protected by tough tunic and spines. Young 1986 Mar Biol 91: 513; Young 1985 Mar Biol 84: 309.


  • Ascidia paratropa no known defenses
  • Ascidia callesa no known defenses
  • Chelyosoma productum no known defenses
  • Botenia villosa< may be protected by living on other tunicates (see Research Study 3 below)
  • Styela gibbsii may be protected by living on other tunicates (see Research Study 3 below)
photographs of protective spines on tunicate Pyura haustor courtesy Young 1986 Mar Biol 91: 513.photographs of protective spines on tunicate Halocynthis igaboja courtesy Young 1986 Mar Biol 91: 513.


  • Pyura haustor protected by a tough leathery tunic that has sand grains and small rocks incorporated into it during growth (upper photo on the far Right), and by spines around the siphon edges (lower photo on the far Right). Photos and information from Young 1986 Mar Biol 91: 513.
  • Cnemidocarpa finmarkiensis no known defenses
  • Halocynthia igaboja protected by stiff spines 1-2cm in length growing from the tunic (photos on near Right). If these are shaved off with a razor blade and the denuded sea squirts exposed to Fusitriton predators along with normal unshaved individuals, the shaved ones are eaten more quickly. Photos and information from Young 1986 Mar Biol 91: 513.

Research study 3: Snails

photograph of tunicate Boltenia villosa Hairy sea-squirt Boltenia villosa 1X

Both Boltenia villosa and Styela gibbsii live epizoitically on Halocynthia igaboja and Pyura haustor. These last are graph showing % settlement of larvae of Boltenia villosa on different substrata including another tunicate Pyura haustorprotected from predators, respectively, by sharp spines and inorganic inclusions in the test (see Research Study 2 above). In laboratory experiments, larvae of Boltenia villosa preferentially settle on pieces of tunics of the 2 host species, and settlement may be delayed in their absence. By living on their protected hosts, the 2 species are provided spatial refuge from the predator. Young 1989 Ophelia 30:131.

NOTE an organism escapes predation by living in a place that is inaccessible to a predator. In this case, Fusitriton cannot reach the Boltenia or Styela when they are perched on their hosts. Other types of refuge are size and temporal – the first being a potential prey that is safe by having grown too large for a predator to eat; the second, a prey that is safe by not being around at the same time that a predator is around

Sea stars

Research study 1: Sea stars

photograph of sea star Evasterias troschelii

On soft-bottom habitats in areas of Washington the diet of mottled sea stars Evasterias troschelii may exceed 90% ascidians. How an ascidian is eaten correlates with toughness of tunic, but few species are immune to attack. Thus, the soft Chelysoma productum is easily torn apart by the arms and tube feet of the sea star, while the leathery Styela gibbsii and Pyura haustor require that holes be digested in them for insertion of the stomach. Young 1984 p.577 In, Echinodermata (Keegan & O’Connor, eds.) AA Balkema, Rotterdam.

Mottled star Evasterias troschelii on an intertidal rock 0.5X

Research study 2: Sea stars

Several attacks by sea stars on tunicates and some defensive behaviours of the intended prey are shown in these photographs. There appear to be few published accounts of predator-prey relationships between sea stars and tunicates.

photograph of leather star Dermasterias imbricata attacking a colonial tunicate Cystodytes lobatusA leather star Dermasterias imbricata making a meal of a colonial tunicate Cystodytes lobatus 0.2X
photograph of sea star Orthasterias koehleri attacking a tunicate Halocynthis aurantiumSea star Orthasterias koehleri attacks a solitary tunicate Halocynthia aurantium 0.25X
photograph of sea star Orthasterias koehleri attacking a tunicate Halocynthis aurantiumClose view of another such encounter between Orthasterias and Halocynthia 0.7X
photograph of a leather star Dermasterias imbricata possibly eating tunicates Corella inflataA leather star Dermasterias imbricata is in apparent digestive mode on an aggregation of Corella inflata 0.3X
photograph of tunicate Halocynthis aurantium in defensive posture against a sunflower star Pycnopodia helianthoidesA juvenile Pycnopodia helianthoides elicits a typical siphon-closing response in Halocynthia aurantium 0.7X
photograph of several juvenile sunflower stars Pycnopodia helianthoides attacking a group of tunicates Cnemidocarpa finmarkiensisJuvenile sunflower stars P. helianthoides in apparent attack on an a group of Cnemidocarpa finmarkiensis 0.3X

Alarm pheromones

Research study 1: Alarm pheromones

Studies on the clonal tunicate Clavelina huntsmani disclose the presence of a possible alarm pheromone, produced by injured individuals and eliciting neurophysiological responses in neighbouring members of the same species. Pelletier 2004 Mar Biol 145: 1159.

Light-bulb tunicates Clavelina huntsmani are social (clonal), with the zooids being interconnected by stolons along which nutrients flow for the common benefit of all 0.25X