Predators & defenses
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photographj of a sunflower star Pycnopodia helianthoides approaching a Didemnum-type colonial tunicate

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. After some introductory material, the topic of predation on larvae is considered, and PREDATION ON ADULTS is dealt with in another section.



A sunflower star Pycnopodia helianthoides approaches a
Didemnum-type colonial tunicate but is unlikely to find it
palatable. A partially closed cup coral is being overgrown
by the tunicate, and a colorful shrimp is wandering over
the surface of the tunicate (upper Right) 0.15X

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Before getting too far into the subject, here is a review (in the form of a quiz) of the kinds of defensive strategies that adult sea squirts might employ against predators.  About half of the following statements are true. Try to identify them, then CLICK HERE for more information.

Some species have unpalatable granules or spicules of calcium carbonate.

Camouflage is a common defense. 

Presence of sulphuric acid makes some species unpalatable. 

The tough outer covering is a barrier to predators. 

Different types of escape behaviours are employed by adult tunicates. 

Tunicates are known to have toxic flesh. 

Tissues of tunicates are nutritionally deficient. 

Siphons are closeable for protection.

Presence of heavy metals adds to unpalatability. 

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Research study 1
 

photograph showing locations of cupular organs on the branchial basket of a sea squirt Corella sp., courtesy Mackie & singla 2004 Invert Biol 123: 269photograph of solitary tunicate Chelyosoma productum courtesy Dave Cowles, Walla Walla University, WashingtonTunicates are not well endowed with sensory organs, and the thick chemically inert tunic would seem to exclude the location there of possible defensive chemo- and mechano-sensors. However, sensory devices located on the atrial surface of the branchial basket known as cupular organs have been identified in species of Ciona and Corella (see illustration on Right) and identified as capsular organs in the branchial basket of Chelyosoma productum. The sensory organs are similarly structured, with each main sensory cell having a large cilium surrounded by microvilli. These sensory cells in tunicates are structured similarly to those comprising the vibration-sensitive lateral-line system of fishes, and both types of receptors are sensitive to mechanical vibrations in the water.  Thus, the cupular and capsular organs are thought to be similarly involved in perception of vibrations and/or changes in water flow through the atrial cavity of the tunicate.  Bone & Ryan 1978 J Zool, Lond 186: 417; Mackie & Singla 2004 Invert Biol 123: 269; Mackie & Singla 2003 Brain Behav Evol 61: 45. Photograph of C. productum courtesy Dave Cowles, Walla Walla University, Washington.

NOTE there are about 200 such organs in the atrial wall of the branchial basket in C. productum

NOTE determined from neurophysiological recordings from the organs when subjected to vibrations generated from a loudspeaker-probe system. Most of the axons from the capsular organs go to the brain via the visceral nerve. If this is cut, behavioral responses by the tunicates to vibrations cease

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  Predation on larvae
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Research study 1
 

Tadpole larve would seem to be highly susceptible to being eaten.  The tadpoles are plump with yolk, seem to have little in the way of physical defenses, and are usually released in daytime when visually foraging predators are abundant.  Even some adult sea squirts are known to eat them.  Corella inflata is a gregarious species; thus, its larvae preferentially settle near to adults and consequently are at risk of being eaten by them.  Intuitively, we would expect that gregarious species would avoid eating their own eggs and larvae or that the larvae themselves would somehow avoid being eaten.  This is tested in a study in which eggs and larvae of different species are pipetted directly into the branchial siphons of the same and other species.  The species tested included ones that live gregariously and ones that live non-gregariously. 

The hypothesis to be tested is that the gregarious species Pyura haustor, Styela gibbsii, and Chelysoma productum will tend not to eat their own conspecific eggs and larvae, but might eat eggs and larvae of other species.  Conversely, species living non-gregariously, such as Ascidia paratropa, Corella inflata, C. willmeriana, Cnemidocarpa finmarkiensis, and Halocynthia aurantium will tend to be less fussy.  With so many species there are many possible interactions but, generally, the results of the study show that the 3 gregarious species eat only 0-20% of conspecific eggs/larvae and 10-83% of other species eggs/larvae, while the non-gregarious species eat 100% of conspecific eggs/larvae and 50-100% of other species’ eggs/larvae. The results generally support the author’s hypothesis.  Rejections involve squirtings from the branchial siphons after the eggs/larvae touch the inside surface of the siphons or the sensory tentacles within.  Sometimes, however, complete closure of the branchial siphon will result in the eggs/larvae being eaten. Young 1988 J Exp Mar Biol Ecol 117: 9.

photograph of a tunicate Halocythia aurantium showing expanded siphonsNOTE another study by the author not done on a west-coast species, but with research-promoting potential, involves warning coloration (aposematism) in a species in Florida. Larvae of the colonial ascidian Ecteinascidia turbinata are about 4mm long, are conspicuously orange in colour, and appear to contain an unpalatable chemical. Larvae of E. turbinata are rejected by juvenile pinfishes. Fishes that have recently tasted the larvae of Ecteinascidia will reject the otherwise palatable larvae of Clavelina oblongata when the tadpoles are dyed orange to resemble the larvae of E. turbinata. There might be a nice parallel experiment to perform on similarly coloured west-coast species. Young & Bingham 1987 Mar Biol 96: 539

 

View from above of the relatively large siphonal
openings of the solitary tunicate Halocynthia aurantium.
The branchial (inhalent) siphon is on the Left 1X

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