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Explanations for answers to tunicate-defenses quiz. The correct "yes" answers are highlighted in purple:

Some species have unpalatable granules or spicules of calcium carbonate. Yes.  Didemnum species are notable in their possession of such spicules1 they contribute a whitish caste to their colours), but it is not known whether the spicules actually make the tunicates unpalatable.  Lewin & Cheng 1975 Phycologia 14: 149.

Camouflage is a common defense.  No.  A feature of many west-coast species is their visual obviousness.  To a human viewer, at least, selection has been for characteristics other than camouflage.

Presence of sulphuric acid makes some species unpalatable.  Yes, this may be true for some species.  The acid is present in relatively high concentrations in the tunic, and is considered to be involved in tunic growth and repair in conjunction with certain participating heavy metals.  Studies on Bermudan ascidians suggest that high tunic acidity is correlated with low levels of fouling microorganisms and algae.  Stoecker 1980: Mar Ecol Progr Ser 3: 257; Stoecker 1980 Ecology 61: 1327. photograph of sea squirt Halocynthia igaboja

The tough outer covering is a barrier to predators.  Yes.  The outer covering, or tunic, is composed of a tough, cellulose-like material called tunicin.  In many species it is leathery in texture, and this feature may make it hard for predators to bite, chew, and digest their intended prey. In some species it is textured or drawn into spikes (see photo of Halocynthia igaboja on Right, 1X).

Different types of escape behaviours are employed by adult tunicates.  No.  They attach themselves immovably to the substratum.

Tunicates are known to have toxic flesh.  Yes.  Tunicates, both larvae and adults, are a rich source of secondary metabolites.  These are organic chemicals that are not known to be involved in an animal’s general metabolism, but may serve as deterrents to being eaten by predators.  They are best described for tropical ascidians.  For example, a group of alkaloids called “didemnimides”, named after the Caribbean genus Didemnum in which they were first discovered, are the most potent antipredator chemical defense yet described from tunicates.  Vervoort et al. 1998 Mar Ecol Progr Ser 164: 228; Lindquist et al. 1992 Ecol Monogr 62: 547.

Tissues of tunicates are nutritionally deficient.  No, there is no evidence for this.  One Japanese species of solitary tunicate, Halocynthia roretzi (known as maboya), is considered a delicacy2 to Japanese gourmands (of course, this does not prove that the tissues are NOT nutritionally deficient).

Siphons are closeable for protection. Yes. Many species are able to close off their siphons in response to shadows, and to physical and chemical disturbances. 

Presence of heavy metals adds to unpalatability.  No. While many metals are found in the tunics of ascidians, including vanadium3 and iron, these do not appear to play a role in defense.  In fact, studies with the predatory snail Fusitriton oregonensis (see below) show that certain ascidians readily eaten by the snail, for example, Ascidia spp. and Chelysoma productum, actually have high concentrations of vanadium.

NOTE1  in 5d settled juveniles (the oozooid) of the colonial tunicate Cystodytes lobatus along the central California coast (Pacific Grove), spicules first appear at the anterior end of the abdomen and migrate to the inner layer of the tunic where they form a sac surrounding but separated from the abdomen of the zooid.  When the oozooid buds the spicular sac becomes disrupted and the spicules are re-allocated to the two daughter zooids.  Lambert 1979 Biol Bull 157: 464

NOTE2  those who have eaten the flesh of maboya (represented by the orange soft parts after the test has been removed) would probably agree that it is definitely an “aquired taste”.  Our local species Halocynthia aurantium and H. igaboja may be equally palatable, were someone to try them, and the tissues could be tested for nutritional value with various types of predators 

NOTE3  vanadium is concentrated in ascidians at levels 100,000-1,000,000 times that in seawater, is present in specialized cells found in and around the tunic, and appears to be involved in growth and wound repair of the tunic.  Danskin 1978 Can J Zool 56: 547; Smith 1970 Biol Bull 138: 3

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