Spitting on your enemy just got a whole new meaning when considering the defense mechanism adopted by the Parrotfish. As a preemptive defense strategy they cocoon themselves in a spit bubble when at their most vulnerable, while sleeping. This spit cocoon envelopes the Parrotfish completely and is thought to mask olfactory cues unintentionally sent out to their predators (Grutter et al 2011).
However, a recent paper published by Grutter et al (2011) found that there is little evidence to support the claim that the cocoon does protect them against predation. One paper did comment on the observed behaviors of Parrotfish in the wild (Sazima & Ferreira 2006), saying that they fled when their cocoon was touched by an outsider. Sazima & Ferreira (2006) suggested that this cocoon might work as an early warning system. One major counterargument to this is the energy expenditure on this defense mechanism, estimated to be at 2.5% of their daily energy budget (Grutter et al 2011). This indicates that the mechanism is of high value by not being selected against in nature. Therefore, functioning purely as an early warning system is highly unlikely. Still, there would have to be a use for it.
Grutter et al (2011) found statistically sound proof of the cocoon being an outstanding parasitic preventive tool, especially towards the Gnathiid parasite, which has been found to be a vector for blood transmitted parasites (Grutter et al 2011). Therefore, the cocoon is more likely to have evolved to prevent parasitism than predation.
Another thing to note is that Parrotfish are not the only fish that can produce these mucus bubbles, as seen in this video showing a Wrasse performing the same defensive strategy.
Once again we are presented with the very rare homoplasy situation where two different species, however, of the same suborder (Labroidei), have evolved the same defensive strategy. Of the Parrotfish only the genus Scarus seems to be able to produce this mucus cocoon (Sazima & Ferreira 2006), which would be phylogenetically distant from the Wrasse. This increases the likelihood that this cocoon is of great importance for improved fitness, however, with lacking research it is impossible to say why it is of such high importance. Quite possibly it could be a wide range of applications.
Regardless of what use the mechanism is for, what question usually interests me is how these impressive and interesting defense mechanisms evolved. There is a known wide range of application of mucus within the fish “community” in general, for example the Clown fish that cover themselves in it in order to protect themselves against the nomatocysts of the sea anemones. However, to specifically blow up a mucus bubble, that, in addition to imaginably being complicated to make, takes a long time to create, suggests that the first individual would not have accidentally made one and discovered its potential. At the moment this is an extremely under explored subject, which is interestingly enough, seeing as it has some medical prospects as discovered by Videler et al (1999). Apparently, the mucus cocoons Parrotfish produce hold antibiotics, which at the moment does not seem to be of use for humans. However, with more research it could hold a great deal of potential for aquaculture for example., while simultaneously providing some much needed insight into the rare adaptation, which at the moment there is close to none.
Next week we will look into some more mucus production, this time produced by the infamous Hagfish.