Parrotfish and Their Role in the Ecosystem

 

Parrotfish are colorful fish found throughout the worlds coral reefs.  There are about 90 species.  They are characterized by a beak-like structure that they use to feed.  They are mostly herbivores and feed on algae, but also feed on corals and sponges.  Their beak heps them scrape algae and crush the hard limestone of corals.  They then excrete any of this as sand, which then creates the white sandy beaches of the tropics.  Below is a somewhat humerus visualization of this process.

Parrotfish are also a part of a group of organisms called sequential hermaphrodites.  A sequential hermaphrodite is an animal that goes through an initial and terminal phase during its adult life.  Each phase is characterized by being one gender.  So, for example, parrotfish start as female in their initial phase, then change to male in the terminal phase.  This change usually takes place due to an environmental cue, such as the loss of the dominant male.  Probably the most interesting thing though about parrotfish is their role in coral reefs.

Stoplight parrotfish, terminal phase (CC SA 3.0)

Coral reefs are under a lot of stressors including ocean acidification, rising ocean temperatures, and changing ecosystem balance due to overfishing and bycatch.  Analysis shows though that the number one thing we can do to protect the health of coral reefs is to limit the amount of parrotfish we remove from the environment.  In the Caribbean, parrotfish are the primary herbivore on the reef at middepth, helping keep macroalgae in check.  A shift to macroalgae dominated habitat would offer little value to fisheries, as most of the nutrients is lost to detrital pathways.  The problem faced is that parrotfish are also a commonly fished species in the Caribbean.  A recent study suggests that if we implement a size restriction of less than 30cm there is a win:win outcome in the short term.  This would have both ecological and economical benefits as it would also lead to an increase in coral reef health and production.  A more long term benefit requires a more strict harvest limitation to combat the ever-increasing threats to coral reefs.

Parrotfish are also one of the primary grazers of sponges.  Sponges are also now the primary habitat-forming organism on Caribbean reefs.  Loh and Pawlik found that parrotfish, along with other spongivores, would graze on sponges that lacked chemical defenses over sponges that possess secondary metabolites.  Due to this grazing, the palatable sponges tend to heal and grow faster, as well as have a higher rate of recruitment and reproduction.  This allows them to compete with sponges that are left relatively untouched.  They also determined that species composition depended more on the abundance of spongivores instead of geographic location.  A decrease in the number of these species would result in a top-down effect, leaving more of the faster spreading palatable sponges to out compete the slower defended sponges and reef-building corals, worsening the state of these coral reef communities.  Below is a figure from this paper which compares different sites and the percent of sponge communities and how it relates to abundance of spongivores.

Figure 2 from the Loh and Pawlik paper