Parrotfishes (Family Scaridae), which derive their name from their beak-like jaws, include approximately nine genera and 83 species. They are abundant in tropical reefs around the world, and often form one of the most dominant species biomass on such coral reefs; moreover, they are well known to divers for their striking coloration and noisy feeding as they scrape epiphytic algae from dead coral. Parrotfishes exhibit several types of complex mating systems that vary by geographic location, even within a single species. They also have considerable ecological impacts on coral reefs through herbivory and bioerosion.
Parrotfishes are characterized by their distinctive beak-like jaws, in which the teeth are fused together in most species, and a pharyngeal apparatus, which acts as a second set of jaws in the throat. In the pharyngeal apparatus, the teeth are arranged in rows and are highly specialized to grind, crop, and crush food as it is processed. Parrotfishes have large scales, usually with 22 to 24 scales along the lateral line. Some parrotfishes have a complex socio-sexual (socially influenced sexual change) system punctuated by three phases, and each phase change results in a different color pattern . For instance, juveniles tend to have a drab mixture of browns, grays and blacks, but as they mature a distinct coloration emerges with the addition of red tones.
A third set of colors is donned by males and by females that have recently undergone sex change into males. As these males mature, they exhibit bright, intricate patterns of reds, greens, and blues. This type of color change has been documented in Scarus, Sparisoma, Nicholsina, Bolbometapon, and Cryptotomus, but there are some monochromic (fishes that do not exhibit sexual color change) species that exhibit different types of sexual dimorphism. Scarus coelestinus and Scarus coeruleus in the eastern Pacific and Scarus niger in the Indo-West Pacific exhibit no color differences. However, mature males of Scarus coelestinus and Scarus coeruleus are larger and develop more squared-off and prominent foreheads than younger males, while Scarus niger exhibits no physical differences other than size. Finally, fleshy tips on the upper and lower lobes of the caudal fin can be observed in mature males of Scarus rubroviolaceus, but are poorly developed on small males and females.
Parrotfishes are found primarily in tropical waters throughout the Atlantic, Indian, and Pacific Oceans. However, some species inhabit subtropical waters, and some, such as Scarus ghobban, may venture far from reef environments.
Most parrotfishes exclusively inhabit offshore coral reefs in tropical regions. However, a few species feed primarily on sea grasses and are most common in the Caribbean Basin. Two other species, Nicholsina denticulata and Sparisoma cretensis are common over rocky reefs of the Gulf of California and Mediterranean Sea, respectively.
Parrotfishes are primarily herbivorous, grazing intensively on dead, algae-coated coral, vegetable material, and in some species sea grasses. Bolbometopon muricatum in the Pacific, and Sparisoma amplum in the South Atlantic, exemplify an interesting exception; both consume significant amounts of live coral. Key to the success of parrotfishes is their ability to take up plant material, detritus and calcareous sediment and process it through the action of the pharyngeal jaw. This chewing mechanism grinds ingested material into a fine paste and breaks down algal cells, releasing the cellular material for digestion. This process makes available to parrotfish a broad menu of algae not available to other grazers.
Like Acanthuridae, parrotfishes can form large feeding groups, sometimes with multiple species.
Parrotfishes are most well known for their complex social structures. Most are organized into male-dominated harems but others breed cooperatively or in pairs. Some parrotfishes are highly territorial while others are mainly nomadic, with the home range increasing as the size of the fish increases. Large foraging groups of up to 500 individuals can form for spawning. Parrotfishes feed continuously throughout the day and seek shelter in reefs at night. Most known forms of communication in parrotfishes are related to reproduction and are discussed below. However, in some species male coloration intensifies when defending its territory, which suggests that visual cues are used to deter invaders.
A unique feature of some parrotfishes is the production of a mucous envelope at night before resting. The envelope takes about 30 minutes to construct and is open at both ends to allow water flow. The secreted envelope is foul smelling and tasting, which may serve to deter nighttime predators that hunt by scent. Reef-dwelling parrotfishes seek out caves and ledges in the reef for protection at night; some seagrass grazers such as Cryptotomus roseus bury themselves in the sand like Labridae. After creating a hole in the sand Cryptotomus then produces its mucous nightgown.
Parrotfishes utilize some of the most complex and unusual reproduction systems known to fishes. Males can be either primary, i.e. born male, or secondary, i.e. females that have undergone sex change. In some species there are no secondary males while in others all individuals are born female (monandric) and change sex when necessary. In the most complex systems, species are diandric – both primary and secondary males exist in the population. In these species, individuals proceed through three distinct phases, marked by color differences.
In fact, the color differences are so pronounced that for over 200 years researchers regarded some phases as distinct species. Sexually immature and drab colored juveniles represent the first phase. The second, known as the initial, phase (IP) can include sexually mature males or females, which are impossible to tell apart without internal examination or observation during spawning. The terminal phase (TP) includes only mature males, which display brilliant colors. TP males usually dominate reproductive activity through a harem-based social system. The death of a TP male serves as a social cue for an IP female to change sex and behavior. The morphology and behavior of IP males may also change in response to the death of a TP male.
In some cases IP males attempt to infiltrate a TP male’s harem by masquerading as a female. In the so called “sneak spawning” attempt, IP males follow spawning pairs into the water column and release a large cloud of gametes at peak spawning in an attempt to overwhelm fertilization by the TP male. IP males are well equipped to perform “sneak spawning” as they have larger testes and so are able to produce more gametes, while TP males have smaller testes and rely on aggression to deter other males.
The type of reproductive behavior described above and whether it involves paired, foraging group or mass spawning depends on a complex set of behavioral and geographic factors. For instance, some species, such as Scarus croicensis, exhibit a wide range of reproductive behaviors depending on the area in which they are found. In Panama, Scarus croicensis employs a system involving three classes of individuals: territorials, stationeries and foragers. Territorials are organized into groups that consist of a dominant female, several subordinate females and usually, but not always, a terminal (TP) male. Paired spawning occurs within the territory, which both males and females defend. Stationaries consistently use the same area for spawning but do not defend it, and foragers include groups of up to 500 individuals, mostly females. In Puerto Rico, initial phase (IP) and terminal phase (TP) individuals migrate to temporary spawning areas in deep water, usually in pairs. Finally, in Jamaica Scarus croicensis emphasizes aspects of the foraging group system and spawning only takes place in groups. These three examples illustrate the flexibility of the socio-sexual mating systems found in parrotfishes. The reasons that different aspects of the basic spawning system manifest in different areas range from population density to competition for spawning sites and other resources to geographic factors like seasons and water temperature.
In general, parrotfishes spawn year-round, usually at dusk. However, peak spawning occurs in summer for many species and there is evidence that some species have defined non-spawning periods. As discussed above, many species migrate to the outer edges of the reef to spawn but some spawn within defined territories. There is evidence that some scarids respond to the lunar cycle during spawning, but in others, spawning correlates closely with high tide, regardless of the time of the lunar month. In species that spawn several times during the day, the tidal cycle is followed closely since this is the optimal time for egg dispersal. There is no evidence of parental behavior in parrotfishes
The maximum age of most parrotfishes is less than 20 years and most live less than five years. There is a general trend in the scarids for larger species to live longer. Subsequently, the largest scarid, Bolbometopon muricatum, is the one exception to the 20 year maximum age.
Parrotfishes have a major impact on coral reefs through intensive grazing and associated bioerosion, as their digestive process grinds coral skeletons into sediment. The grazing patterns of large schools of parrotfish have the effect of selecting for certain species of corals and algae, and preventing algae from choking out corals. Many parrotfishes feed on calcareous algae (hard algae that contain calcium carbonate, like corals do) growing on dead, exposed coral by biting off chunks of algae and coral substrate. This type of grazing contributes significantly to the process of bioerosion and the creation of sediment on reefs. For instance, it has been calculated that a single large parrotfish, Bolbometapon muricatum (bump-head parrotfish), consumes approximately one cubic meter of coral skeletons per year, and turns it into fine sediment.
In this way large schools of Bolbometapon muricatum determine the fine-scale topography of coral reefs. A separate ecological consequence of intense herbivory in parrotfishes is the conversion of plant material into fish flesh. The success of parrotfishes in consuming plant material unavailable to most other fishes and the large size of parrotfish populations makes them an important link in the food chain.
Parrotfishes are naturally abundant, but because of their complex mating systems, the impact of fishing pressure, which is heavier on large individuals, can be difficult to predict. Parrotfish harvest is often largely sportfishing and subsistence fishing and goes largely unreported, complicating the prediction further (Westerns Pacific Regional Fishery Management Council, 2008). They are not a major commercial catch and their flesh tends not to keep well, but they are a popular fresh catch in some areas. Territorial populations, since they are more localized, may be more vulnerable (FAO, 2002). One scarid, Scarus trispinosus (Greenback parrotfish), is listed as Endangered. Bolbometopon muricatum is listed as Vulnerable, and Chlorurus bowersi and Scarus hypselopterus are Near Threatened. (The World Conservation Union, 2011).
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