The Bahamas: A Closer Look at the colorful and Unique Parrotfish

This topic submitted by Tim Smith ( smithtj1@miamioh.edu) at 1:15 PM on 5/30/02.

The "Three Amigos" pose in the Bahamas

Tropical Field Courses -Western Program-Miami University


THE BAHAMAS: A CLOSER LOOK AT THE COLORFUL AND UNIQUE PARROTFISH

Tim Smith

Coral reefs are inhabited by some of the most diverse biological communities in the world. For example, more than 4,000 species of fishes (one-third of all marine fish species) live on coral reefs (Pitkin, 2001). Of these species, one of the more colorful and better-known fish that depends on the reef for food and protection is the parrotfish. This superior competitor among the herbivorous reef fish is found in both the Atlantic and Pacific Oceans along with Caribbean waters.

Characteristics

The herbivorous parrotfish can be distinguished from other Bahaman reef fish by their size combined with colorful appearance, large and heavy scales in regular rows on the head and body, and beak-like mouth. The unusual mouth in which their teeth are fused together to form a beak-like jaw bears strong resemblance to that of the tropical bird, and hence, is how their name was derived. They possess a unique pharyngeal dentition in which the upper interlocking pharyngeal bones located above the gills rest plush against the lower pharyngeal bone to form the pharyngeal mill (can be thought of as molar-like teeth in their throats). This mill is used to grind up the hard coral skeleton that contains microscopic algae called zooxanthellae (gives coral its color) on which parrotfish feed (Davidson, 1998). The crushed calcareous material travels through the fish's digestive system and is voided on the reef as white coral sand. Some fish will return to the same location to deposit this calcareous powder resulting in the formation of small hills over time. A study on a Bermuda reef reported that parrotfish produce an estimated one-ton of coral sand per acre of reef per year (Bšhlke & Chaplin, 1993).

Most parrotfish live on reefs from which they rarely wander far with the exception of the species Scarus guacamaia (rainbow parrotfish). This species is thought to use the sun for navigation to travel from its nocturnal cave in deeper water to the shore to feed during the day, returning by direct course in the evening. A unique feature of all parrotfish is their method of swimming. They use the pectoral fins located behind the gills for propulsion unlike most other fish which rely on their caudal or tail fins.

Parrotfish feed continuously and only during the day, mainly in small groups although a few species travel in solitary, nibbling on seaweed, rocks, corals, and an occasional mollusk (Hiatt& Strasburg, 1960). In addition to scraping algae from substrate, some parrotfish browse on sea grasses and other plants that they are able to directly bite. At night, each fish separates to search for a suitable place within the reef to sleep.

The large, thick scales of the parrotfish are strong enough to stop a spear in some species and have been used by man to decorate basketwork and shellflower arrangements. Because its flesh is soft and spoils quickly, the parrotfish is not known as a food fish in the Bahamas. In Hawaii however, they are eaten raw and at one time were reserved for royalty. One parrotfish from the Indo-Pacific and the blue parrotfish (Scarus coeruleus) are the only Scaridae suspected of carrying ciguatera-producing toxins that result in illness when consumed.

Adaptations and Reproduction

As mentioned earlier, parrotfish retire to the reef bottom to sleep at night. Some burrow into the sand like wrasses while some species of Scarus have developed the ability to secrete a filmy mucus cocoon. Some individuals also produce mucus cocoons under anoxic conditions. The mucus envelope is secreted in thirty minutes and masks its scent, affording the parrotfish protection from coral reef night predators such as sharks and moray eels. Six series of experiments were performed to determine the effectiveness of the mucus envelope in reduction of predation using the common spotted moray eel and three species of parrotfish (Winn & Bardach, 1959). Only one of the parrotfish (Scarus croicensis) was capable of secreting a mucus cocoon. The results indicated an increased tendency for the moray eel to prey on the species of Sparisoma (apparently do not secrete mucus cocoons) rather than Scarus croicensis. The moray uses the senses of smell, taste, and touch in its feeding activities. A grasping reflex is initiated and the food is swallowed immediately upon touch. In these experiments, the grasping reflex was not initiated when the head of the moray was exposed to the mucus.

Another fascinating adaptation of the parrotfish is its ability to undergo sex reversal in which female fish become males. Parrotfish that are born male remain this gender throughout their lives and are called primary males. The female born fish may change sex and color to become male and are termed secondary males or sometimes referred to as supermales or terminal males. This gender change occurs after a sexually mature female lays its eggs and is apparent by a color transformation from light to dark. Females and primary males are similar in coloration (red, gray, brown, and black), while secondary males are bright green, yellow, blue, and red. This evolutionary adaptation is thought to occur because many of the fish die young and therefore, there is a need for many females to release eggs that can be fertilized by only a few males. Some parrotfish are even capable of playing the role of chameleon, changing their colors to match their surroundings.

Parrotfish spawn throughout the year although this activity is more frequent during the summer months. Supermales pair up with one female while primary males spawn in small groups containing several males and only one female. In some species of Scarus, one male and several females form together similar to that of the cow-bull relationship in mammalian groups. This behavior helps in increasing reproductive efficiency. In Scarus, the eggs are fusiform in shape and the chromatophores are branched, while in Sparisoma, the eggs are spherical and the chromatophores are disc-shaped. Fertilized eggs hatch after twenty five hours with the larvae being 1.7 mm in length, lacking eyes, pigment, and a mouth which appears after three days in the larval stage. The length of the parrotfish larval stage is unknown.

Classification and Identification

The identification of parrotfish by early naturalists was based on the distinct color differences between young and adults and the contrasting color phases of both males and females. As a result, over 350 different species of parrotfish were improperly identified based on their different color forms. In actuality, there are 80 species of parrotfish belonging to the family Scaridae (Bšhlke & Chaplin, 1993).

Parrotfish reach maturity in three to five years and have average longevities of eight to twelve years. The length of an adult ranges from seven inches to an astonishing four feet (for example, rainbow and blue parrotfishes). The wrasses (Labridae) are closely related to the parrotfish from which scientists believed the Scaridae evolved (Randall, 1983). In contrast to the parrotfish, however, wrasses are not herbivorous, feeding on zooplankton and larger invertebrates, and their teeth are not fused together.

Of the Atlantic species, Scarus and Sparisoma are the two major genera recognized. They can be distinguished from one another by how their upper and lower teeth join. The upper teeth of the genus Scarus stick out and cover the lower resembling a parrot's beak. Scarus are the more colorful and largest of the parrotfish (up to four feet in length), swim in schools, and some males develop a hump on their forehead. In contrast, the upper teeth fit inside the lower teeth of Sparisoma, which represent the smaller species of parrotfish (generally under one foot in length). They are less colorful than their counterparts (reds, browns, and grays) and swim alone or in small groups.

Sparisoma is found in the Caribbean and Atlantic Ocean and includes species such as Sparisoma aurofrenatum (redband parrotfish) and Sparisoma viride (stoplight parrotfish). Scarus has been found in tropical waters worldwide and includes Scarus coeruleus (blue parrotfish) and Scarus coelestinus (midnight parrotfish). Since these four species are commonly sited among Bahaman reefs, they will be described in detail below.

Sparisoma aurofrenatum (redband parrotfish)

The mature male is brown or greenish-brown with blue radiating on the back and sides, becoming red ventrally. It also exhibits a red slash from the corner of the mouth to below the eye. The terminal-phase male is greenish gray on back with reddish tones on the sides and a yellow spot above the pectoral fin containing black spots. Juveniles and females have a white mark at the middle of the gill cover and both sexes have a white spot across the dorsum behind the last dorsal-fin ray (best underwater recognition marking). The redband parrotfish can attain a maximum length of eleven inches with the supermale being larger than the juvenile and mature male and female.

Sparisoma viride (stoplight parrotfish)

Both males and females have a brown head with the upper two-thirds of the body having scales with pale centers and dark edges and the lower third being bright red. The young display a checkerboard of white dots on the body with a white bar at the caudal-fin base. The terminal phase male is green with three diagonal orange bands on the upper half of the head and can be distinguished by its two bright yellow markings: a small round spot behind the mouth and below the eye (upper end of opercle) and the bar at the base of the caudal fin. This parrotfish can attain a maximum length of twenty-one inches and 3.5 pounds in weight with the supermale being larger than the juvenile and mature male and female.

Scarus coeruleus (blue parrotfish)

Adults are light to dark blue in color with a distinct hump on the forehead (absent in young). The mouth of the young is set lower with the snout being blunt and more prominent than other Bahaman parrotfishes. This species has been observed secreting a mucus cocoon at night to protect against predation and is reported to attain a maximum length of four feet.

Scarus coelestinus (midnight parrotfish)

The midnight parrotfish is the only Scarus in the Bahamas that is not striped when young. The young and adults of both sexes are dark violet in color with light blue markings on the cheeks, chin, and centers of body scales. The adult has green teeth, sometimes schools with surgeonfish, and can attain a maximum length of thirty inches.

Conclusion

The vibrantly colored parrotfish plays a major role in maintaining the cycle of reef growth and erosion, perpetuating the diversity of the reef community. They prefer to feed on dead coral containing microscopic algae (Bruggemann, van Kessel, van Rooij, & Breeman, 1996) and their grazing removes weed filaments that limit the growth of algal mats, exposing bare surfaces. These cleared sites are also colonized by coralline algae, which help to stabilize areas of loose rubble, and by invertebrate larvae, some of which are the early stages of reef building corals. Do not be alarmed if you experience a sudden drift of sediment or hear the crunching sound of coral the next time you are snorkeling or diving along a coral reef in the Bahamas. It is just a parrotfish doing its job.

References

Bšhlke, J.E., and C.G. Chaplin 1993. Fishes of the Bahamas and Adjacent Tropical Waters. University of Texas Press, Austin, 464-482.

Bruggemann, J.H., A.M. van Kessel, J.M. van Rooij, & A.M. Breeman 1996. Bioerosion and Sediment ingestion by the Caribbean parrotfish Scarus vetula and Sparisoma viride: implications of fish size, feeding mode and habitat use. Marine Ecology Progress Series 134: 59-71.

Davidson, Osha Gray 1998. The Enchanted Braid: coming to terms with nature on the coral reef. John Wiley and Sons, Inc., Canada, 14-16.

Hiatt, R.W. and D.W. Strasburg 1960. Ecological Relationships of the Fish Fauna on Coral Reefs of the Marshall Islands. Ecological Monographs 30 (1): 103.

Pitkin, Linda 2001. Coral Fish. Smithsonian Institution Press, Washington D.C., 4.

Randall, John 1983. Caribbean Reef Fishes. T.F.H. Publications, New Jersey, 199, 217-235.

Winn, H.E. and John E. Bardach 1959. Differential Food Selection by Moray Eels and a Possible Role of the Mucous Envelope of Parrot fishes in Reduction of Predation. Ecology 40 (2): 296-298.




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