Known Predators:
S. rivulatus has also been known as S. rivulatus, S. rivulatoides, and S. micrognathos. S. rivulatus is found in most articles as the valid scientific name, as in 1862 the first revisor, Bleeker, chose rivulatus as the species name (Randall and Choat, 1980).
Perception Channels: tactile ; chemical
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: least concern
The larval stage of S. rivulatus is estimated to last between 28-47 days(Lou, 1993). Once the young have become juveniles they grow continuously throughout their lives. Choat et al. (1996) were unable to determine an asymptotic size for S. rivulatus, but it is likely that older fish die before reaching such a size.
No adverse effects on humans are known.
Parrotfishes, including S. rivulatus, although often part of reef catches, are not important to the fishing industry (Dalzell et al., 1996).
S. rivulatus, as are all other scarids, are herbivores. They feed on many types microscopic algae that grow on calcareous material, such as coral skeletons. S. rivulatus only graze during daylight hours, and have been observed to increase their feeding rate in the late afternoon.
Scarids have fused beak-like jaws which they use to graze on algae. Scarus rivulatus has been characterized as a ‘scraping’ scarid, as it scrapes algae and other materials from the surface of the reef substratum. The jaws are relatively weak, but are able to move a great deal, due to a highly mobile synovial joint between the maxilla and the premaxilla. Its shallow dental plates and weak teeth form an even cutting edge, which although not capable great force, are able to shear algae off of the substratum with ease (Bellwood and Choat 1990; Choat and Clements 1993).
Plant Foods: algae
Primary Diet: herbivore (Algivore)
Scarus rivulatus are distributed throughout the western Pacific. Their range includes the southern Great Barrier Reef, as well as New Caledonia and Thailand and extends north to the Ryukyu Islands. They have also been found along the east coast of Malaysia, Okinawa and Ponape (Randall, 1997; Randall and Choat, 1980).
Biogeographic Regions: oriental (Native ); australian (Native ); pacific ocean (Native )
Scarus rivulatus inhabit coral reefs and are most abundant in the mid-shelf region. They may also inhabit inshore reefs. Unlike other scarid species, they often move onto the reef flat at high tide to feed and therefore may be seen in tidal pools (Choat and Randall, 1986).
Habitat Regions: tropical ; saltwater or marine
Aquatic Biomes: reef
Other Habitat Features: intertidal or littoral
It appears that the maximum lifespan of wild S. rivulatus is about 8 to 10 years. Terminal phase individuals, tend to be, on average, older than fish in the initial phase, and have a short post-transition life span, which is attributed to increased predation due to their brighter colors. It is unknown how long S. rivulatus may live in captivity (Choat et al., 1996).
Range lifespan
Status: wild: 10 (high) years.
Mature S. rivulatus can range in size from 167 to 400 mm SL. Initial phase individuals usually measure between 167 and 292 mm SL. In the initial phase, primary males and females are generally the same color and size, although males, on average, are slightly smaller than females of the same age. Both initial phase males and females may transition to a terminal phase male, which are larger and more colorful, from 190 to 400 mm SL.
Like many other parrotfish, the coloration of S. rivulatus changes dramatically at different stages of life. Juvenile S. rivulatus (to about 60mm SL) have a light brownish to pale olive-tan body, dorsal, and anal fin. The pectoral fins are hyaline, a homogeneous transluscent bluish white color. The caudal fin and caudal peduncle usually have a pale yellow hue. A striped pattern may also occur in solitary fish or in those who are schooling with other young scarid species.
Initial phase S. rivulatus have different colorations depending on their reproductive status and the size of their school. In large schools they usually have a pale gray to light brown body with two or three pale stripes on the lower abdominal region. Solitary fish and those in small social groups may have a more yellow hue on the body with areas of darker shading. Reproductively active individuals may have a uniform dark gray body with pale abdominal striping and a pale margin on the spinous region of the dorsal fin.
S. rivulatus in the terminal phase exhibit a strikingly beautiful color pattern. The body scales are green and each has an orange basal bar. The face has bright green lines on an orange background and a bright orange operculum. Parts of the dorsal and anal fins may be tri-colored with an orange central region surrounded by blue borders. The caudal fin is also orange with blue margins along the top and bottom of the fin. The pelvic fins are orange and have a bright blue lateral margin. The pectoral fins are chartreuse. In reproductively active terminal individuals the anterior half of the body becomes a dark green color, giving the bodies a distinct bi-colored appearance.
Diagnosis of S. rivulatus (from Choat and Randall, 1980): Median predorsal scales 6; scale rows on cheek the lower row with 2 or 3 scales (usually 2); pectoral rays 14; caudal fin varying from slightly rounded to truncate with the lobe tips slightly prolonged; dental plates covered by lips; initial phase without canines posteriorly on side of dental plates; terminal phase usually with 2 small upper and 0 to 1 lower canines (Bellwood and Choat, 1975; Choat and Randall, 1986; Randall and Choat, 1980).
Range length: 167 to 400 mm.
Other Physical Features: bilateral symmetry
Initial and terminal phase males have different mating behaviors. Initial males usually mate in large spawning groups made up of several males and females. Terminal males almost always pair spawn with a single other female, but have also been observed to participate in group spawning alongside initial phase males.
Mating System: polygynandrous (promiscuous)
S. rivulatus are sequential hermaphrodites. Pair spawning takes place in deeper areas than those used for feeing. Aggressive mating behaviors are not very common, but a mating pair will defend the area in which they are spawning.
Fertilization occurs externally after the eggs have been released from the female
Breeding season: Year round breeding
Key Reproductive Features: iteroparous ; year-round breeding ; sequential hermaphrodite (Protogynous ); sexual ; fertilization (External )
The eggs are simply scattered onto the substratum or released into open water during spawning. Neither males nor females guard the eggs or fry (Choat and Robertson, 1975; Choat and Randall, 1986).
Parental Investment: no parental involvement
