The Bluehead (scientific name: Thalassoma bifasciatum) is a member of the wrasse family (Family Labridae) that lives on coral reefs in the Western Atlantic Ocean and Caribbean Sea. This species, also termed Blue-headed wrasse, possesses the ability for a gender transformation, which becomes a permanent sex change.
Bluehead. Source: ''Reef Fish Identification'', New World Publications © 1994
Initial phase/juvenile bluehead. Source: ''Reef Fish Identification'', New World Publications © 1994
Kingdom: Animalia (Animals)
Blueheads have long, semi-cylindrical or cigar-shaped bodies with round and flat scales. Their pointed snout contains teeth. Their size and color, however, depend on whether they are terminal phase males, initial phase males, or females. Terminal phase (TP) males, also known as "supermales," have blue heads and green bodies. Three stripes (black, white, and black again) divide the colors of the head and body. Terminal phase males measure about 70 to 80 millimeters in length while initial phase males are approximately 60 millimeters. Initial phase (IP) females and males are colored in two different ways. One type has a yellow upper half of the body followed by a slight green/black area and then a white lower half. Females and initial phase males are also known to be white both above and below the dark area. A dark spot is found on the anterior dorsal fin of both types of females and initial phase males.
Both females and initial phase males have the ability to change into terminal phase males and this switch includes a change of size and coloration. Once there is a transformation from female or initial phase male to terminal phase male, the change is permanent.
Blueheads are common to the tropical waters of the western Atlantic Ocean. Their primary range includes the region around Bermuda and the waters south of Florida, the Caribbean Sea, reaching to northern South America and extending west into the southeast area of the Gulf of Mexico.
Blueheads generally live among coral reefs, but are also known to inhabit other areas, such as inshore bays and seagrass beds. Herds of bluehead wrasse that consist of initial males and females feed in reefs, rubble beds, and gorgonian fields. Bluehead wrasses inhabit areas that vary in size, the size of the area usually reflecting population size, with about 200 fish living on a small reef and up to 10,000 fish on some of the larger reefs. Tagging studies have shown that blueheads do not move from reef to reef. Blueheads are typically found from zero to 40 meters in depth.
These fish forage for zooplankton, worms, mollusks, echinoderms, shrimp, and other small crustaceans at depths of one to 27 meters. Initial phase bluehead wrasses primarily eat zooplankton found in the water current, but herds of females and initial phase males also hunt daily during daylight hours. By hunting in packs, IP bluehead wrasses steal eggs from egg-laying fishes including redlip blennies (Ophioblennius atlanticus), bicolor damselfish (Stegastes partitus), and sergeant majors (Abudefduf saxatilis). A herd of bluehead wrasses are able to interrupt the defenses of the nest's guardian and steal eggs for several minutes before they are driven away.
Bluehead diet also consists of parasites found on other fish. Although initial phase bluehead wrasse are one of the primary cleaners of the Caribbean, they only account for 10% of the cleaning activity on the reefs because they are also frequently preyed upon by the fish they clean. Nonetheless, bluehead wrasses remove parasites and reduce disease among fish that stop at cleaning stations. By cleaning wounds of injured fish, it is speculated that they speed up the healing process. Terminal phase males rarely function as cleaners because of their well-developed teeth which enable them to eat hard-shelled invertebrates.
Blueheads live in populations that roam freely. They have no distinct home ranges; their territories seem to overlap with one another. Females do not seem to stray from their original sites of spawning even when terminal phase males leave or are taken away. New males quickly replace the previous ones in the spawning sites, but their previous partners do not follow. It could be possible that females prefer to stay in areas they are used to, especially since the threat of predators is constant. It has also been speculated that movement to different spawning sites is a behavior that is learned.
A social system exists among the three different kinds of bluehead wrasses. Terminal phase (TP) males exhibit the most aggression and rank above initial phase males. Initial phase (IP) males only get to mate as sneaker males or in groups within large populations. TP males change color to indicate courtship and aggression. When chasing IP males, they become a bright, metallic green. When courting females, they become a pink-grey opalescent and develop distictive black circles on the pectoral fins. The color changes occur within a few seconds.
Blueheads are able to change sex. Initial phase males and initial phase females are able to change into terminal phase males. This change involves a change in color, size, and functioning gonads (in the case of the females changing to males). This process of sex change is not reversible. Once an initial phase fish becomes a terminal phase male, they remain that way for life. A good deal of research has been done on these fish in order to further explore their ability to transform. Some research has been conducted by removing terminal phase males from a certain population. Results show that females do in fact make the change into males in the course of just a few days. As their body color and size change into that of a terminal phase male, they become more aggressive. Researchers are beginning to identify this change as a response to the social environment (the removal or absence of terminal phase males), which may then be related to the neuropeptide arginine vasotocin (AVT). It has been suggested that AVT is involved in the process of gonadal hormones altering behavior.
Blueheads swim with fins alone using a rowing or flapping action of the pectoral fins. This provides precise swimming movements that are useful when moving among coral reefs. The term labriform swimming derives from the wrasses (family Labridae) swimming behavior.
Prior to mating, it has been observed that terminal phase males seem to communicate with females in an attempt to attract them. Terminal phase males wait in their spawning sites for females to arrive. The terminal phase males then perform signal jumps, which involve quick, vertical dashes repeated over and over. The signal jumps are a way for terminal phase males to communicate to females that they are present and ready to mate and also to show that their site is free from predators. It was also observed that a terminal phase male sometimes extends this communication, which takes place prior to mating, by descending to the female and making contact with her dorsal area.
Females, on the other hand, are not known to exhibit behaviors analogous to the terminal phase males. A female communicates to the male that she is ready to mate by turning her head upwards and then dashing towards the surface of the water.
Terminal phase males exhibit aggressive behavior to other fish, especially just before mating. This form of communication is demonstrated when they chase other organisms from the site they pick to spawn. Initial phase males and females are not typically aggressive unless they are developing into terminal phase males. This characteristic of aggression helps distinguish social rank among the terminal phase males, initial phase males, and initial phase females with the terminal phase males being the only group to behave aggressively towards other fish.
Body color appears to be important in communication, as color communicates social status and mood during aggressive encounters.
Blueheads are capable of changing sexes. They are protogynous, meaning that females are capable of becoming males. Females, IP males, and TP males, however, are capable of reproducing. The density of TP males in the spawning sites is associated with the size of the reef. On small reefs (under 600 sq. m) with fewer than 200 bluehead wrasses, there are very few IP males and TP males defend territories of a small number of females. On large reefs (over 1000 sq. m) with over 400 bluehead wrasses, IP males may make up 50% of the population. Here, group-spawning is much more common particularly in downcurrent areas. In areas of high population density, TP males tend to be found primarily in upcurrent sites.
Terminal phase males aggressively defend breeding sites and therefore gain exclusive access to females visiting the site. They may mate with more than 100 females per day, but the mean range is from 30 to 50. Initial phase males exhibit no breeding site defense, are much less aggressive than TP males and often sneak-mate with a TP male/ female pair. The IP males mate in large aggregations where the operational sex-ratio can exceed 50 IP males per females. Initial phase females have no breeding site defense, show little aggression toward other IP fish and visit spawning sites containing either single TP males or IP male aggregations.
Transitional sex changers aggressively defend their breeding sites from the first day with an increase over the next several days. They display a full repertoire of mating behavior including spawning, but do not contribute gametes until the sex change is complete, which happens within 7 to 10 days.
Fertilization is occurs in the water column resulting in pelagic larvae. After six to eight weeks in the larvae settle in sandy areas where they burrow into the sand and metamorphose in to juveniles. Juveniles live together in the sea grass or near coral reefs. The maximum lifespan is estimated to be 3 years for individuals reaching the terminal pha
Blueheads are described as one of the most successful cleaner fish in the tropical Atlantic. They feed on the parasites of other fish that come to wrasses to be cleaned. Initial phase Bluehead wrasse are classified as primary cleaners, accounting for ten percent of all cleaning activities that take place within Caribbean reefs. Initial phase Bluehead wrasse have also been spotted cleaning parasites from the mouths of other fish as well as purifying the wounds of injured fish.
Bluehead are among the most common fishes living in the Caribbean Sea; correspondingly the species is not deemed to be at risk.
- Thalassoma bifasciatum (Bloch, 1791)
- Humann, P. and N. Deloach (Editor), 1994. Reef Fish Identification: Florida, Caribbean, Bahamas. New World Publications, Inc. Jacksonville, FL. ISBN: 1878348078
- Deloach, N. 1999. Reef Fish Behavior, Florida, Caribbean, Bahamas. New World Publications, Inc. Jacksonville, FL. ISBN: 1878348280