Thalassoma bifasciatum is a is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of saltwater fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 in the wrasse

Wrasse

The wrasses are a family, Labridae, of marine fish, many of which are brightly colored. The family is large and diverse, with over 600 species in 82 genera, which are divided into nine subgroups or tribes....

 family (family Labridae) of order Perciformes

Perciformes

The Perciformes, also called the Percomorphi or Acanthopteri, is one of the largest orders of vertebrates, containing about 40% of all bony fish. Perciformes means perch-like. They belong to the class of ray-finned fish and comprise over 7,000 species found in almost all aquatic environments...

 native to the coral reef

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s of the Caribbean Sea

Caribbean Sea

The Caribbean Sea is a sea of the Atlantic Ocean located in the tropics of the Western hemisphere. It is bounded by Mexico and Central America to the west and southwest, to the north by the Greater Antilles, and to the east by the Lesser Antilles....

. Like its relative T. amblycephalum

Thalassoma amblycephalum

Thalassoma amblycephalum, or the Blunt-headed wrasse, is a species of fish in the Labridae family....

it is commonly called "blue-headed wrasse", "blunt-headed wrasse" or "bluehead". Blueheads are small (less than 110 mm standard length) and rarely live longer than 2 years. They form large schools

Shoaling and schooling

In biology, any group of fish that stay together for social reasons are said to be shoaling , and if, in addition, the group is swimming in the same direction in a coordinated manner, they are said to be schooling . In common usage, the terms are sometimes used rather loosely...

 over the reef and feed primarily on planktonic copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s as well as small benthic

Benthos

Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s.

Like many other wrasse

Wrasse

The wrasses are a family, Labridae, of marine fish, many of which are brightly colored. The family is large and diverse, with over 600 species in 82 genera, which are divided into nine subgroups or tribes....

 species, the bluehead is a protogynous sequential hermaphrodite

Hermaphrodite

In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

: individuals may begin life either as males or females, but females can change sex later in life and function as males. Young/small females and males are yellow and white in color, often with black lateral stripes and occasionally dark vertical bars. This coloration is known as the Initial Phase. These individuals can rapidly alter the presence or intensity of their yellow color, stripes, and bars, and these color changes appear to correspond to behavioral changes. Large females and some males can permanently change coloration and/or sex and enter the Terminal Phase coloration, which has a blue head, black and white bars behind the head, and a green body. It is this color phase that gives the species its name.

The initial phase males ('IP males') have comparatively larger testes than the larger, terminal phase males. This enables the initial phase males to produce lots of sperm for the snatched opportunities they must take when trying to fertilise the eggs of the females in the guarded harem. Initial phase males also achieve fertilizations through participating in group spawns. These groups consist of 20-50 or more IP males that congregate at specific sites during the daily spawning period on medium and larger sized reefs. Females visit these groups to spawn and release eggs in a 'spawning rush'. IP males attempt to position themselves next to a female when she releases her eggs as this maximizes their probability of fertilizing these eggs. Releasing large numbers of sperm also increases this probability and this is thought to also help explain the large testis size observed in IP males. This type of mating competition is referred to as 'sperm competition' and is seen in many species.

The bluehead wrasse and its congener, the Hawaiian saddleback wrasse (Thalassoma duperrey), have become important models for understanding the physiological and neurobiological bases of sex change. Sex change can be induced socially in both species by making large females the largest members of social groups. Sex change in experimental pens by saddleback wrasses involves complete gonadal transformation with associated decreases in a key steroid hormones (estradiol and 11-ketotestosterone) and steroid hormone synthesizing enzymes in the gonads. Sex changing saddleback wrasses also show substantial changes in brain levels of the monoamine neurotransmitters serotonin, dopamine, and norepinephrine.

Sex change has been studied in bluehead wrasses primarily using field manipulations, where it can be induced in large females by removing dominant TP males from small reefs. Gonadotropin-releasing hormone (GnRH) neurons differ across sexual phenotypes in the hypothalamus of bluehead wrasses and also with androgen implants that induce sex change. Behavioral sex change is very rapid in bluehead wrasses under field conditions, with male-typical behaviors being observed within minutes to hours after dominant terminal phase males are removed. Interestingly, behavioral sex change occurs even in females whose gonads (ovaries) have been surgically removed prior to becoming socially dominant. Behavioral sex change is associated with increases in expression of a neuropeptide hormone termed arginine vasotocin or AVT and these increases occur regardless of whether sex changing females have gonads or not. Injections of AVT can induce sexual and aggressive behaviors in terminal phase male bluehead wrasses while injections of fluoxetine (tradename: Prozac) can reduce aggressive behaviors by terminal phase males.