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Schärer Group
Evolutionary Biology
Zoological Institute
University of Basel
Vesalgasse 1
CH-4051 Basel

Thalassoma bifasciatum

The bluehead wrasse, Thalassoma bifasciatum, is a protogynous sequential hermaphrodite, i.e. females can change sex to become functional males. It has two adult colour phases: the initial phase (IP) includes females and small males, and the terminal phase (TP) includes only large males (see the pictures below). TP males can develop from IP males (i.e. primary TP males) and from females (i.e. secondary TP males). Mating takes place daily throughout the year, fertilization is external, eggs are pelagic and there is no parental care. Whereas males can spawn many times a day, a female releases her day’s egg production in one spawning act.

During the spawning period, TP males are territorial and defend spawning sites at the downcurrent reef edge and pair-spawn with single females. TP males therefore experience sperm competition only if an individual IP male joins a spawning pair just as they release their gametes. This so-called streaking occurs in 3–5% of all pair-spawns by TP males. In contrast IP males are non-territorial, and form spawning aggregations of dozens to hundreds of males, where typically 5-15 IP males group-spawn with single females. IP males therefore usually experience very high levels of sperm competition. The different life histories of IP males, TP males and females allows interesting sex allocation research.

Courting terminal phase (TP) male

initial phase (IP) individual (could be a male or a female)
only a good look at the genital papilla allows to distinguish IP males and females
Sperm morphology and Sex Allocation

the sperm of Thalassoma bifasciatum (DIC)

Sperm morphology and sex allocation of Thalassoma bifasciatum was the subject of my Diploma thesis at the University of Basel (1994 to 1996). I was supervised by David Senn and Ross Robertson, and the field work was carried out at the San Blas field station of the Smithsonian Tropical Research Institute in Panama.

As outlined in the Introduction the different life histories of IP males, TP males and females allow for interesting comparisons regarding A) the effect of sperm competition on sperm morphology and sperm concentration. Whereas sperm of IP males are almost always under intense sperm competition the sperm of TP males usually do not experience sperm competition. We checked if this is reflected in a different sperm size and/or a different sperm concentration in the milt of these two male morphs. B) we can compare the effect of sperm competition on sex allocation. The different sperm competition levels experienced by IP and TP males are well reflected in the size of their testes, with IP males having 3-4 times larger testes on absolute terms. We wanted to quantify how much energy is allocated to the production of sperm and to compare this to how much energy females allocated to the production of eggs.

The results of this study were published in 1999 in Journal of Fish Biology (pdf).

A role for the parasite Kudoa ovivora in sex change

The influence of the parasite Kudoa ovivora parasite on the timing of sex change in Thalassoma bifasciatum was the topic of my first PostDoc at the Université P.&M. Curie, in Paris. The field work was done in the Bocas del Toro Province in Panama.

Sex allocation theory for sequential hermaphrodites is based on the size-advantage model, which predicts that sex change can occur if the relationships between reproductive success and individual size differ between the sexes. If these relationships are known, the theory can predict in which sex an individual will start life, and at which size it will change sex. In the bluehead wrasse female fitness increases monotonically after the female reaches reproductive maturity, and females reproduce once per reproductive period (on up to 2 out of 3 days year-round). Large TP males can spawn up to 100 times per day and can therefore attain much higher fitness than females. Females are thus expected to change sex once they reach a competitive size. Similarly, IP males are competitively excluded from mating opportunities by TP males, so that IP males also have low daily fitness. Once IP males reach a competitive size they are predicted to turn into TP males. Moreover, females can be infected by an ovarian parasite, the myxozoan Kudoa ovivora, whose transmission stage infects and kills eggs. The parasite is thus expected to reduce the fitness that a female derives from a given reproductive investment. We therefore predicted that infected females should reach the point where it pays to change sex at a smaller size than uninfected females.

In order to test this prediction we studied the timing of sex change in the above host-parasite system by following infected and uninfected, individually marked females in the field over several months. We checked for differences in sex change between infected and uninfected females, both in terms of the timing and the size at sex change. Further, we estimated the reduction in fitness associated with infection by collecting natural spawns of marked females, and determining egg production and the proportion of eggs that are infected.

The results of this study were published in 2003 in Behavioral Ecology and Sociobiology (pdf).

Aquatic field work (for terrestrial pictures see here)

oorr (orange orange red red)
an IP female
marked with visible implant elastomere tags (photo by Kuno von Wattenwyl)

Jono Wilson is catching a spawn (photo by Kuno von Wattenwyl)

a dynamic pair spawn (photo by Kuno von Wattenwyl)

a map of our main study reef (drawing by Dita Vizoso)

this page was last updated on Tuesday, December 22, 2015