Let’s Talk About Sex


Well, not my sex life, or your sex life. Those two conversations would not be nearly as interesting as talking about mangrove rivulus’ sex life, or lack thereof I suppose. Rivulus’ unusual sexual behavior is one of the principal reasons that this little fish is so popular among researchers. Rivulus are the only known vertebrate that is a self-fertilizing simultaneous hermaphrodite. In other words, this fish has both male and female reproductive parts at the same time and prefers self-fertilization to doing it the traditional way. You know what they say, if you want something done right, do it yourself!

When rivulus decide that it is time to start a family, it takes matters into its own fins. No partner required. I know…how empowering! These little fish are able to fertilize themselves (here’s where having both male and female reproductive parts is particularly helpful). The babies hatch as genetic ‘clones’ of their parent. I put ‘clone’ in quotations because it’s not technically cloning. Nothing could ever be so matter-of-fact with rivulus. Although genetically identical, rivulus’ babies cannot be considered true clones because they are the result of sexual reproduction (albeit sex within the same fish, but it’s still considered sex). Clones, on the other hand, are the product of asexual reproduction. Technicalities aside, the final product is essentially the same. Being “basically clones” is an important attribute of the mangrove rivulus. When a fish reproduces, its babies are all genetically identical. You can imagine how this could be helpful during experiments. We can expose individuals of each clone to a variety of different environmental conditions and see how it affects their behavior, physiology, morphology, or any other trait of interest. Since there is no genetic variation among siblings, the only differences we might see among them are the direct result of environmental factors. Pretty cool, right? Since they are all basically clones we know that any noticeable differences are the direct result of environmental influence! We can also put many different genotypes (animals with different genotypes) into the same environment to look at how genes are associated with the traits that we study. This allows scientists to tease apart genetic and environmental factors and how they are affecting the animal! Taking things one step further, we have the ability to put many different genotypes into many different environments and look at the interaction between genes and the environment. This type of research is becoming increasingly important in many types of biology including conservation and medicine. Rivulus’ extraordinary, and entirely unique (at least among vertebrates) reproductive behavior is of great interest to researchers as it offers a lot of insight into a reproductive process that, thus far, is unparalleled in vertebrates.

As if their reproductive tendencies were not weird enough, the mangrove rivulus is also capable of changing sex. No surgery required. I know I said that rivulus are simultaneous hermaphrodites, but that is not always true. While the majority of individuals within a population are indeed hermaphrodites, there are also a smattering of males. Hermaphrodites are able to undergo a sex change in which they dismantle their female parts and begin to invest heavily in their male parts, which eventually leads to them becoming fully male.

But, why? Why would a hermaphrodite that possesses all the equipment necessary to happily procreate without the nagging of a partner, suddenly assume a fully male identity? That is the million-dollar question driving a lot of our research. While we might not know the answer to this question yet, what we do know is how these males appear in populations. Basically, it can occur in one of two ways. Baby fish can either develop directly into males when exposed to triggers in the environment (we think this is pretty uncommon in wild populations). These males are referred to as primary males. Secondary males, on the other hand, result from adult hermaphrodites transitioning into males. They stop investing energy into their ovarian tissue, and actually get rid of it altogether. They then add on more testicular tissue to create a fully functioning male testis.

Now, let’s take a moment and consider how seriously crazy this is: these guys literally morph into another sex. How do they just spontaneously turn into males though? Well, first of all it doesn’t seem to be spontaneous by any means. We know there are certain cues that can trigger the transition – keeping them in hot temperatures, for example. However, these cues, and what exactly they encompass, are not very well understood. This, among other things, makes them of particular interest to us in the Earley lab as we work to reveal secrets behind their strange sex life. If this has got you interested, keep an eye out for publications on this topic from our lab in the upcoming year! We’ve got something cooking!



Earley RL, Hanninen AF, Fuller A, Garcia MJ, Lee EA. 2012. Phenotypic plasticity and integration in the mangrove rivulus (Kryptolebias marmoratus): a prospectus. Integr Comp Biol 52:814–27.

Harrington RW Jr. 1967. Environmentally controlled induction of primary male gonochorists from eggs of the self-fertilizing hermaphroditic fish, Rivulus marmoratus. Biol Bull. 132:174–199.



Ana Preda-Naumescu



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