When people typically think of extreme fish, visually spectacular or odd species such as the oceanic sunfish or the deep-sea angler fish come to mind. While the human mind is inevitably attracted to visually exciting species, not all extreme fish are apparent. Kryptolebias marmoratus or mangrove rivulus is a small New World euryhaline killifish that inhabits mangrove forests through the Caribbean, Central America, and North America (Tatarenkov et al. 2017). Visually, you may lump it together with the other small coastal/estuarine fish, but biologically rivulus is very different.

     Rivulus populations are filled with primarily hermaphrodites, a few males, and no females (androdiecious mating system). Hermaphrodites primarily reproduce through self-fertilization, leaving few opportunities for males to leave offspring. However, outcrossing between males and hermaphrodites does occur at low frequencies (Costa, Lima, and Bartolette 2010), although researchers have been unsuccessful at observing outcrossing events in laboratory settings. Rivulus and their sister species (Kryptolebias hermaphroditus) are the only self-fertilizing hermaphroditic vertebrates in the world. Furthermore, rivulus hermaphrodites can reabsorb their ovarian tissue and irreversibly change sex into males when the going gets tough. This sex change mechanism has been suggested to be a survival strategy under extreme environmental stress (Gresham et al. 2020). Therefore, a single fertilized egg can theoretically establish an entirely new population of fish, who would all be basically clones of each other! (Caveat: These “clones” are not exact clones because self-fertilization is still sexual reproduction.)

     While self-fertilizing hermaphroditic fish are cool, rivulus continues to amaze with their environmental tolerances. This fish species can tolerate a wide range of salinities from 0 ppt to 80 ppt (Kristensen 1970; Taylor, Davis, and Turner 1995; Taylor 2000). For reference, typical ocean water is 35 ppt; therefore, rivulus can survive in double seawater! Furthermore, rivulus can survive in a wide range of temperatures (7 – 38°C) (Taylor, Davis, and Turner 1995), levels of dissolved oxygen (Dunson and Dunson 1999), and hydrogen sulfide concentrations (Carlson et al. 1983; Abel, Koenig, and Davis 1987; Rey et al. 1992). Rivulus can even breathe through their skin; they can survive on land for up to 66 days (Wright 2012; Taylor 2012; Blanchard et al. 2019). Once on land, their movement is not the expected random tail thrashing implied by the “fish out of water” sentiment. Rivulus utilize a directed tail-flip jump to move across terrestrial habitat, possibly to find new, more suitable aquatic habitats (Pronko, Perlman, and Ashley-Ross 2013).

     The mating system and environmental tolerances of rivulus have made the species an invaluable tool for research investigating osmoregulation, behavior, and a variety of other evolutionary vital traits. Researchers can even utilize this species and its relatives to study the evolution of sex! Additionally, the pseudo-clonal reproduction of rivulus enables researchers to conduct extensive experiments with many genetic replicates to uncover the potential genetic basis for various phenotypes. The amazing scientific insight we can gain from studying rivulus is not the take-home message. Rivulus is a prime example of not judging a book by its cover. Species do not need to be large, charismatic, or conventionally beautiful to be interesting and warrant study. Sometimes, the unique and intriguing is right outside your door.

Works Cited
Abel, Daniel C., Christopher C. Koenig, and William P. Davis. 1987. “Emersion in the Mangrove Forest Fish Rivulus Marmoratus: A Unique Response to Hydrogen Sulfide.” Environmental Biology of Fishes 18 (1): 67–72. https://doi.org/10.1007/BF00002329.
Blanchard, Tessa S., Andrew Whitehead, Yunwei W. Dong, and Patricia A. Wright. 2019. “Phenotypic Flexibility in Respiratory Traits Is Associated with Improved Aerial Respiration in an Amphibious Fish out of Water.” Journal of Experimental Biology 222 (2). https://doi.org/10.1242/jeb.186486.
Carlson, Paul R, Laura A Yarbro, Carl F Zimmermann, and John R Montgomery. 1983. “Pore Water Chemistry of an Overwash Mangrove Island.” Florida Scientist 46: 239–49.
Costa, Wilson J E M, Sergio Maia Queiroz Lima, and Renata Bartolette. 2010. “Androdioecy in Kryptolebias Killifish and the Evolution of Self-Fertilizing Hermaphroditism.” Biological Journal of the Linnean Society 99 (2): 344–49. https://doi.org/10.1111/j.1095-8312.2009.01359.x.
Dunson, William A., and David B. Dunson. 1999. “Factors Influencing Growth and Survival of the Killifish, Rivulus Marmoratus, Held inside Enclosures in Mangrove Swamps.” Copeia 1999 (3): 661. https://doi.org/10.2307/1447598.
Gresham, Jennifer D., Kristine M. Marson, Andrey Tatarenkov, and Ryan L. Earley. 2020. “Sex Change as a Survival Strategy.” Evolutionary Ecology 34 (1): 27–40. https://doi.org/10.1007/s10682-019-10023-2.
Kristensen, Ingvar. 1970. “Competition in Three Cyprinodont Fish Species in the Netherlands Antilles.” Studies on the Fauna of Curaçao and Other Caribbean Islands 32 (1): 82–101.
Pronko, Alexander J., Benjamin M. Perlman, and Miriam A. Ashley-Ross. 2013. “Launches, Squiggles and Pounces, Oh My! The Water-Land Transition in Mangrove Rivulus (Kryptolebias Marmoratus).” Journal of Experimental Biology 216 (21): 3988–95. https://doi.org/10.1242/jeb.089961.
Rey, Jorge R., John Shaffer, Tim Kain, Robert Stahl, and Roy Crossman. 1992. “Sulfide Variation in the Pore and Surface Waters of Artificial Salt-Marsh Ditches and a Natural Tidal Creek.” Estuaries 15 (3): 257–69. https://doi.org/10.2307/1352774.
Tatarenkov, Andrey, Sergio M Q Lima, Ryan L Earley, Waldir M Berbel-Filho, Frans B M Vermeulen, D Scott Taylor, Kristine Marson, Bruce J Turner, and John C Avise. 2017. “Deep and Concordant Subdivisions in the Self-Fertilizing Mangrove Killifishes (Kryptolebias) Revealed by Nuclear and MtDNA Markers.” Biological Journal of the Linnean Society 122 (3): 558–78. https://doi.org/10.1093/biolinnean/blx103.
Taylor, D Scott. 2012. “Twenty-Four Years in the Mud: What Have We Learned About the Natural History and Ecology of the Mangrove Rivulus, Kryptolebias Marmoratus?” Integrative and Comparative Biology 52 (6): 724–36. https://doi.org/10.1093/icb/ics062.
Taylor, Scott D. 2000. “Biology and Ecology of Rivulus Marmoratus: New Insights and a Review.” Florida Scientist 63 (4): 242–55.
Taylor, Scott D., Wm. P. Davis, and Bruce J. Turner. 1995. “Rivulus Marmoratus: Ecology of Distributional Patterns in Florida and the Central Indian River Lagoon.” Bulletin of Marine Science 57 (1): 202–7.
Wright, Patricia A. 2012. “Environmental Physiology of the Mangrove Rivulus, Kryptolebias Marmoratus, a Cutaneously Breathing Fish That Survives for Weeks out of Water.” In Integrative and Comparative Biology, 52:792–800. Oxford Academic. https://doi.org/10.1093/icb/ics091.