The self-fertilizing, hermaphroditic mangrove rivulus Kryptolebias marmoratus
inhabits mangrove forests of the southern US and South/Central America. These tiny fish (~100 mg) tolerate a wide range in water temperatures, pH, salinities, oxygen levels, hydrogen sulfide concentration, as well as prolonged air exposure. We are interested in several aspects of their extremophilic nature. What strategies are used to cope with life out of water? We discovered that rivulus excrete gaseous NH3 during air exposure, a nitrogen excretion strategy similar to terrestrial gastropods and isopods. We have used microelectrodes to follow changes in the composition of the cutaneous boundary layer. We have measured cutaneous blood vessel reactivity following air exposure using video microscopy and quantified angiogenesis using histological techniques. We discovered that angiogenesis occurs in the buccal, opercular and cutaneous surfaces after 1 week out of water. We have found that gill morphology is plastic. When air exposed, a cell mass grows between secondary lamellae (interlamellar cell mass, ILCM), but when they return to water the gills shed the ILCM. This reduction in functional gill surface area may prevent excessive water loss and/or help to support the delicate lamellar structures out of water. Recent studies have shown that hemoglobin-oxygen affinity increases in fish out of water, but not during hypoxia exposure. Thus, several traits linked to respiratory performance are reversibly changed in K. marmoratus
exposed to air.
We have investigated iono- and osmoregulation when mangrove rivulus are air exposed, particularly focusing on cell composition of the gills and skin. We are also interested in water flux across the skin and whether water flux rates are altered with exposure to air. We are interested in behavioural differences between genotypically identical fish. Why do some fish emerse more frequently than others? Why are some fish able to survive out of water for longer periods than others? These are just some of the avenues of investigation that are underway to more fully understand the remarkable physiological capability of these versatile fish.