Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus ( Kryptolebias marmoratus ) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na + -K + -ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.

Seeing in the swamp: hydrogen sulfide inhibits eye metabolism and visual acuity in a sulfide-tolerant fish

In fish, vision may be impaired when eye tissue is in direct contact with environmental conditions that limit aerobic ATP production. We hypothesized that the visual acuity of fishes exposed to hydrogen sulfide (H 2 S)-rich water would be altered owing to changes in cytochrome c oxidase (COX) activity. Using the H 2 S-tolerant mangrove rivulus ( Kryptolebias marmoratus ), we showed that a 10 min exposure to greater than or equal to 200 µM of H 2 S impaired visual acuity and COX activity in the eye. Visual acuity and COX activity were restored in fish allowed to recover in H 2 S-free water for up to 1 h. Since K. marmoratus are found in mangrove pools with H 2 S concentrations exceeding 1000 µM, visual impairment may impact predator avoidance, navigation and foraging behaviour in the wild.

Social experience alters different types of learning abilities controlled by distinct brain nuclei in Kryptolebias marmoratus

Fighting experiences strongly influence aggressive behavior and physiology (winner-loser effects). These effects are conserved from invertebrates to vertebrates, but the underlying mechanisms remain unclear. Recent studies indicate that the brain social decision-making network (SDN) plays a key role in guiding experience-induced behavioral change. Also, while most studies have focused on how winning and losing experiences alter aggression, growing evidence points to these experiences driving multiple behavioral effects, including changes in the ability to learn. In mangrove rivulus fish (Kryptolebias marmoratus), we discovered that single winning experiences significantly improved spatial learning but not risk-avoidance learning, whereas single losing experiences drove the exact opposite to occur. These results provide strong evidence that winning and losing modulate diverse behaviors served by key nodes within the SDN, specifically the dorsolateral pallium (Dl; fish homolog to mammalian hippocampus, which serves spatial learning) and dorsomedial pallium (Dm; fish homolog to mammalian basolateral amygdala, which responds to fear). We therefore quantified whole-proteome expression within the forebrain (where Dm and Dl are located) of adult rivulus with divergent social experiences. We discovered 23 proteins were significantly differentially expressed in the forebrains of winners and losers. Differentially expressed proteins in losers related to modulation of cellular processes, apoptosis and learning while those in winners related to neuronal plasticity, neuroendocrine homeostasis, energy utilization, and learning. These results imply that winner-loser effects might be governed by very different patterns of protein expression, which could explain why winners and losers show such pronounced differences in behavioral performance.

Mitochondrial physiology and responses to elevated hydrogen sulphide in two isogenic lineages of an amphibious mangrove fish

ABSTRACT Hydrogen sulphide (H2S) is toxic and can act as a selective pressure on aquatic organisms, facilitating a wide range of adaptations for life in sulphidic environments. Mangrove rivulus (Kryptolebias marmoratus) inhabit mangrove swamps and have developed high tolerance to environmental H2S. They are hermaphroditic and can self-fertilize, producing distinct isogenic lineages with different sensitivity to H2S. Here, we tested the hypothesis that observed differences in responses to H2S are the result of differences in mitochondrial functions. For this purpose, we performed two experimental series, testing (1) the overall mitochondrial oxidizing capacities and (2) the kinetics of apparent H2S mitochondrial oxidation and inhibition in two distinct lineages of mangrove rivulus, originally collected from Belize and Honduras. We used permeabilized livers from both lineages, measured mitochondrial oxidation, and monitored changes during gradual increases of sulphide. Ultimately, we determined that each lineage has a distinct strategy for coping with elevated H2S, indicating divergences in mitochondrial function and metabolism. The Honduras lineage has higher anaerobic capacity substantiated by higher lactate dehydrogenase activity and higher apparent H2S oxidation rates, likely enabling them to tolerate H2S by escaping aquatic H2S in a terrestrial environment. However, Belize fish have increased cytochrome c oxidase and citrate synthase activities as well as increased succinate contribution to mitochondrial respiration, allowing them to tolerate higher levels of aquatic H2S without inhibition of mitochondrial oxygen consumption. Our study reveals distinct physiological strategies in genetic lineages of a single species, indicating possible genetic and/or functional adaptations to sulphidic environments at the mitochondrial level.

Male mate choice unresolved in the mangrove rivulus

Mate choice has the potential to drive phenotypic evolution because it can determine traits that increase an individual’s likelihood to reproduce (courtship behaviors, elaborate ornamentation). These traits, however, can also be detrimental for health or survival, often antagonizing the evolution of extreme phenotypes. Mangrove rivulus fish (Kryptolebias marmoratus) develop as self-fertilizing simultaneous hermaphrodites. Hermaphrodites overwhelmingly self-fertilize their eggs internally, but occasionally oviposit unfertilized eggs. Some individuals change sex to male after sexual maturity, essentially forgoing the reproductive assurance of selfing. In a continuing effort to understand how sex change to male is maintained this species, I designed an experiment to determine whether males act as choosers to increase their likelihood of finding unfertilized eggs for reproduction. I hypothesized that males would prefer to associate with younger hermaphrodites when given a dichotomous choice, as they lay a greater proportion of unfertilized eggs compared to older hermaphrodites. The males in this study did not show a preference for either the younger or older hermaphrodite but exhibited greater within individual variance across subtrials than among individual variation. I discuss alternative hypotheses concerning male mate choice in mangrove rivulus, which may illuminate hypotheses to be tested in this and other hermaphroditic species.

Variation in self-compatibility among genotypes and across ontogeny in a self-fertilizing vertebrate

Mixed mating strategies offer the benefits of both self-fertilizing one’s own eggs (selfing) and outcrossing, while limiting the costs of both methods. The economics of mixed mating is further determined by individual self-compatibility. In gynodioecious (hermaphrodites, females) and androdioecious (hermaphrodites, males) species, the level of self-compatibility of the hermaphrodites also acts as a selection pressure on the fitness of the other sex. Mangrove rivulus fish populations are comprised of selfing hermaphrodites and males that result from hermaphrodites changing sex. Although hermaphrodites overwhelmingly reproduce through internal selfing, they occasionally oviposit unfertilized eggs. Males can externally fertilize these eggs. Here, we reveal that fecundity and self-compatibility varies within individuals across ontogeny until about 365 days post hatch, and among individuals derived from lineages that vary in their propensity to change sex. Hermaphrodites from high sex changing lineages were significantly less fecund and self-compatible than hermaphrodites from low sex changing lineages. These differences in self-compatibility and fecundity have the potential to drive evolutionary changes on mating strategy and the fitness of males in populations of the mangrove rivulus. This study also illustrates the importance of including lineage variation when estimating the costs and benefits of mixed mating strategies.

Outbreeding depression as a selective force on mixed mating in the mangrove rivulus fish,Kryptolebias marmoratus

Mixed mating, a reproduction strategy utilized by many plants and invertebrates, optimizes the cost to benefit ratio of a labile mating system. One type of mixed mating includes outcrossing with conspecifics and self-fertilizing one’s own eggs. The mangrove rivulus fish (Kryptolebias marmoratus)is one of two vertebrates known to employ both self-fertilization (selfing) and outcrossing. Variation in rates of outcrossing and selfing within and among populations produces individuals with diverse levels of heterozygosity. I designed an experiment to explore the consequences of variable heterozygosity across four ecologically relevant conditions of salinity and water availability (10‰, 25‰, and 40‰ salinity, and twice daily tide changes). I report a significant increase in mortality in the high salinity (40‰) treatment. I also report significant effects on fecundity measures with increasing heterozygosity. The odds of laying eggs decreased with increasing heterozygosity across all treatments, and the number of eggs laid decreased with increasing heterozygosity in the 10‰ and 25‰ treatments. Increasing heterozygosity also was associated with a reduction liver mass and body condition in all treatments. My results highlight the fitness challenges that accompany living in mangrove forests ecosystem and provide the first evidence for outbreeding depression on reproductive and condition-related traits.