Reproductive cycles in a reared strain of the mummichog, a daily spawner

Annual, lunar, and diel samplings were taken from a strain of mummichog (Arasaki strain) reared in outdoor tanks under natural conditions, to examine gonadal maturity. Gonads of yearling fish were quite immature in September. During late autumn and winter, a gradual increase in the GSI of both sexes was observed, and the growth of cortical alveolus phase oocytes in females and basal spermatogenesis in males progressed. In late February, a rapid increase in the GSI of both sexes, vitellogenesis in females, and active spermatogenesis in males, occurred. The spawning period of the yearling fish was from late March to August judging from the presence of milt-producing males and ovulated females. The spawning period of the underyearling fish started in the same month as the yearlings, but terminated 1 month earlier. Plasma levels of oestradiol-17 β (E₂) in females and testosterone in males were high during the spawning period in the yearlings. In the underyearlings, however, E₂ levels peaked in early spring, and declined in the latter part of the spawning period. Neither a lunar nor semilunar cycle was evident in the reproductive activity of this fish, which proved to be a typical daily spawner. Females showed an apparent daily reproductive cycle; oocyte maturation commenced at about 1200 hours, germinal vesicle breakdown (GVBD) occurred at about 2400 hours, and ovulation was completed by 2400 hours, 24 h after GVBD. Such clear annual and daily reproductive cycles make this strain of mummichog a suitable model for the study of environmental and endocrine regulation of reproductive cycles in marine and estuarine teleosts.     

Evolution,ecology and physiology of amphibious killifishes (Cyprinodontiformes)

The order Cyprinodontiformes contains an exceptional diversity of amphibious taxa, including at least 34 species from six families. These cyprinodontiforms often inhabit intertidal or ephemeral habitats characterized by low dissolved oxygen or otherwise poor water quality, conditions that have been hypothesized to drive the evolution of terrestriality. Most of the amphibious species are found in the Rivulidae, Nothobranchiidae and Fundulidae. It is currently unclear whether the pattern of amphibiousness observed in the Cyprinodontiformes is the result of repeated, independent evolutions, or stems from an amphibious common ancestor. Amphibious cyprinodontiforms leave water for a variety of reasons: some species emerse only briefly, to escape predation or capture prey, while others occupy ephemeral habitats by living for months at a time out of water. Fishes able to tolerate months of emersion must maintain respiratory gas exchange, nitrogen excretion and water and salt balance, but to date knowledge of the mechanisms that facilitate homeostasis on land is largely restricted to model species. This review synthesizes the available literature describing amphibious lifestyles in cyprinodontiforms, compares the behavioural and physiological strategies used to exploit the terrestrial environment and suggests directions and ideas for future research.     

Aspects of the life history of a freshwater population of the mummichog,Fundulus heteroclitus (Pisces: Cyprinodontidae), in the Bronx River,New York,U.S.A.

A freshwater population of the mummichog, Fundulus heteroclitus, was sampled during summer and autumn 1979, from the Bronx River, New York. Other occurrences in freshwater of this common estuarine cyprinodontid fish have been noted in Pennsylvania, New Hampshire and Canada. In the Bronx River, this species is a significant part of the fauna and is one of the few species found throughout the river system. Specimens were examined to determine the growth and diet. It appears that this freshwater population does not differ from its brackish water conspecifics with respect to age structure, growth rate and feeding behavior.     

Calcium regulation in the freshwater-adapted mummichog

In light of recent findings of an unusual pattern of ionoregulation (high Na⁺ uptake and negligible Cl^- uptake) in the freshwater-adapted mummichog Fundulus heteroclitus, the pattern of Ca²⁺ regulation was examined. Under control conditions (water Ca²⁺= 200μEq l^-1), unidirectional Ca²⁺ influx was 11 ± 4 nEq g^-1 h^-1. Acute variation of external Ca²⁺ levels revealed a saturable Ca²⁺ uptake system with a relatively high affinity (K_m= 125 ± 36 μEq 1^-1) and a transport capacity (J_max= 31 ± 4 nEq g^-1 h^-1) comparable to those of other teleosts. Lanthanum (equimolar to [Ca²⁺]) significantly blocked Ca²⁺ uptake by 67% whereas magnesium had no effect. Chronic low Ca²⁺ exposure (50 μEq 1^-1) stimulated Ca²⁺ uptake almost three-fold above control values, whereas chronic high Ca²⁺ exposure (20000 μEq 1^-1) had no effect. Lanthanum and chronic low Ca²⁺ treatments disturbed the normally positive Ca²⁺ and Na⁺ balances of the animals whereas acid-base balance and ammonia excretion were undisturbed. The results indicate that Ca²⁺ regulation by the mummichog conforms to the model for freshwater Ca²⁺ transport whereby chloride cells on the gills take up Ca²⁺ actively from the water. However, the absence of extra-intestinal Cl^- uptake and the recent demonstration of significant Ca²⁺ uptake by opercular epithelia raise questions about the relative roles of branchial and opercular epithelial chloride cells in freshwater F. heteroclitus.     

Variation Among Four Health Indices in Natural Populations of the Estuarine Fish, Fundulus heteroclitus (Pisces, Cyprinodontidae), from Five Geographically Proximate Estuaries

Variation among four commonly used health indices was examined in the mummichog, Fundulus heteroclitus. The four indices were liver glycogen content (LGC), liver-somatic index (LSI), condition index (K) and RNA–DNA ratio. Fish were collected from five coastal locations in southeastern Connecticut. Fish health, as determined by these four indices, varied considerably among estuaries and between sexes. The relationship between each index and specimen length was significantly different among estuaries for either sex. When regressed against length, the slopes for the indices ranged from positive to negative. For each index, significant differences existed among some of the length-centered means at each estuary for either sex. Estuary rank for one index did not necessarily correlate with the estuary rank for another index. The significance of this variability and its impact on the use of the indices as bioindicators of environmental perturbation is discussed.     

Predator/prey interactions: A link between the individual level and both higher and lower level effects of toxicants in aquatic ecosystems

Behavior can demonstrate linkages ofcontaminant effects at different levels oforganization from the biochemical/cellular to theorganism, population, and community levels.Mummichogs, Fundulus heteroclitus, from acontaminated area were previously found to havereduced condition, growth, and longevity, comparedwith conspecifics from clean sites. Thispopulation-level observation may be due to theirimpaired predator/prey behavior, which is associatedwith altered levels of serotonin in their brains. Theyare slow, less able to capture live food, and eat muchnon-nutritious detritus, which may contribute to theirdecreased growth and condition. They are also lessable to avoid being captured by blue crabs, which canalso contribute to their reduced life span andsize-structure. Grass shrimp, Palaemonetespugio, a major prey species, are not behaviorallyimpaired at this polluted site, but are more numerousand larger in size than at the reference site,probably due to reduced mummichog predation.     

Steep,coincident, and concordant clines in mitochondrial and nuclear‐encoded genes in a hybrid zone between subspecies of Atlantic killifish,Fundulus heteroclitus

Steep genetic clines resulting from recent secondary contact between previously isolated taxa can either gradually erode over time or be stabilized by factors such as ecological selection or selection against hybrids. We used patterns of variation in 30 nuclear and two mitochondrial SNPs to examine the factors that could be involved in stabilizing clines across a hybrid zone between two subspecies of the Atlantic killifish, Fundulus heteroclitus. Increased heterozygote deficit and cytonuclear disequilibrium in populations near the center of the mtDNA cline suggest that some form of reproductive isolation such as assortative mating or selection against hybrids may be acting in this hybrid zone. However, only a small number of loci exhibited these signatures, suggesting locus‐specific, rather than genomewide, factors. Fourteen of the 32 loci surveyed had cline widths inconsistent with neutral expectations, with two SNPs in the mitochondrial genome exhibiting the steepest clines. Seven of the 12 putatively non‐neutral nuclear clines were for SNPs in genes related to oxidative metabolism. Among these putatively non‐neutral nuclear clines, SNPs in two nuclear‐encoded mitochondrial genes (SLC25A3 and HDDC2), as well as SNPs in the myoglobin, 40S ribosomal protein S17, and actin‐binding LIM protein genes, had clines that were coincident and concordant with the mitochondrial clines. When hybrid index was calculated using this subset of loci, the frequency distribution of hybrid indices for a population located at the mtDNA cline center was non‐unimodal, suggesting selection against advanced‐generation hybrids, possibly due to effects on processes involved in oxidative metabolism.     

Conspicuous behaviour of Fundulus heteroclitus associated with high digenean metacercariae gill abundances

Fundulus heteroclitus from six sites throughout the Hackensack Meadowlands District in northern NJ, U.S.A., were examined. Differences in behaviour (surfacing, conspicuousness and activity) were compared to gill infection intensity. Fish from populations infected with >1500 digenean metacercariae of Ascocotyle phagicola diminuta and Echinochasmus schwartzi , spent significantly more time at the water surface and exhibited significantly more conspicuous behaviour ( e.g. jerking) than fish from less parasitized populations. This more conspicuous behaviour has the potential to increase trophic transmission of the parasite to its definitive wading-bird host.