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Does the Autecology of the Mangrove Rivulus Fish (<I>Rivulus

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Does the Autecology of the Mangrove Rivulus Fish (<I>Rivulus BULLETIN OF MARINE SCIENCE. 57(1): 208-214. 1995 DOES THE AUTECOLOGY OF THE MANGROVE RIVULUS FISH (RIVULUS MARMORATUS) REFLECT A PARADIGM FOR MANGROVE ECOSYSTEM SENSITIVITY? William P. Davis, D. Scott Taylor and Bruce J. Turner ABSTRACT The killifish Rivulus marmoratus, mangrove rivulus, represents the one of the two poten- tially truly "mangrove dependent" fish species in western Atlantic mangrove ecosystems. The distribution of this species closely parallels the range of red mangroves. These plants and fish exhibit parallel ecological and physiological tolerances to the wide ranges of tropical temperatures and salinities, as well as substrate and hydrological conditions of mangrove habitats. The mangrove rivulus, R. marmoratus, is, as well, the only truly marine represen- tative of a speciose genus of otherwise freshwater fish species. Many of the biological spe- cializations of this species characterize the specific challenges to survival in mangrove forest conditions. As recent studies report, this fish species, once considered "rare," has been shown to be very abundant in specific substrate microhabitats of the mangal. Among the unique specializations of this fish are amphibious emersion from water, survival in moist detrital substrate during periods of low water or drought, and reproduction through internal self- fertilization producing homozygous clones. The autecology of this species provides fascinat- ing insights and generates a wealth of questions regarding evolution of specific adaptations for distribution, dispersal, colonization, population genetics and the interrelationships between adaptation and specialization. The Indian River Lagoon (IRL) represents both the site of rediscovery of the fish as well as the northern frontier of the species distribution range, habitat, and ecosystem. This suggests close interrelationships and parallels in the parameters to which the species has adapted, perhaps representing a commonage, of ecological associ- ation. These aspects are discussed with respect to details and insight needed to develop strategies for the management of unique communities and ecosystems, especially along their natural distributional borders. The topic raises such questions as: Are some "exotic species" actually examples of newly arrived colonizers, representing dynamic biotic responses to cli- mate change and/or anthropogenic habitat modification? The capture of Rivulus marmoratus in the IRL by the Harringtons and W. L. Bidlingmayer in 1955 (Harrington and Rivas, 1958) represented rediscovery of a species previously known only from Cuba (Poey, 1880), and perhaps Brazil (Hu- ber, 1992). The mangrove rivulus has continued to fascinate a wide variety of biologists and generate a plethora of research questions and studies. With the description of reproduction through self-fertilization, each new examination of this fish has revealed yet another discovery that challenged previous assumptions of established fact. Eleanor S. Harrington described to us how they had observed mangrove rivulus out of water on several occasions, attached to the underside of mangrove foliage. These researchers decided at that time such bizarre field sight- ings should not be intermingled with their descriptions of the exciting homozy- gous lineages resulting from internal fertilization in an ovitestes unique among vertebrates. The biology of mangrove rivulus continues to reveal new insights and understanding, leading us, perhaps, to a new synthesis of a way species' specializations and associations are interwoven to produce the ecosystems we recognize. As naturalists or ecologists we have traditionally classified habitats by com- munity and species associations, or by their physical, geomorphic, and/or func- tional characteristics. Further ecological research has involved definition and enu- meration of parameters of physical, productivity or nutritional exchange rates or 208 DA VIS ET AL.: AUTECOLOGY OF RIVULUS 209 reclassifications by diversity, nutritional, guild, or other functionally based schemes. The word "biodiversity," which has emerged during the I980's as a generic code word denoting species richness, has acquired broad public usage, as well as a highly variable meaning among its many users (Noss, 1990). The concept of protecting and "managing biodiversity" has become so strongly embraced that it has acquired significant political clout. As noted by Redford (1994), in most tropical areas too little is known about many species to allow their use in con- servation strategies, so presently, communities and ecosystems receive the focus of attention; implied of course, is that biodiversity encapsulates the ecological values therein. Another term, "assessment," is also acquiring special bio-political connotation and representing the interlinking "word-bridge" between scientific data and pol- icy interpretation, particularly in discussions of ecological risk assessment (Davis et aI., 1994). As these and other terminologies enter into the public forum of environmental communication and policy, we must carefully examine and con- sider how various basic principles of natural history and ecology may become transformed, or potentially lost, as meaning and interpretation change through the levels of bureaucracy and policy formulation. The effects of such transformations may be recognized as tools or consequences of "the delicate process of nomen- clatural metamorphosis" which reflect the different special interest groups com- peting for limited resources. Comprehension of terms by the public, differentiating between the subtleties of specialized scientific (uncommon), into advertisement (common) knowledge represents challenge to resource utilization. For example: we all surely know a definition for "life history." However, there really is not a generic or collective term for the "critical threshold number of completed life histories" required to sustain a species from extinction. Survival, a potential an- swer, does represent or transmit enough clout in the present political arena to deliver sufficient public support to research and determine these thresholds for many of our important resource species and communities. To assume that we can "manage sustainability" further requires scaling our measurements through com- munity and ecosystem levels. Therefore, are not the basic needs of comprehending ecological sustainability related to reducing "risk" to completion of ontogenies or life history cycles? However, this collage of critical questions and parameters eventually evolves and is termed, we submit that it represents part of the "commonage" of sustained living systems. "Commonage" was first used, to our knowledge, in a biological context, by John Couch in the published debate whether fish or rodents serve as better assay organisms with which to screen for carcinogenic responses from exposure to chemicals and situations (Dawe and Couch, 1984). Couch states: "We biologists or life scientists generally tend to accept with evidence, and intuitively perhaps, the idea that there are common threads among all living forms. I call this 'biologic commonage.'" As a concept, commonage appropriately alludes to "the tragedy of the commons" (Hardin, 1968) and may be used among different levels of scale, from biochemical, cellular, organism, through communities, eco- systems to global landscapes. In this brief paper, we intend to illustrate the potential use of commonage by examination of the adaptations of a fish species, mangrove rivulus (Rivulus mar- moratus), and the plant (and community) that this fish, ecologically, is most close- ly allied with, the red mangrove (Rhizophora mangle). On a cellular level, all life forms share the commonage of DNA, mitochondria, and therefore, certain risks of dysfunction to the processes that are related to specializations. At the scale 210 BULLETIN OF MARINE SCIENCE, VOL, 57. NO. I. 1995 level of ecosystem, would it not be useful to describe factors that are part of the commonage of associated species, as a means to understand risks to the sustain- ability of whole systems or their components? Can this approach and knowledge contribute to development of sound policy decisions and ecological management? AUTECOLOGY OF MANGROVE RIVULUS AND MANGROVES The mangrove rivulus is the single species of the widely distlibuted Central and South American speciose genus Rivulus of the killifish family, Aplocheilidae, to be: (1) wholely associated with marine habitats (estuarine, coastal, insular) and (2) naturally occur on the North American continent. Statement (1) may represent species-level ecophysiological adaptation, whereas (2) may reflect the biogeo- graphic distribution of the associated habitats of the mangal, red and black man- grove communities. Inasmuch as the mangal, as used by Macnae (1968), repre- sents associations of aquatic "wet" or "bottom" rain forest tree species that are also specifically adapted to saline conditions, we present here the association of plant(s) and fish as an example of a bio-ecological commonage. Generally, rivulin fishes are predators on invertebrates and insects, but readily attack anything that moves which can be swallowed whole or in bitten-off parts (including other fishes or tadpoles). What distinguishes mangrove rivulus from most of the other fishes associated with Atlantic mangrove forests (Odum and Heald, 1972) is their routine, periodic emersion from standing water, especially in response to increasing anaerobic/H2S conditions (Abel et a\., 1987). Emersion is an adaptive
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