BULLETIN OF MARINE SCIENCE. 57(1): 208-214. 1995

DOES THE AUTECOLOGY OF THE MANGROVE (RIVULUS MARMORATUS) REFLECT A PARADIGM FOR MANGROVE ECOSYSTEM SENSITIVITY?

William P. Davis, D. Scott Taylor and Bruce J. Turner

ABSTRACT The killifish Rivulus marmoratus, , 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 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 . 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 (). 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 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 response to maintain respiratory function, escaping the periodic anoxia characteristic of warm stagnant waters, yet allowing a rapid reentry to the rich source of prey invertebrates during alternative periods of inundation of aer- ated water. The emersion habit seems to be relatively common among the freshwater ri- vulins (TeeVan, 1922; Vaz-Ferreira and Sierra, 1972; Fromm, 1986) and a number of other fish families as well (eleotrids, synbranchids). In freshwater habitats, emersion represents an amphibian life-style. We suggest that it is likely that the ancestors of R. marmoratus exercised this specialized behavior before adapting to sustained high salinities, considering that mangrove rivulus represent the single marine species among hundreds of other related freshwater forms with the ability to emerse. A common characteristic among the diverse plant species of the mangal is ecophysiological tolerance to fluctuating dissolved oxygen concentrations, often concomitantly with high levels of hydrogen sulfide (H2S). This is accomplished in mangroves through the structural extensions of prop roots or pneumatophores. Hydrogen sulfide is a powerful toxin. The ever-present anaerobic bacteria and fungi in the moist detrital substrates of tropical forests represent the real potential

to quickly produce H2S whenever dissolved O2 diminishes, especially during high temperatures and still waters typical of the mangal. Both the mangroves and mangrove rivulus become key representative associated species of the Western Atlantic mangal, community members which share special adaptations and colo- nize ecological sites which are not suitable and thereby select against other po- tential "wetland competitors" from colonizing these habitats of the shallow trop- ical marine littoral zone. Extension of the mangal into coastal waters provides many of the same detrital substrates and microhabitats to which terrestrial counterpart species are adapted. Therefore, much of fauna and some flora associated with mangroves are forms which are also associated with coastal wetlands of varying salinities. The limiting factors for continuous survival in the mangal, therefore, include tolerance to wide DA VIS ET AL.: AUTECOLOGY OF RIVULUS 211

ranges of temperature, salinity and H2S, with concomitant respiratory adaptation through the entire ontogenic cycle of each species, including reproduction and developmental stages. Adaptive endurance of fishes in extreme thermal events in mangrove lagoon is reported by Heath et a!. (1993). It is in the category of reproduction that mangrove rivulus exhibit their truly most unique feature: self-fertilization within the individual , gen- erally resulting in populations composed predominantly of homozygous clones. During our collaborative studies, we felt that the clonal composition of popula- tions, unreported for any other , would likely reveal various patterns that reflected adaptation, geographic migration or dynamics of the relationships among separated mangrove rivulus populations. Analyses reported by Turner et al. (1990, I992b, Lubinsky et aI. in press) revealed the number of individual clones to be astonishingly large. Rivulus marmoratus populations composed of diverse clonal forms may potentially represent an "escape from" or an alternative strategy to the more common rules and patterns of genetic diversity that generally predominate among biota. The mode of reproduction and resulting genetic pat- terns of mangrove rivulus populations most resemble the case reported for species of self-fertilizing "cleistogamous" plants. These plants have been described as examples of suppression of expression of male traits which is thought to provide "conservation of energy," an adaptation enabling colonization of widely dis- persed Pacific oceanic islands without resource expenditure in male structure (Lloyd, 1988). Although this is an appealing hypothesis for the available obser- vations of mangrove rivulus, it remains a substantial challenge to experimentally demonstrate. In addition, mangrove rivulus, in contrast to known examples of selfing homozygous plants, are apparently uniquely able to functionally "retrieve" cross-fertilization. Secondary males can occur in hermaphroditic fish from loss of ovarian tissue and function, concomitant with phenotypic color change (SOlOand Noakes, 1994). Among fish sampled from populations, we have found relatively high (10-20%) proportions of male fish (Turner et a!., 1992a), as well as heterozygous individuals (Lubinski, 1993). This finding is in distinct contrast to extreme rarity of males in wild populations of this fish sampled in Florida and elsewhere. Male fish, however, can be readily produced by thermal manipulations during embryonic development in the laboratory (Taylor et a!., 1995). This poses an interesting hypothesis: does genetic heterozygosity, at least in the case of this clonal species, potentially reflect response to ecological stresses or disturbances? There is an intriguing suggestion of this possibility in our Belize studies which exhibit more heterozygous fish from one site affected by man modifications, com- pared to samples from less disturbed sites on adjacent islands (Lubinski, 1993). One of the three males we have captured in the environs of South Florida (Thom- erson, 1966) also was captured in a site highly impacted by human intrusion (mangrove removal and ditching). The other males that we captured occurred in the environs of Rookery Bay, Collier Co., Florida, where intensive mosquito spraying has been conducted for decades. However, aside from these half dozen specimens, males have been very rare among the thousand or more fish sampled from south Florida (including both coasts, Everglades and the Keys). Substantial additional testing is necessary to investigate for potential correlation between occurrence of male rivulus and specific ecological factors. However, this area of research represents an intriguing approach to assessment and evaluation of inter- relationships between ecological stress and biodiversity. Does specialization for colonization represent another, but incompletely under- stood, example of biological commonage? Does R. marmoratus distribution in Florida represent an example and later stage of colonization of mangrove eco- 212 BULLETIN OF MARINE SCIENCE, VOL. 57, NO. I, 1995 system associates that preceded the recent arrival and establishment in Florida of "exotic species"? Such species as the pike killifish (Belonesox belizanus) or Maya cichlid (Cichlasoma urophthalmus) are typical members of the lagoon fish faunal associations found in Central America mangrove forests. These fish species, as well as the mangroves, are all at the edge of their biogeographical limit. Florida populations are periodically impacted by freeze events as exemplified by the re- sponse of R. marmoratus to one freeze event (Taylor, 1993). Dispersal is an essential element of a biotic association; the floating propagules of the mangrove plant species are well known. Only one investigator has noticed occurrence of juvenile R. marmoratus among the numerous ichthyoplankton sam- ples from Florida mangrove tidal creeks (R. E. Matheson, Fla. Dept. Env. Prot., pers. comm.). However, frequent occurrence of newly hatched mangrove rivulus in isolated pools after periods of drought suggests aestivation of embryos (Ritchie and Davis, 1986). Furthermore, although we have emphasized association of R. marmoratus with mangrove substrates, there have been several observations of the species inhabiting sites with solution holes, wells or caverns in calcium car- bonate areas of the Bahamas, Turks and Caicos islands and Yucatan (P. Colin; R. G. Gilmore; R. Heard; W. Loftus, pers. comm.). These diverse piecemeal obser- vations, together with the fish's known ability to traverse the forest floor (Huehner et aI., 1985) during rainy periods, represent the few reported hints of this species potential for local movement and ultimately dispersal through the incredible dis- tributional range of mangrove rivulus in the tropical West Atlantic. Among community enumeration assessment reports of mangrove ecosystems, mangrove rivulus have typically been "tacked-on" (even mistakenly as an insect!) to species associations of the mangrove community, perhaps as a result of the misimpression of its rarity. Milward (1982) examines the concept of "mangrove- dependent biota" for mangrove ecosystems of Australia, where associations are much more complex and diverse than reported for the Caribbean and West Indian Province. A scan of the vertebrates listed by Milward (1982) reveals what we call "substrate associations," i.e., species which one would expect to find in analogous terrestrial habitats, i.e., not limited by salinity extremes, anoxic epi- sodes or the diverse pathogenic biota of the mangal. Guild associations with specific microhabitats of South Florida mangrove forests have been recently re- ported by Gilmore and Snedaker (1993). The spatial approach reported by these authors appears to indicate that of the vertebrate fauna unique to mangroves, virtually each form represents a subspecies of a terrestrial species (except man- grove rivulus and Gambusia rhizophorae). This calls to mind the fact that man- groves have advanced and retreated during geologic history since the Pleistocene in this region, coupled with patterns of global climate change (Woodroffe and Grindgrod, 1991). Does this reflect potential relationships between the geological history or continuity of an ecosystem in the context of the present biodiversity structure? Another approach to ecological relationships, carbon exchange, among mangal species is emerging from CfN/S stable isotope analyses. Preliminary results re- ported by McIvor (1992, pers. comm.) indicate that among the fish species, man- grove rivulus exhibit the strongest "mangrove stable isotope signal," probably a reflection of the insect/invertebrate diet (Huehner et aI., 1985; Taylor, 1992) and a lack of foraging in adjacent habitats. Utilization of microhabitats by R. mar- moratus within the mangrove forest is now well documented (Davis et al., 1990; Taylor, 1988, 1990). In the context of aspects of the management for biodiversity, the IRL mangal represents the northern edge or frontier of the biogeographic range of this eco- DAVIS ET AL.: AUTECOLOGY OF RIVULUS 213

system. This poses interesting problems and tests the ability to develop policy incorporating scientific knowledge. There is little debate whether developmental pressure for use of wetlands exists; however, whether we can arrive at intelligent solutions in decision making is another matter. The questions are especially dicey when the habitat in question may arguably be at the fringe of existence and/or periodically destroyed by natural climatic events (hurricanes, hard freezes). The ecological uniqueness of an edge of a biogeographic range represents a straight forward and easy "management solution" decision to an ecologist; simply pre- serve and manage the mangal habitat for longterm study and monitoring. How- ever, ecologists rarely have the last word in policy or decision making (Davis et aI., 1994). Developers argue that mangroves represent a "nuisance habitat" more valuable if replaced with habitation for mankind or "treated" to reduce the habitat for mosquitos (Taylor et aI., 1995). How do we deal with the challenge of mea- sures selected specifically to restrict biological commonage to those species which are not considered nuisance to humans? Is it possible to accomplish ecological manipulations in the IRL or elsewhere in Florida without replacing a rich biodi- versity with "monoculture of golf-course ecosystems"?

SUMMARY The red mangrove and the mangrove rivulus are two forms representative of a specific ecosystem that exemplify parallel adaptations through levels of scale from biochemical, ecophysiological, through community and landscape. The mangrove forest represents a very important "substrate" in coastal and insular areas of tropical marine region, including the subtropical areas of south Florida. The man- grove rivulus represents a fish species whose range is associated throughout the Western Atlantic with mangrove forest ecosystems, and curiously does not, as other vertebrate associations, venture to any significant extent into adjacent hab- itats. If one approaches the so-called unique attributes of mangrove rivulus from a concept of biological commonage, this fish represents a clear example, if not of a mangrove-dependent species, then a case comparable to certain insect species associations with specific habitat types. The unique specializations of this killifish offer potential approaches to testing and assessment of questions of broad interest to our understanding of ecology and biodiversity. The concept of ecological com- monage may prove a useful approach for viewing the specializations of species, and connecting these to the environmental processes of ecosystems, thus filling some of the unresolved "chinks" in our comprehension of biodiversity.

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DATEACCEPTED: November 7, 1994.

ADDRESSES: (WP.D.) Environmental Research Laboratory (EPA), Gulf Breeze, Florida 32561-5299; (D.S. T.) Brevard Mosquito Control, 2870 Greenbrook St., Valkaria, Florida 32950; (B.J. T.) Dept. of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0406.