The Northwestern Sierra Madre Occidental of Mexico is a rugged mountain range covering portions of Sonora, Chihuahua, Durango and Sinaloa, and is drained by multiple Pacific Slope Rivers to the west and the Casas Grandes, Conchos, and Nazas to the east. The overall area is topographically, climatically and biotically diverse, ranging from endorheic basins (Casas Grandes) to mountainous areas elevations up to 3348 m, average mean temperatures from 10-20°C and precipitation from 250- 1100 mm/yr. The region is also geological complex that, combined with these other variables, provides a great diversity of both aquatic and terrestrial habitats conducive to both biotic endemicity and diversity. The overall diversity of the region has contribiited to the recent listing of this region by Conservation International as one of six new high-priority biodiversity hotspots. Our understanding of the aquatic and terrestrial biodiversity of this region, however, is poor and in urgent need of rapid investigation by collaborative communities. The combination of an incredibly rugged landscape, drug and bandit activities and indigenous peoples that have not always been welcoming to visitors has resulted in a general paucity of roads in the region, and the lack of access has limited inventory studies. Recently, however, access to the region has changed dramatically and many of the areas are now accessible enough for logging, mining, and agriculture practices, all exerting extreme pressures in some areas on the biodiversity. In addition to human-induced changes to these diverse ecosystems, impacts of invasive aquatic species are becoming more and more apparent, and the potential for severely reducing population sizes of species or their extirpation or extinction is real. While several invasive or exotic species are identified as potentially destructive to these communities, the exotic Rainbow Trout (Oncorhynchus mykiss) that is derived from hatcheries or culture facilities within the region represents one of the most critical threats to the aquatic and semi-aquatic biodiversity. The native trout of mainland Mexico represent the southern- most salmonids, and are at imminent risk of introgression and/or replacement by feral Rainbow Trout, Oncorhynchus mykiss. Our recent survey efforts have expanded the known diversity and ranges of each of several distinct forms, which we feel represent valid species. We discuss our discovery of multiple new species from the Sierra Madre Occidental and focus on a new species of trout restricted to the upper Conchos drainage, the first native species of Oncorhynchus known to occupy the Atlantic Slope in Mexico. Many of these taxa are restricted to small areas of intact habitat in headwater areas of high-elevation streams, and are at risk from a suite of human-associated perturbations, emerging diseases, and introduced species. These fishes occupy unique habitats, and represent a diverse portion of the Mexican montane ichthyofauna. The habitats on which they depend support a wide range of other aquatic organisms, most of which are grossly understudied. The discovery of the Conchos Trout derived primarily by GARP niche modeling of a subset of localities of previously sampled undescribed native Mexican trout, provides only a snapshot of the biodiversity awaiting to be discovered in this region. The rugged landscape of the Sierra Madre Occidental simply precludes routine sampling at easily accesible locations of streams and most sampling locations require time-intensive access. In an effort to rapidly assess the biodiversity of rivers of this region we employed this method to aid in predicting the most suitable and highly probable Mexican trout niches in the region. This method offered highly efficient and powerful results that not only predicted the occurrence of a previously unknown trout in the upper Rio Conchos but also provided excellent predictions of available habitats in drainages where previously unknown trout have been discovered by the Truchas Mexicanas team in the last nine years. Multiple threats exist to the biodiversity of the northern Sierra Madre Occidental, including uncontrolled introductions of exotic and invasive species, emerging diseases such as whirling disease, Myxobolus cerebralis, infectious pancreatic necrosis (IPN), iridioviruses and pathogenic water mold, Saprolegnia ferax, land-use practices leading to habitat degradation via overgrazing, logging, deforestation and road construction, increasing human population growth, over-fishing or overharvesting of aquatic resources and global climate cliange reducing surface and ground water in the area and creating environments more conducive to the spread of invasive species, congregated and dense human populations, and emerging diseases. Immediate actions need to be developed to aid in public education as to the threats to these ecosystems, protection of areas, assessment of diversity, and sustainable development throughout the region that incorporates a likely highly successful ecotourism system for the region.

A complex of little-studied catfishes of the genus ⬚Ictalurus⬚ is native to Pacific drainages of the Sierra Madre Occidental and interior drainages of Northwest Mexico. Its only described member, ⬚Ictalurus pricei⬚, is protected by the Mexican and US governments and given varied conservation status by diverse other conservation organizations. We contribute new data and a review of previous work to partially rectify prior inadquacies of genetic and conservation status information on the complex. Sequence data for the cytochrome b (⬚cytb⬚) and 12S ribosomal (⬚12SrRNA⬚) mitochondrial genes provide new diagnostic molecular characters for ⬚I. pricei⬚ and both genes display intra-specific variation. The only ⬚I. pricei⬚ specimens we could obtain, however, despite extensive field collection efforts, were from a captive broodstock established starting 20 years ago for conservation purposes. We provide basic, previously unpublished information about the history and, unfortunately, recent apparent demise of that captive stock. Non-native catfishes are widely introduced throughout the range of the ⬚pricei⬚ complex and, along with habitat degradation, represent a significant, but still very little-studied threat to its genetic integrity and persistence. Conservation of the Yaqui catfish is clearly a complicated issue with much remaining to be learned, but it is clear that current levels of protection do not adequately reflect its critical conservation plight and management efforts to date have failed to provide appropriate, long-term conservation actions and much-needed research.

Este proyecto se refiere a dos grupos de peces incluidos en la Norma Oficial Mexicana-059-SEMARNAT-2001 (NOM-059). El primero es un grupo taxonómico, la familia Cichlidae (mojarras de agua dulce), del cual aparecen en la NOM-059 ocho especies, seis de ellas carentes todavía de la ficha correspondiente; todas se asignaron en la NOM erróneamente al género Cichlasoma, a pesar de que desde hace más de 20 años quedó demostrado que ese taxon no existe en México (Kullander, 1983). De las seis especies sin ficha, en el sur de México están C. grammodes (excluida de la presente propuesta por estar ya incluida en otra), C. (= Vieja) hartwegi, C. intermedium (= V. intermedia), C. (= Thorichthys) socolofi y, en el norte, C. (= Herichthys) steindachneri y C. (= Herichthys) minckleyi, esta última con un interesante polimorfismo trófico. El segundo grupo abordado en esta propuesta se refiere a los peces de la frontera sur (entendida aquí como toda la península de Yucatán y el estado de Chiapas) enlistados en la NOM-059. Además de los cíclidos mencionados, este segundo grupo consiste en once especies: Astyanax armandoi, los Cyprinodon endémicos de la laguna de Chichancanab, Quintana Roo (C. beltrani, C. labiosus, C. maya, C. simus y C. verecundus – la NOM no toma en cuenta todavía al sexto, C. esconditus Strecker 2002, ni al séptimo, C. suavium Strecker 2005), Poecilia velifera, Profundulus hildebrandi, Potamarius nelsoni, Ogilbia (=Typhliasina) pearsei y Ophisternon infernale; la NOM contempla además a Rhamdia guatemalensis, que ya cuenta con una ficha, a pesar de que su validez taxonómica ha sido cuestionada (Silfvergrip 1996), y a pesar de su amplísima distribución y abundancia. El objetivo central de la contribución fue elaborar las fichas correspondientes y obtener el valor que asigna a cada una de estas 16 especies el Método de Evaluación del Riesgo de Extinción de las especies silvestres en México (MER), para recomendar su reclasificación o su exclusión de la lista, si fuera necesario. La información necesaria se ha tomado de la literatura, tanto publicada como “gris”, ésta última integrada fundamentalmente por datos inéditos de los investigadores participantes, sus estudiantes y otros colegas. Como resultado de lo anterior, se anexan tres propuestas formales de cambio de categoría de riesgo: C. beltrani (de [P] a [A]), Th. socolofi (de [Pr] a [A]) y V. hartwegi (de [Pr] a [A]). En el caso de A. armandoi, la ficha incluye y fundamenta una observación sobre el carácter dudoso de su identidad taxonómica, aunque, por principio precautorio, no se propone (todavía) excluirla de la NOM. Las demás especies estudiadas permanecen sin cambios en su categoría de riesgo. Además, siempre con base en la aplicación del MER, se recomienda la inclusión de otras siete especies de la frontera sur hasta ahora ignoradas por la NOM-059: Astyanax altior, Cyprinodon esconditus, C. suavium, Fundulus grandissimus, F. persimilis, Lacantunia enigmatica y Menidia colei. Finalmente, se recomienda formalmente la exclusión de Rhamdia guatemalensis de la NOM-059.

Este proyecto se refiere a dos grupos de peces incluidos en la Norma Oficial Mexicana-059-SEMARNAT-2001 (NOM-059). El primero es un grupo taxonómico, la familia Cichlidae (mojarras de agua dulce), del cual aparecen en la NOM-059 ocho especies, seis de ellas carentes todavía de la ficha correspondiente; todas se asignaron en la NOM erróneamente al género Cichlasoma, a pesar de que desde hace más de 20 años quedó demostrado que ese taxon no existe en México (Kullander, 1983). De las seis especies sin ficha, en el sur de México están C. grammodes (excluida de la presente propuesta por estar ya incluida en otra), C. (= Vieja) hartwegi, C. intermedium (= V. intermedia), C. (= Thorichthys) socolofi y, en el norte, C. (= Herichthys) steindachneri y C. (= Herichthys) minckleyi, esta última con un interesante polimorfismo trófico. El segundo grupo abordado en esta propuesta se refiere a los peces de la frontera sur (entendida aquí como toda la península de Yucatán y el estado de Chiapas) enlistados en la NOM-059. Además de los cíclidos mencionados, este segundo grupo consiste en once especies: Astyanax armandoi, los Cyprinodon endémicos de la laguna de Chichancanab, Quintana Roo (C. beltrani, C. labiosus, C. maya, C. simus y C. verecundus – la NOM no toma en cuenta todavía al sexto, C. esconditus Strecker 2002, ni al séptimo, C. suavium Strecker 2005), Poecilia velifera, Profundulus hildebrandi, Potamarius nelsoni, Ogilbia (=Typhliasina) pearsei y Ophisternon infernale; la NOM contempla además a Rhamdia guatemalensis, que ya cuenta con una ficha, a pesar de que su validez taxonómica ha sido cuestionada (Silfvergrip 1996), y a pesar de su amplísima distribución y abundancia. El objetivo central de la contribución fue elaborar las fichas correspondientes y obtener el valor que asigna a cada una de estas 16 especies el Método de Evaluación del Riesgo de Extinción de las especies silvestres en México (MER), para recomendar su reclasificación o su exclusión de la lista, si fuera necesario. La información necesaria se ha tomado de la literatura, tanto publicada como “gris”, ésta última integrada fundamentalmente por datos inéditos de los investigadores participantes, sus estudiantes y otros colegas. Como resultado de lo anterior, se anexan tres propuestas formales de cambio de categoría de riesgo: C. beltrani (de [P] a [A]), Th. socolofi (de [Pr] a [A]) y V. hartwegi (de [Pr] a [A]). En el caso de A. armandoi, la ficha incluye y fundamenta una observación sobre el carácter dudoso de su identidad taxonómica, aunque, por principio precautorio, no se propone (todavía) excluirla de la NOM. Las demás especies estudiadas permanecen sin cambios en su categoría de riesgo. Además, siempre con base en la aplicación del MER, se recomienda la inclusión de otras siete especies de la frontera sur hasta ahora ignoradas por la NOM-059: Astyanax altior, Cyprinodon esconditus, C. suavium, Fundulus grandissimus, F. persimilis, Lacantunia enigmatica y Menidia colei. Finalmente, se recomienda formalmente la exclusión de Rhamdia guatemalensis de la NOM-059.

A grant has been awarded to Dr. Richard L. Mayden at Saint Louis University to conduct field and laboratory research in collaboration with other USA scientists and several scientists from Mexico on the environmental status, distribution, conservation and genetics of native trout populations of Mexico. While biologists have been aware of the existence of native trout in Mexico for over a century, they have received little study. Beginning in the mid-1990's a group of ichthyologists and conservation biologists from both Mexico and USA jointly formed an international alliance, Truchas Mexicanas, to better understand the native trout of the Sierra Madre Occidental. Through these efforts, scientists have discovered previously unknown trout species restricted to Mexico and introduced (non-native) populations of rainbow trout. The existence of these non-native rainbow trout originating from streams in the US and Canada in the ranges of the unique Mexican trout species is a significant threat to the future existence of native species. Rainbow trout are known to displace native trout species through competition or genetically "swamp out" their gene pools through aggressive hybridization. Other native populations of trout are also predicted to occur in the upper Rio Conchos; these populations are predicted to also be in danger from habitat destruction and introductions of rainbow trout. In this study we will inventory many river systems of the sierras for native and introduced trout species, including the upper Conchos River system. Genetic samples will be examined from all of these populations to evaluate genetic variability of native species and determine if any native populations are compromised genetically through hybridization with introduced rainbow trout. This research supports the development of an international alliance investigating native trout populations of Mexico and southwestern United States. Trout species are important game and forage species for human populations, constituting a major element of diets in some regions. Trout fisheries are also very important economic elements to some regions as trout are highly prized sport fish species. Understanding native trout species diversity in Mexico and its conservation and maintenance in these streams has a major impact on both fisheries and economic areas. Mexican trout species are the southern-most trout and are adapted to theses unique ecosystems. Introductions of non-native rainbow trouts will seriously jeopardize these adapted populations and lead to their extirpation from the region. This will have serious sociological and economic impacts for the native people of the region.

A broad-scale survey of potential trout habitats in upper tributaries of the Río Conchos of Chihuahua, México resulted in the re-discovery in 2005 of a long-lost native, endemic, and now endangered, undescribed trout species that we call the "Conchos Trout." We are currently in the process of scientifically describing this rare trout, the only native Mexican trout known from an Atlantic drainage. Our extensive field efforts to date clearly indicate that this species was formerly much more widely distributed historically. Though surveys should continue, our extensive field surveys found only one small isolated and extremely vulnerable population, leaving us less than optimistic that many other, if any, additional populations will be found. Persistence of this new critically endangered endemic Conchos Trout clearly requires rapid conservation action. Our consensus expert opinion is that we cannot over-emphasize the urgency of protection for this critically endangered population of this unique trout, and so we have produced this "white paper" in hopes that it might encourage others to join us in initiating appropriate conservation programs. The potentially viable population is restricted to a short reach of a very small stream where it and another newly discovered, undescribed fish species, a sucker (and likely another unique taxon), could be relatively easily protected, studied and managed. Additionally, adjoining arroyos where the species recently occurred offer the possibility of restoring native fish populations to them following appropriate protection and habitat restoration efforts. All members of our highly qualified and diversely specialized, binational "Truchas Mexicanas" team stand prepared to join collaborators and invest personal resources in hands-on actions to help this new species persist. We recommend immediate establishment of a small reserve centered around currently occupied habitat and adjacent streams having suitable habitat, combined with education and compensation of local residents for their cooperation with reserve management. We recognize that we are largely ignorant of the local political and cultural systems via which conservation actions will have to proceed and are limited with regard to our ability to raise funds. We thus hope that readers of this white paper will add their financial and in-kind support and additional expertise to help us conserve the Conchos Trout!

The gut contents of Herichthys minckleyi were examined to determine if variation in jaw morphology in this trophically polymorphic cichlid fish is associated with feeding specialization. First, individual H. minckleyi were categorized as having molariform, papilliform, or undetermined pharyngeal morphology. Molariforms possess large molar-like teeth and robust crushing musculature, papilliforms exhibit only small pointed papilliform teeth on their more gracile jaws, and undetermined individuals (\textless 50 mm standard length) were not clearly assignable to one of the other two morphotypes. Undetermined individuals did not consume as great a percentage of plant material as papilliforms and never crushed snails. Aquatic arthropods comprised a small proportion of prey material recovered from the three pharyngeal morphotypes. Papilliforms, once size was accounted for as a covariate, consumed substantially more plant detritus than molariforms, while molariforms consumed snails more frequently than papilliforms. Even when only comparing molariforms and papilliforms that did consume snails, molariforms consistently crushed a greater number of the robust and extremely abundant snails Mexipyrgus churinceanus, Mexithauma quadripaludium, and Nymphopilus minckleyi. Contrary to expectation, there was no relationship between molariform standard length and the number of snails crushed. However, greater molariform tooth number, adjusted for standard length, was correlated with the inclusion of snails in molariform gut contents. The diet differences recovered between molariform and papilliform H. minckleyi suggest current ecological differentiation plays a role in maintaining this trophic polymorphism.

Stromatolites, the dominant Precambrian life form, declined in the Phanerozoic to occur today in only a few sites. This decline has been attributed to evolution of metazoan grazers, but stromatolites in our study site, Cuatro Cie'negas, Coahuila, Me'xico, harbor diverse macroinvertebrates. Drawing on food chain theory, we hypothesized that fish predation on invertebrates controls invertebrate populations, allowing stromatolites to flourish in Cuatro Cie'negas. Our experiment used small mesh (1 mm) cages to exclude all but larval fishes, and larger (6.5 mm) cages to exclude all larger fishes (including the molluscivorous and omnivorous endemic polymorphic cichlid, Herichthys minckleyi), but allow access to all sizes of the abundant endemic pupfish, Cyprinodon bifasciatus. No effects of treatments on invertebrate densities were noted at 6 week, but significant effects were observed on specific taxonomic groups after 3 month. In absence of fishes, hydrobiidae snails and ceratopogonids increased 3- and 5-fold, respectively, and invertebrate assemblage composition varied among treatments. Algal biomass was not affected by treatments, but algal species composition appeared to change. Overall results suggest that fish assemblages structure invertebrate assemblages, and that fishes may also be factors in determining algal communities.

A new family (Lacantuniidae), genus and species of catfish, Lacantunia enigmatica, is described from the Río Usumacinta basin of Chiapas, México. This odd siluriform is diagnosed by five distinctively autapomorphic and anatomically complex structures. The fifth (last) infraorbital bone is relatively large, anteriorly convex and remote from a prominent sphenotic process. The lateral margin of the frontal, lateral ethmoid and sphenotic bones are thick at the origins of much enlarged adductor mandibulae and levator arcus palatini muscles; otherwise the skull roof is constricted and flat. One pair of cone-shaped "pseudo-pharyngobranchial" bones is present at the anterior tips of enlarged cartilages medial to the first epibranchial. A hypertrophied, axe-shaped uncinate process emerges dorsally from the third epibranchial. The gas bladder has paired spherical, unencapsulated diverticulae protruding from its anterodorsal wall. Lacantunia enigmatica cannot be placed within or as a basal sister lineage to any known catfish family or multifamily clade except Siluroidei. This species may represent an ancient group, perhaps of early Tertiary age or older, and it adds another biogeographic puzzle to the historically complex Mesoamerican biota.

With an origin dating back 400 million years, fishes represent the most ancient group of vertebrates (Helfman et al. 1997). They are also the most diverse, with more than 25,000 species. Of the more than 2200 species known from Mexico, about 500 live in freshwater. The Mexican Official Norm NOM-059-ECOL-2001 lists only 186 fish species among the 1515 vertebrates "At Risk" in Mexico (SEMARNAT 2002; see chapter 4). Fishes thus account for only 12.3% of all listed species in Mexico, compared to 30.8% for reptiles, 24.8% birds, and 19.5% mammals. Why are so few fishes listed in Mexico? The answer probably has little to do with actual conservation status and more to do with other factors. First, the great taxonomic diversity of fishes renders any comprehensive evaluation of their conservation status quite daunting. Not only are fishes more than half of all vertebrate species, but new species continue to be described every year (Helfman et al. 1997). Because fishes live only in water, they are more difficult to observe than are most other vertebrates. Finally, fishes show a high degree of intraspecific phenotypic variation that makes them highly sensitive to environmental factors and often difficult to identify (Allendorf et al. 1987; Allendorf 1988). Fishes are important to humans because they represent an important source of food. Their commercial and recreational value has led to fish farming on an industrial scale, both for easy exploitation and as a means to recover overharvested natural populations. Scientific interest in fishes is also considerable. Those species easy to manage in captivity can be used in laboratory experiments. Additionally, freshwater fishes in particular can be used as biogeographic indicators, contributing important information to our understanding of the history of river basins and serving as indicators of aquatic ecosystem health. Though their aquatic habitats perhaps make wild fish populations more difficult to study than terrestrial organisms, they clearly deserve greater emphasis in the field of biological conservation. The northern part of Mexico harbors 3 aquatic ecoregions known as the Sonoran, ChihuahuanPotosian, and Tamaulipan regions (ContrerasBalderas 1969). The Tamaulipan ecoregion is located between the Sierra Madre Oriental and the Gulf of Mexico, within the Mexican states of Coahuila, Nuevo Leon, and Tamaulipas (CONABIO 2000); the last of these states is the focus of this chapter. To the north, Tamaulipas is bounded by the Rio Grande (Rio Bravo), which marks the border with Texas. To the west, Tamaulipas is bounded by the Mexican states of Nuevo Leon and San Luis Potosi, to the east by the Gulf of Mexico, and to the south by the states of Veracruz and San Luis Potosi (fig. 7.1). The geomorphology of watersheds influences species richness (Eadie et al. 1986). For example, river discharge is a direct measure of availability of habitat for freshwater fishes (Livingstone et al. Patterns of Species Diversity and Ecological Importance of Natural Ecosystems 1982), and there is a positive correlation between species richness and surface area of a river basin (Horwitz 1978). Thus, anthropogenic alterations of a watershed can drastically reduce its associated biological diversity (Sheldon 1987). Decreases in total habitat area and habitat fragmentation (typically a result of dams) occur conjunctively, reducing not only the size of many populations but also the potential for dispersal and genetic flow (Frankham et al. 2001). Additionally, diversion canals linking once separate waterways, and the introduction of exotic species, are both leading to homogenization of aquatic faunas (Sheldon 1988). Due to the rapid increase in hu.man populations, northern Mexico has experienced major, humaninduced alterations and fragmentation of its watersheds and associated changes in the distribution of taxa and loss of biodiversity (Contreras-Balderas 1978). Clearly, any conservation effort requires an inventory of the ichthyofauna using a taxonomic and biogeographic approach, focusing on documenting and maintaining overall biodiversity, but also including the rare and endangered species. The specific objectives of the study described in this chapter were to evaluate the diversity of freshwater fishes in Tamaulipas, to characterize each watershed and analyze the status of its ichthyofauna, and to determine the level of anthropogenic impact on freshwater fish communities statewide. We begin with a description of the watersheds of Tamaulipas, then present a synthesis of the state of knowledge of taxonomy, biology, genetics, evolution, exploitation, and conservation of fishes in Tamaulipas and provide the first list of freshwater fishes assembled for the state.

A new family (Lacantuniidae), genus and species of catfish, Lacantunia enigmatica, is described from the Río Usumacinta basin of Chiapas, México. This odd siluriform is diagnosed by five distinctively autapomorphic and anatomically complex structures. The fifth (last) infraorbital bone is relatively large, anteriorly convex and remote from a prominent sphenotic process. The lateral margin of the frontal, lateral ethmoid and sphenotic bones are thick at the origins of much enlarged adductor mandibulae and levator arcus palatini muscles; otherwise the skull roof is constricted and flat. One pair of cone-shaped "pseudo-pharyngobranchial" bones is present at the anterior tips of enlarged cartilages medial to the first epibranchial. A hypertrophied, axe-shaped uncinate process emerges dorsally from the third epibranchial. The gas bladder has paired spherical, unencapsulated diverticulae protruding from its anterodorsal wall. Lacantunia enigmatica cannot be placed within or as a basal sister lineage to any known catfish family or multifamily clade except Siluroidei. This species may represent an ancient group, perhaps of early Tertiary age or older, and it adds another biogeographic puzzle to the historically complex Mesoamerican biota.

A recent survey of benthic macroinvertebrates of the Cuatro Cienegas basin found 118 species in the 21 sites collected. Four exotic macroinvertebrates that could threaten the native biota were found within or near the basin.

Herichthys minckleyi Kornfield & Taylor 1983 is an endangered, trophically polymorphic cichlid endemic to the Cuatro Ciénegas basin of Coahuila, Mexico. A "molariform" morph has stout pharyngeal teeth while a "papilliform" morph has numerous fine pharyngeal teeth. Individuals with intermediate pharyngeal dentition also exist, as does yet another morph, called the "piscivore," that has a comparatively more fusiform body than do "normal-bodied" individuals. Previous studies indicated that morphs utilize different food sources, thus suggesting morph-specific spatial segregation would be likely since food resource availability is spatially heterogeneous. We present data from an observational study of all morphs (but focusing on the 3 most common, normal-bodied ones - molariform, papilliform and intermediate) in a single spring pool, Poza Mojarral Oeste. We analyzed morph distribution in relation to habitat types, and document morph-specific differences in feeding behavior. Habitat partitioning on both spatial and temporal axes was also investigated. Habitat use by each molariform, papilliform, and intermediate pharyngeal morphs was found to be non-random. Morphs differed in habitat use, albeit with considerable overlap. Strong segregation among morphs was not detected in any season or time of day, but habitat use patterns varied seasonally within each morph and were consistently different among morphs. Morphs of this species displayed a variety of feeding behaviors, but in general all behave as feeding generalists. This endangered species may prove more difficult to manage than are other, non-polymorphic species. It is clearly important to manage not only for the maintenance of the species, but also for maintenance of its different morphs, which our study indicates may each require different mixes of habitat types⬚. We thus hypothesize that any changes in habitat heterogeneity will lead to altered proportions of the different morphs of the species.⬚

We studied how pharyngeal jaw morphology influences feeding performance and prey use in the trophically polymorphic cichlid fish Herichthys minckleyi. In this species, the pharyngeal jaw exhibits two discrete morphologies. "Molariforms" possess robust jaw structure with flattened teeth and enlarged muscles, and "papilliforms" exhibit more gracile jaws, pointed teeth, and smaller muscles. The threefold difference in molariform and papilliform pharyngeal muscle mass found in an ontogenetic series of individuals (Range: 65 mm to 146 mm) support the hypothesis that the morphotypes differ in crushing force used to process prey. For both morphotypes, we also tested for tradeoffs in handling time and the ability to process several types of prey that occur in H. minckleyi's native habitat, Cuatro Ciƒ\_snegas. Unexpectedly, handling time on plants and arthropods was only slightly different between morphotypes. However, papilliforms consistently shredded plants more finely than molariforms, while only molariforms appeared capable of crushing snails. We inferred molariforms routinely employed their maximum force producing capabilities in the wild to crush the exceptionally hard snails endemic to Cuatro Ciƒ\_snegas. Comparisons with other molluskivorous fish suggested that the amount of hard-shelled prey H. minckleyi ingests is not unusual, but the mass of its pharyngeal musculature and the force used to crush snails is unparalleled.