[Back to Meeting 2003]

Patterns and Processes in the Evolution of Fishes

Symposium organized by
Francesco Santini and Gustavo Ybazeta
Department of Zoology, University of Toronto
Ramsay Wright Zoological Laboratories, 25 Harbord Street, Toronto, M5S 3G5, Canada
Fax: ++1-416-978 8532
Email: fsantini@zoo.utoronto.ca, gybazeta@zoo.utoronto.ca.

Fishes comprises over 50% of all described species of vertebrates. They are present in, and constitute a large part of, the biodiversity of every aquatic ecosystem, and often play a dominant ecological role. In spite of their importance, fishes are the least known and least studied vertebrates. The patterns of their evolution, and the processes that have promoted them are much less understood than those of the various clades of tetrapods. Furthermore, an understanding of fish evolution is hampered by the lack of communication that has historically existed between pattern-oriented biologists, such as systematists and paleontologists, and process-oriented scientists such as ecologists, biogeographers and developmental biologists. This symposium aims at bringing together pattern- and process-scientists, in the search for a better understanding of the evolution of non-tetrapod vertebrates. Our program will bring together systematists and comparative morphologists with developmental biologists and genome scientists, for exciting discussions about the origin of novel morphologies and their roles in the diversifications of the various fish clades. Historical biogeographers and paleobiologists will have the opportunity to talk with community ecologists and ecological biogeographers about patterns of diversifications and their causes.
This symposium is extremely timely. First, it will be a clear example of the importance of integrative and comparative biology, and most talks will have a strong phylogenetic component. The symposium will provide an opportunity to consolidate cutting edge information in fish systematics. Furthermore, certain fish taxa, such as Takifugu rubripes and Danio rerio have been selected as model organisms for the study of the vertebrate genome and vertebrate development. This symposium will bring together scientists who work on model organisms with scientists working on non-model organisms, promoting ichthyological research of greater depth and breadth.


DAY 1 - Monday January 6th, P.M.
12.50 - 13.00Francesco Santini & Gustavo Ybazeta. "Welcome and introduction to the symposium."
13.00 - 13.40John G. Maisey. "CT scanning and the evolution of the elasmobranch braincase."
13.40 - 14.20Gloria Arratia. "Origin and early radiation of Teleosteii."
14.20 - 15.00G. David Johnson. "Diversity of the Acanthomorpha."
15.00 - 15.20COFFEE BREAK
15.20 - 15.40Masaki Miya. "Major patterns of actinopterygian phylogenies: A new perspective based on >200 complete mitochondrial DNA sequences."
15.40 - 16.00Agnes Dettai. "New clades within the acanthomorph radiation from taxonomic congruence among nuclear and mitochondrial genes."
16.00 - 16.20Lynne R. Parenti. "Evolution and Phylogeny of Gonad Morphology in Bony Fishes."
16.20 - 16.40Andrew P. Martin. "Gene duplications and phylogenetic inference of vertebrates: the perils of paralogy"
16.40 - 17.00Questions and answers.

DAY 2 - Tuesday, January 7th, A.M.
8.20 - 8.40John H. Postlethwait. "The role of genome duplication in the origin of developmental novelties and radiation of ray-fin fish."
8.40 - 9.00Edmund Stellwag. "Ray-finned fish genome duplication, organismal complexity and gene value optima."
9.00 - 9.20Paula M. Mabee. "Evolution of Median Fin Modules in the Axial Skeleton of Fishes."
9.20 - 9.40Mark W. Westneat. "Functional Morphology of Feeding in Fishes: Phylogenetic Trends in Mechanical Design of Cranial Levers and Linkages."
9.40 - 10.00Questions and answers.
10.00 - 10.20COFFEE BREAK
10.20 - 10.40Eldredge Bermingham. "Evolutionary assembly of the Mesoamerican freshwater fish fauna."
10.40 - 11.00Anindo Choudhury. "Parascript studies and the historical biogeography of North American freshwater fishes."
11.00 - 11.20Peter Sale. "Connectivity and Structure of Reef Fish Communities."
11.20 - 11.40Pedro Peres Neto. "Patterns in the co-occurrence of fish species in streams: the role of site suitability, morphology and phylogeny versus species interactions."
11.40 - 12.00Questions and answers.


Dr. John Maisey
Division of Paleontology
American Museum of Natural History
Central Park West at 79th Street
New York, New York 10024, U.S.A.
Email: maisey@amnh.org
CT scanning and the evolution of the elasmobranch braincase.
The archetypal status of the elasmobranch braincase is challenged by findings from CT scan analyses of both Recent and fossil shark neurocrania. Inner ear morphology in modern elasmobranchs is highly derived, and many of the same specializations are recognized in extinct hybodonts, although more primitive taxa such as xenacanths and symmoriids lack these features.
An osteichthyan like cranial fissure is primitively present in elasmobranchs, and the braincase of the cladistically primitive early Devonian chondrichthyan Pucapampella is anatomically quite similar to that of a primitive actinopterygian, Ligulalepis (also early Devonian), despite lacking a series of dermal bones. The platybasic arrangement of most elasmobranch braincases is probably primitive; the tropibasic arrangement (in which the interorbital septum is very narrow) is rarely developed, and characterizes chimaeroids and extinct symmoriid sharks. This seems to have come about by enlargement and posterior migration of the eyeballs, as in actinopterygians and some arthrodires.

Dr. Gloria Arratia
Humboldt-Universität zu Berlin
Museum für Naturkunde
Institute of Palaeontology
D-10099 Berlin, Germany
Email: gloria.arratia@museum.hu-berlin.de
Origin and Early radiation of Teleostei
The Teleostei and its sister-group the Halecomorphi are currently included in the Division Halecostomi. During the 30 last years, fundamental work concerning the monophyly of the Teleostei, as well as of the Halecomorphi, and the phylogenetic relationships among halecostome subgroups has been generated. Although there is apparently no doubt concerning the monophyly of both the Halecomorphi and Teleostei, new investigations on Triassic and Jurassic 'pholidophoriforms' raise important questions concerning (1) the monophyly of these groups, (2) the evolutionary significance of certain features,  (3) the age of the Teleostei versus that of the stem-groups, and (4) the content of the halecostome subgroups. For instance, the three characters (e.g. symplectic articulating with lower jaw; one supramaxilla) currently accepted as supporting the monophyly of Halecomorphi turn into homoplasies because they are present in some of the Triassic pholidophorid genera and in stem-group teleosts. The monophyly of the Teleostei stands only on one uniquely derived morphological character (quadrate with long posteroventral process), and all other characters previously proposed in the literature as uniquely derived become homoplastic. The study of Triassic pholidophorids, at the base of the Teleostei in the cladogram, reveals that 'true' teleosts are much older than most of the taxa that have been suggested as stem-group teleosts; thus, the group arose probably in the Middle-Early Triassic (~240 m.y. ago). Although by the Late Triassic the teleosts were represented by several genera, it was in the Jurassic, particularly the Late Jurassic (~151 m.y. ago), when most of the stem and some of the modern teleostean lineages (e.g. elopiforms) arose. In contrast, the osteoglossomorphs do not appear in the record until about 137 m.y. ago.

Dr. David G. Johnson
Division of Fishes
Smithsonian Institution
National Museum of Natural History
Washington, DC 20013-7012, U.S.A.
Email: johnson.dave@nmnh.si.edu
Diversity of the Acanthomorpha
The spiny-rayed fishes, Acanthomorpha, are the crown group of the Teleostei. With over 300 families and 16,000 species, they comprise more than 60% of extant teleosts and about one-third of living vertebrates. They exhibit staggering diversity in size (1cm-17m), body form, skeletal and soft anatomy, habitat, behavior, etc., much of which may differ strikingly through ontogeny. This presentation will attempt to illustrate some sense of the breadth of morphological diversity, including an ontogenetic perspective. Fossil acanthomorphs first appear sparsely at the base of the Upper Cretaceous. By the late Paleocene, the fauna is a bit more diverse, but at the Middle-Eocene, an explosive radiation essentially laid out the majority of acanthomorph diversity. Three unique innovations probably contributed to the extensive radiation of acanthomorphs: reductive restructuring of the dorsal gill arches, true fin spines, and a single median rostral cartilage facilitating upper jaw protrusion. Acanthomorph monophyly is well-supported morphologically, but hypotheses of intrarelationships are controversial or non-existent. Johnson and Patterson (1993) proposed an admittedly imperfect hypothesis of relationships among basal acanthomorph groups and redefined the crown group, Percomorpha, comprising about 245 families and 14,000 species, that best represent Nelson's (1989)unresolved "bush at the top." Despite considerable progress in establishing monophyly and composition of many percomorph orders, suborders and families, relationships among the majority remain obscure. Even the largest and most diverse order of vertebrates, the Perciformes (9800 species in over 200 families), remain uncharacterized. Clearly, THE outstanding problem in systematic ichthyology is sorting out the percomorph/perciform morass, which undoubtedly will also alter our current concept of basal acanthomorph relationships.

Dr. Andrew P. Martin
Department of Biology, University of Colorado
Boulder, Colorado, 80309, U.S.A.
Gene duplications and phylogenetic inference of vertebrates: the perils of paralogy
Genes are often members of multigene families. Members of multigene families are related by descent. The size of gene families depends on the rate of birth and death of individual genes. Genes are born by gene duplication and are lost by either deletion or through the accumulation of mutations. Comparative analyses of metazoan genes revealed rates of gene duplication can be on the order of the average gene substitution rate (10-6 -10-7 per gene per generation). In addition, duplicate genes can be retained for relatively long periods of time (i.e. millions of years). The continuous remodeling of gene families through the birth and death of genes has important implications for organismal phylogenetic inference based on gene trees. In particular, phylogenetic analysis of nuclear genes may suffer from hidden paralogy-namely, that genes sampled from different taxa are related through gene duplication, not speciation. Paralogy of sampled genes will cause an overestimation of the divergence time between species and can confound accurate inference of relationships among species. The problem of hidden paralogy is illustrated with the HSP70 gene from sharks of the order Lamniformes. Because HSP70 has been widely used for inferring organismal phylogenies, the results have implications for published trees. Moreover, the results from the HSP70 analysis underscore the distinction between gene and species trees and highlight an under-appreciated source of discordance between gene trees and organismal phylogeny; namely, that due to unrecognized paralogy of sampled genes.

Dr. Masaki Miya
Department of Zoology
Natural History Museum & Institute, Chiba
955-2 Aoba-cho, Chuo-ku, Chiba 260-8652
E-mail: miya@chiba-muse.or.jp
Major patterns of actinopterygian phylogenies: A new perspective based on >200 complete mitochondrial DNA sequences.
Actinopterygii, the ray-finned fish, is the most diversified group of all vertebrates, currently comprising over 25,000 species placed in 42 orders, 431 families, and 4075 genera. Because of the enormous species diversity involved, ancient origin that goes back some 400 million years ago, and the wide ranging variations not only in morphology but also in behavior, ecology, and physiology, there remains much controversy over the higher-level relationships of actinopterygians. Our research group has determined complete mitochondrial DNA sequences from unprecedentedly large number of species (ca. 400) that covers a whole spectrum of actinopterygian diversity. We will present results from phylogenetic analyses of this large data set which is divided into the following three hierarchical levels: 1) basal actinopterygians; 2) higher actinopterygians; and 3) percomorphs. We have discovered many examples of previously-unrecognized, major comprehensive groups of actinopterygians with high statistical support through preliminary phylogenetic analysis.

Ms. Agnes Dettai
Muséum National d'Histoire Naturelle
Laboratoire d'Ichtyologie Générale et Appliquée
43, rue Cuvier, F-75231 Paris Cedex 05, France
Email: adettai@mnhn.fr
New clades within the acanthomorph radiation from taxonomic congruence among nuclear genes.
In the last two years, decisive steps have been made in favor of a resolution of the famous "bush at the top" of the teleostean tree, that includes several large and diverse assemblages of true spiny fishes (Acanthomorpha) and comprises more than 60 % of "fish" species. To investigate acanthomorph interrelationships, we are working on the separate and combined analyses of several independent molecular datasets for a large number of taxa (more than 90). The separate analyses allow a comparison of the resulting trees, identify the groups found in several or all of the datasets: those are considered to be probably due to the common phylogenetic signal. The groups obtained for only one dataset are dismissed (at least provisionally, until more information can be gathered) as results of the biases peculiar to that dataset. The results for mitochondrial ribosomal genes 12S and 16S, partial 28S nuclear ribosomal gene and the nuclear gene encoding rhodopsin have been presented before, but 1100 bp of the nuclear gene MLL (mixed-lineage leukemia like) have been obtained, adding a new dataset, and more important, adding new recurrent (henceforth considered as reliable) clades and confirming others. These clades can be proposed for future research in morphology and/or molecular phylogenetics of acanthomorphs.

Dr. Lynne Parenti
Division of Fishes
Smithsonian Institution
National Museum of Natural History,
Washington, DC 20013-7012, U.S.A.
Email: parenti.lynne@nmnh.si.edu
Evolution and Phylogeny of Gonad Morphology in Bony Fishes
Gonad morphology at the gross anatomical or histological levels has long been studied to identify annual reproductive cycles and length of breeding season, among other goals. Comparative surveys across vertebrate taxa have not been detailed enough, however, 1) to describe fully the differences and similarities between gonads of bony fishes and other vertebrates, and 2) to use gonad morphology in phylogenetic analyses. These are our aims. First, an emerging constant among vertebrates is the similar morphology of the germinal epithelium. All osteichthyans have a germinal epithelium composed of somatic and germ cells in male and female gonads. Activity within the germinal epithelium allows us to understand why lower vertebrates have indeterminate fecundity whereas higher vertebrates do not: in mammals and birds, the germinal epithelium produces oocytes only during embryonic development, whereas in fishes the germinal epithelium produces oocytes throughout the reproductive life of a female. Second, we present examples at higher and lower taxonomic levels, focusing on model organisms, of the relationship between gonad morphology and phylogeny. For example, lower teleosts have an anastomosing tubular testis which we identify as primitive for bony fishes; whereas higher teleosts, such as the medaka, Oryzias latipes have a derived lobular testis. Further, atherinomorph fishes, the medaka and allies, have a unique type of lobular testis and egg morphology correlated with an array of reproductive modifications such as sperm-bundle formation, internal fertilization, embryo retention, and live-bearing. The use of reproductive morphology to identify phylogenetic patterns within bony fishes is explored.

Dr. Paula Mabee
Department of Biology
414 East Clark Street
University of South Dakota
Vermillion, SD 57069-2390, U.S.A.
Email: pmabee@usd.edu
Evolution of Median Fin Modules in the Axial Skeleton of Fishes
Broadly conserved phylogenetic patterns in the directions of development within the median fins of fishes were identified from a survey of the developmental morphology of actinopterygian fishes. Four modules involved in their positioning and patterning were hypothesized. The earliest actinopterygians likely had dorsal and anal fins that were symmetrically positioned via a positioning module. The common patterning (differentiation) of skeletal elements within the dorsal and anal fins may have been set into motion by linkage to this positioning module. Frequent evolutionary changes in dorsal and anal fin position indicate a high level of dissociability of the positioning module from the patterning module. In contrast, the patterning of the dorsal and anal fins remains linked: in nearly all fishes, the endo- and exoskeletal elements of the two fins co-differentiate. In all fishes the exoskeletal fin rays differentiate in the same directions as the endoskeletal supports, indicating complete developmental integration. In acanthopterygians, a new first dorsal fin module evolved via duplication and divergence. The evolutionary sequence of their hierarchical assembly and secondary dissociation provides an example of how hierarchical assemblages of modules change over time. The changes in these modules during the evolution of fishes appear to be produced through dissociation, duplication and divergence, and co-option. Although the relationship between identified median fin modules and underlying mechanisms is unclear, Hox addresses may be correlated. Comparing homologous gene expression and function in various fishes may test these predictions. The median fins provide an example of how basic modularity is maintained over 400 million years of evolution.

Dr. John H. Postlethwait
Institute of Neuroscience, University of Oregon
1425 E. 13th Ave.
Eugene, OR, 97403, U.S.A.
The role of genome duplication in the origin of developmental novelties and radiation of ray-fin fish
Ecological and genomic factors have both contributed to the remarkable diversity of ray-fin fish. Our maps of the zebrafish genome revealed a genome duplication event, and comparative genomic analysis showed that this occurred before the teleost radiation but after the divergence of ray-fin and lobe-fin fish. To investigate the role of genome duplication in evolution, we are studying molecular developmental mechanisms in a phylogenetically nested set of ray fin fish {amia[zebrafish(stickleback)(pufferfish)]} and comparing them to the lobe-fin fish Homo sapiens. For about 30% of human genes, zebrafish has two orthologues (called co-orthologues). For the other 70%, one of the duplicate copies has mutated to become a pseudogene or has been lost entirely. We have investigated the roles of duplicated Hox, Sox, and Krox genes during development by expression analysis and gene ablation studies. In general, duplicate genes appear to have been preserved by subfunctionalization, the parcelling out of ancestral gene functions between the two fish co-orthologues. In some cases, different gene copies have been preserved in the different species. In other cases, orthologous gene duplicates have evolved different developmental functions in the different species. We propose that a genome duplication event at the base of the teleost radiation may have provided genetic redundancies that facilitated the evolution of novel morphologies, and thus may have contributed to the wonderful diversity of teleost fish.

Dr. Edmund Stellwag
Department of Biology
East Carolina University
Greenville, NC 27858, U.S.A.
Email: stellwage@mail.ecu.edu
Ray-finned fish genome duplication, organismal complexity and gene value optima
A growing body of evidence supports the hypothesis that genome expansion among chordates was fueled by a series of complete genome duplications. A corollary hypothesis proposes that these genome level duplications provided the genetic basis for the continuous increase in organismal complexity observed during chordate evolution. The discovery of an additional genomic duplication exclusive to the actinopterygian lineage, unparalleled by a similar duplication in sarcopterygians, has called into question the relationship between genome expansion and organismal complexity. Our laboratory is exploring a concept that we refer to as the Total Gene Value Optimum (TGVO). We hypothesize the existence of an optimum total gene value (Go) required for the maintenance of a particular level of organismal complexity. Given that genome duplications sharply increase the total gene value, we argue that selection will favor reduction in the total gene value toward Go, particularly in cases when genome duplications are unlinked to increases in organismal complexity. In a related fashion, reductions in the total gene value below Go may result in concomitant decreases in organismal complexity. The TGVO concept is useful in that it predicts that lineages exceeding Go resulting from genome level duplication will undergo gene loss toward the value of Go. Examination of the TGVO concept in actinopterygian and sarcopterygian lineages provides support for the TGVO concept, at least for the limited collection of developmental regulatory genes we have examined to date. Moreover, comparative genomic analyses suggest that actinopterygians are undergoing a reduction in gene content relative to sarcopterygians consistent with the TGVO concept.

Dr. Mark W. Westneat
Department of Zoology
Field Museum of Natural History
1400 S Lakeshore Dr
Chicago, IL 60605-2496, U.S.A.
Email: mwestneat@fieldmuseum.org
Functional Morphology of Feeding in Fishes: Phylogenetic Trends in Mechanical Design of Cranial Levers and Linkages
Fishes have met the challenge of capturing or ingesting food in a sensational diversity of ways. One of the hallmarks of this diversity is the complexity of the kinetic skull in fishes which may have 20 or more major skeletal elements capable of movement, driven by a score of different muscles. The feeding system in fishes is one of the central model systems in the field of vertebrate biomechanics. Recent research has focused on biomechanical models of the jaws that represent hypotheses of how force and motion are transferred from muscle through tendon to bone. This study develops a more complete set of such models for the many movable elements in the skulls of fishes. The lever mechanism of the lower jaw is joined by lever mechanisms for the skull, maxilla, opercle, and pectoral complex. New morphometric protocols and software applications for analysis of lever and linkage design are introduced. The geometric position and contraction physiology of muscles are critical to accurate estimations of lever and linkage function. A survey of skull mechanical designs throughout the phylogeny of fishes reveals a wide range of strategies for transfering force and motion during feeding. For example, the mechanical advantage of the mandible ranges from 0.05 in gar to 0.70 in damselfishes. Major evolutionary changes in the feeding mechanisms of fishes are illustrated from a morphological and mechanical perspective.

Dr. Giacomo Bernardi Department of Ecology and Evolutionary Biology
University of California Santa Cruz
100 Shaffer Road, Santa Cruz
95060, California, U.S.A.
Recruitment patterns of the coral reef three-spot damselfish using molecular markers
The three-spot damelfish, Dascyllus trimaculatus , displays a bipartite life history with a pelagic larval phase and a benthic adult phase. Juveniles recruit and live sheltered in anemones, and once large enough, leave the anemones and swim freely over coral heads. A genetic survey of Indo-Pacific populations showed that the species partitions in five clades: 1. Red Sea - Indian Ocean, 2. Hawaii, 3. Marquesas islands, 4. French Polynesia, 5. Pacific Rim. In the case of French Polynesian individuals, approximately 4% of the adults harbor Pacific Rim haplotypes. This rare molecular marker was used to determine recruitment patterns in the island of Moorea, French Polynesia. By placing anemones at different distances from the reef crest, inside the lagoon, we were able to catch new recruits and score their haplotypes. We found that frequencies of the Pacific Rim haplotype were vastly different between recruits and adults, and between locations of recruitment. Hedgecock effects and natural selection hypotheses are discussed.

Dr. Eldredge Bermingham Smithsonian Tropical Research Institute
Box 2072
Balboa, Ancon, Republic of Panama
Evolutionary assembly of the Mesoamerican freshwater fish fauna
Over the past 2-7 million years there has been extensive intercontinental exchange of flora and fauna between North and South America across the isthmian bridge of Panama, a phenomenon known as the Great American Interchange because of its importance for New World biogeography. Although freshwater fishes participated in the Great American Interchange, biogeographic studies of this group are few in comparison to the detailed and instructive studies of mammals. Yet, because the dispersal of primary freshwater fishes depends on direct connections between drainage basins, historical biogeographic analysis of freshwater fishes permits strong inference regarding the biotic and geologic evolution of Mesoamerica. Using molecular systematic approaches, we have taken advantage of the unique isthmian experiment to investigate the modern assembly and diversification of a biota. We show that the primary freshwater fish fauna of Mesoamerica assembled in a relatively brief period of time, and posit several distinct, but relatively recent waves of invasion from putative source populations in northwestern Colombia. In subsequent colonization episodes the geographic scale of the dispersion of lineages was progressively more limited, a pattern we attribute to both biological contingency and landscape evolution. Thus, the fish eye view of Mesoamerica suggests a complex biogeographic history of overlaid cycles of colonization, diversification, sorting and extinction of lineages.

Dr. Anindo Choudhury
Division of Natural Sciences
St. Norbert College
100 Grant Street
DePere, Wisconsin 54115, U.S.A.
Email: anindo.choudhury@snc.edu
Parascript studies and the historical biogeography of North American freshwater fishes.
The historical biogeography of enduring associations between North American freshwater fishes and their metazoan parasites is explored in the tradition of parascript studies (using the language of parasites to reveal the history and biology of the hosts). Patterns of associations of selected groups of parasites (trematode flukes and nematodes) in three groups of freshwater fishes, the basal Acipenseriformes, and the ostariophysian families Ictaluridae and Catostomidae are examined/reviewed. Results suggest that: a) acipenserids were primitively diadromous, b) catostomidae are strong candidates as basal cypriniforms, and c) ictalurids do not have close biogeographical relationships with neotropical siluriforms, rather their affinities likely lie with the Afro-Asian siluriforms. The nature of such associations in the Mexican transition zone indicates little parasite exchange between neotropical and nearctic siluriforms. Hypotheses of historical colonization of North American freshwater environments from marine environments by ancestors of hosts such as Centrarchidae and Aplodinotus grunniens (the only truly freshwater Nearctic sciaenid) are strongly corroborated by the evidence from parasite systematics.

Dr. Peter Sale
Biological Sciences
University of Windsor
Room 119 BIO, 401 Sunset
Windsor Ontario N9B 3P4, Canada
Email: sale@uwindsor.ca
Connectivity and Structure of Reef Fish Communities.
While descriptive data can suggest that the diverse communities of fish on coral reefs are closely co-evolved, equilibrial assemblages of species, these are highly dynamic, non-equilibrial assemblages with structure driven more by patterns of recruitment of individual species, than by patterns of resource allocation among differently adapted phenotypes. As a consequence, local assemblages differ in structure, and structure wanders through time. Individual fish are confronted by different mixes of species in different times and places. The recruitment process that drives these dynamics is complex, being driven by several mechanisms, and local populations receive some portion of their recruitment from distant sources. Information on this connectivity among local populations is critically important for management which is based increasingly on use of marine protected areas (no-take zones) both to conserve, and to provide sustainable fisheries. At present, however, we do not know the spatial scale or the extent of this connectivity, and this critical knowledge gap impedes both management, and fundamental understanding. I will outline, and present preliminary results from ECONAR, a regional scale, multi-disciplinary project to delineate the scale and extent of this connectivity.

Dr. Pedro R. Peres-Neto
Départment de chimie-biologie - GREA
Université du Québec à Trois-Rivières
Trois-Rivières, Québec G9A 5H7
Email: pperes@uqtr.ca
Patterns in the co-occurrence of fish species in streams: the role of site suitability, morphology and phylogeny versus species interactions.
A number of studies at large scales have pointed out that abiotic factors and recolonization dynamics appear to be more important in structuring stream-fish assemblages than biotic interactions. In contrast, experimental and field studies at small scales show the importance of competition among stream fishes. However, given the highly variable nature of stream systems over time, competition may not be intense enough to generate complementary distributions via competitive exclusion at larger scales. Complementary distribution is a recurrent pattern observed in fish communities across stream gradients, though it is not clear the instances whether this pattern may be due to competitive interactions or to individual species requirements. In this study, I introduce a series of null models developed to provide a more robust evaluation of species associations by facilitating the distinction between different processes that may shape species distributions and community assembly. These null models were applied to test whether conspicuous patterns in species co-occurrences are more consistent with their differences in habitat use, morphological features and/or phylogenetic constraints, or with species interactions in fish communities in streams of a eastern Brazil watershed. I conclude that patterns in species co-occurrences within the studied system are driven by common species-habitat relationships and species interactions may not play a significant role in structuring these communities. I conclude by suggesting that large-scale studies, where adequate designs and robust analytical tools are applied can contribute substantially in understanding the importance of different types of processes in structuring stream-fish communities. Where appropriate, subsequent experiments at small scales, can determine whether the putative process is reasonable at smaller scales so that the power of explanation at larger scales is reinforced.