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    V: ASIH Symposium: Why are there so many kinds of fishes? A showcase of early-career ichthyologists, II

    2021-07-27   10:00 - 12:15

    Moderator: Luke Tornabene

    1.  10:00  VIRTUAL    Genomic architecture of lacustrine speciation and gene flow in the Waccamaw Darter (Etheostoma perlongum). Daniel MacGuigan*, University at Buffalo; Trevor Krabbenhoft, University at Buffalo; Nathan Backenstose, University at Buffalo; Tianying Lan, University at Buffalo; Thomas Near, Yale University

    Species are the product of myriad evolutionary forces, yet in many groups of organisms, speciation is primarily the product of geographic isolation. Darters, a clade of North American freshwater fishes, exemplify this pattern; nearly all darter sister species pairs occur in allopatry and are separated by millions of years of divergence. One of the only exceptions is the lacustrine Etheostoma perlongum, endemic to Lake Waccamaw, North Carolina, and the presumed closest relative of E. maculaticeps in the Waccamaw River. With no geographic isolation, natural selection in lotic versus lentic habitats is hypothesized to have driven speciation.Here, we examine morphological and genomic evidence of lacustrine speciation utilizing geometric morphometrics, traditional taxonomic characters, double digest restriction-site associated DNA (ddRAD) sequencing, and a de novo genome assembly for E. perlongum. Although E. perlongumis phylogenetically nested within the widespread E. maculaticeps, there is a sharp genetic and morphological break coinciding with the lake-river boundary. Demographic analyses reveal that E. perlongumand E. maculaticepsdiverged 8,000-39,000 years ago, consistent with the young age of Lake Waccamaw. Despite ongoing gene flow, we detect a 10 Mb putative chromosomal inversion with elevated divergence between E. perlongumand E. maculaticeps, consistent ecological speciation theory. Our results provide the first glimpse of the genomic architecture of speciation in darters and pave the way for future speciation studies of behavior, ecology, plasticity, and genomics. Etheostoma perlongumillustrates that rapid ecological speciation with gene flow is possible even in clades where geographic isolation is the dominant mechanism driving diversification.

    2.  10:15  VIRTUAL    Early Career: Why are there so many kinds of fishes? The evolution of species richness in fish adaptive radiations. Catherine Wagner*, University of Wyoming

    African cichlids provide well-known examples of adaptive radiation, and the replicated nature of these radiations presents opportunities for examining predictors of their composition and species richness. Previous work has shown that species richness in African cichlid radiations is best predicted by lake surface area, and that the evolutionary species-area relationship is nonlinear. This pattern is better supported by models assuming a relationship between total diversity and area, rather than a relationship between diversification rate and area, a finding consistent with diversity-dependent diversification. Although the concept of diversity dependent diversification has been met with controversy in recent years, I here argue that a large part of this controversy is due to semantic issues rather than to a fundamental disagreement with the implied evolutionary processes. I further discuss the gaps in our understanding of the ecology of adaptive radiations that may help us to better understand diversification dynamics and the evolution of species richness in adaptive radiation.

    3.  10:45  VIRTUAL    A Paleontological Perspective on the Early Evolution of the Largest Living Vertebrate Group. Sam Giles*, University of Birmingham; Stephanie Pierce, Harvard University; Matt Friedman, University of Michigan

    Ray-finned fishes comprise half of living vertebrates and include animals typically seen in an aquarium or on a dinner plate, such as trout, seahorses and flatfish. Their deep evolutionary roots--which stretch back nearly half a billion years--and inaccessible internal anatomy--which is often hidden in fossil taxa-- obscure both the relationships between fossil and living fishes and the age of origin of the living groups. Recent integration of molecular and morphological data led to a revised timescale for the origin of the crown node of ray-finned fishes, bringing it roughly in line with the End-Devonian Mass Extinction and suggesting major diversification in the aftermath of this event. Here we present a new articulated ray-finned fish from the Late Devonian ‘Chemung facies’ of Warren, Pennsylvania, USA. Although this specimen has been known for over a century, its miniscule size (ca. 60 mm total length) has precluded detailed investigation. MicroCT scanning reveals crucial anatomical details, especially of the head, that are surprisingly derived features for a Devonian ray fin. These features, such as a fused dermohyal and perforate aortic canal, are typically restricted to stratigraphically younger taxa. Phylogenetic analysis places the Warren fish among post-Devonian forms, implying cryptic diversification of many lineages prior to the Devonian-Carboniferous boundary. These results support latest Devonian and earliest Carboniferous forms as being critical to understanding evolutionary dynamics in early ray-finned fishes as well as the impact of the Hangenberg mass extinction on shaping the largest living vertebrate group.

    4.  11:00  VIRTUAL    Mega-phylogenies Elucidate the Pervasiveness and Ecology of the ‘Living-Fossil’ Phenomenon. John Sime*, University of Pennsylvania; Jonathan Chang, Los Angeles County Department of Public Health; Michael Alfaro, University of California, Los Angeles; Lauren Sallan, University of Pennsylvania

    Amid the many kinds of extant ray-finned fishes, clades with low diversity are apparently as common a feature as large radiations. The presumed failure of some ray-fin lineages to diversify while persisting over geological timescales would seem anomalous in light of accepted drivers of diversification. The factors responsible for low-diversity persistence are still a puzzle more than 150 years after Charles Darwin described freshwater, non-teleost fishes (bichirs, bowfins, paddlefish, sturgeon, and gars) as "living fossils." Are living fossils the result of a blend of active processes? How widespread are persistent, depauperate lineages in phylogenies? We propose that living fossils are a widespread phenomenon; in the fossil record, they may appear as "dead clades walking" following mass extinctions. The ecological traits of fishes that exemplify these phenomena--large body size, life in freshwater, and filter feeding--lead us to hypothesize that they have converged on similar niches. A molecular, fossil-calibrated, mega-phylogeny of extant ray-finned fishes (~32,000 tips) allows a test of that prediction using clade (im)balance, a measure of sister clade parity. Preliminary analysis shows that the phylogeny is more imbalanced than null simulations. A large plurality of sister clades over 20 million years old have species ratios of 1:10; most smaller clades have five or fewer species. Thus, the branching pattern of Darwin’s living-fossil fishes, as successive outgroups to the hyper-diverse teleosts, is mirrored within teleosts. We are now analyzing the distribution of ecological traits across imbalanced sister clades to test whether there are significant markers of “living fossil” status.

    5.  11:15  VIRTUAL    The Fossil Record of Extant Elasmobranchs. Catalina Pimiento*, University of Zurich

    Sharks and their relatives (Elasmobranchii) are disproportionally threatened with extinction due to various anthropogenic pressures. They have a long evolutionary history and an abundant fossil record that consists mainly of teeth. Many fossil taxa have living representatives. However, little is known about the representation of extant elasmobranchs in the fossil record, the quality of such record and how far back in time does it go. We addressed these questions by quantifying the fossil record of elasmobranchs living in our oceans today across taxonomic levels, ecological traits and extinction risk status. Our results challenge the assumption that the fossil record becomes worse backwards in time, and reveal a robust representation of higher taxonomic ranks, with all orders, most families and over half of the extant genera having a fossil record. Further, they show that 10% of the current global species diversity is represented in the geological past. While the fossil record of extant genera extends as far back as ~190 Ma, the fossil record of extant species extends ~66 Ma. No significant differences were found in the extent of the fossil record between ecological traits; however, we found that species currently threatened with extinction have a significantly older fossil record than non–threatened species. Our study demonstrates that the geological history of extant elasmobranchs has great potential to advance our understanding of how species currently facing extinction have responded to different stressors in the past, thereby providing a Deep Time perspective to conservation.

    6.  11:30  VIRTUAL    Early Career: Fins for the Win. Brooke Flammang*, New Jersey Institute of Technology

    The water environment presents an opportunity for organisms to move in any direction and be relatively freed from the constraints of gravity. If an organism in this fluid environment has a body shape and stiffness that allows it to push against and through the water effectively, it can capitalize on speed and maneuverability within a broad fitness landscape. Early fishes and secondarily aquatic vertebrates all converged on a similar stiff, lobate fin shape for propulsion, which makes good hydrodynamic sense for generating lift and may be the most effective paddle for animals of substantial inertial mass. However, highly flexible propulsors, like those in ray-finned fishes, offered new advantages to manipulate fluid and increase maneuverability. In addition, flexible fins have been the basis for many new behavioral strategies and novel functional morphologies. Herein we will discuss performance-selected fin morphologies that have allowed fishes to not only rule all depths of the sea, but to take to the land and air as well. I will focus on my and my lab’s research on fin shape and stiffness, maneuverability, and a broad range of locomotor strategies through the perspectives of functional morphology, comparative biomechanics, and bioinspired robotics.

    7.  11:45  VIRTUAL    Early Career: An Evolutionary-Ecological Perspective on the Lives of Charismatic Cartilaginous Fishes. Kayla Hall*, University of Washington; Peter Hundt, University of Minnesota; Matt Kolmann, University of Michigan; Adam Summers, University of Washington; Allison Bronson, CSU Humbolt

    Chondrichthyes (chimaeras, sharks, skates and rays) are a 400+ million year old vertebrate lineage with skeletons composed entirely of cartilage. Throughout their long evolutionary history, chondrichthyans diversified into a variety of freshwater and marine habitats, distinctive ecological roles, and an array of life history traits. Chondrichthyans have evolved nearly all types of known vertebrate reproductive modes, ranging from egg-laying to live birth: oviparity, multipletypes of aplacental viviparity, and placental viviparity. They have alsoevolved many modes of locomotion, from axial-swimming (tail undulations),undulating or oscillating pectoral fins, to walking (or ‘punting’) along thesubstrate with pelvic fins. We described the ecological structuring of extantchondrichthyan communities over time, with the aim of exploring correlates andmechanisms that have shaped modern assemblages. We used body size, reproductivemodes, and locomotive modes to establish the ecospace occupation of extantspecies from all aquatic ecosystems. This meta-analysis allows us to viewecosystem and community structures in 3D ecospace and contrast the functionaltraits and ecological roles across species. The relevant ecospace thesecommunities fill is limited compared to the theoretical ecospace availablethroughout the world’s waterways, especially in the deep-sea for instance. Wesimulated ancestral states of reproduction on a complete modern phylogeny,which supports the evolution of viviparity much earlier in chondrichthyanhistory than previously suspected. Overall, these results corroborate what has been found in the paleontological record, and allows us to reinterpret the evolution of vertebrate reproductive modes and cartilaginous fish communities in deep time.

    8.  12:00  VIRTUAL    Early Career: Flexible Stems and Ecological Opportunities - the Role of Phenotypic Plasticity in Generating Fish Diversity. Dina Navon*, Rutgers University

    Understanding the generation and maintenance of biodiversity remains an elusive but critical goal of evolutionary biology, despite many fruitful attempts to furnish this understanding. Fish are arguably the most diverse lineage in the vertebrate family tree – therefore, identifying the factors that have led to the evolution of fish diversity is crucial to understanding biodiversity as a whole. Phenotypic plasticity, where a single genotype gives rise to multiple phenotypes in the face of alternate environmental regimes, is likely instrumental in the evolutionary success of fish, as it can provide a mechanism for these animals to thrive even when faced with novel or unpredictable habitats. Plasticity can also influence evolutionary trajectories, as an ancestral plastic population may give rise to independent lineages that evolve along the axis of variation initially established by the plastic response. In this talk, I will describe two important examples of plasticity facilitating fish evolution: in threespine stickleback and African cichlids. Both evolve and are plastic along a common, major axis of fish variation, the benthopelagic axis. This axis encompasses coordinated shifts in behavior, ecology, and morphology related to the animals’ preferred feeding habitat and mode. I will discuss how these two models have furthered our understanding of the genetic basis of plasticity as well as its consequences on the evolutionary landscape. Ultimately, my talk will show that one main answer for the question, “why are there so many fishes?”, lies in this important trait – phenotypic plasticity, which gives fish the flexibility to react immediately to environmental input.

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