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1. Specimen alignment with limited point-based homology: 3D morphometrics of disparate bivalve shells (Mollusca: Bivalvia)

   https://doi.org/10.7717/peerj.13617  

   Edie, Stewart M. ; Collins, Katie S. ; Jablonski, David ( January 2022 , PeerJ)

   Background Comparative morphology fundamentally relies on the orientation and alignment of specimens. In the era of geometric morphometrics, point-based homologies are commonly deployed to register specimens and their landmarks in a shared coordinate system. However, the number of point-based homologies commonly diminishes with increasing phylogenetic breadth. These situations invite alternative, often conflicting, approaches to alignment. The bivalve shell (Mollusca: Bivalvia) exemplifies a homologous structure with few universally homologous points—only one can be identified across the Class, the shell ‘beak’. Here, we develop an axis-based framework, grounded in the homology of shell features, to orient shells for landmark-based, comparative morphology. Methods Using 3D scans of species that span the disparity of shell morphology across the Class, multiple modes of scaling, translation, and rotation were applied to test for differences in shell shape. Point-based homologies were used to define body axes, which were then standardized to facilitate specimen alignment via rotation. Resulting alignments were compared using pairwise distances between specimen shapes as defined by surface semilandmarks. Results Analysis of 45 possible alignment schemes finds general conformity among the shape differences of ‘typical’ equilateral shells, but the shape differences among atypical shells can change considerably, particularly those with distinctive modes of growth. Each alignment corresponds to a hypothesis about the ecological, developmental, or evolutionary basis of morphological differences, but we suggest orientation via the hinge line for many analyses of shell shape across the Class, a formalization of the most common approach to morphometrics of shell form. This axis-based approach to aligning specimens facilitates the comparison of approximately continuous differences in shape among phylogenetically broad and morphologically disparate samples, not only within bivalves but across many other clades. 

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2. Documenting Environmental and Oceanic Variability in the Down East Coastal Region of the Gulf of Maine using the long-lived bivalve Arctica islandica

   Walton, A. ; Griffin, S.M. ; Wanamaker, A.D. ; Jarosinski, L. ( April 2022 , PAGES Open Science Meeting)

   The Gulf of Maine (GoM) is currently undergoing rapid environmental and ecological changes and this system is projected to become increasingly stressed in the coming decades. Thus understanding past spatial and temporal conditions of this region is key to understanding how future environmental changes and extreme events may impact fisheries and ecosystem dynamics in the GoM. Changes in the physical and chemical variations in the shells of mollusks can be used as a powerful proxy for studying past climates and environments. In this study, we used the growth and geochemistry signatures in the long-lived marine bivalve Arctica islandica collected from the Down East coastal region in the Gulf of Maine (Jonesport, ME) to evaluate past climatic and hydrographic variability in the northwestern North Atlantic Ocean. The recent collection of shells extends a previously developed master shell growth chronology by 11 years and now spans from 1954 to 2020 CE. Based on visual crossdating techniques, shell growth variability is highly coherent among the population indicating that environmental conditions are driving growth. Variability in annually resolved shell growth increments and stable oxygen isotope values are largely related to sea surface temperatures (SSTs) and water mass properties of the Eastern Maine Coastal Current. This master shell growth chronology and annually-resolved isotope series will fill data gaps prior to instrumental records and allow us to better understand the spatial oceanographic variability in the GoM. 

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3. An inversion supergene in Drosophila underpins latitudinal clines in survival traits

   https://doi.org/10.1111/jeb.13310  

   Durmaz, Esra ; Benson, Clare ; Kapun, Martin ; Schmidt, Paul ; Flatt, Thomas ( June 2018 , Journal of Evolutionary Biology)

   Chromosomal inversions often contribute to local adaptation across latitudinal clines, but the underlying selective mechanisms remain poorly understood. We and others have previously shown that a clinal inversion polymorphism inDrosophila melanogaster,In(3R)Payne, underpins body size clines along the North American and Australian east coasts. Here, we ask whether this polymorphism also contributes to clinal variation in other fitness‐related traits, namely survival traits (lifespan, survival upon starvation and survival upon cold shock). We generated homokaryon lines, either carrying the inverted or standard chromosomal arrangement, isolated from populations approximating the endpoints of the North American cline (Florida, Maine) and phenotyped the flies at two growth temperatures (18 °C, 25 °C). Across both temperatures, high‐latitude flies from Maine lived longer and were more stress resistant than low‐latitude flies from Florida, as previously observed. Interestingly, we find that this latitudinal pattern is partly explained by the clinal distribution of theIn(3R)Ppolymorphism, which is at ~ 50% frequency in Florida but absent in Maine: inverted karyotypes tended to be shorter‐lived and less stress resistant than uninverted karyotypes. We also detected an interaction between karyotype and temperature on survival traits. AsIn(3R)Pinfluences body size and multiple survival traits, it can be viewed as a ‘supergene’, a cluster of tightly linked loci affecting multiple complex phenotypes. We conjecture that the inversion cline is maintained by fitness trade‐offs and balancing selection across geography; elucidating the mechanisms whereby this inversion affects alternative, locally adapted phenotypes across the cline is an important task for future work.

    

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4. Testing for allocation strategies and evolutionary tradeoffs in the bivalve shell

   https://doi.org/10.1093/mollus/eyae047  

   Edie, Stewart_M ; Collins, Katie_S ; Jablonski, David ( November 2024 , Journal of Molluscan Studies)

   Tradeoffs are a fundamental aspect of evolution, such that organismal morphologies often reflect a combination of developmental constraints and functional requirements. The net result is a striking variety of allocation strategies—where and how resources are shunted into aspects of the body plan to meet these competing demands. The bivalve shell records aspects of the animal's allocation strategies as the relative investment in the carbonate valves and the internal soft parts, generally reflected by the volume of the internal cavity. The carbonate volume can be further partitioned among different components of the shell, including the sculptural elements thought to evolve partly under ecological controls. Assuming there are general limits to energy acquisition, bivalve individuals may show different allocations between the volumes of the inner cavity and shell, and possibly for the sculpture. Using 3D scans of 385 valves representing 339 extant marine species occurring in the Florida Keys, USA, we find a variety of allocation strategies but that tradeoffs in shell construction are uncommon. Instead, species with relatively high allocations to their shell thickness also have prominent sculpture, suggesting a structural relationship. Neither outer shell mineralogy nor microstructure appears to partition the tradeoff space, suggesting varied pathways to the fabrication of both convergent and divergent shell forms. Framing new morphological and physiological analyses of bivalves by allocation strategies recorded in their well-fossilized shells opens new opportunities for understanding an under-studied, macroevolutionary dynamic: how tradeoffs have shaped the class's 500 million-year evolutionary and ecological history.

    

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5. Evidence of coherent warming in the northwest Atlantic Ocean from Arctica islandica oxygen isotope records

   Thatcher, D. ; Whitney, N. ; Walton, A. ; Sun, H. ; Jarosinski, L. ; Wanamaker, A. ; LaVigne, M. ; Williams, B. ; Stewart, J. ( May 2023 , International Sclerochronology Conference)

   Parts of the northwest Atlantic Ocean, including the Gulf of Maine, along the northeastern coast of the United States, are warming at a rate as much as four times faster than the global ocean, according to instrumental and satellite records. In addition to the longer-term temperature increases, the frequency and severity of marine heat waves have been increasing. Combined, the ecological impacts are numerous and concerning, yet our understanding of past climate in this region is incomplete due to sparse and short-duration instrumental records. Here we present annually resolved oxygen isotope records from the marine bivalve, Arctica islandica, from five locations ranging from Jonesport and Seguin Island in the Gulf of Maine to Long Island, New York, Cape May, New Jersey, and Ocean City, Maryland in the Mid-Atlantic Bight, a span of over 870 km along the Atlantic coast. Several of the isotope records span the last 100 years or more and all records show coherent, substantial warming since at least 1980 CE. The level of warming indicated in the shell oxygen isotopes is comparable to the 0.5 °C per decade (1980-2020 CE) warming also shown in the instrumental record of sea surface temperature from Boothbay Harbor along the central coast in the Gulf of Maine. These five spatially distant isotope records span different oceanographic conditions and dynamics, including water mass sources, yet they all indicate a substantial warming in recent decades, likely related to increased anthropogenic warming. Beyond reconstructing seawater temperature prior to instrumental records, a major goal of this work is to disentangle the global warming signal from these records to better understand the underlying ocean dynamics also influencing these records. 

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