The first explicit model of population differentiation and speciation in the deep-sea fauna, the depth-differentiation hypothesis, was formulated in the early 1990s. According to this theory, the potential for population differentiation decreases with depth because the bathyal zone (200- 4000 m) has stronger selective gradients and more opportunity for geographic isolation to impede gene flow than does the more extensive and environmentally uniform abyssal plain (>4000 m). To determine whether depth-related variation is genetic, and therefore a consequence of evolutionary change, the PI has developed new methods to extract and sequence mitochondrial DNA from archived deep-sea molluscan species collected in earlier expeditions that had been fixed in formalin and preserved in alcohol. These preliminary studies supported the depth-differentiation hypothesis. They also revealed the limitations of using preserved material. For this project, the PI describes 2 hypotheses about evolution in the deep sea that emerged from the previous work: 1) The depth differentiation hypothesis suggests population divergence decreases with depth; and 2) A strong break in population structure at 3300 m might represent an unrecognized phylogeographic barrier.

The PI will test each of these hypotheses with multiple independent loci using deep-sea protobranch bivalves and recently developed statistical phylogeographic and phylogenetic models. The aforementioned work relied on formalin-fixed tissues, restricting analyses to a single locus (mtDNA). Nuclear loci are essential as independent measures of population structure, gene flow and historical influences, but are also critical to establish whether some of the remarkable divergences the PI documented represent cryptic species. The new material collected during the previous round of funding allowed the PI to develop the necessary nuclear loci and assess their utility for this work. The primary focus of this proposal is to use these new markers to test each of these hypotheses and distinguish intra- versus inter-specific variation.

Intellectual Merit: The deep-sea supports one of the most diverse and unique marine communities, the evolutionary and historical development of which is virtually unknown. The proposed research will contribute very significantly to answering the two most basic questions about evolutionary diversification in this vast and remote environment: Where does it occur, and how? Analysis of the strong bathymetric divergence in Deminucula atacellana will provide the first detailed investigation of potential incipient speciation in a deep-sea organism (apart from reducing environments) and possibly identify the scales and mechanisms involved. It will also create a solid conceptual and methodological context for future evolutionary studies in the deep sea and lay the groundwork for understanding bathymetric and geographic variation at much larger scales (e.g., among ocean basins or pan-Atlantic).

Broader Impacts: The research program will train undergraduate and graduate students and public school teachers. At UMass-Boston, students and faculty have outstanding opportunities to be directly involved in science education and public outreach at all levels. Currently, the biology Department has an NSF-Research Experiences for Undergraduate program, an NIH Initiative for Maximizing Student Diversity (IMSD) Program, a Bridges to Baccalaureate Program, and an NSF Undergraduate Mentoring in Environmental Biology program. All are oriented toward underrepresented minorities, and students from each of these initiatives have and will continue to be been involved in the PI's research. The campus has a Boston Science Partnership Grant to upgrade high school science curricula and is a Center of Ocean Science Education Excellence to foster public awareness of ocean science. The PI will recruit undergraduates, graduate students, and teachers into this research program, and topics in deep-sea biology will be incorporated into university and high school curricula. The research also has broad relevance for conservation and sustainable development of the deep-sea ecosystem. Genetic population structure is a crucial component of biodiversity, and has important implications for extinction potential from deep-sea exploitation.

Project Report

The deep sea (below 200m) is by far the largest ecosystem on the planet covering more than two-thirds of the surface. It supports a highly diverse and largely endemic fauna, yet virtually nothing is known about how all these species formed. Species formation requires the isolation of gene pools, but few obvious barriers exist in the deep sea that would impede gene flow and allow new species to form. The high diversity, lack of obvious isolating barriers, and broad-scale distribution of many taxa raise intriguing questions about how and where new species form in the deep sea. Our research represents the first concerted effort to study the genetic basis of how populations diverge in this vast ecosystem. We quantified the population genetic structure of several molluscan species arrayed along a depth gradient in the western North Atlantic. Genetic divergence among populations decreased with depth suggesting that the potential for population differentiation and speciation varied bathymetrically. Depth differences were considerably more important in fostering genetic divergence than geographic distances at the same depth. Patterns of genetic variation also indicated that deep-sea macrofauna could have strong population structure over small spatial scales, despite the lack of obvious isolating barriers. The small-scale divergence was often associated with depth differences and likely reflected the strong environmental gradients that attend changes in depth. Genetic divergence was sufficiently large for some species that they may represent cryptic species complexes. The presence of cryptic species suggests that geographic distributions may be greatly overestimated and biodiversity underestimated, which will have important implications for identifying the ecological forces that shape local and regional levels of diversity, understanding the evolutionary processes that promote diversification, and protecting the ecosystem properties essential for managing and preserving the deep-water fauna. These emerging phylogeographic patterns suggest that the environmental gradients paralleling changes in depth likely play an important role in the formation of new species in deep-water ecosystems and in the genesis and adaptive radiation of the deep-water fauna. Our research is providing the genetic tools to explore population structure in the deep sea, and producing the first critical evidence of how and where evolutionary differentiation occurs in this vast, remote and complex ecosystem. Unraveling how and where evolution unfolds is critical for explaining biogeographic patterns of diversity, predicting how deep-sea ecosystems might respond to climate change, developing conservation and management strategies to mitigate the intense exploitation of deep-sea resources and identifying appropriate locations and scales for MPAs. Broader Impacts This work has fostered a collaboration among biologists (UMB), and physical and paleo oceanographers (at WHOI) to better understand larval dispersal and connectivity in the deep ocean. In addition to our genetic work, we simulated larval dispersal at various depths in the western North Atlantic to test hypotheses about population connectivity and the potential role of the Deep Western Boundary Current (DWBC) in impeding gene flow between depths. Larval dispersal was estimated from Lagrangian particle trajectories simulated based on a deep-ocean circulation model (FLAME). Larval dispersal appeared to be quite likely between upper and lower bathyal populations because of strong across-slope transport mechanisms. This indicated that the DWBC was unlikely to preclude larval exchange between depth regimes and was unlikely to be responsible for the strong genetic divergence we typically find between upper and lower bathyal depths. This work also provided the first estimates of the nature and scale of larval dispersal at bathyal depths in the deep ocean. A number of students and postdocs have been involved in all aspects of this research (1 postdoc, 3 PhD students and 5 undergraduates). Undergraduate students were drawn from a NSF supported Research Experiences for Undergraduate program, an NIH Initiative for Maximizing Student Diversity (IMSD) Program, and a NSF Bridges to Baccalaureate Program all oriented toward underrepresented minorities. Our research has produced 1 PhD thesis and 8 publications so far with several others submitted or in preparation. The results from our work have broad relevance for conservation and sustainable development of the deep-sea ecosystem. Genetic population structure is a fundamental component of biodiversity, essential for accurately quantifying the geographic and bathymetric distribution of the deep-water fauna, and is critical for estimating extinction potential from deep-sea exploitation.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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David L. Garrison
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University of Massachusetts Boston
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