My research focuses on the environmental constraints imposed on marine and estuarine species. More specifically, this encompasses (1) physiological, behavioral, and ecological responses to environmental change and environmental stress, (2) environmental effects on life histories, distributions, and population dynamics, and (3) anthropogenic impacts on organism-environment interactions. My research lies at the intersection of several disciplines and combines field and laboratory experimentation with quantitative and spatial analyses of long-term datasets to better understand the interactions between marine organisms and their environments. This research also has a strong applied focus, using ecophysiological approaches to address fisheries management and conservation questions. Please look through the information on my research and teaching, and contact me if you have any questions or are interested in joining my lab.
Department of Biological Sciences
Nicholls State University
P.O. Box 2021
Thibodaux, LA 70310
Phone: (985) 448-4709
Interactions between natural and sexual selection in thermally stressful environments
Sexually selected traits evolve in a complex ecological context, with interactions between multiple selective pressures driving the evolution of these traits. Such traits, including elaborate ornaments and armaments, can have substantial effects on heat exchange between the organism and the environment and thus affect thermal stress, performance, and fitness. Much of my recent research has focused on the ecological implications of sexual dimorphism in fiddler crabs. Fiddler crabs exhibit extreme sexual dimorphism and have mating behaviors that are often constrained by thermal stress during the peak activity season. This research resulted in the first direct evidence of a thermoregulatory function of the enlarged male claw. I have also recently examined sex-specific color change responses to thermal stress, and thermal constraints on sexual selection via endurance rivalry. I am interested in understanding how the need to maintain body temperature and thermoregulatory ability constrains sexual dimorphism and mating behavior, and the implications of these constraints for mating success and the evolution of sexual dimorphism under climate change.
-BBC Nature , ScienceDaily, and Science News coverage of Darnell and Munguia (2011)
Thermal adaptation and acclimation ability as a driver of species distributions
temperatures and coastal climates are steadily changing, and organisms will face challenges such as changes in heat and desiccation stress and synchronization of mating and gamete cycles. At any location, climate shifts create new microhabitats and can shift the species composition. The degree to which species are impacted by a changing climate, however, will depend on the species’ adaptability, acclimation ability, and capacity for behavioral thermoregulation, all of which can buffer effects of climate change. My research in this area has focused on fiddler crabs, a group of over 95 intertidal species with vastly different latitudinal ranges. I have examined thermoregulatory mechanisms and acclimation ability, in the context of their potential implications for species distributions and mating behavior. I am particularly interested in the role of thermal acclimation, adaptation, and behavioral thermoregulation in driving species distribution shifts under changing climatic regimes.
Population-level consequences of phenotypic plasticity in crustacean reproduction
Phenotypic plasticity in maternal investment and reproductive traits can be driven by environmental fluctuations or changes in population density and demography, and can have important consequences for performance and fitness of offspring. These effects can scale up to affect abundance, distribution, and population dynamics. Crustaceans in particular exhibit an astounding diversity of reproductive strategies and degrees of maternal investment, and often show remarkable plasticity in response to environmental conditions. My dissertation research examined blue crab reproductive biology including spawning behavior, reproductive timing, temperature-dependent reproductive output, and pheromonal stimulation of maternal care behaviors. The wide latitudinal range and intensive fisheries in many areas make blue crabs (Callinectes sapidus) an ideal species with which to study phenotypic plasticity in traits such as reproductive timing, degree of maternal investment, and clutch size. My research in this area is focused on phenotypic plasticity in reproductive traits in response to exploitation, environmental stochasticity, climate change, and range shifts (including both anthropogenic and climate-triggered invasions of novel habitats) and will also consider the impacts of reproductive plasticity and maternal investment on performance and survival of early life history stages, stages that typically experience high mortality.
Ecology, physiology, and behavior of migration
Marine organisms often have migratory life cycles, with migratory ability optimized by complex interactions between physiological changes, orientation mechanisms, and movement behaviors. My long-term research interests in migratory ecology are focused on the mechanisms that optimize migratory ability of both adults and larvae under a variety of biotic and abiotic conditions and the implications of these mechanisms for the abundance and distribution of the species. I am especially interested in species that use selective tidal-stream transport to increase migratory ability or decrease the energetic costs of migration. This works involves a combination of laboratory behavioral experiments to examine migratory mechanisms and decision rules, multiple telemetry techniques to monitor large-scale migrations in the field, and the use of small, experimentally tractable estuarine systems such as Lake Mattamuskeet, NC to test hypotheses in the field.
Responses of estuarine organisms to altered freshwater inflows
WUnder climate change, increasing variability in precipitation and increasing frequency of droughts and floods will alter patterns of freshwater inflow into estuaries. Coupled with increasing global temperatures, this variability will have substantial impacts on many organisms, including those that support valuable commercial and recreational fisheries. These effects may be exacerbated by increasing human development and diversion of freshwater. I am interested in the effects of altered freshwater inflows and environmental stochasticity on estuarine-dependent organisms, especially effects on recruitment, dispersal and migration, and interspecific interactions. I am currently examining responses of estuarine dependent crustaceans to variation in freshwater inflows, focusing on blue crabs, white shrimp, and brown shrimp. This research relies on a combination of field and lab experimentation, surveys, and GIS-based analyses of fishery-dependent and fishery-independent datasets, with the goal of developing predictive tools for assessing potential responses of these species to future alterations in freshwater inflow patterns.
Linking population dynamics of blue crabs to conservation of whooping cranes
Whooping cranes are one of the rarest birds in North America. Currently, the only natural migratory population of whooping cranes winters along the Texas coast in the Aransas National Wildlife Refuge (NWR). During the winter, cranes acquire 60-98% of their energy by preying on blue crabs and decreased crab abundance has been linked to high crane mortality. I am investigating the important linkage between crab population dynamics and crane population health, including foraging behavior of whooping cranes on blue crabs, recruitment of juvenile crabs into the complex marsh system used by the cranes, and movement patterns and habitat use of crabs within the crane habitat. This research also involves analyses of long-term datasets to assess environmental impacts on crab and crane distribution and abundance and to estimate carrying capacity of the current crane habitat. Results will be used not only to improve conservation efforts for cranes and crabs in Texas but also to develop future reintroduction plans for whooping cranes in areas such as St. Marks NWR, Florida.
Biological timing and endogenous rhythmicity
Regardless of habitat, organisms encounter numerous environmental cycles on a variety of temporal scales, from tidal cycles of salinity and hydrostatic pressure (for marine/estuarine organisms) to seasonal cycles of photoperiod, temperature, and food availability. Endogenous biological rhythms allow an organism to alter physiology or behavior in anticipation of these cycles, rather than in response to these cycles. I have previously examined endogenous biological rhythms in blue crabs, amphipods, fiddler crabs, and terrestrial crabs. My interests in biological rhythms lie in their functional significance as adaptations to fluctuating environmental conditions. I am especially interested in rhythms related to reproduction and migration, and focus not only the overt expression of the rhythm, but also the entrainment cues used for synchronization and the selective advantage provided by these rhythms.
Please email me if you are unable to access a PDF of any of these papers.
(17) Darnell, M.Z., K.K. Fowler*, P. Munguia. 2013. Sex-specific thermal constraints on fiddler crab behavior. Behavioral Ecology 24:991-1003. Link
(16) Kronstadt, S.M.*, M.Z. Darnell, P. Munguia. 2013. Background and temperature effects on Uca panacea color change. Marine Biology 160:1373-1381. Link
(15) Darnell, M.Z. 2012. Ecological physiology of the circadian pigmentation rhythm in the fiddler crab Uca panacea. Journal of Experimental Marine Biology and Ecology 426-427:39-47. Link
(14) Darnell, M.Z., T.G. Wolcott, D. Rittschof. 2012. Environmental and endogenous control of selective tidal-stream transport behavior during blue crab Callinectes sapidus spawning migrations. Marine Biology 159:621-631. Link
(13) Darnell, M.Z., P. Munguia. 2011. Thermoregulation as an alternate function of the sexually dimorphic fiddler crab claw. The American Naturalist 178:419-428. PDF
(12) Hines, A.H., E.G. Johnson, M.Z. Darnell, D. Rittschof, T.J. Miller, L.J. Bauer, P. Rodgers, R. Aguilar. 2010. Predicting effects of climate change on blue crabs in Chesapeake Bay. In: Kruse, G.H., G.L. Eckert, R.J. For, R.N. Lipcius, B. Sainte-Marie, D.L. Stram, D. Woodby (eds) Biology and Management of Exploited Crab Populations Under Climate Change. Alaska Sea Grant College Program, Fairbanks. PDF
(11) Rittschof, D., M.Z. Darnell, K.M. Darnell, M. Goldman, M.B. Ogburn, R.E. McDowell. 2010. Estimating relative abundance of the female blue crab spawning stock in North Carolina. In: Kruse, G.H., G.L. Eckert, R.J. For, R.N. Lipcius, B. Sainte-Marie, D.L. Stram, D. Woodby (eds) Biology and Management of Exploited Crab Populations Under Climate Change. Alaska Sea Grant College Program, Fairbanks. PDF
(10) Darnell, M.Z., K.M. Darnell, R.E. McDowell, D. Rittschof. 2010. Postcapture survival and future reproductive potential of ovigerous blue crabs Callinectes sapidus caught in the central North Carolina pot fishery. Transactions of the American Fisheries Society 139:1677-1687. PDF
(9) Darnell, M.Z., D. Rittschof, R.B. Forward Jr. 2010. Endogenous swimming rhythms underlying the spawning migration of the blue crab, Callinectes sapidus: ontogeny and variation with ambient tidal regime. Marine Biology 157:2415-2425. PDF
(8) Darnell, M.Z., D. Rittschof. 2010. Role of larval release pheromones and peptide mimics in abdominal pumping and swimming behavior of ovigerous blue crabs, Callinectes sapidus. Journal of Experimental Marine Biology and Ecology 391:112-117. PDF
(7) Darnell, M.Z., D. Rittschof, K.M. Darnell, R.E. McDowell. 2009. Lifetime repoductive potential of female blue crabs Callinectes sapidus in North Carolina, USA. Marine Ecology Progress Series 394:153-163. PDF
(6) Forward Jr, R.B., M.H. Bourla*, M.Z. Darnell, J.H. Cohen. 2009. Entrainment of the circadian rhythm of the supratidal amphipod Talorchestia longicornis by light and temperature: mechanisms of detection and hierarchical organization. Marine and Freshwater Behaviour and Physiology 42:233-247. PDF
(5) Ramach, S.M., M.Z. Darnell, N.G. Avissar, D. Rittschof. 2009. Habitat use and population dynamics of blue crabs, Callinectes sapidus, in a high-salinity embayment. Journal of Shellfish Research 28:635-640.PDF
(4) Darnell, M.Z., M.B. Ogburn, H. Diaz. 2008. A novel running wheel apparatus to monitor locomotor rhythms in land crabs. Marine and Freshwater Behaviour and Physiology 41:205-210. PDF
(3) Welch, M.E., M.Z. Darnell, D.E. McCauley. 2006. Variable populations within variable populations: quantifying mitochondrial heteroplasmy in natural populations of the gynodioecious plant Silene vulgaris. Genetics 174: 829-837. PDF
(2) Forward Jr, R.B., J.H. Cohen, M.Z. Darnell, A. Saal. 2005. The circatidal rhythm in vertical swimming of female blue crabs, Callinectes sapidus, during their spawning migration: A reconsideration. Journal of Shellfish Research 24:587-590. PDF
(1) McCauley, D.E., M.F. Bailey, N.A. Sherman, M.Z. Darnell. 2005. Evidence for paternal transmission and heteroplasmy in the mitochondrial genome of Silene vulgaris, a gynodioecious plant. Heredity 95:50-58. PDF
I currently teach 4 courses:
Marine and Environmental Biology I (BIOL 551) is a core course in our M.S. program in Marine and Environmental Biology. My goals for this course are for students to to gain a thorough understanding of ecological principles from readings and discussions of foundational papers and to stimulate critical thinking about ecological theory, experimental design, and the current state of research in the field. This is a discussion-based course with discussions focusing on the primary literature, including both foundational papers as well as more current papers.
Marine Field Ecology is offered at LUMCON during the summer. This course focuses on introducting students to the marine environment through a number of excursions to local habitats throughout the Louisiana Coastal Zone. Students spend the first two weeks of the course exploring the local environment and learning basic principles of marine ecology. The second two weeks of the course are devoted to original research projects conducted in groups of 2–3 students.
Introduction to Marine Biology (BIOL 283/284) is offered during the Fall semester. This course has two primary objectives. The first is for students to gain an understanding of and appreciation for life in the the marine environment. The second goal is for students to develop a mechanistic understanding of the processes structuring marine populations, communities, and ecosystems. We will take a process-based approach to study the diversity of life in the oceans, the interactions between these organisms, and the interactions between organisms and their environment. The 2014 syllabi can be downloaded here (Biol 283, lecture) and here (Biol 284, lab).
Invertebrate Zoology (BIOL 354) is offered during the Spring semester. The primary objective of this course is for students to gain an understanding of and appreciation for the diversity of invertebrate life in marine, freshwater, and terrestrial environments. We will work our way through the invertebrate phyla from Porifera to Chordata, focusing on form, function, and phylogeny. In addition to lectures, dissections and labs will be used to explore invertebrate anatomy, morphology, and behavior, and field trips to local habitats will be used to collect invertebrate organisms and explore relationships between invertebrates and their environment. The 2014 syllabus can be downloaded here.
Prior to coming to Nicholls, I taught several courses at the University of Texas at Austin, including Marine Invertebrates, Marine Environmental Science, and Laboratory Studies in Marine Ecology.