The current American administration is excited about its space program on extraterrestrial exploration and discovery. A mission to the moon, several ones to Mars, and perhaps others someday to other planets are part of the current funding plan. NASA has chosen Jezero Crater as the landing site for its upcoming Mars 2020 rover mission after almost six years of scrutinizing and debating which location might be optimal. This rover mission will include rock and soil collections to find signs of habitable conditions and microbial life. Jazero Crater is located just north of the Martian equator. The 45 kilometers wide crater had most probably been a huge river delta in ancient Mars times more than 3 billion years ago. The explorers hope to find preserved ancient organic molecules in the delta’s sediment and learn about any type of previous and current life on Mars.
Since August 2018, we also know about liquid water under Mars’ southern ice cap thanks to a study published in Science by Roberto Orosei et al. (2018). These authors detected a 20 kilometer wide lake of liquid water underneath solid ice, similar to an aquifer, using a MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding). ‘The presence of liquid water on Mars has implications for astrobiology, evolution and future human exploration’ (as the authors state). Now I can understand why Jeremy Y. is watching First Contact on a Sunday night. The idea of finding water sources on other planets, studying extraterrestrial molecules, and learning about Martian ecology is so romantic! How exciting it would be if we could just take off and start human settlements on other planets?! Now that we have officially entered the Anthropocene and humankind has heralded Earth’s sixth major extinction event, it only makes sense to consider migration as an option.
In the last 50 years alone, algal biomass has declined substantially (~40%) in the world’s oceans (Shiermeier 2010). Why care about algae? Well, these little organisms carry out a large portion of the photosynthesis on the planet and help reduce CO2 levels in our atmosphere. In 2018, many of us have heard about corals needing help. Coral reefs are dying by the hour. While we are watching Meg, a funny movie about a superpredator in the ocean that feeds on mammals, we don’t realize that since the dawn of human civilization, ~80% of wild mammals have already disappeared as a result of human population growth and overconsumption (Ceballos, Ehrlich, and Dirzo 2017). Meg might be more scared of us than we are of her, and that is why she is hiding in the deep sea.
I brought my family to the Monterey Bay Aquarium a few weeks ago to show them the Mission to the Deep exhibition. We walked into a dark room and submerged into a 360-degree video projection that took us far below the ocean’s surface. My daughter (6y) navigated a benthic rover across the seafloor and we looked for animals in the mud. We did not spot Meg. On the way back up, we saw deep sea squids, fish with oversized mouths, and many other sinister looking creatures. The Monterey Canyon was less than 20km away from us, but it felt like we knew more about Mars than about the oceans in our backyard. Moreover, many creatures might never be documented because they will go extinct before we even learn about them (Apprill 2017). Food for thought. This is why I decided to write about a paper on my to-read list by the International Council for the Exploration of the Sea (ICES). It made me aware of how little we actually know about marine systems.
Comparison of marine and terrestrial ecosystems: suggestions of an evolutionary perspective influenced by environmental variation
by John H. Steele, Kenneth H. Brink, and Beth E. Scott
Steele et al. argue that environmental dynamics, particularly temperature and currents, are orders of magnitude less variable in the ocean than in terrestrial systems. Hence, terrestrial vertebrates, invertebrates, and plants have evolved strategies after they had moved on land to minimize detrimental consequences of atmospheric variability. Are they therefore better at reacting to rapid climate change in the future?
The authors paint in very broad strokes and represent the marine realm as an orderly system, where plants, herbivores and carnivores increase in a regular fashion in body size, life span and range; in contrast to non-systematic groupings on land with short and long-lived plants, large warm-bodied herbivores, and social insects with lifespans not related to any temporal or spatial scale. It is worth noting that by ‘ocean’, the authors refer to the pelagic environment, with taxa floating freely. Transitioning from the more stable environment in the ocean to land required three major solutions: (i) evolving longer lifespans to endure environmental perturbations, (ii) building nests to protect offspring from environmental variability, and (iii) creating their own internal environment to incubate offspring by becoming warm blooded and viviparous (Figure 2 below).
The article focuses on early life stages of organisms. The vast majority of marine life evolved ways to make use of their physical environment as a part of their reproductive plan while most taxa on land did not. In other words, it is quite common in oceans that huge amounts of eggs, sperm and larvae are released into the water column without any parental care. Predictable currents or tides transport the young to habitats with food. The new generation starts feeding early and can live from the nutrients floating in its proximate surroundings. The whole cycle is perfectly timed to maximize offspring survival.
On land there seems to occur the opposite with a decoupling from the use of the physical environment. Here, animals use strategies and invest a significant amount of energy and resources to shield their offspring from large and unpredictable fluctuations in their environment. The authors refer to a book by E.O. Wilson who compared marine arthropods to terrestrial insects (Wilson 2012). While the former use a planktonic phase as an essential part of the life cycle, insects on land tend to build nests for their offspring. The more complex structures of defense achieve greater isolation from external conditions but also require more division of labor, which gave rise to several forms of eusociality on land. Wilson goes a step further and calls dispersal a barrier for eusociality. Hence, eusociality evolved many times independently in arthropods on land but only once in the ocean (Duffy 1996).
What caused these major differences? Is it because organisms in the ocean are mostly floating in a soup of nutrients? Apparently not only. The main difference between floating in pelagic environments in the ocean and living on land is presumably environmental variability. In the ocean, time scales are longer and spatial scales are shorter compared to land. Now what does this mean? The authors refer to a paper by Hasselmann (1976) who developed spectra for stochastic climate models. He patched together his models using temperature, sea level and geophysical data. Terrestrial systems show variation on the scale from days to months, whereas oceanic systems show much less variation and are more predictable with cycles of months and years. This transformation in scales has important implications for ecological and evolutionary processes in the two different systems. On land, we experience atmospheric fronts with high and low pressure systems sweeping erratically across the continents. Ironically, while I was writing this blog post, there was a huge storm raging outside. I partly wrote this blog post in the dark because of a power outage. However, in the oceans, the physical environment is much more predictable. This predictability, including for example the Gulf stream in the North Atlantic or tidal motions at coastlines, allowed taxa to adapt strategies to make use of predictable currents and ultimately increase their fitness.
The article ends relatively abruptly. I am left thinking about the life-span of mouth breeders, seahorses, and nest builders in the sea. Have you seen the puffer fish’s masterpiece of love? The pufferfish’s mating ritual allows for sexual selection on good genes and intensifies natural selection. Recently, I met J. Emmett Duffy at a workshop in Panama. Emmett reported eusociality for the first time in a marine animal, the eusocial shrimp Synalpheus regalis. This species lives in sponges, only one individual – the queen – reproduces, and most colony members are full siblings. Eusocial shrimp have been found in the last 20 years in several different species of sponges throughout the Caribbean.
Her large snapping claws are replaced with smaller claws used for feeding. Her reproductive monopoly in the cast is not maintained by her own aggression but evidently enforced by the colony’s workers. Photo generously contributed by J. Emmett Duffy
I think that the evolvability of a population depends basically on its adaptive genetic diversity. Individuals need to be able to acquire novel functions through genetic change that will help the organisms survive and reproduce. Further, it will depend on population size, mutation rate, sexual reproduction, and dispersal rates. Evolvability in bacteria has been shown to increase by generating more variation when populations are stressed (Frenoy and Bonhoeffer 2018). Large population sizes of pelagic fish increase threshold values of the selection coefficient above which selection (e.g. environmental variation) becomes an important player. Does the difference in spatial and temporal variation in the pelagic ocean compared to terrestrial systems affect the evolvability of populations living in them? I would say yes it does with regard to dispersal, modes of reproduction and population sizes, at least.
Although it seems that environmental variability is much higher in terrestrial systems than in the open ocean, there might be variability in the ocean soup that is not obvious at first sight. Variation can be caused by niche differentiation, clines (e.g., salinity), cryptic gene flow barriers (e.g., timing of spawning in corals), spatial autocorrelation of selection, or selection against migrants (Richardson et al. 2014).
While there are many restoration and conservation efforts in terrestrial systems, only a few marine protected areas have been established. We are obviously more excited about Mars than the ocean. ‘The deep ocean is still one of the least explored frontiers in the solar system,’ said Principal Investigator Robert Zierenberg. ‘Maps of our planet are not as detailed as those of Mercury, Venus, Mars or the moon, because it is hard to map underwater. This is the frontier.‘
Full disclosure: The paper by Steele et al. (2018) was part of my homework reading for a ‘global’ journal club. We are going to ‘meet’ (by slack) for the first time in less than two weeks. The goal is to connect oceanographers, marine scientists, and evolutionary biologists to develop integrated frameworks for studying adaptation to ocean change. The Research Coordinated Network for Evolution in Changing Seas (RCN) is a network of scientists sponsored by the National Science Foundation. Together we want to advance knowledge in marine sciences. I just read the first article on our reading list. We will discuss them every two weeks. Sign up here if you would like to join us and engage in lengthy discussions about the future of our oceans. RCN is going to organize three major network meetings in the upcoming months (2019 Synthesis workshop; 2020 Genomics Committee; 2021 Training and Integration). RCN is currently looking for a postdoc, and their twitter handle @EvolvingSeas is regularly taken over by volunteers sharing updates from relevant conferences, suggestions for great papers, and progress reports from the lab or field.
References:
Apprill, Amy. 2017. “Marine Animal Microbiomes: Toward Understanding Host–Microbiome Interactions in a Changing Ocean.” Frontiers in Marine Science 4 (July): 1518. https://doi.org/10.3389/fmars.2017.00222.
Ceballos, Gerardo, Paul R. Ehrlich, and Rodolfo Dirzo. 2017. “Biological Annihilation via the Ongoing Sixth Mass Extinction Signaled by Vertebrate Population Losses and Declines.” Proceedings of the National Academy of Sciences of the United States of America 114 (30): E6089–96. https://doi.org/10.1073/pnas.1704949114.
Duffy, J. Emmett. 1996. “Eusociality in a Coral-Reef Shrimp.” Nature 381 (June): 512. https://doi.org/10.1038/381512a0.
Frenoy, Antoine, and Sebastian Bonhoeffer. 2018. “Death and Population Dynamics Affect Mutation Rate Estimates and Evolvability under Stress in Bacteria.” Edited by Arjan de Visser. https://doi.org/10.1371/journal.pbio.2005056.
Hasselmann, K. 1976. “Stochastic Climate Models Part I. Theory.” Tell’Us 28 (6): 473–85. https://doi.org/10.3402/tellusa.v28i6.11316.
Orosei, R., S. E. Lauro, E. Pettinelli, A. Cicchetti, M. Coradini, B. Cosciotti, F. Di Paolo, et al. 2018. “Radar Evidence of Subglacial Liquid Water on Mars.” Science 361 (6401): 490–93. https://doi.org/10.1126/science.aar7268.
Richardson, Jonathan L., Mark C. Urban, Daniel I. Bolnick, and David K. Skelly. 2014. “Microgeographic Adaptation and the Spatial Scale of Evolution.” Trends in Ecology & Evolution 29 (3): 165–76. https://doi.org/10.1016/j.tree.2014.01.002.
Steele, John H., Kenneth H. Brink, and Beth E. Scott. 2018. “Comparison of Marine and Terrestrial Ecosystems: Suggestions of an Evolutionary Perspective Influenced by Environmental Variation.” ICES Journal of Marine Science: Journal Du Conseil, November. https://doi.org/10.1093/icesjms/fsy149.
Schiermeier, Quirin. 2010. “Ocean Greenery under Warming Stress.” Nature News, July. https://doi.org/10.1038/news.2010.379.
Wilson, Edward O. 2012. The Social Conquest of Earth. W. W. Norton & Company. https://market.android.com/details?id=book-StLT0zJOczkC.