Sam Bashevkin, a marine ecologist in the Graduate Group in Ecology at UC Davis talks about looking at (mostly) transparent plankton, getting a lift with the Policía Ecológica and accepting imperfect studies.
Plankton (the community of small free-floating organisms too weak to swim against currents) are predominantly transparent organisms with small spots of pigments. These pigments are thought to protect from ultraviolet radiation (UVR; as does the pigmentation in our skin) at the expense of increasing visibility to fish predators. Transparent plankton are almost invisible to predators but completely exposed to UVR. More pigmented plankton are well defended from UVR but easy to find by fish predators. This results in a pigmentation trade-off between UVR and predation.
Most of the research on this topic has been conducted in freshwater lakes. Lakes naturally differ in the presence or absence of fish, resulting in pigmented plankton in fish lakes and transparent plankton in fish-free lakes. In addition, almost all studies have been conducted on holoplankton (species that spend their entire life cycle in the plankton). We decided to look at this trade-off in the sea, where fish are ubiquitous, and in larval crabs, which only occupy the plankton during the larval life stage; thus, they may be more constrained in their adaptations to planktonic selective forces. Furthermore, larval crabs vary widely among species in their degree of pigmentation and the lengths of their defensive spines that deter fish predators.
We asked two main questions in this study to dig into the drivers of interspecific variability in pigmentation and spine length in larval crabs: 1) Do more pigmented species compensate for their expected increase in predation risk by growing longer defensive spines? and 2) what constraints are imposed on spine length and pigmentation that may inhibit optimal adaptation?
Unexpectedly, we found that more pigmented species tend to have shorter spines. This may indicate that individuals are limited energetically and cannot optimally adapt to both UVR and predation simultaneously. Instead, species specialize in defending from either UVR (dark pigments) or predation (long spines) but not both. Furthermore, we found significant phylogenetic constraint in spine lengths, indicating that the evolution of spine length may be constrained by a species’ evolutionary history. On the other hand, pigmentation was not phylogenetically constrained and may be freer to evolve. Lastly, we found that spine lengths were closely tied to larval size, but grow disproportionately larger with larval carapace length increase.
Overall, our study suggests that larval crabs may be specializing in defending from UVR or predation. We expect that this specialization would be related to the relative strengths of UVR and predation faced by each species, which is likely determined by larval habitat. Predatory fishes are more common nearshore while UVR is stronger in clearer offshore waters. UVR is strongest at the very surface of the water column while predatory fishes may congregate a little lower in the water column since they are also damaged by UVR. Since spine length is phylogenetically constrained, we may also expect that related species would occupy habitats with similar predation threats. These relationships among larval traits, relatedness, and habitat may lay the groundwork for a better understanding of the habitat use and dispersal of crab larvae, which are difficult to find and measure in the field.
This study came from a fascination with the beauty and diversity of larval forms, as well as an interest in photography. The paper was built on fieldwork, photography, and statistical modeling.
I conducted this study primarily at a small marine lab on the Caribbean coast of Panama, the Galeta Marine Lab, which is a research station of the Smithsonian Tropical Research Institute. Although underutilized by researchers (I was almost always the only researcher at the lab), Galeta offers easy access to a diversity of tropical habitats: seagrass, sandy beach, rocky cobble shore, coral reef, and mangroves. As of 1976, over 80 species of decapods have been discovered near Galeta island (Abele, 1976). This easy access to diversity inspired my study design. During my two summers in Panama, I embarked on a mission to collect and photograph as many species of crab larvae as I could find.
While more sophisticated methods of measuring pigments with chemical extractions and analysis would have been ideal, I quickly realized that remote field labs did not lend themselves to this type of lab work. There were no microbalances, deionized water, or -80 freezers available and bench space was limited. Instead, Galeta had flow-through seawater and abundant crabs that would often walk right into the lab for collection. I made the decision to sacrifice fine chemical precision for rougher pigment estimates with photography since I believed that the decapod diversity and easy access to the field at Galeta were far more valuable.
While collecting crabs in the presence of three armed men was a new experience for me, the policemen were often genuinely interested in my work
Crabs are easiest to catch on land at night. Like a deer in the headlights, they freeze when illuminated with a bright light. I would collect crabs after sunset with my field assistant, a security guard from the marine lab, and a couple resident members of the Policía Ecológica, a division of the Policía Nacional de Panamá tasked with protecting natural resources like the no-fishing zone around Galeta island. While collecting crabs in the presence of three armed men was a new experience for me, the policemen were often genuinely interested in my work. I also greatly appreciated the rides on an ATV or golf cart (when the ATV broke down) they offered me through the mangroves at night.
We would only collect female crabs carrying eggs, which I would then keep in individual “crab houses” until they released larvae. Each morning we would change the water in every crab house while checking for freshly released larvae. When we found larvae, we would isolate them and photograph 20 individuals.
After returning from these trips to my home base at the Bodega Marine Laboratory in Bodega Bay, California, I applied the same methods to a few local species of crabs that I had collected for other experiments or were donated to me by staff at the lab. In all, I had photographs of 21 species (and 1,601 individuals) of crab larvae representing a wide diversity of pigmentations and spine lengths (Bashevkin, Christy, & Morgan, 2019). This dataset, combined with new statistical methods I had picked up at UC Davis, allowed me to address some of the questions I had developed during countless hours over the microscope and in the field with these crabs.
The combination of field, lab, and computer work was integral to the completion of this study. Fieldwork gave me a firsthand glimpse into the lives of these animals, as well as the diversity of adult forms and life history strategies. This opened up new questions while grounding my hypotheses in ecological reality. Detailed examinations of live crab larvae under a microscope and more freely swimming in the water helped me understand how they move and use the bodies I was studying. Lastly, sophisticated statistical modeling helped me tease out the predominant patterns in the data while accounting for non-independence due to phylogenetic and familial relatedness.
The biggest takeaway I gained from this study was a realization that no study is perfect
The biggest takeaway I gained from this study was a realization that no study is perfect. Mine would have been improved with more sophisticated chemical analyses, more species, higher geographic coverage, and many other changes. Nevertheless, as imperfect as this study may be, it improves our understanding of adaptation in the face of trade-offs and constraint, the drivers of plankton morphology, and more specifically the adaptations and morphology of larval crabs. This information may help us predict habitat usage of larval crabs based on their morphology, or responses as their environment is altered by climate change. No paper is perfect and all papers that expand our understanding are valuable.
I first became involved in research during my freshman year of college. A graduate student in my marine biology professor’s lab hurt his back so I was hired to help maintain the phytoplankton cultures (which involved hauling a lot of water around). I quickly became hooked on the lab’s research into a decidedly non-charismatic (except to those who study it) marine snail, Crepidula fornicata. I eventually transitioned into independent research on environmental determinants of larval and juvenile growth and survival in this species, which became my senior thesis.
After finishing my Ph.D. at UC Davis this year, I decided I wanted to do work with more of a direct connection to policy. As much as I loved conducting basic research in marine ecology, I wanted to try something new. Therefore, I accepted a position in the Delta Science Program at the Delta Stewardship Council, a California State Agency. The Delta Science Program is tasked with providing the best possible scientific information to inform management of the Sacramento San Joaquin Delta, which supplies water to 2/3 of California’s people and a large proportion of its farmland. The Delta faces many challenges from endangered species to rising sea levels, and I saw an opportunity to apply my ecological understanding and data science skills toward generating new synthetic insights to guide effective management of this system.
I think the best and worst thing about being an ecologist is the complexity of ecological systems. Ecologists are often teased for commonly answering questions with “it depends.” Most people (editors, reviewers, journalists, the public) want a straight answer, which we are often unable to honestly provide. However, diving into these dependencies, teasing apart relationships, and gaining a more mechanistic understanding of the system are fascinating and important challenges to tackle. They might not result in the most satisfying answers, but they push the field further than broad-brush generalities that often fail to hold true. It always depends, but the real question is how?
Abele, L. G. (1976). Comparative species composition and relative abundance of decapod crustaceans in marine habitats of Panamá. Marine Biology, 38(3), 263–278.
Bashevkin, SM, Christy, JH, Morgan, SG. Adaptive specialization and constraint in morphological defences of planktonic larvae. Funct Ecol. 2019; 00: 1– 12. https://doi.org/10.1111/1365-2435.13464
Bashevkin, S. M., Christy, J. H., & Morgan, S. G. (2019). Data from: Adaptive specialization and constraint in morphological defenses of planktonic larvae. Dryad Digital Repository, http://doi.org/10.5061/dryad.sxksn02z8.