Introducing my study system: a resilient copepod

When I started my PhD, I did not have a clear sense of what my dissertation research would be. I was fresh out of undergrad, immersed in my department’s core courses of Ecology, Evolution, and Biometry, and surrounded with new ideas. Once it was time to develop my proposal, I was generally interested in marine invertebrate zoology, the responses of populations to global change, and the interactions between adaptation and phenotypic plasticity. I wanted to conduct experiments on invertebrate populations across multiple generations, to test whether they are able to acclimate or adapt to stressors (such as changes in pH, temperature, and salinity) over time. To complete this work within my PhD, I had to find a system with a short generation time that would be relatively easy to work with in the lab. After a good amount of searching, thinking, and discussing, I decided to work with the copepod Tigriopus californicus, a small (~2 mm long) crustacean on the west coast of North America that I had never heard of before.

Now that three years of experiments, field work, and reading everything I could find about this copepod have passed, I’ve become very familiar with this cool little animal. I plan to start research on other invertebrate systems for the last part of my dissertation, but Tigriopus californicus (I call them “Tigs”), will always hold a special place in my heart! In the rest of this post, I will introduce the natural history of Tigs and some of the really interesting research that has been done on them.

In future posts, I plan to share findings from my own research.

What is a Tigriopus californicus? The natural history story.

Tigriopus californicus is a harpacticoid copepod that can be found along the west coast of North America, ranging from Baja California to southern Alaska [1]. It inhabits the intertidal zone of rocky shores, but is only found in pools above mean high water, which can be isolated from seawater input for days or weeks at a time depending on the tides, seasons, and weather. Dr. Megan Dethier of the University of Washington’s Friday Harbor Labs has worked out the ecology underlying their curious distribution on the shore. Through a series of field transplant and laboratory experiments, she found that the copepods are restricted to these high pools due to intense predation by sculpins, anemones, and likely many other animals in lower pools. Although Tigs can survive abiotic conditions throughout the intertidal zone, they are quickly eaten if they are washed into lower parts of the shore. Few fish can resist a bright red, chunky copepod snack. The isolated, high pools serve as a refuge for Tigs, where conditions are too stressful and variable for their predators to survive [1].

Tigs live in an extreme habitat. High intertidal pools are strongly influenced by processes such as evaporation, precipitation, respiration, and photosynthesis. Abiotic conditions including temperature, salinity, oxygen, and pH can undergo wide diurnal fluctuations, as well as seasonal shifts. Conditions may also be different among pools, depending on their location, size (surface area and depth) and the activities of their inhabitants [2]. Although these pools are very stressful places to live, where relatively few organisms persist, Tigs flourish within them. Populations can be so dense that the pools are tinted red from the copepods’ carotenoid pigments (obtained from their algal food). Unlike many other copepods, Tigs do not have resting stages and therefore they cannot just shut down to get through tough times- they must physiologically deal with harsh conditions. Some pools are isolated from seawater input for so long that they completely dry out, leading to the extinction of their residents. Throughout the year, pools within an outcrop may go through many cycles of Tigriopus extinction and recolonization [1]. Understanding how Tigs withstand such dynamic and often extreme conditions has been of great interest to scientists for decades.

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A dense population of Tigs in a high shore pool at Cattle Point on San Juan Island in Washington in Fall 2018. It is easier to see their bright red bodies as they jerkily swim through the water above my hand!

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Tigs are mobile detritivores that are strongly associated with the benthos. Even though all of their life stages are free swimming, they are not good at migrating around.  Tigs cannot crawl over more than 1 cm of dry rock, and it seems that most of their movement occurs during storms, when pools are washed out and temporarily connected by streams of water [3]. This species displays metapopulation dynamics, in which there is migration among pools within single rocky outcrops, but almost no migration between separated outcrops. The majority of genetic variation for this species is partitioned among, not within, populations. This genetic homogeneity within outcrops and strong differentiation among even geographically close, but separate outcrops was initially discovered in allozyme studies by Burton and colleagues [3,4]. Northern populations are less genetically diverse than southern populations, which may be a result of colonization following recent glaciations [5].

The “marine fruit fly”?

Tigriopus californicus is well-studied and is the center of many interesting evolution and ecology research programs. It is a great marine invertebrate model system for many reasons [6]:

• Easy to collect– You can often find Tigs in dense monoculture within their pools. You can suck them up with a turkey baster and put them in a container, with no need to sort them out from other critters.
• They thrive in the lab– Tigs are easy to culture in large quantities. You just need some seawater and food (Spirulina powder or fish food work, and adding live microalgae helps maintain their pigment) and they can be kept in static containers with infrequent water changes.
• Reproductive system makes crossing studies feasible– Males clasp immature females, a behavior called mate-guarding, and hold them until their final molt to complete insemination. The female then stores the sperm and uses it to make multiple clutches throughout her lifetime. Thus it is easy to isolate males and virgin females to set up desired crosses of different populations [7].
• Short generation time of about 4 weeks– This facilitates multigenerational experiments and rapid population growth.
• Wealth of genetic resources– Including a high-quality reference genome, re-sequenced genomes of multiple populations, a range of population specific markers, and multiple transcriptomes [8].

What scientific questions are being explored using the Tigriopus study system?

• Understanding local adaptation and tolerance to stressors
• Acclimation and adaptation to global change
• Exploring hybrid breakdown & the mechanisms of speciation

These topics will be discussed in more detail in upcoming blog posts- stay tuned!

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References

[1]  Dethier M. N. 1980. Tidepools as refuges: Predation and the limits of the harpacticoid copepod Tigriopus californicus (Baker). Journal of Experimental Marine Biology and Ecology. 42:99-111.

[2]  Morris S. & A.C. Taylor 1983. Diurnal and seasonal variation in physico-chemical conditions within intertidal rock pools. Estuarine, Coastal and Shelf Science. 17:339-355.

[3]  Burton R.S., M.W. Feldman, & J.W. Curtsinger 1979. Population genetics of Tigriopus californicus (Copepoda: Harpacticoida): I. Population structure along the central California coast. Marine Ecology Progress Series. 1:29-39.

[4]  Burton R.S. & M.W. Feldman 1981. Population genetics of Tigriopus  californicus: II. Differentiation among neighboring populations. Evolution. 35:1192-1205.

[5]  Edmands S. 2001. Phylogeography of the marine copepod Tigriopus californicus reveals substantially reduced interpopulation divergence at northern latitudes. Molecular Ecology. 10:1743-1750.

[6]  Raisuddin S., K.W.H. Kwok, K.M.Y. Leung, D. Schlenk, & J.S. Lee 2007. The copepod Tigriopus: a promising marine model organism for ecotoxicology and environmental genomics. Aquatic Toxicology. 83:161-173.

[7]  Burton R.S. 1985.  Mating system of the intertidal copepod Tigriopus californicus. Marine Biology. 86:247-252.

[8]  Barreto F.S., E.T. Watson, T.G. Lima, C.S. Willett, S. Edmands, W. Li, & R.S. Burton 2018. Genomic signatures of mitonuclear coevolution across populations of Tigriopus californicus. Nature Ecology and Evolution. 2:1250-1257.

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Interested in Tigriopus californicus research? Check out some of the labs that study them:

Burton Lab

Edmands Lab

Kelly Lab

Willett Lab

Barreto Lab