In the Pursuit of Species

by Celicia Cordes

Ahh, taxonomy; the age-old bloodbath that is biologists quarrelling over the remarkably capricious systematic organisation of things living and dead. Throughout the study of taxonomy from the 18th century to the modern day, there has been an ever-persisting and controversial matter that is the question: How do we define a species? Now, I’m sure that your high school biology teacher may have told you about the biological concept of species which defines it as a group of organisms that can reproduce and have fertile offspring. Unsurprisingly, this is not the most widely accepted definition and does not nearly come close to detailing the heavy implications of describing one.


To this day, we have no solid answer for what a species is. However, it can be roughly defined as the smallest quantifiable group of organisms that is the most morphologically and molecularly similar to one another. In essence, a species can be thought of as the smallest possible unit in taxonomic classification, equivalent to how an element in chemistry is the smallest possible unit of unique composition in a given atom that cannot be broken down into more distinct units. The species problem is one that has been discussed and fought over for centuries of biological studies and has become one of the most controversial subjects in all of biology. So, if any scientist were to tell you that they can offer a concrete definition to quantify what a species is; they’re either full of it or in line to win a Nobel Prize. Although we are still far from an exact definition, biologists have come up with extraordinary ways to assist in the classification of taxa. In comes phylogenetics, the study of the evolutionary history and relationships between existing groups that are primarily based on morphological characters and molecular data. However, in recent years, paleobiologists have started to include fossil data in their analyses of taxonomic clades. Here is where my story begins.

Over the course of two weeks between July and August 2022, I had the privilege to be a part of the Harvard University Pre-College Program for high school students where I took a course on Evolution and Paleobiology Research Methods at the Museum of Comparative Zoology (MCZ), under Harvard Ph.D. candidate Sarah Losso. In this course, we covered a range of topics that included; evolutionary trends, Earth history, phylogenetics and taxonomy, morphometrics, fossil evidence, familiarising ourselves with primary literature, and, of course, the species problem. This course allowed me to broaden my understanding of the various scientific processes involved in systematic classification and emphasised just how volatile a species’ relationships on a phylogenetic tree can be.

As part of the program, students were expected to complete a final project. We acquired a number of extant and extinct specimens from the MCZ fossil teaching collection of a singular clade of marine organisms and were tasked with providing a research question, proposed methodology, and possible solution for our investigation of the specimens. My chosen data set consisted of fossilised Ordovician to Permian period corals of the orders Tabulata and Rugosa and various present-day scleractinian corals. I decided that I wanted to study polyp and gape size variation in solitary versus colonial hard corals by measuring the mouth size of each sample in conjunction with classifying every specimen into groups by polyp distance (or coenosteum width) so that I could plot polyp size, polyp distance, and polyp nature (solitary versus colonial) on a scattergram. I concluded that this method would allow us to visualise any grouping or trends that occur between the three given variables.


As for my incredibly brilliant solution; after a late night of literature review and pestering a biologist with questions, I suggested that mouth size variation allows for more diversification in the diets of these anthozoans and that by studying modern gape size variation, we could potentially ascertain the feeding habits of prehistoric corals. Because hard corals cannot engulf their prey as other cnidarians can, a larger mouth allows solitary corals to ingest larger prey1. Modern-day solitary scleractinian corals have been observed to consume salps and even certain sacoglossan sea slugs1,2. However, the polyps in smaller-mouthed colonial corals can utilise a teamwork-centric solution called protocooperation to capture and consume more sizeable prey. Observations of polystomatous corals have demonstrated their ability to catch and ingest jellyfish and other large planktonic prey without predation being restricted by polyp size3,4,5. I argued that studying the behaviour of extant coral species may be able to give us a better understanding of the potential proto-cooperative feeding habits between colonial tabulate and colonial rugose corals or the possible prey items of our favourite chalice-shaped solitary rugose corals. Thereby, giving us a greater understanding of the ecology of mid-Ordovician to late Permian phototrophic zones 470 million years ago.

After all of this talk about studying species, you may be wondering what the benefit or even the purpose of such research could be. Why work to systematise different organisms into taxa? Conservation, including marine conservation, is a long and painful road that is made slightly smoother by the investigation and classification of species. By studying their ecological roles and relationships with other organisms, biologists use this information to form increasingly more accurate theories of how the natural world around us functions because of these beings. Moreover, studying the ecological significances and traits of organisms allows us further methods of classification, making the identity of a species more distinct through its ecology. For instance, the sea slugs Phestilla fuscostriata and Phestilla viei, while similar-looking morphologically, were identified as different species based on many factors including their prey preferences. P. fuscostriata has only ever been documented feeding on the coral Pavona decussata whereas P. viei feeds exclusively on Pavona explanulata. Consequently, the exploration of species allows us to determine their ecological importance and role in the balance of life on Earth. Take, for example, the widely known IUCN Red List; a worldwide index compiled of ecological and taxonomic research in an endeavour to conserve the diversity of life.


The pursuit of species is an arduous, yet satisfying, venture to record and preserve Earth’s biodiversity. It is the foundation of all natural biology and zoology; classifying organisms from bacteria and fungi to plants and megafauna. Though systematics is a centuries-old field of research, we are nowhere near the completion of our journey. Millions of species alive today have yet to be described and countless others have gone extinct with poor or nonexistent fossil records to diagnose them. We are in an age where taxonomy has progressed beyond simple morphometric analyses; where molecular investigations into organisms are more readily accessed by the average researcher. As someone who aspires to one day do a great deal of (hopefully) good quality work studying marine biology, I am undoubtedly beyond excited to see how the pursuit of species evolves in the future.

  • 1Mehrotra, R., Scott, C.M. and Hoeksema, B.W., 2016. A large gape facilitates predation on salps by Heteropsammia corals. Marine Biodiversity, 46(2), pp.323-324. 
  • 2Mehrotra, R., Monchanin, C., Scott, C.M., Phongsuwan, N., Caballer Gutierrez, M., Chavanich, S. and Hoeksema, B.W., 2019. Selective consumption of sacoglossan sea slugs (Mollusca: Gastropoda) by scleractinian corals (Cnidaria: Anthozoa). PloS one, 14(4), p.e0215063.
  • 3Musco, L., Fernández, T.V., Caroselli, E., Roberts, J.M. and Badalamenti, F., 2018. Protocooperation among small polyps allows the coral Astroides calycularis to prey on large jellyfish. Bulletin of the Ecological Society of America, 99(4), pp.1-6.
  • 4Gregorin, C., Musco, L. and Puce, S., 2022. Protocooperation in Tubastraea cf. micranthus to catch large planktonic prey. Marine Biodiversity, 52(3), pp.1-2.
  • 5Ter Horst, L.J. and Hoeksema, B.W., 2021. Salpivory by colonial reef corals at Curaçao, Southern Caribbean. Diversity, 13(11), p.560.