Explaining Science – crustacean symbionts with sea anemones

Explaining Science – a new series of blog posts bringing scientific discoveries focused around biodiversity to your living room.

Are you ready to learn about phylogeography, population genetics and symbiosis, great topics to impress your boss and co-workers at the next Holiday party?

Let’s set the scene: If you have been scuba-diving off the coast of Florida you are probably familiar with the coral reefs found offshore from the coast, the only living coral barrier reef in the continental United States and the third largest coral barrier reef system in the world. It is a hotspot for biodiversity, meaning that you were probably awed (if you looked closely) by the diversity of animals that call this reef their home. Given the large diversity, not only of species but also in strategies that allow them to be successful in this environment, coral reefs are great places for scientists to study relationships among living beings. We refer to this part of science as phylogeography, the study of why species live where they do, how they got there and how they are related with each other.

Here at the MBD we are lucky to have the Laboratory of Marine Invertebrate Diversity led by Professor Meg Daly and her students who explore biodiversity of sea anemones and the diversity in marine symbioses of crustaceans with some anemones. Benjamin Titus, one of the PhD students in Daly’s lab, recently published a study entitled “Specialist and generalist symbionts show counterintuitive levels of genetic diversity and discordant demographic histories along the Florida Reef Tract” in the scientific journal Coral Reefs. I was intrigued and asked Ben to explain the purpose and the significance of his study. Following is a series of clips from my interview with Ben or you can scroll down to a brief take-home-message, summarizing the main results.

  • Introduction: Every good scientific study tests a hypothesis that developed from previous research, here Ben tests these two hypotheses:


One of these hypotheses, the specialist-generalist variation hypothesis, predicts that specialists, species that rely on a close association with one particular species, show reduced genetic variation and genetically structured populations.


  • Study site: Map of coral reefs sampled along the Florida Reef Tract

Ben collected samples along the Florida Reef tract:


  • Study system: Crustaceans living with sea anemones are small animals, many have not (yet) been studied closely. Thus we often do not even know how many species exist within a genus. It is not surprising that genetic analyses reveal new relationships. Ben’s study subjects are 1 sea anemone and 6 crustacean species:
corkscrew sea anemone Bartholomea annulata

corkscrew sea anemone Bartholomea annulata

At least 6 species of crustaceans can live symbiotically with sea anemones. Ben focused on the corkscrew anemone, Bartholomea annulata, as a host for red snapping shrimp (Genus Alpheus), the Pederson cleaning shrimp (Genus Ancylomenes), the spotted cleaner shrimp (Genus Pericilemenes), the sexy shrimp (Genus Thor), and the arrow crab (Genus Stenorhynchus).

These species differ in their life history strategies, what they do to strive and survive:

The red snapping shrimp is most often found buried in the sand beneath the anemone’s column; it is considered an obligate symbiont, because it depends on the anemone for survival.

The Pederson cleaning shrimp is commonly found within the anemone’s tentacles; it is also considered an obligate symbiont but a host generalist, in that it will associate with a variety of anemone species.

The spotted cleaner shrimp lives among the tentacles of several species of sea anemones; like the Pederson cleaning shrimp, it is an obligate symbiont and host generalist.

The sexy shrimp is a generalist symbiont found with a variety of anemones and corals; it is a facultative generalist, for whom a close relationship with an anemone is not necessary for its survival.

The arrow crab is a facultative symbiont, often living in association with an anemone, but such a relationship is not necessary for its survival.

  • Sample collection: Collecting crustaceans for the genetic analysis required a couple of field trips to Florida, scuba-diving offshore and occasionally an inventive technique to knock-out pesky invertebrates.


  • Genetic Analysis: Back in the lab at OSU Ben extracted genetic material, specifically mitochondrial DNA, from the samples he collected in the field and for this study used one particular marker within this mitochondrial genome: CO1.


  • Results – hypothesis 1: Against his expectations (recall the specialist-generalist variation hypothesis from above), Ben found that specialists showed greater genetic diversity than the two generalists he studied and overall the sampled populations are not structured along the Florida Reef Tract.


  • Cryptic species are individuals within a species that are morphologically similar, appear identical, but do not breed with each other because they are genetically quite distinct. Crustaceans living with sea anemones are small animals and many have not (yet) been studied closely. Thus we often do not even know how many species exist within a genus. It is not surprising that genetic analyses reveal new relationships. Based on his previous research findings, Ben suspected cryptic species in some genera, and found evidence  both within the snapping and the cleaner shrimp (genera Ancylomenes and Periclimenes). Ben discovered species that had so far been unknown to us.


  •  Results – hypothesis 2: The second hypothesis regarding shared diversification by co-occurring species, was not supported by the collected data. Most populations did not show a recent expansion since the last changes in sea levels with the end of the ice age some 15,000 years ago. Only one of the snapping shrimp species, Alpheus immaculatus (Fig.6b), showed an increase in population size about 300,000 years ago.


  • Conclusion: So what do these findings mean? Finding new species highlights that the Florida Reef Tract is a biodiversity hotspot, currently maybe even underappreciated because not all species within this diversity have been detected and described yet.

Understanding the genetic structure of crustacean populations is important because they play fundamental roles in the marine ecosystem, in symbiotic relationships with anemones and corals, the building blocks of coral reefs.


Take-home-message: The Florida Reef tract is an important biodiversity hotspot with yet undescribed species. Specialists may have greater genetic diversity than generalists in some, particularly marine ecosystems, where larvae disperse far distances. When host availability is not a limiting factor, species that co-occur on the same host and would be expected to show similar diversification patterns, species may follow quite different trajectories of phylogeographic history.


Titus, B. M., & Daly, M. (2016). Specialist and generalist symbionts show counterintuitive levels of genetic diversity and discordant demographic histories along the Florida Reef Tract. Coral Reefs, 1-16.

Did you know that …

Biodiversity refers to the variety of living beings on our planet. Invertebrates are estimated to have the greatest diversity among all animals. The famous biologist and author E.O. Wilson estimates the total number of existing species close to 7 millions (only ~20% of these have actually been described) in his recent book Half-Earth – a great read if you are interested in finding out what it takes to preserve the diversity of animals and plants on planet Earth.

Sea anemones are animals and they are close relatives of jellyfish and corals, all members of the phylum Cnidaria. You may know anemones for their close relation with clownfish, the small orange, black and white fish that make their home within the sea anemones’ tentacles. But these are not the only symbionts living with anemones; some species of crustaceans also use the tentacles for protection.

Symbionts are animals or plants that live closely together. In mutualisms, the symbionts each benefit from this close proximity. For example, clownfish clean the anemones of parasites, the stinging tentacles of the anemone provide protection to the clownfish – watch some clownfish in their sea anemone habitat during your next visit at the aquarium at the Columbus Zoo!

Some symbionts only exist with one particular other species, these are called host specialists. Other symbionts can live with a wide range of species, you guessed right, these are called host generalists.


About the Author: Angelika Nelson is the social media manager at the Museum of Biological Diversity, here in an interview with Benjamin Titus, PhD candidate in Meg Daly’s Laboratory of Marine Invertebrate Diversity at the Ohio State University.

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