The fascinating relationship between red snapping shrimp and corkscrew anemones.

By Charlotte Kratovil-Lavelle (Dickinson College) and Clara Masseau (University of Colorado) ~ Edited by C.E. O’Brien, Ph.D. (The School for Field Studies, Center for Marine Resource Studies, South Caicos, Turks & Caicos Islands)

Courtesy of Disney’s Finding Nemo (2003), many of us are familiar with the way clownfish make their homes in sea anemones, having adapted to the anemone’s sting. What is relatively less known among the general public is the relationship between the red snapping shrimp (Alpheus armatus) and the corkscrew anemone (Bartholomea annulata). Native to the Western Atlantic, these species have been found to coexist in a way that benefits both, a relationship known as obligatory mutualism.

A red snapping shrimp lives inside its corkscrew anemone “host.”

Much like a homeowner, the red snapping shrimp will choose, clean, and protect a corkscrew anemone. But unlike your traditional brick or wooden home, the corkscrew anemone is a living being with an active incentive to protect and aid its invertebrate inhabitant. This relationship has piqued the interest of researchers and has been the subject of numerous studies throughout the Western Atlantic. What are the mechanisms of this relationship? Why do red snapping shrimp show preference to the corkscrew anemone? Are there efforts being made to observe and protect these symbiotic species? These are a few of the questions we will dive into as we investigate this oceanic perfect pairing.

In any ecosystem, organisms are constantly interacting with one another in a variety of ways. Symbiosis refers to the wide array of prolonged, close interactions that occur between individuals from two or more species within an ecosystem. You can think of the relationship between snapping shrimp and corkscrew anemones like an ecological quid pro quo; a “tit for tat,” in layman’s terms. In scientific terms, we call this mutualism. Due to the nature of the interactions between red snapping shrimp and corkscrew anemone we would refer to it, more specifically, as obligatory mutualism, but more on that later.

We begin our discussion of this symbiotic mutualism with the corkscrew anemone: the host of the relationship. The anemone’s physical structure serves as the habitat, providing its crustacean companion with a place to call home. Interestingly, red snapping shrimp have been shown to prefer spending their time under the anemone, whereas other symbiotic crustaceans will be on or near their anemone.

An anemone’s tentacles are armed with nematocysts, specialized cells that uncoil like a harpoon to inject a toxin into its victim. Fortunately for red snapping shrimp, they have evolved the ability to develop immunity against their host’s sting through a process known as acclimation. This makes the anemone habitable for red snapping shrimp while also deterring predators looking to make a meal out of the shrimp. Aside from shelter, the shrimp gain a food source by removing mucus, inorganic debris, and necrotic tissue from the anemone. This tissue, defined as excessive, diseased, or injured, and non-natural, would harm the anemone if not removed by the shrimp.

This relationship is, of course, not a one-way relationship; the shrimp must also take care of its home. A 2021 study investigating this symbiosis found that red snapping shrimp undertook the important role of removing sand and sediment from the anemone. This cleaning was shown to aid the anemone in more effectively contracting into its column, the body of the anemone, thus providing both species with better protection from predators.

In addition to being tidy, red snapping shrimp are a territorial species that has been known to defend their host anemone against predators. A 2014 study found that corkscrew anemone individuals that hosted red snapping shrimp were less likely to suffer damage from the bearded fireworm (Hermodice carunculata) when predated on. The feisty shrimp aided in the defense of their host anemone by rushing the fireworm and snapping at and pinching the worm with its chelipeds (its pair of legs with pincers). 

A Pederson cleaner shrimp at left and a yellowline arrow crab at right perch near, but not under, a corkscrew anemone.

Another benefit provided by the snapping shrimp is the creation of nitrogenous waste, which can act as fertilizer to the single-celled, microscopic algae known as endosymbiotic zooxanthellae. Zooxanthellae are single celled organisms that live in the tissues of a wide range of marine organisms, including anemones, and operate in mutualistic relationships. One well-known example of this is the crucial relationship between zooxanthellae and coral polyps, which provide coral with their color and a food source. Similarly to their relationship with coral, zooxanthellae live amongst corkscrew anemones and provide photosynthetic benefits in which they produce sugars in exchange for carbon dioxide, water, and sunlight.

Though the relationship between red snapping shrimp and corkscrew anemone may not be one-way, it is one-sided in terms of dependence. For the anemone, this arrangement is advantageous, but not essential for its survival, and it would therefore be considered facultative. But for the shrimp, the use of the anemone as shelter is crucial for their survival. In fact, a red snapping shrimp will remain with the same host anemone for all three to four years of its life (talk about a satisfied homeowner). For the shrimp, the relationship is therefore known as obligatory. Even if only one of the species involved in a mutualistic symbiosis relies on that interaction for their survival, we still categorize the relationship as obligatory.

The snapping shrimp are not the only organisms that the corkscrew anemone hosts. In their commonality among Caribbean reefs, corkscrew anemones host several crustaceans from the order Decapoda, including other shrimp and even crabs. Pederson cleaner shrimp (Ancylomenes pedersoni) are particularly common tenants. These little shrimp make their living by eating parasites off of passing fish, another form of mutualistic symbiosis (see “Cleaning Stations: The Greatest Marine Restaurant and Spa”—Times of the Islands Winter 2022/23). Pederson shrimp will position themselves near their home anemone and wave their antennae to advertise their services to potential “clients.” Other species include the yellowlined arrow crab (Stenorhynchus seticornis), and spotted cleaner shrimp (Periclimenes yucatanicus). Among the many symbionts, red snapping shrimp are the most territorial and aggressive, especially when not mated.

Corkscrew anemones are also not the only anemone that forms partnership with other organisms in the Turks & Caicos waters. Giant anemones (Condylactis gigantea), branching anemones (Lebrunia danae), sun anemones (Stichodactyla helianthus) and knobby anemones (Ragactis lucida) are all known to host a variety of organisms. One example is pictured here: a banded clinging crab (Mithraculus cinctimanus), a small crab that can be associated with anemones, sponges, or coral, hangs on to the tentacles of a giant anemone (Condylactis gigantea). This relationship is considered commensal, rather than mutualistic as with snapping shrimp, since the crab benefits from living there but the anemone is neither helped nor harmed. 

Studies have indicated that not all anemones are created equal in the eyes of their prospective tenants. A 2012 study conducted in the Puerto Morelos coral reef found that the probability of a corkscrew anemone hosting a crustacean such as red snapping shrimp was influenced by reef zone and season. The likelihood of a corkscrew anemone hosting any crustacean was found to be significantly lower in the reef channels as compared to the back reef or fore reef. One proposed explanation is that anemones tend to be smaller in the reef channels as compared to the fore reef to compensate for the channels’ high flow, which can dislodge an anemone. Why would this matter? Well, it appears that in the eyes of red snapping shrimp, bigger is better. For corkscrew anemones, the probability of harboring crustaceans increased significantly with surface area. This has interesting implications on the distribution of red snapping shrimp and corkscrew anemone symbiosis throughout the Western Atlantic.

A banded clinging crab hangs onto the tentacles of a giant anemone.

Every species plays a greater role in its ecosystem, including our perfect pair. Unfortunately, due largely to human activity, the corkscrew anemone is under threat. Anemone populations have been found to rely on frequent recruitment, the replenishment of young anemone into the existing population to promote individual growth and sustain viable populations. Sea anemones have short lifespans, and they need to be constantly adding more individuals to their population. In Florida, researchers concluded that survival and recruitment rates of corkscrew anemones were higher among areas less impacted by humans and with cooler summer water temperatures. Rising ocean temperatures and human impact, defined as areas easily accessible to human traffic or visibly polluted with debris, pose threats to the corkscrew anemone populations. Corkscrew anemones are harvested and sold to aquariums for decorative purposes, but without further research conservation efforts and management cannot be supported. Investigating the population dynamics of the anemone is also important to understanding the implications for the shrimp symbiont. 

In the world of the big blue ocean, it must be nice to have a place to call home, and a companion that has your back. The symbiotic relationship between the red snapping shrimp and corkscrew anemone is a great example of obligatory mutualism in the Western Atlantic—each species benefits the other, making life in the ocean a little easier for both. As we continue to learn about these species, we can better understand how our human activity affects them and how we can better protect them. In the long run, a healthy ecosystem tends to be better for everyone involved, especially humans who call the Western Atlantic home. Overall, the fascinating relationship between the red snapping shrimp and the corkscrew anemone warrants further investigation (and perhaps its own blockbuster animated film).

For detailed article references or more information about The School for Field Studies, contact Director Heidi Hertler on South Caicos at hhertler@fieldstudies.org or visit www.fieldstudies.org.