It’s difficult to overstate the ocean’s vastness and diversity of life: 2.1 million marine species have already been documented by biologists (only 9% of the estimated total), and each one has specifically evolved to exist in a part of the multitude of ecosystems the ocean contains^[1]Mora, C., Tittensor, D. P., Adl, S., Simpson, A. G. B. & Worm, B. How Many Species Are There on Earth and in the Ocean? PLoS Biol 9, e1001127 (2011).. It’s hard to overstate the importance of coral reefs — one-quarter of all marine organisms rely on them during at least one stage of their lives^[2]US EPA, O. Basic Information about Coral Reefs. https://www.epa.gov/coral-reefs/basic-information-about-coral-reefs (2017).. As such, it’s worth learning more about the creatures that make these ecosystems possible. Although it’s easy to only consider the most charismatic species like sea turtles (Chelonioidea) or dolphins (Delphinus delphis) when discussing coral reef ecosystems, there are many lesser-known organisms contributing greatly to the function of the reefs. For example, you may gloss right over cryptobenthic reef fish, who (despite being tiny, shy, and generally inconspicuous) play a crucial role in the health and structure of this essential habitat.

Characteristics of Cryptobenthic Reef Fish

The first defining characteristic of cryptobenthic reef fish can be found in its name. The word “cryptobenthic” can be broken down into two parts, crypto– (meaning “hidden”), and –benthic (meaning “on the bottom”), which rightfully describes the shy behavior of these fish and their habitation of crevices and cracks on the bottom of coral reefs^[3]Matishov, D. G. & Matishov, G. G. The benthic zone. in Radioecology in Northern European Seas (eds. Matishov, D. G. & Matishov, G. G.) 103–160 (Springer, 2004). … Continue reading^[4]Bickford, D. et al. Cryptic species as a window on diversity and conservation. Trends in Ecology & Evolution 22, 148–155 (2007).. There are currently over 2,000 species of cataloged cryptobenthic reef fish, but the most well-known and studied species include gobies (Gobiidae), blennies (Blenniidae), cardinalfishes (Apogonidae), dottybacks (Pseudochromidae), and dragonets (Callionymidae)^[5]Goatley, C. H. R. & Brandl, S. J. Cryptobenthic reef fishes. Current Biology 27, R452–R454 (2017).. Cryptobenthic reef fish are small but flashy species, generally less than 5 centimeters in length, and brightly colored^[6]Reese, D. Tiny Fishes Fuel Coral Reefs | Smithsonian Ocean. https://ocean.si.edu/ocean-life/fish/tiny-fishes-fuel-coral-reefs.. Many cryptobenthics can be recognized by the large eyespots commonly located on their upper body, which are markings or coloration within the fish’s scales to look like a large eye, a defense mechanism used to trick predators into thinking they are being watched, and dissuading them from attacking. Considering the high rates at which cryptobenthics are eaten, they need these types of defenses to aid in population regulation^[7]Hemingson, C. R., Siqueira, A. C., Cowman, P. F. & Bellwood, D. R. Drivers of eyespot evolution in coral reef fishes. Evolution 75, 903–914 (2021)..

Cryptobenthic Reef Fish Diversity (Goatley & Brandl 2017) https://doi.org/10.1016/j.cub.2017.03.051

Cryptobenthic reef fish can also be categorized by their high fecundity rates, which describes the rate at which an organism can produce offspring. If a species has a high fecundity, it means that they have a high potential for reproduction^[8]Fecundity definition and meaning | Collins English Dictionary. https://www.collinsdictionary.com/us/dictionary/english/fecundity.. Some cryptobenthic fish species can reach a reproductive rate of 7.4 generations within one year! This high reproductive rate, coupled with uniquely high levels of parental care leads to large and successful populations, with a very large and likely potential for the formation of new species^[9]Brandl, S. J., Goatley, C. H. R., Bellwood, D. R. & Tornabene, L. The hidden half: ecology and evolution of cryptobenthic fishes on coral reefs. Biological Reviews 93, 1846–1873 (2018).. Cryptobenthic fishes’ proclivity for forming new species, a phenomenon known as “speciation,” is noteworthy because it may help bolster their populations against anthropogenic climate change. Large and genetically diverse populations have more opportunities for traits to arise that protect against increasing ocean temperature and acidity, hopefully allowing the fish to survive (and continue acting as a food source) as the ocean changes ^[10]Speciation | National Geographic Society. https://education.nationalgeographic.org/resource/speciation.^[11]Dias, R. et al. Different speciation processes in a cryptobenthic reef fish from the Western Tropical Atlantic. Hydrobiologia 837, (2019)..

Why are Cryptobenthic Reef Fish Important to Marine Ecosystems?

Cryptobenthics’ fast reproduction and large numbers make them integral to marine food chains and ecosystem function, even in areas with high predation. It is estimated that 60% of the fish consumed on coral reefs are cryptobenthic species^[12]Brandl, S. J. et al. Demographic dynamics of the smallest marine vertebrates fuel coral reef ecosystem functioning. Science 364, 1189–1192 (2019).. This rapid production and consumption of cryptobenthic fish is an example of “dark productivity”: although cryptobenthics are responsible for the majority of fish biomass (the total quantity and weight of organisms in an ecosystem) within coral reef ecosystems, the biomass production is not often recognized because the fish are consumed as quickly as they can reproduce and grow^[13]Depczynski, M. & Bellwood, D. The role of cryptobenthic reef fishes in coral reef trophodynamics. Mar. Ecol. Prog. Ser. 256, 183–191 (2003).. 

Alongside their role as a food source, cryptobenthic reef fish are also ecosystem indicators. Ecosystem indicators are species that are heavily impacted by changes in an ecosystem and can be used to monitor and identify when these changes occur^[14]Landres, P. B., Verner, J. & Thomas, J. W. Ecological Uses of Vertebrate Indicator Species: A Critique. Conservation Biology 2, 316–328 (1988).. For example, cryptobenthic fish are habitat specialists and associate with particular organisms for food and/or shelter, so changes in cryptobenthic fish populations may be an indicator of changes in other, harder-to-observe populations. Understanding indicator species allows us to identify possible larger-scale issues within an ecosystem, including those brought on by anthropogenic climate change^[15]De Brauwer, M., Hobbs, J.-P. A. & Jompa, J. Widespread low abundance despite habitat availability elevates extinction risk in pygmy seahorses. Coral Reefs 39, 847–852 (2020).^[16]Extreme environmental conditions reduce coral reef fish biodiversity and productivity | Nature Communications. https://www.nature.com/articles/s41467-020-17731-2..

Cryptobenthics at Risk, and Why We Should Care

Although cryptobenthic reef fish reproduce quickly and experience rapid speciation and adaptation events, they require habitats with lots of living coral, which do not. As coral reefs suffer from ocean acidification and warming, so too do cryptobenthic fish. Furthermore, these reef fish do not migrate, nor are they equipped or able to travel and seek out new habitats. They are very susceptible to temperature fluctuations due to their decreased gill surface area (meaning they take in less oxygen per unit of weight^[17]Gill development and water temperatures limit fish growth • Earth.com. https://www.earth.com/news/gill-development-and-water-temperatures-limit-fish-growth/.), and have high mass-specific metabolisms, meaning that they quickly burn the energy provided by their food, and must continually eat to survive (another factor that prevents them from engaging in large-scale migration)^[18]Wang, Z., Bosy-Westphal, A., Schautz, B. & Müller, M. Mechanistic model of mass-specific basal metabolic rate: evaluation in healthy young adults. Int J Body Compos Res 9, 147 (2011).. As anthropogenic environmental impacts continue, we are cornering our cryptobenthics into failing ecosystems, and when their populations start to drop, this will cause catastrophic damage throughout the food chain. 

We should prioritize the conservation of cryptobenthic species and their habitats for many reasons. Whether the conservation efforts come from a desire to protect and strengthen the food chains in marine systems or protect for the sake of the species, the continued abundance of cryptobenthic reef fish is crucial for the health and function of our marine organisms and ecosystems. Although they may not be a showstopping species, and largely keep themselves hidden from human admiration, cryptobenthic reef fish continually prove their astounding abilities to support and maintain massive ecosystems and thousands of other species.

Emily Padden

Research and Writing Intern

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