Tiny Fishes Fuel Coral Reefs

A small gray fish with two orange stripes, hides in between corals.
Girdled cardinalfish (Archamia zosterophora) on a reef. (Rickard Zerpe via Flickr, CC-BY)

Encountering a healthy coral reef while snorkeling or diving is like coming upon a flash mob in the desert. Hordes of flamboyant fishes generate a constantly moving scene against a waving backdrop of sponges, corals, and sea anemones. 

Scientists have long debated what makes reefs such exceptional hubs of ocean productivity. For an ecosystem to be productive, it must be accessing and processing lots of nutrients. Yet, coral reefs are veritable oases in a sea of nutrient-poor open waters. The first statement of this conundrum (referred to as “Darwin’s paradox”) was credited to Charles Darwin’s observation, 200 years ago, that Pacific coral reefs thrive in otherwise barren waters. Since then, scientists have put forth various theories about reef productivity.

Recent studies by Simon Brandl and coauthors, including Smithsonian’s Carole Baldwin, have shed new light on the nutrients that fuel the copious life on reefs. Hiding in a reef’s crevices are multitudes of tiny colorful fishes, most of them less than an inch long. Snorkeling the reefs, you might perceive them as flecks of color, like sparkles in a prism, that tease your peripheral vision. 

Cryptobenthic fishes are so-called for their habits of darting away to hide out on or near the seafloor (crypto = “hidden”; benthic = “on the bottom”). With names like dragonets, gobies, blennies, cardinalfishes, dottybacks, and sandgazers, the thousands of species sound like characters in a science fiction novel. In fact, however, these diminutive characters are the most abundant and diverse fishes on coral reefs. 

Sampling of reefs in Belize, French Polynesia, and Australia by Brandl and his coauthors turned up impressive densities of 10 to 20 tiny fishes per single square meter of reef. Their results suggest that cryptobenthic fishes may be the elusive source of the nutrients that keep reef ecosystems going. While the little fishes are expert at hiding, they can be snapped up as they flit in and out to feed. Larger reef fishes move like vacuum cleaners over a reef, suctioning up the cryptobenthic morsels. 

If they are such popular menu items, how do small cryptobenthic fishes continue to persist on reefs?

The secret is in the young, or larval, stage—cryptobenthic fish larvae stay near the reef instead of moving away on ocean currents. The reef may even provide some shelter during their entire lives, something biologists are sorting out. However, the total lifespan of a cryptobenthic fish might be just a few weeks or months, in contrast to large predatory fishes, such as grouper or barracuda, that measure their lives in years or decades. 

The study’s authors also modeled reef fish population dynamics and nutrient cycling using lots of data from this and other studies. While the total biomass of these fish morsels at a given moment on a reef is nothing to write home about, their transfer of biomass to larger fishes over time is staggering. The authors found that 60 percent of the fish flesh consumed on a reef is from cryptobenthic fishes. 

Quick generation times, Carole Baldwin points out, provide “a continuous stream of new generations of tiny fish as a food source for other reef creatures.” Since they stay close to home and rapidly replenish their populations, cryptobenthic fishes are daily, reliable cafeteria items for predators on the reef system. 

When later research challenged some of the modeling results, the study authors ran a more nuanced model in response. They extended the modeling time frame and changed the constraints to better mimic real world coral ecosystem dynamics. Looking at the contribution of the tiny fishes to the productivity on a reef in this new model, they came to the same conclusion: cryptobenthic fishes make a significant contribution to the daily biomass consumed by predators on a reef.

Despite their sizable role in the food webs on reefs, cryptobenthic fishes can be quite picky in their own preferences, such as living with only one species of coral. As reefs suffer continuing damage from warming and acidifying oceans, these tiny fishes that inhabit their nooks and crannies are likely to suffer changes. As Brandl says, cryptobenthic fishes might be one of the most “sensitive indicators of the impacts of disturbances on coral reefs.” They are small, but enormously significant in reef food webs. 

These small fishes are a reminder to focus our attention not just on the charismatic species, but also on the species that are less obvious to the human eye. As ocean conditions continue to change with climate, we might look to these tiny denizens for clues towards understanding and thwarting the impacts.

March 2020