What are cryptobenthic reef fishes? On reefs around the world, fishes display a remarkable abundance and diversity of shapes and colours. However, perhaps even more remarkable is that most casual observers on reefs probably do not see half of the fishes that live there. This ‘hidden half’ of the fish community is comprised of cryptobenthic reef fishes — adult fishes typically less than 5 cm long that are visually or behaviourally cryptic, and live near to or even within the seabed ( Figure 1 Cryptobenthic reef fish diversity. Show full caption Cryptobenthic fishes from the Caribbean (left panel, photographed in photo-tanks) and Indo-Pacific (right panels, photographed in situ). Photo-tank species (top to bottom): Scartella cristata (Blenniidae), Starksia weigti (Labrisomidae), Acanthemblemaria aspera (Chaenopsidae), Lythrypnus okapia (Gobiidae), Labrisomus guppyi (Labrisomidae), Lucayablennius zingaro (Chaenopsidae), Priolepis hipoliti (Gobiidae). In situ species (top row, left–right): Eviota pellucida (Gobiidae), Enneapterygius atrogulare (Tripterygiidae), Nemateleotris magnifica (Microdesmidae), (middle row, all from the Gobiidae) Paragobiodon xanthosoma, Trimma lantana, Amblyeleotris wheeleri, (bottom row) Eviota prasites (Gobiidae), Ecsenius stictus (Blenniidae), Luposicya lupus (Gobiidae). Photos by R.M. Bonaldo, S.J. Brandl, J.M. Casey, C.H.R. Goatley and J.P. Krajewski. On reefs around the world, fishes display a remarkable abundance and diversity of shapes and colours. However, perhaps even more remarkable is that most casual observers on reefs probably do not see half of the fishes that live there. This ‘hidden half’ of the fish community is comprised of cryptobenthic reef fishes — adult fishes typically less than 5 cm long that are visually or behaviourally cryptic, and live near to or even within the seabed ( Figure 1 ). Cryptobenthic reef fish communities can be comprised of many species and families, but perhaps the most common and widespread members are the gobies (family Gobiidae) and the blenny-like fishes (suborder Blennioidei). While small camouflaged fishes may not sound particularly exciting at first, cryptobenthic reef fishes display extraordinary diversity and are of critical ecological importance on reefs worldwide.

What do cryptobenthic reef fishes do? In short, cryptobenthic reef fishes live fast and die young. For most of them, growth, reproduction and death happen at extraordinary rates. After settlement on the reef, often in a very specific microhabitat, adult fishes spend most of their time feasting on microscopic prey, such as small invertebrates (e.g. copepods), filamentous algae, coral mucus or detritus. Most of the energy gained from feeding is used for growth, which lets cryptobenthic reef fishes grow rapidly throughout their lives. The remaining time and energy is spent largely on a surprising variety of different social and reproductive strategies, including sex changes, territoriality, and both monogamous and polygamous reproduction. Some species, like the dwarf goby Eviota sigillata, can produce 7.4 generations per year, which results in a steady stream of new recruits of cryptobenthic fishes to the reef. After a few weeks, it’s all over. In fact, with a maximum age of just 59 days in the wild, E. sigillata currently holds the record for the shortest lifespan of all vertebrates, and the goby Coryphopterus kuna is the only known fish that has a larval stage that lasts longer than its adult lifespan.

Where are cryptobenthic fishes found? Everywhere! Wherever researchers have used specific techniques to look for cryptobenthic reef fishes — usually involving anaesthetising all fishes within a small area — they have found them to be surprisingly abundant. This has been the case for studies conducted on tropical, subtropical and temperate reefs throughout the Indo-Pacific, Caribbean, Gulf of California, Mediterranean and Atlantic. In almost every habitat, in every sea, fish communities are numerically dominated by these small cryptic fishes. While communities of cryptobenthic fishes are found almost everywhere, individual species of cryptobenthic reef fishes can be very picky about where they live. For example, coral gobies (genera including Gobiodon, Luposicya and Pleurosicya) are often dependent upon single species of coral for shelter. While these are extreme examples, most other cryptobenthic reef fishes have some specific habitat requirements. These needs, combined with the complexity of reef habitats, lead to cryptobenthic fish communities being highly variable among reefs.

The community composition of cryptobenthic reef fishes is also affected by depth. Recent work has begun to highlight that cryptobenthic reef fish communities are not restricted to shallow reefs, and many species are only found in deeper waters. In fact, sampling of deep reefs in the Caribbean using manned submersibles in depths of 300 m or more continues to yield a plethora of new species. Keeping in mind that sampling tiny fishes in a complex but dimly lit environment using a manned submersible is like searching for a needle in a haystack in a dark barn using a forklift, we will probably continue to find new cryptobenthic reef fish species on deep-water reefs from locations worldwide. In addition, new species from shallow reefs are described regularly, studies that sample cryptobenthic reef fishes with appropriate methods are still geographically patchy and cryptobenthic fish species generally have small geographic ranges. All of this suggests that we are probably grossly underestimating the biodiversity of cryptobenthic fishes worldwide.

How many cryptobenthic fishes are there? Lots. On tropical reefs around the world, the average abundance of cryptobenthic reef fishes is frequently around ten individuals per square meter, and in rubble beds of the Great Barrier Reef there are often more than 20 individuals per square meter. This makes cryptobenthic reef fishes by far the most abundant group of vertebrates on reefs. Similarly, their diversity is unrivalled by any other marine vertebrate. Shallow lagoons of the Great Barrier Reef often harbour more than ten species per square meter. In particular, the gobies (Gobiidae), which often dominate cryptobenthic reef fish communities, consist of more species than any other family of fishes. It is likely that the extreme diversity of cryptobenthic reef fishes, along with ecological differences among species, helps to ensure the (more or less) peaceful coexistence of these densely populated communities.

Why should we care about cryptobenthic fishes? Beyond their visual appeal ( Beyond their visual appeal ( Figure 1 ), their fast paced lifestyles make cryptobenthic reef fishes a vital part of coral reef food webs. Incredibly, up to 8% of the population of cryptobenthic reef fishes are eaten every day, and almost any fish capable of catching and eating cryptobenthic reef fishes will readily do so. In other words, by growing quickly and steadily replenishing their populations, cryptobenthic reef fishes transfer energy from their microscopic prey into more accessible food (fish tissue) for larger predators. Thus, cryptobenthic reef fishes are a fundamental part of complex coral-reef food webs, helping to support large reef fishes and coastal populations that depend on these fish stocks. Cryptobenthic fishes also offer scientists precious insights into the process of evolution in the world’s oceans. While populations of many marine species are connected across vast areas of ocean through the dispersal of planktonic larvae, the short lifespans of cryptobenthic fishes mean that they often do not move very far, and populations are easily isolated from one another. Combined with short generation times, this isolation appears to have led to the relatively rapid formation of new species. A good example are dwarf gobies (Eviota spp.), many of which have evolved relatively recently (within the last half-million years) from common ancestors. Now, each species occupies specific regions or microhabitats. Finally, despite their abundance and short life-cycles, the highly specific needs of cryptobenthic reef fishes mean they are often the first species to succumb to environmental changes. Thus, cryptobenthic reef fishes may serve as sensitive indicators of the impacts of disturbances on coral reefs, allowing the identification of threats and an increased window of opportunity for management responses to be applied.

What next for cryptobenthic reef fish research? We still have much to learn about these tiny, but fascinating, animals. Most importantly, the actual number of species and individuals of cryptobenthic reef fishes and their ecological role is unknown in most locations. Better ways of sampling cryptobenthic reef fishes (such as UV-fluorescence surveys or standardized habitat modules) may be useful. These would permit direct comparisons of cryptobenthic fish communities from different locations and facilitate their exploration in areas that are difficult to access.