Cryptobenthic Fish Research Articles

Widespread low abundance despite habitat availability elevates extinction risk in pygmy seahorses

Due to difficulties with surveying, cryptobenthic fishes are understudied, which limits assessments of their extinction risk. Using a novel survey method (a combination of underwater visual census and underwater biofluorescence census), we surveyed the densities of two gorgonian seafan-associated pygmy seahorses (Hippocampus bargibanti and H. denise) in 63 sites across Indonesia. Densities were low for both seahorses (≤ 0.4 seahorses/host) and their hosts (≤ 0.25 seafans/200 m2). These low densities, combined with habitat specialisation, increase extinction risk. We failed to detect regional or depth patterns in seahorse abundance per host or group size in occupied hosts. Host occupancy rates were low (< 13.7%), suggesting that other factors than habitat availability constrain pygmy seahorse population size. The low densities found in this study highlight the high extinction vulnerability for habitat specialists on coral reefs.

A quantitative assessment of the diel influence on the cryptobenthic fish assemblage of the shallow Mediterranean infralittoral zone

Cryptobenthic fishes are an underestimated and probably important component of coastal marine ecosystems that are usually overlooked by standard methods for collecting and studying benthic fishes. Studies focusing on cryptobenthic fishes have been rare and all have been based on samples taken during daytime. The present study tested the difference in epibenthic and cryptobenthic fish composition, diel differences in cryptobenthic fish assemblage and diel shifts of infralittoral fish species between hidden and open bottom spaces. It also looked for the significant habitat variables structuring the cryptobenthic fish assemblage. The daylight, sunset and night samples of epibenthic and cryptobenthic fish assemblages were collected from 78 squares of 1 m2 shallow water plots (0.5 to 3 m) on Brač island in the eastern Adriatic. The study recorded 27 species, among which the family Gobiidae dominated fish diversity with 14 species. Cryptobenthic specimens highly outnumbered epibenthic specimens, with a ratio of 7.5 to 1. Species composition of cryptobenthic and epibenthic fish assemblages differed significantly. No diel variation in composition, species richness or abundance of the cryptobenthic fish assemblage was detected. Occurrence frequencies in hidden and open bottom spaces of ambivalent species did not change significantly between times of day, so no dial switches between open and hidden places were apparent. In combination, these results suggest that the cryptobenthic fish assemblage has diel stability and is mostly composed of permanent inhabitants of hidden spaces with domination of miniature gobies. Significant habitat variables for species occurrence were the presence of multiple layers, bottom inclination and the presence of cobbles, while depth also had a large but not significant effect.

Habitat Preferences and Spatial Distribution of the Cryptobenthic Fish Assemblages around the Gökçeada Island (North Aegean Sea)

The aim of the present study is to assess the spatial distribution and habitat preferences of the cryptobenthic fish assemblages around the Gokceada Island, where no study on these species were previously conducted. A total of 51 quadrats of 1 m2 were applied to 12 sites at the depths of 0–15 m in June, August and October 2017. The cryptobenthic fishes were studied by anaesthetic sampling. A total of 187 cryptobenthic fish specimens of 23 species were observed and 111 of them were obtained. The results showed that the 0–1 m depth zone is dominated by the blennies and shows greater density than 1–15 depth zone, where gobies are outnumbered. The average abundance of cryptobenthic fish was 3.7 individuals/m2 and Gobius incognitus represented 21% of all individuals. Among the studied species, Parablennius incognitus and P. zvonimiri were only found within 0–1 m, preferring rocky substrates with short thallus algae. Coryphoblennius galerita was found on bare rocky surfaces, mostly in 0–1 m depth interval and Microlipophrys canevae preferred rocky surfaces with short thallus algae covers in as well very shallow waters. While Lepadogasterlepadogaster preferred the depths between 3–5 m, L. candolii was observed in a wider depth range (1 and 6 m).

Dietary and habitat niche partitioning in congeneric cryptobenthic reef fish species

Ecological niches hold critical information concerning the eco-evolutionary dynamics that govern biodiversity and abundance patterns. Cryptobenthic reef fishes account for approximately half of all reef fish species and are an abundant and important group on coral reefs worldwide. Yet, due to their small size and inconspicuous lifestyles, relatively little is known about the ecological niches of most cryptobenthic species. Here, we use gut content DNA metabarcoding to determine dietary niche overlap and prey richness in four sympatric species of cryptobenthic reef fishes in two genera (Acanthemblemaria aspera, A. spinosa, Enneanectes altivelis, and E. matador). Furthermore, we test whether dietary differentiation corresponds with differences in species distribution patterns across twelve sites on the Mesoamerican Barrier Reef in Belize. Our approach reveals dietary partitioning among the four species, which is further supported by low edge density and high modularity in the resulting trophic network. A. spinosa and E. matador consume a significantly higher richness of prey items than their congeners. This result corresponds with non-random distributions and co-occurrence patterns in both species pairs: the two high prey richness species (A. spinosa and E. matador) co-occur more frequently than predicted by chance, but they are exclusive to exposed forereef sites with high wave action. In contrast, their congeners occur across exposed forereef and sheltered backreef sites, but they do not increase in numbers at sheltered sites. Our findings suggest that A. spinosa and E. matador monopolize a wide variety of prey in exposed habitats, but they are unable to meet the energetic demands of their adaptation to high-flow habitats in sheltered areas, possibly due to lower prey availability. This, in turn, indicates strong ecological differentiation among closely related species of cryptobenthic fishes, driven by links between diet, physiology, prey availability, and wave exposure.

Potential invasion of the Gulf of Mexico by Protemblemaria punctata (Teleostei: Chaenopsidae), a cryptobenthic fish endemic to Venezuela

The movement of species to areas outside their native ranges has greatly intensified over the past few decades. However, detecting invasive species often requires detailed and regular surveys, especially for animals with small body sizes. A cryptobenthic fish endemic to the Caribbean coast of Venezuela, Protemblemaria punctata (Cervigón, 1966), is reported here from the southwestern Gulf of Mexico, based on the collection of 8 specimens (5 males and 3 females) from Bajo Mersey, a coral reef in the Parque Nacional Sistema Arrecifal Veracruzano (PNSAV). The collected specimens were taken from sessile worm tubes that grew on PVC structures previously placed on the reef at a depth of 6 m. The number of P. punctata specimens we collected could indicate that the species has established a viable population on the reefs in the PNSAV. The colonization of P. punctata in the Gulf of Mexico waters may be due to the dispersal of individuals present on offshore oil platforms that are transported from Trinidad to Venezuela and the Gulf of Mexico.

Comment on "Demographic dynamics of the smallest marine vertebrates fuel coral reef ecosystem functioning".

Brandl et al (Reports, 21 June 2019, p. 1189) report that cryptobenthic fishes underpin coral reef ecosystem function by contributing ~60% of "consumed fish" biomass and ~20% of production. These results are artifacts of their simulation. Using their data and model, we show that cryptobenthic species contribute less than 4% to fish production, calling into question the extent to which they contribute to the high productivity of coral reefs.

Response to Comment on "Demographic dynamics of the smallest marine vertebrates fuel coral reef ecosystem functioning".

Allgeier and Cline suggest that our model overestimates the contributions of cryptobenthic fishes to coral reef functioning. However, their 20-year model ignores the basic biological limits of population growth. If incorporated, cryptobenthic contributions to consumed fish biomass remain high (20 to 70%). Disturbance cycles and uncertainties surrounding the fate of large fishes on decadal scales further demonstrate the important role of cryptobenthic fishes.

Conspicuous and cryptic reef fishes from a unique and economically important region in the northern Red Sea.

Al Wajh Bank in the northern Red Sea contains an extensive coral reef system that potentially supports a novel fish community. The large (1500km2) and shallow (< 40m depth) lagoon experiences greater temperature and salinity fluctuations, as well as higher turbidity, than most other Red Sea reefs. Since these conditions often influence coral community structure and introduce physiological challenges to its resident organisms, changes in reef-associated fishes are expected. We present critical baseline data on fish biodiversity and benthic composition for the Al Wajh Bank. Underwater visual census of conspicuous fishes and standardized collections of cryptobenthic fishes were combined to provide a comprehensive assessment of these fish communities. We documented 153 fish species and operational taxonomic units, including undescribed species, within 24 families on reefs largely dominated by hard coral and soft sediment (39% and 32% respectively). The families Pomacentridae and Gobiidae contributed the most towards fish diversity and abundance. Bray-Curtis dissimilarity distances among sampled sites suggest a distinctive fish community within the lagoon, and coefficients of variation for each species show high variation in their distribution across the lagoon. Species accumulation curves predict that additional sampling would document many more species throughout Al Wajh. Our findings provide the most extensive biodiversity survey of fishes from this region to date and record the condition of the reef prior to major coastal development planned to occur in the near future.

Eco-engineering increases habitat availability and utilisation of seawalls by fish

The replacement of natural marine habitats with less structurally complex human infrastructure has been linked to the homogenisation of epibenthic assemblages and associated changes in fish assemblages. To mitigate these impacts, eco-engineering efforts have focussed on increasing the physical and biogenic complexity of artificial structures, in the form of crevices added to seawalls and the seeding of the substrate with habitat-forming organisms such as oysters. While these studies have assessed how these interventions affect epibenthic assemblages, the effect of these strategies on the behaviour, such as feeding and habitat use, of different functional groups of fish (e.g. cryptobenthic and pelagic) remains uncertain. To do this, we manipulated complexity on seawalls by adding concrete tiles with different physical (flat or structured with crevices and ridges) and biogenic (seeding with two common habitat-forming species or naturally recruited fouling) complexities. We assessed pelagic and cryptobenthic fish species composition, abundance, interaction time with the tiles and number of feeding bites on three occasions 8–12 months after deployment. Cryptobenthic fish interacted more with physically complex tiles than flat tiles, regardless of biogenic complexity. In contrast, cryptobenthic fish fed more from flat tiles compared to physically complex tiles, and also appeared to feed more from tiles seeded with oysters. Pelagic fish interacted and fed more from naturally fouled tiles compared to unfouled control tiles, regardless of physical complexity. This study showed that manipulating complexity at the scales used here affects behaviour of fish, but it does not affect fish community. Increasing physical complexity facilitated fish use of seawalls as habitat by providing refuge, while it also hindered fish feeding by providing refuge for their prey. Cryptobenthic fish are important trophic linkages in their ecosystems and we have shown that by changing habitat complexity, we can change the habitat use and feeding activity of these fish, allowing them to fulfil this essential ecosystem role.

Lunar influences in the diet habits and selectivity of larval clingfish Gobiesox marmoratus.

The influence of the lunar cycle on the feeding habits, feeding success and selectivity was tested in situ on larval stages of the clingfish Gobiesox marmoratus (Gobiesocidae). Gobiesox marmoratus larvae and their prey were collected in the water column during the lunar cycle in austral spring 2015 and 2016, in shallow waters (<30 m depth) of El Quisco, central Chile. Feeding incidence was high (80-100%) throughout the moon cycle. The diet was composed of 32 prey items, being gastropod larvae and invertebrate eggs the most important. Prey composition varied among lunar phases with the lowest prey richness during new moon. During 2016, G. marmoratus larvae ingested the lowest number, but the largest prey during new moon. In spring 2015, there were no significant changes in the abundance of each prey taxon in the water column among moon phases, but in spring 2016 there were significant differences in the abundance of cypris and copepod nauplii, particularly between full and new moon. Feeding selectivity index showed that gastropod larvae were positively selected throughout the lunar cycle during spring 2015. In spring 2016, invertebrate eggs were selectively ingested in full moon and third quarter, but at a new moon G. marmoratus larvae selected gastropod larvae. Mean temperature of the water column and its vertical gradient and nocturnal cloud cover influences the feeding success of larval G. marmoratus. Lunar illumination favoured only an increase of richness of prey items. Therefore, nocturnal cloud cover precludes a greater influence of lunar illumination in the larval trophic ecology of this crypto-benthic fish.

Nearshore environmental conditions influence larval growth and shape changes for a temperate rocky reef fish

Coastal upwelling and other oceanographic processes, exert influences on phenotypic traits of early life stages of marine fishes. However, it remains to elucidate the environmental effects on the morphology of fish larvae. Using geometric morphometrics and otolith microstructure of a temperate rocky reef fish, the labrisomid blenny Auchenionchus crinitus (Labrisomidae), we studied the larval growth, mortality and morphospace of four consecutive cohorts, hatched between austral winter and summer off central Chile. We hypothesize that coastal environmental conditions will influence early life traits of this cryptobenthic fish. The winter cohort, which inhabited colder and well-mixed nearshore waters, grew more slowly and had a more hydrodynamic head shape, while the summer cohort, which lived in warmer and stratified waters, grew faster, but had a more robust shape. Cohorts expressed less ontogenetic allometry as seasons progressed, suggesting that the seasonal increment in water temperature reduces the shape changes with size during larval development. Also, slower- and faster-growing larvae showed large variation from the consensus shape, e.g. had the largest shape disparity. Nonetheless, all cohorts showed similar mortality rates during the first month of life. Finally, increases in zonal wind stress (i.e. sea breeze) and decreases in the sea temperatures (i.e. upwelling events) increased the growth rates of older (> 15 days old) larvae. Therefore, for this cryptobenthic species, differences in growth rate and body shape are related to the seasonal increase in water temperature, and local-scale meteorological conditions.

Little fish make a big contribution

Coral Reefs Coral reefs represent one of the most biodiverse and rich ecosystems. Such richness conjures up images of coral heads and large colorful reef fishes. Brandl et al. show, however, that one of the most striking and important parts of the reef ecosystem is almost never seen (see the Perspective by Riginos and Leis). Small cryptobenthic fish, like blennies, make up nearly 40% of reef fish biodiversity. Furthermore, the majority of cryptobenthic fish larvae settle locally, rather than being widely dispersed, and have rapid turnover rates. Such high diversity and densities could thus provide the biomass base for larger, better-known reef fish. Science , this issue p. [1189][1]; see also p. [1128][2] [1]: /lookup/doi/10.1126/science.aav3384 [2]: /lookup/doi/10.1126/science.aax8961

Visual modelling supports the potential for prey detection by means of diurnal active photolocation in a small cryptobenthic fish

Active sensing has been well documented in animals that use echolocation and electrolocation. Active photolocation, or active sensing using light, has received much less attention, and only in bioluminescent nocturnal species. However, evidence has suggested the diurnal triplefin Tripterygion delaisi uses controlled iris radiance, termed ocular sparks, for prey detection. While this form of diurnal active photolocation was behaviourally described, a study exploring the physical process would provide compelling support for this mechanism. In this paper, we investigate the conditions under which diurnal active photolocation could assist T. delaisi in detecting potential prey. In the field, we sampled gammarids (genus Cheirocratus) and characterized the spectral properties of their eyes, which possess strong directional reflectors. In the laboratory, we quantified ocular sparks size and their angle-dependent radiance. Combined with environmental light measurements and known properties of the visual system of T. delaisi, we modeled diurnal active photolocation under various scenarios. Our results corroborate that diurnal active photolocation should help T. delaisi detect gammarids at distances relevant to foraging, 4.5 cm under favourable conditions and up to 2.5 cm under average conditions. To determine the prevalence of diurnal active photolocation for micro-prey, we encourage further theoretical and empirical work.

From scuba diving to social networks: A curious association between two small fish species, Lepadogaster candolii Risso, 1810 and Parablennius rouxi (Cocco, 1833), and Muraena helena (Linnaeus, 1758) coming from citizen science

In this work, we report for the first time the associationbetween two cryptobenthic fish, Lepadogaster candolii and Parablennius rouxi, and the Mediterranean moray, Muraena helena. The nature of these associations and the importance of citizen science and social network in marine biology are discussed. Other associations with two relatively large fishes (Conger conger and Epinephelus marginatus) emerged during the study.

The contrast sensitivity function of a small cryptobenthic marine fish.

Spatial resolution is a key property of eyes when it comes to understanding how animals' visual signals are perceived. This property can be robustly estimated by measuring the contrast sensitivity as a function of different spatial frequencies, defined as the number of achromatic vertical bright and dark stripe pairs within one degree of visual angle. This contrast sensitivity function (CSF) has been estimated for different animal groups, but data on fish are limited to two free-swimming, freshwater species (i.e., goldfish and bluegill sunfish). In this study, we describe the CSF of a small marine cryptobenthic fish (Tripterygion delaisi) using an optokinetic reflex approach. Tripterygion delaisi features a contrast sensitivity that is as excellent as other fish species, up to 125 (reciprocal of Michelson contrast) at the optimal spatial frequency of 0.375 c/°. The maximum spatial resolution is instead relatively coarse, around 2.125 c/°. By comparing our results with acuity values derived from anatomical estimates of ganglion cells' density, we conclude that the optokinetic reflex seems to be adapted to process low spatial frequency information from stimuli in the peripheral visual field and show that small marine fish can feature excellent contrast sensitivity at optimal spatial frequency.

Hidden diversity in pebble beaches - a thus far unrecognized radiation of the cryptobenthic fish genus Gouania (Gobiesocidae)

Event Abstract Back to Event Hidden diversity in pebble beaches - a thus far unrecognized radiation of the cryptobenthic fish genus Gouania (Gobiesocidae) Maximilian Wagner1* 1 University of Graz, Institute of Biology, Austria Gouania willdenowi (Risso 1810), the sole representative of the monotypic clingfish genus Gouania, is endemic to the Mediterranean Sea where it lives a very cryptic and secretive life in the interstices of pebble beaches. With its wormlike body and small eyes, it is perfectly adapted to this extreme environment. Because of a reduced swim bladder and a very patchy distribution of suitable habitats, adult G. willdenowi are very weak active dispersers and dispersal likely happens passively during the pelagic larval phase. Despite being quite common in pebble beaches throughout the Mediterranean, hardly anything is known about its biology, behaviour, phylogeny or taxonomy. However, DNA-barcoding, species tree analysis (*BEAST) including 9 nuclear and one mitochondrial marker, as well as geometric morphometric analyses (GEMO) and MicroCT imaging across the whole Mediterranean Sea, yielded unexpected results regarding the evolutionary history of this genus. Molecular data suggests that the genus Gouania underwent a thus far unrecognized radiation into five highly divergent species that started around three million years ago. Furthermore, GEMO revealed two parallelly evolved morphotypes characterized by a slender (smaller eyes, increased vertebrae number) and a stout (larger eyes, smaller number of vertebrae) body shape. Whereas in the Adriatic and the Eastern Mediterranean basins “slender” and “stout” Gouania occur sympatrically, only the “stout” morphotype was found in the Western Mediterranean. To conclude, the radiation of Gouania represents a promising model system for studying radiation and the independent origin of eco-morphologically similar species pairs, and the role of water currents and ecological potential for driving diversification in the marine environment. Keywords: Gobiesocidae, Cryptic diversity, Cryptobenthic fishes, Gouania, phylogeny Conference: XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019. Presentation Type: Oral Topic: TAXONOMY, PHYLOGENY AND ZOOGEOGRAPHY Citation: Wagner M (2019). Hidden diversity in pebble beaches - a thus far unrecognized radiation of the cryptobenthic fish genus Gouania (Gobiesocidae). Front. Mar. Sci. Conference Abstract: XVI European Congress of Ichthyology. doi: 10.3389/conf.fmars.2019.07.00099 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 10 May 2019; Published Online: 14 Aug 2019. * Correspondence: Mr. Maximilian Wagner, University of Graz, Institute of Biology, Graz, Austria, maximilian.wagner@uni-graz.at Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Maximilian Wagner Google Maximilian Wagner Google Scholar Maximilian Wagner PubMed Maximilian Wagner Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

High diversity, but low abundance of cryptobenthic fishes on soft sediment habitats in Southeast Asia

Cryptobenthic fishes play a crucial role in marine ecosystems as trophic links between the base of the food chain and higher-level consumers. Infralittoral soft sediments are the largest marine habitat, yet little is known about fish assemblages in these ecosystems. This study investigates the cryptobenthic fish abundance and diversity on soft sediment habitats in the centre of tropical marine biodiversity. We surveyed 20 sites across three regions in Indonesia (Bali, Lembeh Strait) and the Philippines (Dauin) using Underwater Visual Surveys (UVC) and roving diver surveys. We tested the effects of depth (6 m, 16 m), benthic cover and sediment grain size characteristics on fish assemblages. Our results showed a high diversity (112 species), but low abundances (mean: 93 individuals/500 m2 ± SE: 28 ind.). Benthic cover on surveyed sites consisted for 90.1% (±SE: 0.7%) of unconsolidated sediments, which were predominantly poorly sorted (σ = 0.975 φ), gravelly sand. PERMANOVA analyses showed that fish assemblages and fish diversity was significantly different between regions (p < 0.001) and depth (p = 0.002). Distance based linear Models (DistLM) explained respectively 25% and 33.5% of the variation in fish assemblages and fish diversity. The high cryptobenthic fish diversity found in this study exceeds that of many coral reefs and contradicts the current view of soft sediment fish communities as depauparate ones. Our results provide valuable insights in a poorly studied marine ecosystem and call for more research in these valuable habitats.

Gamma-diversity partitioning of gobiid fishes (Teleostei: Gobiidae) ensemble along of Eastern Tropical Pacific: Biological inventory, latitudinal variation and species turnover

Gobies are the most diverse marine fish family. Here, we analysed the gamma-diversity (γ-diversity) partitioning of gobiid fishes to evaluate the additive and multiplicative components of α and β-diversity, species replacement and species loss and gain, at four spatial scales: sample units, ecoregions, provinces and realms. The richness of gobies from the realm Eastern Tropical Pacific (ETP) is represented by 87 species. Along latitudinal and longitudinal gradients, we found that the γ-diversity is explained by the β-diversity at both spatial scales, ecoregions and provinces. At the ecoregion scale, species are diverse in the north (Cortezian ecoregion) and south (Panama Bight ecoregion) and between insular and coastal ecoregions. At the province scale, we found that the species turnover between the warm temperate Northeast Pacific (WTNP), Tropical East Pacific (TEaP) and the Galapagos Islands (Gala) was high, and the species nestedness was low. At the ecoregion scale, historical factors, and phylogenetic factors have influenced the hotspots of gobiid fish biodiversity, particularly in the Cortezian, Panama Bight and Cocos Island ecoregions, where species turnover is high across both latitudinal and longitudinal gradients. At the provincial level, we found that the contributions of the β-diversity from north to south, in the WTNP, TEaP and Gala were high, as result of the high number of unique species. Species turnover was also high at this scale, with a low contribution from species nestedness that was probably due to the low species/gene flow within the provinces. These results highlight the importance and successful inclusion of a cryptobenthic fish component in ecological and biogeographical studies.

Comparison of cryptobenthic reef fish communities among microhabitats in the Red Sea

Knowledge of community structure within an ecosystem is essential when trying to understand the function and importance of the system and when making related management decisions. Within the larger ecosystem, microhabitats play an important role by providing inhabitants with a subset of available resources. On coral reefs, cryptobenthic fishes encompass many groups and make up an important proportion of the biodiversity. However, these fishes are relatively small, exhibit extreme visual or behavioral camouflage, and, therefore, are often overlooked. We examined the differences in fish community structure between three common reef microhabitats (live hard coral, dead coral rubble, and sand) using ichthyocide stations in the central Red Sea. Using a combination of morphological and genetic (cytochrome oxidase I (COI) barcoding) techniques, we identified 326 individuals representing 73 species spread across 17 families, from fifteen 1 m2 quadrats. Fish assemblages in the three microhabitats were significantly different from each other. Rubble microhabitats yielded the highest levels of fish abundance, richness, and diversity, followed by hard coral, and then sand. The results show that benthic composition, even at a small scale, influences cryptobenthic communities. This study also provides new COI sequence data to public databases, in order to further the research of cryptobenthic fishes in the Red Sea region.

The hidden half: ecology and evolution of cryptobenthic fishes on coral reefs.

Teleost fishes are the most diverse group of vertebrates on Earth. On tropical coral reefs, their species richness exceeds 6000 species; one tenth of total vertebrate biodiversity. A large proportion of this diversity is composed of cryptobenthic reef fishes (CRFs): bottom-dwelling, morphologically or behaviourally cryptic species typically less than 50 mm in length. Yet, despite their diversity and abundance, these fishes are both poorly defined and understood. Herein we provide a new quantitative definition and synthesise current knowledge on the diversity, distribution and life history of CRFs. First, we use size distributions within families to define 17 core CRF families as characterised by the high prevalence (>10%) of small-bodied species (<50 mm). This stands in strong contrast to 42 families of large reef fishes, in which virtually no small-bodied species have evolved. We posit that small body size has allowed CRFs to diversify at extremely high rates, primarily by allowing for fine partitioning of microhabitats and facilitation of allopatric reproductive isolation; yet, we are far from understanding and documenting the biodiversity of CRFs. Using rates of description since 1758, we predict that approximately 30 new species of cryptobenthic species will be described per year until 2050 (approximately twice the annual rate compared to large fishes). Furthermore, we predict that by the year 2031, more than half of the described coral reef fish biodiversity will consist of CRFs. These fishes are the 'hidden half' of vertebrate biodiversity on coral reefs. Notably, global geographic coverage and spatial resolution of quantitative data on CRF communities is uniformly poor, which further emphasises the remarkable reservoir of biodiversity that is yet to be discovered. Although small body size may have enabled extensive diversification within CRF families, small size also comes with a suite of ecological challenges that affect fishes' capacities to feed, survive and reproduce; we identify a range of life-history adaptations that have enabled CRFs to overcome these limitations. In turn, these adaptations bestow a unique socio-ecological role on CRFs, which includes a key role in coral reef trophodynamics by cycling trophic energy provided by microscopic prey to larger consumers. Although small in body size, the ecology and evolutionary history of CRFs may make them a critical component of coral-reef food webs; yet our review also shows that these fishes are highly susceptible to a variety of anthropogenic disturbances. Understanding the consequences of these changes for CRFs and coral reef ecosystems will require us to shed more light on this frequently overlooked but highly diverse and abundant guild of coral reef fishes.