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

Abstract

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.

Reconstructing hyperdiverse food webs: Gut content metabarcoding as a tool to disentangle trophic interactions on coral reefs

“…The two most dominant taxonomic OTU categories detected in our study, brachyuran crabs and caridean shrimps, are abundant organisms in standardized surveys of marine biodiversity as well as prevalent OTUs across marine fish diets (Brandl, Goatley, Bellwood, & Tornabene, ; Harmelin‐Vivien, Kaim‐Malka, Ledoyer, & Jacob‐Abraham, ; Plaisance, Caley, Brainard, & Knowlton, ; Randall, ). While most studies list these prey items simply as brachyuran crabs and caridean shrimps, our data identify 300 distinct OTUs classified as brachyuran crabs and 286 distinct OTUs classified as caridean shrimps.…”

Repeated cyclone events reveal potential causes of sociality in coral-dwelling Gobiodon fishes

“…Many species of coral-reef fishes, especially habitat-specialists, can be found in social groups [33, 34–36]. The size of these social groups is often related directly or indirectly to the size of the habitat in which they reside [36–39].…”

“…The size of these social groups is often related directly or indirectly to the size of the habitat in which they reside [36–39]. Complex social structures such as size-based dominance hierarchies, in which the largest dominants breed and smaller subordinates are reproductively suppressed, have been documented in these groups [23, 38].…”

“…These species are small (3–4 cm) microbenthic [36] habitat-specialist fishes that live within the structures of branching and plate-forming acroporid and pocilloporid corals [53, 54]. These fishes are highly site attached once settled, but have been shown to move between corals [41].…”

A new genus and species of clingfish from the Rangitāhua Kermadec Islands of New Zealand (Teleostei, Gobiesocidae)

“…Clingfishes are considered archetypal crypto-benthic fishes (Brandl et al 2018) and it is not surprising that new species continue to be discovered and described on an almost annual basis. Since 2010, this includes 19 new species (Fricke et al 2010, 2015, 2016; Allen and Erdman 2012; Moore et al 2012; Sparks and Gruber 2012; Conway et al 2014, 2017b, c, 2018; Fricke 2014; Craig et al 2015; Shinohara and Katayama 2015; Bilecenoğlu et al 2017; Fricke and Wirtz 2017; Hastings and Conway 2017; Fujiwara and Motomura 2018; Fujiwara et al 2018), three of which were also considered to represent new genera at the time of description (Fricke 2014; Fricke et al 2016; Conway et al 2017b).…”