Abstract

Small and cryptic organisms that live within the interstices of reef habitats contribute greatly to coral reef biodiversity, but are poorly studied. Many species of cryptofauna have seemingly obligate associations with live coral and are therefore considered to be very vulnerable to coral mortality. Here we report the unanticipated prevalence of obligate coral-dwelling decapod crustaceans on dead colonies of branching corals in the Chagos Archipelago (British Indian Ocean Territory) in the central Indian Ocean. A total of 205 obligate coral-dwelling decapods, including Trapezia crabs, were recorded from 43 (out of 54) dead coral colonies of Acropora and Pocillopora collected across five different atolls. Trapezia individuals found on dead corals were mainly juveniles, and the few adults were almost exclusively male. Among the shrimps (Pontoniinae), however, it was predominantly adult females found on dead corals. Obligate coral-dwelling species that typically occur only on live Pocillopora hosts (e.g., Trapezia spp.) were recorded on dead Acropora. These findings suggests that these obligate coral-dwelling decapods are not simply persisting on coral hosts that have died, but may be explicitly recruiting to or moving to dead coral hosts at certain stages in their life cycle. Variation in the abundance of live coral among sites had no affect on the presence or abundance of obligate coral-dwelling decapods on dead corals. This study shows that habitat associations of obligate coral-dwelling organisms, and their reliance on different habitat types, are complex and further work is required to establish their vulnerability to widespread habitat degradation on coral reefs.

Highlights

  • Coral reefs exhibit extremely high levels of biodiversity with estimated species richness ranging from 172,000 to over nine million species worldwide (Reaka-Kudla 1997; Small et al 1998; Ruppert et al 2004)

  • We report on the unanticipated abundance of obligate coral-dwelling decapod species on dead colonies of Acropora and Pocillopora in the Chagos Archipelago

  • We refer to the live coral obligates reported here as ‘obligate coral-dwelling’ decapods as we found these individuals on dead coral, and it is unclear whether these organisms are completely dependent on live coral throughout their life cycle

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Summary

Introduction

Coral reefs exhibit extremely high levels of biodiversity with estimated species richness ranging from 172,000 to over nine million species worldwide (Reaka-Kudla 1997; Small et al 1998; Ruppert et al 2004). The vast majority of species living on coral reefs are small, cryptic invertebrate species that live within the interstices of coral reef habitats (Reaka-Kudla 1997). Many of these invertebrates have evolved to form close symbiotic associations with corals, branching Scleractinia, which provide food (mostly, in the form of coral mucus and associated detritus), as well as a physical refuge from predators (Castro 1988; Stella et al 2011a). Coral Reefs (2015) 34:905–915 corals often benefit from the presence of coral-dwelling invertebrates. Obligate coral-dwelling crabs of the family Trapeziidae actively defend their coral hosts against corallivores, e.g., Acanthaster planci (Glynn 1982; Pratchett et al 2000; Pratchett 2001; McKeon and Moore 2014), as well as removing excess sediment in turbid conditions which may otherwise smother the coral (Stewart et al 2006, 2013)

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