Below the Mesophotic

Abstract

Mesophotic coral ecosystems, which occur at depths of ~40 to 150 m, have received recent scientific attention as potential refugia for organisms inhabiting deteriorating shallow reefs. These ecosystems merit research in their own right, as they harbor both depth-generalist species and a distinctive reef-fish fauna. Reef ecosystems just below the mesophotic are globally underexplored, and the scant recent literature that mentions them often suggests that mesophotic ecosystems transition directly into those of the deep sea. Through submersible-based surveys in the Caribbean Sea, we amassed the most extensive database to date on reef-fish diversity between ~40 and 309 m at any single tropical location. Our data reveal a unique reef-fish assemblage living between ~130 and 309 m that, while taxonomically distinct from shallower faunas, shares strong evolutionary affinities with them. Lacking an existing name for this reef-faunal zone immediately below the mesophotic but above the deep aphotic, we propose "rariphotic." Together with the "altiphotic," proposed here for the shallowest reef-faunal zone, and the mesophotic, the rariphotic is part of a depth continuum of discrete faunal zones of tropical reef fishes, and perhaps of reef ecosystems in general, all of which warrant further study in light of global declines of shallow reefs.

Figure 1

Depth distributions of 71 mesophotic and rariphotic fishes derived from 4,436 depth observations between 40 and 309 m off Curaçao, southern Caribbean. Boxes indicate the 25^th and 75th quantiles, whiskers are 1.5 the interquartile, and circles are outliers.

Figure 2

Representative Caribbean fishes inhabiting the rariphotic zone off Curaçao. Haptoclinus dropi (Labrisomidae); Pontinus castor (Scorpaenidae); Anthias asperilinguis (Serranidae); Lipogramma evides (Grammatidae); Serranus notospilus (Serranidae); Polylepion sp. (Labridae). Photograph of A. asperilinguis by Patrick Colin, other photographs by C. C. Baldwin and D. R. Robertson.

Figure 3

Hierarchical clustering dendrogram from the Bray-Curtis dissimilarity analysis of 4,436 depth observations of fishes between 40 and 309 m off Curaçao. Thick solid black lines below clusters indicate groups that have significantly (p < 10⁻⁷) distinct faunal communities based on a SIMPROF analysis. Depth bins are 10-m intervals labeled by the minimum depth in each interval (e.g., “100 m” = 100–109 m).

Figure 4

Nonmetric multidimensional scaling ordination (MDS) plot derived from the Bray-Curtis dissimilarity analysis. Red and blue polygons represent mesophotic and rariphotic zones, respectively. Shaded polygons represent upper and lower mesophotic and rariphotic sections. The two zones and four sections were derived from eight significantly different (p < 10⁻⁷) clusters in the SIMPROV analysis. Each a posteriori zone and section is significantly distinct (PERMANOVA, p < 0.01).

Figure 5

Families of rariphotic reef-fish species off Curaçao analyzed in this study and the predominant depth category to which each can be assigned. Altiphotic/mesophotic families are those for which depth ranges of members are predominantly shallower than 130 m^–. Rariphotic families are those predominantly inhabiting depths > 130 to as deep as 500 m^–. Deep-sea (aphotic) families are those typically inhabiting depths > 500 m^,–.

Figure 6

Classification of faunal zones above the aphotic based on analysis of fish assemblages at Curaçao. Representative coral and fish species are depicted for each zone. Altiphotic: (left) the zooxanthellate coral, Acropora palmata, and (right) Thalassoma bifasciatum (depth off Curaçao 0-35 m); Mesophotic: (left) a zooxanthellate plate coral, Agaricia sp., and (right) Lipogramma levinsoni (depth off Curaçao 91–154 m); Rariphotic: (left) the lace coral Stylaster sp. and gorgonian Nicella sp., and (right) Liopropoma olneyi (depth off Curaçao 112–229 m). Photos by Federico Cabello (upper left), Kevin Bryant (upper right), C. C. Baldwin, D. R. Robertson, and L. Tornabene.

Figure 7

Mean (with range and +/− one standard deviation) temperatures at 10-m intervals between the surface and 309 m recorded from 319 submersible dives off Curaçao.