Abstract
Environmental clines such as latitude and depth that limit species’ distributions may be associated with gradients in habitat suitability that can affect the fitness of an organism. With the global loss of shallow-water photosynthetic coral reefs, mesophotic coral ecosystems (~30–150 m) may be buffered from some environmental stressors, thereby serving as refuges for a range of organisms including mobile obligate reef dwellers. Yet habitat suitability may be diminished at the depth boundary of photosynthetic coral reefs. We assessed the suitability of coral-reef habitats across the majority of the depth distribution of a common demersal reef fish (Stegastes partitus) ranging from shallow shelf (SS, <10 m) and deep shelf (DS, 20–30 m) habitats in the Florida Keys to mesophotic depths (MP, 60–70 m) at Pulley Ridge on the west Florida Shelf. Diet, behavior, and potential energetic trade-offs differed across study sites, but did not always have a monotonic relationship with depth, suggesting that some drivers of habitat suitability are decoupled from depth and may be linked with geographic location or the local environment. Feeding and diet composition differed among depths with the highest consumption of annelids, lowest ingestion of appendicularians, and the lowest gut fullness in DS habitats where predator densities were highest and fish exhibited risk-averse behavior that may restrict foraging. Fish in MP environments had a broader diet niche, higher trophic position, and higher muscle C:N ratios compared to shallower environments. High C:N ratios suggest increased tissue lipid content in fish in MP habitats that coincided with higher investment in reproduction based on gonado-somatic index. These results suggest that peripheral MP reefs are suitable habitats for demersal reef fish and may be important refuges for organisms common on declining shallow coral reefs.
Highlights
Suitability of a habitat for an organism can be influenced by environmental parameters that are associated with ecological clines that limit distributions (Sunday et al 2012) and small-scale habitat variability
The mechanisms that determine habitat suitability may be the result of patchy habitat variability, or be linked to depth gradients or horizontal clines
At the depth boundaries of most coral-reef organisms, S. partitus has high reproductive investment and likely higher fitness than shallow water populations, indicating that habitat suitability may be decoupled from the processes that limit distributions and abundance across depths (Goldstein et al 2016b)
Summary
Suitability of a habitat for an organism can be influenced by environmental parameters that are associated with ecological clines that limit distributions (Sunday et al 2012) and small-scale habitat variability. Trade-offs between condition, growth, reproduction, and mortality can regulate behavioral responses of fishes to predation risk, often at the expense of foraging or reproductive output (Holbrook and Schmitt 1988; Dahlgren and Eggleston 2000). Such differences in habitat suitability affect the surplus of energy available for processes such as tissue repair and maintenance, defense and foraging, or demographic rates and reproduction that define fitness (Booth 1995; Munday 2001)
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