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PLoS One
2018 Jan 01;131:e0190470. doi: 10.1371/journal.pone.0190470.
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Comparative metabolic ecology of tropical herbivorous echinoids on a coral reef.
Lewis LS
,
Smith JE
,
Eynaud Y
.
Abstract
BACKGROUND: The metabolic rate of consumers is a key driver of ecosystem dynamics. On coral reefs, herbivorous echinoids consume fleshy algae, facilitating the growth of reef-building calcified organisms; however, little is known about differences among species in their metabolic and functional ecology. Here, we used log-linear (log-log) regression models to examine the allometric scaling of mass and routine metabolic rate for five common herbivorous echinoids on a Hawaiian coral reef: Echinothrix calamaris, E. diadema, Echinometra matthaei, Heterocentrotus mammillatus, and Tripneustes gratilla. Scaling relationships were then contrasted with empirical observations of echinoid ecology and general metabolic theory to broaden our understanding of diversity in the metabolic and functional ecology of tropical herbivorous echinoids.
RESULTS: Test diameter and species explained 98% of the variation in mass, and mass and species explained 92.4% and 87.5% of the variation in individual (I) and mass-specific (B) metabolic rates, respectively. Scaling exponents did not differ for mass or metabolism; however, normalizing constants differed significantly among species. Mass varied as the cube of test diameter (b = 2.9), with HM exhibiting a significantly higher normalizing constant than other species, likely due to its heavily-calcified spines and skeleton. Individual metabolic rate varied approximately as the 2/5 power of mass (γ = 0.44); significantly smaller than the 3/4 universal scaling coefficient, but inclusive of 2/3 scaling. E. calamaris and H. mammillatus exhibited the lowest normalizing constants, corresponding with their slow-moving, cryptic, rock-boring life-history. In contrast, E. calamaris, E. diadema, and T. gratilla, exhibited higher metabolic rates, likely reflecting their higher levels of activity and ability to freely browse for preferred algae due to chemical anti-predator defenses. Thus, differences in metabolic scaling appeared to correspond with differences in phylogeny, behavior, and ecological function. Such comparative metabolic assessments are central to informing theory, ecological models, and the effective management of ecosystems.
Fig 1. Study design.(A) Herbivorous echinoids used in this study, (B) design of portable flow-through acclimation and assay water baths, (C) design of portable respirometry chamber. Echinoid artwork by Adi Khen.
Fig 2. Allometric relationships of echinoid mass and metabolism.Mass (M, g) vs. test diameter (D, cm) (a-b); and individual (I, mgO2/h) (c-d) and biomass-specific (B, mgO2/g/h) (e-f) metabolic rates vs. mass. Figures on the left (a,c,e) include all individuals pooled; figures on the right (b,d,f) include species as fixed factors (slopes = homogenous). All data were Log10(x) transformed and lines represent ordinary least-square linear fits of log-log transformations (Table 2). Echinoid species codes as in Fig 1a.
Fig 3. Allometric scaling parameters for mass and metabolism.Plots of (a) estimated mass-scaling exponents and predicted values (grey-dashed lines) for mass (b = 3.0), individual metabolic rate (γ = 0.75) and mass-specific metabolic rate (α = -0.25), and (b-d) species-specific normalizing coefficient modifiers (ΔiM,ΔiI,ΔiB). Letters in (b-d) indicate groupings based on 95% confidence intervals. All error bars = ±95% confidence intervals. Red-dashed lines = 0. Echinoid species codes as in Fig 1a.
Fig 4. Back-transformed (raw) metabolic scaling relationships.Echinoid mass (M in g) versus (A) individual metabolic rate (I in mgO2/h), and (B) biomass-specific metabolic rate (B in mgO2/g/h). Lines represent corresponding scaling functions from Table 4. Urchin codes as in Fig 1a.
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